- 00000018WIA30908970GYZ
- id_400257611.3
- Mar 29, 2022 12:37:54 PM
Pixel Values in saved images considerations
When exploring saved READY View images with other viewing applications (such as the Viewer on the MR host system), the pixel values in the saved images may be different from the original values in the functional map.
Important: Under no circumstances should the absolute pixel value from saved functional maps be used for direct measurements.
Functional maps
For saved functional maps that have been saved as processed images, the explanation is the following:
- When images are saved, the pixel values are stored on the image disk as integers.
- To avoid loss of information when a functional map is saved, a scale factor is applied whenever the values returned by the algorithm used are too small to be meaningful when converted to integer values.
- As an example, assume the computation of a ratio by an algorithm that returns a value between 0.0 and 1.0. Storing the resulting functional map as an image with pixel values of either 0 or 1 would obviously be of no use. Therefore the ratio value is multiplied by 100, resulting in a pixel range of 0 to 100.
- When viewing such a saved functional map with another viewing application (such as the Viewer), you multiply the displayed pixel value by a scale factor of 0.01 to obtain the ratio.
| Value returned by function (algorithm or derived function) | Multiply pixel value in saved image by: | To obtain: |
|---|---|---|
| Ratio (division) | 0.01 | Ratio except (spectroscopy) |
| Correlation coefficient | 0.001 | Correlation coefficient in range –1.0 to 1.0 |
| ADC (apparent diffusion coefficient) | 0.000 001 (10 -6) | ADC in mm2/sec |
| Average DC | 0.0000000000014652 (1.4652x10-12) | Average DC in m2/sec |
| FA | 0.0002442 | Fractional Anisotropy |
| Vol Ratio Aniso | 0.0002442 | Volume Ratio Anisotropy |
| Exp attenuation | 0.0002442 | Exponential Attenuation |
| Aniso Index | 0.0002442 | Anisotropy Index |
| Trace | 1 | T2 weighted trace |
| Isotropic image | 1 | Isotropic image |
Note: Certain functions also use a scale factor for the display of the functional maps. In those cases, no further scaling is applied when saving the images, but the scale factor is still required to derive the actual physical quantities from the saved pixel values.