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PURPOSE: Develop and evaluate the effectiveness of a T1-based correction method for errors in proton resonant frequency shift thermometry due to non-local field effects caused by heating in fatty breast tissues. METHODS: Computational models of human breast tissue were created by segmenting MRI data from a healthy human volunteer. MR guided focused ultrasound (MRgFUS) heating and MR thermometry measurements were simulated in several locations in the heterogeneous segmented breast models. A T1 based correction method for PRF thermometry errors was applied and the maximum positive and negative errors and the root mean squared error (RMSE) in a region around each heating location was evaluated with and without correction. The method uses T1 measurements to estimate the temperature change in fatty tissues and correct for their influence. Experimental data from a heating study in cadaver breast tissue was analyzed, and the expected PRFS error computed. RESULTS: The simulated MR thermometry had maximum single voxel errors ranging between 10 and 18% when no correction was applied. Applying the correction led to a considerable improvement, lowering the maximum error range to 2–5%. The 5(th) to 95(th) percentile interval of the temperature error distribution was also lowered with correction, from approximately 3.5 to 1°C. This correction worked even when T1 times were uniformly raised or lowered by 5–10%. The experimental data showed predicted errors of 15%. CONCLUSIONS: This simulation study demonstrates that the T1-based correction method reduces MR thermometry errors due to non-local effects from heating in fatty tissues, potentially improving the accuracy of thermometry measurements during MRgFUS treatments. The presented correction method is reliant on having a patient-specific 3D model of the breast, and may be limited by the accuracy of the fat temperatures which in turn may be limited by noise or bias present in the T1 measurements.

We present a design overview of the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) with a newly designed 835-MHz REBCO insert (H835) as a replacement for the 800-MHz REBCO insert (H800) that was damaged when it quenched during operation in 2018. The new H835 is designed to contribute 19.6 T in a background field of 10.93 T by an LTS NMR magnet that normally rated at 11.74 T (500 MHz): combined, 1.3G generates a total field of 30.53 T corresponding to a proton resonance frequency of 1.3 GHz. H835 is designed to operate stably while meeting 1.3G design constraints. We have also designed H835 to protect it from permanent damage in an improbable event like a quench. Key design features are: 1) a single-coil formation, composed of 38 stacked metal-co-wound no-insulation and 2 stacked no-insulation double-pancake coils, all with mechanically improved cross-over sections; 2) enhanced thermal stability; and 3) reduced current margin with a detect-and-heat method. This paper includes: 1) electromagnetic and mechanical design of H835; 2) cryogenics overview; 3) quench protection strategy; and 3) discussion on the next steps to successfully complete 1.3G.

Magnetic resonance imaging (MRI) has become a popular modality in guiding minimally invasive thermal therapies, due to its advanced, nonionizing, imaging capabilities and its ability to record changes in temperature. A variety of MR thermometry techniques have been developed over the years, and proton resonance frequency (PRF) shift thermometry is the current clinical gold standard to treat a variety of cancers. It is used extensively to guide hyperthermic thermal ablation techniques such as high-intensity focused ultrasound (HIFU) and laser-induced thermal therapy (LITT). Essential attributes of PRF shift thermometry include excellent linearity with temperature, good sensitivity, and independence from tissue type. This noninvasive temperature mapping method gives accurate quantitative measures of the temperature evolution inside biological tissues. In this review, the current status and new developments in the fields of MR-guided HIFU and LITT are presented with an emphasis on breast, prostate, bone, uterine, and brain treatments.Level of Evidence 5Technical Efficacy Stage 3 The project was co-financed by the European Regional Development Fund (ERDF) under Ireland’s European Structural, Investment Funds Programme 2014–2020 and Enterprise Ireland; Grant agreement: CF-2017-0826-P, Irish Research Council postgraduate scholarship GOIPG/2018/82 and the NUI Galway College of Science. peer-reviewed 2021-11-20

[EN] In this work we numerically and experimentally study the performance of large-volume hyperthermia induced by acoustic holographic-modulated high-intensity focused ultrasound (AHmHIFU) in gelatin phantoms. Excellent agreement was found between thermal simulations of the AHmHIFU system and temperature maps characterized with Proton Resonance Frequency Shift MR-thermometry at 1.5 T. ROIs inside which experimental temperature elevation was greater than 4 degrees were 11 mm by 16 mm and 7 mm by 30 mm with and without the holographic lens, respectively. Most importantly, standard deviation inside these ROIs were equal to 12% and 40% of the maximum temperature with and without the lens. This demonstrates that a wider and more uniform heated region can be generated using the holographic lens, showing the capabilities of acoustic holograms to be used as a low-cost alternative for large-volume hyperthermia treatments, This research has been supported by the Spanish Ministry of Science, Innovation and Universities (MICINN) through grants IJC2018-037897-I, FPU19/00601 and PID2019-111436RBC22, by the Agencia Valenciana de la Innovaci o through grants INNCON/2021/8 and INNVA1/2020/92, by Generalitat Valenciana through grant AICO/2020/268. Action co-financed by the European Union through the Programa Operativo del Fondo Europeo de Desarrollo Regional (FEDER) of the Comunitat Valenciana 2014-2020 (IDIFEDER/2018/022) and (IDIFEDER/2021/004).

Structural and metabolic imaging are fundamental for diagnosis, treatment and follow-up in oncology. Beyond the well-established diagnostic imaging applications, ultrasounds are currently emerging in the clinical practice as a noninvasive technology for therapy. Indeed, the sound waves can be used to increase the temperature inside the target solid tumors, leading to apoptosis or necrosis of neoplastic tissues. The Magnetic resonance-guided focused ultrasound surgery (MRgFUS) technology represents a valid application of this ultrasound property, mainly used in oncology and neurology. In this paper, patient safety during MRgFUS treatments was investigated by a series of experiments in a tissue-mimicking phantom and performing ex vivo skin samples, to promptly identify unwanted temperature rises. The acquired MR images, used to evaluate the temperature in the treated areas, were analyzed to compare classical proton resonance frequency (PRF) shift techniques and referenceless thermometry methods to accurately assess the temperature variations. We exploited radial basis function (RBF) neural networks for referenceless thermometry and compared the results against interferometric optical fiber measurements. The experimental measurements were obtained using a set of interferometric optical fibers aimed at quantifying temperature variations directly in the sonication areas. The temperature increases during the treatment were not accurately detected by MRI-based referenceless thermometry methods, and more sensitive measurement systems, such as optical fibers, would be required. In-depth studies about these aspects are needed to monitor temperature and improve safety during MRgFUS treatments.

