The effect of electronically steering a phased array ultrasound transducer on near-field tissue heating

Acoustic holography in biomedical applications

Acoustic holography can be used to construct an arbitrary wavefront at a desired 2D plane or 3D volume by beam shaping an emitted field and is a relatively new technique in the field of biomedical applications. Acoustic holography was first theorized in 1985 following Gabor's work in creating optical holograms in the 1940s. Recent developments in 3D printing have led to an easier and faster way to manufacture monolithic acoustic holographic lenses that can be attached to single-element transducers. As ultrasound passes through the lens material, a phase shift is applied to the waves, causing an interference pattern at the 2D image plane or 3D volume, which forms the desired pressure field. This technology has many applications already in use and has become of increasing interest for the biomedical community, particularly for treating regions that are notoriously difficult to operate on, such as the brain. Acoustic holograms could provide a non-invasive, precise, and patient specific way to deliver drugs, induce hyperthermia, or create tissue cell patterns. However, there are still limitations in acoustic holography, such as the difficulties in creating 3D holograms and the passivity of monolithic lenses. This review aims to outline the biomedical applications of acoustic holograms reported to date and discuss their current limitations and the future work that is needed for them to reach their full potential in the biomedical community.

Overview of Therapeutic Ultrasound Applications and Safety Considerations: 2024 Update

A 2012 review of therapeutic ultrasound was published to educate researchers and physicians on potential applications and concerns for unintended bioeffects (doi: 10.7863/jum.2012.31.4.623). This review serves as an update to the parent article, highlighting advances in therapeutic ultrasound over the past 12 years. In addition to general mechanisms for bioeffects produced by therapeutic ultrasound, current applications, and the pre-clinical and clinical stages are outlined. An overview is provided for image guidance methods to monitor and assess treatment progress. Finally, other topics relevant for the translation of therapeutic ultrasound are discussed, including computational modeling, tissue-mimicking phantoms, and quality assurance protocols.

Design of a 1-MHz Therapeutic Ultrasound Array for Small Volume Blood-Brain Barrier Opening at Cortical Targets in Macaques

[[gabstract]][] Focused ultrasound (FUS) can temporarily open the blood-brain barrier (BBB) and increase the delivery of chemotherapeutics, viral vectors, and other agents to the brain parenchyma. To limit FUS BBB opening to a single brain region, the transcranial acoustic focus of the ultrasound transducer must not be larger than the region targeted. In this work, we design and characterize a therapeutic array optimized for BBB opening at the frontal eye field (FEF) in macaques. We used 115 transcranial simulations in four macaques varying f-number and frequency to optimize the design for focus size, transmission, and small device footprint. The design leverages inward steering for focus tightening, a 1-MHz transmit frequency, and can focus to a simulation predicted 2.5- ± 0.3-mm lateral and 9.5- ± 1.0-mm axial full-width at half-maximum spot size at the FEF without aberration correction. The array is capable of steering axially 35 mm outward, 26 mm inward, and laterally 13 mm with 50% the geometric focus pressure. The simulated design was fabricated, and we characterized the performance of the array using hydrophone beam maps in a water tank and through an ex vivo skull cap to compare measurements with simulation predictions, achieving a 1.8-mm lateral and 9.5-mm axial spot size with a transmission of 37% (transcranial, phase corrected). The transducer produced by this design process is optimized for BBB opening at the FEF in macaques.

Multitarget Transcranial Ultrasound Therapy in Small Animals Based on Phase-Only Acoustic Holographic Lens

Transcranial ultrasound therapy has become a noninvasive method for treating neurological and psychiatric disorders, and studies have further demonstrated that multitarget transcranial ultrasound therapy is a better solution. At present, multitarget transcranial ultrasound therapy in small animals can only be achieved by the multitransducer or phased array. However, multiple transducers may cause spatial interference, and the phased array system is complicated, expensive, and especially unsuitable for small animals. This study is the first to design and fabricate a miniature acoustic holography lens for multitarget transcranial ultrasound therapy in rats. The acoustic holographic lens, working at a frequency of 1.0 MHz, with a size of 10.08 mm ×10.08 mm and a pixel resolution of 0.72 mm, was designed, optimized, and fabricated. The dual-focus transcranial ultrasound generated based on the lens was measured; the full-width at half-maximum (FWHM) of the focal spots in the y -direction was 2.15 and 2.27 mm and in the z -direction was 2.3 and 2.36 mm. The focal length was 5.4 mm, and the distance between the two focuses was 5.6 mm, close to the desired values of 5.4 and 6.0 mm. Finally, the multiple-target blood-brain barrier opening in rats' bilateral secondary visual cortex (mediolateral area, V2ML) was demonstrated using the transcranial ultrasound therapy system based on the lens. These results demonstrate the good performance of the multitarget transcranial ultrasound therapy system for small animals, including high spatial resolution, small size, and low cost.

