de Wit Tim C, Xiao Jianbin, Nijsen J Frank W, van het Schip Fred D, Staelens Steven G, van Rijk Peter P, Beekman Freek J
Department of Nuclear Medicine, Image Sciences Institute, University Medical Centre Utrecht, Universiteitsweg 100, STR 5.203, Utrecht, The Netherlands.
Phys Med Biol. 2006 Oct 7;51(19):4773-87. doi: 10.1088/0031-9155/51/19/004. Epub 2006 Sep 8.
Ho-166 is a combined beta-gamma emitter of which the betas can be used therapeutically. From the 81 keV gammas of Ho-166, SPECT images can be obtained, which give opportunities to guide Ho-166 therapy. Accurate reconstruction of Ho-166 images is currently hampered by photopeak-scatter in the patient, down-scatter in the detector, collimator and patient caused by the 1.4 MeV photons and by bremsstrahlung. We developed and validated a method for quantitative SPECT of Ho-166 that involves correction for both types of scatter plus non-uniform attenuation correction using attenuation maps. Photopeak-scatter (S) is compensated for by a rapid 3D Monte Carlo (MC) method that is incorporated in ordered subset (OS) reconstruction of the emission data, together with simultaneous correction for attenuation (A) and detector response (D); this method is referred to as OS-ADS. Additionally, for correction of down-scatter, we use a 14 keV wide energy window centred at 118 keV (OS-ADSS). Due to a limited number of available energy windows, the same 118 keV energy window was used for down-scatter correction of the simultaneously acquired Gd-153 transmission data. Validations were performed using physical phantom experiments carried out on a dual-head SPECT system; Gd-153 transmission line sources were used for acquiring attenuation maps. For quantitative comparison of OS-ADS and OS-ADSS, bottles filled with Ho-166 were placed in both a cylindrical phantom and an anthropomorphic thorax phantom. Both OS-ADS and OS-ADSS were compared with an ordered subset reconstruction without any scatter correction (OS-AD). Underestimations of about 20% in the attenuation map were reduced to a few per cent after down-scatter correction. The average deviation from the true activity contained in the bottles was +72% with OS-AD. Using OS-ADS, this average overestimation was reduced to +28% and with OS-ADSS the deviation was further reduced to 16%. With OS-AD and OS-ADS, these numbers were more sensitive to the choice of volumes of interest than with OS-ADSS. For the reconstructed activity distributions, erroneous background activity found with OS-AD was reduced by a factor of approximately 2 by applying OS-ADS and reduced by a factor of approximately 4 by applying OS-ADSS. The combined attenuation, photopeak-scatter and down-scatter correction framework proposed here greatly enhanced the quantitative accuracy of Ho-166 imaging, which is of the uppermost importance for image-guided therapies. It is expected that the method, with adapted window settings, also can be applied to other isotopes with high energy peaks that contaminate the photopeak data, such as I-131 or In-111.
钬 - 166是一种β - γ复合发射体，其β射线可用于治疗。利用钬 - 166的81 keVγ射线可获取单光子发射计算机断层扫描（SPECT）图像，这为引导钬 - 166治疗提供了机会。目前，患者体内的光电峰散射、探测器、准直器以及患者中由1.4 MeV光子和轫致辐射引起的向下散射阻碍了钬 - 166图像的精确重建。我们开发并验证了一种用于钬 - 166定量SPECT的方法，该方法涉及对两种散射类型进行校正，并使用衰减图进行非均匀衰减校正。光电峰散射（S）通过一种快速三维蒙特卡罗（MC）方法进行补偿，该方法被纳入发射数据的有序子集（OS）重建中，同时对衰减（A）和探测器响应（D）进行校正；这种方法被称为OS - ADS。此外，为了校正向下散射，我们使用一个以118 keV为中心、宽度为14 keV的能量窗（OS - ADSS）。由于可用能量窗数量有限，相同的118 keV能量窗被用于同时采集的钆 - 153透射数据的向下散射校正。验证是通过在双头SPECT系统上进行的物理体模实验来进行的；钆 - 153透射线源用于获取衰减图。为了对OS - ADS和OS - ADSS进行定量比较，将装有钬 - 166的瓶子放置在圆柱形体模和人体胸部体模中。将OS - ADS和OS - ADSS与未进行任何散射校正的有序子集重建（OS - AD）进行比较。在进行向下散射校正后，衰减图中约20%的低估减少到了百分之几。使用OS - AD时，瓶子中所含真实活度的平均偏差为 +72%。使用OS - ADS时，这种平均高估减少到了 +28%，使用OS - ADSS时偏差进一步减少到了16%。与OS - ADSS相比，使用OS - AD和OS - ADS时，这些数值对感兴趣体积的选择更为敏感。对于重建的活度分布，应用OS - ADS可使OS - AD中发现的错误背景活度降低约2倍，应用OS - ADSS可使其降低约4倍。这里提出的联合衰减、光电峰散射和向下散射校正框架极大地提高了钬 - 166成像的定量准确性，这对于图像引导治疗至关重要。预计该方法通过调整窗设置，也可应用于其他具有高能峰且会污染光电峰数据的同位素，如碘 - 131或铟 - 111。
