Future potential of MRI-guided focused ultrasound brain surgery

Numerical Evaluation of the Influence of Skull Heterogeneity on Transcranial Ultrasonic Focusing

In transcranial penetration, ultrasound undergoes refraction, diffraction, multi-reflection, and mode conversion. These factors lead to phase aberration and waveform distortion, which impede the realization of transcranial ultrasonic imaging and therapy. Ray tracing has been used to correct the phase aberration and is computationally more efficient than traditional full-wave simulation. However, when ray tracing has been used for transcranial investigation, it has generally been on the premise that the skull medium is homogeneous. To find suitable homogeneity that balances computational speed and accuracy, the present work investigates how the focus deviates after phase-aberration compensation with ray tracing using time-reversal theory. The waveforms are synthetized with ray tracing for phase aberration, by which the properties of the skull bone are simplified for refraction calculation as those of either (i) the cortical bone or (ii) the mean of the entire skull bone, and the focusing accuracy is evaluated for each hypothesis. The propagation of ultrasound for transcranial focusing is simulated with the elastic model using the k-space pseudospectral method. Unlike the fluid model, the elastic model does not omit shear waves in the skull bones, and the influence of that omission is investigated, with the fluid model resulting in a focal deflection of 0.5 mm. The focusing deviations are huge when the properties of the skull bone are idealized with ray tracing as those of the mean of the entire skull bone. The focusing accuracy improves when the properties of the skull bone are idealized as those of the cortical bone. The results reveal minimal deviation (8.6, 3.9, and 3.2% in the three Cartesian coordinates) in the focal region and suggest that transcranial focusing deflections are caused mostly by ultrasonic refraction on the surface of the skull bone. A heterogeneous skull bone causes wave bending but minimal focusing deflection. The proposed simplification of a homogeneous skull bone is more accurate for transcranial ultrasonic path estimation and offers promising applications in transcranial ultrasonic focusing and imaging.

Neurosurgical Applications of High-Intensity Focused Ultrasound with Magnetic Resonance Thermometry

Magnetic resonance guided focused ultrasound surgery (MRgFUS) has potential noninvasive effects on targeted tissue. MRgFUS integrates MRI and focused ultrasound surgery (FUS) into a single platform. MRI enables visualization of the target tissue and monitors ultrasound-induced effects in near real-time during FUS treatment. MRgFUS may serve as an adjunct or replace invasive surgery and radiotherapy for specific conditions. Its thermal effects ablate tumors in locations involved in movement disorders and essential tremors. Its nonthermal effects increase blood-brain barrier permeability to enhance delivery of therapeutics and other molecules.

Numerical evaluation of the effect of electronically steering a phased array transducer: axially post-focal shifting

PURPOSE

HIFU has been emerging as an effective and safe modality for the treatment of solid tumours and cancers. The focus shifting range of phased array HIFU transducer is an important safety concern because of the presence of grating lobe in the pre-focal region. However, previous studies were only based on linear acoustic wave model.

MATERIALS AND METHODS

The nonlinear wave propagation from a 256-element phased array through multiple layered media was simulated using the angular spectrum approach (ASA) in marching fractional steps with the consideration of diffraction, attenuation and non-linearity effects by a second-order operator splitting scheme. The distribution of acoustic intensities, temperature elevations, lesion sizes and grating lobe levels were calculated at various axially post-focal shifting distances and driving frequencies.

RESULTS

Axially shifting HIFU focus leads to significant increase of the acoustic intensity at the grating lobe, but decrease at the main lobe. The influences on the acoustic field, thermal field and lesion sizes are determined by the shifting distance and driving frequency, and variations can be fit monotonically and linearly. Prediction accuracies by simple regression models are satisfactory. Irreversible tissue coagulation could be generated by the grating lobe at certain conditions.

CONCLUSIONS

The established nonlinear wave propagation algorithm allows the accurate description of HIFU field and consequently the evaluation of grating lobe and steerability of focus. The influence of focus shifting may be predicted simply. The treatment planning of phased array HIFU ablation could be optimised by setting the appropriate exposure and focus scanning schemes.

