multi-slice ct scanners - buyers’ guide

8/15/2019 Multi-slice CT scanners - Buyers’ guide

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Buyers’ guide

Multi-slice CT scanners

CEP08007

March 2009


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Contents 2

CEP08007: March 2009

Introduction ............................................................................................. 3

Technical considerations......................................................................... 9

Operational considerations.................................................................... 29

Economic considerations ...................................................................... 53

Purchasing............................................................................................ 62

Market review........................................................................................ 72

Acknowledgements ............................................................................. 113

Glossary.............................................................................................. 114

References.......................................................................................... 127

Appendix 1: Supplier contact details ................................................... 142

Appendix 2: EU procurement procedure ............................................. 143

Appendix 3: Supporting sustainable purchasing.................................. 146

Appendix 4: Preparing a specification ................................................. 149

Appendix 5: Example statement of operational requirements.............. 160

Appendix 6: Site visits ......................................................................... 169

Appendix 7: Evaluation scoring........................................................... 176

Author and report information.............................................................. 181


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Introduction 3


Scope and purpose

This buyers’ guide is designed to help purchasers to select a multi-slice CT scannerwhich best meets their needs. It covers technical, operational, and economicconsiderations, provides guidance on the purchasing process, and presents a reviewof the multi-slice scanners available on the UK market. The guide does not coverPET/CT.

It is aimed at a variety of readers involved in the purchasing and decision-makingprocess, including finance directors and capital equipment boards, businessmanagers, radiologists and physicists. Information on the basic principles of CTscanning is provided for the benefit of those with little direct experience of thetechnology. Those involved in specifying equipment to be purchased will find themarket overview and technical specifications useful. Full comparative specificationsare available separately (Table 1).

Table 1: Comparative specification reports fo r multi -slice CT systems

Report number Report title

CEP08024 CT clinical applications software [84]

CEP08025 16 slice CT scanner technical specifications [85]

CEP08026 32 to 40 slice CT scanner technical specifications [86]

CEP08027 64 slice CT scanner technical specifications [87]

CEP08028 128 to 320 slice CT scanner technical specifications [88]

CEP08029 Wide bore CT scanner technical specifications [89]

Overview of a CT scanner

Computed tomography (CT) scanners were first introduced into clinical use in 1972and are now an indispensable tool within the radiology department. The technologyhas progressed greatly since that time, and the range of clinical applicationscontinues to grow.

CT scanning is a cross-sectional imaging modality, and as such offers a keydifference from standard diagnostic X-ray imaging. In the latter, structures overlayeach other in the resultant image and may be hard to differentiate from each other.However in a CT scan, which utilises X-rays in a different geometrical and dataacquisition arrangement, a cross-sectional image results which allows thedifferentiation of overlying structures.


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Introduction 4


A CT scanner may be purchased to replace or supplement an older scanner, to meetincreased demands on the service, or to take advantage of new developments whichenable improved diagnostics, faster throughput or other clinical benefits. CTscanners are, however, expensive both to purchase and to operate, hence it isimportant to select the scanner which offers greatest value for money.

Of equal importance to the hardware, is the clinical applications software. This isavailable for different clinical tasks, and varies in complexity from the standard two-dimensional (2D) reconstructions which are available on the scanner console, tomore complex three-dimensional (3D) reconstructions and software packages which

are often carried out on an ancillary workstation. With the advent of multi-slice CT(MSCT) and volumetric datasets, the facility to perform 3D reconstruction hasbecome an essential component of the CT system. The range of clinical applicationssoftware is extensive, and that of greater complexity is purchased optionally.

As with any imaging modality involving the use of X-rays, under The IonisingRadiation (Medical Exposure) Regulations, 2000 (IR(ME)R 2000) [22], the hazards ofionising radiation must be considered and the use of the technology justified, ie itshould only be employed if the clinical benefit of the examination outweighs the riskof cancer induction or other radiation-induced morbidity. The price of the additionaldiagnostic information obtained from a CT scan is the higher radiation dose

compared with a conventional X-ray; this is by a factor of the order of 10 for head andbody routine imaging to about 200 for chest imaging. The doses are of equivalentorder to those from nuclear medicine studies or conventional angiography. However,manufacturers have invested considerable effort into minimising the radiation dosedelivered by their scanners whilst optimising image quality (see Technicalconsiderations).

Basic princip les of CT scanning

CT scanners are used to image the internal structures of the body. They providedetailed anatomical information by utilising the principle that different types of tissue,depending on their composition and density, absorb varying amounts of X-rays. Thestructures scanned are displayed in the image as different shades of grey.Intravenous or oral contrast media may be used to further enhance discriminationbetween tissues.

The basic components of a CT scanner are an X-ray tube and an arc of detectors,mounted on a gantry with a circular aperture (Figure 1a). Along the patient long axisthere are many rows of these arcs of detectors, giving rise to the term multi-slice CT(Figure 2). Multirow CT, or multidetector CT (MDCT) are also commonly used terms.The extent of patient coverage by the detector rows currently ranges from 12 mm to

160 mm in length, depending on the scanner model.


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Introduction 5


Figure 1. Schematic diagram of the CT scanner (a) ‘end v iew’, and (b) ‘side view’ in helicalacquisition mode

(a) (b)

patient

Power Data

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detectors

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Figure 2. Multi-slice CT scanner X-ray beam and detectors (a) approximately to scale, and (b)schematic

(a) (b)

12 - 160 mmeach < 1 mm

12 - 160 mmeach < 1 mm

The patient lies on an integral couch and the X-ray tube and detectors rotate,continuously monitoring the absorption of X-rays as their path through the bodychanges. Image data can be acquired in sequential mode or in helical mode(Figure 1b).

In sequential mode, sometimes known as axial mode or ‘step and shoot’, the couchis stationary during each rotation, then steps through the gantry to the next position inorder to acquire another set of data. Some newer models have such an extent ofcoverage along the patient axis that for some studies only one rotation is needed.


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Introduction 6


If the couch moves through the gantry at a steady rate, during the irradiation, whilstcontinuously acquiring attenuation data, this is a ‘helical scan’ (Figure 1b). This isalso known as a spiral scan.

CT scanner technology has advanced rapidly in recent years, moving to morepowerful X-ray tubes, more efficient and stable detectors, more refined engineeringand data acquisition systems and electronics, and faster computers.

These developments have been largely directed towards improvements in the ‘threeFs of CT scanning’: faster scanning of further lengths of the patient, using finer slices.

As a result, CT has evolved from a slice-by-slice diagnostic imaging system into atruly volumetric imaging modality, where images can be reconstructed in any planewithout loss of image quality. This has lead to the increased use of multiplanar and3D display modes in diagnosis.

Field of use

Patients are referred to CT scanning from practically all clinical specialties. They maybe referred as in-patients, out-patients, or from the accident and emergencydepartment.

CT can be used in diagnosis, to assess the effectiveness of treatment, and to guideor plan clinical intervention. The majority of CT scans are currently performed toobtain anatomical information on a wide range of organs and tissues, but there areincreasing numbers of functional and interventional applications. Current commonuses are:

• neurology - intra-cranial examinations, including brain perfusion in strokeassessment, examination of sinus and ear canals, spinal investigations

• oncology – diagnosis, staging, follow up and radiotherapy treatment planning

• cardiology – including coronary angiography and calcium scoring

• angiography – whole body including venous, brain carotid, thorax, EVAR(endovascular aortic aneurysm repair) planning and follow up, andperipheral run-off

• thoracic – evaluation of acute chest pain or dyspnoea and diffuse lungdisease

• virtual endoscopy – including colonography and bronchoscopy

• orthopaedics - including surgical planning

• trauma

• image-guidance of interventional procedures – eg biopsy, drainage and RFablation.


