Tomo Low Contrast

This section has 8 acrylic spheres located in a central circular pattern highlighted in blue in the illustration below. The spheres are used to evaluate the scanner’s low contrast capability. The sphere diameters are 0.8, 1, 1.5, 2, 3, 4, 6, 8, and 10mm. Since the target contrast is variable depending on exposing and reconstruction techniques used for the DBT image, the actual target contrasts need to be determined. The actual contrast levels are measured by making region of interest measurements over the target and in the local background area. To determine actual contrast levels, average the measurements made from several scans. It is important to measure the background area adjacent to the measured target because non-uniformity effects may cause variation of pixel numbers from one image region to another. Position the region of interest ROI in the central portion of each target to avoid the target edges. The size of the region of interest should be small compared to the contrast sphere diameter, typically 4 x 4 pixels in diameter. Because low contrast measurements are “noisy” it is advisable to calculate an average of multiple measurements made from several scans. Carefully monitor the mAs and kVp settings because the photon flux will improve with increased x-ray exposure (dose) and the contrast of the sphere may vary. Use the size of the targets visualized at various noise levels to estimate information on contrast detail curves.

Tomophan Manual