Keywords: MR-Guided Focused Ultrasound, MR-Guided Interventions

Motivation: In MRI at 3T, the typical configuration of MR-guided focused Ultrasound (MRgFUS) results in a dark band through the brain in MRI.

Goal(s): We introduce an approach to significantly improving MRI in the whole brain in addition to removing the dark band artifact, demonstrating here with an improved phantom for better results.

Approach: An improved phantom for experimental validation of our method was developed by using a spheroidal compartment rather than a hemispherical compartment to represent the head.

Results: Transmit Efficiency was improved by more than a factor of 4 on average in the ROI using our proposed method. 

Impact: By strategically adjusting electric permittivity of the transfer medium and slotting the transducer ground plane, transmit efficiency of MRI in transcranial MRgFUS can be improved by a factor of up to 4 while also eliminating a well-known dark band.

Keywords: MR-Guided Focused Ultrasound, RF Arrays & Systems, Focus Ultrasound

Motivation: MRI with the focused ultrasound treatment on the spine has low image quality, presumably due to a tailored receiver coil array.

Goal(s): A receiver coil array for 3T MRI is developed to image the spine of a large animal during focused ultrasound (FUS) treatment using a flat, large-aperture (25 cm diameter) phased array transducer.

Approach: A 4-channel receiver coil array was built with a housing curved to conform to the back of the specimen.

Results: Structural images with T₁- and T₂-weighted contrast confirmed an SNR gain of approximately 50% using this coil array compared with images acquired using a body coil.

Impact: The FUS-compatible spine MRI receiver coil array will enhance visualization of BSCB openings. As a clinical application, this coil array will also alleviate intraprocedural imaging challenges.

Keywords: MR-Guided Focused Ultrasound, Bone, Skull, UTE, B1, T1, VFA

Motivation: Unintended heating of the skull and nearby soft tissue during transcranial MR-guided Focused Ultrasound surgery needs to be monitored. However, T1-based thermometry in cortical bone may be biased with variable flip angles.

Goal(s): Our goal was to correct flip angles to improve the T1 mapping accuracy in cortical bone.

Approach: In phantom and human studies, B1 maps of soft tissue were measured with a phase-sensitive method, extrapolating from which B1 maps of bone were generated and were used to correct flip angles for T1 mapping.

Results: Improved accuracy and homogeneity of T1 maps were demonstrated as validation.

Impact: Extrapolated-B1 correction with a phase-sensitive method should improve accuracy and homogeneity of T1 maps in cortical bone and is promising in monitoring unintended heating in the skull and nearby brain tissue during transcranial MR-guided Focused Ultrasound surgery.

Keywords: Thermometry/Thermotherapy, Thermometry

Motivation: Monitoring thermal ablations with PRFS thermometry ensures therapy safety and efficiency. Spectral fat saturation cannot be 100% efficient: the remaining fat signal may affect PRFS temperature measurements.

Goal(s): This work experimentally evaluates the residual signal after spectral fat saturation, and the error it causes in PRFS thermometry.

Approach: The IDEAL algorithm is used to quantify fat signal after spectral fat saturation. The error in PRFS thermometry with spectral fat saturation is evaluated in fat-water phantoms.

Results: The residual fat signal after spectral fat saturation leads to errors in PRFS thermometry, that increase with the fat content, and oscillate with TE and absolute temperature.

Impact: This work demonstrates experimentally that PRFS temperature errors can be significant during hyperthermia despite fat suppression methods. Careful selection of the TE according to spectral fat saturation strategy can mitigate such errors and improve the accuracy of PRFS thermometry.

Keywords: Thermometry/Thermotherapy, Thermometry, Breast, Spectroscopic imaging

Motivation: Clinically available MR thermometry methods only measures relative temperature change. Availability of absolute temperature measurements has high impact potential to improve monitoring and evaluation of thermal therapies.

Goal(s): Develop an efficient and practical approach for spatial absolute MR thermometry in the breast.

Approach: A multi-echo echo-planar spectroscopic imaging pulse sequence was implemented and tested in ex vivo human breast fat samples and healthy volunteers. Comparisons to single voxel spectroscopy was performed.

Results: Water and fat peaks can be detected in both ex vivo samples and in vivo, and converted to absolute temperature measurements. Potential of spatial absolute thermometry is demonstrated.

Impact: Absolute MR thermometry has the potential to improve treatment monitoring and evaluation of thermal therapies, and is demonstrated in the breast using a new efficient high-resolution interleaved echo planar spectroscopic imaging pulse sequence.

Keywords: Thermometry/Thermotherapy, Thermometry

Motivation: Proton-resonance-frequency (PRF) shift-based thermometry is used to monitor the temperature change during Laser-interstitial-thermal-therapy (LITT). Recent LITT developments call for PRF thermometry to achieve larger volume coverage and higher spatiotemporal resolution for enhanced therapeutic efficacy.

Goal(s): Accelerate the PRF thermometry by compressed-sensing (CS) undersampling to enlarge the volume coverage and increase the spatiotemporal resolution. Use a neural network for real-time reconstruction of the undersampled data.