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Introduction 7


Each trust will have identified the CT investigations it undertakes as part of itsdiagnostic imaging service, based on the current CT capabilities and availability ofother local diagnostic services.

Clinical impact

As a key element within the diagnostic imaging department, the purchase of a CTscanner will have a major impact on the provision of services within the hospital. Thenew scanner may enable specialist investigations to be undertaken that werepreviously referred externally, and may also reduce the demand on other modalities.It can therefore impact upon healthcare targets such as the Department of Health(DH) 18 week referral to treatment time (RTT) target [1], which includes a 6 weektarget for time of referral to diagnostic procedure.

The generally non-invasive nature of CT eliminates the need for hospitalisation dueto possible morbidity resulting from invasive procedures such as angiography andendoscopy. Both CT angiography (CTA) and CT colonography (CTC) investigationshave benefited from the advances in MSCT technology. CT angiography is becomingthe method of choice for the investigation of suspected pulmonary embolism [2], andin cardiac CT angiography (CCTA), significant improvement in diagnosticperformance has been shown for 16 and 64 slice scanners, compared with 4 slice

devices [3]. CT colonography (CTC) is also gaining rapid clinical acceptance [4], andis more tolerable for patients compared to conventional colonography [5].

MSCT scanners have significantly reduced scan times, minimising motion artefacts.The acquisition of longer volumes, with finer slices, has improved image quality,enhancing 3D resolution.

National guidance

The Royal College of Radiologists (RCR) has published guidance on which

examinations are best suited to CT [6]. The National Institute for Health and ClinicalExcellence (NICE) [7] and the Department of Health (DH) have also issued guidance;key recommendations are summarised in Table 2.


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Introduction 8


Table 2. NICE and DH guidance - key recommendations

Issuer Investigation Summary of recommendation

NICE Head Injury [8] Scan within either 1 or 8 hours of request, depending on riskfactors

NICE CT colonography [9] To replace conventional colonoscopy and double contrastbarium examinations

NICE Lung cancer [10] Follow up to abnormal chest X-ray to confirm staging ofdisease

DH Stroke [11] Scan in next available slot for requests during working hours

Scan within 1 hour for requests out-of-hours

There is also guidance associated with the use of personally initiated CT scans forthe health assessment of asymptomatic individuals, published by COMARE(Committee on Medical Aspects of Radiation in the Environment) [12]. For thesecircumstances, CT is not generally recommended, although certain allowances,under strict conditions, are made for coronary calcium scoring and colonography.


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Technical considerations 9


A good understanding of technical considerations will underpin selection of anappropriate CT scanner. Technical specifications available for CT scanners are oftenquite extensive. Although it is helpful to review these for each scanner component,such an exercise may not reflect the relative clinical performance of the systems. Thedetailed specifications provide a guide to the level of performance expected and canhighlight differences between manufacturers. These are given in the accompanyingCEP Comparative specification reports [84]-[89].

However it is also important to recognise that the performance in practice will dependon the trade-off between image quality and radiation dose. Each system should

therefore also be assessed in terms of the clinical, output-based specification it canmeet, with an observation of the radiation dose utilised, and this is often best done inconjunction with a site visit (appendix 6).

Key technical factors impacting on clinical performance are described in this chapter.Differences between categories of MSCT scanners (ie 16 slice, 64 slice etc) arehighlighted, as well as the impact of new technological developments, particularly intheir contribution to the advancement of the ‘three Fs of CT scanning’: to scan faster,further, and with finer slices.

Figure 3 illustrates the rapid pace of developments in scanner technology over thelast twenty years, and especially the acceleration of development in the last ten yearsfrom four to 320 slice scanners.

Figure 3. Technological advances in CT scanner technology, 1985 - 2008

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is available with two tubes, mounted at 90 degrees to each other, requires only a halfrotation of data, so is effectively even faster. This has specialist cardiac applications.For general body scanning, 0.5 second rotations are usually more than adequate,and for head scanning, 1 second rotation times are often sufficient.

Higher tube currents will be required for these faster rotation times, and whencombined with long scan lengths there will be a need for a high anode heat capacityor high anode cooling rate. This effect is off-set by the use of longer detector arraylengths.

Detector array length64 slice scanners cover a patient volume between 20 and 40 mm in length perrotation, and the latest diagnostic MSCT scanners can image patient volumes of upto 160 mm per rotation.

The length of detector array will determine the number of rotations needed to coverthe total scan length, and thus the overall scan time. The example in Figure 4 showshow the total scan time will be halved by doubling the array length .The ability to scana given length with fewer rotations also helps to minimise heat load on the X-raytube, thereby allowing the scanning of longer lengths.

Figure 4. Effect of detector array on number of ro tations and scan time

Detector arrays are broadly divided into two types; ‘fixed’ and ‘variable’, sometimesknown also as ‘matrix’ and ‘hybrid’. Fixed arrays have detectors of equal z-axisdimension over the full extent of the array, whereas on variable arrays, the centralportion comprises finer detectors. With variable arrays, the total scan time for a givenlength, for the finest slice acquisition, will be longer, because the z-axis coverage isreduced (Figure 5).

eg 20 mm eg 40 mmeg 20 mm eg 40 mmeg 20 mm eg 40 mm


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Figure 5. Example of 16 slice detector wi th reduced coverage for f ine slices

Examples of a range of actual detector configurations for different manufacturers areshown in Figure 6 (all detector array designs are given in the Market review). Thisfigure illustrates that there is no fixed pattern when manufacturers move from 16 to64 slice systems. Manufacturers A and B changed from a variable to a fixed arraydesign. However, manufacturer A doubled the length of the detector array, whereasmanufacturer B kept the same length. Manufacturer C retained the variable arraydesign for their initial 64 slice scanner, with little change in the overall array length.Not shown is their newest design where they kept the same length but changed to

the fixed array; that is detectors of all the same dimension.

All the scanners available with greater than 64 slice acquisition have a fixed array.

The evolution of designs reflects different strategies to accommodate futuredevelopments and allow for production costs. There is also some small dose savingwhere larger detector elements are used on the lower slice category scanners

16 x 1.25 mm16 x 0.625 mm

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Figure 6. Examples o f fi xed and variable z-axis detector arrays

Complete coverage of an organ, such as the brain or the heart, offers advantages forboth dynamic perfusion and cardiac studies. The z-axis detector array lengths on thecurrent 64 slice scanners, of up to 40 mm, are adequate to cover these organs inonly a few rotations. A coverage length of 160 mm usually allows complete organcoverage in a single rotation, so the function of the whole organ can be monitoredover time.

Techniques have recently been developed to extend the effective coverage fordynamic perfusion studies on scanners where the whole organ, or required part ofthe organ, is not completely covered by the detector array. There are two approachesto this. One is to perform consecutive, sequential scans by repeatedly ‘jogging’ thepatient couch between two z-axis positions, effectively doubling the length of organthat can be monitored. The second approach is to perform a ‘helical shuttle’ scan,whereby the organ is scanned in helical mode in alternating directions. The length ofcoverage in this mode is dependent largely on the frequency with which the organneeds to be monitored.

