No abstract

The aim of this study was to test a prototype MRI-compatible focused ultrasound phased array system for trans-skull brain tissue ablation. Rabbit thigh muscle and brain were sonicated with a prototype, hemispherical 500-element ultrasound phased array operating at frequencies of 700 -800 kHz. An ex vivo human skull sample was placed between the array and the animal tissue. The temperature elevation during 20 -30-sec sonications was monitored using MRI thermometry. The induced focal lesions were observed in T 2 and contrast-enhanced T 1 -weighted fast spin echo images. Whole brain histology evaluation was performed after the sonications. The results showed that sharp temperature elevations can be produced both in the thigh muscle and in the brain. High-power sonications (600 -1080 W) produced peak temperatures up to 55°C and focal lesions that were consistent with thermal tissue damage. The lesion size was found to increase with increasing peak temperature. The device was then modified to operate in the orientation that will be used in the clinic and successfully tested in phantom experiments. As a conclusion, this study demonstrates that it is possible to create ultrasound-induced lesions in vivo through a human skull under MRI guidance with this large-scale phased array. Magn Reson Med 52: 100 -107, 2004.

MR imaging thermometry and dosimetry provide an index to predict the threshold for tissue damage in vivo. This index offers improved online control over minimally invasive thermal treatments and should allow for more accurate target volume coagulation.

In this study, we propose a focused ultrasound surgery protocol that induces and then uses gas bubbles at the focus to enhance the ultrasound absorption and ultimately create larger lesions in vivo. MRI and ultrasound visualization and monitoring methods for this heating method are also investigated. Larger lesions created with a carefully monitored single ultrasound exposure could greatly improve the speed of tumour coagulation with focused ultrasound. All experiments were performed under MRI (clinical, 1.5 T) guidance with one of two eight-sector, spherically curved piezoelectric transducers. The transducer, either a 1.1 or 1.7 MHz array, was driven by a multi-channel RF driving system. The transducer was mounted in an MRI-compatible manual positioning system and the rabbit was situated on top of the system. An ultrasound detector ring was fixed with the therapy transducer to monitor gas bubble activity during treatment. Focused ultrasound surgery exposures were delivered to the thighs of seven New Zealand while rabbits. The experimental, gas-bubble-enhanced heating exposures consisted of a high amplitude 300 acoustic watt, half second pulse followed by a 7 W, 14 W or 21 W continuous wave exposure for 19.5 s. The respective control sonications were 20 s exposures of 14 W, 21 W and 28 W. During the exposures, MR thermometry was obtained from the temperature dependency of the proton resonance frequency shift. MRT2-enhanced imaging was used to evaluate the resulting lesions. Specific metrics were used to evaluate the differences between the gas-bubble-enhanced exposures and their respective control sonications: temperatures with respect to time and space, lesion size and shape, and their agreement with thermal dose predictions. The bubble-enhanced exposures showed a faster temperature rise within the first 4 s and higher overall temperatures than the sonications without bubble formation. The spatial temperature maps and the thermal dose maps derived from the MRI thermometry closely correlated with the resulting lesion as examined by T2-weighted imaging. The lesions created with the gas-bubble-enhanced heating exposures were 2-3 times larger by volume, consistently more spherical in shape and closer to the transducer than the control exposures. The study demonstrates that gas bubbles can reliably be used to create significantly larger lesions in vivo. MRI thermometry techniques were successfully used to monitor the thermal effects mediated by the bubble-enhanced exposures.

A single-voxel Carr-Purcell-Meibloom-Gill sequence was developed to measure localized T 2 relaxation times of 13 C-labeled metabolites in vivo for the first time. Following hyperpolarized [1-13 C]pyruvate injections, pyruvate and its metabolic products, alanine and lactate, were observed in the liver of five rats with hepatocellular carcinoma and five healthy control rats. The T 2 relaxation times of alanine and lactate were both significantly longer in HCC tumors than in normal livers (p < 0.002). The HCC tumors also showed significantly higher alanine signal relative to the total 13 C signal than normal livers (p < 0.006). The intra-and inter-subject variations of the alanine T 2 relaxation time were 11% and 13%, respectively. The intra-and inter-subject variations of the lactate T 2 relaxation time were 6% and 7%, respectively. The intra-subject variability of alanine to total carbon ratio was 16% and the inter-subject variability 28%. The intra-subject variability of lactate to total carbon ratio was 14% and the inter-subject variability 20%. The study results show that the signal level and relaxivity of [1-13 C]alanine may be promising biomarkers for HCC tumors. Its diagnostic values in HCC staging and treatment monitoring are yet to be explored.

A tissue-mimicking material (TMM) for the acoustic and thermal characterization of high-intensity focused ultrasound (HIFU) devices has been developed. The material is a high-temperature hydrogel matrix (gellan gum) combined with different sizes of aluminum oxide particles and other chemicals. The ultrasonic properties (attenuation coefficient, speed of sound, acoustical impedance, and the thermal conductivity and diffusivity) were characterized as a function of temperature from 20 to 70°C. The backscatter coefficient and nonlinearity parameter B/A were measured at room temperature. Importantly, the attenuation coefficient has essentially linear frequency dependence, as is the case for most mammalian tissues at 37°C. The mean value is 0.64f(0.95) dB·cm(-1) at 20°C, based on measurements from 2 to 8 MHz. Most of the other relevant physical parameters are also close to the reported values, although backscatter signals are low compared with typical human soft tissues. Repeatable and consistent temperature elevations of 40°C were produced under 20-s HIFU exposures in the TMM. This TMM is appropriate for developing standardized dosimetry techniques, validating numerical models, and determining the safety and efficacy of HIFU devices.

No abstract

No abstract