Bilateral thermal capsulotomy with MR-guided focused ultrasound for patients with treatment-refractory obsessive-compulsive disorder: a proof-of-concept study

Despite optimal pharmacotherapy and cognitive-behavioral treatments, a proportion of patients with obsessive-compulsive disorder (OCD) remain refractory to treatment. Neurosurgical ablative or nondestructive stimulation procedures to treat these refractory patients have been investigated. However, despite the potential benefits of these surgical procedures, patients show significant surgery-related complications. This preliminary study investigated the use of bilateral thermal capsulotomy for patients with treatment-refractory OCD using magnetic resonance-guided focused ultrasound (MRgFUS) as a novel, minimally invasive, non-cranium-opening surgical technique. Between February and May 2013, four patients with medically refractory OCD were treated with MRgFUS to ablate the anterior limb of the internal capsule. Patients underwent comprehensive neuropsychological evaluations and imaging at baseline, 1 week, 1 month and 6 months following treatment. Outcomes were measured with the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), the Hamilton Rating Scale for Depression (HAM-D) and the Hamilton Rating Scale for Anxiety (HAM-A), and treatment-related adverse events were evaluated. The results showed gradual improvements in Y-BOCS scores (a mean improvement of 33%) over the 6-month follow-up period, and all patients showed almost immediate and sustained improvements in depression (a mean reduction of 61.1%) and anxiety (a mean reduction of 69.4%). No patients demonstrated any side effects (physical or neuropsychological) in relation to the procedure. In addition, there were no significant differences found in the comprehensive neuropsychological test scores between the baseline and 6-month time points. This study demonstrates that bilateral thermal capsulotomy with MRgFUS can be used without inducing side effects to treat patients with medically refractory OCD. If larger trials validate the safety, effectiveness and long-term durability of this new approach, this procedure could considerably change the clinical management of treatment-refractory OCD.

Transcranial MRI-Guided Focused Ultrasound: A Review of the Technologic and Neurologic Applications

OBJECTIVE

This article reviews the physical principles of MRI-guided focused ultra-sound and discusses current and potential applications of this exciting technology.

CONCLUSION

MRI-guided focused ultrasound is a new minimally invasive method of targeted tissue thermal ablation that may be of use to treat central neuropathic pain, essential tremor, Parkinson tremor, and brain tumors. The system has also been used to temporarily disrupt the blood-brain barrier to allow targeted drug delivery to brain tumors.

Protective role of female gender in programmed accelerated renal aging in the rat

The aging kidney exhibits a progressive decline in glomerular filtration rate, accompanied by inflammatory and oxidative damage. We hypothesized that accelerated, age-related progression of renal injury is ovarian hormones-dependant. To address this we used an established model of developmentally programmed accelerated renal aging in the rat, superimposed by ovariectomy to assess interactions between ovarian hormones and the aging process. Under our experimental conditions, we found that kidney function worsens with age, that is GFR reduces over 18 month analyzed time-course and this was worsened by fetal exposure to maternal low-protein diet and absence of estrogen. Reduction in GFR was followed by increases in albuminuria, proteinuria, inflammatory markers, and tissue carbonyls, all suggesting inflammatory response and oxidative stress. This was associated with changes in AGTR2 expression which was greater at 18 months of age compared to earlier time points, but in MLP offspring only. Our studies show an influence of ovarian hormones on programmed accelerated renal aging and the AGTR2 across the lifespan. The main findings are that ovariectomy is a risk factor for increased aging-related renal injury and that this and oxidative damage might be related to changes in AGTR2 expression.

Magnetic resonance-guided focused ultrasound treatment of facet joint pain: summary of preclinical phase

STUDY DESIGN

A phantom experiment, two thermocouple experiments, three in vivo pig experiments, and a simulated treatment on a healthy human volunteer were conducted to test the feasibility, safety, and efficacy of magnetic resonance-guided focused ultrasound (MRgFUS) for treating facet joint pain.

