Patterns of Thermal Deposition in the Skull During Transcranial Focused Ultrasound Surgery

Experimental demonstration of passive acoustic imaging in the human skull cavity using CT-based aberration corrections

PURPOSE

Experimentally verify a previously described technique for performing passive acoustic imaging through an intact human skull using noninvasive, computed tomography (CT)-based aberration corrections Jones et al. [Phys. Med. Biol. 58, 4981-5005 (2013)].

METHODS

A sparse hemispherical receiver array (30 cm diameter) consisting of 128 piezoceramic discs (2.5 mm diameter, 612 kHz center frequency) was used to passively listen through ex vivo human skullcaps (n = 4) to acoustic emissions from a narrow-band fixed source (1 mm diameter, 516 kHz center frequency) and from ultrasound-stimulated (5 cycle bursts, 1 Hz pulse repetition frequency, estimated in situ peak negative pressure 0.11-0.33 MPa, 306 kHz driving frequency) Definity™ microbubbles flowing through a thin-walled tube phantom. Initial in vivo feasibility testing of the method was performed. The performance of the method was assessed through comparisons to images generated without skull corrections, with invasive source-based corrections, and with water-path control images.

RESULTS

For source locations at least 25 mm from the inner skull surface, the modified reconstruction algorithm successfully restored a single focus within the skull cavity at a location within 1.25 mm from the true position of the narrow-band source. The results obtained from imaging single bubbles are in good agreement with numerical simulations of point source emitters and the authors' previous experimental measurements using source-based skull corrections O'Reilly et al. [IEEE Trans. Biomed. Eng. 61, 1285-1294 (2014)]. In a rat model, microbubble activity was mapped through an intact human skull at pressure levels below and above the threshold for focused ultrasound-induced blood-brain barrier opening. During bursts that led to coherent bubble activity, the location of maximum intensity in images generated with CT-based skull corrections was found to deviate by less than 1 mm, on average, from the position obtained using source-based corrections.

CONCLUSIONS

Taken together, these results demonstrate the feasibility of using the method to guide bubble-mediated ultrasound therapies in the brain. The technique may also have application in ultrasound-based cerebral angiography.

Comparison of analytical and numerical approaches for CT-based aberration correction in transcranial passive acoustic imaging

Computed tomography (CT)-based aberration corrections are employed in transcranial ultrasound both for therapy and imaging. In this study, analytical and numerical approaches for calculating aberration corrections based on CT data were compared, with a particular focus on their application to transcranial passive imaging. Two models were investigated: a three-dimensional full-wave numerical model (Connor and Hynynen 2004 IEEE Trans. Biomed. Eng. 51 1693-706) based on the Westervelt equation, and an analytical method (Clement and Hynynen 2002 Ultrasound Med. Biol. 28 617-24) similar to that currently employed by commercial brain therapy systems. Trans-skull time delay corrections calculated from each model were applied to data acquired by a sparse hemispherical (30 cm diameter) receiver array (128 piezoceramic discs: 2.5 mm diameter, 612 kHz center frequency) passively listening through ex vivo human skullcaps (n  =  4) to emissions from a narrow-band, fixed source emitter (1 mm diameter, 516 kHz center frequency). Measurements were taken at various locations within the cranial cavity by moving the source around the field using a three-axis positioning system. Images generated through passive beamforming using CT-based skull corrections were compared with those obtained through an invasive source-based approach, as well as images formed without skull corrections, using the main lobe volume, positional shift, peak sidelobe ratio, and image signal-to-noise ratio as metrics for image quality. For each CT-based model, corrections achieved by allowing for heterogeneous skull acoustical parameters in simulation outperformed the corresponding case where homogeneous parameters were assumed. Of the CT-based methods investigated, the full-wave model provided the best imaging results at the cost of computational complexity. These results highlight the importance of accurately modeling trans-skull propagation when calculating CT-based aberration corrections. Although presented in an imaging context, our results may also be applicable to the problem of transmit focusing through the skull.

Sampling strategies for subsampled segmented EPI PRF thermometry in MR guided high intensity focused ultrasound

PURPOSE

To investigate k-space subsampling strategies to achieve fast, large field-of-view (FOV) temperature monitoring using segmented echo planar imaging (EPI) proton resonance frequency shift thermometry for MR guided high intensity focused ultrasound (MRgHIFU) applications.

