Issue with SSIM Values Exceeding 1 in MONAI Metrics

[MiguelHGDC]

Context

I am encountering an issue with the SSIM and MS-SSIM metrics when using MONAI's implementation. Specifically, the SSIM values sometimes exceed 1, which should not happen as the theoretical range for SSIM is [0, 1].

The input data consists of normalized CT images with values in the range [0, 1], and I’ve ensured that both the original images and reconstructions are clamped to this range. Here are some key observations and debugging details:

Data Ranges:

• Image Max: 1.0, Min: 0.0
• Reconstruction Max: 1.0, Min: 0.0
• Both images are correctly normalized to the [0, 1] range.

SSIM Outputs:

• SSIM exceeds 1, e.g., SSIM: 1.0341
• MS-SSIM also exceeds 1, e.g., MS-SSIM: 1.0176

Debugging Outputs:

First Pass:

• Converted y_pred max: 1.0, min: 0.0
• Converted y max: 1.0, min: 0.0
• c1: 0.0001, c2: 0.0009
• mu_x max: 0.9824, min: 0.0000015
• mu_y max: 0.9844, min: 0.0000053
• sigma_x max: 0.0471, min: -0.000381
• sigma_y max: 0.0461, min: -0.000571
• sigma_xy max: 0.0464, min: -0.000488
• SSIM max: 20.1017, min: -0.3385

Second Pass:

• Converted y_pred max: 1.0, min: 0.0
• Converted y max: 1.0, min: 0.0
• c1: 0.0001, c2: 0.0009
• mu_x max: 1.0, min: 0.0
• mu_y max: 1.0, min: 0.0
• sigma_x max: 0.0319, min: -0.000858
• sigma_y max: 0.0308, min: -0.001054
• sigma_xy max: 0.0298, min: -0.000977
• SSIM max: 158.7476, min: -50.2216

Third Pass:

• Converted y_pred max: 0.9999, min: 0.0
• Converted y max: 1.0, min: 0.0
• c1: 0.0001, c2: 0.0009
• mu_x max: 0.9946, min: 0.0
• mu_y max: 0.9980, min: 0.0
• sigma_x max: 0.0382, min: -0.000670
• sigma_y max: 0.0377, min: -0.000706
• sigma_xy max: 0.0381, min: -0.000488
• SSIM max: 5.3155, min: -5.8543

Fourth Pass:

• Converted y_pred max: 0.9909, min: 0.0
• Converted y max: 0.9934, min: 0.0
• c1: 0.0001, c2: 0.0009
• mu_x max: 0.9414, min: 0.0000036
• mu_y max: 0.9429, min: 0.0000264
• sigma_x max: 0.0650, min: -0.000462
• sigma_y max: 0.0644, min: -0.000459
• sigma_xy max: 0.0647, min: -0.000488
• SSIM max: 4.2530, min: -0.1976

Fifth Pass:

• Converted y_pred max: 0.9245, min: 0.0000036
• Converted y max: 0.9247, min: 0.0000444
• c1: 0.0001, c2: 0.0009
• mu_x max: 0.7886, min: 0.000368
• mu_y max: 0.7896, min: 0.000695
• sigma_x max: 0.0898, min: -0.000309
• sigma_y max: 0.0903, min: -0.000309
• sigma_xy max: 0.0900, min: -0.000244
• SSIM max: 2.3628, min: 0.5758

Sixth Pass:

• Converted y_pred max: 0.7804, min: 0.000368
• Converted y max: 0.7836, min: 0.000788
• c1: 0.0001, c2: 0.0009
• mu_x max: 0.6177, min: 0.1516
• mu_y max: 0.6187, min: 0.1527
• sigma_x max: 0.0813, min: -0.000276
• sigma_y max: 0.0813, min: 0.000015
• sigma_xy max: 0.0813, min: 0.0
• SSIM max: 1.3410, min: 0.9381

Final Results:

• Image 2 - SSIM: 1.0341
• Image 2 - MS-SSIM: 1.0176