Effect of k-space-weighted image contrast and ultrasound focus size on the accuracy of proton resonance frequency thermometry.

PURPOSE: To construct a predictive model that describes how the duration and symmetry of a k-space-weighted image contrast (KWIC) window affects the temporal resolution of differently sized ultrasound foci when using a pseudo-golden angle stack-of-stars acquisition. METHODS: We performed a modulation analysis of proton resonance frequency temperature measurements to create the temporal modulation transfer function for KWIC windows of different symmetry and temporal duration. We reconstructed simulated ultrasound heating trajectories and stack-of-stars k-space data as well as experimental phantom data using the same trajectories. Images were reconstructed using symmetric and asymmetric KWIC windows of 3 different temporal durations. Simulated results were compared against temporal modulation transfer function predictions, experimental results, and the original simulated temperatures. RESULTS: The temporal modulation transfer function shows that temporal resolution with KWIC reconstructions depend on the object size. The KWIC window duration affected SNR and severity of undersampling artifacts. Accuracy and response delay improved as the KWIC window duration decreased or the size of the heated region within the KWIC plane increased. Precision worsened as the window duration decreased. Using a symmetric window eliminated the response delay to heated region size but introduced a large reconstruction delay. CONCLUSION: The accuracy and precision of proton resonance frequency temperature measurements from a stack-of-stars acquisition using a sliding KWIC window reconstruction are dependent on the size of the KWIC window and the size and shape of the heated region. The temporal modulation transfer function of KWIC reconstructions for any object size can predict the temporal response to changes in signal being acquired, such as temperature and contrast enhancement.

Validation of hybrid angular spectrum acoustic and thermal modelling in phantoms.

In focused ultrasound (FUS) thermal ablation of diseased tissue, acoustic beam and thermal simulations enable treatment planning and optimization. In this study, a treatment-planning methodology that uses the hybrid angular spectrum (HAS) method and the Pennes' bioheat equation (PBHE) is experimentally validated in homogeneous tissue-mimicking phantoms. Simulated three-dimensional temperature profiles are compared to volumetric MR thermometry imaging (MRTI) of FUS sonications in the phantoms, whose acoustic and thermal properties are independently measured. Additionally, Monte Carlo (MC) uncertainty analysis is performed to quantify the effect of tissue property uncertainties on simulation results. The mean error between simulated and experimental spatiotemporal peak temperature rise was +0.33°C (+6.9%). Despite this error, the experimental temperature rise fell within the expected uncertainty of the simulation, as determined by the MC analysis. The average errors of the simulated transverse and longitudinal full width half maximum (FWHM) of the profiles were -1.9% and 7.5%, respectively. A linear regression and local sensitivity analysis revealed that simulated temperature amplitude is more sensitive to uncertainties in simulation inputs than in the profile width and shape. Acoustic power, acoustic attenuation and thermal conductivity had the greatest impact on peak temperature rise uncertainty; thermal conductivity and volumetric heat capacity had the greatest impact on FWHM uncertainty. This study validates that using the HAS and PBHE method can adequately predict temperature profiles from single sonications in homogeneous media. Further, it informs the need to accurately measure or predict patient-specific properties for improved treatment planning of ablative FUS surgeries.

Experimental assessment of phase aberration correction for breast MRgFUS therapy.

PURPOSE: This study validates that phase aberrations in breast magnetic resonance-guided focussed ultrasound (MRgFUS) therapies can be corrected in a clinically relevant time frame to generate more intense, smaller and more spatially accurate foci. MATERIALS AND METHODS: Hybrid angular spectrum (HAS) ultrasound calculations in an magnetic resonance imaging (MRI)-based tissue model, were used to compute phase aberration corrections for improved experimental MRgFUS heating in four heterogeneous breast-mimicking phantoms (n = 18 total locations). Magnetic resonance(MR) temperature imaging was used to evaluate the maximum temperature rise, focus volume and focus accuracy for uncorrected and phase aberration-corrected sonications. Thermal simulations assessed the effectiveness of the phase aberration correction implementation. RESULTS: In 13 of 18 locations, the maximum temperature rise increased by an average of 30%, focus volume was reduced by 40% and focus accuracy improved from 4.6 to 3.6 mm. Mixed results were observed in five of the 18 locations, with focus accuracy improving from 6.1 to 2.5 mm and the maximum temperature rise decreasing by 8% and focus volume increasing by 10%. Overall, the study demonstrated significant improvements (p < 0.005) in maximum temperature rise, focus volume and focus accuracy. Simulations predicted greater improvements than observed experimentally, suggesting potential for improvement in implementing the technique. The complete phase aberration correction procedure, including model generation, segmentation and phase aberration computations, required less than 45 min per sonication location. CONCLUSION: The significant improvements demonstrated in this study i.e., focus intensity, size and accuracy from phase aberration correction have the potential to improve the efficacy, time-efficiency and safety of breast MRgFUS therapies.

Thermal diffusivity and perfusion constants from in vivo MR-guided focussed ultrasound treatments: a feasibility study.

