Intra-operative visualisation of 3D temperature maps and 3D navigation during tissue cryoablation

GPU-based real-time approximation of the ablation zone for radiofrequency ablation

Percutaneous radiofrequency ablation (RFA) is becoming a standard minimally invasive clinical procedure for the treatment of liver tumors. However, planning the applicator placement such that the malignant tissue is completely destroyed, is a demanding task that requires considerable experience. In this work, we present a fast GPU-based real-time approximation of the ablation zone incorporating the cooling effect of liver vessels. Weighted distance fields of varying RF applicator types are derived from complex numerical simulations to allow a fast estimation of the ablation zone. Furthermore, the heat-sink effect of the cooling blood flow close to the applicator's electrode is estimated by means of a preprocessed thermal equilibrium representation of the liver parenchyma and blood vessels. Utilizing the graphics card, the weighted distance field incorporating the cooling blood flow is calculated using a modular shader framework, which facilitates the real-time visualization of the ablation zone in projected slice views and in volume rendering. The proposed methods are integrated in our software assistant prototype for planning RFA therapy. The software allows the physician to interactively place virtual RF applicator models. The real-time visualization of the corresponding approximated ablation zone facilitates interactive evaluation of the tumor coverage in order to optimize the applicator's placement such that all cancer cells are destroyed by the ablation.

In vivo assessment of cold adaptation in insect larvae by magnetic resonance imaging and magnetic resonance spectroscopy

BACKGROUND

Temperatures below the freezing point of water and the ensuing ice crystal formation pose serious challenges to cell structure and function. Consequently, species living in seasonally cold environments have evolved a multitude of strategies to reorganize their cellular architecture and metabolism, and the underlying mechanisms are crucial to our understanding of life. In multicellular organisms, and poikilotherm animals in particular, our knowledge about these processes is almost exclusively due to invasive studies, thereby limiting the range of conclusions that can be drawn about intact living systems.

METHODOLOGY

Given that non-destructive techniques like (1)H Magnetic Resonance (MR) imaging and spectroscopy have proven useful for in vivo investigations of a wide range of biological systems, we aimed at evaluating their potential to observe cold adaptations in living insect larvae. Specifically, we chose two cold-hardy insect species that frequently serve as cryobiological model systems--the freeze-avoiding gall moth Epiblema scudderiana and the freeze-tolerant gall fly Eurosta solidaginis.

RESULTS

In vivo MR images were acquired from autumn-collected larvae at temperatures between 0 degrees C and about -70 degrees C and at spatial resolutions down to 27 microm. These images revealed three-dimensional (3D) larval anatomy at a level of detail currently not in reach of other in vivo techniques. Furthermore, they allowed visualization of the 3D distribution of the remaining liquid water and of the endogenous cryoprotectants at subzero temperatures, and temperature-weighted images of these distributions could be derived. Finally, individual fat body cells and their nuclei could be identified in intact frozen Eurosta larvae.

CONCLUSIONS

These findings suggest that high resolution MR techniques provide for interesting methodological options in comparative cryobiological investigations, especially in vivo.

The Performance of 17-gauge Cryoprobes In Vitro

In cryosurgery it is crucial that the performance of cryoprobes is predictable and constant. In this study we tested the intra- and interneedle variation between 17-gauge cryoprobes in two homogeneous mediums. Also, a multiprobe setup was tested. Cryoprobe performance was defined as the time it takes one cryoprobe to lower the temperature from 0 to −20 °C as measured by four thermosensors each at 3 mm distance from the cryoprobe. In agar eight cryoprobes were tested during six freeze cycles, and in gel four cryoprobes during four freeze cycles; each freeze cycle in a different cup of agar or gel. Using more accurate ‘bare’ thermosensors three cryoprobes were tested in gel during two freeze cycles. A multiprobe configuration with four cryoprobes was tested during two freeze cycles in both agar and gel. Statistical analyses were done using ANOVA for repeated measures.

There was no significant intraneedle variation, whereas both in agar and gel there was a significant interneedle variation (p<0.05). Mean performance in gel was better than in agar (p<0.001). Also, there was a significant variation between the four thermosensors (p< 0.001). Using bare thermosensors mean performance was 2.7 times faster compared to measurements by regular thermosensors (p<0.001). In a multiprobe configuration, overall performance seems less variable and more reproducible compared to a single cryoprobe.

In conclusion, the performance of cryoprobes differs depending on the medium and measuring device used. Cryoprobes deliver reproducible freeze cycles, although there is variation between different cryoprobes. In a multiprobe configuration performance seems less variable.

Thermal maps around two adjacent cryoprobes creating overlapping ablations in porcine liver, lung, and kidney

PURPOSE

To determine cryoprobe spacing requirements in order to achieve overlapping ablation zones using the same ablation protocol in porcine liver, lung, and kidney.

MATERIALS AND METHODS

Six female pigs underwent cryoablation of the liver, lung, and kidney. Two 2.4-mm cryoprobes were spaced 20-mm apart with seven 16-gauge thermometers placed linearly and in axis with the cryoprobes at 5-mm increments from one another. The placement of the thermometers was such that three were placed between the two probes and two were placed laterally to each probe. Simultaneous use of the cryoprobes, using 12- and 8-minute double-freeze cycles, was performed with intratissue temperature monitoring during the procedure.

RESULTS

The center temperatures between the two cryoprobes in kidney, lung, and liver were -25.87( degrees )C +/- 1.91, -6.47(degrees )C +/- 3.94, and 0.48( degrees )C +/- 6.69, respectively. Dual 2.4-mm cryoprobes in our model achieved acute pathological complete coagulative necrosis zone at the center of the ablation zone between the cryoprobes only in the kidney tissue where a mean diameter of the acute complete coagulative necrosis zone was 39.6 mm +/- 0.76 mm.

CONCLUSIONS

The critical temperature of -20 degrees C was not reached at the midpoint between the probes with the 20-mm spacing arrangement in the lung and liver. These results emphasize the need for individualized organ ablation treatment protocols.

Magnetic resonance guidance of thermal ablation

The aim of this study is to describe all aspects of the process of using MR imaging to control thermal ablation procedures and the strengths and weaknesses of the individual thermal ablation modalities in relation to their use in the MR environment. Magnetic resonance thermal sequences, MR scanner configurations, and the different thermal ablation modalities are discussed in the context of how they are commonly used in MR scanners to provide optimal image guidance of therapy. The outcomes of completed research on some of the applications of thermal tissue ablation using MR guidance are described to indicate how these processes may impact patient treatment. At the end of this review, the reader should have an understanding of how MR guidance of thermal ablation may be carried out, in what areas it is currently most used, and were it may develop in the near future.