64-element intraluminal ultrasound cylindrical phased array for transesophageal thermal ablation under fast MR temperature mapping: an ex vivo study

This work was undertaken to investigate the feasibility of using a cylindrical phased array for transoesophaeal thermal ablation under magnetic resonance (MR) imaging guidance. Sixty-four transducers (0.45 mm wide by 15 mm tall), operating at 4.6 MHz, were spread around the periphery of a 10.6-mm-diam cylinder. The head of the applicator was covered with a 65-microm thick latex balloon attached using watertight seals. This envelope was inflated with degassed water to provide acoustic coupling between the transducer and the tissues. The underlying operating principle of this applicator is to rotate a plane ultrasound beam electronically. For this purpose, eight adjacent transducers were excited with appropriate delay times so as to generate a plane wave. The exposure direction was changed by exciting a different set of eight elements. Ex vivo experiments conducted on 47 samples of pig liver under MR temperature monitoring demonstrated the ability of this applicator to generate cylindrical or sector-based coagulation necroses at depths up to 19 mm with excellent angular precision by applying 20 W/cm2. MR thermometry was performed in "real-time" with segmented echo-planar imaging gradient echo sequences. The temporal resolution was approximately 3 s/ image. The average value for the temperature baseline in liver tissue close to the applicator was 0.3 degrees C (+/- 0.6 degrees C). The thermal dose delivered in tissues was computed on-line during temperature imaging. Excellent MR compatibility was demonstrated, all MR acquisitions were performed without susceptibility artifacts or radio-frequency interferences with the ultrasound device. Thermal lesions identified on post-treatment follow up showed good correlation with online MR thermometry data. The individual differences between measurements performed visually and using MRI thermal dose maps were about 11% of volume. This study demonstrated the feasibility of thermal ablation using a phased array intraluminal ultrasound applicator and on-line MR monitoring.     

Feasibility of fast MR-thermometry during cardiac radiofrequency ablation

Online MR temperature monitoring during radiofrequency (RF) ablation of cardiac arrhythmias may improve the efficacy and safety of the treatment. MR thermometry at 1.5 T using the proton resonance frequency (PRF) method was assessed in 10 healthy volunteers under normal breathing conditions, using a multi-slice, ECG-gated, echo planar imaging (EPI) sequence in combination with slice tracking. Temperature images were post-processed to remove residual motion-related artifacts. Using an MR-compatible steerable catheter and electromagnetic noise filter, RF ablation was performed in the ventricles of two sheep in vivo. The standard deviation of the temperature evolution in time (TSD) was computed. Temperature mapping of the left ventricle was achieved at an update rate of approximately 1 Hz with a mean TSD of 3.6 ± 0.9 °C. TSD measurements at the septum showed a higher precision (2.8 ± 0.9 °C) than at the myocardial regions at the heart-lung and heart-liver interfaces (4.1 ± 0.9 °C). Temperature rose maximally by 9 °C and 16 °C during 5 W and 10 W RF applications, respectively, for 60 s each. Tissue temperature can be monitored at an update rate of approximately 1 Hz in five slices. Typical temperature changes observed during clinical RF application can be monitored with an acceptable level of precision.     

