Analysis of rapid multi-focal-zone ARFI imaging

Abstract

Acoustic radiation force impulse (ARFI) imaging has shown promise for visualizing structure and pathology within multiple organs; however, because the contrast depends on the push beam excitation width, image quality suffers outside of the region of excitation. Multi-focal-zone ARFI imaging has previously been used to extend the region of excitation (ROE), but the increased acquisition duration and acoustic exposure have limited its utility. Supersonic shear wave imaging has previously demonstrated that through technological improvements in ultrasound scanners and power supplies, it is possible to rapidly push at multiple locations before tracking displacements, facilitating extended depth of field shear wave sources. Similarly, ARFI imaging can utilize these same radiation force excitations to achieve tight pushing beams with a large depth of field. Finite element method simulations and experimental data are presented, demonstrating that single- and rapid multi-focal-zone ARFI have comparable image quality (less than 20% loss in contrast), but the multi-focal-zone approach has an extended axial region of excitation. Additionally, as compared with single-push sequences, the rapid multi-focalzone acquisitions improve the contrast-to-noise ratio by up to 40% in an example 4-mm-diameter lesion.

Fig. 1

Simulated displacement though time profiles (without simulated ultrasonic tracking) are shown for each focal configuration and at each focal depth, with the appropriate time delays applied according to when the pushes started in the multi-focal zone configurations. The top row of images compares the profiles when pushing at the shallowest focal depth first and the bottom row of images utilized the deep-to-shallow configuration. In each plot, the dashed gray line indicates the time-delayed and summed output of the three individual focal zone simulations and is nearly identical to the solid black line, which is the multi-focal zone simulation. As expected for a linear system, the RMS difference between the time-delayed and summed displacement profiles and the multi-focal zone simulations is less than 0.01μm in all configurations.

Fig. 2

Experimentally acquired displacement though time profiles in a uniform region of the phantom are shown for each focal configuration and at each focal depth in the same configuration as figure 1. The error bars indicate the standard deviation over 9 independent speckle realizations. In each plot, the solid black line (the multi-focal zone data) is in agreement with the dashed gray line (the sum of the individual focal zone acquisitions). The RMS difference through time between the time-delayed and summed displacement profiles and the multi-focal zone acquisitions is less than 0.2μm in all configurations, which is consistent with the simulations results in figure 1.

Fig. 3

The jitter magnitude relative to the mean displacement of the experimentally acquired data from figure 2 is shown here for each focal depth. For each of the 9 speckle realizations, the jitter was computed independently, and the error bars indicate the standard deviation of the jitter magnitude. Since the jitter magnitude is computed relative to the displacement, a high jitter magnitude is expected whenever the displacement magnitude is low, such as late in time.

Fig. 4

The experimental displacement amplitude jitter magnitude relative to the mean displacement comparing single push, single focal zone, triple push, single focal zone, and rapid multi-focal zone imaging sequences. For each of the 9 speckle realizations, the displacement magnitude and jitter was computed independently, and the error bars indicate the standard deviation of the jitter magnitude. The triple push sequence has the highest overall displacement and its peak displacement occurs earlier in time as compared with the multi-focal zone sequence.

Fig. 5

Experimentally acquired ARFI images of the 4 mm cylindrical target, displayed 0.6 ms after force cessation. The top row of images shows 3 different multi-focal zone sequences and the bottom row displays the triple push, single focal zone acquisitions. Comparable image quality is achieved in all of the multi-focal zone sequences since each configuration included the 20 mm focal depth, which is centered on the lesion. Conversely, as demonstrated by the 10 mm and 30 mm single focal zone acquisitions, when the push is only focused away from the target, the image quality is severely degraded.

Fig. 6

Contrast, noise, and CNR of the 4 mm cylindrical target are given, where the error bars indicate the standard deviation over 9 independent speckle realizations. The data are shown as a function of time for the triple push, single focal zone sequences at 20 mm and 25 mm. The 20 mm focus has the highest contrast, but the 25 mm focus sequence has the highest CNR due to the temporal evolution of both the contrast and the image noise.

Fig. 7

Contrast, noise, and CNR of the 4 mm cylindrical target are given, where the error bars indicate the standard deviation over 9 independent speckle realizations. The bar plots portray the data from the time step where the maximum contrast through time was observed. As expected per the derivation in III, the 20 mm push focal configuration yields the highest contrast (paired Student’s two-tailed t-test, p < 0.01).

Fig. 8

Contrast, noise, and CNR of the 4 mm cylindrical target are given, where the error bars indicate the standard deviation over 9 independent speckle realizations. The bar plots portray the data form the time step where the maximum CNR through time was observed. In many of the focal configurations, by looking later in time after force cessation, the noise decreases, increasing the CNR of the target. The triple push, 25 mm focus sequence had the highest CNR (paired Student’s two-tailed t-test, p < 0.01), but comparing the multi-focal zone sequences to the single push, single focal zone sequences, the 20 → 25 → 30 mm combined focal zone acquisition had the highest CNR (p < 0.01).