Abstract
BackgroundStorage issues and bandwidth over networks have led to a need to optimally compress medical imaging files while leaving clinical image quality uncompromised.MethodsTo determine the range of clinically acceptable medical image compression across multiple modalities (CT, MR, and XR), we performed psychometric analysis of image distortion thresholds using physician readers and also performed subtraction analysis of medical image distortion by varying degrees of compression.ResultsWhen physician readers were asked to determine the threshold of compression beyond which images were clinically compromised, the mean image distortion threshold was a JPEG Q value of 23.1 ± 7.0. In Receiver-Operator Characteristics (ROC) plot analysis, compressed images could not be reliably distinguished from original images at any compression level between Q = 50 and Q = 95. Below this range, some readers were able to discriminate the compressed and original images, but high sensitivity and specificity for this discrimination was only encountered at the lowest JPEG Q value tested (Q = 5). Analysis of directly measured magnitude of image distortion from subtracted image pairs showed that the relationship between JPEG Q value and degree of image distortion underwent an upward inflection in the region of the two thresholds determined psychometrically (approximately Q = 25 to Q = 50), with 75 % of the image distortion occurring between Q = 50 and Q = 1.ConclusionIt is possible to apply lossy JPEG compression to medical images without compromise of clinical image quality. Modest degrees of compression, with a JPEG Q value of 50 or higher (corresponding approximately to a compression ratio of 15:1 or less), can be applied to medical images while leaving the images indistinguishable from the original.
Highlights
Storage issues and bandwidth over networks have led to a need to optimally compress medical imaging files while leaving clinical image quality uncompromised
Psychometric Experiment 1: Clinically important distortion When physician readers were asked to determine the degree of compression beyond which images were clinically compromised, the mean image distortion threshold was a JPEG Q value of 23.1 ± 7.0 (Figure 2)
Psychometric Experiment 2: ReceiverOperator Characteristics (ROC) plot analysis of discrimination between compressed and original image pairs In ROC plot analysis, compressed images could not be reliably distinguished from original images at any compression level between Q = 50 and Q = 95 (Figure 3A)
Summary
Storage issues and bandwidth over networks have led to a need to optimally compress medical imaging files while leaving clinical image quality uncompromised. As medical imaging has transitioned to digital formats such as DICOM and archives grow in size, [1] optimal settings for image compression are needed to facilitate long-term mass storage requirements. One definition of optimal medical image compression is a degree of compression that decreases file size substantially but produces a degree of image distortion that is not clinically significant. We assessed the crossover point for distortion of grayscale medical images (CT, MR, and XR modalities) by JPEG compression according to two different definitions: (1) the point at which distortion is clinically significant to the viewer and (2) the point at which any distortion can be reliably discriminated by the viewer. We performed analysis of subtracted images to correlate the accumulation of increasing error pixel burden at lower JPEG Q values with the thresholds determined psychometrically
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