Towards optimized MR thermometry of the human heart at 3T

Catheter ablation using radio frequency (RF) has been used increasingly for the treatment of cardiac arrhythmias and may be combined with proton resonance frequency shift (PRFS) ?based MR thermometry to determine the therapy endpoint. We evaluated the suitability of two different MR thermometry sequences (TFE and TFE‐EPI) and three blood suppression techniques. Experiments were performed without heating, using an optimized imaging protocol including navigator respiratory compensation, cardiac triggering, and image processing for the compensation of motion and susceptibility artefacts. Blood suppression performance and its effect on temperature stability were evaluated in the ventricular septum of eight healthy volunteers using multislice double inversion recovery (MDIR), motion sensitized driven equilibrium (MSDE), and inflow saturation by saturation slabs (IS). It was shown that blood suppression during MR thermometry improves the contrast‐to‐noise ratio (CNR), the robustness of the applied motion correction algorithm as well as the temperature stability. A gradient echo sequence accelerated by an EPI readout and parallel imaging (SENSE) and using inflow saturation blood suppression was shown to achieve the best results. Temperature stabilities of 2 °C or better in the ventricular septum with a spatial resolution of 3.5 × 3.5 × 8mm³ and a temporal resolution corresponding to the heart rate of the volunteer, were observed. Our results indicate that blood suppression improves the temperature stability when performing cardiac MR thermometry. The proposed MR thermometry protocol, which optimizes temperature stability in the ventricular septum, represents a step towards PRFS‐based MR thermometry of the heart at 3 T. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparison of four magnetic resonance methods for mapping small temperature changes

Non-invasive detection of small temperature changes (< 1 degree C) is pivotal to the further advance of regional hyperthermia as a treatment modality for deep-seated tumours. Magnetic resonance (MR) thermography methods are considered to be a promising approach. Four methods exploiting temperature-dependent parameters were evaluated in phantom experiments. The investigated temperature indicators were spin-lattice relaxation time T1, diffusion coefficient D, shift of water proton resonance frequency (water PRF) and resonance frequency shift of the methoxy group of the praseodymium complex (Pr probe). The respective pulse sequences employed to detect temperature-dependent signal changes were the multiple readout single inversion recovery (T One by Multiple Read Out Pulses; TOMROP), the pulsed gradient spin echo (PGSE), the fast low-angle shot (FLASH) with phase difference reconstruction, and the classical chemical shift imaging (CSI). Applying these sequences, experiments were performed in two separate and consecutive steps. In the first step, calibration curves were recorded for all four methods. In the second step, applying these calibration data, maps of temperature changes were generated and verified. With the equal total acquisition time of approximately 4 min for all four methods, the uncertainties of temperature changes derived from the calibration curves were less than 1 degree C (Pr probe 0.11 degrees C, water PRF 0.22 degrees C, D 0.48 degrees C and T1 0.93 degrees C). The corresponding maps of temperature changes exhibited slightly higher errors but still in the range or less than 1 degree C (0.97 degrees C, 0.41 degrees C, 0.70 degrees C, 1.06 degrees C respectively). The calibration results indicate the Pr probe method to be most sensitive and accurate. However, this advantage could only be partially transferred to the thermographic maps because of the coarse 16 x 16 matrix of the classical CSI sequence. Therefore, at present the water PRF method appears to be most suitable for MR monitoring of small temperature changes during hyperthermia treatment.     

Temperature monitoring with line scan echo planar spectroscopic imaging

A new magnetic resonance imaging method, line scan echo planar spectroscopic imaging (LSEPSI), is shown capable of providing rapid, internally referenced temperature monitoring from water and fat chemical shifts. METHODS: Orthogonal 90 degrees and 180 degrees slice selective RF pulses inclined by 45 degrees from the image plane solicit a spin echo from a tissue column. The echo is read by asymmetric sampling of 32 gradient echoes spaced 1.4-1.8 ms apart. Sixty-four adjacent columns are sequentially sampled in 4.2-6.4 s with 4,096 voxels sampled with voxel volumes of 0.08-0.13 cm3. Mixed mayonnaise/water phantoms were used to correlate LSEPSI-derived chemical shifts and thermocouple-based temperature measurements from 23 to 60 degrees C with a 1.5 T scanner. Measurement artifacts unrelated to temperature were investigated with the phantom, as was the feasibility of applying the sequence in human breast in vivo. RESULTS: The correlation between LSEPSI and thermocouple-based temperature measurements in the phantom was excellent (r2>0.99). Field drifts affecting the temperature measurements using the water peak alone were corrected by using the water/lipid peak difference. The sequence had an average temperature resolution of 1.4 degrees C in the phantom. The frequency difference measurement reduced the sensitivity to artifacts related to temperature. Both water and lipid peaks were detectable throughout many locations in the breast, suggesting the applicability of LSEPSI in this organ. DISCUSSION: T1-saturation losses occur in conventional and echo-planar based 2D CSI sequences using phase encoding methods with short TR periods. These losses are eliminated when individual columns are sampled in snapshot fashion with LSEPSI since the effective TR becomes the time between scans rather than excitations. T1 saturation can make small spectral peaks difficult to detect at high temperatures and generally lowers the signal-to-noise ratio of the spectra. The rapid acquisition and insensitivity to T1 saturation effects make LSEPSI an attractive technique for monitoring thermal therapies in breast using the internally referenced fat/water frequency separation.     

