Real-time Magnetic Resonance Thermometry Research Articles

Dual-Applicator MR Imaging–Guided Microwave Ablation with Real-Time MR Thermometry: Phantom and Porcine Tissue Model Experiments

PurposeTo investigate the effect of simultaneous use of dual applicators on the image quality of real-time magnetic resonance (MR) thermometry and to characterize the dual-applicator treatment zone pattern during MR imaging–guided microwave ablation (MWA). Materials and MethodsMWA experiments were performed on a 1.5-T MR scanner with 2 commercial microwave systems (902–928 MHz). Phantom experiments were first performed to evaluate the effect of dual-applicator MWA on the image quality of MR. Then, porcine tissue model experiments were conducted with real-time MR thermometry using either a single applicator or dual applicators inserted 2.6, 3.6, and 4.6 cm apart. Fiberoptic thermal probes were used to measure the temperature changes at the tissue surface. ResultsSimultaneous use of dual applicators resulted in a decrease in the relative signal-to-noise ratio (SNR) in the MR thermometry images to 55% ± 2.9% when compared with that of a single applicator (86.2% ± 2.0%). Despite the lower SNR, the temperature and ablation zone maps were of adequate quality to allow visualization of the ablation zone(s). The extents of increase in the temperature at the tissue surface using dual applicators (19.7 °C ± 2.6 °C) and a single applicator (18.2 °C ± 3.3 °C) were not significantly different (P = .40). Treatment zones were significantly larger (P < .05) in dual-applicator ablations (29.4 ± 0.4, 39.9 ± 0.6, and 42.6 ± 0.9 cm2 with 2.6-, 3.6-, and 4.6-cm spacing, respectively) at the end of the ablation procedure than in the single-applicator MWA (18.6 ± 0.9 cm2). ConclusionsMR imaging–guided dual-applicator MWA produced larger ablation zones while allowing adequate real-time MR thermometry image quality for monitoring the evolution of the treatment zone.

Magnetic Resonance-Guided Focused Ultrasound: A Brief Review With Emphasis on the Treatment of Extra-abdominal Desmoid Tumors.

Magnetic resonance-guided focused ultrasound (MRgFUS) utilizes high-intensity focused ultrasound to noninvasively, thermally ablate lesions within the body while sparing the intervening tissues. Magnetic resonance imaging provides treatment planning and guidance, and real-time magnetic resonance thermometry provides continuous monitoring during therapy. Magnetic resonance-guided focused ultrasound is ideally suited for the treatment of extra-abdominal desmoid fibromatosis due to its noninvasiveness, lack of ionizing radiation, low morbidity, and good safety profile. Conventional treatments for these benign tumors, including surgery, radiation, and chemotherapy, can carry significant morbidity. Magnetic resonance-guided focused ultrasound provides a safe and effective alternative treatment in this often-young and otherwise healthy patient population. While there is considerable experience with MRgFUS for treatment of uterine fibroids, painful bone lesions, and essential tremor, there are few reports in the literature of its use for treatment of benign or malignant soft tissue tumors. This article reviews the principles and biologic effects of high-intensity focused ultrasound, provides an overview of the MRgFUS treatment system and use of magnetic resonance thermometry, discusses the use of MRgFUS for the treatment of extra-abdominal desmoid tumors, and provides several case examples.

Assessment of left ventricle magnetic resonance temperature stability in patients in the presence of arrhythmias.

Magnetic resonance (MR) thermometry allows visualization of lesion formation in real-time during cardiac radiofrequency (RF) ablation. The present study was performed to evaluate the precision of MR thermometry without RF heating in patients exhibiting cardiac arrhythmia in a clinical setting. The evaluation relied on quantification of changes in temperature measurements caused by noise and physiological motion. Fourteen patients referred for cardiovascular magnetic resonance imaging underwent an extra sequence to test the temperature mapping stability during free-breathing acquisition. Phase images were acquired using a multi-slice, cardiac-triggered, single-shot echo planar imaging sequence. Temperature maps were calculated and displayed in real-time while the electrocardiogram (ECG) was recorded. The precision of temperature measurement was assessed by measuring the temporal standard deviation and temporal mean of consecutive temperature maps over a period of three minutes. The cardiac cycle was analyzed from ECG recordings to quantify the impact of arrhythmia events on the precision of temperature measurement. Finally, two retrospective strategies were tested to remove acquisition dynamics related either to arrhythmia events or sudden breathing motion. ECG synchronization allowed categorization of inter-beat intervals (RR) into distinct beat morphologies. Five patients were in stable sinus rhythm, while nine patients showed irregular RR intervals due to ectopic beats. An average temporal standard deviation of temperature of 1.6°C was observed in patients under sinus rhythm with a frame rate corresponding to the heart rate of the patient. The temporal standard deviation rose to 2.5°C in patients with arrhythmia. The retrospective rejection strategies increased the temperature precision measurement while maintaining a sufficient frame rate. Our results indicated that real-time cardiac MR thermometry shows good precision in patients under clinical conditions, even in the presence of arrhythmia. By providing real-time visualization of temperature distribution within the myocardium during RF delivery, MR thermometry could prevent insufficient or excessive heating and thus improve safety and efficacy.

