Poels Kenneth, Verellen Dirk, Van de Vondel Iwein, El Mazghari Rafik, Depuydt Tom, De Ridder Mark
Department Radiotherapy, Universitair Ziekenhuis Brussel, Vrije Universiteit Brussel, 1090 Brussel, Belgium.
Department Radiotherapy, Universitair Ziekenhuis Leuven, Katholieke Universiteit Leuven, 3000 Leuven, Belgium and Department Radiotherapy, Universitair Ziekenhuis Brussel, Vrije Universiteit Brussel, 1090 Brussel, Belgium.
Med Phys. 2014 Oct;41(10):101911. doi: 10.1118/1.4896116.
Because frame rates on current clinical available electronic portal imaging devices (EPID's) are limited to 7.5 Hz, a new commercially available PerkinElmer EPID (XRD 1642 AP19) with a maximum frame rate of 30 Hz and a new scintillator (Kyokko PI200) with improved sensitivity (light output) for megavolt (MV) irradiation was evaluated. In this work, the influence of MV pulse artifacts and pulsing artifact suppression techniques on fiducial marker and marker-less detection of a lung lesion was investigated, because target localization is an important component of uncertainty in geometrical verification of real-time tumor tracking.
Visicoil™ markers with a diameter of 0.05 and 0.075 cm were used for MV marker tracking with a frame rate of, respectively, 7.5, 15, and 30 Hz. A 30 Hz readout of the detector was obtained by a 2 × 2 pixel binning, reducing spatial resolution. Static marker detection was conducted in function of increasing phantom thickness. Additionally, marker-less tracking was conducted and compared with the ground-truth fiducial marker motion. Performance of MV target detection was investigated by comparing the least-square sine wave fit of the detected marker positions with the predefined sine wave motion. For fiducial marker detection, a Laplacian-of-Gaussian enhancement was applied after which normalized cross correlation was used to find the most probable marker position. Marker-less detection was performed by using the scale and orientation adaptive mean shift tracking algorithm. For each MV fluoroscopy, a free running (FR-nF) (ignoring MV pulsing during readout) acquisition mode was compared with two acquisition modes intending to reduce MV pulsing artifacts, i.e., combined wavelet-FFT filtering (FR-wF) and electronic readout synchronized with respect to MV pulses.
A 0.05 cm Visicoil marker resulted in an unacceptable root-mean square error (RMSE) > 0.2 cm with a maximum frame rate of 30 Hz during FR-nF readout. With a 30 Hz synchronized readout (S-nF) and during 15 Hz readout (independent of readout mode), RMSE was submillimeter for a static 0.05 cm Visicoil. A dynamic 0.05 cm Visicoil was not detectable on the XRD 1642 AP19, despite a fast synchronized readout. For a 0.075 cm Visicoil, deviations of sine wave motion were submillimeter (RMSE < 0.08 cm), independent of the acquisition mode (FR, S). For marker-less tumor detection, FR-nF images resulted in RMSE > 0.3 cm, while for MV fluoroscopy in S-mode RMSE < 0.1 cm for 15 Hz and RMSE < 0.16 cm for 30 Hz. Largest consistency in target localization was experienced during 15 Hz S-nF readout.
In general, marker contrast decreased in function of higher frame rates, which was detrimental for marker detection success. In this work, Visicoils with a thickness of 0.075 cm were showing best results for a 15 Hz frame rate, while non-MV compatible 0.05 cm Visicoil markers were not visible on the new EPID with improved sensitivity compared to EPID models based on a Kodak Lanex Fast scintillator. No noticeable influence of pulsing artifacts on the detection of a 0.075 cm Visicoil was observed, while a synchronized readout provided most reliable detection of a marker-less soft-tissue structure.
由于当前临床可用的电子射野影像装置（EPID）的帧率限制为7.5Hz，因此对一款新的市售珀金埃尔默EPID（XRD 1642 AP19，最大帧率为30Hz）以及一种用于兆伏（MV）照射且具有更高灵敏度（光输出）的新型闪烁体（京科PI200）进行了评估。在本研究中，研究了MV脉冲伪影和脉冲伪影抑制技术对肺病变的基准标记和无标记检测的影响，因为目标定位是实时肿瘤跟踪几何验证中不确定性的一个重要组成部分。
使用直径为0.05和0.075cm的Visicoil™标记进行MV标记跟踪，帧率分别为7.5、15和30Hz。通过2×2像素合并获得探测器的30Hz读出，从而降低空间分辨率。在体模厚度增加的情况下进行静态标记检测。此外，进行了无标记跟踪并与基准标记的真实运动进行比较。通过将检测到的标记位置的最小二乘正弦波拟合与预定义的正弦波运动进行比较，研究了MV目标检测的性能。对于基准标记检测，应用高斯-拉普拉斯增强，然后使用归一化互相关来找到最可能的标记位置。无标记检测通过使用尺度和方向自适应均值漂移跟踪算法进行。对于每次MV透视，将自由运行（FR-nF）（在读出期间忽略MV脉冲）采集模式与两种旨在减少MV脉冲伪影的采集模式进行比较，即组合小波-FFT滤波（FR-wF）和与MV脉冲同步的电子读出。
在FR-nF读出期间，对于0.05cm的Visicoil标记，当最大帧率为30Hz时，均方根误差（RMSE）>0.2cm，不可接受。对于30Hz同步读出（S-nF）以及15Hz读出（与读出模式无关），对于静止的0.05cm Visicoil，RMSE为亚毫米级。尽管有快速同步读出，但在XRD 1642 AP19上仍无法检测到动态的0.05cm Visicoil。对于0.075cm的Visicoil，正弦波运动的偏差为亚毫米级（RMSE<0.08cm），与采集模式（FR、S）无关。对于无标记肿瘤检测，FR-nF图像的RMSE>0.3cm，而对于S模式的MV透视，15Hz时RMSE<0.1cm，30Hz时RMSE<0.16cm。在15Hz S-nF读出期间，目标定位的一致性最高。
一般来说，标记对比度随着帧率的提高而降低，这对标记检测的成功不利。在本研究中，对于15Hz的帧率，厚度为0.075cm的Visicoil表现出最佳结果，而与基于柯达Lanex Fast闪烁体的EPID模型相比，在新型具有更高灵敏度的EPID上，不兼容MV的0.05cm Visicoil标记不可见。未观察到脉冲伪影对0.075cm Visicoil检测有明显影响，而同步读出为无标记软组织结构提供了最可靠的检测。