METHODS

Five different k-space sampling approaches were investigated, varying sample spacing (equally vs nonequally spaced within the echo train), sampling density (variable sampling density in zero, one, and two dimensions), and utilizing sequential or centric sampling. Three of the schemes utilized sequential sampling with the sampling density varied in zero, one, and two dimensions, to investigate sampling the k-space center more frequently. Two of the schemes utilized centric sampling to acquire the k-space center with a longer echo time for improved phase measurements, and vary the sampling density in zero and two dimensions, respectively. Phantom experiments and a theoretical point spread function analysis were performed to investigate their performance. Variable density sampling in zero and two dimensions was also implemented in a non-EPI GRE pulse sequence for comparison. All subsampled data were reconstructed with a previously described temporally constrained reconstruction (TCR) algorithm.

RESULTS

The accuracy of each sampling strategy in measuring the temperature rise in the HIFU focal spot was measured in terms of the root-mean-square-error (RMSE) compared to fully sampled "truth." For the schemes utilizing sequential sampling, the accuracy was found to improve with the dimensionality of the variable density sampling, giving values of 0.65 °C, 0.49 °C, and 0.35 °C for density variation in zero, one, and two dimensions, respectively. The schemes utilizing centric sampling were found to underestimate the temperature rise, with RMSE values of 1.05 °C and 1.31 °C, for variable density sampling in zero and two dimensions, respectively. Similar subsampling schemes with variable density sampling implemented in zero and two dimensions in a non-EPI GRE pulse sequence both resulted in accurate temperature measurements (RMSE of 0.70 °C and 0.63 °C, respectively). With sequential sampling in the described EPI implementation, temperature monitoring over a 192×144×135 mm3 FOV with a temporal resolution of 3.6 s was achieved, while keeping the RMSE compared to fully sampled "truth" below 0.35 °C.

CONCLUSIONS

When segmented EPI readouts are used in conjunction with k-space subsampling for MR thermometry applications, sampling schemes with sequential sampling, with or without variable density sampling, obtain accurate phase and temperature measurements when using a TCR reconstruction algorithm. Improved temperature measurement accuracy can be achieved with variable density sampling. Centric sampling leads to phase bias, resulting in temperature underestimations.

Transcranial Assessment and Visualization of Acoustic Cavitation: Modeling and Experimental Validation

The interaction of ultrasonically-controlled microbubble oscillations with tissues and biological media has been shown to induce a wide range of bioeffects that may have significant impact on therapy and diagnosis of brain diseases and disorders. However, the inherently non-linear microbubble oscillations combined with the micrometer and microsecond scales involved in these interactions and the limited methods to assess and visualize them transcranially hinder both their optimal use and translation to the clinics. To overcome these challenges, we present a framework that combines numerical simulations with multimodality imaging to assess and visualize the microbubble oscillations transcranially. In the present work, microbubble oscillations were studied with an integrated US and MR imaging guided clinical FUS system. A high-resolution brain CT scan was also co-registered to the US and MR images and the derived acoustic properties were used as inputs to two- and three-dimensional Finite Difference Time Domain simulations that matched the experimental conditions and geometry. Synthetic point sources by either a Gaussian function or the output of a microbubble dynamics model were numerically excited and propagated through the skull towards a virtual US imaging array. Using passive acoustic mapping (PAM) that was refined to incorporate variable speed of sound, we were able to correct the aberrations introduced by the skull and substantially improve the PAM resolution. The good agreement between the simulations incorporating microbubble emissions and experimentally-determined PAMs suggest that this integrated approach can provide a clinically-relevant framework and more control over this nonlinear and dynamic process.

Feasibility and Safety of MR-Guided Focused Ultrasound Lesioning in the Setting of Deep Brain Stimulation

BACKGROUND

Patients treated with deep brain stimulation (DBS) often develop symptom progression. If safe, focused ultrasound (FUS) lesioning could be used for patients unable to undergo further DBS surgery.

OBJECTIVE

To test the feasibility and safety of MR-guided FUS surgery in the setting of a previously implanted DBS system.

METHODS

Three preclinical experiments were designed to test feasibility and safety. Hydrogels were implanted with an electrode, and FUS lesions were targeted adjacently. Cadavers were implanted with a thalamic electrode, and FUS lesions were targeted in the contralateral thalamus. Finally, DBS systems were implanted in swine, and FUS lesioning was targeted to the contralateral thalamus, MRI was used to assess the treatments, and histological analyses were performed at 2 days and at 1 month.

