Lossy JPEG compression in quantitative angiography: the role of X-ray quantum noise

Abstract

In medical imaging, contrary to applications in the consumer market, the use of irreversible or lossy compression is still in its beginnings. This is due to the suspected risk of compromising the diagnostic content. Many studies have been performed, but it was not until 2008 that national activities in different countries resulted in recommendations for the safe use of irreversible image compression in clinical practice. Quantitative coronary angiography (QCA), however, poses a special problem, since here a large variation in published maximum compression factors has strengthened the general concerns about the use of lossy techniques. Up to now, the reason for the variation has not been thoroughly investigated. Reasons for the discrepancies in published compression factors are determined in this study. Since JPEG compression reduces the quantum noise of the X-ray images, the impact of compression is overestimated when interpreting any change in local diameter as an error. By taking into consideration the quantitative effect of quantum noise in QCA, it is shown that the influence of JPEG compression can be neglected for compression factors up to ten at clinically applicable X-ray doses. This limit is comparable to that found by visual analysis for aesthetic image quality. Future studies on image compression effects should take the interaction with quantum noise explicitly into consideration.

Fig 1

Relationship between compression factor and JPEG quality factor for coronary angiograms.

Fig 2

Convolution kernels used for edge detection (left edge).

Fig 3

Compression-induced changes of the minimal luminal diameter of defined vessel segments of coronary angiograms.

Fig 4

Compression-induced change of the local diameter of perspex phantom vessels.

Fig 5

Compression-induced changes of the diameter of perspex phantom vessels.

Fig 6

Distributions of the compression-induced changes of diameter value as a function of the change of diameter caused by quantum noise in the uncompressed image (simulated phantom images, JPEG quality factor 90).

Fig 7

Division of the total standard deviation of diameter into the part due to quantum noise and the compression-induced part, for a perspex vessel image (vessel diameter =  3 mm).