Experimental evaluation of the near-field and far-field heating of focused ultrasound using the thermal dose concept

BACKGROUND

Conventional motion algorithms utilized during High Intensity Focused Ultrasound (HIFU) procedures usually sonicate successive tissue cells, thereby inducing excess deposition of thermal dose in the pre-focal region. Long delays (~60 s) are used to reduce the heating around the focal region. In the present study the experimental evaluation of six motion algorithms so as to examine the required delay and algorithm for which the pre-focal (near-field) and post-focal (far-field) heating can be reduced using thermal dose estimations is presented.

MATERIALS AND METHODS

A single element spherically focused transducer operating at 1.1 MHz and focusing beam at 9 cm, was utilized for sonication on a 400 mm2 area of an agar-based phantom. Movement of the transducer was performed with each algorithm, using 0-60 s (10 s step) delays between sonications. Temperatures were recorded at both near and far-field regions and thermal dose calculations were implemented.

RESULTS

With the algorithms used in the present study, a delay of 50-60 s was required to reduce heating in the near-field region. A 30 s delay induced a safe thermal dose in the far-field region using all algorithms except sequential which still required 60 s delay.

CONCLUSIONS

The study verified the conservative need for 60 s delay for the sequential plan treatment. Nevertheless, present findings suggest that prolonged treatment times can be significantly reduced in homogeneous tissues by selection of the optimized nonlinear algorithm and delay.

AAPM Task Group 241: A medical physicist's guide to MRI-guided focused ultrasound body systems

Magnetic resonance-guided focused ultrasound (MRgFUS) is a completely non-invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report addresses the issues of interest to the medical physics community, specific to the body MRgFUS system configuration, and provides recommendations on how to successfully implement and maintain a clinical MRgFUS program. The following sections describe the key features of typical MRgFUS systems and clinical workflow and provide key points and best practices for the medical physicist. Commonly used terms, metrics and physics are defined and sources of uncertainty that affect MRgFUS procedures are described. Finally, safety and quality assurance procedures are explained, the recommended role of the medical physicist in MRgFUS procedures is described, and regulatory requirements for planning clinical trials are detailed. Although this report is limited in scope to clinical body MRgFUS systems that are approved or currently undergoing clinical trials in the United States, much of the material presented is also applicable to systems designed for other applications.

Design and evaluation of an open-source, conformable skin-cooling system for body magnetic resonance guided focused ultrasound treatments

PURPOSE

Magnetic resonance guided focused ultrasound (MRgFUS) treatment of tumors uses inter-sonication delays to allow heat to dissipate from the skin and other near-field tissues. Despite inter-sonication delays, treatment of tumors close to the skin risks skin burns. This work has designed and evaluated an open-source, conformable, skin-cooling system for body MRgFUS treatments to reduce skin burns and enable ablation closer to the skin.

METHODS

A MR-compatible skin cooling system is described that features a conformable skin-cooling pad assembly with feedback control allowing continuous flow and pressure maintenance during the procedure. System performance was evaluated with hydrophone, phantom and in vivo porcine studies. Sonications were performed 10 and 5 mm from the skin surface under both control and forced convective skin-cooling conditions. 3D MR temperature imaging was acquired in real time and the accumulated thermal dose volume was measured. Gross analysis of the skin post-sonication was further performed. Device conformability was demonstrated at several body locations.

RESULTS

Hydrophone studies demonstrated no beam aberration, but a 5-12% reduction of the peak pressure due to the presence of the skin-cooling pad assembly in the acoustic near field. Phantom evaluation demonstrated there is no MR temperature imaging precision reduction or any other artifacts present due to the coolant flow during MRgFUS sonication. The porcine studies demonstrated skin burns were reduced in size or eliminated when compared to the control condition.

CONCLUSION

An open-source design of an MRgFUS active skin cooling system demonstrates device conformability with a reduction of skin burns while ablating superficial tissues.

