Simultaneous temperature and magnetization transfer (MT) monitoring during high‐intensity focused ultrasound (HIFU) treatment: Preliminary investigation on ex vivo porcine muscle

Abstract

Purpose:To measure temperature change and magnetization transfer ratio (MTR) simultaneously during high-intensity focused ultrasound (HIFU) treatment. Materials and Methods:This study proposed an interleaved dual gradient-echo technique to monitor the heat and tissue damage brought to the heated tissue. The technique was applied to tissue samples to test its efficacy. Results:Ex vivo experiments on the porcine muscle demonstrated that both temperature changes and MTR exhibited high consistency in localizing the heated regions. As the heat dissipated after the treatment, the temperature of the heated regions decreased rapidly but MTR continued to be elevated. Moreover, thermal dose (TD) maps derived from the temperature curves demonstrated a sharp margin in the heated regions, but MTR maps may show a spatial gradient of tissue damage, suggesting complimentary information provided by these two measures.Conclusion:In a protocol of spot-by-spot heating over a large volume of tissue, MTR provides additional values to mark the locations of previously heated regions. By continuously recording the locations of heated spots, MTR maps could help plan the next target spots appropriately, potentially improving the efficiency of HIFU treatment and reducing undesirable damage to the normal tissue. RECENT DEVELOPMENT OF HIGH-INTENSITY FO-CUSED ULTRASOUND (HIFU) technology has offered a potentially new approach to the local ablation of malignant or benign tumors (1,2). By focusing an ultrasound beam at the target tissue, HIFU maximizes the heating damage to the targeted tissue while preserving the surrounding normal tissue. To improve treatment efficiency and ensure patient safety, it is crucial to perform continuous and real-time monitoring of local heat deposition during HIFU treatment. Owing to satisfactory spatial and temporal resolution, high tissue contrast, and no ionizing radiation, MRI is advantageous over other imaging modalities in guiding, monitoring, and controlling the HIFU treatment, and has therefore been used for preheating localization of the targets or postheating evaluation of the treatment effect (3). Several MR parameters, such as the equilibrium magnetization (M 0 ), T1, T2, and the diffusion coefficient of water molecules, are known to exhibit temperature dependence (4 -10). The temperature sensitivity of these parameters, however, is tissue-dependent, nonlinear, or varies with tissue conditions such as thermal coagulation or denaturation (5,11,12). Currently, the MR parameter that is sensitive to temperature and stable under various tissue conditions is the water proton resonant frequency (PRF) shift (5). The method of PRF shift deter-

Multiparametric MRI Analysis for the Identification of High Intensity Focused Ultrasound-Treated Tumor Tissue

“…Both a decreased [34] and increased [23] MTR have been observed in ex vivo thermally treated samples. An MTR decrease could be explained by denaturation of structural proteins [34], whereas an increase might be caused by increased access of water molecules to macromolecules due to disruption of barriers.…”

“…The MTR is a measure for the level of magnetization exchange between water protons and semi-solid macromolecular protons in tissue [22]. An increase in tissue MTR has been observed after thermal treatment of ex vivo porcine muscle tissue [23].…”

Proton resonance frequency shift‐weighted imaging for monitoring MR‐guided high‐intensity focused ultrasound transmissions

“…A real-time HIFU monitoring system capable of reconstructing both SWI and PRFSWI simultaneously could extend the future applicability of this method. In addition, the sequence used in this study is able to perform MTR (29), PRFSWI, and PRF-shift measurements simultaneously. By monitoring both temperature and the progress of tissue coagulation, this sequence has value in the application of HIFU.…”

“…A single-element focused piezoelectric transducer (central frequency 1.85 MHz, 10-cm diameter, 12.5-cm curvature, Imasonic, Besançon, France) was used as a source of sonication built into the heating apparatus. The imaging sequence was the interleaved dual gradient echo technique as the imaging sequence for measuring the changes in temperature and magnetization transfer ratio (MTR) simultaneously (29). Because MTR is beyond the scope of this study, PRFSWI postprocessing was used only on images without MT preparation.…”

“…As a result, the aforementioned methods, i.e., PD, T1, T2, diffusion and MTR, are not favorable for monitoring temperature in HIFU applications. However, by optimizing MR pulse sequences, several alternative techniques are capable of detecting phase transitions resulting from the coagulation of tissue (27)(28)(29) and relatively low power deposition (30,31). Although these techniques cannot provide quantitative measurements of temperature, they possess the unique advantage of monitoring phase transitions within the treatment process, and, therefore, warrant further investigation.…”