Using Phase Data From MR Temperature Imaging to Visualize Anatomy During MRI-Guided Focused Ultrasound Neurosurgery

Magnetic resonance cavitation imaging for the monitoring of ultrasound therapies

Objective.Focused ultrasound (FUS) is a promising non-invasive therapeutic approach that can be used to generate thermal and non-thermal bioeffects. Several non-thermal FUS therapies rely on FUS-induced oscillations of microbubbles (MBs), a phenomenon referred to as cavitation. Cavitation monitoring in real time is essential to ensure both the efficacy and the safety of FUS therapies. This study aims to introduce a new magnetic resonance (MR) method for cavitation monitoring during FUS therapies.Approach.By finely synchronizing the FUS pulse with an accelerated turbo spin-echo MR sequence, the cavitation effect could be quantitatively estimated on the acquired images at 1-Hz refresh rate. The proposed method was assessed in vitro in a water bath. A series of FUS pulses were generated on a silicone tube filled with MBs at different acoustic pressures (0.07-2.07 MPa) and pulse durations (20-2000μs). MR images and passive cavitation detection (PCD) signals were simultaneously acquired for each FUS pulse.Main results.Inertial cavitation was found to induce a quantitatively interpretable signal loss on the MR image. The transition from stable to inertial cavitation was identified on MR cavitation maps with high repeatability. These results were found to be in good agreement with PCD measurements in terms of pressure thresholds between stable and inertial cavitation. MR cavitation imaging was shown to be sensitive to short and even ultrashort FUS pulses, from 2 ms down to 20μs. The presented theoretical model suggests that the signal loss in MR cavitation imaging relies on susceptibility changes related to the diameter of the oscillating MBs.Significance.The proposed MR cavitation imaging method can both locate and characterize cavitation activity. It has therefore the potential to improve the efficacy and safety of FUS therapies, particularly for localized drug delivery applications.

Ultrasound-Induced Membrane Hyperpolarization in Motor Axons and Muscle Fibers of the Crayfish Neuromuscular Junction

OBJECTIVE

Focused ultrasound (FUS) can modulate neuronal activity by depolarization or hyperpolarization. Although FUS-evoked depolarization has been studied extensively, the mechanisms underlying FUS-evoked hyperpolarization (FUSH) have received little attention. In the study described here, we developed a procedure using FUS to selectively hyperpolarize motor axons in crayfish. As a previous study had reported that these axons express mechano- and thermosensitive two-pore domain potassium (K2P) channels, we tested the hypothesis that K2P channels underlie FUSH.

METHODS

Intracellular recordings from a motor axon and a muscle fiber were obtained simultaneously from the crayfish opener neuromuscular preparation. FUSH was examined while K2P channel activities were modulated by varying temperature or by K2P channel blockers.

RESULTS

FUSH in the axons did not exhibit a coherent temperature dependence, consistent with predicted K2P channel behavior, although changes in the resting membrane potential of the same axons indicated well-behaved K2P channel temperature dependence. The same conclusion was supported by pharmacological data; namely, FUSH was not suppressed by K2P channel blockers. Comparison between the FUS-evoked responses recorded in motor axons and muscle fibers revealed that the latter exhibited very little FUSH, indicating that the FUSH was specific to the axons.

CONCLUSION

It is not likely that K2P channels are the underlying mechanism for FUSH in motor axons. Alternative mechanisms such as sonophore and axon-specific potassium channels were considered. Although the sonophore hypothesis could account for electrophysiological features of axonal recordings, it is not consistent with the lack of FUSH in muscle fibers. An axon-specific and mechanosensitive potassium channel is also a possible explanation.

Longitudinal MR imaging after unilateral MR-guided focused ultrasound thalamotomy: clinical and radiological correlation

Introduction

Magnetic-resonance-guided focused ultrasound (MRgFUS) thalamotomy uses multiple converging high-energy ultrasonic beams to produce thermal lesions in the thalamus. Early postoperative MR imaging demonstrates the location and extent of the lesion, but there is no consensus on the utility or frequency of postoperative imaging. We aimed to evaluate the evolution of MRgFUS lesions and describe the incidence, predictors, and clinical effects of lesion persistence in a large patient cohort.

Methods

A total of 215 unilateral MRgFUS thalamotomy procedures for essential tremor (ET) by a single surgeon were retrospectively analyzed. All patients had MR imaging 1 day postoperatively; 106 had imaging at 3 months and 32 had imaging at 1 year. Thin cut (2 mm) axial and coronal T2-weighted MRIs at these timepoints were analyzed visually on a binary scale for lesion presence and when visible, lesion volumes were measured. SWI and DWI sequences were also analyzed when available. Clinical outcomes including tremor scores and side effects were recorded at these same time points. We analyzed if patient characteristics (age, skull density ratio), preoperative tremor score, and sonication parameters influenced lesion evolution and if imaging characteristics correlated with clinical outcomes.