Development of Scalable 2D Plane Array for Transcranial Ultrasonic Neuromodulation on Non-Human Primates: An Ex Vivo Study

Ultrasonic neuromodulation on large animals, like non-human primates, requires the array transducer to have a good steering ability to arbitrarily stimulate various brain locations. Moreover, due to the different sizes of the animal heads, the array is preferred to be conveniently adjustable to different aperture sizes. To meet these requirements, a scalable 2D plane array, which can consist of up to tens of 256-element square modules, has been designed and fabricated in this study. Arrays with large apertures, consisting of 12 and 48 modules (including 3072 and 12288 elements, respectively), have been assembled and tested. The square modules were driven at a center frequency of 1.04 MHz. The transcranial and steering abilities of the modules were verified in a water tank on an ex vivo macaque skull. A focused beam transmitted by single module could pass through the skull with its peak acoustic pressure reduced to 18.1% of that in the free-field, with an acceptable transverse steering range up to 10 mm. Moreover, a much smaller focal spot was obtained by simultaneous transmission using two vertical-placed modules. Compared to the single module, the focal zone axial size was reduced from 29.7 mm to 3.3 mm, and the "stripes" pattern in the focal zone caused by standing waves was eliminated using a difference-frequency transmitting strategy. Such scalable 2D plane array, which can provide a small-size focal spot and has a large steering range and an acceptable transcranial ability, can be useful in research on ultrasonic neuromodulation on non-human primates.

A variable flip angle golden-angle-ordered 3D stack-of-radial MRI technique for simultaneous proton resonant frequency shift and T1 -based thermometry

PURPOSE

To develop and evaluate a variable-flip-angle golden-angle-ordered 3D stack-of-radial MRI technique for simultaneous proton resonance frequency shift (PRF) and T₁ -based thermometry in aqueous and adipose tissues, respectively.

METHODS

The proposed technique acquires multiecho radial k-space data in segments with alternating flip angles to measure 3D temperature maps dynamically on the basis of PRF and T₁ . A sliding-window k-space weighted image contrast filter is used to increase temporal resolution. PRF is measured in aqueous tissues and T₁ in adipose tissues using fat/water masks. The accuracy for T₁ quantification was evaluated in a reference T₁ /T₂ phantom. In vivo nonheating experiments were conducted in healthy subjects to evaluate the stability of PRF and T₁ in the brain, prostate, and breast. The proposed technique was used to monitor high-intensity focused ultrasound (HIFU) ablation in ex vivo porcine fat/muscle tissues and compared to temperature probe readings.

RESULTS

The proposed technique achieved 3D coverage with 1.1-mm to 1.3-mm in-plane resolution and 2-s to 5-s temporal resolution. During 20 to 30 min of nonheating in vivo scans, the temporal coefficient of variation for T₁ was <5% in the brain, prostate, and breast fatty tissues, while the standard deviation of relative PRF temperature change was within 3°C in aqueous tissues. During ex vivo HIFU ablation, the temperatures measured by PRF and T₁ were consistent with temperature probe readings, with an absolute mean difference within 2°C.

CONCLUSION

The proposed technique achieves simultaneous PRF and T₁ -based dynamic 3D MR temperature mapping in aqueous and adipose tissues. It may be used to improve MRI-guided thermal procedures.

Focused Ultrasound Effects on Osteosarcoma Cell Lines

MRI guided Focused Ultrasound (MRgFUS) has shown to be effective therapeutic modality for non-invasive clinical interventions in ablating of uterine fibroids, in bone metastasis palliative treatments, and in breast, liver, and prostate cancer ablation. MRgFUS combines high intensity focused ultrasound (HIFU) with MRI images for treatment planning and real time thermometry monitoring, thus enabling non-invasive ablation of tumor tissue. Although in the literature there are several studies on the Ultrasound (US) effects on cell in culture, there is no systematic evidence of the biological effect of Magnetic Resonance guided Focused Ultrasound Surgery (MRgFUS) treatment on osteosarcoma cells, especially in lower dose regions, where tissues receive sub-lethal acoustic power. The effect of MRgFUS treatment at different levels of acoustic intensity (15.5-49 W/cm²) was investigated on Mg-63 and Saos-2 cell lines to evaluate the impact of the dissipation of acoustic energy delivered outside the focal area, in terms of cell viability and osteogenic differentiation at 24 h, 7 days, and 14 days after treatment. Results suggested that the attenuation of FUS acoustic intensities from the focal area (higher intensities) to the “far field” (lower intensities) zones might determine different osteosarcoma cell responses, which range from decrease of cell proliferation rates (from 49 W/cm² to 38.9 W/cm²) to the selection of a subpopulation of heterogeneous and immature living cells (from 31.1 W/cm² to 15.5 W/cm²), which can clearly preserve bone tumor cells.

