Effects of Grayscale Window/Level on Breast Lesion Detectability

1. Effects of Grayscale Window/Level on Breast Lesion Detectability Jeffrey Johnson, PhD a John Nafziger, PhD a Elizabeth Krupinski, PhD b Hans Roehrig, PhD b a b Supported by U. S. Army Medical Research and Materiel Command, grant DAMD-17-01-1-0621

2. Rationale • Nearly 50% of breast lesions missed at initial screening are visible retrospectively • Digital mammography could reduce perceptual errors by enhancing lesion conspicuity with image processing • Perceptual models could be useful tools for automating and optimizing techniques for image enhancement

3. Overview • This study evaluated the use of a visual discrimination model (VDM) for predicting effects of one type of image enhancement - grayscale window width and level (W/L) - on the detectability of breast lesions • Compared model and observer performance in two experiments: • 2AFC detection thresholds with simulated mammograms and nonmedical observers • ROC observer performance study with radiologists and digitized mammograms

4. Methods: Simulated Mammograms • Backgrounds • Filtered noise, 1/f3 noise power spectrum • Two groups: Bright and Dark central regions • Lesion signals • Mass: 2D Gaussian (d=50 arcmin) • Microcalcification cluster: six blurred disks or “specks” (disk d=8 arcmin, cluster d=40 arcmin)

5. Methods: W/L Conditions • P-value transformations: • Fully stretched • Understretched (-25%) • Overstretched (±25%) • Bright shifted (+25%) • Dark shifted (-25%) • Applied to full 512x512 pixel image or 170x170 pixel central region of interest containing lesion

6. Example Test Images Specks Full W/L Dark Center Specks Central W/L Dark Center Gaussian Full W/L Bright Center Gaussian Central W/L Bright Center Fully stretched (FS) Under stretched (US) Over stretched (OS)

7. Example Test Images Specks Full W/L Dark Center Specks Central W/L Dark Center Gaussian Full W/L Bright Center Gaussian Central W/L Bright Center Bright shifted (BS) Dark shifted (DS)

8. 2AFC Threshold Detection • Side-by-side presentation of same background with/without signal • Signal amplitude varied in 1-up/3-down staircase procedure; detection threshold at ~80% correct • Five W/L conditions interleaved in same session • Separate sessions for two signal and two background types

9. Test Conditions • Siemens 5M-pixel CRT monitor (P45) • Luminance range = 0.3 to 290 cd/m2 • Barco 10-bit display controller • DICOM-14 grayscale display function • Three nonmedical observers • Viewing distance = 52 cm; chin rest • Ambient lights off

10. Results: Detection Thresholds for Gaussian Signals Bright Backgrounds Dark Backgrounds Error bars show 95% confidence intervals

11. Results: Detection Thresholds for Speck Clusters Bright Backgrounds Dark Backgrounds Error bars show 95% confidence intervals

12. Experimental Detection Thresholds • Significant variations across W/L conditions • Generally lower for central vs. full W/L • due to local contrast enhancement - fully stretched not always optimal • Full W/L: Lowest thresholds for … • fully stretched, understretched (specks only) • dark shifted on bright, bright shifted on dark • Central W/L: Lowest thresholds for … • overstretched for Gaussians and specks on dark • dark shifted on bright, bright shifted on dark

13. Visual Discrimination Modeling • Simulates physiological response of human visual system to visual stimuli: luminance patterns from images & video • Output is a deterministic prediction of feature or image discriminability as function of spatial location, spatial frequency, and time • Discriminability measured in units of Just Noticeable Differences (JND)

14. VDM Architecture Contrast Pyramid (visual cortex) Pair of input images Spatial orientation responses Display luminance Within-band Masking … Spatial frequency bands Crossband Masking … Optics Contrast Pyramid JND Distance Probability Display & Ocular Processing Combin. Rule JND scalar JND map

15. VDM vs. Experimental Thresholds for Gaussians on Bright Backgrounds Full W/L Central W/L Error bars show 95% confidence intervals

16. VDM vs. Experimental Thresholds for Gaussians on Dark Backgrounds Full W/L Central W/L Error bars show 95% confidence intervals

17. VDM vs. Experimental Thresholds for Specks on Bright Backgrounds Full W/L Central W/L Error bars show 95% confidence intervals

18. VDM vs. Experimental Thresholds for Specks on Dark Backgrounds Full W/L Central W/L Error bars show 95% confidence intervals

19. VDM vs. Experimental Thresholds: Simulated Lesions & Backgrounds • Generally good agreement between model and experimental detection thresholds and variations across W/L conditions • Consistently reduced thresholds with central (local ROI) vs. full-image W/L • Largest modeling discrepancies for specks, especially on dark backgrounds

20. ROC Observer Study • Determine effects of W/L functions and size on detection of microcalcification clusters by mammographers • Evaluate utility of localized ROI contrast enhancement (central vs. full W/L)

21. ROC Observer Study: Image Preparation • Digitized mammograms (n=15) from Digital Database for Screening Mammography • Extracted 512x512-pixel sections with single, centered microcalcification cluster • Removed calcifications by median filtering • Generated five lesion-contrast levels: 0, 25, 50, 75, and 100% • Applied three W/L functions: Fully stretched, under and over stretched by 15% • Full and Central W/L sizes

22. ROC Observer Study: Test Conditions • 6 radiologists at Univ. of Arizona • 225 images/session • 2 reading sessions ~2 weeks apart • Decision confidence on 6-point scale • No image processing, no time limits, ambient lights off; viewed at ~25 cm • Siemens 5M-pixel CRT monitor (P45) • Luminance = 0.8 to 500 cd/m2 • DICOM-14 grayscale display function

23. Examples of Test Images Overstretched (OS, 15%) Understretched (US, 15%) Fully stretched (FS, 0-4095) Full W/L Central W/L

24. ROC Observer Study: Results • Compared central vs. full W/L across all W/L functions, all lesion contrasts • Observer performance statistically better (p<0.05) for FULL W/L size Az Values

25. ROC Observer Study: Results • No statistically significant variations: • between central and full W/L sizes for a single W/L function (all lesion contrasts) • between central and full W/L sizes for a single combination of W/L function and lesion contrast (except FS, 50%) • across W/L functions in central and full W/L sizes considered separately (all lesion contrasts)

26. ROC Observer Study: Analysis • Central W/L enhanced lesion contrast but changed appearance of parenchymal tissue relative to surrounding areas • Decision confidence lowered by nonuniform appearance of background tissue characteristics • Conclusion: Calcifications may be easier to perceive (due to higher contrast) but more difficult to interpret (due to cognitive factors, past experience)

27. Conclusions • For simulated lesions and backgrounds, VDM was generally a reliable predictor of W/L conditions for optimal detectability • Results with simulated images suggested benefits of localized contrast enhancement • Decision confidence and performance of mammographers actually lower with localized W/L, probably due to nonuniform tissue appearance

28. Future Directions • Allow toggling between full and local W/L modes (combine uniform contextual data with local contrast enhancement) • Evaluate effects of W/L on detection of very subtle lesions (low contrast, near threshold) • Model refinements: • improved crossband masking for higher frequency signals: specks/calcifications • include effects of background noise via statistical observer model