Real time 3D ultrasound imaging

Acoustic Radiation Force Impulse (ARFI) imaging exploits differences in the mechanical properties of soft tissue to delineate tissue structure that is not necessarily apparent with conventional B-Mode ultrasound. In ARFI imaging, an impulse of relative high acoustic energy is transmitted into the body to deliver spatially and temporally localized radiation force at the imaging focus in a manner that subtly pushes tissue away from the imaging transducer (tissue displacement is on the order of microns). Each ARFI impulse is followed by ensembles of conventional ultrasonic transmit-receive lines, which serve to generate data for ARFI-induced axial motion tracking with one-dimensional cross-correlation. Displacements measured in space and time may then be rendered into graphical and parametric image representations that depict differences in tissue mechanical properties. Given that ARFI imaging is implemented using a conventional diagnostic ultrasound system (specially equipped for research purposes) and conventional ultrasound transducers, virtually simultaneous matched B-Mode and Doppler imaging is possible. The physical basis of ARFI imaging is presented with extensive detail by Nightingale et al. (Nightingale, 2003). In comparison to alternative approaches to noninvasive screening for CVD, such as MR or X-ray based imaging methods, ARFI imaging is advantageous in that is fundamentally an ultrasonic imaging technology. Therefore, it is non-ionizing, relatively inexpensive, real-time, and portable. In addition, matched B-Mode and Doppler ultrasonography is readily realizable.

A matched B-mode image of the left iliac artery of a 1 year 8 month-old familial hypercholesterolemic pig  is presented in (a), with a focal atherosclerotic plaque (red arrow, ‘AP’) and hyper-echogenicity of the leading plaque edge (yellow arrow) apparent.  The profiles of (b) represent initial ARFI-induced displacement and recovery for a non-atherosclerotic vessel wall region (solid) and atherosclerotic region (dashed) under intraluminal pressures of 3 (blue) to 12 (red) kPa.  The non-atherosclerotic and atherosclerotic regions examined are marked on the B-Mode image of (a) by blue dots.  A two-dimensional parametric image of peak ARFI-induced displacements is shown in (c), with color representing peak displacement in microns, per the adjacent colorbar.  Masked blood signal is mapped to the color gray.  Time to 67% recovery from peak ARFI-induced displacement is illustrated in (d), with color coding representing recovery time in milliseconds.  The arterial wall beneath the raised atherosclerotic focal plaque is boxed.

Acoustic radiation force has been demonstrated for a variety of clinical imaging applications, including differentiating malignant lesions from fluid-filled cysts in the breast (Nightingale et al., 1999), manipulating the vitreous humor of the eye (Walker et al., 1999), and vibrating tissue at confocal transducers’ beat frequencies (Fatemi et al., 1999). More recently, ARFI imaging has been implemented for assessing blood clot formation in vitro (Viola et al., 2004), monitoring chemical and thermal ablations in vivo (Fahey et al., 2004) , and streaming blood in arteries and veins in vivo. In application to CVD, ARFI imaging has been successful for isolating regions of atherosclerosis via surveying arterial wall mechanical properties in in vivo and ex vivo human investigations (Trahey et al., 2004).

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