In vivo simultaneous proton resonance frequency shift thermometry and single reference variable flip angle T1 measurements.

The purpose of this study is to demonstrate the first work of T 1-based magnetic resonance thermometry using magnetic resonance fingerprinting (dubbed MRFT). We compared temperature estimation of MRFT with proton resonance frequency shift (PRFS) thermometry on ex vivo bovine muscle. We demonstrated MRFT’s feasibility in predicting temperature on ex vivo bovine muscles with deep brain stimulation (DBS) lead. B 0 maps generated from MRFT were compared with gold standard B 0 maps near the DBS lead. MRFT and PRFS estimated temperatures were compared in the presence of motion. All experiments were performed on a 3 Tesla whole-body GE Premier system with a 21-channel receive head coil (GE Healthcare, Milwaukee, WI). Four fluoroptic probes were used to measure the temperature at the center of a cold muscle (probe 1), the room temperature water bottle (probe 2), and the center and periphery of the heated muscle (probes 3 and 4). We selected regions of interest (ROIs) around the location of the probes and used simple linear regression to generate the temperature sensitivity calibration equations that convert T 1 maps and Δs maps to temperature maps. We then repeated the same setup and compared MRFT and PRFS thermometry temperature estimation with gold standard probe measurements. For the MRFT experiment on DBS lead, we taped the probe to the tip of the DBS lead and used a turbo spin echo sequence to induce heating near the lead. We selected ROIs around the tip of the lead to compare MRFT temperature estimation with probe measurements and compared with PRFS temperature estimation. Vendor-supplied B 0 mapping sequence was acquired to compare with MRFT-generated B 0 maps. We found strong linear relationships (R 2 > 0.958) between T 1 and temperature and Δs and temperatures in our temperature sensitivity calibration experiment. MRFT and PRFS thermometry both accurately predict temperature (RMSE < 1.55 °C) compared to probe measurements. MRFT estimated temperature near DBS lead has a similar trend as the probe temperature. Both B 0 maps show inhomogeneities around the lead. MRFT estimated temperature is less sensitive to motion.

Referenceless proton resonance frequency (PRF) shift thermometry is robust to B 0 drift and inter-scan tissue displacement in temperature change measurement during minimally invasive thermotherapy procedures such as High Intensity Focused Ultrasound (HIFU). In this method, the baseline phase of the heated region is extrapolated from unheated region by a polynomial model. The procedure would involve the time-consuming phase unwrapping procedure necessary for polynomial modeling. We propose here a novel method for referenceless PRF without the need for phase unwrapping. The method fits the spatial derivative of phase image to the polynomial model to avoid phase unwrapping. The baseline phase of heated region is then numerically integrated from the predicted phase derivative image based on the polynomial model. Our experiment demonstrated that the new method improved computational efficiency by 2 to 4 times with slightly better accuracy to the original referenceless thermometry. The speed improvement would facilitate the application of the technique to multi-slice and 3D near-real-time MR thermometry.

Referenceless proton resonance frequency (PRF) shift thermometry provides a means to measure temperature changes during minimally invasive thermotherapy that is inherently robust to motion and tissue displacement. However, if the referenceless method is used to determine temperature changes during prostate ablation, phase gaps between water and fat in image regions used to determine the background phase can confound the phase estimation. We demonstrate an extension to referenceless thermometry which eliminates this problem by allowing background phase estimation in the presence of phase discontinuities between aqueous and fatty tissue. In this method, images are acquired with a multi‐echo sequence and binary water and fat maps are generated from a Dixon reconstruction. For the background phase estimation, water and fat regions are treated separately and the phase offset between the two tissue types is determined. The method is demonstrated during in vivo thermal ablation of canine prostate.

Independent assessment of MRI-induced temperature change and SAR distributions in phantoms

The T1 and T2 temperature dependence of female breast adipose tissue was investigated at 1.5 T in order to evaluate the applicability of relaxation-based MR thermometry in fat for the monitoring of thermal therapies in the breast. Relaxation times T1 , T2 and T2TSE (the apparent T2 measured using a turbo spin echo readout sequence) were measured in seven fresh adipose breast samples for temperatures from 25 to 65 °C. Spectral water suppression was used to reduce the influence of the residual water signal. The temperature dependence of the relaxation times was characterized. The expected maximum temperature measurement errors based on average calibration lines were calculated. In addition, the heating-cooling reversibility was investigated for two samples. The T1 and T2TSE temperature (T) dependence could be fitted well with an exponential function of 1/T. A linear relationship between T2 and temperature was found. The temperature coefficients (mean ± inter-sample standard deviation) of T1 and T2TSE increased from 25 °C (dT1/dT = 5.35 ± 0.08 ms/°C, dT2TSE/dT = 3.82 ± 0.06 ms/°C) to 65 °C (dT1 /dT = 9.50 ± 0.16 ms/°C, dT2TSE/dT = 7.99 ± 0.38 ms/°C). The temperature coefficient of T2 was 0.90 ± 0.03 ms/°C. The temperature-induced changes in the relaxation times were found to be reversible after heating to 65 °C. Given the small inter-sample variation of the temperature coefficients, relaxation-based MR thermometry appears to be feasible in breast adipose tissue, and may be used as an adjunct to proton resonance frequency shift (PRFS) thermometry in aqueous tissue (glandular + tumor).