Manufacturer B:Variable array

Coverage:

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Figure 7. CT perfusion with ‘j og’ or ‘shuttle’ scan

X-ray tube

Modern CT scanning techniques place a high heat load on the X-ray tube due to theneed for high tube current values (mA) in order to give enough photons in the imagewhen scanning with fast rotations and fine slices. Increasing rates of obesity in theUK mean that the size of the average patient is an added burden on the X-ray tube,as higher tube currents need to be used in order to generate enough photons to givereasonable image quality. To scan a sufficiently long length, whilst avoidingoverheating, X-ray tubes have generally been developed to have high anode heatcapacities and high cooling rates. Some designs have low anode heat capacities, butvery high cooling rates to compensate. These two specifications, heat capacity and

cooling rate, therefore need to be considered jointly to assess the overall heat loadcapability. Implications in clinical practice can be enquired of during site visits.

Some designs that improve cooling rates are spiral-groove bearings with liquid metallubrication, and anodes with direct oil cooling.

Image quality

The principal parameters that describe image quality are spatial resolution, contrastresolution, temporal resolution, and the prevalence of artefacts. Manufacturers

provide performance specifications on spatial resolution and contrast resolution. TheInternational Electrotechnical Commission (IEC) has issued standards relating to the

40 mm detector

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measurement of some of these parameters [13],[25]. However, for some parameters,particularly those for contrast resolution, these can be difficult to compare betweendifferent systems, due to the differing methodologies employed. All manufacturerswill have some approaches to artefact reduction, depending on the type of artefact.Little objective comparative information, if any, is provided by manufacturers withrespect to artefacts, as there are no standard methods for their quantification.

The image quality actually achieved on any scanner will depend not only on scannerdesign features, but also on scan parameters selected and patient-related factors,and will always be a compromise between image quality and radiation dose. The

following sections deal with the impact of scanner design features on image qualityand radiation dose separately.

It is therefore important to be supported through the equipment selection andprocurement process by the local medical physics service, who should be invited toassist during site visits with objective and subjective comparisons of image quality,together with the radiation dose required to produce the image.

Spatial resolution

Spatial resolution is the ability of the system to image an object without blurring. It is

often described as the ‘sharpness’ of an image (Figure 8). It may be quoted as thesmallest object size able to be discerned, and as such is evaluated using highcontrast test objects where signal to noise level is high and does not influenceperception.

It can also be specified in terms of spatial frequency, in line pairs per cm (lp/cm), forparticular levels of the modulation transfer function (MTF); usually at the 50%, 10%and 2% or 0% levels. The 0% MTF level is referred to as the ‘cut-off frequency’ andreflects the limit of the spatial resolution. The visual limit of spatial resolution, as theminimum size of high contrast objects, in millimetres, that can be distinguished, moregenerally relates to the frequency values between approximately the 2 and 5%

modulation of the MTF. Sometimes a visual limit value is given by the manufacturers,either from a visual test object, or by converting the 2% value on the MTF to its sizein mm.

Modern MSCT scanners should be capable of achieving isotropic resolution: a z-axisresolution that is equal to, or approaching, the scan plane resolution, as this isessential for good quality multiplanar and 3D reconstructions.

In practice, it is helpful to remember that the cost of high spatial resolution is either inhigh image noise, or in high radiation dose when the tube current is raised to reduce

the image noise.


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Figure 8. Test object with line pairs of varying frequencies for assessment of scan plane spatialresolution

The following scanner design features affect the x-y plane spatial resolution:

• focal spot size (x-dimension)

• focal spot stability

• detector size (x-dimension)

• number of ‘views’ per rotation (sampling frequency)

• ‘over-sampling’ techniques

• quarter-detector shift

• flying/dynamic focal spot

• attenuating grid (x-y plane)

The focal spot size and detector size determine the ‘sampling aperture’. Thesampling frequency is the number of times data from the detectors is ‘read’ during arotation, and together with the sampling aperture determines the sampling density, iehow finely the object is sampled.

Over-sampling techniques are aimed at further enhancing the spatial resolution bysampling the object at intervals smaller than the sampling aperture. All modernscanners employ the quarter-detector shift approach, in which data from the second

180° of each rotation are off-set from the first 180° (Figure 9a). Some manufacturersalso use a dynamic or flying focal spot, effectively obtaining two sets of data, or‘views’ at each angular sampling position, increasing the sampling density still further(Figure 9b).

9 lp/cm

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Figure 9. Diagram of methods for improv ing sampling density (a) quarter-detector shift, (b)flying focal spot

For the highest spatial resolution, such as that required for imaging the internalauditory canal, a technique using an attenuating grid or ‘comb’ is available on some

scanners (Figure 10). This grid effectively reduces the detector size but should beused only when necessary, as it reduces dose efficiency. In other words; the imagenoise is increased for the same patient dose, or the tube current can be increased tocompensate, reducing the noise but increasing the patient dose.

Figure 10. Reduction of effective detector size with attenuating grid

Any unplanned movement of the focal spot will cause additional blurring and reducespatial resolution, and this can be a particular problem with fast rotation speeds.Developments in X-ray tube technology, such as dual-support anodes andsegmented anodes are aimed at improving focal spot stability.

(a) (b)(a) (b)

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The z-axis resolution is often referred to as z-sensitivity and is quoted in terms of thefull width at half maximum (FWHM) of the imaged slice dose profile, but it may alsobe determined by the MTF. It is governed by similar factors as the x-y planeresolution:

• focal spot size (z-dimension)

• focal spot stability

• detector size (z-dimension)

• ‘over-sampling’ techniques

• optimal pitch values

• z-axis flying/dynamic focal spot• attenuating grid (z-axis)

The z-axis resolution is primarily determined by the z-axis detector dimensions.Z-axis detector array design on MSCT scanners varies considerably betweensystems, with minimum dimensions ranging from 0.50 to 0.75 mm. As describedearlier, some arrays are fixed design, whilst others are a variable design (Figure 6).With variable arrays, the z-axis spatial resolution will be reduced when the full extentof the array is used for imaging, as data from adjacent detectors are combined,increasing the effective detector size.

Contrast resolution

Contrast resolution is the ability to resolve an object from its surroundings where theCT numbers are similar (eg in the imaging of liver metastases). It is sometimesreferred to as low contrast resolution or low contrast detectability. The ability to detectan object will be dependent on its contrast, the level of image noise and its size.Contrast resolution is usually specified as the minimum size of object of a givencontrast difference that can be resolved for a specified set of scan and reconstructionparameters (Figure 11).

Figure 11. Test object for contrast resolu tion measurements


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Generator power is an important factor in low contrast examinations. Low noiseimages require high tube current (mA) values, particularly when coupled with fastrotation speeds and narrow slice acquisitions. Fast rotation speeds reducemovement artefacts, thin slices improve spatial resolution as well as reduce partialvolume effects.

Dose efficiency of the scanner is a significant factor in these types of examinations,as it will determine the dose required for a given level of contrast resolution. Contrastresolution specifications should give a guide to a scanner’s dose efficiency. However,there is no standard methodology of data acquisition and image quality scoring to

enable a good comparison of manufacturers’ data.

Temporal resolution

In CT, temporal resolution is usually considered in the context of cardiac scanning.The aim, in cardiac CT, is to minimise image artefacts due to the motion of the heart.This can be achieved using ECG-gating techniques, and imaging the heart during theperiod of least movement in the cardiac cycle, for a time interval of about 10% of thecycle. This results in a temporal resolution requirement of about 100 ms for a heartrate of 60 beats per minute.

The temporal resolution is defined as the time taken to acquire a segment of data forimage reconstruction. For ‘single segment’ reconstruction, it will be the time taken to

acquire180° of data, ie the time for half a gantry rotation. However, for higher heartrates this can still result in unacceptable cardiac motion artefacts. In this situationdata from multiple, smaller segments, acquired from successive rotations, can be

summed in order to obtain the 180° dataset (Figure 12). Using the multi-segmentreconstruction approach requires an asynchrony between the gantry rotation and thepatient’s heart rate so that data from the successive segments are not acquired atthe same angular positions.

