A beamforming study for implementation of vibro-acoustography with a 1.75-D array transducer

Abstract

Vibro-acoustography (VA) is an ultrasound-based imaging modality that uses radiation force produced by two cofocused ultrasound beams separated by a small frequency difference, Δf, to vibrate tissue at Δf. An acoustic field is created by the object vibration and measured with a nearby hydrophone. This method has recently been implemented on a clinical ultrasound system using one-dimensional (1D) linear array transducers. In this article, we discuss VA beamforming and image formation using a 1.75D array transducer. A 1.75D array transducer has several rows of elements in the elevation direction which can be controlled independently for focusing. The advantage of the 1.75D array over a 1D linear array transducer is that multiple rows of elements can be used for improving elevation focus for imaging formation. Six configurations for subaperture design for the two ultrasound beams necessary for VA imaging were analyzed. The point-spread functions for these different configurations were evaluated using a numerical simulation model. Four of these configurations were then chosen for experimental evaluation with a needle hydrophone as well as for scanning two phantoms. Images were formed by scanning a urethane breast phantom and an ex vivo human prostate. VA imaging using a 1.75D array transducer offers several advantages over scanning with a linear array transducer including improved image resolution and contrast due to better elevation focusing of the imaging point-spread function.

Feasibility of vibro‐acoustography with a quasi‐2D ultrasound array transducer for detection and localizing of permanent prostate brachytherapy seeds: A pilot ex vivo study

“…These two beams intersect and generate a low frequency modulated ARF at f = 51.5464 kHz at the focal region. The spatial distribution of ARF by using the described configuration is studied by Urban et al 36 During the scanning, a digitally controlled multiplexer was utilized to translate and steer the active aperture along the azimuthal direction and over the total aperture of the transducer. The probe is moved by a motorized linear axis [not shown in Fig.…”

“…The Q2D array US transducer was designed for the operating frequencies between 5 and 8 MHz. Since the ultrasound scanner was capable of simultaneously driving 128 channels, a subset of the 12 × 12 elements were selected to form an "active" VA aperture 36 [ Fig. 1(b)].…”

“…37 While a 2D reconfigurable array is potentially an ideal solution for clinical implementation of VA, we have recently developed a VA system using a Q2D (in technical term, it is called 1.75D array transducer) array transducer as an intermediate step toward a desired 2D VA system. 36,38 This Q2D array US transducer has multiple rows of elements in elevation which can be electronically configured to optimally generate a close-to-symmetric ARF. Beam mixing and generating ARF by using a Q2D array US transducer is closer to the ideal confocal VA and thus can improve VA images as compared to a 1D linear array transducer.…”

“…While VA operating with a confocal probe provides an ARF with a symmetric spatial distribution in the focal plane, 34,35 the 1D lineararray transducer produces an asymmetric ARF. 36 Therefore, objects and features can appear elongated or blurred in 1D VA images. 32,33 As a result, although 1D VA can reveal larger objects and features in tissues, smaller objects such as PPB seeds cannot be well-visualized by using 1D array transducers.…”

“…Beam mixing and generating ARF by using a Q2D array US transducer is closer to the ideal confocal VA and thus can improve VA images as compared to a 1D linear array transducer. 36,38 Vibro-acoustography has shown promising results in various medical diagnostic applications ranging from detection of breast tumors 29,31 and calcifications in arteries, 28 to exogenous objects such as brachytherapy seeds 39 and metal implants. Our research group has previously demonstrated the functionality of confocal VA in prostate imaging 30,40 and detecting brachytherapy seeds in excised prostate specimens.…”