Multi-slice CTPrinciples and Perspectives Mindy M. Horrow, MD, FACR Director of Body Imaging Albert Einstein Medical CenterAll photos retain the copyrights of their original owners 2005 Mindy Horrow, MDTo advance slide, use spacebar; use left arrow for previous slide; use browsers back button to exit program

Before CTEntire areas of body inaccessible to radiography (brain, retroperitoneum, etc.) Some useful diagnostic procedures were either potentially harmful or considerably uncomfortable (exploratory laparotomy, pneumoencephalography)

Principles of CTRadiographic tube emits x-rays while rotating axially around patient Array of detectors on opposite side of patient detects x-rays transmitted through patient Computer algorithms use digitized data from detectors to create axial tomographic images of body. CT = tomography + algorithms + high speed digital computers

Tomography- CT actually eliminates unwanted material, outside of scan plane instead of just blurring it (1921-conventional tomography) Reconstruction algorithms- Fast Fourier Transformation: allows mapping of function of space into a function of frequency using the theorem that any function can be decoded into a sum of sine and cosine functions (described by Fourier in 1811)Principles of CT

Development of Workable CT Scanner1963- Cormack in S. Africa develops algorithm for accurate reconstruction of images from radiographic projections 1971- Hounsfield, a computer engineer in England produces first working CT scanner used clinically on patients. Produced 70 head CTs in 6 mos, at 4 min per slice, recorded on magnetic tape with two days reconstruction time per case. Cormack and Hounsfield awarded Nobel Prize in medicine and physiology in 1979

Sir Godfrey Hounsfield with a prototype CT scanner in 1974

Head CT circa 1975 with 128 x 128 matrix

Radiology: Volume 119, 1976Davis, Taveras, New, et al. Diagnosis of Epidermoid Tumor by Computed Tomography Hahn, et al The Normal Range and position of the Pineal Gland on Computed Tomography Huckman, Ramsey, et al. Computed Tomography in the Diagnosis of Pseudotumor Cerebri Messina, Potts, et al. Computed Tomography: Evaluation of the Posterior Third Ventricle OConnor, et al. Computed Tomography in a Community Hospital Sagel, Stanley, Evens. Early Clinical Experience with Motionless Whole Body Computed Tomography

Sagel, Stanley, Levitt, et al. Computed Tomography of the Kidney Radiology 124:359-370, 1977Computed tomography is an extremely accurate method of obtaining more definitive diagnostic information about a renal mass discovered on a urogram. Benign renal cysts are readily distinguished from solid renal neoplasms, and CT is often valuable in characterizing possible juxtarenal masses. The cause of a nonfunctioning kidney(s) on a urogram can often be discerned, and hydronephrosis is easily detected.

Proliferation of CTBy 1976, 3 years after Hounsfields publications, 22 companies were manufacturing CT scanners By 1979 1000 scanners were operating in 50 countries Competition produced rapid technological sophistication Introduction of fan beam-scanning decreased scan time from 300 sec to 2 sec per slice in 4 years

Conventional CT scannersEmploy fan of x-ray beams and a large detector array 3 types of gantries: translate-rotate, rotate-rotate, rotate-stationary Involves alternating patient translation and x-ray exposure Each rotation of x-ray tube generates data from which a corresponding axial image is reconstructed

Helical (spiral) CTSimultaneous patient translation and x-ray scanning generates volume of data X-ray beam traces a helix of raw data from which axial images must be generated Each rotation generates data specific to an angled plane of section To create true axial image, data points above and below desired section must be interpolated to estimate value in axial plane Thus, interval between reconstructed transsexual images can be chosen retrospectively

Technological considerations of helical CTSlip-ring technology (no electrical cables connecting gantry to ground) allows source detector assembly to rotate continuously. Previously, frequent, abrupt changes between scans were necessary to permit winding and unwinding of cables More robust x-ray tubes and generators were developed to allow high tube current for prolonged duration. Also needed to be lightweight enough to be mounted in slip ring gantry

Comparison of single slice and multi-slice CTDetector configuration Reconstructions Detector design Definition of pitch Pitch and image quality Spatial resolution

Configuration of detectorsSS- long, narrow array with length of single detector aligned in z axis MS- detector array segmented in z axis, a mosaic Allows for simultaneous acquisition of multiple images in scan plane with ONE rotation

Mosaic Detector 16 cells in Z direction --each cell 1.25 mm (in Z) 16 cells (Z) x 912 cells (transverse) = 14592 total cells Signal collected from 4 channels/2 flex connectors

ReconstructionsSS- reconstruct images of SAME thickness with different image indexing (table increment intervals) MS- acquire 3D raw data that are contiguous in space. Therefore can reconstruct images at various thicknesses AND at different intervals If image index < image thickness, results in overlapping slices Must have raw data available for any type of reconstruction