There is an optimum combination of pitch, gantry rotation time, and number of

segments for a given heart rate. Manufacturers’ software may have a combination ofautomatic and semi automatic adjustment of these parameters with differing amountsof user input required. Some manufacturers’ software automatically adapts the gantryrotation speed to the heart rate, others have automatic algorithms for calculatingpitch and the number of multi-segments.


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Figure 12. Principle of mult i-segment reconstruct ion in retrospectively gated CCTA

Although multi-segment reconstruction provides a method for improving the temporalresolution, it is prone to mismatch artefacts, particularly for unstable heart rates.

Single segment reconstruction is therefore the preferable approach, requiringscanners with high gantry rotation speeds.

Another approach to improving temporal resolution is with multiple X-ray sources. Ascanner with two X-ray tubes and two detector arrays is currently available (Figure 13). The two assemblies are positioned orthogonally in the scan plane and

simultaneously acquire a 90° segment of data. In this way, a temporal resolution ofquarter of the rotation time is achieved, thereby improving the temporal resolution bya factor of two compared to a single source system using single segmentreconstruction.

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Figure 13. Schematic diagram of a dual source CT scanner

Image artefacts

Artefacts are defined as structures in the image that are not present in the object. Animaging system will invariably produce some level of artefact, but it becomes anissue if it obscures an abnormality, resulting in a false negative diagnosis, or mimics

an abnormality, giving a false positive result.

Artefacts can be due to patient factors, scanner design factors or the reconstructionprocess, which by necessity involves some approximations. Image artefactscommonly encountered are due to:

• patient motion

• partial volume

• photon starvation

• metal objects

• beam hardening• helical scanning

• cone-beam geometry.

For MSCT scanners, patient motion and partial volume artefacts will generally bereduced due to the decreased scan time and the ability to acquire with narrow slices.Photon starvation artefacts, ie streaks arising from the high attenuation in lateralprojections of areas such as the shoulders and pelvis (Figure 14) can be reducedwith angular tube current modulation (see Ionising radiation and patient dose below).Other artefacts, such as those resulting from the extended X-ray beam along thez-axis, will be increased. These are generally referred to as ‘cone-beam’ artefacts.


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Figure 14. CT scan through the shoulders, demonst rating pho ton starvation artefacts

Traditional back-projection methods of reconstruction in CT assume parallel-beamgeometry in the y-z plane. As the z-axis beam extent is increased, this assumptionbreaks down and ‘cone-beam’ reconstruction methods must be used to avoidexcessive artefacts. Some manufacturers employ adaptations of the back-projectionapproach, whereas others use 3D methods such as approximations of the Feldkampreconstruction [14], [15]. Although 3D methods are more exact, they may requirelonger reconstruction times. Cone-beam reconstructions are generally onlyimplemented in helical scanning, therefore in sequential scanning, the extent of beamused, or the narrow slice reconstructions, may be limited. The latest scanners, withbeam extents of 80 mm to 160 mm, by necessity also use cone-beam reconstruction

methods in sequential scan mode.

Ionising radiation and patient dose

As with any imaging modality involving the use of X-rays, The CT scanner and itsuse, falls under both The Ionising Radiations Regulations,1999 (IRR 99) and TheIonising Radiation (Medical Exposure) Regulations, 2000 (IR(ME)R 2000) [21],[22].Scanners have different design and safety features that affect the level of radiationdose. These will need to be considered at the time of purchase, and also in theoperation of the scanner.

Doses from CT examinations are generally significantly higher than those forconventional X-ray, although a CT scan provides more diagnostic information. TheCT doses may be typically factors of 10s higher for standard head and abdomenexaminations, and factors of 100s for chest examinations [20]. Recent UK surveysreport conventional X-ray examinations with average doses of 0.04 mSv for headexaminations, 0.7 mSv for abdomen, and 0.02 mSv for chest examinations [16],[18]. Asimilar survey for CT examinations gave values of 1.5, 5, and 6 mSv respectively forthe same examination regions [17]. These figures represent average values from theuse of a wide range of operational parameters, such as tube current and voltage,however they can be used as a guide.


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Doses in CT are of the order of those received in nuclear medicine studies andinterventional X-ray [19],[20].

The standard reference parameters used to describe dose in CT are the CTDIvol (volume computed tomography dose index) and the DLP (dose length product).

The CTDIvol is calculated from measurements, made with a 100mm long pencil ionchamber, in standard sized polymethymethacrylate (PMMA) head and bodyphantoms which have been irradiated at the halfway position, along the length, with asingle beam rotation.

However, as a dose descriptor, it is important to think of the CTDIvol as representingthe average dose in a slice of tissue, halfway along a 100 mm irradiated length. TheDLP represents the total amount of irradiation given, and as such gives an indicatorof risk (without taking into account the radiosensitivity of particular organs).

The CTDIvol is a very useful dose descriptor for comparing dose from differentprotocols or different scanners. However comparisons should only be done for scansundertaken on standard size patients.

Figure 15. a) PMMA body phantom used for measurement of CT doses b) illustration of CTDIvol representing average dose at central slice position of 100mm irradiation length

(a) (b)

The CTDIvol (and sometimes the DLP) values are displayed on the scanner console.It is always invaluable to look at these figures when reviewing patient images for anassessment of the image quality and dose performance of a scanner. Both theCTDIvol and the DLP are used when comparing with dose reference levels (DRLs)[22],[17].

MSCT scanners have the potential to give higher radiation doses compared to singleslice scanners. Their flexibility in scanning long lengths with high mAs values, andthe ease with which they perform dual and even triple-phase contrast studies, can

equivalent to an

irradiation length

100 mm

CTDIvol represents

average dose at

central slice position

standard sizePerspex

phantom

equivalent to an

irradiation length

100 mm

CTDIvol represents

average dose at

central slice position

standard sizePerspex

phantom


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lead to high patient doses. In addition, there are some intrinsic features of currentMSCT design which can give rise to slightly higher doses and these are discussedbelow.

Manufacturers have invested a great deal of effort in measures to minimise radiationdose. Nevertheless, the optimisation of scan protocols, in order to keep doses as lowas reasonably practicable (ALARP), is a legal obligation [21][22]. This is of particularimportance in paediatric studies.

Over-beaming

Over-beaming in MSCT is the extent of the X-ray beam penumbra along the z-axiswhich is not utilised for imaging, so the true irradiated volume per rotation is greaterthan the nominal imaged volume. It is quantified in terms of geometric efficiency(GEff), and the user is alerted by a display on the console if a scan protocol results ina GEff of less than 70%.

Over-beaming is necessitated in MSCT so that all active detectors are exposed tothe same intensity of X-rays. The extent of the penumbra is generally 2 to 3 mmeither side. For narrow z-axis beams, the over-beaming will therefore affect the dosesignificantly. For example, with a nominal imaged length of 2 mm, the actual

irradiated length will be 4 to 5 mm, resulting in a doubling of dose or more, comparedwith a single slice scanner where only 2 mm would be irradiated. As the extent of thebeam increases, this penumbra is proportionately less significant (Figure 16), so thatfor nominal collimations of 20 mm or more, GEffs comparable to those on single slicescanners are achieved.

