Improved Thermal Sensitivity Using Virtual Monochromatic Imaging Derived from Photon Counting Detector CT Data Sets: Ex Vivo Results of CT-Guided Cryoablation in Porcine Liver

Abstract

Purpose: To investigate differences in thermal sensitivity of virtual monoenergetic imaging (VMI) series generated from photon-counting detector (PCD) CT data sets, regarding their use to improve discrimination of the ablation zone during percutaneous cryoablation.

Materials and methods: CT-guided cryoablation was performed using an ex vivo model of porcine liver on a PCD-CT system. The ablation zone was imaged continuously for 8 min by acquiring a CT scan every 5 s. Tissue temperature was measured using fiberoptic temperature probes placed parallel to the cryoprobe. CT-values and noise were measured at the tip of the temperature probes on each scan and on VMI series from 40 to 130 keV. Correlation of CT-values and temperature was assessed using linear regression analyses.

Results: For the whole temperature range of [- 40, + 20] °C, we observed a linear correlation between CT-values and temperature in reference 70 keV images (R² = 0.60, p < 0.001) with a thermal sensitivity of 1.4^HU/_°C. For the most dynamic range of [- 15, + 20] °C, the sensitivity increased to 2.4^HU/_°C (R² = 0.50, p < 0.001). Using VMI reconstructions, the thermal sensitivity increased from 1.4 ^HU/_°C at 70 keV to 1.5, 1.7 and 2.0^HU/_°C at 60, 50 and 40 keV, respectively (range [- 40, + 20] °C). For [- 15, + 20]°C, the thermal sensitivity increased from 2.4^HU/_°C at 70 keV to 2.5, 2.6 and 2.7^HU/_°C at 60, 50 and 40 keV, respectively. Both CT-values and noise also increased with decreasing VMI keV-levels.

Conclusion: During CT-guided cryoablation of porcine liver, low-keV VMI reconstructions derived from PCD-CT data sets exhibit improved thermal sensitivity being highest between + 20 and - 15 °C.

Keywords: Computed tomography; Cryoablation; Liver; Thermal analysis.