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

A combined numerical-experimental iterative procedure, based on the Gauss-Newton algorithm, has been developed for control of magnetic resonance (MR)-guided hyperthermia (HT) applications in a hybrid MR-HT system BSD 2000 3D-MRI. In this MR-HT system, composed of a 3-D HT applicator Sigma-Eye placed inside a tunnel-type MR tomograph Siemens MAGNETOM Symphony (1.5 T), the temperature rise due to the HT radiation can be measured on-line in three dimensions by use of the proton resonance frequency shift (PRFS) method. The basic idea of our iterative procedure is the improvement of the system's characterization by a step-by-step modification of the theoretical HT antenna profiles (electric fields radiated by single antennas). The adaptation of antenna profiles is efficient if the initial estimates are radiation fields calculated from a good a priori electromagnetic model. Throughout the iterative procedure, the calculated antenna fields (FDTD) are step-by-step modified by comparing the calculated and experimental data, the latter obtained using the PRFS method. The procedure has been experimentally tested on homogeneous and inhomogeneous phantoms. It is shown that only few comparison steps are necessary for obtaining a dramatic improvement of the general predictability and quality of the specific absorption rate (SAR) inside the MR-HT hybrid system.

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).

MR-Guided Interventions of the Breast

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.

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

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.

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

A novel method for improved planning and control of the magnetic resonance (MR)-guided hyperthermia (HT) has been developed and tested in the combined Berlin MR-HT hybrid system, consisting of the 12-channel HT applicator Sigma-Eye and of the MR tomograph Siemens MAGNETOM Symphony 1.5 T. The method is based on a step-by-step adaptation of antenna profiles (single antenna fields) using MR temperature rise measurements obtained by the proton resonance frequency shift (PRFS) method. A Gauss-Newton algorithm is applied as an iterative solver for adaptation. It is shown that, using the finite-difference time-domain (FDTD) method as initial estimate for antenna profiles, already after few adaptation steps a significant improvement of the specific absorption rate (SAR) predictability in the MR-HT hybrid system is reached.

Respiratory induced resonance offset (RIRO) is a periodic disturbance of a magnetic field due to breathing. Such disturbance handicaps the accuracy of the proton resonance frequency shift (PRFS) method of MRI temperature mapping in anatomies situated nearby the lungs and chest wall. In this work, we propose a method capable of minimizing errors caused by RIRO in PRFS temperature maps. In this method, a set of baseline images characterizing RIRO at a variety of respiratory cycle instants is acquired before the thermal treatment starts. During the treatment, the temperature evolution is found from two successive images. Then, the calculated temperature changes are corrected for the additional contribution caused by RIRO using the pre-treatment baseline images acquired at the identical instances of the respiratory cycle. Our method is shown to improve the accuracy and stability of PRFS temperature maps in the presence of RIRO and inter-scan motion in phantom and volunteers' breathing experiments. Our method is also shown to be applicable to anatomies moving during breathing if a proper registration procedure is applied.

The single reference variable flip angle sequence with a multi-echo stack of stars acquisition (SR-VFA-SoS) simultaneously measures temperature change using proton resonance frequency (PRF) shift and T1-based thermometry methods. This work evaluates SR-VFA-SoS thermometry in MR-guided focused ultrasound in an in vivo rabbit model. Simultaneous PRF shift thermometry and T1-based thermometry were obtained in a New Zealand white rabbit model (n = 7) during MR-guided focused ultrasound surgery using the SR-VFA-SoS sequence at 3 T. Distinct locations in muscle (n = 16), fat (n = 12), or the interface of both tissues (n = 23) were heated. The T1-temperature coefficient of fat was determined using least-squares fitting of inversion recovery-based T1 maps of untreated fat harvested from the animal and was applied to the in vivo measured heat-induced T1 changes to create temperature maps. Using k-space weighted image contrast reconstruction, temporal resolution of 1.71 s was achieved for simultaneous thermometry at 1.5 × 1.5 × 2 mm voxel resolution. PRF shift thermometry was not sensitive to heating in fat. T1 changes were observed in fat at the ultrasound focus. The mean T1-temperature coefficient for fat was determined to be 1.9%/°C ± 0.2%/°C. Precision was 0.76°C ± 0.18°C for PRF shift thermometry in muscle and 1.93°C ± 0.60°C for T1-based thermometry in fat. Sonications in muscle showed an increase in T1 of 2.4%/°C ± 0.9%/°C. The SR-VFA-SoS sequence was shown to simultaneously measure temperature change using PRF shift and T1-based methods in an in vivo model, providing thermometry for both aqueous and fat tissues.

