Acoustic Radiation Force Impulse (ARFI) Imaging: a Review

Abstract

Acoustic radiation force based elasticity imaging methods are under investigation by many groups. These methods differ from traditional ultrasonic elasticity imaging methods in that they do not require compression of the transducer, and are thus expected to be less operator dependent. Methods have been developed that utilize impulsive (i.e. < 1 ms), harmonic (pulsed), and steady state radiation force excitations. The work discussed herein utilizes impulsive methods, for which two imaging approaches have been pursued: 1) monitoring the tissue response within the radiation force region of excitation (ROE) and generating images of relative differences in tissue stiffness (Acoustic Radiation Force Impulse (ARFI) imaging); and 2) monitoring the speed of shear wave propagation away from the ROE to quantify tissue stiffness (Shear Wave Elasticity Imaging (SWEI)). For these methods, a single ultrasound transducer on a commercial ultrasound system can be used to both generate acoustic radiation force in tissue, and to monitor the tissue displacement response. The response of tissue to this transient excitation is complicated and depends upon tissue geometry, radiation force field geometry, and tissue mechanical and acoustic properties. Higher shear wave speeds and smaller displacements are associated with stiffer tissues, and slower shear wave speeds and larger displacements occur with more compliant tissues. ARFI images have spatial resolution comparable to that of B-mode, often with greater contrast, providing matched, adjunctive information. SWEI images provide quantitative information about the tissue stiffness, typically with lower spatial resolution. A review these methods and examples of clinical applications are presented herein.

Fig. 1

Images (a)–(c) portray the simulated axial displacement response in a 3D, homogeneous, isotropic, elastic tissue (E, the elastic Young’s modulus, = 4kPa) to a focused (F/1.3 focal configuration, frequency= 6.7 MHz), impulsive (pulse duration = 50 µs), radiation force excitation, at three snapshots in time after the excitation (0.3ms, 2 ms, and 4ms). The transducer is located at the top of the images, focused in the center of the plane, at 20 mm in depth (marked by the blue X in the images). The red circle is located at 1.5 mm laterally, and the green square at 3 mm laterally in the images. The 3D tissue volume has been bisected so that the central axial/lateral plane is visible. The displacement response at each symbol (x, circle, square) as a function of time after excitation is shown in (d). Positive displacement indicates displacement in the direction of acoustic wave propagation, or down in the images. Initially, (image (a)), the axial displacement response is spatially distributed in the geometry of the applied radiation force, confined within the ROE, with the largest displacement near the focus; the peak focal displacement of 30 µm occurs at 0.4 ms after excitation (blue line in (d)). With increasing time after excitation, a shear wave propagates away from the ROE, decreasing in amplitude with propagation distance due to geometric spreading. In viscoelastic media such as tissue, shear wave attenuation would further decrease the shear wave amplitude with increasing propagation distance. The peak displacement amplitude is inversely proportional to the tissue elastic (shear) modulus, and the speed of the shear wave propagation is quadratically related to the shear modulus.

Fig. 2

Experimental data: matched B-mode (a) and normalized ARFI displacement images, (b)–(d), of a Computerized Imaging Reference Systems, Inc. (CIRS, Norfolk, VA) custom tissue mimicking phantom (E=4 kPa) with two 3 mm spherical lesions (E=58kPa). The lesion contrast in the ARFI images is largest at t=0.3 ms (b), decreases with time after excitation (c), and reverses later in time (d). In addition, the lesion size appears to grow with time post-force, which is caused by shear wave propagation and reflection at lesion boundaries (35). Note also the ’posterior enhancement’, or increase of displacement beneath the lesions (b), (c). This is because the lesions were slightly less attenuating than the surrounding tissue, thus the tissue beneath the lesions experienced larger radiation force than that adjacent to it. Figure reproduced with permission from:(36)

Fig. 3

(a) Simulated temperature increases in the axial-lateral plane (centered in elevation) for a single ARFI interrogation using a relatively high frequency (7 MHz), and tight focal configuration (F/1.3). The transducer is centered along the top of the image, spanning a total of 1.52 cm laterally. The focus is at 2.0 cm. (b) Temperature increases after a single frame of two-dimensional ARFI imaging using the pulse from (a) in a high contrast beam sequence (50 tightly focused, closely spaced (0.35 mm) push pulses, with a time delay of 5 ms between successive locations temporally), assuming a tissue. absorption of 0.5 dB/cm/MHz. Note that the maximum temperature increase has shifted from the focal depth of 2.0 cm (the location of maximum temperature increase for a single ARFI interrogation), to 1.45 cm, which is the region of maximum overlap for the sequence. (c) Real-time, two-dimensional ARFI imaging, using the same sequence as in (b), performed at a frame rate of 3 frames per second for a total duration of 3 seconds. Note that the temperature scale, in degrees Celsius, is different for each figure. Figure reproduced with permission from:(43)