Fast spectroscopic imaging for non-invasive thermometry using the Pr[MOE-DO3A] complex.

The praseodymium complex of 10-(2-methoxyethyl)-1,4,7,10-tetraaza-cyclododecane-1,4,7-tr iacetate) was evaluated as a temperature-sensitive contrast agent using the temperature dependence (approximately 0.12 ppm degrees C(-1)) of the chemical shift of its methoxy side group signal. Pr[MOE-DO3A] was employed in combination with spectroscopic imaging (SI) methods for the determination of spatially resolved 2D and 3D temperature distributions in phantoms. Conventional SI and fast echo planar SI sequences (EPSI) were implemented on a 4.7 T MR imaging system fulfilling the demands for non-invasive thermometry (NIT) with respect to thermal and temporal resolution, being <1 degree C and <20 s total measuring time, respectively. The sequences are based on a fast spin echo SI method taking into account the very short relaxation times of the Pr complex methoxy group (T1 = 28 ms, T2 = 13 ms) and its chemical shift difference (-24 ppm) from water. Calibration curves were measured in a uniformly heated water phantom and 2D SI methods were applied to dynamic heating experiments. The average differences between the temperatures measured via fibreoptic thermometer and those derived from the spectroscopic methods were < or =0.2 degrees C. Furthermore, 3D EPSI experiments with a 16 x 16 x 16 matrix size yielded temperature measurements within 17 s from voxels of size 3 x 3 x 3 mm3.     

A self-reference PRF-shift MR thermometry method utilizing the phase gradient

In magnetic resonance (MR) imaging, the most widely used and accurate method for measuring temperature is based on the shift in proton resonance frequency (PRF). However, inter-scan motion and bulk magnetic field shifts can lead to inaccurate temperature measurements in the PRF-shift MR thermometry method. The self-reference PRF-shift MR thermometry method was introduced to overcome such problems by deriving a reference image from the heated or treated image, and approximates the reference phase map with low-order polynomial functions. In this note, a new approach is presented to calculate the baseline phase map in self-reference PRF-shift MR thermometry. The proposed method utilizes the phase gradient to remove the phase unwrapping step inherent to other self-reference PRF-shift MR thermometry methods. The performance of the proposed method was evaluated using numerical simulations with temperature distributions following a two-dimensional Gaussian function as well as phantom and in vivo experimental data sets. The results from both the numerical simulations and experimental data show that the proposed method is a promising technique for measuring temperature.     