Clinical application of high-temperature thermal therapy as a treatment forsolid tumours requires an accurate and close to real-time method forassessing tissue damage. Imaging methods that detect structural changesduring heating may underestimate the extent of thermal damage. This is dueto the occurrence of delayed damage manifested at tissue locations exposedto temperatures lower than those required to cause immediate structuralchanges. An alternative approach is to measure temperature and thencalculate the expected damage based on the temperature history at eachtissue location. Magnetic resonance (MR) imaging methods now allowtemperature maps of the target and surrounding tissues to be generated inalmost real-time. The aim of this work was to evaluate whether thermaldamage zones calculated on the basis of MR thermometry maps measured duringheating correspond to actual tissue damage as measured after treatment byhistological methods and MR imaging.Four male rabbits were treated with high-temperature thermal therapydelivered in the brain by a single microwave antenna operating at 915 MHz.MR scanning was performed before, during and after treatment in a 1.5 Twhole-body scanner. Temperature maps were produced using the protonresonance frequency (PRF) shift method of MR thermometry. In addition,conventional T1-weighted and T2-weighted spin-echo images wereacquired after treatment. Thermal damage zones corresponding to cell death,microvascular blood flow stasis and protein coagulation were calculatedusing an Arrhenius analysis of the MR temperature/time course data. Thecalculated zones were compared with the lesions seen on histopathologicalexamination of the brains which were removed within 6-8 h of treatment.The results showed that calculated damage zones based on MR thermometryagreed well with areas of damage as assessed using histology after heatingwas completed. The data suggest that real-time calculations of finalexpected thermal damage based on an Arrhenius analysis of MR temperaturedata may provide a useful method of real-time monitoring of thermal therapywhen combined with conventional T2-weighted images taken aftertreatment.

This study aimed to explore the accuracy of non-invasive temperature measurement based on proton resonance frequency (PRF) phase subtraction in microwave ablation (MWA). The signal change of the agar phantom during the ablation process was monitored by the gradient echo sequence under 1.5 T superconducting magnetic resonance imagining (MRI), and the temperature change was converted by the phase subtraction method of the PRF, which was compared with the temperature measured using an optical fiber. SPSS software version 22.0 was used for data processing, and the independent sample t-test was used for comparative analysis. P < 0.05 indicated statistical significance. The maximum error between the MRI temperature measurement and the standard value was 3.61°C, whereas the minimum and average errors were 0.01°C and 1.19°C ± 0.78°C, respectively. The temperature measurement technology, which is based on the PRF phase method, has good accuracy in MRI-guided MWA.

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.

Purpose: Since mild hyperthermia therapy (MHT) requires maintaining the temperature within a narrow window (e.g. 40–43 °C) for an extended duration (up to 1 h), accurate and precise temperature measurements are essential for ensuring safe and effective treatment. This study evaluated the precision and accuracy of MR thermometry in healthy volunteers at different anatomical sites for long scan times.Methods: A proton resonance frequency shift method was used for MR thermometry. Eight volunteers were subjected to a 5-min scanning protocol, targeting chest wall, bladder wall, and leg muscles. Six volunteers were subjected to a 30-min scanning protocol and three volunteers were subjected to a 60-min scanning protocol, both targeting the leg muscles. The precision and accuracy of the MR thermometry were quantified. Both the mean precision and accuracy <1 °C were used as criteria for acceptable thermometry.Results: Drift-corrected MR thermometry measurements based on 5-min scans of the chest wall, bladder wall, and leg muscles had accuracies of 1.41 ± 0.65, 1.86 ± 1.20, and 0.34 ± 0.44 °C, and precisions of 2.30 ± 1.21, 1.64 ± 0.56, and 0.48 ± 0.05 °C, respectively. Measurements based on 30-min scans of the leg muscles had accuracy and precision of 0.56 ± 0.05 °C and 0.42 ± 0.50 °C, respectively, while the 60-min scans had accuracy and precision of 0.49 ± 0.03 °C and 0.56 ± 0.05 °C, respectively.Conclusions: Respiration, cardiac, and digestive-related motion pose challenges to MR thermometry of the chest wall and bladder wall. The leg muscles had satisfactory temperature accuracy and precision per the chosen criteria. These results indicate that extremity locations may be preferable targets for MR-guided MHT using the existing MR thermometry technique.

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.