OBJECTIVE

The goal of the current study was to develop a novel method for accurate and safe noninvasive facet joint ablation using MRgFUS.

SUMMARY OF BACKGROUND DATA

Facet joints are a common source of chronic back pain. Direct facet joint interventions include medial branch nerve ablation and intra-articular injections, which are widely used, but limited in the short and long term. MRgFUS is a breakthrough technology that enables accurate delivery of high-intensity focused ultrasound energy to create a localized temperature rise for tissue ablation, using MR guidance for treatment planning and real-time feedback.

METHODS

We validated the feasibility, safety, and efficacy of MRgFUS for facet joint ablation using the ExAblate 2000® System (InSightec Ltd., Tirat Carmel, Israel) and confirmed the system's ability to ablate the edge of the facet joint and all terminal nerves innervating the joint. A phantom experiment, two thermocouple experiments, three in vivo pig experiments, and a simulated treatment on a healthy human volunteer were conducted.

RESULTS

The experiments showed that targeting the facet joint with energies of 150-450 J provides controlled and accurate heating at the facet joint edge without penetration to the vertebral body, spinal canal, or root foramina. Treating with reduced diameter of the acoustic beam is recommended since a narrower beam improves access to the targeted areas.

CONCLUSIONS

MRgFUS can safely and effectively target and ablate the facet joint. These results are highly significant, given that this is the first study to demonstrate the potential of MRgFUS to treat facet joint pain.

Magnetic resonance-guided focused ultrasound: a new technology for clinical neurosciences

Transcranial MRI-guided focused ultrasound (TcMRgFUS) is an old idea but a new technology that may change the entire clinical field of the neurosciences. TcMRgFUS has no cumulative effect, and it is applicable for repeatable treatments, controlled by real-time dosimetry, and capable of immediate tissue destruction. Most importantly, it has extremely accurate targeting and constant monitoring. It is potentially more precise than proton beam therapy and definitely more cost effective. Neuro-oncology may be the most promising area of future TcMRgFUS applications.

MR-acoustic radiation force imaging (MR-ARFI) and susceptibility weighted imaging (SWI) to visualize calcifications in ex vivo swine brain

PURPOSE

To present the use of MR-acoustic radiation force imaging (MR-ARFI) and susceptibility weighted imaging (SWI) to visualize calcifications in ex vivo brain tissue as a planning indicator for MR-guided focused ultrasound (MRgFUS).

MATERIALS AND METHODS

Calcifications were implanted in ex vivo swine brain and imaged using SWI, MR-ARFI, and computed tomography (CT). SWI-filtered phase images used 3D gradient recalled echo (GRE) images with a Fourier-based unwrapping algorithm. The MR-ARFI pulse sequence used a 2DFT spin-echo with repeated bipolar encoding gradients in the direction of the longitudinal ultrasound beam. MR-ARFI interrogations scanned a subregion (14 × 10 × 12 mm) of the brain surrounding the calcification. They were combined into a single displacement weighted map, using the sum of squares method. Calcification size estimates were based on image profiles plotted along the ±x and ±z direction, at the full-width half-maximum.

RESULTS

Both MR-ARFI and SWI were able to visualize the calcifications. The contrast ratio was 150 for CT, 12 for SWI, and 12 for MR-ARFI. Profile measures were 1.35 × 1.28 mm on CT, 1.24 × 1.73 mm on SWI, and 2.45 × 3.02 mm on MR-ARFI. MR-ARFI displacement showed a linear increase with acoustic power (20-80 W), and also increased with calcification size.

CONCLUSION

The use of SWI-filtered phase and MR-ARFI have the potential to provide a clinical indicator of calcification relevance in the planning of a transcranial MRgFUS treatment.

Comparison of temperature processing methods for monitoring focused ultrasound ablation in the brain

PURPOSE

To investigate the performance of different reconstruction methods for monitoring temperature changes during transcranial magnetic resonance imaging (MRI)-guided focused ultrasound (MRgFUS).