Wideband Self‐Grounded Bow‐Tie Antenna for Thermal MR

The objective of this study was the design, implementation, evaluation and application of a compact wideband self‐grounded bow‐tie (SGBT) radiofrequency (RF) antenna building block that supports anatomical proton (¹H) MRI, fluorine (¹⁹F) MRI, MR thermometry and broadband thermal intervention integrated in a whole‐body 7.0 T system. Design considerations and optimizations were conducted with numerical electromagnetic field (EMF) simulations to facilitate a broadband thermal intervention frequency of the RF antenna building block. RF transmission (B₁⁺) field efficiency and specific absorption rate (SAR) were obtained in a phantom, and the thigh of human voxel models (Ella, Duke) for ¹H and ¹⁹F MRI at 7.0 T. B₁⁺ efficiency simulations were validated with actual flip‐angle imaging measurements. The feasibility of thermal intervention was examined by temperature simulations (f = 300, 400 and 500 MHz) in a phantom. The RF heating intervention (P_in = 100 W, t = 120 seconds) was validated experimentally using the proton resonance shift method and fiberoptic probes for temperature monitoring. The applicability of the SGBT RF antenna building block for in vivo ¹H and ¹⁹F MRI was demonstrated for the thigh and forearm of a healthy volunteer. The SGBT RF antenna building block facilitated ¹⁹F and ¹H MRI at 7.0 T as well as broadband thermal intervention (234‐561 MHz). For the thigh of the human voxel models, a B₁⁺ efficiency ≥11.8 μT/√kW was achieved at a depth of 50 mm. Temperature simulations and heating experiments in a phantom demonstrated a temperature increase ΔT >7 K at a depth of 10 mm. The compact SGBT antenna building block provides technology for the design of integrated high‐density RF applicators and for the study of the role of temperature in (patho‐) physiological processes by adding a thermal intervention dimension to an MRI device (Thermal MR).     

梯度回波成象技术分析

当今磁共振快速成象技术大多是建立在梯度回波脉冲序更的基础上，这种序列采用小的射频（ＲＦ）偏转角、短重复时间（ＴＲ）。当ＴＲ小到自与自旋－自旋驰豫时间Ｔ２同数量组或更短时，序列上的细微差异将引起图象对比上的较大变化，从而产生了多种多样的梯度回波序列，它们在结构上大致相似，但通过一些细微的差异保留各自的特色。梯度回波的物理基础是磁共振波谱学中早期发展的稳态自由进动即ＳＳＦＰ技术，本文从这一基本理论出发     

Real-time volumetric MRI thermometry of focused ultrasound ablation in vivo: a feasibility study in pig liver and kidney

MR thermometry offers the possibility to precisely guide high-intensity focused ultrasound (HIFU) for the noninvasive treatment of kidney and liver tumours. The objectives of this study were to demonstrate therapy guidance by motion-compensated, rapid and volumetric MR temperature monitoring and to evaluate the feasibility of MR-guided HIFU ablation in these organs. Fourteen HIFU sonications were performed in the kidney and liver of five pigs under general anaesthesia using an MR-compatible Philips HIFU platform prototype. HIFU sonication power and duration were varied. Volumetric MR thermometry was performed continuously at 1.5 T using the proton resonance frequency shift method employing a multi-slice, single-shot, echo-planar imaging sequence with an update frequency of 2.5 Hz. Motion-related suceptibility artefacts were compensated for using multi-baseline reference images acquired prior to sonication. At the end of the experiment, the animals were sacrificed for macroscopic and microscopic examinations of the kidney, liver and skin. The standard deviation of the temperature measured prior to heating in the sonicated area was approximately 1 °C in kidney and liver, and 2.5 °C near the skin. The maximum temperature rise was 30 °C for a sonication of 1.2 MHz in the liver over 15 s at 300 W. The thermal dose reached the lethal threshold (240 CEM(43) ) in two of six cases in the kidney and four of eight cases in the liver, but remained below this value in skin regions in the beam path. These findings were in agreement with histological analysis. Volumetric thermometry allows real-time monitoring of the temperature at the target location in liver and kidney, as well as in surrounding tissues. Thermal ablation was more difficult to achieve in renal than in hepatic tissue even using higher acoustic energy, probably because of a more efficient heat evacuation in the kidney by perfusion.     

Non-invasive temperature imaging with thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethyl-1,4,7,10-tetraacetic acid (TmDOTMA-)