RESULTS

In gel experiments and cadavers, FUS resulted in target heating to 29-32°C without any heating at the electrode. In animal experiments, there were no FUS-related MRI signal changes near the electrode. Histological analysis showed typical FUS lesions with no evidence of damage surrounding the electrode tracts.

CONCLUSIONS

FUS is feasible in the setting of a preimplanted DBS device. There was minimal heating of the device during the procedure and no apparent FUS-related tissue injury. © 2015 S. Karger AG, Basel.

Safety of thrombolytic therapy with rt-PA and transcranial color Doppler ultrasound (TCCS) combined with microbubbles: a histopathologic study on rabbit brain tissues

OBJECTIVE

To evaluate effect of thrombolytic therapy with rt-PA (recombinant tissue plasminogen activator) and transcranial color Doppler ultrasound (TCCS) combined with microbubbles on histology of brain tissue.

METHODS

New Zealand rabbits were subjected to TCCS based thrombolytic therapy, in 8 groups depending on dose of rt-PA, exposure duration of TCCS and presence of attenuation by skull bone window, 2 animals/group: (1) skull+1/2 rt-PA+TCCS+MBs, 10 min, (2) skull+rt-PA+TCCS+MBs, 10 min, (3) skull+1/2 rt-PA+TCCS+MBs, 20 min, (4) skull+rt-PA+TCCS+MBs, 20 min, (5) skull+1/2 rt-PA+TCCS+MBs, 30 min, (6) skull+rt-PA+TCCS+MBs, 30 min, (7) 1/2 rt-PA+TCCS+MBs, 10 min, (8) 1/2 rt-PA+TCCS+MBs, 20 min. The brain tissues were harvested after therapies and submitted for microscopic, electronic microscope and immunohistochemical examination. The histological changes were scored.

RESULTS

TCCS caused exposure duration dependent brain tissue damage. With attenuation by bone window, TCCS based therapies for 10-20 min caused minimal tissue damage. However, significant tissue damage was observed upon TCCS for 30 min in presence of skull bone window, presenting as hemorrhage, misdistribution of organelles, demyelination of nerve fibers, and thinning of basement membrane in blood-brain barrier, which was milder than that after 20 min of exposure to TCCS in absence of bone window. Dose of rt-PA did not affect brain histology in all groups.

CONCLUSION

Short treatment of brain tissue with TCCS through a bone window is relative safe. And skull bone window protected brain tissue from TCCS induced damage.

Numerical simulations of clinical focused ultrasound functional neurosurgery

A computational model utilizing grid and finite difference methods were developed to simulate focused ultrasound functional neurosurgery interventions. The model couples the propagation of ultrasound in fluids (soft tissues) and solids (skull) with acoustic and visco-elastic wave equations. The computational model was applied to simulate clinical focused ultrasound functional neurosurgery treatments performed in patients suffering from therapy resistant chronic neuropathic pain. Datasets of five patients were used to derive the treatment geometry. Eight sonications performed in the treatments were then simulated with the developed model. Computations were performed by driving the simulated phased array ultrasound transducer with the acoustic parameters used in the treatments. Resulting focal temperatures and size of the thermal foci were compared quantitatively, in addition to qualitative inspection of the simulated pressure and temperature fields. This study found that the computational model and the simulation parameters predicted an average of 24 ± 13% lower focal temperature elevations than observed in the treatments. The size of the simulated thermal focus was found to be 40 ± 13% smaller in the anterior-posterior direction and 22 ± 14% smaller in the inferior-superior direction than in the treatments. The location of the simulated thermal focus was off from the prescribed target by 0.3 ± 0.1 mm, while the peak focal temperature elevation observed in the measurements was off by 1.6 ± 0.6 mm. Although the results of the simulations suggest that there could be some inaccuracies in either the tissue parameters used, or in the simulation methods, the simulations were able to predict the focal spot locations and temperature elevations adequately for initial treatment planning performed to assess, for example, the feasibility of sonication. The accuracy of the simulations could be improved if more precise ultrasound tissue properties (especially of the skull bone) could be obtained.