Accurate estimation of SAR is critical to safeguarding vulnerable patients who require an MRI procedure. The increased static field strength and RF duty cycle capabilities in modern MRI scanners mean that systems can easily exceed safe SAR levels for patients. Advisory protocols routinely used to establish quality assurance protocols are not required to advise on the testing of MRI SAR levels and is not routinely measured in annual medical physics quality assurance checks. This study aims to develop a head phantom and protocol that can independently verify global SAR for MRI clinical scanners. A four-channel birdcage head coil was used for RF transmission and signal reception. Proton resonance shift thermometry was used to estimate SAR. The SAR estimates were verified by comparing results against two other independent measures, then applied to a further four scanners at field strengths of 1.5¿T and 3¿T. Scanner output SAR values ranged from 0.42 to 1.52¿W/kg. Percentage SAR differences between independently estimated values and those calculated by the scanners differed by 0-2.3%. We have developed a quality assurance protocol to independently verify the SAR output of MRI scanners. The project was co-financed by the European Regional Development Fund (ERDF) under Ireland’s European Structural, Investment Funds Programme 2014–2020 and Enterprise Ireland; Grant agreement: CF-2017-0826-P, Irish Research Council postgraduate scholarship GOIPG/2018/82 and the NUI Galway College of Science. Thank you to the NUI Galway Microbiology and physics department for their help creating the phantom. Authors would like to thank Maja Drapiewska and David Connolly from the NUI Galway College of Engineering and Informatics and Eileen Smith from Netzsch for help with DSC measurements. peer-reviewed 2020-10-05

Real-time MR-imaging has been clinically adapted for monitoring thermal therapies since it can provide on-the-fly temperature maps simultaneously with anatomical information. However, proton resonance frequency based thermometry of moving targets remains challenging since temperature artifacts are induced by the respiratory as well as physiological motion. If left uncorrected, these artifacts lead to severe errors in temperature estimates and impair therapy guidance. In this study, we evaluated deep learning for on-line correction of motion related errors in abdominal MR-thermometry. For this, a convolutional neural network (CNN) was designed to learn the apparent temperature perturbation from images acquired during a preparative learning stage prior to hyperthermia. The input of the designed CNN is the most recent magnitude image and no surrogate of motion is needed. During the subsequent hyperthermia procedure, the recent magnitude image is used as an input for the CNN-model in order to generate an on-line correction for the current temperature map. The method's artifact suppression performance was evaluated on 12 free breathing volunteers and was found robust and artifact-free in all examined cases. Furthermore, thermometric precision and accuracy was assessed for in vivo ablation using high intensity focused ultrasound. All calculations involved at the different stages of the proposed workflow were designed to be compatible with the clinical time constraints of a therapeutic procedure., 21 pages, 9 figures, 1 table

A computer implemented method for measuring T1 in an anatomical region of interest during a dynamic procedure includes acquiring a reference MR image of the anatomical region of interest using a first flip angle. A first set of dynamic MR images of the anatomical region of interest are acquired using a second flip angle. The reference MR image and the first set are used to calculate a reference T1 value for tissue in the anatomical region of interest. During an intervention where the T1 value may change, a second set of dynamic MR images of the anatomical region of interest is acquired using the second flip angle. The reference MR image and the second set are used to calculate an estimated T1 value. The reference T1 value, the estimated T1 value, and the first and second flip angles may then be used to correct the estimated T1 value.

PURPOSE: To perform multi-echo water/fat separated proton resonance frequency (PRF)-shift temperature mapping. METHODS: State-of-the-art, iterative multi-echo water/fat separation algorithms produce high-quality water and fat images in the absence of heating but are not suitable for real-time imaging due to their long compute times and potential errors in heated regions. Existing fat-referenced PRF-shift temperature reconstruction methods partially address these limitations but do not address motion or large time-varying and spatially inhomogeneous B(0) shifts. We describe a model-based temperature reconstruction method that overcomes these limitations by fitting a library of separated water and fat images measured before heating directly to multi-echo data measured during heating, while accounting for the PRF shift with temperature. RESULTS: Simulations in a mixed water/fat phantom with focal heating showed that the proposed algorithm reconstructed more accurate temperature maps in mixed tissues compared to a fat-referenced thermometry method. In a porcine phantom experiment with focused ultrasound heating at 1.5 Tesla, temperature maps were accurate to within 1°C of fiber optic probe temperature measurements and were calculated in 0.47 s per time point. Free-breathing breast and liver imaging experiments demonstrated motion and off-resonance compensation. The algorithm can also accurately reconstruct water/fat separated temperature maps from a single echo during heating. CONCLUSIONS: The proposed model-based water/fat separated algorithm produces accurate PRF-shift temperature maps in mixed water and fat tissues in the presence of spatiotemporally varying off-resonance and motion.

Purpose Simultaneous fat-referenced proton resonance frequency shift (FRPRFS) thermometry combined with MR elastography (MRE) is proposed, to continuously monitor thermal ablations for all types of soft tissues, including fat-containing tissues. Fat-referenced proton resonance frequency shift thermometry makes it possible to measure temperature even in the water fraction of fat-containing tissues while enabling local field-drift correction. Magnetic resonance elastography allows measuring the mechanical properties of tissues that are related to tissue structural damage. Methods A gradient-echo MR sequence framework was proposed that combines the need for multiple TE acquisitions for the water-fat separation of FRPRFS, and the need for multiple MRE phase offsets for elastogram reconstructions. Feasibility was first assessed in a fat-containing gelatin phantom undergoing moderate heating by a hot water circulation system. Subsequently, high intensity focused ultrasound heating was conducted in porcine muscle tissue ex vivo (N = 4; 2 samples, 2 locations/sample). Results Both FRPRFS temperature maps and elastograms were updated every 4.1 seconds. In the gelatin phantom, FRPRFS was in good agreement with optical fiber thermometry (average difference 1.2 ± 1°C). In ex vivo high-intensity focused ultrasound experiments on muscle tissue, the shear modulus was found to decrease significantly by 34.3% ± 7.7% (experiment 1, sample 1), 17.9% ± 10.0% (experiment 2, sample 1), 55.1% ± 8.7% (experiment 3, sample 2), and 34.7% ± 8.4% (experiment 4, sample 2) as a result of temperature increase (ΔT = 22.5°C ± 4.2°C, 14.0°C ± 2.8°C, 14.7°C ± 3.7°C, and 14.5°C ± 3.0°C, respectively). Conclusion This study demonstrated the feasibility of monitoring thermal ablations with FRPRFS thermometry together with MRE, even in fat-containing tissues. The acquisition time is similar to non-FRPRFS thermometry combined with MRE.