A random phased-array for MR-guided transcranial ultrasound neuromodulation in non-human primates

Transcranial focused ultrasound (FUS) is a non-invasive technique for therapy and study of brain neural activation. Here we report on the design and characterization of a new MR-guided FUS transducer for neuromodulation in non-human primates at 650 kHz. The array is randomized with 128 elements 6.6 mm in diameter, radius of curvature 7.2 cm, opening diameter 10.3 cm (focal ratio 0.7), and 46% coverage. Simulations were used to optimize transducer geometry with respect to focus size, grating lobes, and directivity. Focus size and grating lobes during electronic steering were quantified using hydrophone measurements in water and a three-axis stage. A novel combination of optical tracking and acoustic mapping enabled measurement of the 3D pressure distribution in the cortical region of an ex vivo skull to within ~3.5 mm of the surface, and allowed accurate modelling of the experiment via non-homogeneous 3D acoustic simulations. The data demonstrates acoustic focusing beyond the skull bone, with the focus slightly broadened and shifted proximal to the skull. The fabricated design is capable of targeting regions within the S1 sensorimotor cortex of macaques.

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.

Predicting variation in subject thermal response during transcranial magnetic resonance guided focused ultrasound surgery: Comparison in seventeen subject datasets

PURPOSE

In transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) treatments, the acoustic and spatial heterogeneity of the skull cause reflection, absorption, and scattering of the acoustic beams. These effects depend on skull-specific parameters and can lead to patient-specific thermal responses to the same transducer power. In this work, the authors develop a simulation tool to help predict these different experimental responses using 3D heterogeneous tissue models based on the subject CT images. The authors then validate and compare the predicted skull efficiencies to an experimental metric based on the subject thermal responses during tcMRgFUS treatments in a dataset of seventeen human subjects.

METHODS

Seventeen human head CT scans were used to create tissue acoustic models, simulating the effects of reflection, absorption, and scattering of the acoustic beam as it propagates through a heterogeneous skull. The hybrid angular spectrum technique was used to model the acoustic beam propagation of the InSightec ExAblate 4000 head transducer for each subject, yielding maps of the specific absorption rate (SAR). The simulation assumed the transducer was geometrically focused to the thalamus of each subject, and the focal SAR at the target was used as a measure of the simulated skull efficiency. Experimental skull efficiency for each subject was calculated using the thermal temperature maps from the tcMRgFUS treatments. Axial temperature images (with no artifacts) were reconstructed with a single baseline, corrected using a referenceless algorithm. The experimental skull efficiency was calculated by dividing the reconstructed temperature rise 8.8 s after sonication by the applied acoustic power.

RESULTS

The simulated skull efficiency using individual-specific heterogeneous models predicts well (R(2) = 0.84) the experimental energy efficiency.

CONCLUSIONS

This paper presents a simulation model to predict the variation in thermal responses measured in clinical ctMRGFYS treatments while being computationally feasible.

Image-guided ultrasound phased arrays are a disruptive technology for non-invasive therapy

Focused ultrasound offers a non-invasive way of depositing acoustic energy deep into the body, which can be harnessed for a broad spectrum of therapeutic purposes, including tissue ablation, the targeting of therapeutic agents, and stem cell delivery. Phased array transducers enable electronic control over the beam geometry and direction, and can be tailored to provide optimal energy deposition patterns for a given therapeutic application. Their use in combination with modern medical imaging for therapy guidance allows precise targeting, online monitoring, and post-treatment evaluation of the ultrasound-mediated bioeffects. In the past there have been some technical obstacles hindering the construction of large aperture, high-power, densely-populated phased arrays and, as a result, they have not been fully exploited for therapy delivery to date. However, recent research has made the construction of such arrays feasible, and it is expected that their continued development will both greatly improve the safety and efficacy of existing ultrasound therapies as well as enable treatments that are not currently possible with existing technology. This review will summarize the basic principles, current statures, and future potential of image-guided ultrasound phased arrays for therapy.

Frequency considerations for deep ablation with high-intensity focused ultrasound: A simulation study

PURPOSE

The objective of this study was to explore frequency considerations for large-volume, deep thermal ablations with focused ultrasound. Though focal patterns, focal steering rate, and the size of focal clusters have all been explored in this context, frequency studies have generally explored shallower depths and hyperthermia applications. This study examines both treatment efficiency and near-field heating rate as functions of frequency and depth.

METHODS

Flat, 150 mm transducer arrays were simulated to operate at frequencies of 250, 500, 750, 1000, 1250, and 1500 kHz. Each array had λ2 interelement spacing yielding arrays of 2000-70 000 piston-shaped elements arranged in concentric rings. Depths of 50, 100, and 150 mm were explored, with attenuation (α) values of 2.5-10 (Np/m)/MHz. Ultrasound propagation was simulated with the Rayleigh-Sommerfeld integral over a volume of homogeneous simulated tissue. Absorbed power density was determined from the acoustic pressure which, in turn, was modeled with the Pennes bioheat transfer equation. Using this knowledge of temperature over time, thermal dose function of Sapareto and Dewey was used to model the resulting bioeffect of each simulated sonication. Initially, single foci at each depth, frequency, and α were examined with either fixed peak temperatures or fixed powers. Based on the size of the resulting, single foci lesions, larger compound sonications were designed with foci packed together in multiple layers and rings. For each depth, focal patterns were chosen to produce a similar total ablated volume for each frequency. These compound sonications were performed with a fixed peak temperature at each focus. The resulting energy efficiency (volume ablated per acoustic energy applied), near-field heating rate (temperature increase in the anterior third of the simulation space per unit volume ablated), and near- and far-field margins were assessed.

