M.J. McAllister

University of Virginia, Charlottesville, VA, United States

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Publications (8)0 Total impact

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    ABSTRACT: Conventional coherent imaging systems map the energy which is reflected directly back towards the transducer. While extremely useful, these systems fail to utilize information in the energy field which has been scattered at other angles. Angular scatter imaging attempts to form images from the scattered energy field at angles other than the 180° backscattered path. We propose a synthetic aperture based imaging scheme for acquiring angular scatter data in medical ultrasound. We describe this technique in k-space and provide an intuitive explanation of the imaging system's behavior. This method, which we term Synthetic Aperture Angular Scatter (SAAS) imaging effectively uses single element geometries to acquire data at a range of scattering angles. In this paper, we present experimental results implementing SAAS on a GE Logiq 700MR system. We applied the SAAS method to form angular scatter images of a 5-wire depth of field (DOF) phantom and a tissue mimicking 3-wire phantom (steel, nylon and cotton). We present results from this data and discuss the degree of uniformity necessary in element response for successful SAAS imaging. Results from these experiments show new image information previously unavailable in conventional B-mode images and suggest that angular scatter imaging may have applications in the breast, thyroid and peripheral vasculature.
    Signals, Systems and Computers, 2004. Conference Record of the Thirty-Eighth Asilomar Conference on; 12/2004
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    ABSTRACT: Angular scatter imaging has been proposed as a new source of image contrast in medical ultrasound and as a parameter for tissue characterization. We describe a new method that combines the translating apertures algorithm (TAA) with synthetic aperture methods to coherently obtain angular scatter information with high resolution in both space and scattering angle. This method, which we term synthetic aperture angular scatter (SAAS) imaging effectively applies the TAA to single array elements and then focuses the data synthetically to form high resolution images at precisely defined scattering angles. In this paper, we present experimental results implementing SAAS to form angular scatter images of a 5-wire depth of field phantom, a tissue mimicking 3-wire phantom, and in vivo human thyroid. We discuss the degree of uniformity necessary in element response for successful SAAS imaging. These experiments show new image information previously unavailable in conventional B-mode images and suggest that angular scatter imaging may have applications in the breast, thyroid, and peripheral vasculature.
    Ultrasonics Symposium, 2004 IEEE; 09/2004
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    ABSTRACT: Angular scatter offers a new source of tissue contrast and an opportunity for tissue characterization in ultrasound imaging. We have previously described the application of the translating apertures algorithm (TAA) to coherently acquire angular scatter data over a range of scattering angles. While this approach works well at the focus, it suffers from poor depth of field (DOF) due to a finite aperture size. Furthermore, application of the TAA with large focused apertures entails a tradeoff between spatial resolution and scattering angle resolution. While large multielement apertures improve spatial resolution, they encompass many permutations of transmit/receive element pairs. This results in the simultaneous interrogation of multiple scattering angles, limiting angular resolution. We propose a synthetic aperture imaging scheme that achieves both high spatial resolution and high angular resolution. In backscatter acquisition mode, we transmit successively from single transducer elements, while receiving on the same element. Other scattering angles are interrogated by successively transmitting and receiving on different single elements chosen with the appropriate spatial separation between them. Thus any given image is formed using only transmit/receive element pairs at a single separation. This synthetic aperture approach minimizes averaging across scattering angles, and yields excellent angular resolution. Likewise, synthetic aperture methods allow us to build large effective apertures to maintain a high spatial resolution. Synthetic dynamic focusing and dynamic apodization are applied to further improve spatial resolution and DOF. We present simulation results and experimental results obtained using a GE Logiq 700MR system modified to obtain synthetic aperture TAA data. Images of wire targets exhibit high DOF and spatial resolution. We also present a novel approach for combining angular scatter data to effectively reduce grating lobes. With this approach we have been able to push the grating lobes below -50 dB in simulation and effectively eliminate their presence in the experimental wire target images.
    Proc SPIE 01/2004;
  • W.F. Walker, M.J. McAllister
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    ABSTRACT: Human tissues exhibit variation in scattering magnitude as the angle between transmission and reception is changed. These angular scatter variations result from intrinsic acoustic properties and sub-resolution structure. We have developed a clinical imaging system that uses the translating apertures algorithm to obtain statistically reliable, local angular scatter measurements. The obtained data can be processed to yield novel images. A significant problem with angular scatter imaging is limited depth of field (DOF). We describe a new method to improve DOF by applying shift variant filters to the data obtained at each angle. We show that this approach is optimal in a minimum sum squared error sense. The filter coefficients used in the technique can be determined via experiment or simulation. Unlike prior methods, this approach does not assume a model for the source of decorrelation, rather it includes all sources of decorrelation implicitly. We present simulation results showing the improvements in DOF obtained using this technique. We present experimental angular scatter data from phantoms and human subjects. In one phantom, designed to mimic microcalcifications in soft tissue, experimental data shows the angular scatter from 500 um glass spheres falling off by 50% over a 20 degree range of interrogation angles. In the same phantom the angular scatter from 50 um sephadex spheres fell off by only 10% over the same range. In the human calf muscle, brightness fell off by 60% over 20 degrees, while tendon brightness dropped by only 20%. Interestingly, the brightest target in the phantom (glass spheres) exhibited the greatest angular scatter variation, while the brightest target in the calf (tendon) exhibited the least angular scatter variation. These results provide compelling evidence that angular scatter properties are uncorrelated to b-mode image brightness.
