Digital Radiology

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  • Digital Radiology

    1. 1. Diagnostic Radiology Part : Digital Radiology 1 Chapter 12Chapter 12 Image Quality inImage Quality in Digital RadiologyDigital Radiology
    2. 2. Diagnostic Radiology Part : Digital Radiology 2 Transition from conventional to digitalTransition from conventional to digital radiologyradiology  Digital images can be numerically processedDigital images can be numerically processed  Digital images can be easily transmitted throughDigital images can be easily transmitted through networks and archivednetworks and archived  Attention should be paid to the potential increaseAttention 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 needed producing higher image quality notproducing higher image quality not necessarily required for the clinical purposenecessarily required for the clinical purpose
    3. 3. Diagnostic Radiology Part : Digital Radiology 3 Radiation dose in digital radiologyRadiation dose in digital radiology  Conventional films allow to detectConventional films allow to detect mistakes if a wrong radiographicmistakes if a wrong radiographic technique is used :technique is used : images are tooimages are too white or too blackwhite or too black  Digital technology provides userDigital technology provides user always with a “always with a “good imagegood image” since” since its dynamic range compensates forits dynamic range compensates for wrong settings even if the dose iswrong settings even if the dose is higher than necessaryhigher than necessary
    4. 4. Diagnostic Radiology Part : Digital Radiology 4 What is “dynamic range”?What is “dynamic range”? Wide dose range to the detector, allows aWide dose range to the detector, allows a ““reasonablereasonable” image quality to be obtained” image quality to be obtained Flat panel detectors (discussed later) haveFlat 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 haswhile a screen-film system has approximately 10approximately 101.51.5
    5. 5. Diagnostic Radiology Part : Digital Radiology 5 Characteristic curve of CRCharacteristic curve of CR systemsystem HR-III CEA Film-Fuji Mammofine CR response Air Kerma (mGy) 0.001 0.01 0.1 1 3.5 3 2.5 2 1.5 1 0.5 0 Density
    6. 6. Diagnostic Radiology Part : Digital Radiology 6 Digitizing conventional filmsDigitizing conventional films Conventional radiographic images can beConventional radiographic images can be converted into digital information by aconverted into digital information by a ““digitizerdigitizer”, and electronically stored”, and electronically stored Such a conversion also allows someSuch a conversion also allows some numerical post-processingnumerical post-processing Such a technique cannot be consideredSuch a technique cannot be considered as a “ digital radiology” technique.as a “ digital radiology” technique.
    7. 7. Diagnostic Radiology Part : Digital Radiology 7 Analogue versus digitalAnalogue versus digital 0 5 10 15 20 1 2 3 4 5 6 7 8 9 10 Digital: A givenDigital: A given parameter can onlyparameter can only have discrete valueshave discrete values Analogue: A givenAnalogue: A given parameter can haveparameter can have continuous valuescontinuous values 0 10 20 1 2 3 4 5 6 7 8 9 10 C1
    8. 8. Diagnostic Radiology Part : Digital Radiology 8 Different number of pixels per image: originalDifferent number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8was 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).
    9. 9. Diagnostic Radiology Part : Digital Radiology 9 Different number of pixels per image: originalDifferent number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8was 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).
    10. 10. Diagnostic Radiology Part : Digital Radiology 10 Different number of pixels per image: originalDifferent number of pixels per image: original was 3732 x 3062 pixels x 256 grey levels (21.8was 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)
    11. 11. Diagnostic Radiology Part : Digital Radiology 11 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 toImportance of time allocated to retrieve imagesretrieve images
    12. 12. Diagnostic Radiology Part : Digital Radiology 12 Image qualityImage quality • DiagnosticDiagnostic information contentinformation content in digitalin digital radiology is generally higher than inradiology is generally higher than in conventional radiology ifconventional radiology if equivalentequivalent dosedose parameters are usedparameters are used • TheThe wider dynamic rangewider dynamic range of the digitalof the digital detectors and the capabilities of postdetectors and the capabilities of post processing allow to obtain moreprocessing allow to obtain more information from radiographic imagesinformation from radiographic images
    13. 13. Diagnostic Radiology Part : Digital Radiology 13 Tendency to increase dose ?Tendency to increase dose ?  For digital detectors,For digital detectors, higher doses result inhigher doses result in a better image qualitya better image quality (less “noisy” images)(less “noisy” images)  When increasing dose, the signal to noiseWhen increasing dose, the signal to noise ratio is improvedratio is improved  Thus, a certain tendency to increase dosesThus, a certain tendency to increase doses could happen specially in thosecould happen specially in those examinations where automatic exposureexaminations where automatic exposure control is not usually available.control is not usually available.