When magnetic resonance (MR) thermometry is performed for temperature monitoring during time-consuming thermal therapy like hyperthermia, tiny main magnetic field (B0) drifts may cause significant errors in temperature readings inside the human subject. We propose a correction method of B0 drift effects in MR thermometry, which is based on temperature-dependent proton resonance frequency shift (PRFS) of water molecules. We placed magnetic field monitoring (MFM) probes around the subject and we read the center frequency of MFM signals. By interpolating the center frequencies of MFM signals on the imaging slice, we computed phase correction maps for MR thermometry. We intermittently acquired MFM signals with performing MR thermometry at 3 Tesla during radiofrequency (RF) heating of a tissue-mimicking phantom. With the B0 drift effect correction, the temperature readings of MR thermometry maps became similar to the temperature readings of an optic fiber temperature sensor embedded at the center of the phantom. We believe the proposed correction method can be used for MRI-guided thermal therapy in which precise temperature monitoring is critical.

Purpose To demonstrate that proton resonance frequency shift MR thermometry (PRFS-MRT) acquisition with nonselective free induction decay (FID), combined with coil sensitivity profiles, allows spatially resolved B0 drift-corrected thermometry. Methods Phantom experiments were performed at 1.5T and 3T. Acquisition of PRFS-MRT and FID were performed during MR-guided high-intensity focused ultrasound heating. The phase of the FIDs was used to estimate the change in angular frequency δωdrift per coil element. Two correction methods were investigated: (1) using the average δωdrift over all coil elements (0th-order) and (2) using coil sensitivity profiles for spatially resolved correction. Optical probes were used for independent temperature verification. In-vivo feasibility of the methods was evaluated in the leg of 1 healthy volunteer at 1.5T. Results In 30 minutes, B0 drift led to an apparent temperature change of up to -18°C and -98°C at 1.5T and 3T, respectively. In the sonicated area, both corrections had a median error of 0.19°C at 1.5T and -0.54°C at 3T. At 1.5T, the measured median error with respect to the optical probe was -1.28°C with the 0th-order correction and improved to 0.43°C with the spatially resolved correction. In vivo, without correction the spatiotemporal median of the apparent temperature was at -4.3°C and interquartile range (IQR) of 9.31°C. The 0th-order correction had a median of 0.75°C and IQR of 0.96°C. The spatially resolved method had the lowest median at 0.33°C and IQR of 0.80°C. Conclusion FID phase information from individual receive coil elements allows spatially resolved B0 drift correction in PRFS-based MRT.

Purpose To reconstruct proton resonance frequency-shift temperature maps free of chemical shift distortions. Theory and Methods Tissue heating created by thermal therapies such as focused ultrasound surgery results in a change in proton resonance frequency that causes geometric distortions in the image and calculated temperature maps, in the same manner as other chemical shift and off-resonance distortions if left uncorrected. We propose an online-compatible algorithm to correct these distortions in 2DFT and echo-planar imaging acquisitions, which is based on a k-space signal model that accounts for proton resonance frequency change-induced phase shifts both up to and during the readout. The method was evaluated with simulations, gel phantoms, and in vivo temperature maps from brain, soft tissue tumor, and uterine fibroid focused ultrasound surgery treatments. Results Without chemical shift correction, peak temperature and thermal dose measurements were spatially offset by approximately 1 mm in vivo. Spatial shifts increased as readout bandwidth decreased, as shown by up to 4-fold greater temperature hot spot asymmetry in uncorrected temperature maps. In most cases, the computation times to correct maps at peak heat were less than 10 ms, without parallelization. Conclusion Heat-induced proton resonance frequency changes create chemical shift distortions in temperature maps resulting from MR-guided focused ultrasound surgery ablations, but the distortions can be corrected using an online-compatible algorithm. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.

Background Because protons in fat do not exhibit a temperature-dependent frequency shift, proton resonance frequency shift (PRFS)-based MR thermometry always suffers from disturbances due to the presence of fats or lipids. Objective A new fat suppression method for PRFS-based MR thermometry is proposed to obtain accurate variation of phase angle. Methods Similar to the approach of separating fat and water with the two-point Dixon technique, we first scan a complex MR image for reference and then scan another complex image varying with temperature at the same TE point. Based on the conventional PRFS method, we use geometric relationships to remove the effect of fat on the variation of the phase angle. Results Two phantoms with different water-to-fat ratios are involved in the temperature mapping test. Experimental results show that the temperature images of two phantoms are approximated under the same conditions. Conclusions The proposed fat suppression method is simple and effective for PRFS-based MR thermometry.

Applications for noninvasive biologic temperature monitoring are widespread in biomedicine and of particular interest in the context of brain temperature regulation, where traditionally costly and invasive monitoring schemes limit their applicability in many settings. Brain thermal regulation, therefore, remains controversial, motivating the development of noninvasive approaches such as temperature-sensitive nuclear MR phenomena. The purpose of this work was to compare the utility of competing approaches to MR thermometry by using proton resonance frequency chemical shift. We tested 3 methodologies, hypothesizing the feasibility of a fast and accurate approach to chemical shift thermometry, in a phantom study at 3T.A conventional, paired approach (difference [DIFF]-1), an accelerated single-scan approach (DIFF-2), and a new, further accelerated strategy (DIFF-3) were tested. Phantom temperatures were modulated during real-time fiber optic temperature monitoring, with MR thermometry derived simultaneously from temperature-sensitive changes in the water proton chemical shift (∼0.01 ppm/°C). MR thermometry was subsequently performed in a series of in vivo nonhuman primate experiments under physiologic and ischemic conditions, testing its reproducibility and overall performance.Chemical shift thermometry demonstrated excellent agreement with phantom temperatures for all 3 approaches (DIFF-1: linear regression R(2) = 0.994; P.001; acquisition time = 4 minutes 40 seconds; DIFF-2: R(2) = 0.996; P.001; acquisition time = 4 minutes; DIFF-3: R(2) = 0.998; P.001; acquisition time = 40 seconds).These findings confirm the comparability in performance of 3 competing approaches to MR thermometry and present in vivo applications under physiologic and ischemic conditions in a primate stroke model.

