Evaluation of Different Registration Algorithms to Reduce Motion Artifacts in CT-Thermography (CTT)

Abstract

Computed tomography (CT)-based Thermography (CTT) is currently being investigated as a non-invasive temperature monitoring method during ablation procedures. Since multiple CT scans with defined time intervals were acquired during this procedure, interscan motion artifacts can occur between the images, so registration is required. The aim of this study was to investigate different registration algorithms and their combinations for minimizing inter-scan motion artifacts during thermal ablation. Four CTT datasets were acquired using microwave ablation (MWA) of normal liver tissue performed in an in vivo porcine model. During each ablation, spectral CT volume scans were sequentially acquired. Based on initial reconstructions, rigid or elastic registration, or a combination of these, were carried out and rated by 15 radiologists. Friedman's test was used to compare rating results in reader assessments and revealed significant differences for the ablation probe movement rating only (p = 0.006; range, 5.3-6.6 points). Regarding this parameter, readers assessed rigid registration as inferior to other registrations. Quantitative analysis of ablation probe movement yielded a significantly decreased distance for combined registration as compared with unregistered data. In this study, registration was found to have the greatest influence on ablation probe movement, with connected registration being superior to only one registration process.

Keywords: CT; CT thermography; CTT; computed tomography; elastic; registration; rigid; thermoablation.

Figure 1

Experimental setup of computed tomography (CT)-based thermography (CCT): (a) Intraoperative photograph showing the animal under general anesthesia with the liver exposed and the microwave ablation (MWA) probe inserted; (b) CT image showing inserted MWA probe in the liver at the end of heating phase reaching maximum temperature (T_max) before cooling down. In the same axis with a clear representation of the ablation probe and zone, the images for the rating were set.

Figure 2

Boxplots of scores assigned for overall registration quality and ablation probe movement. Plus (+): mean value (a) Overall registration quality; n = 240; 3 ablations × 20 time points per ablation × 4 registrations; 15 readers. (b) Assessment of ablation probe movement; n = 300; 3 ablations × 20 time points per ablation × 5 (4 registrations + nonregistered dataset); 15 readers.

Figure 3

Boxplots of measured probe tip movement in mm. Mean value of region of interest (ROI) movement was measured for comparison, including all four CTT datasets acquired. Mean values (+) were calculated of all 19 ROI-distance measures for each ablation and registration algorithm, respectively. n = 400; 4 CTT datasets × 20 time points per ablation × 5 (4 registrations + nonregistered); p < 0.001 for differences between mean values and p < 0.001 for variances.