Abstract No. 489 Evaluation of microwave ablation zone temperature following administration of a novel thermal accelerant agent in swine: An MR thermometry study

To evaluate the effects of a novel thermal accelerant (TA) agent on microwave ablation zone temperature using real-time magnetic resonance thermometry (MRT) in porcine liver and skeletal muscle. This study was performed following IACUC approval. Castrated adult male domestic swine underwent microwave ablation under general endotracheal anesthesia. The Surblate system (Vision Medical, Santa Clara, CA) was used for all ablation treatments at a generator power of 100W and a frequency of 2,450 MHz for 5 or 10 minutes. Treatments were performed percutaneously with and without injection of 2-3 mL TA via a 5 Fr catheter at a distance of approximately 1.5 cm from the ablation antenna within normal liver and skeletal muscle. Separately, ablations were also performed in ex vivo whole livers. Real-time MRT was conducted on a 3.0 Tesla scanner (Siemens, Erlangen, DE) using a multigradient echo sequence with fat suppression and respiratory gating. Frequency-derived thermal maps were generated according to the slope of the phase vs. TE line on a per-pixel basis with temperature change calculated from the known temperature dependence of the water proton chemical shift. Ablation zone temperature change was quantified according to the volume of tissue that achieved a cytotoxic temperature greater than or equal to 60°C (V60). Data were analyzed with SAS/GLIMMIX using generalized mixed modeling and sandwich estimation assuming a lognormal distribution. Statistical significance was set at α = 0.05. A total of 34 ablations were performed (in vivo liver: 8 TA, 8 control; ex vivo liver: 7 TA, 5 control; in vivo muscle: 3 TA, 3 control). Mean V60 was significantly higher among TA ablations in both the in vivo and ex vivo liver groups when compared with controls (27.6 cm3 vs. 18.6 cm3 and 63.1 cm3 vs. 43.7 cm3, respectively; p<0.01). A trend toward significance was observed in the TA muscle ablation group (42.0 cm3 vs. 28.8 cm3; p = 0.07). In all three experimental conditions, an approximately 50% increase in V60 was seen with TA use. The use of thermal accelerant results in higher ablation zone temperatures as assessed by MRT. Further investigations in animal and human subjects are planned.

Feasibility of real-time MR thermal dose mapping for predicting radiofrequency ablation outcome in the myocardium in vivo

BackgroundClinical treatment of cardiac arrhythmia by radiofrequency ablation (RFA) currently lacks quantitative and precise visualization of lesion formation in the myocardium during the procedure. This study aims at evaluating thermal dose (TD) imaging obtained from real-time magnetic resonance (MR) thermometry on the heart as a relevant indicator of the thermal lesion extent.MethodsMR temperature mapping based on the Proton Resonance Frequency Shift (PRFS) method was performed at 1.5 T on the heart, with 4 to 5 slices acquired per heartbeat. Respiratory motion was compensated using navigator-based slice tracking. Residual in-plane motion and related magnetic susceptibility artifacts were corrected online. The standard deviation of temperature was measured on healthy volunteers (N = 5) in both ventricles. On animals, the MR-compatible catheter was positioned and visualized in the left ventricle (LV) using a bSSFP pulse sequence with active catheter tracking. Twelve MR-guided RFA were performed on three sheep in vivo at various locations in left ventricle (LV). The dimensions of the thermal lesions measured on thermal dose images, on 3D T1-weighted (T1-w) images acquired immediately after the ablation and at gross pathology were correlated.ResultsMR thermometry uncertainty was 1.5 °C on average over more than 96% of the pixels covering the left and right ventricles, on each volunteer. On animals, catheter repositioning in the LV with active slice tracking was successfully performed and each ablation could be monitored in real-time by MR thermometry and thermal dosimetry. Thermal lesion dimensions on TD maps were found to be highly correlated with those observed on post-ablation T1-w images (R = 0.87) that also correlated (R = 0.89) with measurements at gross pathology.ConclusionsQuantitative TD mapping from real-time rapid CMR thermometry during catheter-based RFA is feasible. It provides a direct assessment of the lesion extent in the myocardium with precision in the range of one millimeter. Real-time MR thermometry and thermal dosimetry may improve safety and efficacy of the RFA procedure by offering a reliable indicator of therapy outcome during the procedure.