Figure 16. Reduced in fluence of over-beaming for larger z-axis beam col limations

z-axisz-axis


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Reduced X-ray beam collimations (and therefore lower GEff) are required onscanners with adaptive arrays, when acquiring narrow slices. Scanners with fixedarrays will therefore have dose advantages, as they can utilise the full extent of thearray for narrow slice acquisition.

Over-ranging in helical scanning

Irradiation extending beyond the imaged length is required in helical scanning. Thisover-ranging results from the extra rotations necessary for reconstruction of the firstand last images in the imaged volume. Their contribution to patient dose becomesmore significant for wider z-axis beam collimations (Figure 17). Therefore, the

increase in dose contribution with wide collimations from over-ranging counteractsthe improvements in dose efficiency of over-beaming. The increase in dose fromover-ranging is particularly significant for short scan lengths, and at some point it ispreferable to reduce the collimated beam, or even to use sequential scan mode ifexamination time is not an issue. Sequential scan mode may also be preferred inorder to avoid certain radiosensitive organs at the beginning and end of the imagedvolume.

Figure 17. Increased dose contribut ion from over-ranging with wider X-ray collimations

Some manufacturers have sought to address the problem of excess dose from over-

ranging, and certain scanner models have a feature which dynamically adjusts thebeam collimation at the beginning and the end of a scan to minimise the dose whilststill allowing full reconstruction of the required imaged volume (Figure 18). The totaldose savings depends on the length of the scan, the X-ray beam collimation, therotation time and the pitch, but are estimated to be between 10 and 25%.

Imaged volume

Extra rotations

Imaged volume

Extra rotations

Imaged volume

Extra rotations


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Figure 18. Dynamic col limation to reduce dose at the extremities of a scan

Automatic tube current control in CT

Traditionally, the X-ray tube current (mA) in CT was selected for a particular protocol,and remained constant throughout a scan. Any changes to accommodate different-sized patients had to be estimated, and implemented manually. Modern scanners areequipped with automatic exposure control mechanisms, which adjust the tube currentfor changing patient attenuation throughout a scan. The adjustment can be made to

compensate for changing attenuation (Figure 19): (a) in different-sized patients; (b)along the patient’s long axis; and (c) throughout a gantry rotation.

Most modern systems have the capability to operate all three compensation modes,which are generally implemented simultaneously. Most scanners will allow manualde-selection of one or more modes, and on others the de-selection may beimplemented automatically within a protocol, according to the clinical region scanned.

Figure 19. Automatic tube current contro l in CT

Imaged volume

Dynamic

collimators

X-ray beam

Dose saving

Imaged volume

Dynamic

collimators

X-ray beam

Dose saving

Imaged volume

Dynamic

collimators

X-ray beam

Dose saving

m A

Low mA

H i g h m A

High mA

Low mA

(a) (b) (c)

m A

Low mA

H i g h m A

High mA

Low mA

(a) (b) (c)


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Cardiac scanningDose considerations are a particular issue in cardiac scanning. A standard chest CTscan would give an effective dose of approximately 6 mSv [17], whereas effectivedoses for cardiac CT angiography (CCTA) scans, using retrospectively gatedreconstructions from low pitch, helical scans, are typically about 16 mSv, but can bemuch higher, with 32 mSv being reported in the literature [20].

To reduce doses for these types of scans, manufacturers have introduced ECG-gated tube current modulation on their cardiac-enabled scanners (Figure 20).Outside the cardiac phase used for reconstruction, the tube current can generally be

reduced to 20% of its peak value, although one range of scanners allows a reductiondown to 4%. ECG-gated tube current modulation is only effective for patients withstable heart rates.

Figure 20. ECG-gated tube current modulation

Another approach for reducing doses in CCTA is to use prospectively gatedsequential scanning, where the tube current is only switched on during the cardiacphase of interest. Prospectively gated reconstruction has been used for some time incardiac calcification scoring, but not in CCTA, as the thinner slice acquisition requiredled to reduced beam collimation and extended the examination time unacceptably.

However, on some scanners of 64 slices and above, prospectively gated CCTAscans are possible. Although prospective gating has potential for large dosereductions it requires a steady heart rate for good results as the examination time isincreased. However, the wide beam acquisition systems that can acquire the wholeimaged volume in one rotation can employ prospective gating with unstable heartrates.

New and future applications

Dual energy applications

Dual energy applications in CT are still evolving. They are aimed at identifying and

discriminating between materials of similar CT number, such as soft plaque and fattytissue, or calcified plaque and contrast media. They make use of the differing

Imaging windowECG signal

100%

20%

Tube current Imaging windowECG signal

100%

20%

Tube current Imaging windowImaging windowECG signal

100%

20%

Tube currentTube current


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chemical composition and attenuation properties of the materials, and the changewith different X-ray energies. There a number of ways in which dual energy scanningcan be implemented. One manufacturer’s dual source (two tube) system employsone tube operating at 80 kV and the other at 140 kV, to acquire data at the twoenergies almost simultaneously. An approach being developed by anothermanufacturer with a single X-ray tube scanner, alternates the tube kV at half rotationintervals. A third manufacturer has developed a dual-layered detector to discriminatebetween energies. The top-layer detects the low energies, and the bottom layer thehigher ones. The dual source method currently has the most highly developed clinicalapplications.

Future proofing the decision

The rate of change of CT scanners has been high in the last few years, and if itsustains this rate of growth it is difficult to predict what the forthcoming years willbring. However it is always important to discuss upgrade options with themanufacturer.

Some systems are on a clear upgradeability path, particularly if a lower specificationmodel has been purchased. For example the lower slice category scanners with fixedarrays can theoretically be easily upgraded to a higher slice category without achange of detectors, though with other software and some changes required.

Upgrade paths, however, are often much more complex than this, and some may bemore of a ‘fork lift’ upgrade, in that the whole system will be replaced. Eachmanufacturer should be asked for a clear description and potential costs.

The longer arrays (80 mm, 160 mm), announced at the end of 2007, have broughtgreater capability in cardiac scanning and in perfusion studies, and furtherapplications are developing. These scanners, as high cost, high performancescanners, are currently regarded as specialist systems, and are likely to remain sofor a number of years.

Sustainability

Intrinsically MSCT is more energy efficient as the wider coverage requires fewerrotations for a given scan length, resulting in lower X-ray tube heat load and longertube life. In practice, this energy and tube life saving may not be achieved due tochanges in scan protocols. Developments in tube technology, such as spiral-groovebearings with liquid-metal lubrication and anodes with direct oil cooling, can alsoextend tube life.

Some systems have contactless transmission for data transfer, which eliminates the

need for carbon brush replacement. One scanner has a slip ring design with air-bearings which might also reduce maintenance requirements.


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Operational considerations 29


This chapter addresses the operational issues which impact on selection of a multi-slice CT scanner (Table 3).

Table 3. Operational issues affecting MSCT selection

Area Aim Topic / issue

Scanner installation Effective installation

Scanner location

Room requirements

Ancillary equipmentErgonomics

Acceptance and commissioning

Training

Decommissioning

Ongoing considerationsMaintaining quality andproviding service

Periodic maintenance

Consumables

Quality Assurance (QA)

Patient workflowEfficient patientthroughput

Staffing

ReferralScheduling

Patient preparation

Scan set-up

Image reconstruction

Further processing

Specialist applications

Information workflowEfficient processing andreporting

CT scanner workstations

PACS workstation

Remote access client

ReportingData management

InteroperabilityIntegration with otherhospital systems

Systems integration

Existing modalities

Specialist systems

Information security

Safety Safe working practicesIonising radiation

Infection control


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Installation

Overview

To ensure an efficient and effective installation process, it is recommended that aproject manager be appointed to manage the project to completion. A CT scannerrequires supporting infrastructure that is wide ranging. The estates department, theIT department, the local radiation protection advisor, and PACS or RIS (radiologyinformation system) managers should all be involved in planning the installation.Plans for scanner suites should also be checked with local infection control officers.

