O'Shea Tuathan P, Garcia Leo J, Rosser Karen E, Harris Emma J, Evans Philip M, Bamber Jeffrey C
Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS foundation Trust, Sutton and London, UK.
Phys Med Biol. 2014 Apr 7;59(7):1701-20. doi: 10.1088/0031-9155/59/7/1701. Epub 2014 Mar 12.
This study investigates the use of a mechanically-swept 3D ultrasound (3D-US) probe for soft-tissue displacement monitoring during prostate irradiation, with emphasis on quantifying the accuracy relative to CyberKnife® x-ray fiducial tracking. An US phantom, implanted with x-ray fiducial markers was placed on a motion platform and translated in 3D using five real prostate motion traces acquired using the Calypso system. Motion traces were representative of all types of motion as classified by studying Calypso data for 22 patients. The phantom was imaged using a 3D swept linear-array probe (to mimic trans-perineal imaging) and, subsequently, the kV x-ray imaging system on CyberKnife. A 3D cross-correlation block-matching algorithm was used to track speckle in the ultrasound data. Fiducial and US data were each compared with known phantom displacement. Trans-perineal 3D-US imaging could track superior-inferior (SI) and anterior-posterior (AP) motion to ≤0.81 mm root-mean-square error (RMSE) at a 1.7 Hz volume rate. The maximum kV x-ray tracking RMSE was 0.74 mm, however the prostate motion was sampled at a significantly lower imaging rate (mean: 0.04 Hz). Initial elevational (right-left; RL) US displacement estimates showed reduced accuracy but could be improved (RMSE <2.0 mm) using a correlation threshold in the ultrasound tracking code to remove erroneous inter-volume displacement estimates. Mechanically-swept 3D-US can track the major components of intra-fraction prostate motion accurately but exhibits some limitations. The largest US RMSE was for elevational (RL) motion. For the AP and SI axes, accuracy was sub-millimetre. It may be feasible to track prostate motion in 2D only. 3D-US also has the potential to improve high tracking accuracy for all motion types. It would be advisable to use US in conjunction with a small (∼2.0 mm) centre-of-mass displacement threshold in which case it would be possible to take full advantage of the accuracy and high imaging rate capability.
本研究调查了在前列腺放疗期间使用机械扫描三维超声（3D-US）探头进行软组织位移监测，重点是相对于射波刀（CyberKnife®）X射线基准标记跟踪来量化其准确性。将植入X射线基准标记的超声体模放置在运动平台上，并使用通过Calypso系统获取的五条真实前列腺运动轨迹在三维空间中进行平移。通过研究22例患者的Calypso数据，运动轨迹代表了所有类型的运动。使用三维扫描线性阵列探头（模拟经会阴成像）对体模进行成像，随后使用射波刀上的千伏X射线成像系统成像。采用三维互相关块匹配算法跟踪超声数据中的散斑。将基准标记数据和超声数据分别与已知的体模位移进行比较。经会阴三维超声成像能够以1.7Hz的容积速率跟踪上下（SI）和前后（AP）运动，均方根误差（RMSE）≤0.81mm。千伏X射线跟踪的最大RMSE为0.74mm，然而前列腺运动的采样成像速率要低得多（平均：0.04Hz）。最初的纵向（左右；RL）超声位移估计显示准确性较低，但在超声跟踪代码中使用相关阈值以去除错误的容积间位移估计后，准确性可以提高（RMSE<2.0mm）。机械扫描三维超声能够准确跟踪分次内前列腺运动的主要成分，但存在一些局限性。超声最大RMSE出现在纵向（RL）运动中。对于AP和SI轴，准确性达到亚毫米级。仅在二维空间中跟踪前列腺运动可能是可行的。三维超声还有提高所有运动类型跟踪准确性的潜力。建议将超声与约2.0mm的质心位移小阈值结合使用，在这种情况下，可以充分利用其准确性和高成像速率能力。