Results

Visible lesions were present in all patients 1 day post- MRgFUS and measured 307.4 ± 128.7 mm3. At 3 months, residual lesions (excluding patients where lesions were not visible) were 83.6% smaller and detectable in only 54.7% of patients (n = 58). At 1 year, residual lesions were detected in 50.0% of patients (n = 16) and were 90.7% smaller than 24 h and 46.5% smaller than 3 months. Lesions were more frequently visible on SWI (100%, n = 17), DWI (n = 38, 97.4%) and ADC (n = 36, 92.3%). At 3 months, fewer treatment sonications, higher maximum power, and greater distance between individual sonications led to larger lesion volumes. Volume at 24 h did not predict if a lesion was visible later. Lesion visibility at 3 months predicted sensory side effects but was not correlated with tremor outcomes.

Discussion

Overall, lesions are visible on T2-weighted MRI in about half of patients at both 3 months and 1 year post-MRgFUS thalamotomy. Certain sonication parameters significantly predicted persistent volume, but residual lesions did not correlate with tremor outcomes.

Focused Ultrasound Thalamotomy for Tremor in Parkinson's Disease: Outcomes in a Large, Prospective Cohort

BACKGROUND

Magnetic resonance guided focused ultrasound (MRgFUS) is United States Food and Drug Administration approved for the treatment of tremor-dominant Parkinson's disease (TdPD), but only limited studies have been described in practice.

OBJECTIVES

To report the largest prospective experience of unilateral MRgFUS thalamotomy for the treatment of medically refractory TdPD.

METHODS

Clinical outcomes of 48 patients with medically refractory TdPD who underwent MRgFUS thalamotomy were evaluated. Tremor outcomes were assessed using the Fahn-Tolosa-Marin scale and adverse effects were categorized using a structured questionnaire and clinical exam at 1 month (n = 44), 3 months (n = 34), 1 year (n = 22), 2 years (n = 5), and 3 years (n = 2). Patients underwent magnetic resonance imaging <24 hours post-procedure.

RESULTS

Significant tremor control persisted at all follow-ups (P < 0.001). All side effects were mild. At 3 months, these included gait imbalance (38.24%), sensory deficits (26.47%), motor weakness (17.65%), dysgeusia (5.88%), and dysarthria (5.88%), with some persisting at 1 year.

CONCLUSIONS

MRgFUS thalamotomy is an effective treatment for sustained tremor control in patients with TdPD. © 2023 International Parkinson and Movement Disorder Society.

Susceptibility phase imaging of deep gray matter: Presenting the effects of slice orientation

Susceptibility-weighted image (SWI) is a T2* gradient echo sequence, which is highly sensitive to substances that have magnetic properties. The phase and magnitude of SWI can play an important role in the diagnosis of several diseases. The phase data is highly affected by spatial variations in the main magnetic field of the magnetic resonance imaging (MRI) scanner. The axial acquisition is the frequent plane alignment while acquiring SWI in diagnostic imaging. Clinical requirements often lead to changing of the alignment angles due to variability in patient positioning and anatomy. For many patients undergoing brain MRI, the line of the anterior and posterior commissure AC-PC can vary in direction with respect to the transverse plane of the MRI system. We investigated whether there exist significant effect on phase data of SWI, and this is due to oblique orientation. The obtained results showed significant differences in phase values between axial and anatomically alignments.

Application of Ultrasound Combined with Magnetic Resonance Imaging in the Diagnosis and Grading of Patients with Prenatal Placenta Accreta

In order to study the clinical application value of placenta accreta (PIA) diagnosis and grading, the authors propose a method based on ultrasound combined with magnetic resonance imaging in the diagnosis and grading of prenatal placenta accreta patients. This method is adopted in materials and methods: a retrospective analysis of hospital patients with high suspicion of placenta accreta by clinical or ultrasonography between October 2019 and October 2021, the imaging and clinical data of 312 patients who underwent placental MRI examination. The MRI imaging data of all patients were jointly analyzed, and the main observation indicators are as follows: (1) dark zone in the placenta, (2) disruption of the border of the myometrium, (3) disruption of the myometrium, (4) abnormal blood vessels in the placenta, (5) enlargement of the lower part of the uterus, and (6) local bulge of the bladder/or invasion of the adjacent tissues of the uterus. The results show the following: in MRI combined with ultrasonography (P < 0.05), there was no statistical significance in the specificity and accuracy of MRI combined with ultrasound to diagnose PIA (P > 0.05). The comparison of graded diagnostic accuracy showed that in ultrasound alone < MRI alone < MRI combined with ultrasound, the differences were statistically significant (P < 0.05). Ultrasound combined with MRI in the diagnosis of placenta accreta is in good agreement with the clinical and surgical pathological results; MRI examination can be used as an important method for prenatal placenta accreta screening. MRI can classify placenta accreta to some extent.