Non-invasive thermometry using hyperfine-shifted MR signals from paramagnetic lanthanide complexes has attracted attention recently because the chemical shifts of these complexes are many times more sensitive to temperature than the water 1H signal. Among all the lanthanide complexes examined thus far, thulium tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (TmDOTMA-) appears to be the most suitable for MR thermometry. In this paper, the feasibility of imaging the methyl 1H signal from TmDOTMA- using a frequency-selective radiofrequency excitation pulse and chemical shift-selective (CHESS) water suppression is demonstrated. A temperature imaging method using a phase-sensitive spin-echo imaging sequence was validated in phantom experiments. A comparison of regional temperature changes measured with fiber-optic probes and the temperatures calculated from the phase shift near each probe showed that the accuracy of imaging the temperature with TmDOTMA- is at least 0.1-0.2 degrees C. The feasibility of imaging temperature changes in an intact rat at 0.5-0.6 mmol/kg dose in only a few minutes is demonstrated. Similar to commonly used MRI contrast agents, the lanthanide complex does not cross the blood-brain barrier. TmDOTMA- may prove useful for temperature imaging in many biomedical applications but further studies relating to acceptable dose and signal-to-noise ratio are necessary before clinical applications.     

Sodium MRI of the human heart at 7.0 T: preliminary results

The objective of this work was to examine the feasibility of three‐dimensional (3D) and whole heart coverage ²³Na cardiac MRI at 7.0 T including single‐cardiac‐phase and cinematic (cine) regimes. A four‐channel transceiver RF coil array tailored for ²³Na MRI of the heart at 7.0 T (f = 78.5 MHz) is proposed. An integrated bow‐tie antenna building block is used for ¹H MR to support shimming, localization and planning in a clinical workflow. Signal absorption rate simulations and assessment of RF power deposition were performed to meet the RF safety requirements. ²³Na cardiac MR was conducted in an in vivo feasibility study. 3D gradient echo (GRE) imaging in conjunction with Cartesian phase encoding (total acquisition time T_AQ = 6 min 16 s) and whole heart coverage imaging employing a density‐adapted 3D radial acquisition technique (T_AQ = 18 min 20 s) were used. For 3D GRE‐based ²³Na MRI, acquisition of standard views of the heart using a nominal in‐plane resolution of (5.0 × 5.0) mm² and a slice thickness of 15 mm were feasible. For whole heart coverage 3D density‐adapted radial ²³Na acquisitions a nominal isotropic spatial resolution of 6 mm was accomplished. This improvement versus 3D conventional GRE acquisitions reduced partial volume effects along the slice direction and enabled retrospective image reconstruction of standard or arbitrary views of the heart. Sodium cine imaging capabilities were achieved with the proposed RF coil configuration in conjunction with 3D radial acquisitions and cardiac gating. Cardiac‐gated reconstruction provided an enhancement in blood–myocardium contrast of 20% versus the same data reconstructed without cardiac gating. The proposed transceiver array enables ²³Na MR of the human heart at 7.0 T within clinical acceptable scan times. This capability is in positive alignment with the needs of explorations that are designed to examine the potential of ²³Na MRI for the assessment of cardiovascular and metabolic diseases. Copyright © 2015 John Wiley & Sons, Ltd.     

Artefacts in intracavitary temperature measurements during regional hyperthermia

For adequate hyperthermia treatments, reliable temperature information during treatment is essential. During regional hyperthermia, temperature information is preferably obtained non-invasively from intracavitary or intraluminal measurements to avoid implant risks for the patient. However, for intracavitary or intraluminal thermometry optimal tissue contact is less natural as for invasive thermometry. In this study, the reliability of intraluminal/intracavitary measurements was examined in phantom experiments and in a numerical model for various extents of thermal contact between thermometry and the surroundings. Both thermocouple probes and fibre optic probes were investigated. Temperature rises after a 30 s power pulse of the 70 MHz AMC-4 hyperthermia system were measured in a tissue-equivalent phantom using a multisensor thermocouple probe placed centrally in a hollow tube. The tube was filled with (1) air, (2) distilled water or (3) saline solution that mimics the properties of tissue, simulating situations with (1) bad thermal contact and no power dissipation in the tube, (2) good thermal contact but no power dissipation or (3) good thermal contact and tissue representative power dissipation. For numerical simulations, a cylindrical symmetric model of a thermocouple probe or a fibre optic probe in a cavity was developed. The cavity was modelled as air, distilled water or saline solution. A generalised E-Field distribution was assumed, resulting in a power deposition. With this power deposition, the temperature rise after a 30 s power pulse was calculated. When thermal contact was bad (1), both phantom measurements and simulations with a thermocouple probe showed very high temperature rises (>0.5 degrees C), which are artefacts due to self-heating of the thermocouple probe, since no power is dissipated in air. Simulations with a fibre optic probe showed almost no temperature rise when the cavity was filled with air. When thermal contact was good, but no power was dissipated in the tube (2), artefacts due to self-heating were not significant and the observed temperature rises were very low ( approximately 0-0.1 degrees C). For the situation, with tissue representative power dissipation (3), a temperature rise of approximately 0.23 degrees C was observed for both measurements and simulations. A clinical example of a regional hyperthermia treatment of a patient with a cervix uteri carcinoma showed that the artefacts observed in the case of bad thermal contact also affect the steady-state temperature measurements. Good tissue contact must be assured for reliable intraluminal or intracavitary measurements.     