Quantitative and non-invasive temperature mapping using magnetic resonance imaging (MRI) provides a unique way to measure temperature evolution inside biological tissues. The method is widely used in thermal ablation procedures with magnetic fields at or below 3T. In this paper, the sensitivity of the MRI thermometry at 7T was studied using a proton resonance frequency (PRF)-based technique. We first used an agarose gel phantom with MR-compatible thermometry to calibrate the temperature coefficient, and then this temperature coefficient was employed to measure the internal temperature in both ex vivo (beef muscle) and in vivo (rat) experiments using focused ultrasound heating. The temperature coefficient calibrated by the phantom was 0.0095 ppm/°C, and both the ex vivo and in vivo experiments exhibited clear temperature evolution. This quantitative study confirmed the sensitivity (

Purpose To describe how B0 inhomogeneities can cause errors in proton resonance frequency (PRF) shift thermometry, and to correct for these errors. Methods With PRF thermometry, measured phase shifts are converted into temperature measurements through the use of a scaling factor proportional to the echo time, TE. However, B0 inhomogeneities can deform, spread, and translate MR echoes, potentially making the “true” echo time vary spatially within the imaged object and take on values that differ from the prescribed TE value. Acquisition and reconstruction methods able to avoid or correct for such errors are presented. Results Tests were performed in a gel phantom during sonication, and temperature measurements were made with proper shimming as well as with intentionally introduced B0 inhomogeneities. Errors caused by B0 inhomogeneities were observed, described, and corrected by the proposed methods. No statistical difference was found between the corrected results and the reference results obtained with proper shimming, while errors by more than 10% in temperature elevation were corrected for. The approach was also applied to an abdominal in vivo dataset. Conclusion Field variations induce errors in measured field values, which can be detected and corrected. The approach was validated for a PRF thermometry application. Magn Reson Med 73:2142–2151, 2015. © 2014 Wiley Periodicals, Inc.

We demonstrate the potential utility of spin crossover iron(II) complexes as temperature-responsive paramagnetic chemical exchange saturation transfer (PARACEST) contrast agents in magnetic resonance imaging (MRI) thermometry. This approach is illustrated in the two molecular complexes [Fe(3-bpp)2]2+ (3-bpp = 2,6-di(pyrazol-3-yl)pyridine) and [(Me2NPY5Me2)Fe(H2O)]2+ (Me2NPY5Me2 = 4-dimethylamino-2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine). Variable-temperature magnetic susceptibility data collected for aqueous solutions of these complexes reveal that they exhibit spin crossover behaviour in H2O over the temperature range 20–60 °C. Selective presaturation of pyrazolyl and coordinated water protons in these complexes, respectively, leads to a significant decrease in the NMR signal intensity of bulk water protons through CEST. The corresponding Z-spectra reveal a strong linear temperature dependence of chemical shift of those protons, 0.23(1) ppm °C−1 and 1.02(1) ppm °C−1, respectively, arising from thermal conversion between low-spin S = 0 and high-spin S = 2 iron(II), representing 23- and 100-fold higher sensitivity than that afforded by conventional proton resonance frequency shift thermometry. Finally, temperature maps generated for an aqueous solution containing [(Me2NPY5Me2)Fe(H2O)]2+ show excellent agreement with independently measured temperatures of the solution.

Purpose To characterize the confounding effect of temperature on chemical shift-encoded (CSE) fat quantification. Methods The proton resonance frequency of water, unlike triglycerides, depends on temperature. This leads to a temperature dependence of the spectral models of fat (relative to water) that are commonly used by CSE-MRI methods. Simulation analysis was performed for 1.5 Tesla CSE fat–water signals at various temperatures and echo time combinations. Oil–water phantoms were constructed and scanned at temperatures between 0 and 40°C using spectroscopy and CSE imaging at three echo time combinations. An explanted human liver, rejected for transplantation due to steatosis, was scanned using spectroscopy and CSE imaging. Fat–water reconstructions were performed using four different techniques: magnitude and complex fitting, with standard or temperature-corrected signal modeling. Results In all experiments, magnitude fitting with standard signal modeling resulted in large fat quantification errors. Errors were largest for echo time combinations near TEinit ≈ 1.3 ms, ΔTE ≈ 2.2 ms. Errors in fat quantification caused by temperature-related frequency shifts were smaller with complex fitting, and were avoided using a temperature-corrected signal model. Conclusion Temperature is a confounding factor for fat quantification. If not accounted for, it can result in large errors in fat quantifications in phantom and ex vivo acquisitions. Magn Reson Med 72:464–470, 2014. © 2013 Wiley Periodicals, Inc.

Purpose To present a three-dimensional (3D) segmented echoplanar imaging (EPI) pulse sequence implementation that provides simultaneously the proton resonance frequency shift temperature of aqueous tissue and the longitudinal relaxation time (T1) of fat during thermal ablation. Methods The hybrid sequence was implemented by combining a 3D segmented flyback EPI sequence, the extended two-point Dixon fat and water separation, and the double flip angle T1 mapping techniques. High-intensity focused ultrasound (HIFU) heating experiments were performed at three different acoustic powers on excised human breast fat embedded in ex vivo porcine muscle. Furthermore, T1 calibrations with temperature in four different excised breast fat samples were performed, yielding an estimate of the average and variation of dT1/dT across subjects. Results The water only images were used to mask the complex original data before computing the proton resonance frequency shift. T1 values were calculated from the fat-only images. The relative temperature coefficients were found in five fat tissue samples from different patients and ranged from 1.2% to 2.6%/°C. Conclusion The results demonstrate the capability of real-time simultaneous temperature mapping in aqueous tissue and T1 mapping in fat during HIFU ablation, providing a potential tool for treatment monitoring in organs with large fat content, such as the breast. Magn Reson Med 72:178–187, 2014. © 2013 Wiley Periodicals, Inc.

Purpose To compare numerically simulated and experimentally measured temperature increase due to specific energy absorption rate from radiofrequency fields. Methods Temperature increase induced in both a phantom and in the human forearm when driving an adjacent circular surface coil was mapped using the proton resonance frequency shift technique of magnetic resonance thermography. The phantom and forearm were also modeled from magnetic resonance image data, and both specific energy absorption rate and temperature change as induced by the same coil were simulated numerically. Results The simulated and measured temperature increase distributions were generally in good agreement for the phantom. The relative distributions for the human forearm were very similar, with the simulations giving maximum temperature increase about 25% higher than measured. Conclusion Although a number of parameters and uncertainties are involved, it should be possible to use numerical simulations to produce reasonably accurate and conservative estimates of temperature distribution to ensure safety in magnetic resonance imaging. Magn Reson Med 71:1923–1931, 2014. © 2013 Wiley Periodicals, Inc.