MATERIALS AND METHODS

Four different temperature reconstruction methods were compared in volunteers (without heating) and patients undergoing transcranial MRgFUS: single baseline subtraction, multibaseline subtraction, hybrid single baseline/referenceless reconstruction, and hybrid multibaseline/referenceless reconstruction. Absolute temperature error and temporal temperature uncertainty of the different reconstruction methods were analyzed and compared.

RESULTS

Absolute temperature errors and temporal temperature uncertainty were highest with single baseline subtraction and lowest with hybrid multibaseline/referenceless reconstruction in all areas of the brain. Pulsation of the brain and susceptibility changes from tongue motion or swallowing caused substantial temperature errors when single or multibaseline subtraction was used, which were much reduced when the referenceless component was added to the reconstruction.

CONCLUSION

Hybrid multibaseline/referenceless thermometry accurately measures temperature changes in the brain with fewer artifacts and errors due to motion than pure baseline subtraction methods.

MR temperature imaging of nanoshell mediated laser ablation

Minimally invasive thermal therapy using high-power diode lasers is an active area of clinical research. Gold nanoshells (AuNS) can be tuned to absorb light in the range used for laser ablation and may facilitate more conformal tumor heating and sparing of normal tissue via enhanced tumor specific heating. This concept was investigated in a xenograft model of prostate cancer (PC-3) using MR temperature imaging (MRTI) in a 1.5T scanner to characterize the spatiotemporal temperature distribution resulting from nanoparticle mediated heating. Tumors with and without intravenously injected AuNS were exposed to an external laser tuned to 808 nm for 180 sec at 4 W/cm(2) under real-time monitoring with proton resonance frequency shift based MRTI. Microscopy indicated that these nanoparticles (140-150 nm) accumulated passively in the tumor and remained close to the tumor microvasculature. MRTI measured a statistically significant (p < 0.001) increase in maximum temperature in the tumor cortex (mean = 21 ± 7°C) in +AuNS tumors versus control tumors. Analysis of the temperature maps helped demonstrate that the overall distribution of temperature within +AuNS tumors was demonstrably higher versus control, and resulted in damage visible on histopathology. This research demonstrates that passive uptake of intravenously injected AuNS in PC-3 xenografts converts the tumor vasculature into a potent heating source for nanoparticle mediated ablation at power levels which do not generate significant damage in normal tissue. When used in conjunction with MRTI, this has implications for development and validation of more conformal delivery of therapy for interstitial laser ablations.

Time-reversal transcranial ultrasound beam focusing using a k-space method

This paper proposes the use of a k-space method to obtain the correction for transcranial ultrasound beam focusing. Mirroring past approaches, a synthetic point source at the focal point is numerically excited, and propagated through the skull, using acoustic properties acquired from registered computed tomography of the skull being studied. The received data outside the skull contain the correction information and can be phase conjugated (time reversed) and then physically generated to achieve a tight focusing inside the skull, by assuming quasi-plane transmission where shear waves are not present or their contribution can be neglected. Compared with the conventional finite-difference time-domain method for wave propagation simulation, it will be shown that the k-space method is significantly more accurate even for a relatively coarse spatial resolution, leading to a dramatically reduced computation time. Both numerical simulations and experiments conducted on an ex vivo human skull demonstrate that precise focusing can be realized using the k-space method with a spatial resolution as low as only 2.56 grid points per wavelength, thus allowing treatment planning computation on the order of minutes.

Multimodality intraoperative MRI for brain tumor surgery

Intraoperative MRI has already fundamentally changed the way current brain tumor surgery is performed. The ability to integrate high-field MRI into the operating room has allowed intraoperative MRI to emerge as an important adjunct to CNS tumor treatment. Furthermore, the ability of MRI to successfully couple with molecular imaging (PET and/or optical imaging), neuroendoscopy and therapeutic devices, such as focused ultrasound, will allow it to emerge as an important image-guidance modality for improving brain tumor therapy and outcomes.