MRI-guided Thermal Ablation Therapy: Model and Parameter Estimates to Predict Cell Death from MR Thermometry Images

Solid tumors and other pathologies can be treated using laser thermal ablation under interventional magnetic resonance imaging (iMRI) guidance. A model was developed to predict cell death from magnetic resonance (MR) thermometry measurements based on the temperature–time history, and validated using in vivo rabbit brain data. To align post-ablation T2-weighted spin-echo MR lesion images to gradient-echo MR images, from which temperature is derived, a registration method was used that aligned fiducials placed near the thermal lesion. The outer boundary of the hyperintense rim in the post-ablation MR lesion image was used as the boundary for cell death, as verified from histology. Model parameters were simultaneously estimated using an iterative optimization algorithm applied to every interesting voxel in 328 images from multiple experiments having various temperature histories. For a necrotic region of 766 voxels across all lesions, the model provided a voxel specificity and sensitivity of 98.1 and 78.5%, respectively. Mislabeled voxels were typically within one voxel from the segmented necrotic boundary with median distances of 0.77 and 0.22 mm for false positives (FP) and false negatives (FN), respectively. As compared to the critical temperature cell death model and the generalized Arrhenius model, our model typically predicted fewer FP and FN. This is good evidence that iMRI temperature maps can be used with our model to predict therapeutic regions in real-time during treatment.     

Conformal thermal therapy using planar ultrasound transducers and adaptive closed-loop MR temperature control: demonstration in gel phantoms and ex vivo tissues

MRI-guided transurethral ultrasound therapy offers a minimally invasive approach for the treatment of localized prostate cancer. Integrating a multi-element planar transducer with active MR temperature feedback can enable three-dimensional conformal thermal therapy of a target region within the prostate gland while sparing surrounding normal tissues. Continuous measurement of the temperature distribution in tissue enables dynamic compensation for unknown changes in blood flow and tissue properties during treatment. The main goal of this study was to evaluate the feasibility of using active temperature feedback on a clinical 1.5 T MR imager for conformal thermal therapy. MR thermometry was performed during heating in both gel phantoms and excised tissue with a transurethral heating applicator, and the rotation rate and power were varied based on the thermal measurements. The capability to produce a region of thermal damage that matched a target boundary was evaluated. The influence of a cooling gradient (to simulate cooling of the rectum or urethra) on the desired pattern of thermal damage was also investigated in gel phantoms. Results showed high correlation between the desired target boundary and the 55 degrees C isotherm generated during heating with an average distance error of 0.9 mm +/- 0.4 mm (n = 6) in turkey breasts, 1.4 mm +/- 0.6 mm (n = 4) in gel phantoms without rectal cooling and 1.4 mm +/- 0.6 mm (n = 3) in gel phantoms with rectal cooling. The results were obtained using a temporal update rate of 5 s, a spatial resolution of 3 x 3 x 10 mm for the control point, and a temperature uncertainty of approximately 1 degrees C. The performance of the control algorithm under these conditions was comparable to that of simulations conducted previously by our group. Overall, the feasibility of generating targeted regions of thermal damage with a transurethral heating applicator and active MR temperature feedback has been demonstrated experimentally. This method of treatment appears capable of accounting for unpredictable and varying tissue properties during the treatment.     

