Bio

Honors & Awards


  • Highly Cited Article IEEE, Transactions on Ultrasonics, Ferroelectrics, and Frequency Control (November 2013)
  • Editor's Selection of Articles in IEEE, Transactions on Ultrasonics, Ferroelectrics, and Frequency Control (July 2013)
  • Brain Program Contract Award, Focused Ultrasound Surgery Foundation (August 2012)
  • Finalist Graduate Student Paper Award, International Society of Therapeutic Ultrasound (April 2011)
  • Young Investigator Award, Focused Ultrasound Surgery Foundation (October 2010)
  • New Investigator Award, Society of Thermal Medicine (April 2010)
  • Best Poster Award, 4th Annual Mountain West Biomedical Engineering Conference, Park City, Utah. (2008)

Professional Education


  • Doctor of Philosophy, University of Utah (2011)
  • Bachelor of Engineering, Shri Govindram Seksaria Institute of Tech&Sciences (2006)

Stanford Advisors


Publications

Journal Articles


  • Transcranial phase aberration correction using beam simulations and MR-ARFI. Medical physics Vyas, U., Kaye, E., Pauly, K. B. 2014; 41 (3): 032901-?

    Abstract

    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.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.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.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.

    View details for DOI 10.1118/1.4865778

    View details for PubMedID 24593740

  • Ultrasound Beam Simulations in Inhomogeneous Tissue Geometries Using the Hybrid Angular Spectrum Method IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL Vyas, U., Christensen, D. 2012; 59 (6): 1093-1100

    Abstract

    The angular spectrum method is a fast, accurate and computationally efficient method for modeling wave propagation. However, the traditional angular spectrum method assumes that the region of propagation has homogenous properties. In this paper, the angular spectrum method is extended to calculate ultrasound wave propagation in inhomogeneous tissue geometries, important for clinical efficacy, patient safety, and treatment reliability in MR-guided focused ultrasound surgery. The inhomogeneous tissue region to be modeled is segmented into voxels, each voxel having a unique speed of sound, attenuation coefficient, and density. The pressure pattern in the 3-D model is calculated by alternating between the space domain and the spatial-frequency domain for each plane of voxels in the model. The new technique was compared with the finite-difference time-domain technique for a model containing attenuation, refraction, and reflection and for a segmented human breast model; although yielding essentially the same pattern, it results in a reduction in calculation times of at least two orders of magnitude.

    View details for DOI 10.1109/TUFFC.2012.2300

    View details for Web of Science ID 000305760000003

    View details for PubMedID 22711405

  • Rapid aberration correction for transcranial magnetic resonance-guided focused ultrasound surgery using a hybrid simulation and magnetic resonance-acoustic radiation force imaging method. journal of the Acoustical Society of America Vyas, U., Butts Pauly, K. 2013; 134 (5): 4183-?

    Abstract

    Transcranial magnetic resonance-guided focused ultrasound surgery is a technique for causing tissue necrosis in the brain though the intact skull. Skull spatial and acoustic heterogeneities cause changes in the location, shape, and intensity of the focus. Current techniques use computed tomography (CT) imaging or MR-acoustic radiation force images (MR-ARFI) to correct these aberrations. CT-based techniques approximate acoustic parameters from Hounsfield units but suffer from co-registration concerns. MR-ARFI-based techniques use MR images as feedback to manipulate transducer phases, but require many image acquisitions (~4000) for one correction [Herbert, IEEE-TUFFC 56(11)2388-2399]. We demonstrate here a hybrid technique that uses one MR-ARFI image to improve the focal intensity. The hybrid simulation-MR-ARFI technique used an optimization routine to iteratively modify the simulation aberrations to minimize the difference between simulated and experimental radiation force patterns. Experiments were conducted by applying skull-based aberrations to a 1024-element, 550 kHz phased-array transducer. The experimental MR-ARFI image of the aberrated focus was used with the simulation pattern from the hybrid angular spectrum [Vyas, IEEE-TUFFC 59(6)1093-1100] beam propagation technique to estimate aberrations. The experiment was repeated three times. The hybrid simulation-MR-ARFI technique resulted in an average increase in focal MR-ARFI phase of 44%, and recovered 83% of the ideal MR-ARFI phase.

