diagnostic radiology part : digital radiology1 image quality in digital radiology image quality in...

Diagnostic Radiology Part : Digital Radiology 1

Image Quality inImage Quality inDigital Radiology Digital Radiology


Transition from conventional to digital Transition from conventional to digital radiologyradiology

Digital images can be numerically processed Digital images can be numerically processed

Digital images can be easily transmitted through Digital images can be easily transmitted through networks and archivednetworks and archived

Attention should be paid to the potential increase Attention should be paid to the potential increase of patient doses due to tendency of : of patient doses due to tendency of : producing more images than neededproducing more images than neededproducing higher image quality not producing higher image quality not

necessarily required for the clinical purposenecessarily required for the clinical purpose


Radiation dose in digital radiologyRadiation dose in digital radiology

Conventional films allow to detect Conventional films allow to detect mistakes if a wrong radiographic mistakes if a wrong radiographic technique is used : technique is used : images are too images are too white or too blackwhite or too black

Digital technology provides user Digital technology provides user always with a “always with a “good imagegood image” since ” since its dynamic range compensates for its dynamic range compensates for wrong settings even if the dose is wrong settings even if the dose is higher than necessaryhigher than necessary


What is “dynamic range”?What is “dynamic range”?

Wide dose range to the detector, allows a Wide dose range to the detector, allows a ““reasonablereasonable” image quality to be obtained” image quality to be obtained

Flat panel detectors (discussed later) have Flat panel detectors (discussed later) have a dynamic range of 10a dynamic range of 1044 (from 1 to 10,000) (from 1 to 10,000) while a screen-film system has while a screen-film system has approximately 10approximately 101.51.5


Characteristic curve of CR Characteristic curve of CR systemsystem

HR-IIICEA Film-Fuji Mammofine

CR response

Air Kerma (mGy)

0.001 0.01 0.1 1

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Digitizing conventional filmsDigitizing conventional films

Conventional radiographic images can be Conventional radiographic images can be converted into digital information by a converted into digital information by a ““digitizerdigitizer”, and electronically stored”, and electronically stored

Such a conversion also allows some Such a conversion also allows some numerical post-processingnumerical post-processing

Such a technique cannot be considered Such a technique cannot be considered as a “ digital radiology” technique.as a “ digital radiology” technique.


Analogue versus digitalAnalogue versus digital

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Digital: A given Digital: A given parameter can only parameter can only have discrete valueshave discrete values

Analogue: A given Analogue: A given parameter can have parameter can have continuous valuescontinuous values

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Different number of pixels per image: original Different number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8 was 3732 x 3062 pixels x 256 grey levels (21.8 Mbytes). Here, resized at 1024 x 840 (1.6 MB).Mbytes). Here, resized at 1024 x 840 (1.6 MB).


Different number of pixels per image: original Different number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8 was 3732 x 3062 pixels x 256 grey levels (21.8 Mbytes). Here, resized at 128 x 105 (26.2 kB).Mbytes). Here, resized at 128 x 105 (26.2 kB).


Different number of pixels per image: original Different number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8 was 3732 x 3062 pixels x 256 grey levels (21.8 Mbytes). Here, resized at 64 x 53 (6.6 kB)Mbytes). Here, resized at 64 x 53 (6.6 kB)


Digital radiology processDigital radiology process

Image acquisitionImage acquisition Image processingImage processing Image displayImage display

Importance of viewing conditionsImportance of viewing conditions Image archiving (PACS)Image archiving (PACS) Image retrievingImage retrieving

Importance of time allocated to Importance of time allocated to retrieve images retrieve images


Image qualityImage quality

• Diagnostic Diagnostic information contentinformation content in digital in digital radiology is generally higher than in radiology is generally higher than in conventional radiology if conventional radiology if equivalent equivalent dosedose parameters are used parameters are used

• The The wider dynamic rangewider dynamic range of the digital of the digital detectors and the capabilities of post detectors and the capabilities of post processing allow to obtain more processing allow to obtain more information from radiographic images information from radiographic images


Tendency to increase dose ?Tendency to increase dose ?

For digital detectors, For digital detectors, higher doses result in higher doses result in a better image qualitya better image quality (less “noisy” images) (less “noisy” images)

When increasing dose, the signal to noise When increasing dose, the signal to noise ratio is improvedratio is improved

Thus, a certain tendency to increase doses Thus, a certain tendency to increase doses could happen specially in those could happen specially in those examinations where automatic exposure examinations where automatic exposure control is not usually available. control is not usually available.


