Results and discussion

7.4.1 Results

To give a general impression of the reconstructed images, Figure 7.6b shows an example reconstruction of the phantom (using measurement pose set 3SD, t_exp = 10s), next to a photograph of the phantom.

Experiment A

Here we evaluate the influence of the chosen acquisition poses on the results. Figure 7.7 shows the reconstruction results of Experiment A as projection images along the z-axis as well as the y-axis for all six pose sets using texp = 10s, while Table 7.2 lists the quality measures for each reconstruction. The results of pose sets 1S and 2S show elongations and artifacts along the scanning directions. Adding more sides (3S) or diagonal orientations (1SD, 2SD, 3SD) allows to increasingly eliminate the elongations and artifacts and to confine the reconstructed activity in the expected areas.

(a)Pose set 1S (one side, orthogonal scans)

(b)Pose set 2S (two sides, orthogo-nal scans)

(c)Pose set 3S (three sides, orthog-onal scans)

(d)Pose set 1SD (one side, orthog-onal and diagorthog-onal scans)

(e)Pose set 2SD (two sides, orthog-onal and diagorthog-onal scans)

(f )Pose set 3SD (three sides, orthog-onal and diagorthog-onal scans)

Figure 7.7: Results ofExperiment A shown as projections along the z-axis (top row) and y-axis (bottom row). The projections are individually normalized for better visualization. The ground truth is depicted as an overlay of white circles representing the spheres of the phantom[77].

Experiment B

Here we evaluate the influence of the exposure time per pose for the pose sets 1SD, 2SD and 3SD using exposure times ranging from 1sto 10s. The quality measuresaverage localization error and ratio of uptake values do not show a significant difference (up to 10% for pose set 1SD, less than 5% for pose sets 2SD and 3SD). A more visible impact of the exposure time is shown in theartifact quality measure where longer exposure times reduce the number of the artifacts and their uptake, see Figure 7.8.

Experiment C

Using a fixed total acquisition duration of approximately 45s, we evaluate the pose sets 1SD, 2SD and 3SD with corresponding exposure times t_exp. The results in Table 7.3 show,

Step and Shoot Robotic freehand SPECT using Gamma Camera 75

Activity uptake Artifacts Localization Sørensen- Data set Pose set # 1 # 2 # 3 # Upt. error Dice coeff. # poses

Table 7.2: Experiment A: quality measures for all six pose sets (using texp = 10s). From left to right: ratio of uptake values for the three spheres, number and uptake of artifacts, average localization error, Sørensen-Dice similarity coefficient, data set information (number of poses). The last row displays the ground truth (GT) uptake values for comparison[77].

Figure 7.8: Experiment B: influence of the exposure time on the artifacts. Gray: total number of artifacts in volume (using left axis); Orange: percentage of activity in the artifacts compared to total reconstructed activity (using right axis)[77].

Activity uptake Artifacts Localization Data set Total Pose set # 1 # 2 # 3 # Upt. error # poses texp duration

1SD 42% 19% 36% 11 3% 1.6 mm 9 5 s 45 s

2SD 40% 17% 42% 6 1% 1.0 mm 15 3 s 45 s

3SD 39% 17% 44% 2 0% 0.7 mm 21 2 s 42 s

GT 39% 16% 45%

Table 7.3: Experiment C: quality measures for a fixed total acquisition duration of approximately 45susing selected pose sets. From left to right: ratio of uptake values for the three spheres, number and uptake of the artifacts, average localization error, and data set information (number of poses, exposure timet_exp, total acquisition duration). The last row displays the ground truth (GT) uptake values for comparison[77].

that more poses achieve better results compared to longer exposure times.

7.4.2 Discussion

As shown in the previous section, it is possible to segment and locate hot spots with high accuracy, given that the measurements cover the phantom sufficiently. The exposure time per pose also plays a role, but it is less crucial when considering the overall reconstruction quality.

Experiment Ashows that the number of sides covered by the projections have great impact on the reconstruction quality. Having diagonal views on the sides helps improving the quality of the reconstruction, making Figure 7.7f the best reconstruction. The improvements made by adding diagonal views is also clearly less crucial than the improvement gained by adding poses from another, orthogonal side. Having poses from three orthogonal sides plus diagonal views provides the best image quality. Unfortunately, this type of configuration is not always possible within the operating room, due to the patient anatomy and limited access to the region of interest. Consequently, studying constrained scenarios is key to understanding the feasibility of the technology within actual clinical settings. Judging from the results, it is clear that the 2SD case can also provide acceptable reconstructions.

Experiment B shows the existence of a clear dependency between the exposure time on each pose and the quality of the reconstructed images. Theaverage localization errors andratios of uptake values do not improve much, but the artifact quality measure improves by at least 50% when increasing the exposure time from 1s to 10s, independent of the number of poses covered. But not only the number of artifacts is important, also the accumulated activity plays a role. For the acquisition sets 1SD and 2SD, this measure does not improve much with longer exposure time per pose, whereas for set 3SD it improved significantly. But since in set 3SD the number of artifacts and their total activity is already quite low, this improvement has to be put into perspective.

Experiment C clearly shows that the main factor for improved image quality is the number of distinct poses. All our quality measures improve within the fixed amount of approximately 45s scanning time: The ratio of uptake values approaches the ground truth values when adding more poses, the number of artifacts and their uptake reduces significantly and the localization error also improves by more than 50%.

Experiments B and C show that increased exposure times do not improve the quality of the reconstructions very much. In particular, it is much more effective to distribute the available time to cover more poses, and in doing so increase the angular coverage of the region of interest.

Increasing the number of poses has a direct influence in the reduction of the artifacts, as long as the three orthogonal views are still covered. Furthermore, the activity percentages in the segmented hotspots in all our reconstructions correspond nicely with the actual radioactivity in the spheres, with only slight deviations.

Compared to [79], the hardware setup was upgraded and software changes were introduced.

The localization errors of the acquisition scan similar to pose set 3S are much lower in this work, and the acquisition times were reduced tenfold in the shortest scans.

Step and Shoot Robotic freehand SPECT using Gamma Camera 77 We believe those improvements are mainly caused by a much better, high-resolution and exhaustive characterization of the gamma camera’s measurement model. The voxel size was also reduced from about 16mm³ to 1mm³, and as a result we can now measure the error with much higher accuracy than before.

7.5 Conclusion

The results show that it is possible to obtain accurate reconstructions with less than one millimeter average localization error in less than one minute of acquisition. The key param-eter for image quality was shown to be the number of significantly different measurement poses.

The tested configurations were comparable to an intraoperative sentinel lymph node biopsy, where the results obtained show that this setup can be used as guidance for the surgeon to accurately locate and resect the radioactive lymph nodes.

Compared to the results from the previous chapter using the gamma probe, the localization error of the point sources is about one order of magnitude lower. This opened us the door for more challenging reconstruction scenarios, presented in the next chapter.

Chapter 8

Continuously Moving Robotic