Heating induced near deep brain stimulation lead electrodes during magnetic resonance imaging with a 3 T transceive volume head coil

Heating induced near deep brain stimulation (DBS) lead electrodes during magnetic resonance imaging with a 3 T transceive head coil was measured, modeled, and imaged in three cadaveric porcine heads (mean body weight = 85.47 ± 3.19?kg, mean head weight = 5.78 ± 0.32?kg). The effect of the placement of the extra-cranial portion of the DBS lead on the heating was investigated by looping the extra-cranial lead on the top, side, and back of the head, and placing it parallel to the coil's longitudinal axial direction. The heating was induced using a 641?s long turbo spin echo sequence with the mean whole head average specific absorption rate of 3.16?W kg(-1). Temperatures were measured using fluoroptic probes at the scalp, first and second electrodes from the distal lead tip, and 6?mm distal from electrode 1 (T(6?mm)). The heating was modeled using the maximum T(6?mm)?and imaged using a proton resonance frequency shift-based MR thermometry method. Results showed that the heating was significantly reduced when the extra-cranial lead was placed in the longitudinal direction compared to the other placements (peak temperature change = 1.5-3.2?°C versus 5.1-24.7?°C). Thermal modeling and MR thermometry may be used together to determine the heating and improve patient safety online.     

Multi-modality tissue-mimicking phantom for thermal therapy

A tissue-mimicking phantom material has been developed for use with thermal therapy devices and techniques. This material has magnetic resonance properties (primarily T2) which change drastically upon thermal coagulation, enabling its use for device characterization and treatment verification using simple T2-weighted imaging techniques. The coagulation temperature of the phantom can be changed from 50-60 degrees C by adjusting the pH from 4.3 to 4.7. The energy absorption properties can be adjusted to match the acoustical and optical properties of tissues. T2 relaxation measurements are provided as a function of temperature, along with T2-weighted MR images to illustrate the visualization of heating patterns. A complete recipe for fabricating phantoms is provided.     

A heterogeneous human tissue mimicking phantom for RF heating and MRI thermal monitoring verification

This paper describes a heterogeneous phantom that mimics a human thigh with a deep-seated tumor, for the purpose of studying the performance of radiofrequency (RF) heating equipment and non-invasive temperature monitoring with magnetic resonance imaging (MRI). The heterogeneous cylindrical phantom was constructed with an outer fat layer surrounding an inner core of phantom material mimicking muscle, tumor and marrow-filled bone. The component materials were formulated to have dielectric and thermal properties similar to human tissues. The dielectric properties of the tissue mimicking phantom materials were measured with a microwave vector network analyzer and impedance probe over the frequency range of 80-500 MHz and at temperatures of 24, 37 and 45 °C. The specific heat values of the component materials were measured using a differential scanning calorimeter over the temperature range of 15-55 °C. The thermal conductivity value was obtained from fitting the curves obtained from one-dimensional heat transfer measurement. The phantom was used to verify the operation of a cylindrical four-antenna annular phased array extremity applicator (140 MHz) by examining the proton resonance frequency shift (PRFS) thermal imaging patterns for various magnitude/phase settings (including settings to focus heating in tumors). For muscle and tumor materials, MRI was also used to measure T1/T2* values (1.5 T) and to obtain the slope of the PRFS phase change versus temperature change curve. The dielectric and thermal properties of the phantom materials were in close agreement to well-accepted published results for human tissues. The phantom was able to successfully demonstrate satisfactory operation of the tested heating equipment. The MRI-measured thermal distributions matched the expected patterns for various magnitude/phase settings of the applicator, allowing the phantom to be used as a quality assurance tool. Importantly, the material formulations for the various tissue types may be used to construct customized phantoms that are tailored for different anatomical sites.     