Computed radiography versus Computed radiography versus film screenfilm screen

• In computed radiography (CR) the “image density” is In computed radiography (CR) the “image density” is automatically adjusted by the image processing, automatically adjusted by the image processing, no no mattermatter of the applied dose. of the applied dose.

• This is one of the key advantages of the CR which This is one of the key advantages of the CR which helps to reduce significantly the retakes rate.helps to reduce significantly the retakes rate.

• Underexposures Underexposures are easilyare easily corrected by corrected by radiographers (too noisy image).radiographers (too noisy image).

• Overexposures Overexposures cannot becannot be detected unless patient detected unless patient dose measurements are performeddose measurements are performed


Underexposure results in a “too noisy” image Overexposure yields good images with

unnecessary high dose to the patient Over range of digitiser may result in uniformly

black area with potential loss of information

Exposure level 2,98 Exposure level 2,36


An underexposed image is “too noisy”.

Exposure level 1,15 Exposure level 1,87


Exposure levelExposure level Some digital systems provide the user with an Some digital systems provide the user with an

““exposure levelexposure level” index which expresses the dose level ” index which expresses the dose level received at the digital detectorreceived at the digital detector

It orientates the operator about the goodness of the It orientates the operator about the goodness of the radiographic technique usedradiographic technique used

The relation between The relation between dosedose and and exposure levelexposure level is usually is usually logarithmic: doubling the dose to the detector, will logarithmic: doubling the dose to the detector, will increase the “exposure level” to a factor of 0.3 = log(2).increase the “exposure level” to a factor of 0.3 = log(2).

Some digital systems offer a Some digital systems offer a color codecolor code or a or a bar bar in the in the

previsualization monitor. This bar indicates whether the previsualization monitor. This bar indicates whether the dose received by the detector is in the normal range dose received by the detector is in the normal range (green) or it is too high (red color). (green) or it is too high (red color).


• Example of bar in Example of bar in the image showing the image showing the level of dose the level of dose received by the received by the digital detectordigital detector


Risk to increase dosesRisk to increase doses

The The wide dynamic range wide dynamic range of digital of digital detectors allows to obtaindetectors allows to obtain good good image quality image quality while using while using high dosehigh dose technique at the entrance of the detector and technique at the entrance of the detector and at the entrance of the patient at the entrance of the patient

With conventional screen film systems such a With conventional screen film systems such a choice is not possible since high dose choice is not possible since high dose technique always results in a “too black” technique always results in a “too black” image. image.


Dose in Digital fluoroscopyDose in Digital fluoroscopy In digital fluoroscopy there is a direct link In digital fluoroscopy there is a direct link

between diagnostic information (between diagnostic information (number of number of images and quality of the imagesimages and quality of the images) and ) and patient dose patient dose

Digital fluoroscopy allows producing Digital fluoroscopy allows producing very very easilyeasily a great number of images a great number of images..

As a consequence of that : As a consequence of that : dose to the patient dose to the patient

is likely to increaseis likely to increase without any benefit without any benefit


Actions that can influence image quality Actions that can influence image quality

Avoid bad viewing conditions (e.g. lack Avoid bad viewing conditions (e.g. lack of monitor brightness or contrast, poor of monitor brightness or contrast, poor spatial resolution, etc) spatial resolution, etc)

Improve insufficient skill to use the Improve insufficient skill to use the workstation capabilities to visualize workstation capabilities to visualize images (window level, inversion, images (window level, inversion, magnification, etc). magnification, etc).

Reduce artifacts due to incorrect digital Reduce artifacts due to incorrect digital post-processing (creation of false post-processing (creation of false lesions or pathologies) lesions or pathologies)


Influence of the different image Influence of the different image compression levels compression levels Image compression may:Image compression may:

required for image required for image storage capibilitystorage capibility in the PACS in the PACS modify the time necessary to have the images modify the time necessary to have the images

available (available (transmission speedtransmission speed in the intranet) in the intranet)

A too high level of A too high level of image compressionimage compression may may result in a loss of image quality and, result in a loss of image quality and, consequently, in a possible repetition of the consequently, in a possible repetition of the examination.examination.