RESULTS

Lesions of comparable volume were created with different frequencies at different depths. The results reflect the interconnected nature of frequency as it effects focal size (decreasing with frequency), peak pressure (generally increasing with frequency), and attenuation (also increasing with frequency). The ablation efficiency was the highest for α = 5 (Np/m)/MHz at a frequency of 750 kHz at each depth. For α = 10 (Np/m)/MHz, efficiency was the highest at 750 kHz for a depth of 50 mm, and 500 kHz at depths of 100 and 150 mm. At all sonication depths, near-field heating was minimized with lower frequencies of 250 and 500 kHz.

CONCLUSIONS

Large-volume ablations are most efficient at frequencies of 500-750 kHz at depths of 100-150 mm. When one considers that near-field heat accumulation tends to be the rate limiting factor in large-volume ablations like uterine fibroid surgery, the results show that frequencies as low as 500 kHz are favored for their ability to reduce heating in the near-field.

Renal sympathetic denervation using MR-guided high-intensity focused ultrasound in a porcine model

Background

Initial catheter-based renal sympathetic denervation (RSD) studies demonstrated promising results in showing a significant reduction of blood pressure, while recent data were less successful. As an alternative approach, the objective of this study was to evaluate the feasibility of using magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU) to perform RSD in a porcine model.

Methods

An intravascular fiber optic temperature probe was used to confirm energy delivery during MRgHIFU. This technique was evaluated both in a vascular phantom and in a normotensive pig model. Five animals underwent unilateral RSD using MRgHIFU, and both safety and efficacy were assessed. MRI was used to evaluate the acoustic window, target sonications, monitor the near-field treatment region using MR thermometry imaging, and assess the status of tissues post-procedure. An intravascular fiber optic temperature probe verified energy delivery. Animals were sacrificed 6 to 9 days post-treatment, and pathological analysis was performed. The norepinephrine present in the kidney medulla was assessed post-mortem.

Results

All animals tolerated the procedure well with no observed complications. The fiber optic temperature probe placed in the target renal artery confirmed energy delivery during MRgHIFU, measuring larger temperature rises when the MRgHIFU beam location was focused closer to the tip of the probe. Following ablation, a significant reduction (p = 0.04) of cross-sectional area of nerve bundles between the treated and untreated renal arteries was observed in all of the animals with treated nerves presenting increased cellular infiltrate and fibrosis. A reduction of norepinephrine (p = 0.14) in the kidney medulla tissue was also observed. There was no indication of tissue damage in arterial walls.

Conclusions

Performing renal denervation non-invasively with MRgHIFU was shown to be both safe and effective as determined by norepinephrine levels in a porcine model. This approach may be a promising alternative to catheter-based strategies.

HIFU Tissue Ablation: Concept and Devices

High intensity focused ultrasound (HIFU) is rapidly gaining clinical acceptance as a technique capable of providing non-invasive heating and ablation for a wide range of applications. Usually requiring only a single session, treatments are often conducted as day case procedures, with the patient either fully conscious, lightly sedated or under light general anesthesia. HIFU scores over other thermal ablation techniques because of the lack of necessity for the transcutaneous insertion of probes into the target tissue. Sources placed either outside the body (for treatment of tumors or abnormalities of the liver, kidney, breast, uterus, pancreas brain and bone), or in the rectum (for treatment of the prostate), provide rapid heating of a target tissue volume, the highly focused nature of the field leaving tissue in the ultrasound propagation path relatively unaffected. Numerous extra-corporeal, transrectal and interstitial devices have been designed to optimize application-specific treatment delivery for the wide-ranging areas of application that are now being explored with HIFU. Their principle of operation is described here, and an overview of their design principles is given.

