Dewaraja Y K, Ljungberg M, Koral K F
Internal Medicine Department, The University of Michigan Medical Center, Ann Arbor 48109-0552, USA.
J Nucl Med. 2000 Oct;41(10):1760-7.
Accuracy of 131I tumor quantification after radioimmunotherapy (RIT) was investigated for SPECT imaging with an ultra-high-energy (UHE) collimator designed for imaging 511-keV photons.
First, measurements and Monte Carlo simulations were carried out to compare the UHE collimator with a conventionally used, high-energy collimator. On the basis of this comparison, the UHE collimator was selected for this investigation, which was carried out by simulation of spherical tumors in a phantom. Reconstruction was by an expectation-maximization algorithm that included scatter and attenuation correction. Keeping the tumor activity constant, simulations were carried out to assess how volume-of-interest (VOI) counts vary with background activity, radius of rotation (ROR), tumor location, and size. The constant calibration factor for quantification was determined from VOI counts corresponding to a 3.63-cm-radius sphere of known activity. Tight VOIs corresponding to the physical size of the spheres or tumors were used.
Use of the UHE collimator resulted in a large reduction in 131I penetration, which is especially significant in RIT where background uptake is high. With the UHE collimator, typical patient images showed an improvement in contrast. Considering the desired geometric events, sensitivity was reduced, but only by a factor of 1.6. Simulation results for a 3.63-cm-radius tumor showed that VOI counts vary with background, location, and ROR by less than 3.2%, 3%, and 5.3%, respectively. The variation with tumor size was more significant and was a function of the background. Good quantification accuracy (<6.5% error) was achieved when tumor size was the same as the sphere size used in the calibration, irrespective of the other parameters. For smaller tumors, activities were underestimated by up to -15% for the 2.88-cm-radius sphere, -23% for the 2.29-cm-radius sphere, and -47% for the 1.68-cm-radius sphere.
Reasonable accuracy can be achieved for VOI quantification of 131I using SPECT with an UHE collimator and a constant calibration factor. Difference in tumor size relative to the size of the calibration sphere had the biggest effect on accuracy, and recovery coefficients are needed to improve quantification of small tumors.
研究了使用专为成像511 keV光子设计的超高能（UHE）准直器进行单光子发射计算机断层显像（SPECT）成像时，放射免疫治疗（RIT）后131I肿瘤定量的准确性。
首先，进行测量和蒙特卡罗模拟，以比较UHE准直器与常规使用的高能准直器。基于此比较，选择UHE准直器进行本研究，该研究通过在体模中模拟球形肿瘤来进行。重建采用包括散射和衰减校正的期望最大化算法。在保持肿瘤活性恒定的情况下，进行模拟以评估感兴趣体积（VOI）计数如何随背景活性、旋转半径（ROR）、肿瘤位置和大小而变化。根据对应于已知活性的3.63 cm半径球体的VOI计数确定定量的恒定校准因子。使用对应于球体或肿瘤物理尺寸的紧密VOI。
使用UHE准直器可大幅降低131I的穿透，这在背景摄取较高的RIT中尤为显著。使用UHE准直器时，典型的患者图像对比度有所改善。考虑到所需的几何事件，灵敏度降低，但仅降低了1.6倍。对于半径为3.63 cm的肿瘤，模拟结果表明VOI计数随背景、位置和ROR的变化分别小于3.2%、3%和5.3%。随肿瘤大小的变化更为显著，并且是背景的函数。当肿瘤大小与校准中使用的球体大小相同时，无论其他参数如何，均可实现良好的定量准确性（误差<6.5%）。对于较小的肿瘤，对于半径为2.88 cm的球体，活性低估高达-15%，对于半径为2.29 cm的球体，低估-23%，对于半径为1.68 cm的球体，低估-47%。
使用UHE准直器和恒定校准因子的SPECT对131I的VOI定量可实现合理的准确性。肿瘤大小相对于校准球体大小的差异对准确性影响最大，需要恢复系数来改善小肿瘤的定量。