Digital radiography: initial pitfallsDigital radiography: initial pitfallsLack of trainingLack of training and knowledge of the and knowledge of the

viewing possibilities on the monitors and viewing possibilities on the monitors and post-processing capabilities.post-processing capabilities.

Lack of a preliminary image visualization on Lack of a preliminary image visualization on the monitors may result in a the monitors may result in a loss of diagnostic loss of diagnostic informationinformation (wrong contrast and window (wrong contrast and window levels selection)levels selection)

Drastic changes in radiographic Drastic changes in radiographic techniques or geometric parameters techniques or geometric parameters without paying attention to without paying attention to patient dosespatient doses..


Important aspects for the QA programs in Important aspects for the QA programs in digital radiologydigital radiology

Availability of procedures avoiding Availability of procedures avoiding loss of imagesloss of images due to network problems or electric power supply.due to network problems or electric power supply.

How to avoid that radiographers How to avoid that radiographers delete imagesdelete images. .

Information Information confidentialityconfidentialityCompromise between image quality and Compromise between image quality and

compression levelcompression level in the images in the images

Recommended Recommended minimum timeminimum time to archive images to archive imagesSpecific Specific reference levelsreference levels


Initial basic quality controlInitial basic quality control

A first tentative approach could be:A first tentative approach could be:

to obtain images of a test object under to obtain images of a test object under different radiographic conditions different radiographic conditions

to decide the best compromise considering to decide the best compromise considering both image quality and patient dose both image quality and patient dose aspectsaspects


Optimisation technique

TOR(CDR) plus ANSI phantom to simulate chest and abdomen examinations and to

evaluate image quality


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Optimization technique for Abdomen AP

Simulation with TOR(CDR) + ANSI phantom

81 kVp, 100 cm (focus-film distance)

1.6 mGy


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Optimisation technique for Chest PA

Simulation with TOR(CDR) + ANSI phantom

125 kVp, 180 cm (focus-film distance)

* Grid focalised at 130 cm

0.25 mGy


Image quality comparison

Exam. Type Resolution

(lp/mm)

Low contrast sensitivity threshold

High contrast sensitivity threshold

Conv 2.50 7 9 Abdomen

CR 3.15 9 9

Conv 3.55 8 6 Chest

CR 2.24 7 6

Conv 7.10 11 14 TOR(CDR)+1.5 mm Cu

CR 2.80 16 16


Routine QC programme Not affected by change to CR

Patient dose evaluation (when optimised)Tube-generator controls (except. AEC)

Affected by change to CRImage quality evaluation with test objectImage quality evaluation with clinical

criteriaImage receptors (film-screen, viewing...)Automatic processorsImage processing


QC equipment

AvailableTOR(CDR) image quality test PhotometerDensitometerDosimetersCR image quality test objectSMPTE image testPencil type photometer


Where to Get More InformationWhere to Get More Information

• Balter S. Interventional fluoroscopy. Physics, Balter S. Interventional fluoroscopy. Physics, technology and safety. Wiley-Liss, New York, 2001.technology and safety. Wiley-Liss, New York, 2001.

• ICRP draft on Dose Management in Digital ICRP draft on Dose Management in Digital Radiology. Expected for 2003.Radiology. Expected for 2003.

• PACS. Basic Principles and Applications. Huang PACS. Basic Principles and Applications. Huang HK. Wiley – Liss, New York, 1999.HK. Wiley – Liss, New York, 1999.

• Vano E, Fernandez JM, Gracia A, Guibelalde E, Vano E, Fernandez JM, Gracia A, Guibelalde E, Gonzalez L. Routine Quality Control in Digital Gonzalez L. Routine Quality Control in Digital versus Analog Radiology. Physica Medica 1999; versus Analog Radiology. Physica Medica 1999; XV(4): 319-321.XV(4): 319-321.


Where to Get More Information (2)Where to Get More Information (2)

• http://www.gemedicalsystems.com/rad/xr/education/dig_xray_intro.html (last access 22 August 2002). (last access 22 August 2002).

• http://www.agfa.com/healthcare/ (last (last access 22 August 2002).access 22 August 2002).

http://www.gemedicalsystems.com/rad/xr/education/dig_xray_intro.html

http://www.gemedicalsystems.com/rad/xr/education/dig_xray_intro.html

http://www.agfa.com/healthcare/

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