Magnetic resonance temperature imaging-based quantification of blood flow-related energy losses

This study presents a new approach for evaluating bioheat transfer equation (BHTE) models used in treatment planning, control and evaluation of all thermal therapies. First, 3D magnetic resonance temperature imaging (MRTI) data are used to quantify blood flow-related energy losses, including the effects of perfusion and convection. Second, this information is used to calculate parameters of a BHTE model: in this paper the widely used Pennes BHTE. As a self-consistency check, the BHTE parameters are utilized to predict the temperatures from which they were initially derived. The approach is evaluated with finite-difference simulations and implemented experimentally with focused ultrasound heating of an ex vivo porcine kidney perfused at 0, 20 and 40 ml/min (n = 4 each). The simulation results demonstrate accurate quantification of blood flow-related energy losses, except in regions of sharp blood flow discontinuities, where the transitions are spatially smoothed. The smoothed transitions propagate into estimates of the Pennes perfusion parameter but have limited effect on the accuracy of temperature predictions using these estimates. Longer acquisition time periods mitigate the effects of MRTI noise, but worsen the effect of flow discontinuities. For the no-flow kidney experiments the estimates of a uniform, constant Pennes perfusion parameter are approximately zero, and at 20 and 40 ml/min the average estimates increase with flow rate to 3.0 and 4.2 kg/m(3) /s, respectively. When Pennes perfusion parameter values are allowed to vary spatially, but remain temporally constant, BHTE temperature predictions are more accurate than when using spatially uniform, constant Pennes perfusion values, with reductions in RMSE values of up to 79%. Locations with large estimated perfusion values correspond to high flow regions of the kidney observed in T1 -weighted MR images. This novel, MRTI-based technique holds promise for improving understanding of thermal therapy biophysics and for evaluating biothermal models.

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 phase aberration correction using beam simulations and MR-ARFI

PURPOSE

Transcranial magnetic resonance-guided focused ultrasound surgery is a noninvasive technique for causing selective tissue necrosis. Variations in density, thickness, and shape of the skull cause aberrations in the location and shape of the focal zone. In this paper, the authors propose a hybrid simulation-MR-ARFI technique to achieve aberration correction for transcranial MR-guided focused ultrasound surgery. The technique uses ultrasound beam propagation simulations with MR Acoustic Radiation Force Imaging (MR-ARFI) to correct skull-caused phase aberrations.

METHODS

Skull-based numerical aberrations were obtained from a MR-guided focused ultrasound patient treatment and were added to all elements of the InSightec conformal bone focused ultrasound surgery transducer during transmission. In the first experiment, the 1024 aberrations derived from a human skull were condensed into 16 aberrations by averaging over the transducer area of 64 elements. In the second experiment, all 1024 aberrations were applied to the transducer. The aberrated MR-ARFI images were used in the hybrid simulation-MR-ARFI technique to find 16 estimated aberrations. These estimated aberrations were subtracted from the original aberrations to result in the corrected images. Each aberration experiment (16-aberration and 1024-aberration) was repeated three times.

RESULTS

The corrected MR-ARFI image was compared to the aberrated image and the ideal image (image with zero aberrations) for each experiment. The hybrid simulation-MR-ARFI technique resulted in an average increase in focal MR-ARFI phase of 44% for the 16-aberration case and 52% for the 1024-aberration case, and recovered 83% and 39% of the ideal MR-ARFI phase for the 16-aberrations and 1024-aberration case, respectively.

CONCLUSIONS

Using one MR-ARFI image and noa priori information about the applied phase aberrations, the hybrid simulation-MR-ARFI technique improved the maximum MR-ARFI phase of the beam's focus.

Respiration based steering for high intensity focused ultrasound liver ablation

PURPOSE

Respiratory motion makes hepatic ablation using high intensity focused ultrasound (HIFO) challenging. Previous HIFU liver treatment had required apnea induced during general anesthesia. We describe and test a system that allows treatment of the liver in the presence of breathing motion.

METHODS

Mapping a signal from an external respiratory bellow to treatment locations within the liver allows the ultrasound transducer to be steered in real time to the target location. Using a moving phantom, three metrics were used to compare static, steered, and unsteered sonications: the area of sonications once a temperature rise of 15°C was achieved, the energy deposition required to reach that temperature, and the average rate of temperature rise during the first 10 s of sonication. Steered HIFU in vivo ablations of the porcine liver were also performed and compared to breath-hold ablations.

RESULTS

For the last phantom metric, all groups were found to be statistically significantly different (P ≤ 0.003). However, in the other two metrics, the static and unsteered sonications were not statistically different (P > 0.9999). Steered in vivo HIFU ablations were not statistically significantly different from ablations during breath-holding.

CONCLUSIONS

A system for performing HIFU steering during ablation of the liver with breathing motion is presented and shown to achieve results equivalent to ablation performed with breath-holding.

In vivo evaluation of a breast-specific magnetic resonance guided focused ultrasound system in a goat udder model

PURPOSE

This work further evaluates the functionality, efficacy, and safety of a new breast-specific magnetic resonance guided high intensity focused ultrasound (MRgFUS) system in an in vivo goat udder model.