Approach: A recurrent reconstruction network (RRN) was proposed to reconstruct the highly undersampled PRF data. Retrospective and prospective undersampling experiments were conducted.

Results: RRN demonstrated good image reconstruction quality in retrospective experiments, with promising results in prospective experiments.

Impact: The introduction of the RRN offers a solution for real-time and high-resolution PRF thermometry during LITT, potentially improving treatment outcomes.

Keywords: Thermometry/Thermotherapy, Thermometry

Motivation: Improved MR thermometry is needed for thermal-based therapies.

Goal(s): Improve precision while maintaining accuracy by denoising proton resonance frequency shift (PRFS)-based thermometry images using a deep learning-based reconstruction (DLR).

Approach: 2D fast spoiled gradient-echo (FSPGR) images were acquired on a variety of phantoms with and without heating. Complex images were reconstructed with and without DLR to calculate temperature change maps.  The mean and standard deviation of ROIs were analyzed to demonstrate any changes in accuracy and precision. 

Results: DLR improves precision and maintains accuracy in PRFS temperature change maps in phantoms. 

Impact: The improvements indicate an opportunity to increase spatial and/or temporal resolution in MRI thermometry.  It may be possible to improve MRI-based heating optimization during clinical thermal therapies.       

Keywords: Thermometry/Thermotherapy, Motion Correction

Motivation: To improve the performance of convolutional neural network (CNN) for motion artifacts correction and demonstrate the feasibility of using motion-related information provided by principal component analysis (PCA).

Goal(s): To achieve high accuracy of temperature mapping for motion existing organs like abdominal and thus expand a wide range of MR-thermometry in clinical applications. 

Approach: We proposed a combination method of PCA and basic CNN model to correct artifacts in abdominal MR-thermometry. 

Results: Preliminary results showed that the proposed method outperforms conventional CNN in terms of temperature mapping accuracy. The new method reduces the motion-related phase errors by leveraging PCA. 

Impact: Our proposed method has a high potential to handle motion organs with non-rigid motion. The PCA method in combination of CNN for its efficient reduction of motion induced errors may improve the feasibility and accessibility of MR-thermometry in abdominal applications.

Keywords: Thermometry/Thermotherapy, Thermometry, Phantom

Motivation: Quantitative Magnetic Resonance Imaging (qMRI) is becoming popular and is often evaluated in phantoms. However, MRI properties (T1, T2, diffusion, etc.) depend on temperature, so accurate absolute temperature measurements of the phantom is needed.

Goal(s): Being able to measure absolute temperature throughout qMRI phantoms in a practical, efficient, and accurate way.

Approach: A straight forward spectroscopic imaging approach is used to image ethylene glycol-filled vials and an automatic processing pipeline reconstructs temperatures.

Results: Rapid (15 s/slice) and accurate (up to 0.5 °C) absolute temperature measurements are demonstrated at two different 3T scanners.

Impact: With the described approach rapid and accurate MRI-based absolute phantom temperature measurements can be acquired in a practical way. This will help researchers developing quantitative measurements of temperature-sensitive parameters such as T1,T2, and diffusion, using phantoms for validation and benchmarking.

Keywords: MR-Guided Interventions, Interventional Devices, Active Tracking

Motivation: MR-thermometry is a promising approach for real-time monitoring of lesion formation during MRI-guided cardiac ablation. However, ablation catheters can potentially drift during ablation and lead to inaccurate lesion formation.

Goal(s): To develop a cardiac MR-thermometry sequence with integrated catheter drift detection.

Approach: Continuous active tracking modules were added between the cardiac triggered acquisitions of a cardiac MR-thermometry sequence. Low-pass temporal filtering of the active tracking signal was then applied to remove cardiac and respiratory components and estimate catheter drift. This approach was evaluated in a phantom.

Results: This study showed that catheter drift detection is possible within active tracking in an MR-thermometry sequence.

Impact: This study demonstrates the potential for simultaneous catheter drift detection and cardiac MR-thermometry, which may improve the safety of the procedure and the accuracy of lesion formation. Further evaluation in-vivo is now warranted.  

Keywords: MR-Guided Focused Ultrasound, MR-Guided Interventions, Focused Ultrasound, Histotripsy

Motivation: Transcranial histotripsy requires pre-therapy targeting to ensure treatment at the intended region of interest. MR-ARFI and MR-Thermometry are used for targeting HIFU treatments, but are not tested for targeting of transcranial histotripsy treatments since they use different ultrasound treatment parameters.

Goal(s): We evaluate the accuracy of MR-thermometry and MR-ARFI for targeting histotripsy treatments in ex-vivo tissues.

Approach: Low intensity MR-thermometry and MR-ARFI were done prior to histotripsy treatment on bovine brain tissues. The lesion location was compared with the peak temperature/displacement to quantify targeting error.

Results: Both MR-thermometry and MR-ARFI can perform accurate targeting of histotripsy treatments with mean errors of 2mm.