Multi-slice detector design (GE 4 slice scanner)16 equal elements in z axis, 20mm maximum collimator width * Can acquire 1, 2, or 4 images per rotation For example: with collimator at 10mm can make 4 images @ 2.5mm, 2 images @ 5mm or 1 image @ 10mm Thinnest slice thickness that can be reconstructed depends entirely on combination of slice thickness and table speed* A single 1.25 detector is made of two .63 detectors

Axial Configurations4 x 2.5 mm4 signals collected from eight 1.25 mm detectors with 2 detectors contributing to each signal 2.5 mm is the minimum slice thickness because two 1.25 mm detectors are combined per signal Cells can be combined to form 4 slices @ 2.5 mm each or 2 slices @ 5 mm each or 1 slice @ 10 mm12341214i mode = each set of 2 cells becomes a slice @ 2.5 mm each1i mode = 4 sets (of 2 cells each) are combined to form 1 slice @ 10 mm2i mode = 2 sets (of 2 cells each) are combined to form 2 slices @ 5 mm each

Pitch SS = table travel per rotation image thickness If table travel > slice thickness, pitch > 1 MS = table travel per rotation total active detector width* * = x-ray beam collimation

Table travel/ rotation = 7.5mm Image thickness = 5mm Pitch = 7.5 = 1.5 5.0 Table travel/ rotation = 7.5mm Four Images with thickness = 2.5mm Pitch = 7.5 = .75 (4 x 2.5) SS MS

GE Definition of PitchTable travel per rotation = 7.5 = 3 single image slice thickness 2.5 (High quality mode) i.e.. When 4 images are acquired per tube rotation, associated table travel is 3 times image width

GE Definition of PitchTable travel per rotation = 15 = 6 single image slice thickness 2.5 (High speed mode) i.e.. When 4 images are acquired per tube rotation, associated table travel is 6 times image width

Pitch and Image QualitySS- Image quality decreases as pitch increases MS- GE scanners have unique property of forming images with particularly good quality at 2 specific pitch values, HQ and HS

Pitch and NoiseTo reconstruct image, projections must be collected over 180 gantry rotation and fan angle of x-ray beam (45 ), about 2/3 of spiral Since reconstruction algorithms need fixed number of projections to make image and since pitch only affects how these projections are distributed in spiral, not the number of projections, pitch does not affect noise No difference between SS and MS

Image Quality: Contrast ResolutionAbility of imaging system to detect a single structure that varies only slightly from its surroundings Related to noise AND pitch Less noise, fewer distractions, increased ability to perceive low density object Contrast resolution in x-y plane as pitch for SSCT but does NOT change for MSCT

Contrast ResolutionSS- pitch causes broadening of slice-sensitivity profile. Scanner needs to have enough projections to reconstruct slice and is forced to seek them outside of specific z axis. Some projections may not pass through object in question and results in under-sampling which blurs object

Contrast ResolutionMS- pitch does not broaden SSP because at least one of multiple rows of detectors passes into x-y plane containing object in question. Because of multiple detectors, highly unlikely that projections distant from imaging plane will be needed.

Z axis resolution DoseSS- increased pitch decreases z axis resolution MS- increased pitch has little effect on z axis resolutionSS- increased pitch decreases radiation dose MS increased pitch, machine compensates with increased mA and dose does not change

Dose-Pitch RelationshipFor SS, if pitch>1, dose decreases pitch1, dose similar pitch

Reconstruction AlgorithmsSS and MS are similar First step done by machine, z axis interpolator works on raw data to weight projections nearest the slice location most heavily Second step selected by user: for soft tissue images, want to suppress noise and increase low contrast sensitivity. For bone want higher contrast.

Protocols for MSCTImage thickness, detector configuration, collimation, table speed, interval, reconstruction algorithms IV contrast parameters Length of acquisition Technique: kV, mA, sec Reconstruction algorithm FOV- for scan, for display

Problems/Pitfalls in Protocol DesignTiming of bolus and data acquisition Preset filming Pseudo-enhancement Venous artifacts Increasing numbers of tiny lesions

TimingRoutine chest protocol with 20 cm coverage, table speed 11.25 mm/rotation, takes 14 sec for entire scan at 0.8 sec/slice Using 40 sec prep delay If injection rate is 2cc/sec, use 108 cc If injection rate is 2.5, use 128 cc

Filming Because of high levels of vascular enhancement, classic soft tissue windows will not be appropriate for all organs. Lesions may be obscured in organs that enhance brightly such as kidney and arteries (pulmonary emboli, dissection flaps)

Other issuesPseudo-enhancement of renal cysts surrounded by parenchyma becomes a greater problem because of higher levels of renal enhancement Increasing numbers of tiny lesions in lung, liver, etc. Are these metastases? Venous artifacts, which simulate thrombus become more obvious and frequent

