Slice Sensitivity Profile (SSP)

The slice sensitivity profiles are used to accurately calculate the slice widths, slice increments, and remaining signal using the upper (red) and lower (blue) bead ramp sets.

Figure 6.1: Diagram showing the location of the upper (red) and lower (blue) bead ramp sets.

As described in the Tomophan manual on page 15, the slice sensitivity profile (SSP) is calculated for the slices in which signals from the full bead ramp can be seen. To evaluate slice width, measure the Full Width at Half Maximum (FWHM) of the fitted SSP. Start by determining the half-maximum of the fitted SSP. Subtract the background value (minimum value of fitted SSP) from the maximum value of the fitted SSP and add half of that difference to the background value. In some cases, the background value may already be accounted for. Each peak in the SSP represents a bead in the ramp, so count the number of beads with a maximum intensity over the half-maximum value. Multiply the number of beads by 0.25mm, which is the z-axis spacing, to calculate slice width. Note that as slice thickness increases, the maximum value for the beads decreases due to volume averaging. Presuming the slice thicknesses are accurate, the peak signal over the background in a 1mm slice should be double that of the peak signal over the background in a 2mm slice. This rule, due to the effects of tomographic reconstruction, only loosely applies. 

The center-to-center distance between the peaks of the fitted SSPs for the slices is used to accurately measure the slice increment.

The following plots and measurements are displayed.

Table 6.1 (left): A table of the slice widths measured from the upper and lower bead ramps is displayed along with the mean and standard deviation for these measurements. 

Figure 6.2 (right): Slice sensitivity profiles with their corresponding Full Width at Half Maximums (used to calculate slice width) for upper (lateral) and lower (medial) ramp sets.

Table 6.2 (left): A table of the slice increments measured from the upper and lower bead ramps is displayed along with the mean and standard deviation for these measurements.

Figure 6.3 (right): Slice increment is measured as the center-to-center distance between adjacent peaks. Corresponding SSP plots with slice increments for upper and lower bead ramp sets are displayed.

Figure 6.4: The slice sensitivity plots of the central slice (slice where the four aluminum beads for x-y geometry have the highest pixel intensities) for the lateral and medial sets of beads.

Table 6.3: The remaining signal within the slice increment. This table uses the average slice increment to measure the signal drop off at the edge of the slice. So the percentage is the percentage of maximum fitted SSP signal at the points [slice center- 0.5 x slice increment, slice center + 0.5 x slice increment.