On the assessment of image compression quality by means of quantitative coronary angiography

Many techniques for image compression do exist and are well described in the literature. Lossless image compression is for digital coronary angiograms limited to compression ratios in the order of 3-4. The purpose of this work is about the assessment of the diagnostic image quality of lossy compressed coronary angiograms by means of Quantitative Coronary Angiography (QCA). We measure in the compressed images the diameter of the vessel at several places as a function of the compression ratio and compare this with the original image. The set of representative images (512 by 512 pixels at 8 bits/pixel) is compressed with the ratios 4, 8, 12 and 16. The selected compression algorithms are JPEG, Lapped Orthogonal Transform (LOT) and Modified Fast Lapped Transform (MFLT). The obtained quantitative diameter values start to deviate at images representations down at 0.5 bit per pixel with the JPEG giving the greatest differences (typ. > 5%). The results of LOT and MFLT are performing better with respect to the criterion vessel size of diagnostic relevant vessels. At the greater compression ratios some blocking artifacts (JPEG) or ringing (LOT) starts to become visible. Somewhat to our surprise in our comparison study we have found that there are no great deviations in measured vessel diameter for the compression ratios 3, 8, 12 and 16. At compression ratio 16 JPEG has the largest deviation (typ. > 5%). According to the changes in the quantitative data higher compression ratios are certainly feasible.