MR-guided focused ultrasound surgery, present and future

MR-guided focused ultrasound surgery (MRgFUS) is a quickly developing technology with potential applications across a spectrum of indications traditionally within the domain of radiation oncology. Especially for applications where focal treatment is the preferred technique (for example, radiosurgery), MRgFUS has the potential to be a disruptive technology that could shift traditional patterns of care. While currently cleared in the United States for the noninvasive treatment of uterine fibroids and bone metastases, a wide range of clinical trials are currently underway, and the number of publications describing advances in MRgFUS is increasing. However, for MRgFUS to make the transition from a research curiosity to a clinical standard of care, a variety of challenges, technical, financial, clinical, and practical, must be overcome. This installment of the Vision 20∕20 series examines the current status of MRgFUS, focusing on the hurdles the technology faces before it can cross over from a research technique to a standard fixture in the clinic. It then reviews current and near-term technical developments which may overcome these hurdles and allow MRgFUS to break through into clinical practice.

Phase-shift perfluorocarbon agents enhance high intensity focused ultrasound thermal delivery with reduced near-field heating

Ultrasound contrast agents are known to enhance high intensity focused ultrasound (HIFU) ablation, but these perfluorocarbon microbubbles are limited to the vasculature, have a short half-life in vivo, and may result in unintended heating away from the target site. Herein, a nano-sized (100-300 nm), dual perfluorocarbon (decafluorobutane/dodecafluoropentane) droplet that is stable, is sufficiently small to extravasate, and is convertible to micron-sized bubbles upon acoustic activation was investigated. Microbubbles and nanodroplets were incorporated into tissue-mimicking acrylamide-albumin phantoms. Microbubbles or nanodroplets at 0.1 × 10(6) per cm(3) resulted in mean lesion volumes of 80.4 ± 33.1 mm(3) and 52.8 ± 14.2 mm(3) (mean ± s.e.), respectively, after 20 s of continuous 1 MHz HIFU at a peak negative pressure of 4 MPa, compared to a lesion volume of 1.0 ± 0.8 mm(3) in agent-free control phantoms. Magnetic resonance thermometry mapping during HIFU confirmed undesired surface heating in phantoms containing microbubbles, whereas heating occurred at the acoustic focus of phantoms containing the nanodroplets. Maximal change in temperature at the target site was enhanced by 16.9% and 37.0% by microbubbles and nanodroplets, respectively. This perfluorocarbon nanodroplet has the potential to reduce the time to ablate tumors by one-third during focused ultrasound surgery while also safely enhancing thermal deposition at the target site.

Ultrasound enhanced drug delivery to the brain and central nervous system

There is an increasing interest in the use of ultrasound to enhance drug delivery to the brain and central nervous system. Disorders of the brain and CNS historically have had poor response to drug therapy due to the presence of the blood-brain barrier (BBB). Techniques for circumventing the BBB are typically highly invasive or involve disrupting large portions of the BBB, exposing the brain to pathogens. Ultrasound can be non-invasively delivered to the brain through the intact skull. When combined with preformed microbubbles, ultrasound can safely induce transient, localised and reversible disruption of the BBB, allowing therapeutics to be delivered. Investigations to date have shown positive response to ultrasound BBB disruption combined with therapeutic agent delivery in rodent models of primary and metastatic brain cancer and Alzheimer's disease. Recent work in non-human primates has demonstrated that the technique is feasible for use in humans. This review examines the current status of drug delivery to the brain and CNS both by disruption of the BBB, and by ultrasound enhancement of drug delivery through the already compromised BBB. Cellular and physical mechanisms of disruption are discussed, as well as treatment technique, safety and monitoring.

High-intensity focused ultrasound (HIFU) for dissolution of clots in a rabbit model of embolic stroke