PURPOSE: This study investigates the feasibility of non-invasively determining thermal diffusivity (α) and the Pennes perfusion parameter (w) from pre-clinical and clinical magnetic resonance-guided focussed ultrasound (MRgFUS) temperature data. MATERIALS AND METHODS: Pre-clinical MRgFUS experiments were performed in rabbit muscle (N = 3, 28 sonications) using three-dimensional MR thermometry. Eight sonications were made in a clinical QA phantom with two-dimensional thermometry. Retrospective property determination was performed on clinical uterine fibroid (N = 8, 9 sonications) and desmoid tumour (N = 4, 7 sonications) data. The property determination method fits an analytical solution to MRgFUS temperatures in the coronal MR plane, including all temperatures acquired during heating and one cooling image. When possible, additional cooling data were acquired for property determination. RESULTS: Rabbit α and w from Heating Data (α = 0.164 mm2s-1, w = 7.9 kg m-3 s-1) and Heating and Cooling Data (α = 0.146 mm2s-1, w = 3.3 kg m-3 s-1) were within the range of gold-standard invasive measurements, with >50% reduction in variability by including cooling data. QA phantom property determination with cooling data yielded properties within 3% of expected values (α = 0.144 mm2s-1, w = 0.0 kg m-3 s-1), a difference that was not statistically significant (p = 0.053). Uterine fibroid (Heating Data: α = 0.212 mm2s-1, w = 11.0 kg m-3 s-1) and desmoid tumour (Heating & Cooling Data: α = 0.245 mm2s-1, w = 4.7 kg m-3 s-1) properties are feasible but lack independent verification. CONCLUSIONS: Thermal diffusivity and the Pennes perfusion parameter can be obtained from in vivo data and with clinical MRgFUS protocols. Property values are consistently improved by including cooling data. The utility of this property determination method will increase as clinical protocols implement improved temperature imaging.

Phase aberration simulation study of MRgFUS breast treatments.

PURPOSE: This simulation study evaluates the effects of phase aberration in breast MR-guided focused ultrasound (MRgFUS) ablation treatments performed with a phased-array transducer positioned laterally to the breast. A quantification of these effects in terms of thermal dose delivery and the potential benefits of phase correction is demonstrated in four heterogeneous breast numerical models. METHODS: To evaluate the effects of varying breast tissue properties on the quality of the focus, four female volunteers with confirmed benign fibroadenomas were imaged using 3T MRI. These images were segmented into numerical models with six tissue types, with each tissue type assigned standard acoustic properties from the literature. Simulations for a single-plane 16-point raster-scan treatment trajectory centered in a fibroadenoma in each modeled breast were performed for a breast-specific MRgFUS system. At each of the 16 points, pressure patterns both with and without applying a phase correction technique were determined with the hybrid-angular spectrum method. Corrected phase patterns were obtained using a simulation-based phase aberration correction technique to adjust each element's transmit phase to obtain maximized constructive interference at the desired focus. Thermal simulations were performed for both the corrected and uncorrected pressure patterns using a finite-difference implementation of the Pennes bioheat equation. The effect of phase correction was evaluated through comparison of thermal dose accumulation both within and outside a defined treatment volume. Treatment results using corrected and uncorrected phase aberration simulations were compared by evaluating the power required to achieve a 20 °C temperature rise at the first treatment location. The extent of the volumes that received a minimum thermal dose of 240 CEM at 43 °C inside the intended treatment volume as well as the volume in the remaining breast tissues was also evaluated in the form of a dose volume ratio (DVR), a DVR percent change between corrected and uncorrected phases, and an additional metric that measured phase spread. RESULTS: With phase aberration correction applied, there was an improvement in the focus for all breast anatomies as quantified by a reduction in power required (13%-102%) to reach 20 °C when compared to uncorrected simulations. Also, the DVR percent change increased by 5%-77% in seven out of eight cases, indicating an improvement to the treatment as measured by a reduction in thermal dose deposited to the nontreatment tissues. Breast compositions with a higher degree of heterogeneity along the ultrasound beam path showed greater reductions in thermal dose delivered outside of the treatment volume with correction applied than beam trajectories that propagated through more homogeneous breast compositions. An increasing linear trend was observed between the DVR percent change and the phase-spread metric (R(2) = 0.68). CONCLUSIONS: These results indicate that performing phase aberration correction for breast MRgFUS treatments is beneficial for the small-aperture transducer (14.4 × 9.8 cm) evaluated in this work. While all breast anatomies could benefit from phase aberration correction, greater benefits are observed in more heterogeneous anatomies.

The accuracy and precision of two non-invasive, magnetic resonance-guided focused ultrasound-based thermal diffusivity estimation methods.

PURPOSE: The use of correct tissue thermal diffusivity values is necessary for making accurate thermal modelling predictions during magnetic resonance-guided focused ultrasound (MRgFUS) treatment planning. This study evaluates the accuracy and precision of two non-invasive thermal diffusivity estimation methods, a Gaussian temperature method and a Gaussian specific absorption rate (SAR) method. MATERIALS AND METHODS: Both methods utilise MRgFUS temperature data obtained during cooling following a short (<25 s) heating pulse. The Gaussian SAR method can also use temperatures obtained during heating. Experiments were performed at low heating levels (ΔT∼10 °C) in ex vivo pork muscle and in vivo rabbit back muscle. The non-invasive MRgFUS thermal diffusivity estimates were compared with measurements from two standard invasive methods. RESULTS: Both non-invasive methods accurately estimated thermal diffusivity when using MR temperature cooling data (overall ex vivo error <6%, in vivo <12%). Including heating data in the Gaussian SAR method further reduced errors (ex vivo error <2%, in vivo <3%). The significantly lower standard deviation values (p < 0.03) of the Gaussian SAR method indicated that it had better precision than the Gaussian temperature method. CONCLUSIONS: With repeated sonications, either MR-based method could provide accurate thermal diffusivity values for MRgFUS therapies. Fitting to more data simultaneously likely made the Gaussian SAR method less susceptible to noise, and using heating data helped it converge more consistently to the FUS fitting parameters and thermal diffusivity. These effects led to the improved precision of the Gaussian SAR method.