Salter Bill J, Wang Brian, Szegedi Martin W, Rassiah-Szegedi Prema, Shrieve Dennis C, Cheng Roger, Fuss Martin
Department of Radiation Oncology, University of Utah-Huntsman Cancer Institute, UT, USA.
Phys Med Biol. 2008 Dec 7;53(23):N437-45. doi: 10.1088/0031-9155/53/23/N03. Epub 2008 Nov 12.
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.
放射治疗中使用的超声（US）图像引导系统通常针对软组织应用进行校准，因此当在声速传播不同的介质（如脂肪）中使用时，会在换能器深度表示上引入误差。这种误差通常被称为速度伪像。在本研究中，我们使用标准超声体模来证明当使用商用超声图像引导系统对模拟人体脂肪层进行成像时速度伪像的存在，并将结果与计算/预测值进行比较。一个通用超声体模（声速（SOS）= 1540 m s⁻¹）在多层CT扫描仪上以0.625 mm的层厚和0.5 mm×0.5 mm的轴向像素尺寸进行成像。体模内模拟靶标的金属丝被勾勒轮廓，随后被转移到超声引导系统中。将不同厚度（1 - 8 cm）的商用脂肪模拟材料（SOS = 1435 m s⁻¹）层放置在体模顶部，以研究与深度相关的对准误差。为了证明速度伪像不是由在体模顶部添加额外层引起的，我们在相同设置下使用商用组织模拟材料（SOS = 1540 m s⁻¹）作为顶层重复了这些测量。对于本研究中使用的脂肪模拟材料，我们观察到因速度伪像导致的与深度相关的对准误差大小为每成像1 cm脂肪0.7 mm。对于此处研究的所有不同厚度的脂肪模拟材料，由速度伪像引起的测量对准误差与计算值在一个标准差内相符。我们证明了在使用超声图像引导进行放射治疗对准时，由于速度伪像导致的与深度相关的对准误差，并指出脂肪的存在会使靶标被错误映射到比实际更深的深度。对于当今使用的典型超声引导系统，这将导致仰卧患者的高剂量区域在比预期区域稍靠后的位置交付。在可能的情况下，对于体型较大的患者，在超声对准时应注意避免对厚脂肪层进行成像，或者如果不可避免，医生在制定治疗计划时应考虑由伪像引入的空间不准确性。