It is estimated that only 2-6% of patients receive thrombolytic therapy for acute ischemic stroke suggesting that alternative therapies are necessary. In this study, we investigate the potential for high intensity focused ultrasound (HIFU) to initiate thrombolysis in an embolic model of stroke. Iron-loaded blood clots were injected into the middle cerebral artery (MCA) of New Zealand White rabbits, through the internal carotid artery and blockages were confirmed by angiography. MRI was used to localize the iron-loaded clot and target the HIFU beam for treatment. HIFU pulses (1.5 MHz, 1 ms bursts, 1 Hz pulse repetition frequency, 20 s duration) were applied to initiate thrombolysis. Repeat angiograms and histology were used to assess reperfusion and vessel damage. Using 275 W of acoustic power, there was no evidence of reperfusion in post-treatment angiograms of 3 rabbits tested. In a separate group of animals, 415 W of acoustic power was applied and reperfusion was observed in 2 of the 4 (50%) animals treated. In the last group of animals, acoustic power was further increased to 550 W, which led to the reperfusion in 5 of 7 (∼70%) animals tested. Histological analysis confirmed that the sonicated vessels remained intact after HIFU treatment. Hemorrhage was detected outside of the sonication site, likely due to the proximity of the target vessel with the base of the rabbit skull. These results demonstrate the feasibility of using HIFU, as a stand-alone method, to cause effective thrombolysis without immediate damage to the targeted vessels. HIFU, combined with imaging modalities used to identify and assess stroke patients, could dramatically reduce the time to achieve flow restoration in patients thereby significantly increasing the number of patients which benefit from thrombolysis treatments.

The design of a focused ultrasound transducer array for the treatment of stroke: a simulation study

High intensity focused ultrasound (HIFU) is capable of mechanically disintegrating blood clots at high pressures. Safe thrombolysis may require frequencies higher than those currently utilized by transcranial HIFU. Since the attenuation and focal distortion of ultrasound in bone increases at higher frequencies, resulting focal pressures are diminished. This study investigated the feasibility of using transcranial HIFU for the non-invasive treatment of ischemic stroke. The use of large aperture, 1.1-1.5 MHz phased arrays in targeting four clinically relevant vessel locations was simulated. Resulting focal sizes decreased with frequency, producing a maximum -3 dB depth of field and lateral width of 2.0 and 1.2 mm, respectively. Mean focal gains above an order of magnitude were observed in three of four targets and transducer intensities required to achieve thrombolysis were determined. Required transducer element counts are about an order of magnitude higher than what currently exists and so, although technically feasible, new arrays would need to be developed to realize this as a treatment modality for stroke.

Early rate of contrast extravasation in patients with intracerebral hemorrhage

BACKGROUND AND PURPOSE

For patients with ICH, knowing the rate of CT contrast extravasation may provide insight into the pathophysiology of hematoma expansion. This study assessed whether the PCT-derived PS can measure different rates of CT contrast extravasation for admission CTA spot signs, PCCT, PCL, and regions without extravasation in patients with ICH.

MATERIALS AND METHODS

CT was performed at admission and at 24 hours for 16 patients with ICH with/without contrast extravasation seen on CTA and PCCT. PCT-PS was measured at admission. The Wilcoxon rank sum test with a Bonferroni correction was used to compare PS values from the following regions of interest: 1) spot sign lesions only (9 foci), 2) PCL lesions only (9 foci), 3) hematoma excluding extravasation, 4) regions contralateral to extravasation, 5) hematoma in patients without extravasation, and 6) an area contralateral to that in 5. Additionally, hematoma expansion was determined at 24 hours defined by NCCT.

RESULTS

PS was 6.5 ± 1.60 mL · min(-1) × (100 g)(-1), 0.95 ± 0.39 mL · min(-1) × (100 g)(-1), 0.12 ± 0.39 mL · min(-1) × (100 g)(-1), 0.26 ± 0.09 mL · min(-1) × (100 g)(-1), 0.38 ± 0.26 mL · min(-1) × (100 g)(-1), and 0.09 ± 0.32 mL · min(-1) × (100 g)(-1) for the following: 1) spot sign lesions only (9 foci), 2) PCL lesions only (9 foci), 3) hematoma excluding extravasation, 4) regions contralateral to extravasation, 5) hematoma in patients without extravasation, and 6) an area contralateral to that in 5. PS values from spot sign lesions and PCL lesions were significantly different from each other and all other regions, respectively (P < .05). Hematoma volume increased from 34.1 ± 41.0 mL to 40.2 ± 46.1 mL in extravasation-positive patients and decreased from 19.8 ± 31.8 mL to 17.4 ± 27.3 mL in extravasation-negative patients.

CONCLUSIONS

The PCT-PS parameter measures a higher rate of contrast extravasation for CTA spot sign lesions compared with PCL lesions and hematoma. Early extravasation was associated with hematoma expansion.

