A number of novel imaging modalities have been developed to interrogate the
mechanical properties of tissue. A subset of these methods utilize acoustic
radiation force to mechanically excite tissue and form images from the local
responses of tissue to these excitations. These methods are attractive
because of the ability to focus and steer the excitatory beams and to
control their spatial and temporal characteristics using techniques similar
to those employed in conventional ultrasonic imaging. These capabilities
allow for a wide variety of imaging methods whose features are only
beginning to be explored. However, radiation force based methods also
present significant challenges. Tissue and transducer heating limit the
tissue displacements achievable with radiation force applications and
restrict image frame rates and fields-of-view. The small tissue
displacements are difficult to detect and may be obscured by physiologic
tissue motion. We review the fundamental limits of imaging methods based on
radiation force generated by patient safety concerns and the impact of these
limits on achievable image signal-to-noise ratios and frame rates. We also
review our progress to date in the development and clinical evaluation of
one class of radiation force imaging methods employing very brief impulses
of radiation force.
Acoustic Radiation Force Impulse (ARFI) imaging utilizes brief, high-energy acoustic pulses to excite tissue and ultrasonic correlation based tracking methods to monitor the resulting tissue
displacement, which reflects the relative mechanical properties of tissue (i.e. stiffer tissue displaces less). ARFI image contrast is optimized utilizing tightly focused radiation force excitations at multiple axial and lateral locations throughout a 2D field of view. In an ongoing, IRB approved, clinical study, suspicious breast lesions are interrogated in vivo via multi-focal-zone ARFI prior to undergoing core biopsy. A Siemens SONOLINE Antares (TM) scanner and VF10-5 probe were configured to acquire ARFI data from multiple focal-zones and lateral locations. Data was acquired in real-time, and processed off-line. Processing included: filtering, parametric data analysis, normalization and combination of the multiple focal-zone data, and automatic edge detection. ARFI sequences were designed with varying pushing pulse frequencies and intensities. Contrast to noise ratio was evaluated in a tissue mimicking phantom for lesions at different depths using the different pushing pulse sequences. For shallower lesions (depth=10mm), CNR was higher than for deeper lesions, and did not vary appreciably for the different push sequences. For deeper lesions (depth=20mm), CNR increased with increasing push pulse intensity and decreasing push pulse frequency. With the pushing pulse transmit intensity calibrated (in a homogeneous phantom) to achieve uniform displacement at all axial depths, in vivo results yielded poor SNR at depth and did not achieve overall uniform displacement. In vivo, image quality improved with increasing push pulse intensity. To date, 27 masses have been interrogated using multi-focal-zone ARFI and overall good structural agreement exists between B-mode and ARFI images. Normalization and blending facilitate image generation from ARFI interrogation using different intensities at different focal depths.
Acoustic Radiation Force Impulse (ARFI) imaging uses short duration acoustic pulses to generate and subsequently determine localized displacements in tissue. Time delay estimators, such as normalized cross correlation and phase shift estimation, form the computational basis for ARFI imaging. This paper considers these algorithms and the effects of noise, interpolation, and quadrature demodulation on the accuracy of the time delay estimates. These results are used to implement a real-time ARFI imaging system and in an ex vivo liver ablation study.
We have developed a new method of imaging the mechanical properties of tissues based on very brief (<1msec) and localized applications of acoustic radiation force and the ultrasonic measurement of local tissues' responses to that force. Initial results with this technique demonstrate its ability to image mechanical properties of the medial and adventitial layers within ex vivo and in vivo arteries, and to distinguish hard and soft atherosclerotic plaques from normal vessel wall. We have labeled this method Acoustic Radiation Force Impulse (ARFI) imaging. We describe studies to utilize this technique in the characterization of diffuse and focal atherosclerosis. We describe phantom trials and finite element simulations which explore the fundamental resolution and contrast achievable with this method. We describe in vivo and ex vivo trials in the popliteal, femoral and brachial arteries to assess the relationship between the mechanical properties of healthy and diseased arteries provided by this method and those obtained by alternative methods.
Acoustic radiation force may be used to induce localized displacements within tissue. This phenomenon is used in Acoustic Radiation Force Impulse Imaging (ARFI), where short bursts of ultrasound deliver an impulsive force to a small region. The application of this transient force launches shear waves which propagate normally to the ultrasound beam axis. Measurements of the displacements induced by the propagating shear wave allow reconstruction of the local shear modulus, by wave tracking and inversion techniques. Here we present in vitro, ex vivo and in vivo measurements and images of shear modulus. Data were obtained with a single transducer, a conventional ultrasound scanner and specialized pulse sequences. Young's modulus values of 4 kPa, 13 kPa and 14 kPa were observed for fat, breast fibroadenoma, and skin. Shear modulus anisotropy in beef muscle was observed.
We are investigating a novel ultrasonic method for remote palpation, which provides images of local variations in tissue stiffness. Acoustic radiation force is applied to small volumes of tissue, and the resulting displacement patterns are imaged using ultrasonic correlation based techniques. Tissue displacements are inversely proportional to tissue stiffness, thus a stiffer region of tissue exhibits smaller displacements than a more compliant region. This method also provides information about tissue recovery after force cessation. We will present in vivo experimental results demonstrating the feasability of this method. Using intensities ranging from 120 to 300 W/cm2, peak displacements of up to 50 microns were observed after 1.4 milliseconds of force application. The tissue moved to its peak displacement within 3 milliseconds of force application, and the time constants for tissue recovery varied with tissue type. Tissue displacements appeared to be correlated with tissue structure in matched B-mode images. To our knowledge, these results represent the first in vivo soft tissue images generated using radiation force. These findings support the feasibility of Remote Palpation imaging. We will discuss the technical, safety, and clinical challenges of implementing a real-time Remote Palpation imaging system on a commercial diagnostic scanner.
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