    Ultrasonics Symposium, 2002. Proceedings. 2002 IEEE; 11/2002
  • M.J. McAllister, K.W. Rigby, W.F. Walker
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    ABSTRACT: Traditional ultrasound systems measure backscatter in B-mode, capturing only the acoustic energy that is reflected directly from the target region to the transducer face. These systems fail to utilize the information in the echo field that is scattered in other directions and therefore cannot characterize the angular scattering behavior of the targets being observed. Since target-specific angular scattering has great potential as a source of increased contrast in biological tissues, it is desirable to modify the method of acquisition in order to obtain reliable information about this behavior. However, prior systems used to investigate this information have been clinically unwieldy and statistically inaccurate over small regions. We have implemented a method of acquisition that utilizes the translating apertures algorithm: (TAA) to reliably separate target-specific angular scatter information from the effects of changing acquisition geometry. This acquisition method has been implemented in real-time on a clinical linear array system. Seven interrogation angles are acquired for each imaging line, and the TAA is implemented repeatedly across the array to yield per-pixel maps of angular scatter behavior
    Ultrasonics Symposium, 2001 IEEE; 02/2001
  • Michael J. McAllister, William F. Walker
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    ABSTRACT: A new method of acquiring and processing angular scatter data in ultrasonic imaging is presented. This method, based on the translating apertures algorithm, eliminates system-dependent changes in the received point spread function (psf) that are associated with more conventional methods of angular scatter measurement. This ensures that changes in the received echo are dominated by changes in the angular scattering behavior of insonified targets, and allows for the development of a variety of new imaging methods. Comparison of received echoes acquired at multiple interrogation angles serves to enhance the contrast of targets exhibiting variations in angular scattering behavior relative to the surrounding medium. Emphasis is placed on the improved ability to highlight biological targets that exhibit significant variations in compressibility or density relative to background tissue (e.g. breast microcalcifications, calcified atherosclerotic plaques). Simulation indicates the enhancement of breast microcalcification contrast by 10 - 30 dB over standard b-mode acquisition at 10 MHz. More sophisticated imaging methods involving the frequency dependence of angular scatter and angular speckle coherence are also discussed. Practical implementation and evaluation of this method on a modern imaging system is discussed, and expectations for the performance and utility of this algorithm in clinical diagnosis are investigated.
    Proc SPIE 04/2000;
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    ABSTRACT: Tissue elasticity estimation is a major topic of ultrasound research. While most research focuses on cancer detection, the authors have concentrated on the detection of mechanical changes in the vitreous body of the eye. These changes are believed to contribute to retinal detachment. Thus, an accurate method of imaging vitreous mechanical properties might enable identification of patients at high risk before the presentation of symptoms. The authors are developing a new method of vitreous imaging which attempts to image tissue stiffness by applying acoustic radiation force. Force is applied at either a single location or along a line, with resultant target displacements estimated from returned echoes. This approach yields the dynamic response of targets to the applied forces. Images can be formed of the maximum induced displacement, or the relative elasticity and relative viscosity found by fitting experimental data to the Voigt model. The authors present B-Mode, maximum displacement, relative elasticity, and relative viscosity images of tissue mimicking phantoms and an enzymatically modified porcine eye. Images show the expected correlation between material stiffness and measured displacement. The potential of radiation force imaging was further explored by using a clinical ultrasound system to interrogate a phantom. At 84 mW/cm<sup>2</sup> small displacements were visible in the B-Mode image, suggesting that measurable displacements may be generated at 50 mW/cm<sup>2</sup>, the historical FDA ophthalmic power limit
    Ultrasonics Symposium, 1999. Proceedings. 1999 IEEE; 02/1999
  • W.F. Walker, M.J. McAllister
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    ABSTRACT: Conventional ultrasound systems depict tissue backscatter; that is, the energy reflected directly back to the transmitter. While diagnostically useful, these systems fail to exploit information available in the echoes scattered in other directions (angular scatter). This paper describes a new method of angular scatter imaging. It applies the translating apertures algorithm to maintain a nearly uniform psf over a range of interrogation angles. Data from two angles is processed to form images of the constant scatter with angle and the differential scatter with angle. The authors refer to these images as c-weighted (constant) and d-weighted (difference) images. Since psf uniformity is maintained and speckle variation avoided, these images depict local angular scatter without spatial averaging. The authors present an algorithm and accompanying equations for formation of c- and d-weighted images. They present simulations showing that c- and d-weighted images may offer improved contrast in soft tissues, and that they will significantly improve the detectability of microcalcifications (MCs), important indicators of breast cancer
    Ultrasonics Symposium, 1999. Proceedings. 1999 IEEE; 01/1999