Simulations and measurements of transcranial low-frequency ultrasound therapy: skull-base heating and effective area of treatment

Measurements of temperature elevations induced by sonications in a single intact cadaver skull filled with soft-tissue mimicking phantom material were performed using magnetic resonance thermometry. The sonications were done using a clinical transcranial ultrasound therapy device operating at 230 kHz and the measurements were compared with simulations done using a model incorporating both the longitudinal and shear wave propagation. Both the measurements and simulations showed that in some situations the temperature increase could be higher in the phantom material adjacent to the skull-base than at the focus, which could lead to undesired soft-tissue damage in treatment situations. On average the measurements of the sonicated locations, as well as the comparative simulations, showed 32 ± 64% and 49 ± 32% higher temperature elevations adjacent to the skull-base than at the focus, respectively. The simulation model was used to extend the measurements by simulating multiple sonications of brain tissue in five different skulls with and without correcting the aberrations caused by the skull on the ultrasound. Without aberration correction the closest sonications to the skulls that were treatable in any brain location without undesired tissue damage were at a distance of 19.1 ± 2.6 mm. None of the sonications beyond a distance of 41.2 ± 5.3 mm were found to cause undesired tissue damage. When using the aberration correction closest treatable, safe distances for sonications were found to be 16.0 ± 1.6  and 38.8 ± 3.8 mm, respectively. New active cooling of the skull-base through the nasal cavities was introduced and the treatment area was investigated. The closest treatable distance without aberration correction reduced to 17.4 ± 1.9 mm with the new cooling method. All sonications beyond a distance of 39.7 ± 6.6 mm were found treatable. With the aberration correction no difference in the closest treatable or the safety distance was found in comparison to sonications without nasal cavity cooling. To counteract undesired skull-base heating a new anti-focus within solid media was developed along with a new regularized phasing method. Mathematical bases for both the methods and simulations utilizing them were presented. It was found that utilizing the anti-focus in solid media and regularized phasing, the fraction of temperature increase of the brain tissue at the focus and the peak temperature increase adjacent to the skull-base can be increased from 1.00 to 1.95. This improves the efficiency of the sonication by reducing the energy transfer to the skull-base.

Future potential of MRI-guided focused ultrasound brain surgery

Magnetic resonance image-guided focused ultrasound surgery (MRgFUS) has surfaced as a viable noninvasive image-guided therapeutic method that integrates focused ultrasound (FUS), the therapeutic component, with magnetic resonance imaging (MRI), the image guidance module, into a real-time therapy delivery system with closed-loop control of energy delivery. The main applications for MRgFUS of the brain are thermal ablations for brain tumors and functional neurosurgery, and nonthermal, nonablative uses for disruption of the blood brain barrier (BBB) or blood clot and hematoma dissolution by liquification. The disruption of the BBB by FUS can be used for targeted delivery of chemotherapy and other therapeutic agents. MRI is used preoperatively for target definition and treatment planning, intraoperatively for procedure monitoring and control, and postoperatively for validating treatment success. Although challenges still remain, this integrated noninvasive therapy delivery system is anticipated to change current treatment paradigms in neurosurgery and the clinical neurosciences.

The impact of standing wave effects on transcranial focused ultrasound disruption of the blood-brain barrier in a rat model

Microbubble-mediated disruption of the blood-brain barrier (BBB) for targeted drug delivery using focused ultrasound shows great potential as a therapy for a wide range of brain disorders. This technique is currently at the pre-clinical stage and important work is being conducted in animal models. Measurements of standing waves in ex vivo rat skulls were conducted using an optical hydrophone and a geometry dependence was identified. Standing waves could not be eliminated through the use of swept frequencies, which have been suggested to eliminate standing waves. Definitive standing wave patterns were detected in over 25% of animals used in a single study. Standing waves were successfully eliminated using a wideband composite sharply focused transducer and a reduced duty cycle. The modified pulse parameters were used in vivo to disrupt the BBB in a rat indicating that, unlike some other bioeffects, BBB disruption is not dependent on standing wave conditions. Due to the high variability of standing waves and the inability to correctly estimate in situ pressures given standing wave conditions, attempts to minimize standing waves should be made in all future work in this field to ensure that results are clinically translatable.

Focal beam distortion and treatment planning for transrib focused ultrasound thermal therapy: a feasibility study using a two-dimensional ultrasound phased array

PURPOSE

The purpose of this study is to numerically investigate the feasibility of employing a spherical-section ultrasound phased array for transrib thermal ablation of liver tumors.