Magnetic Resonance–Guided Shielding of Prefocal Acoustic Obstacles in Focused Ultrasound Therapy

The treatment of liver cancer is a major public health issue because the liver is a frequent site for both primary and secondary tumors. Rib heating represents a major obstacle for the application of extracorporeal focused ultrasound to liver ablation. Magnetic resonance (MR)-guided external shielding of acoustic obstacles (eg, the ribs) was investigated here to avoid unwanted prefocal energy deposition in the pathway of the focused ultrasound beam. Ex vivo and in vivo (7 female sheep) experiments were performed in this study. Magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU) was performed using a randomized 256-element phased-array transducer (f∼1 MHz) and a 3-T whole-body clinical MR scanner. A physical mask was inserted in the prefocal beam pathway, external to the body, to block the energy normally targeted on the ribs. The effectiveness of the reflecting material was investigated by characterizing the efficacy of high-intensity focused ultrasound beam reflection and scattering on its surface using Schlieren interferometry. Before high-intensity focused ultrasound sonication, the alignment of the protectors with the conical projections of the ribs was required and achieved in multiple steps using the embedded graphical tools of the MR scanner. Multiplanar near real-time MR thermometry (proton resonance frequency shift method) enabled the simultaneous visualization of the local temperature increase at the focal point and around the exposed ribs. The beam defocusing due to the shielding was evaluated from the MR acoustic radiation force impulse imaging data. Both MR thermometry (performed with hard absorber positioned behind a full-aperture blocking shield) and Schlieren interferometry indicated a very good energy barrier of the shielding material. The specific temperature contrast between rib surface (spatial average) and focus, calculated at the end point of the MRgHIFU sonication, with protectors vs no protectors, indicated an important reduction of the temperature elevation at the ribs' surface, typically by 3.3 ± 0.4 in vivo. This was translated into an exponential reduction in thermal dose by several orders of magnitude. The external shielding covering the full conical shadow of the ribs was more effective when the protectors could be placed close to the ribs' surface and had a tendency to lose its efficiency when placed further from the ribs. Hepatic parenchyma was safely ablated in vivo using this rib-sparing strategy and single-focus independent sonications. A readily available, MR-compatible, effective, and cost-competitive method for rib protection in transcostal MRgHIFU was validated in this study, using specific reflective strips. The current approach permitted safe intercostal ablation of small volumes (0.7 mL) of liver parenchyma.

Pre-Clinical Study of In Vivo Magnetic Resonance-Guided Bubble-Enhanced Heating in Pig Liver

Bubble-enhanced heating (BEH) can be exploited to increase heating efficiency in treatment of liver tumors with non-invasive high-intensity focused ultrasound (HIFU). The objectives of this study were: (i) to demonstrate the feasibility of increasing the heating efficiency of sonication exploiting BEH in pig liver in vivo using a clinical platform; (ii) to determine the acoustic threshold for such effects with real-time, motion-compensated magnetic resonance-guided thermometry; and (iii) to compare the heating patterns and thermal lesion characteristics resulting from continuous sonication and sonication including a burst pulse. The threshold acoustic power for generation of BEH in pig liver in vivo was determined using sonication of 0.5-s duration (“burst pulse”) under real-time magnetic resonance thermometry. In a second step, experimental sonication composed of a burst pulse followed by continuous sonication (14.5 s) was compared with conventional sonication (15 s) of identical energy (1.8 kJ). Modification of the heating pattern at the targeted region located at a liver depth between 20 and 25 mm required 600–800 acoustic watts. The experimental group exhibited near-spherical heating with 40% mean enhancement of the maximal temperature rise as compared with the conventional sonication group, a mean shift of 7 ± 3.3 mm toward the transducer and reduction of the post-focal temperature increase. Magnetic resonance thermometry can be exploited to control acoustic BEH in vivo in the liver. By use of experimental sonication, more efficient heating can be achieved while protecting tissues located beyond the focal point.