Among many methods in tumor treatment, ultrasound hyperthermia is characterized by non-invasiveness, and it has been proven very effective for clinical treatment. But the problem of monitoring temperature limits its development. MRI-based temperature mapping techniques are safe compared with invasive methods and have been applied to detect temperature changes for a variety of applications. Among these techniques, the proton resonance frequency (PRF) method is relatively advanced. With a temperature measuring experiment and experiment conducted on tumors inside rabbit legs, the effectiveness of PRF method has been proved. This paper is to introduce an MRI guided ultrasound superficial tumor hyperthermia instrument based on PRF method.

Proton resonance frequency (PRF) change is used to measure tissue heating, but also distorts the image and causes geometric distortions in temperature estimates in the same manner as other chemical shift distortions if left uncompensated. We propose an algorithm that produces PRF temperature maps free of these distortions by fitting a signal model directly to acquired k-space data that accounts for PRF-induced phase both up to and during the readout. We also introduce a faster method compatible with Cartesian data that corrects distortions from image-domain temperature maps. Gel heating experiments show the proposed CS compensation algorithms correct magnitude image artifacts and hotspot distortions. Without CS compensation, thermal dose values are overestimated in spiral data, and are spatially offset in 2DFT and EPI data. Compensating for heat-induced CS distortions improves the accuracy of temperature change and thermal dose measurements, and can have a significant positive impact on clinical and resea...

Denaturation of macromolecules within the tissues is believed to be the major factor contributing to the damage of tissues upon hyperthermia. As a result, the value of the spin-lattice relaxation time T1 of the tissue water, which is related to the translational and rotational rates of water, represents an intrinsic probe for investigating structural changes in tissues at high temperature. Therefore, the goal of this work is to investigate whether the simultaneous measurement of temperature and T1 using a hybrid proton resonance frequency (PRF)-T1 measurement technique can be used to detect irreversible changes in T1 that might be indicative of tissue damage. A new hybrid PRF-T1 sequence was implemented based on the variable flip angle driven-equilibrium single-pulse observation (DESPOT)1 method from a standard three dimensional segmented echo-planar imaging sequence by alternating two flip angles from measurement to measurement. The structural changes of the heated tissue volumes were analyzed based on the derived T1 values and the corresponding PRF temperatures. Using the hybrid PRF-T1 technique, we demonstrate that the change of spin lattice relaxation time T1 is reversible with temperature for low thermal dose (thermal dose ≤ 240 cumulative equivalent minutes [CEM] 43°C) and irreversible with temperature after significant accumulation of thermal dose in ex vivo chicken breast tissue. These results suggest that the hybrid PRF-T1 method may be a potentially powerful tool to investigate the extent and mechanism of heat damage of biological tissues.

Thermal therapy procedures being carried out under MR guidance would be safer if temperature changes could be accurately monitored in both water-based and fat-based tissues. To this end we present a hybrid PRF/T1 approach for simultaneously measuring proton resonance frequency (PRF) shift temperatures in water-based tissues and T1 changes in fat-based tissues. The hybrid PRF/T1 sequence is a standard RF spoiled gradient echo sequence executed in a dynamic mode with two flip angles alternating every time frame. The PRF information is extracted every time frame using the image phase in the standard approach and the T1 information is extracted every two time frames using a variable flip angle (VFA) approach. Simulation studies, ex vivo high intensity focused ultrasound (HIFU) heating experiments, and in vivo stability experiments were performed to test the feasibility of the approach. The results indicate that the hybrid PRF/T1 approach provides PRF temperature maps of the same quality as those obtained by traditional PRF methods while simultaneously being able to track T1 changes in fat-based tissues. While several potential error sources exist for the T1 measurements, the approach is a promising start towards realizing quantitative temperature measurements in both water-based and fat-based tissues.

Determining the MR compatibility of medical implants and devices is becoming increasingly relevant. In most cases, the heating of conductive implants due to radiefrequency (RF) excitation pulses is measured by fluoroptic temperature sensors in relevant tests for approval. Another common method to determine these heating effects is MR thermometry using the proton resonance frequency. This method gives good results in homogeneous phantoms. However in many cases, technical shortcomings such as susceptibility artifacts prohibit exact proton resonance frequency thermometry near medical implants. Therefore, this work aimed at developing a fast T₁-based method which allows controlled MR-related heating of a medical implant while simultaneously quantifying the spatial and temporal temperature distribution. To this end, an inversion recovery snapshot Fast Low-Angle Shot (FLASH) sequence was modified with additional off-resonant heating pulses. With an accelerated imaging method and a sliding-window technique, every 7.6 s a new temperature map could be generated with a spatial in-plane resolution of 2 mm. The temperature deviation from calculated temperature values to reference fluoroptic probe was found to be smaller than 1 K.

Minimally invasive thermal therapy using high-power diode lasers is an active area of clinical research. Gold nanoshells (AuNS) can be tuned to absorb light in the range used for laser ablation and may facilitate more conformal tumor heating and sparing of normal tissue via enhanced tumor specific heating. This concept was investigated in a xenograft model of prostate cancer (PC-3) using MR temperature imaging (MRTI) in a 1.5T scanner to characterize the spatiotemporal temperature distribution resulting from nanoparticle mediated heating. Tumors with and without intravenously injected AuNS were exposed to an external laser tuned to 808 nm for 180 sec at 4 W/cm(2) under real-time monitoring with proton resonance frequency shift based MRTI. Microscopy indicated that these nanoparticles (140-150 nm) accumulated passively in the tumor and remained close to the tumor microvasculature. MRTI measured a statistically significant (p 0.001) increase in maximum temperature in the tumor cortex (mean = 21 ± 7°C) in +AuNS tumors versus control tumors. Analysis of the temperature maps helped demonstrate that the overall distribution of temperature within +AuNS tumors was demonstrably higher versus control, and resulted in damage visible on histopathology. This research demonstrates that passive uptake of intravenously injected AuNS in PC-3 xenografts converts the tumor vasculature into a potent heating source for nanoparticle mediated ablation at power levels which do not generate significant damage in normal tissue. When used in conjunction with MRTI, this has implications for development and validation of more conformal delivery of therapy for interstitial laser ablations.

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. Copyright © 2011 John Wiley & Sons, Ltd.