Can I reconstruct thinner slices than those printed on image?PE protocol 2.50mm/7.50 1.5:1 Image 2.5mm thickness Table speed = 7.5 mm/rotation Pitch = 1.5:1 (HS mode) 7.5 1.5 = 5mm collimation With 4 slices per rotation, detector size must be 1.25mm and therefore this is thinnest slice thickness that one can reconstruct

16 slice scannerRoutinely 360 rotation in 0.5 sec (798 data points) Can go to 0.4 sec rotation for cardiac scanning For larger patients, increase rotation time Using the large-large FOV, each pixel is 1mm in x-y plane. Thus each vowel is 1 x1 x1mm = ISOTROPIC SCANNING Can also achieve isotropic scanning with small FOV (head, neck, extremity) in which each voxel is 0.5mm

16 slice scannerHelical pitch = table distance per rotation / slice thickness 15mm / 1mm = 15 Beam pitch = helical pitch / image thickness 15 / 16 = 0.938

16 slice scannerProspective Gating: 0.4 sec gantry speed. Machine counts 5 cycles, calculates R-R interval, takes 0.25 sec scan, ending prior to beginning of next R wave. Requires heart rate < 80 bpm. Table moves during next R-R interval. Each scan covers 4 images @ 3mm thickness. Retrospective gating = gated reconstruction. Helical acquisition with ECG over raw data. When ECG is at a given point, take data at that time to make image.

MSCTExamples of Unique Protocols

LungAbnormal CXR- survey exam with 5mm sections, but choose detector set of 4 1.25mm so that retrospective thin slices through a small nodule can be obtained without re-imaging Airway disease- single breath hold with 1 or 1.25mm collimation. Evaluate trachea with overlapping 3-5mm sections, use 1mm sections to assess small airways. Combine inspiratory & expiratory views for physiologic evaluation, air trapping PE- 1.25- 2.5mm, HS mode scan average thorax in 10-12 sec. Use bolus tracking. Helpful to view as reconstructions.

AbdomenPorta hepatis- because of complicated anatomy & oblique orientation, 3mm sections recon w 50% overlap curved or coronal reformats Liver and pancreas- multiple phases in single breath holds Kidneys- 3D reconstructions similar to IVU and angiography

Musculoskeletal System0.5mm slice thickness can result in isotropic data set such that x-, y- and z-axes are equal in size (Will be standard on 8 and 16 slice scanners) Trauma- if scanning chest, abdomen, pelvis can change FOV, recon to thinner slices, change to bone algorithm and do 2D and 3D recons to review spine, without re-imaging as spine protocol

Head and NeckElimination of direct coronal imaging Reformats can avoid artifacts from teeth 3D displays for trauma and congenital anomalies 3D reformats can provide endoscopic views of larynx, hypopharynx and to calculate tumor volumes Angiographic and perfusion studies

Post Processing ApplicationsHuge numbers of images can be generated from original data set and reformatted in different planes, surface displays, angiographic techniques, virtual endoscopy Issues of how to view and store images

QUIZ

1. For SSCT with image thickness of 2.5 mm and table speed of 4.0 mm/rotation, the pitch = __________ 2. For MSCT with tech requesting 4 slices with 3.75 mm thickness and table speed of 11.25 mm/rotation, pitch = ________________

a. SSCTb. MSCTc. bothd. neither3. Re-indexing, or creating overlapping slices is possible on 4. Reconstruction images to different slice thicknesses is possible on 5. X-Y axis resolution (image quality) does not change significantly with higher pitch on

a. SSCTb. MSCTc. bothd. neither6. Noise increases with increasing pitch on 7. As pitch increases, > 1, the radiation dose to the patient decreases on 8. The reconstruction algorithm involves 2 steps, the z axis interpolator and application of specific algorithms such as smooth, standard, bone, etc on

1. For SSCT with image thickness of 2.5 mm and table speed of 4.0 mm/rotation, the pitch = 1.6 2. For MSCT with tech requesting 4 slices with 3.75 mm thickness and table speed of 11.25 mm/rotation, pitch = 0.75

a. SSCTb. MSCTc. bothd. neither3. Re-indexing, or creating overlapping slices is possible on c 4. Reconstruction images to different slice thicknesses is possible on b 5. X-Y axis resolution (image quality) does not change significantly with higher pitch on b

a. SSCTb. MSCTc. bothd. neither6. Noise increases with increasing pitch on d 7. As pitch increases, > 1, the radiation dose to the patient decreases on a 8. The reconstruction algorithm involves 2 steps, the z axis interpolator and application of specific algorithms such as smooth, standard, bone, etc on c

ReferencesBrink JA, Heiken JP, et al. Helical CT: Principles and Technical Considerations. Radiographics 1994; 14:887-893 Friedland GW and Thurber BD. The Birth of CT. AJR; 167: 1365-1370 Silverman PM. multi-slice Computed Tomography- A Practical Approach to Clinical Protocols. Lippincott Williams & Wilkins, 2002; Chapter 1: 1-29 Thanks to GE for providing images

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