Magnetic Resonance Thermometry at 7T for Real-Time Monitoring and Correction of Ultrasound Induced Mild Hyperthermia

While Magnetic Resonance Thermometry (MRT) has been extensively utilized for non-invasive temperature measurement, there is limited data on the use of high field (≥7T) scanners for this purpose. MR-guided Focused Ultrasound (MRgFUS) is a promising non-invasive method for localized hyperthermia and drug delivery. MRT based on the temperature sensitivity of the proton resonance frequency (PRF) has been implemented in both a tissue phantom and in vivo in a mouse Met-1 tumor model, using partial parallel imaging (PPI) to speed acquisition. An MRgFUS system capable of delivering a controlled 3D acoustic dose during real time MRT with proportional, integral, and derivative (PID) feedback control was developed and validated. Real-time MRT was validated in a tofu phantom with fluoroptic temperature measurements, and acoustic heating simulations were in good agreement with MR temperature maps. In an in vivo Met-1 mouse tumor, the real-time PID feedback control is capable of maintaining the desired temperature with high accuracy. We found that real time MR control of hyperthermia is feasible at high field, and k-space based PPI techniques may be implemented for increasing temporal resolution while maintaining temperature accuracy on the order of 1°C.

A method for MRI guidance of intercostal high intensity focused ultrasound ablation in the liver

High intensity focused ultrasound (HIFU) is a promising method for the noninvasive treatment of liver tumors. However, the presence of ribs in the HIFU beam path remains problematic since it may lead to adverse effects (skin burns) by absorption and reflection of the incident beam at or near the bone surface. This article presents a method based on magnetic resonance (MR) imaging for identification of the ribs in the HIFU beam, and for selection of the transducer elements to deactivate. The ribs are visualized on anatomical images acquired prior to heating and manually segmented. The resulting regions of interest surrounding the ribs are projected onto the transducer surface by ray tracing from the focal point. The transducer elements in the "shadow" of the ribs are then deactivated. The method was validated ex vivo and in vivo in pig liver during breathing under multislice real-time MR thermometry, using the proton resonance frequency shift method. Ex vivo and in vivo temperature data showed that the temperature increase near the ribs was substantial when HIFU sonications were performed with all elements active, whereas the temperature was reduced with deactivation of the transducer elements located in front of the ribs. The temperature at the focal point was similar with and without deactivation of the transducer elements, indicative of no loss of heat efficiency with the proposed technique. This method is simple, rapid, and reliable, and enables intercostal HIFU ablation while sparing ribs and their surrounding tissues.

MR-Guided Focused Ultrasound: A Potentially Disruptive Technology

A disruptive technology is a technological innovation that overturns the existing dominant technologies in a market. Magnetic resonance (MR)-guided focused ultrasound (MRgFUS) is a noninvasive procedure based on the combination of real-time MR anatomic guidance, MR thermometry, and high-intensity focused ultrasound. Several hundred transducer elements become convergent at a point under MR guidance, leading to heating and coagulation necrosis. Outside the focal point, there is no significant heating. There is no need to break the skin for procedures in the body or to perform a craniotomy for procedures in the brain. This lack of invasiveness is what makes MRgFUS so disruptive compared with surgery. At present, MRgFUS has been used for the ablation of uterine fibroids, breast tumors, painful bony metastases, and liver tumors. In the brain, it has been used for the ablation of glioblastomas and for functional neurosurgery. Phantom and animal studies suggest future applications for prostate cancer and acute stroke treatment.

Local delivery of magnetic resonance (MR) contrast agent in kidney using thermosensitive liposomes and MR imaging‐guided local hyperthermia: A feasibility study in vivo

To investigate the feasibility of local delivery of a magnetic resonance (MR) contrast agent in vivo using paramagnetic thermosensitive liposomes and infrared (IR) laser-induced local hyperthermia under real-time MR thermometry on rabbit kidney. Respiratory gated, radio frequency (RF)-spoiled gradient-echo sequences were used for precise MR temperature mapping (SD = 1 degrees C). In vivo heating experiments confirmed local release of MR contrast agent from liposomes. T1 decreased from 800 msec to about 500 msec, as measured after tissue cooling, in those locations where the renal parenchyma was heated above the phase transition temperature of the liposome membrane. The release of MR contrast agent has been demonstrated in rabbit kidney in vivo. This may be used as a reporter for simultaneous release of therapeutic agents.