METHODS

Based on CT images, the authors performed a 3D reconstruction of the ribs and the surrounding soft tissues. A 3D pseudospectral time-domain (PSTD) solver was used to assess wave propagation and the distribution of pressure, with the aim of determining the specific absorption rate (SAR) and the resulting thermal doses and dynamics. Phase aberrations caused by the interposed ribs were corrected to assess the efficacy of the device in improving the SAR gain between the ribs and the target positions.

RESULTS

Experimental results supported the usefulness of the PSTD solver for predicting the pressure distribution due to the interfering obstacle. In addition, the method allowed the correction of phase aberrations caused by the ribs, and a significant improvement (176%) in the SAR gain between the ribs and the target points was observed at specific frequencies.

CONCLUSIONS

The method allowed successful tissue targeting without causing overheating of the ribs. One main advantage of this approach is the accurate estimation of phase aberration caused by heterogeneously porous ribs and other interposed tissues. This strategy might prove useful to assess the effectiveness and safety of focused ultrasound thermal ablation prior to transrib treatment.

Detrimental effects of 60 kHz sonothrombolysis in rats with middle cerebral artery occlusion

Recent studies have raised concerns about the safety of low frequency ultrasound in transcranial therapeutic application in cerebral ischemia. This study was designed to evaluate safety aspects and potential deleterious effects of low frequency, 60 kHz ultrasound in treatment of experimental middle cerebral artery occlusion (MCAO) in rats. Forty-five male Wistar rats were submitted to either temporary (90 min; groups I and II) or permanent MCAO (groups III and IV) using the suture technique. All animals received recombinant tissue plasminogen activator (rt-PA) starting 90 min after the beginning of occlusion. Groups I and III were additionally treated with 60 kHz ultrasound (time average acoustic intensity 0.14 W/cm(2), duty cycle 50%). Outcome assessment consisted of magnetic resonance imaging (MRI) and clinical evaluation after 5 and 24 h, and histology (perfusion fixation after 24 h). Overall mortality was higher in animals treated with ultrasound (43% versus 29% in controls). Most animals died during the insonation period (25% in group I, 36% in group III, no animals in the corresponding control groups; p < 0.05). Histology revealed disseminated microscopic intracerebral bleeding and subarachnoid hemorrhage as one possible cause of death. After temporary occlusion, the hemispheric ischemic lesion volume was more than doubled in animals treated with ultrasound (20.3% +/- 14.1% versus 8.6% +/- 5.1% in controls; p < 0.05). No difference in lesion volume was seen after permanent MCAO. Neurological assessment showed impairment of hearing as an additional specific side effect in ultrasound treated animals (65%, no impairment in controls). Although the results are not directly transferable to the human setting, this study clearly demonstrates the potential limitations of low frequency therapeutic ultrasound and the importance of pre-clinical safety assessment.

Prostate thermal therapy with high intensity transurethral ultrasound: the impact of pelvic bone heating on treatment delivery

PURPOSE

This study was designed to assess pelvic bone temperature during typical treatment regimens of transurethral ultrasound thermal ablation of the prostate to establish guidelines for limiting bone heating.

METHODS

Treatment with transurethral planar, curvilinear, and sectored tubular applicators was simulated using an acoustic and biothermal pelvic model that accommodates applicator sweeping, boundary temperature control, and changes in perfusion and attenuation with thermal dose to more accurately model ultrasound energy penetration. The effects of various parameters including power and frequency (5-10 MHz) on bone heating were assessed for a range of prostate cross-sections (3-5 cm) and bone distances (1-3 cm).

RESULTS

All devices can produce significant bone heating (temperatures >50 degrees C, thermal dose >240 EM(43 degrees C)) without optimization of applied frequency or power for bone <3 cm from the prostate boundary. In small glands ( approximately 3 cm) increasing operating frequency of curvilinear and planar devices can increase bone temperatures, whereas the tubular applicator can be used at 10 MHz to avoid likely bone damage. In larger prostates (4-5 cm wide) increasing frequency reduces bone heating but can substantially increase treatment time. Lowering power can reduce bone temperature but may increase thermal dose by increasing treatment duration. All applicators can be used to treat glands 4-5 cm with limited bone heating by selecting appropriate power and frequency.