    View details for DOI 10.1121/1.4831337

    View details for PubMedID 24181669

  • An analytical solution for improved HIFU SAR estimation PHYSICS IN MEDICINE AND BIOLOGY Dillon, C. R., VYAS, U., Payne, A., Christensen, D. A., Roemer, R. B. 2012; 57 (14): 4527-4544

    Abstract

    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.

    View details for DOI 10.1088/0031-9155/57/14/4527

    View details for Web of Science ID 000306072500007

    View details for PubMedID 22722656

  • Reconstruction of fully three-dimensional high spatial and temporal resolution MR temperature maps for retrospective applications MAGNETIC RESONANCE IN MEDICINE Todd, N., Vyas, U., de Bever, J., Payne, A., Parker, D. L. 2012; 67 (3): 724-730

    Abstract

    Many areas of MR-guided thermal therapy research would benefit from temperature maps with high spatial and temporal resolution that cover a large three-dimensional volume. This article describes an approach to achieve these goals, which is suitable for research applications where retrospective reconstruction of the temperature maps is acceptable. The method acquires undersampled data from a modified three-dimensional segmented echo-planar imaging sequence and creates images using a temporally constrained reconstruction algorithm. The three-dimensional images can be zero-filled to arbitrarily small voxel spacing in all directions and then converted into temperature maps using the standard proton resonance frequency shift technique. During high intensity focused ultrasound heating experiments, the proposed method was used to obtain temperature maps with 1.5 mm × 1.5 mm × 3.0 mm resolution, 288 mm × 162 mm × 78 mm field of view, and 1.7 s temporal resolution. The approach is validated to demonstrate that it can accurately capture the spatial characteristics and time dynamics of rapidly changing high intensity focused ultrasound-induced temperature distributions. Example applications from MR-guided high intensity focused ultrasound research are shown to demonstrate the benefits of the large coverage fully three-dimensional temperature maps, including characterization of volumetric heating trajectories and near- and far-field heating.

    View details for DOI 10.1002/mrm.23055

    View details for Web of Science ID 000300683900017

    View details for PubMedID 21702066

  • Design and characterization of a laterally mounted phased-array transducer breast-specific MRgHIFU device with integrated 11-channel receiver array MEDICAL PHYSICS Payne, A., Merrill, R., Minalga, E., VYAS, U., de Bever, J., Todd, N., Hadley, R., Dumont, E., Neumayer, L., Christensen, D., Roemer, R., Parker, D. 2012; 39 (3): 1552-1560

    Abstract

    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.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.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.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.

    View details for DOI 10.1118/1.3685576

    View details for Web of Science ID 000301503400041

    View details for PubMedID 22380387

  • 3D Volume MR Temperature Mapping for HIFU Heating Trajectory Comparisons 11TH INTERNATIONAL SYMPOSIUM ON THERAPEUTIC ULTRASOUND Todd, N., Vyas, U., de Bever, J., Payne, A., Parker, D. L. 2012; 1481: 388-394

    View details for DOI 10.1063/1.4757366

    View details for Web of Science ID 000313182200066

  • Transcranial Phase Aberration Correction Using Beam Simulations and MR-ARFI 12TH INTERNATIONAL SYMPOSIUM ON THERAPEUTIC ULTRASOUND Vyas, U., Kaye, E., Pauly, K. B. 2012; 1503: 185-190

    View details for DOI 10.1063/1.4769941

    View details for Web of Science ID 000312416000035

  • Extension of the angular spectrum method to calculate pressure from a spherically curved acoustic source JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Vyas, U., Christensen, D. A. 2011; 130 (5): 2687-2693