Impact: Transcranial histotripsy is a promising non-invasive treatment method that requires pre-treatment targeting to ensure treatment delivery at the desired location. We showed that both low temperature MR-thermometry and MR-ARFI can be used to accurately estimate histotripsy treatment location.

Keywords: MR-Guided Focused Ultrasound, Focused Ultrasound, neuromodulation

Motivation: MR-ARFI is currently used to target focused ultrasound in the brain, but as a quantitative imaging method it has the potential to provide acoustic dosimetry if the measured tissue displacement can be related back to acoustic intensity or pressure. 

Goal(s):  Develop a method to calculate MR-ARFI images based on high-intensity acoustic simulations of focused ultrasound and finite element modeling.

Approach: We converted acoustic beam simulations to acoustic force, which was input to an FEM solver to calculate time-resolved tissue displacements, and subsequently MR-ARFI images. 

Results: The method enabled a lookup table calculation to recover acoustic intensity from MR-ARFI measurements

Impact: MR-ARFI is used to target focused ultrasound in the brain, but as a quantitative imaging method it could also provide dosimetry. We established the ability to relate simulated acoustic pressure fields to MR-ARFI images via finite element modeling.

Keywords: MR-Guided Focused Ultrasound, Focused Ultrasound

Motivation: The effectiveness of low-intensity focused ultrasound (LIFU) for neuromodulation may be compromised by inaccuracies in target localization and variable energy deposition caused by skull characteristics.

Goal(s): We aimed to improve the estimation of LIFU pressure using MR-acoustic radiation force imaging (ARFI) and test whether the adjusted LIFU intensity correlates with treatment outcomes in a behavioral economic task using sucrose pellets.

Approach: Eight female rats underwent LIFU targeting the nucleus accumbens (NAc) bilaterally with three different pressures. MR-ARFI confirmed the targeting and corrected the intensities during LIFU treatments.

Results: Improved correlations between behavioral outcomes and ARFI-adjusted LIFU intensity were observed.

Impact: Our preliminary findings demonstrate the beneficial effects for using MRI-ARFI not only to verify the FUS target location but also to refine LIFU intensities in neuromodulation procedures.

Keywords: MR-Guided Focused Ultrasound, Focused Ultrasound

Motivation: Previous MR-ARFI optimizations have incorporated tissue response times, but assumed those were constant in a homogenous medium. 

Goal(s): To demonstrate that tissue response times vary with ultrasound parameters such as frequency and depth, and to use those to optimize MR-ARFI. 

Approach: We measured displacement response time in a tissue-mimicking phantom at 0.5MHz and 1.0MHz ultrasound frequencies and depths of 40- and 60-mm. 

Results: With our tissue-mimicking phantom, the time constants were approximately 3ms and 6ms for 1.0MHz and 0.5MHz ultrasound frequencies, respectively. 

Impact: Ultrasound parameters of frequency and depth affect tissue response and can be used to optimize the acquisition to maximize SNR and minimize ultrasound dose.  

Keywords: MR-Guided Focused Ultrasound, Focused Ultrasound, Hyperthermia, focused ultrasound, bone metastasis

Motivation: Mild hyperthermia (HT) induced by magnetic resonance-guided focused ultrasound (MRgHIFU) before radiation therapy (RT) could act as adjuvant in pain relief for bone metastasis.

Goal(s): This study aims to evaluate the feasibility and safety of MRgHIFU procedure in one patient with a posterior right acetabulum

Approach: The target temperature elevation was 6°C. The targeting and temperature monitoring in near real-time were performed by MRI.

Results: The temperature elevation lasting 30-minutes was 6.01°C on average in bone metastasis. The MR-guidance allowed a safe and successful procedure without adverse events. The 25-minutes between HT end point and RT demonstrated the technical feasibility of the procedure.

Impact: This study is a first step to demonstrate MRgHIFU hyperthermia adjuvant to palliative radiotherapy of bone metastases. The non-invasive, safe and accurate use of focused ultrasound to sensitize tumors opens the way to more efficient application of the radiation dose.

Keywords: MR-Guided Focused Ultrasound, Focused Ultrasound, 2-photon microscopy

Motivation: The gold-standard for confirmation of blood-brain barrier (BBB) disruption after FUS is T1-weighted MRI with contrast enhancement. Conventional MRI-based metrics cannot differentiate between BBB leakage at the microvascular level, and thus there is a need for validation with higher spatial and temporal resolution modalities

Goal(s): Use 2-photon microscopy to investigate solute extravasation from the microvasculature into the parenchyma and paravascular space.

Approach: This project is an in-vivo validation of BBB opening after FUS using 2-photon microscopy and MRI.

Results: Results suggest BBB opening occurs in capillaries, and that dye extravasation can be measured within minutes of sonication.

Impact: Sub-micron imaging of the microvasculature after BBB opening will provide insight into which vessels are opened after FUS and improve pharmacokinetic understanding of the paravascular space around arterioles and venules.