    14. 14. Diagnostic Radiology Part : Digital Radiology 14 Computed radiography versusComputed radiography versus film screenfilm screen • In computed radiography (CR) the “image density” isIn computed radiography (CR) the “image density” is automatically adjusted by the image processing,automatically adjusted by the image processing, nono mattermatter of the applied dose.of the applied dose. • This is one of the key advantages of the CR whichThis is one of the key advantages of the CR which helps to reduce significantly the retakes rate.helps to reduce significantly the retakes rate. • UnderexposuresUnderexposures are easilyare easily corrected bycorrected by radiographers (too noisy image).radiographers (too noisy image). • OverexposuresOverexposures cannot becannot be detected unless patientdetected unless patient dose measurements are performeddose measurements are performed
    15. 15. Diagnostic Radiology Part : Digital Radiology 15  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
    16. 16. Diagnostic Radiology Part : Digital Radiology 16 An underexposed image is “too noisy”. Exposure level 1,15 Exposure level 1,87
    17. 17. Diagnostic Radiology Part : Digital Radiology 17
    18. 18. Diagnostic Radiology Part : Digital Radiology 18 Exposure levelExposure level Some digital systems provide the user with anSome 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 theIt orientates the operator about the goodness of the radiographic technique usedradiographic technique used The relation betweenThe relation between dosedose andand exposure levelexposure level is usuallyis usually logarithmic: doubling the dose to the detector, willlogarithmic: 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 aSome digital systems offer a color codecolor code or aor a barbar in thein the previsualization monitor. This bar indicates whether theprevisualization monitor. This bar indicates whether the dose received by the detector is in the normal rangedose 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).
    19. 19. Diagnostic Radiology Part : Digital Radiology 19 • Example of bar inExample of bar in the image showingthe image showing the level of dosethe level of dose received by thereceived by the digital detectordigital detector
    20. 20. Diagnostic Radiology Part : Digital Radiology 20 Risk to increase dosesRisk to increase doses  TheThe wide dynamic rangewide dynamic range of digitalof digital detectors allows to obtaindetectors allows to obtain goodgood image qualityimage quality while usingwhile using high dosehigh dose technique at the entrance of the detector andtechnique at the entrance of the detector and at the entrance of the patientat the entrance of the patient  With conventional screen film systems such aWith conventional screen film systems such a choice is not possible since high dosechoice is not possible since high dose technique always results in a “too black”technique always results in a “too black” image.image.
    21. 21. Diagnostic Radiology Part : Digital Radiology 21 Dose in Digital fluoroscopyDose in Digital fluoroscopy In digital fluoroscopy there is a direct linkIn digital fluoroscopy there is a direct link between diagnostic information (between diagnostic information (number ofnumber of images and quality of the imagesimages and quality of the images) and) and patient dosepatient dose Digital fluoroscopy allows producingDigital fluoroscopy allows producing veryvery easilyeasily a great number of imagesa great number of images.. As a consequence of that :As a consequence of that : dose to the patientdose to the patient is likely to increaseis likely to increase without any benefitwithout any benefit
    22. 22. Diagnostic Radiology Part : Digital Radiology 22 Actions that can influence image qualityActions that can influence image quality Avoid bad viewing conditions (e.g. lackAvoid bad viewing conditions (e.g. lack of monitor brightness or contrast, poorof monitor brightness or contrast, poor spatial resolution, etc)spatial resolution, etc) Improve insufficient skill to use theImprove insufficient skill to use the workstation capabilities to visualizeworkstation capabilities to visualize images (window level, inversion,images (window level, inversion, magnification, etc).magnification, etc). Reduce artifacts due to incorrect digitalReduce artifacts due to incorrect digital post-processing (creation of falsepost-processing (creation of false lesions or pathologies)lesions or pathologies)
    23. 23. Diagnostic Radiology Part : Digital Radiology 23 Influence of the different imageInfluence of the different image compression levelscompression levels Image compression may:Image compression may: required for imagerequired for image storage capibilitystorage capibility in the PACSin the PACS modify the time necessary to have the imagesmodify the time necessary to have the images available (available (transmission speedtransmission speed in the intranet)in the intranet) A too high level ofA too high level of image compressionimage compression maymay result in a loss of image quality and,result in a loss of image quality and, consequently, in a possible repetition of theconsequently, in a possible repetition of the examination.examination.