This study aims to observe and estimate the temperature changes caused by the echo train lengths (ETLs) of fast-spin echo to ensure radiofrequency (RF)-related safety. RF-induced heating on ETL-stepped changes is evaluated using the proton resonance frequency shift (PRF) method and optic-fiber thermography (OFT). The increase in temperature caused by a clinical sequence was compared to that accumulated by the corresponding ETLs. The temperatures are similarly increased in ETLs from 10 to 30 (PRF: 0.109 ± 0.025 ℃, OFT: 0.079 ± 0.009 ℃), suggesting that a large number of ETLs could be utilized at practical sites. The RF-induced heating in the clinical setting (PRF: 0.775 ± 0.069 ℃, OFT: 0.510 ± 0.015 ℃) is comparable to the estimated temperature on the ETL-stepped changes (PRF: 0.780 ℃, OFT: 0.553 ℃). These enable accelerated acquisition with a large number of ETLs to be practically used and RF-induced safety to be thoroughly guaranteed.

MR-guided high-intensity focused ultrasound (HIFU) for treatment of uterine fibroids and painful bone metastases uses the proton resonant frequency shift (PRF) for temperature monitoring in water-based tissues. However, PRF fails to detect temperature changes in tissues with high lipid content, such as fat and bone marrow. Previous studies have shown a change in T2 of subcutaneous fat, red and yellow bone marrow during treatments with focused ultrasound. The lack of calibration data for 3T acquisitions, however, makes it difficult to convert T2 values into maps of tissue temperature. In this study we investigated the dependence of T2 temperature on temperature in porcine adipose tissue and bovine yellow bone marrow at thermal equilibrium at 3T.

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).

Proton resonance frequency shift (PRFS) is an MRI-based simple temperature mapping method that exhibits higher spatial and temporal resolution than temperature mapping methods based on T1 relaxation time and diffusion. PRFS temperature measurements are validated against fiber-optic thermal sensors (FOSs). However, the use of FOSs may introduce temperature errors, leading to both underestimation and overestimation of PRFS measurements, primarily due to material susceptibility changes caused by the thermal sensors. In this study, we demonstrated susceptibility-corrected PRFS (scPRFS) with a high frame rate and accuracy for suitably distributed temperatures. A single-echo-based background removal technique was employed for phase variation correction, primarily owing to magnetic susceptibility, which enabled fast temperature mapping. The scPRFS was used to validate the temperature fidelity by comparing the temperatures of fiber-optic sensors and conventional PRFS through phantom-mimicked human and ex vivo experiments. This study demonstrates that scPRFS measurements in agar-gel are in good agreement with the thermal sensor readings, with a root mean square error (RMSE) of 0.33-0.36 °C in the phantom model and 0.12-0.16 °C in the ex vivo experiment. These results highlight the potential of scPRFS for precise thermal monitoring and ablation in both low- and high-temperature non-invasive therapies.

There is growing interest in performing hyperthermia treatments with clinical magnetic resonance imaging-guided high-intensity focused ultrasound (MR-HIFU) therapy systems designed for tissue ablation. During hyperthermia treatment, however, due to the narrow therapeutic window (41–45 °C), careful evaluation of the accuracy of proton resonant frequency (PRF) shift MR thermometry for these types of exposures is required.Purpose: The purpose of this study was to evaluate the accuracy of MR thermometry using a clinical MR-HIFU system equipped with a hyperthermia treatment algorithm.Methods: Mild heating was performed in a tissue-mimicking phantom with implanted temperature sensors using the clinical MR-HIFU system. The influence of image-acquisition settings and post-acquisition correction algorithms on the accuracy of temperature measurements was investigated. The ability to achieve uniform heating for up to 40 min was evaluated in rabbit experiments.Results: Automatic centre-frequency adjustments prior to image-acquisition corrected the image-shifts in the order of 0.1 mm/min. Zero- and first-order phase variations were observed over time, supporting the use of a combined drift correction algorithm. The temperature accuracy achieved using both centre-frequency adjustment and the combined drift correction algorithm was 0.57° ± 0.58 °C in the heated region and 0.54° ± 0.42 °C in the unheated region.Conclusion: Accurate temperature monitoring of hyperthermia exposures using PRF shift MR thermometry is possible through careful implementation of image-acquisition settings and drift correction algorithms. For the evaluated clinical MR-HIFU system, centre-frequency adjustment eliminated image shifts, and a combined drift correction algorithm achieved temperature measurements with an acceptable accuracy for monitoring and controlling hyperthermia exposures.