Consideration must be given to continuity of clinical services during the installationand commissioning period. Options include use of a mobile CT scanner, eithersupplied by the trust or by the manufacturer as part of the purchase agreement.

Scanner location

There may be little flexibility in the location of the scanner suite, for example, wherean existing scanner is to be replaced. However, a formal assessment of benefits ofbuilding in a new location should be undertaken. If an existing facility is being re-used, it should be properly assessed before the scanner installation commences,with consideration to any revised requirements due to changes in the scanner, theuse, or the workload.

Where there is choice of location, then ease of transfer of patients to the scannersuite is a primary consideration. Certain departments (eg accident and emergency,neurological ITU) are likely to require more frequent access to CT scanning facilitiesthan others. Access routes for both inpatients (including bed access) and outpatientsshould be straightforward.

Separate areas may be required for patient cannulation, and for reportingworkstations. Where more than one scanner is in use, a single shared control roomcan improve efficiency.

The room must have sufficient radiation shielding for a CT scanner. Thesecalculations must be ascertained early in the purchasing process. The local RPAwould need to be involved in this process.

Room requirements

Most manufacturers can provide a checklist of installation requirements in advance.These should be obtained as part of the tendering process. Additional works requiredto meet these requirements may need to be factored into the overall cost.


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The scanner must be housed in a room which is large enough to ensure ease ofaccess for patients and clinical staff, with a floor strong enough to withstand theapplicable loading. There must be sufficient access for an emergency (crash) trolleyfor cardio-pulmonary resuscitation.

Air conditioning may be required to ensure consistent scanner operatingtemperatures, and the comfort of patients and staff. This should be sitedappropriately so as to avoid damage to the scanner from condensation. Additionalventilation with suitable extraction rates may also be required if medical gases orinsufflators are used.

Attention should be paid to the provision of adequate lighting within the scannerroom. Areas housing reporting workstations will have their own specific lightingrequirements.

Power requirements are specified by the manufacturer. It may be necessary toupgrade the main supply to the scanner suite before installation.

Due to the large amount of power required, CT scanners are not normally connectedto the uninterruptible power supply (UPS). However it may be possible to connect to

a UPS that gives sufficient time to allow normal shutdown procedure of the CTscanner without a system crash.

CT scanning is subject to The Ionising Radiations Regulations, 1999 (IRR 99) [21],and The Ionising Radiation (Medical Exposure) Regulations, 2000 (IR(ME)R 2000)[22]. The use of the rooms neighbouring the scanner suite may impose additionalconstraints. A radiation protection advisor should be consulted at an early stage toensure that any such additional constraints are taken into account in calculating theradiation shielding requirement for the walls and ceilings.

Even if a new scanner is being installed in a room previously occupied by a CTscanner, consideration must be given to any change in dose rates. Scatter doserates on multi-slice scanners will generally be larger than for single slice scanners asthe radiation coverage is wider for a single rotation. Multi-slice protocols are alsooften likely to use thinner reconstructed image slices from the wide beam, resulting inpossible increases in tube current and therefore higher scatter doses. Increasedpatient workload will also have a significant effect on scatter doses.

Inadequately shielded scatter radiation can interfere with other equipment, such asgamma and PET cameras. Testing of existing shielding can confirm if this is apotential issue.


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Anci llary equipment As well as the scanner gantry and couch, other equipment may need to be located inthe scanner room itself, or nearby. Manufacturers can supply details of ancillaryequipment, such as computers and cooling plant, indicating any restrictions on wherethese can be located.

Inside the scanner room some ancillary equipment can be ceiling mounted to givemaximum free floor area. Storage for accessories and consumables needs to beincluded in the design of the scanner suite.

Ancillary equipment commonly located in the scanner room includes:

• contrast pumps, warmers, and associated supplies

• insufflator and applicators

• fluoroscopy systems (slave monitor, foot pedal)

• ECG systems

• life support systems / crash carts.

Ancillary equipment commonly located outside the scanner room:

• CT workstation (for post processing and / or reporting)• disc burning unit (MOD and / or DVD)

• RIS workstation (for patient tracking)

• PACS workstation (for other images and/or reporting)

• hard-copy device (for referral forms etc)

• histology station (for biopsies).

Ergonomics

The scanner should be positioned in the scanner room so as to allow free accessaround the couch and the gantry. The couch must be free of obstruction throughout

its full range of motion. It should also be oriented such that staff at the console cansee the patient at all times during the scan procedure, ideally through a shieldedwindow. Some departments now use CCTV systems to view behind the gantry.

Controls for the motion of the couch are often on both sides (left and right) of theaperture, allowing the operator to stand where it is most convenient. Some operatorsfavour controls on both the front and back of the gantry. Foot pedals and couch sidecontrols may also be available.

In order to facilitate patient transfer, it should be possible to adjust the height of the

couch to allow the patient to sit on to the couch. For in-patients, it is necessary to beable to position the bed closely alongside the couch at the same level. Once the


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patient has been transferred to the couch, the bed should be moved away to allowscanning staff full access to the patient.

When support systems, such as ventilation, are required for the patient, there shouldbe facilities to operate these within the scanning room.

The scanner console should be designed to allow the staff to work appropriately. Theheight should be such that staff can operate the system seated or standing. Thesystem should have minimal left and right hand bias. Software interfaces for the set-up and review of the scan must be clear and easy to use. This is best assessed on

site visits.

Guidance for working on computer workstations has been prepared by the RoyalCollege of Radiologists [23].

Any in-room accessories, for example contrast pumps, fluoroscopy monitors, shouldbe fitted such that they do not interrupt the general work of the radiography staff.Ceiling mounted system options offer such an advantage, though they are moreexpensive. Storage of attachments and other occasional accessories should be suchthat there is no lifting hazard when handling.

Critical examination, acceptance and commissioning

Once the scanner has been installed it has to undergo a critical examination inaccordance with the Ionising Radiations Regulations, 1999 (IRR 99) [21], [24]. Theresponsibility for this lies with the manufacturer, and must be carried out inconjunction with a radiation protection advisor (RPA) who may be appointed by thesupplier, or the purchasing trust on behalf of the supplier. There is a need toestablish, preferably at the contract stage, which RPA (the installer’s or thepurchaser’s) will oversee the critical examination.

In addition, the scanner will need to be formally accepted by the purchaser. This is toverify that the contractor has supplied all the equipment specified and has performedadequate tests to demonstrate that the specified requirements in the contract havebeen met. Some trusts will, subject to contract, withhold a percentage of the scannerprice (eg 5%) until the scanner has passed the acceptance process.

The acceptance process also covers electrical and mechanical safety, radiationprotection issues, image quality, and dose performance. There are IEC guidelinesavailable for this process [25]. It is worth considering the requirement of the provisionof phantoms, for acceptance testing purposes, in the tender specification.


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All connectivity issues should also be assessed at this stage. (See Informationworkflow below.)

The final stage of installation is commissioning the scanner. This comprises a set oftests carried out by the purchaser’s representative to ensure that the scanner isready for clinical use and to establish baseline values against which the results ofsubsequent routine quality control performance tests can be compared. This is alsothe process whereby the users bring the system and its protocols to a state ready forclinical use, bearing in mind specific clinical applications and requirements. This mayinvolve scanning phantoms, as well as patients with the attendance of the supplier’s

clinical specialist.