    24. 24. Diagnostic Radiology Part : Digital Radiology 24 Digital radiography: initial pitfallsDigital radiography: initial pitfalls Lack of trainingLack of training and knowledge of theand knowledge of the viewing possibilities on the monitors andviewing possibilities on the monitors and post-processing capabilities.post-processing capabilities. Lack of a preliminary image visualization onLack of a preliminary image visualization on the monitors may result in athe monitors may result in a loss of diagnosticloss of diagnostic informationinformation (wrong contrast and window(wrong contrast and window levels selection)levels selection) Drastic changes in radiographicDrastic changes in radiographic techniques or geometric parameterstechniques or geometric parameters without paying attention towithout paying attention to patient dosespatient doses..
    25. 25. Diagnostic Radiology Part : Digital Radiology 25 Important aspects for the QA programs inImportant aspects for the QA programs in digital radiologydigital radiology Availability of procedures avoidingAvailability 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 radiographersHow to avoid that radiographers delete imagesdelete images.. InformationInformation confidentialityconfidentiality Compromise between image quality andCompromise between image quality and compression levelcompression level in the imagesin the images RecommendedRecommended minimum timeminimum time to archive imagesto archive images SpecificSpecific reference levelsreference levels
    26. 26. Diagnostic Radiology Part : Digital Radiology 26 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 underto obtain images of a test object under different radiographic conditionsdifferent radiographic conditions to decide the best compromise consideringto decide the best compromise considering both image quality and patient doseboth image quality and patient dose aspectsaspects
    27. 27. Diagnostic Radiology Part : Digital Radiology 27 Optimisation technique TOR(CDR) plus ANSI phantom to simulate chest and abdomen examinations and to evaluate image quality
    28. 28. Diagnostic Radiology Part : Digital Radiology 28 0 2 4 6 8 10 12 0 20 40 60 80 mAs numberofobjects 0 0,5 1 1,5 2 2,5 3 lp/mm High cont. (n) Low cont. (n) Resol. (lp/mm) Optimization technique for Abdomen AP Simulation with TOR(CDR) + ANSI phantom 81 kVp, 100 cm (focus-film distance) 1.6 mGy
    29. 29. Diagnostic Radiology Part : Digital Radiology 29 0 2 4 6 8 10 12 14 0 10 20 30 40 50 mAs numberofobjects 0 0,5 1 1,5 2 2,5 3 3,5 lp/mm High cont. (no.) Low cont. (no.) Resol. (lp/mm) 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
    30. 30. Diagnostic Radiology Part : Digital Radiology 30 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
    31. 31. Diagnostic Radiology Part : Digital Radiology 31 Routine QC programme Not affected by change to CR Patient dose evaluation (when optimised) Tube-generator controls (except. AEC) Affected by change to CR Image quality evaluation with test object Image quality evaluation with clinical criteria Image receptors (film-screen, viewing...) Automatic processors Image processing
    32. 32. Diagnostic Radiology Part : Digital Radiology 32 QC equipment Available TOR(CDR) image quality test Photometer Densitometer Dosimeters CR image quality test object SMPTE image test Pencil type photometer
    33. 33. Diagnostic Radiology Part : Digital Radiology 33
    34. 34. Diagnostic Radiology Part : Digital Radiology 34
    35. 35. Diagnostic Radiology Part : Digital Radiology 35 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 DigitalICRP draft on Dose Management in Digital Radiology. Expected for 2003.Radiology. Expected for 2003. • PACS. Basic Principles and Applications. HuangPACS. 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 DigitalGonzalez 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.
    36. 36. Diagnostic Radiology Part : Digital Radiology 36 Where to Get More Information (2)Where to Get More Information (2) • http://www.gemedicalsystems.com/rad/xr/eduhttp://www.gemedicalsystems.com/rad/xr/edu (last access 22 August 2002).(last access 22 August 2002). • http://www.agfa.com/healthcare/http://www.agfa.com/healthcare/ (last(last access 22 August 2002).access 22 August 2002).

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