Focal bone tumor treatments include amputation, limb-sparing surgical excision with bone reconstruction, and high-dose external-beam radiation therapy. Magnetic resonance guided high intensity focused ultrasound (MR-HIFU) is an effective non-invasive thermotherapy for palliative management of bone metastases pain. MR thermometry (MRT) measures the proton resonance frequency shift (PRFS) of water molecules and produces accurate (<1°C) and dynamic (<5s) thermal maps in soft tissues. PRFS-MRT is ineffective in fatty tissues such as yellow bone marrow and, since accurate temperature measurements are required in the bone to ensure adequate thermal dose, MR-HIFU is not indicated for primary bone tumor treatments. Magnetic relaxation times are sensitive to lipid temperature and we hypothesize that bone marrow temperature can be determined accurately by measuring changes in T2, since T2 increases linearly in fat during heating. T2-mapping using dual echo times during a dynamic turbo spin-echo pulse sequence enabled rapid measurement of T2. Calibration of T2-based thermal maps involved heating the marrow in a bovine femur and simultaneously measuring T2 and temperature with a thermocouple. A positive T2 temperature dependence in bone marrow of 20 ms/°C was observed. Dynamic T2-mapping should enable accurate temperature monitoring during MR-HIFU treatment of bone marrow and shows promise for improving the safety and reducing the invasiveness of pediatric bone tumor treatments.

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. 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. 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. 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.

There is a worldwide increasing interest in the use of non- and minimally invasive thermo therapies like radio frequency (RF), laser or focused ultrasound ablation for tumor treatment. Magnetic resonance (MR-)based on-line monitoring of such thermal ablation procedures enables therapists to readjust treatment parameters instantly to optimize therapy outcome. We designed a universal software tool called TAM (Thermal Ablation Monitoring) for MR monitoring of several thermal ablation methods, like RF ablation, laser interstitial induced thermal therapy (LITT) or focused ultrasound surgery (FUS). Based on the proton resonance frequency (PRF) shift method temperature maps, thermal dose and induced tissue damage are calculated and displayed. Moreover, TAM offers simultaneous on-line and off-line evaluation.Accuracy and usability of TAM were evaluated in vitro and in vivo. LITT and FUS experiments were carried out in tissue phantoms and in animals. On-line guidance and monitoring of FUS and LITT were tested. The contours of displayed lethal thermal dose compared to contrast enhanced T1-weighted MR images and macroscopic visible in vivo lesion size was over all smaller than 2 mm. Hence, the accurate monitoring functionality of TAM was demonstrated.KeywordsMR-guided thermotherapyMR thermometryThermal dosimetryInterventional MR

Fast temperature estimation from undersampled k-space with fully-sampled center for MR guided microwave ablation

Noninvasive temperature monitoring is feasible with Magnetic Resonance Imaging (MRI) based on temperature sensitive MR parameters such as <TEX>$T_1$</TEX> and <TEX>$T_2$</TEX> relaxation times, Proton Resonance Frequency shift (PRFs), diffusion, exchange process, magnetization transfer contrast, chemical exchange saturation transfer, etc. While the temperature monitoring is very useful to guide the thermal treatment such as RF hyperthermia or thermal ablation, the optimization of the MR thermometry method is essential because the range of temperature measurement depends on the choice of the measurement methods. Useful temperature range depends on the purpose of treatment methods, for example, <TEX>$42^{\circ}C$</TEX> to <TEX>$45^{\circ}C$</TEX> for RF hyperthermia and over <TEX>$50^{\circ}C$</TEX> for thermal ablation. In this paper, MR thermometry methods using <TEX>$T_1$</TEX> and <TEX>$T_2$</TEX> relaxation times and PRFs-based MR thermometry are tried on a 3.0 T MRI system and their results are reported and compared. In addition, the scanning protocol and temperature calculation algorithms from <TEX>$T_1$</TEX> and <TEX>$T_2$</TEX> relaxation times and PRFs are optimized for the different temperature ranges for the purpose of RF hyperthermia and/or thermal ablation.