Evaluation of alignment error due to a speed artifact in stereotactic ultrasound image guidance

Abstract

Ultrasound (US) image guidance systems used in radiotherapy are typically calibrated for soft tissue applications, thus introducing errors in depth-from-transducer representation when used in media with a different speed of sound propagation (e.g. fat). This error is commonly referred to as the speed artifact. In this study we utilized a standard US phantom to demonstrate the existence of the speed artifact when using a commercial US image guidance system to image through layers of simulated body fat, and we compared the results with calculated/predicted values. A general purpose US phantom (speed of sound (SOS) = 1540 m s(-1)) was imaged on a multi-slice CT scanner at a 0.625 mm slice thickness and 0.5 mm x 0.5 mm axial pixel size. Target-simulating wires inside the phantom were contoured and later transferred to the US guidance system. Layers of various thickness (1-8 cm) of commercially manufactured fat-simulating material (SOS = 1435 m s(-1)) were placed on top of the phantom to study the depth-related alignment error. In order to demonstrate that the speed artifact is not caused by adding additional layers on top of the phantom, we repeated these measurements in an identical setup using commercially manufactured tissue-simulating material (SOS = 1540 m s(-1)) for the top layers. For the fat-simulating material used in this study, we observed the magnitude of the depth-related alignment errors resulting from the speed artifact to be 0.7 mm cm(-1) of fat imaged through. The measured alignment errors caused by the speed artifact agreed with the calculated values within one standard deviation for all of the different thicknesses of fat-simulating material studied here. We demonstrated the depth-related alignment error due to the speed artifact when using US image guidance for radiation treatment alignment and note that the presence of fat causes the target to be aliased to a depth greater than it actually is. For typical US guidance systems in use today, this will lead to delivery of the high dose region at a position slightly posterior to the intended region for a supine patient. When possible, care should be taken to avoid imaging through a thick layer of fat for larger patients in US alignments or, if unavoidable, the spatial inaccuracies introduced by the artifact should be considered by the physician during the formulation of the treatment plan.

Critical assessment of intramodality 3D ultrasound imaging for prostate IGRT compared to fiducial markers

“…Quantitative US scanning may require an image distortion correction when tissues are imaged which have a SOS different than the standard value of 1540 m/s employed by most US systems. [24][25][26][27] Recently, a correction procedure was developed by us, and its clinical use was studied extensively. [25][26][27] However, we need to emphasize that in our study, an intramodality US system is used to compare the position of the prostate on two different US images to derive a correction shift.…”

“…The first generation 2DUS systems 15,17 were intermodality systems, i.e., US images acquired in the treatment room had to be compared to a reference computed tomography (CT) image in several planes. Several publications using 2DUS intermodality prostate localization methods have raised concerns regarding the accuracy of US-based techniques due to issues such as transducer pressure induced prostate motion, [21][22][23] speed of sound (SOS) errors, [24][25][26][27] and image quality. Current 3DUS imaging systems frequently still use the intermodality approach.…”

Quality assurance of U.S.‐guided external beam radiotherapy for prostate cancer: Report of AAPM Task Group 154

“…Szpala et al 41 estimated that the uncertainty for a typical prostate patient with full bladder undergoing U.S. scanning was Ϫ2.7 mm due to combined time-of-flight and refraction uncertainties. Salter et al 42 measured, in phantom, the impact of traversing various thicknesses of fat when using an U.S. image guidance system and observed speed artifact errors of 0.7 mm per cm of fat traversed. 42 In addition, if the tissue interface is not normal to the direction of the propagation of sound waves, refraction at the interface can occur.…”

“…Salter et al 42 measured, in phantom, the impact of traversing various thicknesses of fat when using an U.S. image guidance system and observed speed artifact errors of 0.7 mm per cm of fat traversed. 42 In addition, if the tissue interface is not normal to the direction of the propagation of sound waves, refraction at the interface can occur. Refraction occurs if the U.S. probe is directed at an oblique angle to the abdomen, especially when a fanning scan is performed to obtain a 3D view of the target.…”

A CT based correction method for speed of sound aberration for ultrasound based image guided radiotherapy

“…SOS aberration in US imaging, leading to small but noticeable errors in depth assessments, is a phenomenon that has been reported before. 26 Until now, only partial solutions have been proposed [27][28][29][30] due to the intrinsic difficulty in correcting the aberration using only the information provided by the US image. Also, individual SOS variations in tissues have not been considered previously.…”