The same RPA may undertake the critical examination, acceptance andcommissioning. However, although the tests may be combined, their purpose shouldremain distinct [29].

Training

Under the medical exposures regulations, IR(ME)R 2000 [22], all who act as eitheroperators or practitioners as defined by the regulations, i.e. anyone who eitherauthorises the use of or delivers ionising radiation, are legally required to have

received certified training.

Also, in accordance with good clinical governance, it is essential that all staff usingand supporting the use of the scanner and workstations are suitably trained. Therequirements for such training need to be specified in the tender document. Thesewill cover initial and ongoing training for all users, or for specific users responsible forcascading training to other staff.

The training should cover the basic operation of the scanner, set-up and optimisationof scan protocols, and the function of advanced packages, where installed. Specialistworkstation training will also be required for specific staff. The Royal College ofRadiologists (RCR) has published guidelines for IT systems training in imagingdepartments. The general principles covered by this RCR document are alsoapplicable to training in the use of a CT scanner [26].

Manufacturers generally offer training as part of the purchase package. Training onthe scanner is usually offered in three phases: prior to the scanner installation; at thetime of first patient scanning; and after a few weeks or months as follow up.

Prior to scanner installation it can be helpful for staff to experience some basictraining on a system in use elsewhere, and the manufacturer can often make

arrangements for this. This might involve at least one member of staff spending aweek at another site.


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For the second phase, an application specialist from the manufacturer will be presentat the clinical site, to provide a mixture of training and support during initial patientscanning.

Finally the manufacturer will commonly make provision for a specified level ofapplications specialist availability for future visits, to address training requirements foradvanced functions and issues arising from normal clinical use.

Training on the use of the scanner workstation and specialist software packages iscommonly offered as a separate package for radiologists and other specialists. Some

manufacturers offer training courses off-site for this purpose.

In addition there will be some level of continuing support offered, after the warrantyperiod, as part of a specific level of support contract. This is particularly helpful whenintroducing new applications, or for optimising the use of advanced features such astube-current modulation.

The relationship between the supplier’s applications specialist and the radiologydepartment is important in optimising use of the scanner.

It can be helpful to have specialist staff, such as medical physics experts (MPE) andradiation protection supervisors (RPS), involved in the training, particularly in the useof the specialist features such as those that either modulate radiation dose, or givehigh dose, such as fluoroscopy or cardiac scanning.

Decommissioning

At the end of service life, the scanner will need to be decommissioned.Manufacturers are working towards full WEEE compliance [27], and so will be able toremove and dispose of the scanner as appropriate. This might apply to the removalof an existing scanner, as well as the future decommissioning of the new purchase.

Transfer of existing data may be an issue. Image data stored on a secondary system,such as a picture archiving system (PACS), will not need further migration. Otherdata may need to be transferred or archived for future access.

Ongoing quality

Periodic maintenance

Scanners are generally serviced by engineers from the manufacturer. This is coveredby a service contract, which is normally negotiated at the time of purchase. The

service contract may cover a fixed number of years, or may be renewable annually.


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The type of service contract will depend on local needs to maintain a clinical service.There tend to be two or three levels of service agreement offered, with greater costsassociated with increased availability of response, and inclusion of key hardwareitems, such as X-ray tubes or detectors.

Frequency and duration of routine servicing vary with manufacturer and scanner, butnormally takes up to one day, and occurs every three to six months. At this time thescanner is checked and any planned upgrade to components and softwareundertaken. Hardware and software updates often have implications for imagequality and dose; therefore the local medical physics expert (MPE) and/or radiation

protection advisor (RPA) should always be informed of these visits and giveninformation of any upgrades installed.

For unscheduled support, options are generally available for 24 hour access to anengineer, or for support only during office hours (08:00 – 17:00). Somemanufacturers use remote access systems for an initial test of the scanner. Optionsfor training local engineers in first line trouble-shooting may be requested in thetender document.

When comparing reliability, or ‘up-time’ claims, it should be noted that these figuresmay not be defined in the same way between different manufacturers. It should alsobe noted that then impact of down-time on the clinical service will depend on theclinical investigations carried out, and the demand placed on the scanner.

Consumables

The main consumables of MSCT scanning are associated with contrast systems, andinclude contrast agent and saline, cannulae and injection lines for contrast delivery,and applicators for insufflation. Other consumables are ECG pads for cardiacscanning, paper roll bed liners, personal protective equipment, such as gloves andaprons, cleaning materials, and patient gowns (if single-use gowns are employed).Standard local procedures should be followed for the disposal of the clinical waste

generated from these consumables.

Storage devices such as magneto optical discs (MODs), compact discs (CD), ordigital video discs (DVDs) may constitute significant consumables, dependent onhow information transfer is handled (see Data management below).

The X-ray tube may be included within the maintenance contract. If not included in acontract, it may be regarded as a consumable. The X-ray tube is covered by awarranty that is generally based on number of rotations or exposure time. It might beexpected to last between a year and eighteen months, dependent on patient

throughput and tube technology. The tube technology is a key difference betweensome of the CT manufacturers and claims of tube lifetime vary considerably.


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cannulating the patient outside the scanner room; and having a booking system thatis not dependent on in-patients arriving at specific times. These issues are furtherdiscussed below.

Extending working hours can also improve workflow. However, running the scannerbeyond standard hours routinely will have implications on tube replacementfrequency, service support requirements and cost, and radiation protection issues.

Figure 21. The stages of a CT scan

JustificationScan protocolselection

Patient towaitingarea

Planscan

Scan

Readimages

Transfer images

To archive

REFERRAL Schedule

REPORT

Transfer report to

patient record

Primary and secondaryreconstructions on scanner

Transfer images to CT/MRworkstation and/or PACS workstation

Secondary Reconstructions/Postprocessing on workstation as required

Preparepatient

Staffing

MSCT scanners may only be operated by appropriately trained staff [22], usuallyregistered radiographers. They act under the supervision of ‘practitioners’ who, withinthe terms of the statutory instruments, are a registered medical practitioners (usually

a radiologist) or other health professional who is entitled in accordance with theemployer's procedures to take responsibility for an individual medical exposure.

The operation of the scanning department will be supported by administrative staffwho arrange appointments and carry out the patient reception duties, nurses and/orradiography departmental assistants (RDA), and porters for transfer of in-patients.

Additional general support is provided by radiation protection and medical physicsstaff, from either within the trust or other organisations, who carry out periodic QAtesting of the scanner. They will also advise on issues regarding optimisation ofscanning protocols with respect to patient dose and image quality. These specialiststaff should be trained to nationally recognised levels. Radiation protection advisorsmust hold a certificate of competence from a Health and Safety Executive (HSE)


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recognised assessing body [37], and be experienced in the relevant technologies.The local radiation protection supervisor (RPS) ensures that the ‘local rules’ forradiology practice are followed. (Local rules are mandatory instructions that areformulated to secure compliance with health and safety legislation).

Utilising an appropriate mix of staff skills can reduce staff costs and improve patientworkflow through the scanner. An example scheme is shown in Figure 22.

Figure 22. Examples of allocation of specific ro les to assist patient workflow

Referral

Schedule

– Continues patient set up – Sets up scanner pre-

assigned protocol – Scans – Image reformats etc

Radiographer- Cannulates, ifappropriate- Reassures patient

Helper or Nurse

- Assists patient andradiographer

Patient toscanner room

- own transport (o/p)- or porter (i/p)

Patientpreparation

Patient

– Justification – Scan

protocolselection

Radiologist

Views andReportsimages

Images automaticallysent, as per protocol,

to PACs orworkstation

Radiologist

Referral

Under current legislation [22], where patients are referred for a scan, this must be justified by an entitled practitioner, and a suitable investigation prescribed. Thechoice of investigation, protocol, and scan parameters must comply with the ALARP

(as low as reasonably practical) principle, to ensure that no unwarranted irradiationtakes place. The ALARP principle is a standard tenet in radiation protection andhealth and safety legislation.


