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Evaluate Noise Sensitivity In Vivo

When measuring in people, we do not know the ground truth $T_2$ values. In high SNR images, it is known that NLLS fitting gives accurate results, but in low SNR regions the $T_2$ values are overestimated. In this study we artificially added noise to the in vivoi> images to evaluate how the different methods perform on low-SNR data. The interactive figure below is adapted from Figure 6 in the paper, with a second subject (case B) taken from Figure 5.

This interactive figure is intended to improve understanding of these methods fail differently, which impacts how the maps are interpreted.

Drag the slider to vary the level of noise added to the images and observe how the two methods respond differently to increasing noise. You can also zoom, pan, and vary the noise level with the mouse wheel.

0 (No added noise)
Case A - Benign
Case B - Malignant
NLLS Fit
Case A NLLS
Case B NLLS

This is the standard approach for parameter estimation.

Without added noise, the results in the prostate look good, but you can see some noise amplification in low SNR regions like the muscles and the rectum. As the noise level increases the maps amplify the noise further and are eventually uninterpretable.

CNN
Case A CNN
Case B CNN

Without added noise, the CNN gives similar results as NLLS fitting in the prostate, which has high SNR. At lower levels of added noise, the CNN continues to provide good maps in high-SNR regions at the cost of some blurring. At higher noise levels the CNN gives maps with unusable levels of blurring, warping, and hallucinations.

Consistent with the quantitative analyses in the paper, the CNN and NLLS methods give similar results at high SNR, and the CNN outperforms the NLLS fit in lower SNR levels. At very low SNR levels ($sigma$ > ~0.06) both methods fail.

Image Controls: Right-click + drag ↕ Zoom in/out Middle-click + drag Pan around Double-click Reset zoom/pan Mouse wheel Change noise level