Lossy Compression as a Standard of Practice: A Pan- Canadian ...

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Lossy Compression as a Standard of Practice: A Pan- Canadian ...

Lossy Compression as aStandard of Practice: A Pan-Canadian Study to EvaluateCompression Ratios for use inNational Guidelines.David A. Koff MDPeter Bak PhDPaul Brownrigg MBAAlex Kiss PhDLuigi Lepento MD, FRCPCTracy Michalak MScHarry Shulman MD, FRCPCAndrew Volkening MSc


The CAR PACS Committee Position• The Canadian Association of RadiologistsPACS/Teleradiology committee hasaccepted the principle of irreversible(“lossy”) compression for use in primarydiagnosis and clinical review, usingDICOM JPEG or JPEG-2000 compressionalgorithms, at specific compressionratios set by image type.2


Methodology


Methodology5 modalities (CR/DR, CT, US, MR, NM) and 7 radiologicalareas (Angio, Body, Breast, Chest, MSK, Neuro, Pediatrics)CR/DR CT US MR NMAngio x xBody x x x x xBreast x x xChest x xMSK x x x xNeuro x xPediatrics x x x x x5


MethodologyModalityLowMediumHigh• Images werecompressed inJPEG and JPEGCR/DR 20 25 302000 at 3differentratios, close toCTUS10812101512acceptability,based onMR 16 20 24previousstudies, belowNM 7 9 12and beyond. Angio 8 12 166


MethodologyBased on previous studies, ourevaluation resulted from theassociation of 2 accepted methods:• Diagnostic accuracy evaluation withROC analysis• Image comparison with original-revealed forced choice (JND)7


ROC• Images are a mixed of normalcases and identified pathologies(4 abnormal:1 normal)• Images presented full screencompressed in JPEG and JPEG2000 at 1 of the 3 different ratiosor in original size.8


ROCUsing normal images is importantfor two reasons:• It allows to examine the possibility ofartifacts that simulate abnormalities ina normal image.• It addresses the possibility of bias thatcould occur if readers know that eachimage contains some abnormality.9


ROC• The entire set of images was randomized. Toprevent potential bias due to recognizing agiven image, readers were not shown thesame image twice under differentcompression levels or algorithms.• A restricted number of pathologies were listedin a drop-menu from which the reader had tochose.10


ROC• Reviewer had tospecify in whichsector of the images/he saw thepathology and hadto give aconfidence ratingon a scale from 1to 5 (1=definite absenceof lesion and 5= definitepresence).1 23 411


Image comparison with original-revealed forced choice (JND)• Each compressed image was paired with theoriginal• Observer was asked to rate the degradation inimage quality, which could impair interpretation,on a scale of 1 to 6.30:1 Original12


Image comparison with original-revealed forced choice (JND)654321NoneJust noticeableConspicuousIntermediateSignificantUnacceptableNo Detectable DifferenceNo Loss of Diagnostic InformationBordering on Loss of InformationSubtle Abnormalities Could beoverlookedImportant Diagnostic Information Couldbe Lost, Degradation ImpairsInterpretationInadequate for Diagnosis; Definite Lossof Diagnostic Information13


Methodology• 3 readers per session, 23 sessions,more than 100 readers in total fromall across Canada.• Had to read the images on the DICOMcompliant, calibrated workstation theyuse in their daily activity.• The workstation had to be connectedto the Internet ideally.• Appropriate reading environment withcorrect ambient light.14


Methodology• Each reader received a CD with 70images or image stack of no more than20 images for CT scan.• The session could be interrupted at anytime and resumed automatically wherestopped.• The answers were filled on-line anddirectly transferred to our server.• If user could not connect his workstationto the Internet, an alternate paperbasedsolution was provided.15


Technicaldevelopments


Technical developments• An industry recognized compressionengine from Pegasus Inc. (Tampa,FL): PICTools with JPEG 2000 andLossy JPEG.• DICOM communication: MergeCOM-3 from Merge-Cedara17


Technical developments• A standalone administratorapplication allowing:• Database of anonymized cases• Database of participating radiologists• Compression engine• Creation and compilation of worklists• CD/DVD engraving18


