Performance of ultrasound based measurement of 3D displacement using a curvilinear probe for organ motion tracking

Abstract

Three-dimensional (3D) soft tissue tracking is of interest for monitoring organ motion during therapy. Our goal is to assess the tracking performance of a curvilinear 3D ultrasound probe in terms of the accuracy and precision of measured displacements. The first aim was to examine the depth dependence of the tracking performance. This is of interest because the spatial resolution varies with distance from the elevational focus and because the curvilinear geometry of the transducer Page 2 of 39 causes the spatial sampling frequency to decrease with depth. Our second aim was to assess tracking performance as a function of the spatial sampling setting (low, medium or high sampling). These settings are incorporated onto 3D ultrasound machines to allow the user to control the trade-off between spatial sampling and temporal resolution. Volume images of a speckle-producing phantom were acquired before and after the probe had been moved by a known displacement (1, 2 or 8 mm).This allowed us to assess the optimum performance of the tracking algorithm, in the absence of motion. 3D speckle tracking was performed using 3D cross-correlation and sub-voxel displacements were estimated. The tracking performance was found to be best for y displacements and poorest for z displacements. In general, the performance decreased with depth, although the nature of the depth dependence was complex.Under certain conditions, the tracking performance was sufficient to be useful for monitoring organ motion. For example, at the highest sampling setting, for a 2 mm displacement, good accuracy and precision (an error and standard deviation of <0.4mm) were observed at all depths and for all directions of displacement. The trade-off between spatial sampling, temporal resolution and size of field of view is discussed.

Potential Use of Ultrasound Speckle Tracking for Motion Management During Radiotherapy

“…17 Multiple previous works have demonstrated the accuracy of tissue displacements based on speckle tracking in vivo and in vitro. 27,28,37 These accuracies Relative displacement estimates of a right hepatic lobe hepatocellular carcinoma during full expiration among 4-dimensional computed tomography (4DCT), on-screen motion tracking by a human volunteer, and automated speckle tracking. Values are mean ± SD.…”

“…27,28,37 The degrees of accuracy can be beam profile and depth dependent, but these properties are well known and should be easily accountable when ultrasound is used to monitor tissue motion during radiation oncology treatments. 38 We noted that fractional tracking variations were from about 7% to 15%, with the liver being the highest.…”

“…15,[17][18][19][20][21][22]24,25 A series of papers have suggested that ultrasound speckle tracking could be used to monitor physiologic motion during radiation oncology treatments. [26][27][28][29] They displayed the feasibility of ultrasound speckle tracking or feature/lesion tracking in a radiation treatment environment. Tracking has been tested in a speckle-containing phantom while a treatment beam was on.…”

“…35,36 Multiple experiments performed by us and others have shown that displacement estimates made by speckle-tracking RF signals in ultrasound phantoms and in vivo are highly accurate. 27,28,30 Since the motion was angular, the linear displacement varied across the scan image with displacements increasing with depth from the scan head. We also rocked the ultrasound probe sufficiently rapidly to simulate displacement rates of about 2 to 3 cm/s depending on the distance from the ultrasound probe.…”

“…In addition, tracking of image regions that do not change shape or deform substantially over time (eg, tissue borders, blood vessels, etc) can be used to correct for drift in speckle-tracking estimates. 28 At least 2 previous papers have used electronic 2D array probes for 3D speckle tracking, 27,28 and 3D speckle tracking has been shown to be highly accurate. 16 However, presently 2D ultrasound arrays are still largely developmental, extremely expensive, and not in general use.…”