*3.2. Contrast Recovery*

#### 3.2.1. NEMA IEC Image Quality Phantom

For a T/B ratio of 10:1, a scan time of 10 min per bed position and a high level of noise tolerance (low β), a relatively constant RCavg between 0.8 and 0.9 is found for the biggest four spheres. The RCavg decreases significantly for spheres with a diameter smaller than 17 mm (*p* < 0.001). Increasing the β to 400 and higher and thus effectively smoothing the image, the decrease in RCavg is already seen in the 17 mm-diameter spheres (*p* < 0.05). Shortening the acquisition time to the clinically used two minutes per bed position resulted in apparently higher average recovery coefficients (Figure 2a). The RCpeak of each of the three smallest spheres is lower than that of the three biggest spheres (*p* < 0.001) for both scan times. The higher apparent RCs in the shorter scan do not necessarily correlate with improved lesion detectability due to the increased noise levels.

For the acquisition with a T/B ratio of 20:1 and a scan time of 10 min per bed position, a similar trend was noted. For sphere diameters 17 mm and larger, the average recovery coefficient is similar for all Q.Clear reconstructions. The RCpeak of each of the three smallest spheres is lower than that of the three biggest spheres (*p* < 0.001). For the 10 and 13 mmdiameter spheres a spread developed, with a decrease in average recovery coefficient for increasing β. Reconstructions with data acquired for two minutes per bed position (Figure 2b) showed a similar pattern, but with a slightly higher RCavg overall and a more pronounced spread in RCavg for the 10 and 13 mm-diameter spheres.

The four largest spheres with a T/B ratio of 10:1 exhibit a significantly higher RCavg and RCmax than those with a T/B ratio of 20:1 (*p* < 0.0001), for all reconstructions considered. For the three biggest spheres, RCpeak is similar for both T/B ratios.

**Figure 2.** Recovery coefficients from the Image Quality phantom for multiple T/B ratios. RCavg for T/B ratio 10:1 (**a**) and 20:1 (**b**), with acquisition time per bed position of 2 min. The symbols in the lower part of both graphs denote the significance of the differences between the 13 mm sphere and the four biggest spheres (upper row) and the 10 mm sphere and the four biggest spheres (lower row). An obelus (÷) corresponds to *p* < 0.001, a colon to *p* < 0.01 and a single dot to *p* < 0.05.

#### 3.2.2. Micro Hollow Sphere Phantom

The diameter of the largest sphere in the Micro Hollow Sphere phantom matches with that of the smallest sphere in the NEMA Image Quality phantom. Comparing the two, in general a higher average recovery coefficient is found for the sphere in the Micro Hollow Sphere phantom. These differences in recovery coefficient result from differences in the phantom geometry. An approximate correction factor was introduced to scale the RCs of the Micro Hollow Sphere phantom to those of the NEMA Image Quality phantom. The scaling factor was defined as the ratio between the RC of the matching spheres in the NEMA Image Quality phantom and the Micro Hollow Sphere phantom.

Recovery coefficients are provided for all spheres that could be semi-automatically segmented. For the smaller spheres, the apparent activity concentration in a sphere decreased to less than twice the background value due to the PVE. For these spheres, the region growing algorithm with a threshold 3D isocontour at 50% of the maximum voxel value failed to properly calculate an average recovery coefficient. An increase in β caused a decrease in apparent activity concentration in a sphere and therefore an increase in the number of spheres that could not be properly segmented. A lower T/B ratio also resulted in more difficulties in the segmentation process.

For both phantom scans performed, a large increase in RCavg for one of the spheres at lower β values was observed as can be seen for the scan with a T/B ratio of 10:1 in Figure 3.

Taking RCpeak as a quantitative measure, the obtained recovery coefficients appear to be more robust but lower than the RCavg.

**Figure 3.** Average and peak recovery coefficients from the Micro Hollow Sphere phantom. For an acquisition time of two minutes per bed position, the apparent RCavg (**a**) of the 8 mm sphere measured with T/B ratio 10:1 exceeds that of the bigger spheres for low β, as the center of this sphere happened to coincide with the center of a voxel. Taking RCpeak as a measure for the recovery coefficient (**b**), the recovery coefficients are lower, but more robust.