Technical developments• A specially designed viewing software todisplay images to review at the differentlevels of compression then side by sidepaired with the original when revealed.• Limited processing functions wereprovided:• Zoom• Pan• Window-leveling• Reset19


ROCGROUND GLASS OPACITY20


JNDJPEG 2000 15:1ORIGINAL21


COMPARISON SIDE BY SIDEPNEUMOTHORAX22


JPEG 25:1ORIGINAL23


Results


ResultsOn 23 sessions, only 2 have not been completed yet:Breast CR and Pediatrics CT , and will be shortly.CR/DR CT US MR NMAngio x xBody x x x x xBreast x x xChest x xMSK x x x xNeuro x xPediatrics x x x x x25


ResultsFor 18 anatomical regions/modalities, there was nodifference noted, but discrepancies were noted in 3occurrences.CR/DR CT US MR NMAngio x xBody x x x x xBreast x x xChest x xMSK x x x xNeuro x xPediatrics x x x x x26


Results - ROC• For CT Neuro, statistical differences were foundon sensitivity for the type of compression, withJPEG found to be more sensitive on average(0.95 versus 0.81).• For CR MSK, statistical differences were found onsensitivity for the level of compression with JPEGfound to be more sensitive than JPEG 2000 at25:1 (0.91 vs. 0.80) and at 30:1 (0.97 vs. 0.85).• No statistical difference were found on the othermeasures.27


Results - ROCThe Effect of Compression on Sensitivity of CT NeuroImages% of Pathologies Correctly Identified120100806040200J2000-10 J2000-12 J2000-15 JPEG- 10 JPEG- 12 JPEG-15 RAW28


Results - ROCThe Effect of Compression on Sensitivity of CRMSK Images% of Pathologies CorrectlyIdentified1009080706050403020100J2000-20J2000-25J2000-30JPEG-20JPEG-25JPEG-30RAW29


Results - JND• For CT Body, a significant difference wasfound between type of compression with agreater proportion of readers choosingcategories 1, 2 and 3 (unacceptable tointermediate) for JPEG 2000.• For CT Neuro, a significant difference wasfound between type of compression with agreater proportion of readers choosingcategories 1, 2 and 3 (unacceptable tointermediate) for JPEG 2000.• No statistical difference were found on theother measures.30


Results - JNDThe Effect of Compression on Subjective Assessment of CTBody ImagesNo differenceJust noticeable10.9Conspicious0.8Intermediate0.70.6Significant0.5Unacceptable0.40.30.20.10J2K J2K -10 -10J2K J2K -12 -12J2K J2K -15 -15JPEG JPEG -10 -10JPEG JPEG -12 -12JPEG JPEG -15 -15Uncomp Uncomp31


Results - JND10.90.80.70.60.50.40.30.20.10The Effect of Compression on Subjective Assessment ofCT Neurological ImagesNo differenceJust noticeableConspiciousIntermediateSignificantUnacceptableJ2K-8J2K-10J2K-12JPEG-8JPEG-10JPEG-12Uncomp32


Conclusion• Lossy compression can be used at thelowest levels of compression tested, asthere is no significant loss of diagnosticinformation at those levels.• No difference between Lossy JPEG andJPEG 2000 at the lowest levels ofcompression, but JPEG performs betterthan JPEG 2000 on high frequency lowenergy CT images.33


Recommended valuesCR/DRCTUSMRNMMGAngio10-1516-24Body20-30JPEG 10-15J2K 108-1216-249-11Breast8-1216-2415-25Chest20-3010-15MSKJPEG 20-30J2K 2010-158-1216-24NeuroJPEG 8-12J2K 816-24Ped20-3010-158-1216-249-1134


Recommended valuesCR/DRCTUSMRNMMGAngio10-1516-24Body20-30JPEG 10-15J2K 108-1216-249-11Breast8-1216-2415-25Chest20-3010-15MSKJPEG 20-30J2K 2010-158-1216-24NeuroJPEG 8-12J2K 816-24Ped20-3010-158-1216-249-1120 8-10 16 1535

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