Purpose: To assess the feasibility, precision, and accuracy of real-time temperature mapping (TMap) during laser-induced thermotherapy (LITT) for clinical practice in patients liver with a gradient echo (GRE) sequence using the proton resonance frequency (PRF) method. Materials and Methods: LITT was performed on 34 lesions in 18 patients with simultaneous real-time visualization of relative temperature changes. Correlative contrast-enhanced T1-weighted magnetic resonance (MR) images of the liver were acquired after treatment using the same slice positions and angulations as TMap images acquired during LITT. For each slice, TMap and follow-up images were registered for comparison. Afterwards, segmentation based on temperature (T) >52°C on TMap and based on necrosis seen on follow-up images was performed. These segmented structures were overlaid and divided into zones where the TMap was found to either over- or underestimate necrosis on the postcontrast images. Regions with T>52°C after 20 minutes were defined as necrotic tissue based on data received from two different thermal dose models. Results: The average intersecting region of TMap and necrotic zone was 87% ± 5%, the overestimated 13% ± 4%, and the underestimated 13% ± 5%. Conclusion: This study demonstrates that MR temperature mapping appears reasonably capable of predicting tissue necrosis on the basis of indicating regions having greater temperatures than 52°C and could be used to monitor and adjust the thermal therapy appropriately during treatment. J. Magn. Reson. Imaging 2011;33:704–712. © 2011 Wiley-Liss, Inc.

Conventional spectral-spatial pulses used for water-selective excitation in proton resonance frequency-shift MR thermometry require increased sequence length compared to shorter wideband pulses. This is because spectral-spatial pulses are longer than wideband pulses, and the echo time period starts midway through them. Therefore, for a fixed echo time, one must increase sequence length to accommodate conventional spectral-spatial pulses in proton resonance frequency-shift thermometry. We introduce improved water-selective spectral-spatial pulses for which the echo time period starts near the beginning of excitation. Instead of requiring increased sequence length, these pulses extend into the long echo time periods common to PRF sequences. The new pulses therefore alleviate the traditional tradeoff between sequence length and fat suppression. We experimentally demonstrate an 11% improvement in frame rate in a proton resonance frequency imaging sequence compared to conventional spectral-spatial excitation. We also introduce a novel spectral-spatial pulse design technique that is a hybrid of previous model- and filter-based techniques and that inherits advantages from both. We experimentally validate the pulses' performance in suppressing lipid signal and in reducing sequence length compared to conventional spectral-spatial pulses.

Purpose To validate echo combination as a means to reduce errors caused by fat in temperature measurements with the proton resonance frequency (PRF) shift method. Materials and Methods Computer simulations were performed to study the behavior of temperature measurement errors introduced by fat as a function of echo time. Error reduction by combining temperature images acquired at different echo times was investigated. For experimental verification, three echoes were acquired in a refocused gradient echo acquisition. Temperature images were reconstructed with the PRF shift method for the three echoes and then combined in a weighted average. Temperature measurement errors in the combined image and the individual echoes were compared for pure water and different fractions of fat in a computer simulation and for a phantom containing a homogenous mixture with 20% fat in an MR experiment. Results In both simulation and MR measurement, the presence of fat caused severe temperature underestimation or overestimation in the individual echoes. The errors were substantially reduced after echo combination. Residual errors were about 0.3°C for 10% fat and 1°C for 20% fat. Conclusion Echo combination substantially reduces temperature measurement errors caused by small fractions of fat. This technique then eliminates the need for fat suppression in tissues such as the liver. J. Magn. Reson. Imaging 2007. © 2007 Wiley-Liss, Inc.

A magnetic resonance (MR)-compatible three-dimensional (3-D) hyperthermia applicator was developed and evaluated in the magnetic resonance (MR) tomograph Siemens MAGNETOM Symphony 1.5 T. Radiating elements of this applicator are 12 so-called water coated antenna (WACOA) modules, which are designed as specially shaped and adjustable dipole structures in hermetically closed cassettes that are filled by deionized water. The WACOA modules are arranged in the applicator frame in two transversal antenna subarrays, six antennas per subarray. As a standard load for the applicator an inhomogeneous phantom was fabricated. Details of applicator's realization are presented and a 3-D comparison of calculated and measured temperature data sets is made. A fair agreement is achieved that demonstrates the numerically supported applicator's ability of phase-defined 3-D pattern steering. Further refinement of numerical models and measuring methods is necessary. The applicator's design and the E-field calculations were performed using the finite-difference time-domain (FDTD) method. The calculation and optimization of temperature patterns was obtained using the finite element method (FEM). For MR temperature measurements the proton resonance frequency (PRF) method was used.

In MRI-guided high-intensity focused ultrasound (MR-HIFU) therapy, temperature-dependent proton resonance frequency (PRF) shift is a key factor to quantify and visualize the spatial heating pattern in treated and surrounding tissue. However, during treatment, multiple scanner-related changes can impact the accuracy of the temperature measurements obtained with the PRF shift method and cause an over/under estimation of temperature, which can be a major safety issue for treatments involving real-time temperature control. Hence, it is necessary to apply corrections to ensure accurate temperature measurements during heating. Prior to image acquisition, the central MR frequency F0 can be measured to adjust the F0 of next image acquisition. After acquisition, corrections can be applied to the acquired images to remove scanner-related influences, most importantly phase drift. Different phase drift correction algorithms such as conventional and polynomial adaptive drift correction estimate the background phase by fitting a linear or polynomial to the image phase outside the treatment area and perform the correction accordingly. The goal of this study was to understand the performance of these algorithms for long heating durations as would be experienced during hyperthermia or transurethral HIFU (>20 minutes).

The clinical role of magnetic resonance image-guided high-intensity focused ultrasound (MR-HIFU) is rapidly expanding due to its merit of non-invasiveness. MR thermometry based on a proton resonance frequency shift technique is able to accurately measure HIFU-induced temperature changes, which provides considerable advantages over ultrasonography-guided HIFU in terms of safety and therapeutic efficacy. Recent studies and the resulting technological advances in MR-HIFU such as MR thermometry for moving organs, MR-acoustic radiation force imaging, and a volumetric mild hyperthermia technique further will expand its clinical roles from mere ablation therapy to targeted drug delivery and chemo- or radio-sensitisation for cancer treatment. In this article, MR-HIFU therapy is comprehensively reviewed with an emphasis on the roles of MR imaging in HIFU therapy, techniques of MR monitoring, recent advances in clinical MR-HIFU systems, and potential future applications of MR-HIFU therapy. In addition, the pros and cons of MR-HIFU when compared with ultrasonography-guided HIFU are discussed.