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Some departments have found delays at this stage of the process due to restrictionson the radiologists’ time. Systems that have been used to remove this pressureinclude batch processing of these referrals, and pre-selection of protocols, where astandard scan protocol is selected from a list at the time of justification. In addition, aspecific member of staff may be allocated to screen the referrals to removeunwarranted referral requests according to well defined procedures. Pre-selection ofprotocols ensures there is no delay at the time of scanning. The protocol defines theanatomy to be scanned, and the required slice thicknesses, scan parameters,reconstruction procedures and post processing are all outlined. Systems to easilyidentify the scan protocol from the request form, such as protocol code numbers ortables with tick boxes, can simplify the process and minimise potential for error.

Scheduling

Improvements in scheduling can be obtained in a number of ways, according to thelocal mix of patients. Scheduling patients for particular types of examinations intofixed sessions can improve workflow, especially if specialist clinical support isrequired at the time of scanning to review the images. However, efficiency can clearlybe compromised if there are not enough patients to fill a session for that particularexamination type. If there are too many referrals and they cannot all be fitted into aparticular session, some patients might have to wait up to a week until the nextscheduled session.

Some have found that scheduling patients who do not require contrast injections intosessions at the beginning or end of the day, when there is less likely to be aradiologist available, improves workflow. This ensures that there is no delay andscanning can take place with no delay.

If two identical scanners are procured, then all scans can be booked onto one list andthe first available scanner used. If the scanners differ, then systems need to be inplace to determine which patients can only be scanned on a particular scanner.

The availability of porters can affect the arrival time of in-patients. Some centresovercome this by having a specific porter, or porters, allocated to scanning orradiology. Another approach can be to balance the scheduling of the mix of in-patients and out-patients, so that outpatients can be scanned whilst waiting for latein-patient arrivals, or vice versa.

Efficient procedures for checking the pregnancy status of patients will also ensureminimum effect on workflow.


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Patient preparationImprovements in patient throughput can be achieved by preparing the patient beforetransfer to the scanning room. Patient information and checks for contra-indicationsto the procedures, such as contrast intolerance, can be undertaken whilst the patientis in the waiting area.

In some departments, cannulation is also performed prior to the patient entering thescanner room. This requires a suitable separate area, and trained staff, notnecessarily a radiographer, such as an RDA or a nurse.

Oral contrast administration preparation times can be shortened by changing theprocedure for administration or adopting alternative strategies [34],[35].

Scan set-up

Current scanners can obtain patient information from the modality worklists, whichmay come from the RIS, the PACS, or from a broker acting as an interface betweenthe RIS and/or PACS, and the scanner. Pre-population of patient information fieldson the scanner console during set-up can save operator time, and reduce the chanceof error. Some scanners can also update the RIS or PACS as to the status of thescan, using performed procedure steps.

Patients may need to be instructed about breath-hold and contrast injection duringthe scan. Systems such as visual breath-hold indicators and voice announcementscan ensure that the patient complies with the scanning requirements. Modernsystems offer a selection of language choices, and some systems allow the recordingof additional announcements.

With increased speed of scanning, the timing of the scan relative to contrastadministration is important. Systems generally include a trigger system that willinitiate the scan, based on a time delay or ‘bolus tracking’.

Automated dose reduction systems are intended to optimise the patient exposure toionising radiation whilst maintaining adequate image quality. These require extensiveuser training. Set-up, testing and optimisation of these systems require the support ofa medical physicist.

Image reconstruction

It is generally thought to be good practice for scanner operators to check imagesbefore removing the patient from the scanner. This may take from a few seconds to afew minutes depending on the performance of the reconstruction computer. Systems

that provide fast reconstruction and easy review of images at the console minimisedelay, and help to increase throughput. To facilitate rapid verification of images,


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some scanner manufacturers provide preview image reconstruction at reducedimage quality.

Some centres have a ‘scan and send’ policy, for routine examinations, which doesnot require a radiologist to review the images prior to sending the patient away,reducing delays due to radiologist unavailability.

For ease of reporting and specialist applications, there are a number of additionalprocessing steps that the image data undergo, for example to reconstruct withdifferent convolution algorithms, to reconstruct 3D views, thick or thin slab MIPS or

MiniPS (maximum or minimum intensity projections) etc. Where these secondaryreconstructions are set-up in the protocol, the need for user intervention in post-processing activity may decrease.

However, increasingly, the CT data set is regarded as a volume, and the 3Dreconstructions, MIPS, MPR (multiplanar reformats), etc may be instantly availablerather than secondary reconstructions with the thicker axial slices only reconstructedfor image storage.

The quality of the MPR, MIP and 3D reformatting from image slice data sets is best

when reconstructed from thin slices. Many centres choose to store only the thickerslice data sets, to reduce data storage, however any subsequent reformatting fromthese data sets will result in reduced image quality, compared to thin slice datasources. This is an additional advantage to prescribing these reformats in theprotocol, so that the thin slices are used while they are still available.

Some systems can process the secondary reconstructions whilst the next scan isbeing set up, others may pause one or the other function depending on sharedcomputing resources. The interdependence of scanning and reconstruction mayaffect throughput in departments where a large number of alternative reconstructionsare requested routinely.

The volume of acquired (raw) data is large, and often only stored on the scannercomputer, where the data are typically overwritten cyclically. For many scanners thecycle time may only be a week or a few days, dependent upon the rate of scans, theamount of data acquired for each scan and the storage disc size. Once the raw dataare overwritten, further formatting of images can only be carried out from stored slicedata sets.

Routing of images to stores, such as workstations and archives can also be presetfor given protocols. This may be for the complete study, or for a specific series of

images.


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Further processing Additional processing can be carried out on the scanner console, on the scannerworkstation, or on the PACS workstation. Some functions will be available on allthree systems, others will be limited to scanner console and workstation. For morespecialist applications, such as cardiac analysis, vessel analysis, or automatic lungnodule scoring, software packages will only be available from the scannerworkstation. Systems can also be purchased to allow processing of data on theworkstation through remote access, and thin-client systems.

The usability of the application software will affect the efficiency of workflow at this

stage. Applications that use standard interfaces and inputs, are intuitive, and providesuitable prompts when required, may improve workflow initially or for rarely usedapplications. Some systems allow users to record macros or shortcuts to reduce theinputs required.

Post-processing may be undertaken by a radiographer, technician or clinician.Suitable training must be undertaken for each application.

In studies requiring post-processing or specialist analysis software, suitable worklistsystems should be in place to maintain effective use of reporting radiologists’ time. If

reporting from the workstation, then dictation, a voice recognition system, or anotherreporting system, must be available at, or adjacent to, the workstation.

Specialist applications

Specialist applications, due to their complexity, tend to slow down patient throughput.This can either be due to time in the scanner room during interventional procedures,or in additional preparation and processing time (eg for cardiac examinations).

Interventional use

CT systems can be used for real-time and interventional procedures, such as guidedbiopsies. These may also be called real-time CT, or CT fluoroscopy. Manufacturerssupply additional configurations and accessories for interventional work. Thenecessity for these additions will depend

multi-slice ct scanners - buyers’ guide

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