Thermal dosimetry of a focused ultrasound beam in vivo by magnetic resonance imaging

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-

Section: Discussionsupporting

confidence: 73%

Section: Discussionmentioning

confidence: 99%

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

Peng

Huang

Tseng

et al. 2009

“…While reports exist that the planned treatment volume agrees with histologically-confirmed necrotic tissue (16), there is greater evidence that this threshold is a conservative estimate (17). Studies by Chung et al (8) showed that the ablation volume was 25% larger than that planned in rabbit skeletal muscle, while a two-fold increase was shown in application to uterine fibroid treatment (B.J. Quade, personal communication, 2002).…”

Section: High-intensity Focused Ultrasound (Fus)mentioning

confidence: 93%

“…Planning is usually based on MR-derived thermal indices, in which a coagulation threshold is adopted-in most studies either a dose of 240 minutes at 43°C or a minimum peak temperature of 55°C (8,12). While reports exist that the planned treatment volume agrees with histologically-confirmed necrotic tissue (16), there is greater evidence that this threshold is a conservative estimate (17).…”

Section: High-intensity Focused Ultrasound (Fus)mentioning

confidence: 99%

Prediction of subtle thermal histopathological change using a novel analysis of Gd‐DTPA kinetics

Cheng

Purcell

Bilbao

et al. 2003

Purpose: To investigate Gd-DTPA kinetics as predictors of histopathological changes following focused ultrasound (FUS) thermal ablation for improved planning and assessment. Materials and Methods:Twenty-nine FUS lesions were created in the thigh muscle of eight rabbits under MRguidance at 1.5 Tesla. Three rabbits were killed at four hours; and 11 lesions were analyzed with histopathology. Temperature-sensitive MRI using proton-resonant frequency-shift was used for time-dependent temperature measurements. Analysis of the uptake kinetics of Gd-DTPA was performed after Gd-DTPA injection, within 20 minutes after heating and again at two hours after heating. The resulting kinetic maps, permeability (K trans ) and leakage space (v e ), were correlated to peak temperatures, T 2 -weighted MR, and histopathology.Results: Images of K trans and v e reveal regions of histopathological change not visible on conventional posttherapy MR. At early times after heating, v e predicts the area of injury more accurately than T 2 (7 Ϯ 2% vs. 25 Ϯ 6% underestimation). A circular region of extensive structural/ vascular disruption is indicated only on K trans maps. The sharp decrease in K trans at the boundary of this region occurs at 47.5 Ϯ 0.5°C, and may be a better estimate of cell death than the conventional method of temperature threshold (55°C for coagulation) used in therapy planning. Conclusion:Our results suggest Gd-DTPA kinetics can predict different histopathological changes following FUS ablation and may be valuable for early prediction.

“…At first, only 2D temperature maps were obtained by MRI to allow for accurate targeting of the beam (13). Later these thermal images were used to generate maps of the thermal dose (14) that could then be used to tailor the energy deposition using closed-loop feedback control (15). This preclinical work led to the development of the first commercial system by InSightec (Haifa, Israel), the Exablate 2000, the first implementation of a fully integrated MRgFUS system that combined a phased array transducer, a computer-controlled robotic positioner, and a workstation that integrated control based on MR thermometry.…”

mentioning

confidence: 99%

Current status and future potential of MRI‐guided focused ultrasound surgery

Jólesz

McDannold

2008

Self Cite

112

The combination of the imaging abilities of magnetic resonance imaging (MRI) with the ability to delivery energy to targets deep in the body noninvasively with focused ultrasound presents a disruptive technology with the potential to significantly affect healthcare. MRI offers precise targeting, visualization, and quantification of temperature changes and the ability to immediately evaluate the treatment. By exploiting different mechanisms, focused ultrasound offers a range of therapies, ranging from thermal ablation to targeted drug delivery. This article reviews recent preclinical and tests clinical of this technology. WHILE ITS POTENTIAL has been recognized for decades (1-4), focused ultrasound (FUS) therapy has been in the incubation phase for a long time. Although it has been tested in numerous animal experiments, its widespread clinical realization has lagged, primarily due to the lack of intraprocedural accurate temperature monitoring. When the idea of MRI-guidance was introduced in connection with FUS and MRI thermometry during sonication was demonstrated (5,6), it was anticipated that MR-guided FUS surgery (MRgFUS) would be widely accepted clinically within a few years. Despite these early expectations, there has been relatively slow progress in the clinical implementation of MRgFUS.Early development concentrated primarily on breast tumors (benign fibroadenoma (7,8) and malignant breast cancer (9,10)). These early implementations of the MRgFUS technology motivated the improvement of transducer technology from single-element transducers to MRI-compatible multielement phase arrays (11). There was also significant progress in MRI thermometry and its use to monitor and control sonications. Importantly, too, methods were developed to robustly quantify temperature changes (12). At first, only 2D temperature maps were obtained by MRI to allow for accurate targeting of the beam (13). Later these thermal images were used to generate maps of the thermal dose (14) that could then be used to tailor the energy deposition using closed-loop feedback control (15). This preclinical work led to the development of the first commercial system by InSightec (Haifa, Israel), the Exablate 2000, the first implementation of a fully integrated MRgFUS system that combined a phased array transducer, a computer-controlled robotic positioner, and a workstation that integrated control based on MR thermometry.The early limited clinical trials of breast tumor treatment (8 -10,16,17) were followed by the first major clinical application: the noninvasive therapy of benign uterine fibroids (18,19). The successful Phase I trial was followed by a multicenter . These clinical trials led to the approval of this technology for the treatment of uterine fibroids in several countries, including the United States. Examples of MR thermometry and postablation imaging of a uterine fibroid ablation are shown in Figure 1.The success of uterine fibroid ablation has invigorated the entire field. Technology has rapidly improved and clinical applications ke...