    Abstract

    The angular spectrum method is an accurate and computationally efficient method for modeling acoustic wave propagation. The use of the typical 2D fast Fourier transform algorithm makes this a fast technique but it requires that the source pressure (or velocity) be specified on a plane. Here the angular spectrum method is extended to calculate pressure from a spherical transducer-as used extensively in applications such as magnetic resonance-guided focused ultrasound surgery-to a plane. The approach, called the Ring-Bessel technique, decomposes the curved source into circular rings of increasing radii, each ring a different distance from the intermediate plane, and calculates the angular spectrum of each ring using a Fourier series. Each angular spectrum is then propagated to the intermediate plane where all the propagated angular spectra are summed to obtain the pressure on the plane; subsequent plane-to-plane propagation can be achieved using the traditional angular spectrum method. Since the Ring-Bessel calculations are carried out in the frequency domain, it reduces calculation times by a factor of approximately 24 compared to the Rayleigh-Sommerfeld method and about 82 compared to the Field II technique, while maintaining accuracies of better than 96% as judged by those methods for cases of both solid and phased-array transducers.

    View details for DOI 10.1121/1.3621717

    View details for Web of Science ID 000297486500026

    View details for PubMedID 22087896

  • The effect of electronically steering a phased array ultrasound transducer on near-field tissue heating MEDICAL PHYSICS Payne, A., Vyas, U., Todd, N., de Bever, J., Christensen, D. A., Parker, D. L. 2011; 38 (9): 4971-4981

    Abstract

    This study presents the results obtained from both simulation and experimental techniques that show the effect of mechanically or electronically steering a phased array transducer on proximal tissue heating.The thermal response of a nine-position raster and a 16-mm diameter circle scanning trajectory executed through both electronic and mechanical scanning was evaluated in computer simulations and experimentally in a homogeneous tissue-mimicking phantom. Simulations were performed using power deposition maps obtained from the hybrid angular spectrum (HAS) method and applying a finite-difference approximation of the Pennes' bioheat transfer equation for the experimentally used transducer and also for a fully sampled transducer to demonstrate the effect of acoustic window, ultrasound beam overlap and grating lobe clutter on near-field heating.Both simulation and experimental results show that electronically steering the ultrasound beam for the two trajectories using the 256-element phased array significantly increases the thermal dose deposited in the near-field tissues when compared with the same treatment executed through mechanical steering only. In addition, the individual contributions of both beam overlap and grating lobe clutter to the near-field thermal effects were determined through comparing the simulated ultrasound beam patterns and resulting temperature fields from mechanically and electronically steered trajectories using the 256-randomized element phased array transducer to an electronically steered trajectory using a fully sampled transducer with 40 401 phase-adjusted sample points.Three distinctly different three distinctly different transducers were simulated to analyze the tradeoffs of selected transducer design parameters on near-field heating. Careful consideration of design tradeoffs and accurate patient treatment planning combined with thorough monitoring of the near-field tissue temperature will help to ensure patient safety during an MRgHIFU treatment.

    View details for DOI 10.1118/1.3618729

    View details for Web of Science ID 000294482900007

    View details for PubMedID 21978041

  • The Effects of Spatial Sampling Choices on MR Temperature Measurements MAGNETIC RESONANCE IN MEDICINE Todd, N., Vyas, U., de Bever, J., Payne, A., Parker, D. L. 2011; 65 (2): 515-521