METHODS

Eight female goats underwent an MRgFUS ablation procedure using the breast-specific MRgFUS system. Tissue classification was achieved through the 3D magnetic resonance imaging (MRI) acquisition of several contrasts (T1w, T2w, PDw, 3-point Dixon). The MRgFUS treatment was performed with a grid trajectory executed in one or two planes within the glandular tissue of the goat udder. Temperature was monitored using a 3D proton resonance frequency (PRF) MRI technique. Delayed contrast enhanced-MR images were acquired immediately and 14 days post MRgFUS treatment. A localized tissue excision was performed in one animal and histological analysis was performed. Animals were available for adoption at the conclusion of the study.

RESULTS

The breast-specific MRgFUS system was able to ablate regions ranging in size from 0.4 to 3.6 cm(3) in the goat udder model. Tissue damage was confirmed through the correlation of thermal dose measurements obtained with realtime 3D MR thermometry to delayed contrast enhanced-MR images immediately after the treatment and 14 days postablation. In general, lesions were longer in the ultrasound propagation direction, which is consistent with the dimensions of the ultrasound focal spot. Thermal dose volumes had better agreement with nonenhancing areas of the DCE-MRI images obtained 14 days after the MRgFUS treatment.

CONCLUSIONS

The system was able to successfully ablate lesions up to 3.6 cm(3). The thermal dose volume was found to correlate better with the 14-day postablation nonenhancing delayed contrast enhanced-MR image volumes. While the goat udder is not an ideal model for the human breast, this study has proven the feasibility of using this system on a wide variety of udder shapes and sizes, demonstrating the flexibility that would be required in order to treat human subjects.

Effects of MRTI sampling characteristics on estimation of HIFU SAR and tissue thermal diffusivity

While the non-invasive and three-dimensional nature of magnetic-resonance temperature imaging (MRTI) makes it a valuable tool for high-intensity focused ultrasound (HIFU) treatments, random and systematic errors in MRTI measurements may propagate into temperature-based parameter estimates used for pretreatment planning. This study assesses the MRTI effects of zero-mean Gaussian noise (SD = 0.0-2.0 °C), temporal sampling (tacq = 1.0-8.0 s), and spatial averaging (Res = 0.5-2.0 mm isotropic) on HIFU temperature measurements and temperature-based estimates of the amplitude and full width half maximum (FWHM) of the HIFU specific absorption rate and of tissue thermal diffusivity. The ultrasound beam used in simulations and ex vivo pork loin experiments has lateral and axial FWHM dimensions of 1.4 mm and 7.9 mm respectively. For spatial averaging simulations, beams with lateral FWHM varying from 1.2-2.2 mm are also assessed. Under noisy conditions, parameter estimates are improved by fitting to data from larger voxel regions. Varying the temporal sampling results in minimal changes in measured temperatures (<2% change) and parameter estimates (<5% change). For the HIFU beams studied, a spatial resolution of 1 × 1 × 3 mm(3) or smaller is required to keep errors in temperature and all estimated parameters less than 10%. By quantifying the errors associated with these sampling characteristics, this work provides researchers with appropriate MRTI conditions for obtaining estimates of parameters essential to pretreatment modeling of HIFU thermal therapies.

Design and characterization of a laterally mounted phased-array transducer breast-specific MRgHIFU device with integrated 11-channel receiver array

PURPOSE

This work presents the design and preliminary evaluation of a new laterally mounted phased-array MRI-guided high-intensity focused ultrasound (MRgHIFU) system with an integrated 11-channel phased-array radio frequency (RF) coil intended for breast cancer treatment. The design goals for the system included the ability to treat the majority of tumor locations, to increase the MR image's signal-to-noise ratio (SNR) throughout the treatment volume and to provide adequate comfort for the patient.

METHODS

In order to treat the majority of the breast volume, the device was designed such that the treated breast is suspended in a 17-cm diameter treatment cylinder. A laterally shooting 1-MHz, 256-element phased-array ultrasound transducer with flexible positioning is mounted outside the treatment cylinder. This configuration achieves a reduced water volume to minimize RF coil loading effects, to position the coils closer to the breast for increased signal sensitivity, and to reduce the MR image noise associated with using water as the coupling fluid. This design uses an 11-channel phased-array RF coil that is placed on the outer surface of the cylinder surrounding the breast. Mechanical positioning of the transducer and electronic steering of the focal spot enable placement of the ultrasound focus at arbitrary locations throughout the suspended breast. The treatment platform allows the patient to lie prone in a face-down position. The system was tested for comfort with 18 normal volunteers and SNR capabilities in one normal volunteer and for heating accuracy and stability in homogeneous phantom and inhomogeneous ex vivo porcine tissue.