Lesion location and lesion creation affect outcomes after focused ultrasound thalamotomy

MRI-guided focused ultrasound thalamotomy has been shown to be an effective treatment for medication refractory essential tremor. Here, we report a clinical-radiological analysis of 123 cases of MRI-guided focused ultrasound thalamotomy, and explore the relationships between treatment parameters, lesion characteristics and outcomes. All patients undergoing focused ultrasound thalamotomy by a single surgeon were included. The procedure was performed as previously described, and patients were followed for up to 1 year. MRI was performed 24 h post-treatment, and lesion locations and volumes were calculated. We retrospectively evaluated 118 essential tremor patients and five tremor-dominant Parkinson's disease patients who underwent thalamotomy. At 24 h post-procedure, tremor abated completely in the treated hand in 81 essential tremor patients. Imbalance, sensory disturbances and dysarthria were the most frequent acute adverse events. Patients with any adverse event had significantly larger lesions, while inferolateral lesion margins were associated with a higher incidence of motor-related adverse events. Twenty-three lesions were identified with irregular tails, often extending into the internal capsule; 22 of these patients experienced at least one adverse event. Treatment parameters and lesion characteristics changed with increasing surgeon experience. In later cases, treatments used higher maximum power (normalized to skull density ratio), accelerated more quickly to high power, and delivered energy over fewer sonications. Larger lesions were correlated with a rapid rise in both power delivery and temperature, while increased oedema was associated with rapid rise in temperature and the maximum power delivered. Total energy and total power did not significantly affect lesion size. A support vector regression was trained to predict lesion size and confirmed the most valuable predictors of increased lesion size as higher maximum power, rapid rise to high-power delivery, and rapid rise to high tissue temperatures. These findings may relate to a decrease in the energy efficiency of the treatment, potentially due to changes in acoustic properties of skull and tissue at higher powers and temperatures. We report the largest single surgeon series of focused ultrasound thalamotomy to date, demonstrating tremor relief and adverse events consistent with reported literature. Lesion location and volume impacted adverse events, and an irregular lesion tail was strongly associated with adverse events. High-power delivery early in the treatment course, rapid temperature rise, and maximum power were dominant predictors of lesion volume, while total power, total energy, maximum energy and maximum temperature did not improve prediction of lesion volume. These findings have critical implications for treatment planning in future patients.

Predicting ablation zones with multislice volumetric 2-D magnetic resonance thermal imaging

BACKGROUND

High-intensity focused ultrasound (HIFU) serves as a noninvasive stereotactic system for the ablation of brain metastases; however, treatments are limited to simple geometries and energy delivery is limited by the high acoustic attenuation of the calvarium. Minimally-invasive magnetic resonance-guided robotically-assisted (MRgRA) needle-based therapeutic ultrasound (NBTU) using multislice volumetric 2-D magnetic resonance thermal imaging (MRTI) overcomes these limitations and has potential to produce less collateral tissue damage than current methods.

OBJECTIVE

To correlate multislice volumetric 2-D MRTI volumes with histologically confirmed regions of tissue damage in MRgRA NBTU.

METHODS

Seven swine underwent a total of 8 frontal MRgRA NBTU lesions. MRTI ablation volumes were compared to histologic tissue damage on brain sections stained with 2,3,5-triphenyltetrazolium chloride (TTC). Bland-Altman analyses and correlation trends were used to compare MRTI and TTC ablation volumes.

RESULTS

Data from the initial and third swine's ablations were excluded due to sub-optimal tissue staining. For the remaining ablations (n = 6), the limits of agreement between the MRTI and histologic volumes ranged from -0.149 cm³ to 0.252 cm³ with a mean difference of 0.052 ± 0.042 cm³ (11.1%). There was a high correlation between the MRTI and histology volumes (r² = 0.831) with a strong linear relationship (r = 0.868).

CONCLUSION

We used a volumetric MRTI technique to accurately track thermal changes during MRgRA NBTU in preparation for human trials. Improved volumetric coverage with MRTI enhanced our delivery of therapy and has far-reaching implications for focused ultrasound in the broader clinical setting.

MRI-based thermal dosimetry based on single-slice imaging during focused ultrasound thalamotomy

Transcranial MRI-guided focused ultrasound (MRgFUS) is a noninvasive thermal ablation method approved for the treatment of essential tremor and tremor-dominant Parkinson's disease. This method uses MR temperature imaging (MRTI) to monitor the treatment. Accurately tracking the accumulated thermal dose is important for both safety and efficacy. Currently, MRTI is obtained in a single plane that varies between sonications, preventing direct tracking of the accumulated dose. In this work, we tested a method to estimate this dose during 120 MRgFUS treatments. This method used the MRTI to create simulated thermal images for sonications when the imaging plane was changed. This approach accurately predicted the lesion shapes. The mean Sørensen-Dice similarity coefficient between the lesion segmentations and dose regions at the 17 cumulative min at 43 °C (CEM43) threshold used by the device software was 0.82 but varied among different treatments (range: 0.34-0.95). Tissue swelling appeared to explain when mismatch occurred, although other errors probably contributed. Overall, the mean distance between the lesion segmentations and the 17 CEM43 dose contours was 0.37 ± 0.57 mm. The probability for thermal damage was estimated to be 50% at 13.6 CEM43 and a maximum temperature of 48.6 °C. Due to large thermal gradients, which exceeded 99 CEM43/mm on average, the area where the probability for thermal damage was uncertain was narrow. Overall these results show that the 17 CEM43 threshold is on average a good predictor for thermal lesions, although there will always be a narrow margin where the fate of the tissue is uncertain.