Fig. 4

TOP ROW: Simulated displacement through time profiles, without ultrasonic tracking, at lateral positions offset from the excitation location for elastic media with shear moduli of (a) 1.33 kPa and (b) 8 kPa. Notice that the curve appears more finely sampled in the more compliant medium (1.33 kPa) because of its slower propagation speed and the fixed 10-kHz temporal sampling (simulating a fixed PRF in the experimental system). The vertical dotted lines indicate the TTP values that would be estimated from this data, although experimentally the data would be upsampled using a low-pass interpolation from the acquired PRF to 50 kHz. Notice that the two plots are on different time scales. BOTTOM ROW: (c) Time of peak displacement at the focal depth (20 mm) as a function of lateral position in simulation data for elastic materials, with shear moduli of 1.33 and 2.83 kPa. The inverse slopes of these lines represent the shear wave speeds in these materials. (d) Reconstructed shear moduli over depths from 16–20 mm using the time to peak displacement data at lateral ranges from 2–8 mm outside of the ROE to estimate the shear wave speed for elastic materials with 1.33 (x) and 2.83 (o) kPa shear moduli. The raw FEM data are represented by the red (x) and blue (o) lines, with the mean ± one standard deviation shear modulus estimates over the range of depths represented in each colored text box. The corresponding ultrasonically-displacement-tracked data, using 20 independent speckle realizations, is shown in the black lines. Figure reproduced with permission from:(33)

Fig. 5

Qualitative ARFI images with the co-registered reconstructed shear moduli and matched histological slides from an ex vivo prostate specimen. Images were obtained with a linear array mounted on a translation stage used to interrogate the entire 3D prostate. The central zone (CZ), transition zone with benign prostatic hyperplasia (TZ/BPH) and prostate cancer (PCa) are circled in blue, green and red dashed lines in the ARFI images, respectively. The urethra is indicated by the green arrow. (a) coronal ARFI image with zonal anatomy, PCa and BPH indicated; (b) coronal ARFI image with co-registered reconstructed shear moduli overlaid; (c) axial ARFI image with zonal anatomy, PCa and BPH indicated; (d) co-registered histological slide of the axial ARFI image, in which PCa is masked in red and BPH is circled in black. (a) and (c) are perpendicular to each other and intersect at the dashed lines. These data were obtained with a VF105 linear array mounted to a mechanical translation stage and a Siemens Antares scanner. Figure reproduced with permission from:(46)

Fig. 6

B-mode (left) and matched ARFI maximum displacement image (right) of an in vivo, biopsy-proven reactive lymph node in the breast. The lymph node is the darker oval region in the B-mode image appearing from 12–22 mm in depth. The echogenic center of the lymph node is the normal hilum that corresponds to the region of decreased displacement in the ARFI image (darker region). The node itself appears slightly stiffer (darker) than the surrounding tissue, with an efferent ductal, structure clearly visualized (arrow). These data were obtained with an Antares scanner and a VF105 linear array transducer. Figure reproduced with permission from (59)

Fig. 7

In vivo B-mode (left) and ARFI (right) images of the tibial and common peroneal nerves in a 29 year old subject, just distal to their bifurcation from the sciatic nerve in the popliteal fossa. The ARFI image was generated using two excitation focal zones at 15 and 20 mm with the VF10-5 linear array. The peak displacement in the ARFI image is 4 µm. ECG data acquisition gating was not used in generating the ARFI image. Notice that the two nerves are clearly delineated as stiffer (darker) circular structures in cross-section; their location is not readily apparent in the B-mode image (they have been outlined in yellow (tibial) and green (popliteal) based on the ARFI image boundaries). The improvement in nerve contrast is over 600% in the ARFI image compared with the B-mode image. Figure reproduced with permission from:(54)

Fig. 8

Co-registered coronal sections of B-mode, ARFI images in vivo and the right half histological slide. The basal area of the prostate is at the top of the images, and the apex is at the bottom. Images were obtained in vivo, immediately prior to radical prostatectomy surgery. In the histological slide, cancerous regions were masked in blue by a pathologist. The obvious stiff structure in the right apex region shown in the ARFI image corresponds to the cancerous lesion. The Gleason’s scores are 4+3. The relatively softer structure to the left side of the lesion is the verumontanum. Another suspicious region (darker, asymmetry) is apparent in the upper left side of the ARFI image, however the corresponding pathology was not available. If utilized for biopsy guidance, both suspicious darker regions would likely have been targeted. These data were obtained with an EV9F4, end-firing, mechanical wobbler, curvilinear array and an Antares scanner. Figure reproduced with permission from:(55)

Fig. 9

B-mode, normalized ARFI displacement, and pathology images from an in vivo ovine ablation monitoring experiment. The matched B-mode and ARFI images were acquired 70 min after ablation with the electrode still in place. The imaging was performed in an open abdomen, with the transducer contacting the diaphragm. The pathology image was obtained in a plane closely matched with the imaging plane shown in the B-mode and ARFI images. The dotted line in the b-mode image indicates the boundary between fatty tissue and the tissue surrounding the bowel wall, and the arrows in the B-mode image show the apparent distal boundary of the thermal lesion. The arrow in the ARFI displacement image points to a region of softer tissue separating liver tissue from bowel, which appears to correspond with fatty tissue in the pathology image. The ablation lesion is well visualized in the ARFI image, as a large region of decreased displacement (dark). These data were acquired with the CH6-2 transducer and the Antares scanner. Reproduced with permission from (57)