3D conformal MRI-controlled transurethral ultrasound prostate therapy: validation of numerical simulations and demonstration in tissue-mimicking gel phantoms

MRI-controlled transurethral ultrasound therapy uses a linear array of transducer elements and active temperature feedback to create volumes of thermal coagulation shaped to predefined prostate geometries in 3D. The specific aims of this work were to demonstrate the accuracy and repeatability of producing large volumes of thermal coagulation (>10 cc) that conform to 3D human prostate shapes in a tissue-mimicking gel phantom, and to evaluate quantitatively the accuracy with which numerical simulations predict these 3D heating volumes under carefully controlled conditions. Eleven conformal 3D experiments were performed in a tissue-mimicking phantom within a 1.5T MR imager to obtain non-invasive temperature measurements during heating. Temperature feedback was used to control the rotation rate and ultrasound power of transurethral devices with up to five 3.5 × 5 mm active transducer elements. Heating patterns shaped to human prostate geometries were generated using devices operating at 4.7 or 8.0 MHz with surface acoustic intensities of up to 10 W cm(-2). Simulations were informed by transducer surface velocity measurements acquired with a scanning laser vibrometer enabling improved calculations of the acoustic pressure distribution in a gel phantom. Temperature dynamics were determined according to a FDTD solution to Pennes' BHTE. The 3D heating patterns produced in vitro were shaped very accurately to the prostate target volumes, within the spatial resolution of the MRI thermometry images. The volume of the treatment difference falling outside ± 1 mm of the target boundary was, on average, 0.21 cc or 1.5% of the prostate volume. The numerical simulations predicted the extent and shape of the coagulation boundary produced in gel to within (mean ± stdev [min, max]): 0.5 ± 0.4 [-1.0, 2.1] and -0.05 ± 0.4 [-1.2, 1.4] mm for the treatments at 4.7 and 8.0 MHz, respectively. The temperatures across all MRI thermometry images were predicted within -0.3 ± 1.6 °C and 0.1 ± 0.6 °C, inside and outside the prostate respectively, and the treatment time to within 6.8 min. The simulations also showed excellent agreement in regions of sharp temperature gradients near the transurethral and endorectal cooling devices. Conformal 3D volumes of thermal coagulation can be precisely matched to prostate shapes with transurethral ultrasound devices and active MRI temperature feedback. The accuracy of numerical simulations for MRI-controlled transurethral ultrasound prostate therapy was validated experimentally, reinforcing their utility as an effective treatment planning tool.     

Non-invasive ultrasound-based temperature imaging for monitoring radiofrequency heating-phantom results

Minimally invasive therapies (such as radiofrequency ablation) are becoming more commonly used in the United States for the treatment of hepatocellular carcinomas and liver metastases. Unfortunately, these procedures suffer from high recurrence rates of hepatocellular carcinoma ( approximately 34-55%) or metastases following ablation therapy. The ability to perform real-time temperature imaging while a patient is undergoing radiofrequency ablation could provide a significant reduction in these recurrence rates. In this paper, we demonstrate the feasibility of ultrasound-based temperature imaging on a tissue-mimicking phantom undergoing radiofrequency heating. Ultrasound echo signals undergo time shifts with increasing temperature, which are tracked using 2D correlation-based speckle tracking methods. Time shifts or displacements in the echo signal are accumulated, and the gradient of these time shifts are related to changes in the temperature of the tissue-mimicking phantom material using a calibration curve generated from experimental data. A tissue-mimicking phantom was developed that can undergo repeated radiofrequency heating procedures. Both sound speed and thermal expansion changes of the tissue-mimicking material were measured experimentally and utilized to generate the calibration curve relating temperature to the displacement gradient. Temperature maps were obtained, and specific regions-of-interest on the temperature maps were compared to invasive temperatures obtained using fiber-optic temperature probes at the same location. Temperature elevation during a radiofrequency ablation procedure on the phantom was successfully tracked to within +/-0.5 degrees C.