A parallel tracking method for acoustic radiation force impulse imaging

Abstract

Radiation force-based techniques have been developed by several groups for imaging the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one such method that uses commercially available scanners to generate localized radiation forces in tissue. The response of the tissue to the radiation force is determined using conventional B-mode imaging pulses to track micron-scale displacements in tissue. Current research in ARFI imaging is focused on producing real-time images of tissue displacements and related mechanical properties. Obstacles to producing a real-time ARFI imaging modality include data acquisition, processing power, data transfer rates, heating of the transducer, and patient safety concerns. We propose a parallel receive beamforming technique to reduce transducer heating and patient acoustic exposure, and to facilitate data acquisition for real-time ARFI imaging. Custom beam sequencing was used with a commercially available scanner to track tissue displacements with parallel-receive beamforming in tissue-mimicking phantoms. Using simulations, the effects of material properties on parallel tracking are observed. Transducer and tissue heating for parallel tracking are compared to standard ARFI beam sequencing. The effects of tracking beam position and size of the tracked region are also discussed in relation to the size and temporal response of the region of applied force, and the impact on ARFI image contrast and signal-to-noise ratio are quantified.

Fig. 1

The axial displacement induced by radiation force from a pushing pulse of 5.3 MHz and focused at 20 mm is shown. On the left are images of the displacements, where the transducer is located at the top of the image, and lighter shades indicate greater displacement. On the right is a cross-section of the image taken at the focal point of the displacements. (a) Axial displacements 0.12 ms after force cessation. (b) Axial displacements after 0.6 ms elapsed. Shear waves are beginning to form, however they have not completely separated yet. (c) After 1.4 ms, the shear waves have completely separated and can be seen moving away from the location of force application.

Fig. 2

Measured displacement from a pushing pulse as a function of the parallel-tracking beam’s distance from the push location (Δx) and observation time in a homogeneous 4 kPa phantom.

Fig. 3

The azimuthal PSF for different offsets between the transmit and receive foci of the tracking beam. At an offset of 0 mm, the PSF is identical to that used in conventional tracking. All PSFs are normalized relative to the peak of the PSF with no offset.

Fig. 4

The sidelobe level (left) and positional error (right) of the PSFs as a function of offset between the transmit and receive foci of the tracking beam for three transmit beam f-numbers.

Fig. 5

Images generated from the tissue mimicking phantom using conventional and 4:1, 8:1, and 12:1 parallel tracking. The images were produced 0.48 ms, 0.72 ms, and 0.96 ms after excitation. The images formed by 8:1 and 12:1 tracking were formed synthetically due to limitations of the Antares scanner, however they represent images that would be formed by systems with high parallel tracking capabilities.

Fig. 6

A conventional ARFI image created with 0.53 mm beam spacing is compared to a diagnostically useful parallel tracking scheme with 0.35 mm beam spacing (Note that the conventional image can be generated using 0.35 mm spacing as well, which will improve the spatial resolution, but at the cost of additional heating). These images were created 0.72 ms after application of radiation force.

Fig. 7

(a) The contrast of the lesion in figures 5 and 6 as a function of time. The mechanical contrast of the lesion, 87%, is shown by the horizontal dotted line. (b) The contrast-to-noise ratio of the lesion.

Fig. 8

The simulated, normalized displacement observed by a parallel tracking beam located at an offset of (a) 0.30 mm and (b) 0.71 mm from the center of the push location. The elastic modulus for the homogeneous regions are 1 kPa (——), 5 kPa (– – –), 10 kPa (···· ), and 18 kPa (·–·–).

Fig. 9

The temperature increase in tissue with an absorption of 0.7 dB/cm/MHz is compared for conventional and parallel tracking techniques. The color bar at the top of each column indicates the temperature increase in degrees Celsius. The top row of images show heating associated with conventional tracking and the bottom row shows the heating associated with 4:1 parallel tracking. The temperature change is shown for (a) a single frame, (b) 5 consecutive frames at the same frame rate (2.8 fps), and (c) 2.16 seconds at each mode’s maximum frame rate.