RESULTS

There was a 61% increase in mean relative SNR achieved in a homogeneous phantom using the 11-channel RF coil when compared to using only a single-loop coil around the chest wall. The repeatability of the system's energy delivery in a single location was excellent, with less than 3% variability between repeated temperature measurements at the same location. The execution of a continuously sonicated, predefined 48-point, 8-min trajectory path resulted in an ablation volume of 8.17 cm(3), with one standard deviation of 0.35 cm(3) between inhomogeneous ex vivo tissue samples. Comfort testing resulted in negligible side effects for all volunteers.

CONCLUSIONS

The initial results suggest that this new device will potentially be suitable for MRgHIFU treatment in a wide range of breast sizes and tumor locations.

Toward real-time availability of 3D temperature maps created with temporally constrained reconstruction

PURPOSE

To extend the previously developed temporally constrained reconstruction (TCR) algorithm to allow for real-time availability of three-dimensional (3D) temperature maps capable of monitoring MR-guided high intensity focused ultrasound applications.

METHODS

A real-time TCR (RT-TCR) algorithm is developed that only uses current and previously acquired undersampled k-space data from a 3D segmented EPI pulse sequence, with the image reconstruction done in a graphics processing unit implementation to overcome computation burden. Simulated and experimental data sets of HIFU heating are used to evaluate the performance of the RT-TCR algorithm.

RESULTS

The simulation studies demonstrate that the RT-TCR algorithm has subsecond reconstruction time and can accurately measure HIFU-induced temperature rises of 20°C in 15 s for 3D volumes of 16 slices (RMSE = 0.1°C), 24 slices (RMSE = 0.2°C), and 32 slices (RMSE = 0.3°C). Experimental results in ex vivo porcine muscle demonstrate that the RT-TCR approach can reconstruct temperature maps with 192 × 162 × 66 mm 3D volume coverage, 1.5 × 1.5 × 3.0 mm resolution, and 1.2-s scan time with an accuracy of ±0.5°C.

CONCLUSION

The RT-TCR algorithm offers an approach to obtaining large coverage 3D temperature maps in real-time for monitoring MR-guided high intensity focused ultrasound treatments.

An analytical solution for improved HIFU SAR estimation

Accurate determination of the specific absorption rates (SARs) present during high intensity focused ultrasound (HIFU) experiments and treatments provides a solid physical basis for scientific comparison of results among HIFU studies and is necessary to validate and improve SAR predictive software, which will improve patient treatment planning, control and evaluation. This study develops and tests an analytical solution that significantly improves the accuracy of SAR values obtained from HIFU temperature data. SAR estimates are obtained by fitting the analytical temperature solution for a one-dimensional radial Gaussian heating pattern to the temperature versus time data following a step in applied power and evaluating the initial slope of the analytical solution. The analytical method is evaluated in multiple parametric simulations for which it consistently (except at high perfusions) yields maximum errors of less than 10% at the center of the focal zone compared with errors up to 90% and 55% for the commonly used linear method and an exponential method, respectively. For high perfusion, an extension of the analytical method estimates SAR with less than 10% error. The analytical method is validated experimentally by showing that the temperature elevations predicted using the analytical method's SAR values determined for the entire 3D focal region agree well with the experimental temperature elevations in a HIFU-heated tissue-mimicking phantom.

Hybrid proton resonance frequency/T1 technique for simultaneous temperature monitoring in adipose and aqueous tissues

Thermal therapy procedures being carried out under MR guidance would be safer if temperature changes could be accurately monitored in both water-based and fat-based tissues. To this end, we present a hybrid proton resonance frequency (PRF)/T(1) approach for simultaneously measuring PRF shift temperatures in water-based tissues and T(1) changes in fat-based tissues. The hybrid PRF/T(1) sequence is a standard radiofrequency spoiled gradient echo sequence executed in a dynamic mode with two flip angles alternating every time frame. The PRF information is extracted every time frame using the image phase in the standard approach, and the T(1) information is extracted every two time frames using a variable flip angle approach. Simulation studies, ex vivo high intensity focused ultrasound heating experiments, and in vivo stability experiments were performed to test the feasibility of the approach. The results indicate that the hybrid PRF/T(1) approach provides PRF temperature maps of the same quality as those obtained by traditional PRF methods while simultaneously being able to track T(1) changes in fat-based tissues. Although several potential error sources exist for the T(1) measurements, the approach is a promising start toward realizing quantitative temperature measurements in both water-based and fat-based tissues.