The proton-resonance frequency (PRF) shift method of thermometry has become a promising tool for magnetic resonance image-guided thermal therapies. Although the PRF thermal coefficient has recently been shown to be independent of tissue type when measured ex vivo, significant discrepancy remains on its value for tissues measured in vivo under a variety of experimental conditions. The authors identify a potential source of variation in the PRF thermal coefficient that arises from temperature-induced changes in the volume magnetic susceptibility of tissue and is dependent on the orientation and geometry of the heat-delivery device and its associated heat pattern. This study demonstrates that spatial variations in the apparent PRF thermal coefficient could lead to errors of up to +/-30% in the magnetic resonance estimated temperature change if this effect is ignored.

In this work the temperature dependence of the proton resonance frequency was assessed in agarose gel with a high melting temperature (95 degrees C) and in porcine liver in vitro at temperatures relevant to thermotherapy (25-80 degrees C). Furthermore, an optically tissue-like agarose gel phantom was developed and evaluated for use in MRI. The phantom was used to visualize temperature distributions from a diffusing laser fibre by means of the proton resonance frequency shift method. An approximately linear relationship (0.0085 ppm degrees C(-1)) between proton resonance frequency shift and temperature change was found for agarose gel, whereas deviations from a linear relationship were observed for porcine liver. The optically tissue-like agarose gel allowed reliable MRI temperature monitoring, and the MR relaxation times (T1 and T2) and the optical properties were found to be independently alterable. Temperature distributions around a diffusing laser fibre, during irradiation and subsequent cooling, were assessed with high spatial resolution (voxel size = 4.3 mm3) and with random uncertainties ranging from 0.3 degrees C to 1.4 degrees C (1 SD) with a 40 s scan time.

1H MRS thermometry has been investigated for brain trauma and hypothermia monitoring applications but has not been explored in brain tumours. The proton resonance frequency (PRF) of water is dependent on temperature but is also influenced by microenvironment factors, such as fast proton exchange with macromolecules, ionic concentration and magnetic susceptibility. 1H MRS has been utilized for brain tumour diagnostic and prognostic purposes in children; however, the water PRF measure may provide complementary information to further improve characterization. Water PRF values were investigated from a repository of MRS data acquired from childhood brain tumours and children with apparently normal brains. The cohort consisted of histologically proven glioma (22), medulloblastoma (19) and control groups (28, MRS in both the basal ganglia and parietal white matter regions). All data were acquired at 1.5 T using a short TE (30 ms) single voxel spectroscopy (PRESS) protocol. Water PRF values were calculated using methyl creatine and total choline. Spectral peak amplitude weighted averaging was used to improve the accuracy of the measurements. Mean PRF values were significantly larger for medulloblastoma compared with glioma, with a difference in the means of 0.0147 ppm (p < 0.05), while the mean PRF for glioma was significantly lower than for the healthy cohort, with a difference in the means of 0.0061 ppm (p < 0.05). This would suggest the apparent temperature of the glioma group was ∼1.5 °C higher than the medulloblastomas and ∼0.7 °C higher than a healthy brain. However, the PRF shift may not reflect a change in temperature, given that alterations in protein content, microstructure and ionic concentration contribute to PRF shifts. Measurement of these effects could also be used as a supplementary biomarker, and further investigation is required. This study has shown that the water PRF value has the potential to be used for characterizing childhood brain tumours, which has not been reported previously. © 2014 The Authors. NMR in Biomedicine published by John Wiley & Sons, Ltd.

High energy management in nuclear system and refractory metals productions are equipped with challenging procedures in terms of precise and remote controlling. In order to predict occurrence of contamination and avoidance of huge damages, there are often difficulties to access the equipment during their operation. In addition, estimating the precise and remote nucleation critical temperature of decay and growth of radioactive materials in the nuclear system has also proven to be a great challenge. Other than that, the eutectic crystallization temperature of the refractory metals during production also need to provide a precise estimation. However, it has been understood that the conventional temperature sensors are yet to be applicable to work precisely in such harsh environments. On the other hand, proton resonance frequency thermometry phenomenon have not been utilized or developed to serve as temperature sensors; despite the fact that they are capable to measure temperature in quantum level. Therefore, this article provides a review of the prior art on proton resonance frequency thermometry with its application and reliability, and elaborates on the trajectory of ultra-precise temperature measurement as the latest development.

A method is presented which allows precise temperature and longitudinal (T1) relaxation time measurements with high spatial and temporal resolution. This is achieved by combining dynamic variable flip angle based T1 relaxation mapping with proton resonance frequency shift based thermometry. Herein, dynamic T1 mapping is either used as a complementary measure of temperature or for the detection of T1 contrast agent release. For the first application, the temperature evolution during a high-intensity focused ultrasound tissue ablation experiment was measured in both, porcine fat and muscle, simultaneously. In this application, temperature accuracies of 2.5 K for T1-based thermometry in fat and 1.2 K for proton resonance frequency shift-based thermometry in muscle were observed. The second application relates to MR-guidance of high-intensity focused ultrasound-induced local drug delivery by means of thermo-sensitive liposomes labeled with a T1 contrast agent (Gd-HPDO3A). When the measured temperature exceeded the phase transition temperature of the liposomes, T1 was observed to decrease with a good temporal and spatial correlation due to the release of Gd-HPDO3A. The presented results demonstrate the feasibility of the proposed method for two important applications in MR-guided noninvasive therapy. It offers a high temporal resolution when compared with interleaved Look-Locker based T1 mapping techniques and thus represents an interesting candidate for simultaneous real-time monitoring of T1 and temperature changes. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.

The conventional implementations of proton resonance frequency shift (PRFS) magnetic resonance thermometry (MRT) require the subtraction of single or multiple temporal references, a motion sensitive critical feature. A pilot study was conducted here to investigate the clinical feasibility of near-harmonic two-dimensional (2D) referenceless PRFS MRT, using patient data from MR-guided laser ablation of liver malignancies.PRFS MRT with respiratory-triggered multi-slice gradient-recalled (GRE) acquisition was performed under free breathing in six patients. The precision of the novel referenceless MRT was compared with the reference phase subtraction. Coupling the referenceless MRT with a model-based, real-time compatible regularisation algorithm was also investigated.The precision of MRT was improved by a factor of 3.3 when using the referenceless method as compared to the reference phase subtraction. The approach combining referenceless PRFS MRT and model-based regularisation yielded an estimated precision of 0.7° to 2.1°C, resulting in millimetre-range agreement between the calculated thermal dose and the 24 h post-treatment unperfused regions in liver.The application of the near-harmonic 2D referenceless MRT method was feasible in a clinical scenario of MR-guided laser-induced thermal therapy (LITT) ablation in liver and permitted accurate prediction of the thermal lesion under free breathing in conscious patients, obviating the need for a controlled breathing under general anaesthesia.