    Abstract

    The purpose of this article is to quantify the effects that spatial sampling parameters have on the accuracy of magnetic resonance temperature measurements during high intensity focused ultrasound treatments. Spatial resolution and position of the sampling grid were considered using experimental and simulated data for two different types of high intensity focused ultrasound heating trajectories (a single point and a 4-mm circle) with maximum measured temperature and thermal dose volume as the metrics. It is demonstrated that measurement accuracy is related to the curvature of the temperature distribution, where regions with larger spatial second derivatives require higher resolution. The location of the sampling grid relative temperature distribution has a significant effect on the measured values. When imaging at 1.0 × 1.0 × 3.0 mm(3) resolution, the measured values for maximum temperature and volume dosed to 240 cumulative equivalent minutes (CEM) or greater varied by 17% and 33%, respectively, for the single-point heating case, and by 5% and 18%, respectively, for the 4-mm circle heating case. Accurate measurement of the maximum temperature required imaging at 1.0 × 1.0 × 3.0 mm(3) resolution for the single-point heating case and 2.0 × 2.0 × 5.0 mm(3) resolution for the 4-mm circle heating case.

    View details for DOI 10.1002/mrm.22636

    View details for Web of Science ID 000286643000025

    View details for PubMedID 20882671

  • The Effect of Electronically Steering a Phased Array on Proximal Tissue Heating 9TH INTERNATIONAL SYMPOSIUM ON THERAPEUTIC ULTRASOUND Payne, A., Vyas, U., Todd, N., de Bever, J., Christensen, D. A., Parker, D. L. 2010; 1215: 145-148
  • Minimisation of HIFU pulse heating and interpulse cooling times INTERNATIONAL JOURNAL OF HYPERTHERMIA Payne, A., Vyas, U., Blankespoor, A., Christensen, D., Roemer, R. 2010; 26 (2): 198-208

    Abstract

    This study presents results from a new optimisation technique that reduces HIFU treatment times by minimising individual heating and interpulse cooling times while adhering to normal tissue constraint limits at each sonication position. The potential clinical usefulness of this technique is demonstrated through its implementation in three dimentsional (3D) simulations of HIFU treatments for a range of tumour geometries, normal tissue constraint values, tissue perfusion levels and focal zone scanning path trajectories, all studied as a function of the applied power magnitude. When compared to typical open loop values the optimised treatment times were lower for all conditions studied, including when treatment-limiting normal tissue thermal build-up was present. While use of this technique guarantees minimum pulse heating and interpulse cooling times for each pulse, the total treatment time gains realised depend on the individual clinical treatment configuration. In combination with a judiciously selected scan path, use of the pulse time optimisation procedure reduced treatment times in a small, superficial tumour by 85%. In addition, in all cases studied the use of an increased applied power always decreased the treatment time, including cases when significant normal tissue thermal build-up was present. Importantly, the power maximisation and pulse time minimisation procedures can be applied independently of the optimisation of the focal zone's scan path, size and shape. Given the basic nature, universal applicability and ready clinical adaptability for use in real time model predictive control, the pulse time minimisation and power maximisation approaches have significant clinical promise for reducing HIFU treatment times.

    View details for DOI 10.3109/02656730903436459

    View details for Web of Science ID 000274856800011

    View details for PubMedID 20146573

  • Optimization of HIFU Treatments for Use in Model Predictive Control 8TH INTERNATIONAL SYMPOSIUM ON THERAPEUTIC ULTRASOUND Payne, A., Blankespoor, A., Todd, N., VYAS, U., Christensen, D., Parker, D., Roemer, R. 2009; 1113: 170-174
  • Ultrasound Beam Propagation using the Hybrid Angular Spectrum Method 2008 30TH ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY, VOLS 1-8 Vyas, U., Christensen, D. 2008: 2526-2529

    Abstract

    We introduce a fast and accurate numerical method for simulating the propagation of an ultrasound beam inside inhomogeneous tissue for mapping beam absorption, refraction and diffraction in the body. The technique, called the hybrid angular spectrum method, is an extension of the angular spectrum method to inhomogeneous tissue. Inhomogeneous tissue is modeled using voxels, each with its own speed of sound, density and absorption coefficient. The proposed technique produces very fast simulations, with total calculation times of about one minute for a 201x201x101 model.

    View details for Web of Science ID 000262404501234

    View details for PubMedID 19163217

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