CONCLUSIONS

Pubic bone heating during ultrasound thermal therapy of the prostate can be substantial in certain situations. Successful realization of this therapy will require patient-specific treatment planning to optimally determine power and frequency in order to minimize bone heating.

Thermal impact of an active 3-D microelectrode array implanted in the brain

A chronically implantable, wireless neural interface device will require integrating electronic circuitry with the interfacing microelectrodes in order to eliminate wired connections. Since the integrated circuit (IC) dissipates a certain amount of power, it will raise the temperature in surrounding tissues where it is implanted. In this paper, the thermal influence of the integrated 3-D Utah electrode array (UEA) device implanted in the brain was investigated by numerical simulation using finite element analysis (FEA) and by experimental measurement in vitro as well as in vivo. The numerically calculated and experimentally measured temperature increases due to the UEA implantation were in good agreement. The experimentally validated numerical model predicted that the temperature increases linearly with power dissipation through the UEA, with a slope of 0.029 degree C/mW over the power dissipation levels expected to be used. The influences of blood perfusion, brain metabolism, and UEA geometry on tissue heating were also investigated using the numerical model.

Clinical applications of focused ultrasound-the brain

This paper provides a historic and contemporary overview of the use of focused ultrasound for treating brain disorders.

In vivo transcranial brain surgery with an ultrasonic time reversal mirror

OBJECT

High-intensity focused ultrasonography is known to induce controlled and selective noninvasive destruction of tissues by focusing ultrasonic beams within organs, like a magnifying glass concentrating enough sunlight to burn a hole in paper. Such a technique should be highly interesting for the treatment of deep-seated lesions in the brain. Nevertheless, ultrasonic tissue ablation in the brain has long been hampered by the defocusing effect of the skull bone.

METHODS

In this in vivo study, the authors used a high-power time-reversal mirror specially designed for noninvasive ultrasonic brain treatment to induce thermal lesions through the skulls of 10 sheep. The sheep were divided into three groups and, depending on group, were killed 1, 2, or 3 weeks after treatment. The thermal lesions were confirmed based on findings of posttreatment magnetic resonance imaging and histological examinations. After treatment, the basic neurological functions of the animals were unchanged: the animals recovered from anesthesia without any abnormal delay and did not exhibit signs of paralysis or coma. No major behavioral change was observed.

CONCLUSIONS

The results provide striking evidence that noninvasive ultrasonographic brain surgery is feasible. Thus the authors offer a novel noninvasive method of performing local brain ablation in animals for behavioral studies. This technique may lead the way to noninvasive and nonionizing treatment of brain tumors and neurological disorders by selectively targeting intracranial lesions. Nevertheless, sheep do not represent a good functional model and extensive work will need to be conducted preferably on monkeys to investigate the effects of this treatment.

Pre-clinical testing of a phased array ultrasound system for MRI-guided noninvasive surgery of the brain--a primate study

MRI-guided and monitored focused ultrasound thermal surgery of brain through intact skull was tested in three rhesus monkeys. The aim of this study was to determine the amount of skull heating in an animal model with a head shape similar to that of a human. The ultrasound beam was generated by a 512 channel phased array system (Exablate 3000, InSightec, Haifa, Israel) that was integrated within a 1.5-T MR-scanner. The skin was pre-cooled by degassed temperature controlled water circulating between the array surface and the skin. Skull surface temperature was measured with invasive thermocouple probes. The results showed that by applying surface cooling the skin and skull surface can be protected, and that the brain surface temperature becomes the limiting factor. The MRI thermometry was shown to be useful in detecting the tissue temperature distribution next to the bone, and it should be used to monitor the brain surface temperature. The acoustic intensity values during the 20 s sonications were adequate for thermal ablation in the human brain provided that surface cooling is used.

Preliminary study of the thermal impact of a microelectrode array implanted in the brain

One requirement of a chronically implantable, wireless neural interface device is the integration of electronic circuitry with the microelectrode array. Since the electronic IC dissipates a certain amount of power, it will affect the temperature in the tissues surrounding the implant site. In this paper, the thermal influence of an integrated, 3-dimensional Utah electrode array, to be implanted in the brain was investigated with simulations using the finite element method (FEM). A temperature increase in the brain tissue was predicted using preliminary simulations with simplified models. The model and method used in the simulations were verified by simple in vitro experiments.