Purpose: To combine temperature-related information of phase images and magnitude images acquired from an MR spoiled gradient echo sequence using a postprocessing method referred to as PRF-shift-weighted imaging (PRFSWI). Materials and Methods: Phase images are capable of detecting shifts in proton resonance frequency (PRF) caused by local changes in temperature. Magnitude images provide anatomical information for treatment planning and positioning as well as temperature-related contrast. We used PRFSWI to produce a phase-mask and performed multiplication on the magnitude image to increase temperature-related contrast. Results: Through MRI-guided focused ultrasound (MRIgFUS) experiments (both ex vivo and in vivo), we determined that PRFSWI is capable of enhancing the contrast of a heated area even in the initial stages of transmitting high-intensity focused ultrasound energy. Conclusion: The PRFSWI images are sensitive to changes in temperature and display the heated spot directly in the magnitude images. Although the images do not provide quantitative data related to temperature, this method could be used as a complement to the phase temperature mapping method in the real-time monitoring of MRIgFUS experiments. J. Magn. Reson. Imaging 2011;33:1474–1481. © 2011 Wiley-Liss, Inc.

Conductive implants are in most cases a strict contraindication for MRI examinations, as RF pulses applied during the MRI measurement can lead to severe heating of the surrounding tissue. Understanding and mapping of these heating effects is therefore crucial for determining the circumstances under which patient examinations are safe. The use of fluoroptic probes is the standard procedure for monitoring these heating effects. However, the observed temperature increase is highly dependent on the positioning of such a probe, as it can only determine the temperature locally. Temperature mapping with MRI after RF heating can be used, but cooling effects during imaging lead to a significant underestimation of the heating effect. In this work, an MRI thermometry method was combined with an MRI heating sequence, allowing for temperature mapping during RF heating. This technique may provide new opportunities for implant safety investigations. Magn Reson Med 60:457– 461, 2008. © 2008 Wiley-Liss, Inc.

Purpose To improve the immunity of the proton resonance frequency shift (PRFS) method of MRI temperature mapping against magnetic field disturbances. Since PRFS is a phase-sensitive method, it misinterprets magnetic field disturbances as artifact temperature changes. If not corrected, the resulting temperature artifacts can completely obscure the true temperature estimation, especially if the temperature elevations are small. Materials and Methods Since the fat protons experience the same magnetic field disturbances as the water protons, but no temperature-related frequency shift, the fat signal has been used for correcting PRFS temperature maps for the disturbances. A simple correction method is proposed that has either better compensation capability than the phase correction methods previously reported or higher spatial and temporal resolution than the spectroscopic correction methods previously reported. The evaluated method is based on the utilization of several gradient and spin echoes acquired within one repetition interval with water- and fat-selective scans. Results In a series of phantom experiments, the improved method is shown to enable the reconstruction of accurate temperature maps in spite of interscan motion, suboptimal fat–water separation, and a wide range of magnetic field disturbances. Conclusion Our approach can be used for the guidance of thermal therapies involving tissues containing fat or surrounded by fat. J. Magn. Reson. Imaging 2007. © 2007 Wiley-Liss, Inc.

MR-guided high-intensity focused ultrasound (HIFU) ablation is a promising, noninvasive method for treatment of bone tumors and palliation of pain. During thermal therapy, temperature mapping is necessary to ensure proper heat deposition in targeted tumors as well as to prevent undesired heating in healthy tissues. However, conventional proton resonance frequency-based MR thermometry cannot be applied in cortical bone due to its short T2* relaxation time. Recently, it was shown that ultrashort echo-time MRI can be used to assess T1 change in cortical bone due to temperature change (Han M, ISMRM 2014, P262). This work focused on characterization of temperature dependence of T1 in cortical bone by performing a calibration experiment.

To determine the stability and quality of MR temperature mapping using the proton resonance frequency (PRF) method in the liver of hepatic tumor patients.The standard deviation (SD) of a series of temperature maps was determined in 30 patients (21 patients with cirrhotic livers with carcinoma, and nine patients with noncirrhotic livers with metastasis or angioma) and in five volunteers at normal body temperature under free breathing. A respiratory-gated segmented echo-planar imaging (EPI) sequence (three slices in one expiration phase) was performed with sensitivity encoding (SENSE) acceleration on a 1.5 T scanner. Motion-corrupted images were identified by calculation of the cross-correlation coefficient, and discarded.A T2* range of 10-33 msec was found, with especially low values in advanced cirrhotic livers. The mean temperature SD in patients was 2.3 degrees C (range = 1.5-5.0 degrees C). The stability in healthy livers was slightly better than that in cirrhotic livers, and it was higher in the right liver than in the left liver. The gating failed in 4% of the images when the respiratory cycle was irregular, leading to motion artifacts and errors in the temperature maps.The achieved temperature stability and image quality makes real-time quantitative monitoring of thermal ablation of liver tumors feasible on a clinical scanner.

Of the various techniques employed to quantify temperature changes by MR, proton resonance frequency (PRF) shift-based phase-difference imaging (PDI) is the most accurate and widely used. However, PDI is associated with various artifacts. Motivated by these limitations, we developed a new method to monitor temperature changes by MRI using the balanced steady-state free precession (balanced-SSFP) pulse sequence. Magnitude images obtained with the SSFP pulse sequence were used to find the PRF shift, which is proportional to temperature change. Spatiotemporal temperature maps were successfully reconstructed with this technique in gel phantom experiments and a rabbit model. The results show that the balanced-SSFP-based method is a promising new technique for monitoring temperature.

A technique is presented for the acquisition of temperature maps in the presence of variable respiratory motion using the proton resonance frequency (PRF) shift. The technique uses respiratory triggering, diaphragm position determination with a navigator echo, and the collection of multiple baseline images to generate temperature maps. Laser ablations were performed in an ex vivo liver phantom undergoing variable simulated respiratory motion and in vivo in four porcine livers, demonstrating a reduction of artifacts in the computed temperature maps compared with conventional single baseline techniques, both uncorrected and corrected for motion.