Staelens Steven G, de Wit Tim C, Lemahieu Ignace A, Beekman Freek J
Department of Nuclear Medicine, Image Sciences Institute, University Medical Center Utrecht, Utrecht, The Netherlands.
Eur J Nucl Med Mol Imaging. 2008 May;35(5):922-32. doi: 10.1007/s00259-008-0719-z. Epub 2008 Jan 25.
Thallous (201Tl) chloride is a single-photon emission computed tomography (SPECT) tracer mainly used for assessing perfusion and viability of myocardial tissue. 201Tl emits X-rays around 72 keV and gammas at 167 keV, and has a half-life of 73 h. Regulations allow an intrinsic contamination up to 3-5%, which is mainly caused by 200Tl (368 keV; 26 h) and by 202Tl (439 keV; 12.2 days). Contra-intuitive to the low-level percentages in which these contaminants are present, their impact may be significant because of much higher gamma camera sensitivity for these high-energy photon emissions. Therefore, we investigate the effects of the contaminants in terms of detected fractions of photons in projections and contrast degradation in reconstructed images.
Acquisitions of a digital thorax phantom filled with thallous (201Tl) chloride were simulated with a validated Monte Carlo tool, thereby, modelling 1% of contamination by 200Tl and 202Tl each. In addition, measurements of a thorax phantom on a dual-headed gamma camera were performed. The product used was contaminated by 0.17% of 200Tl and 0.24% of 202Tl at activity reference time (ART). This ART is specified by the manufacturer, thereby, accounting for the difference in half-lives of 201Tl and its contaminants. These measurements were repeated at different dates associated with various contamination levels.
Simulations showed that, with 1% of 200Tl and 202Tl, the total contamination in the 72 keV window can rise up to one out of three detected photons. For the 167 keV window, the contamination is even more pronounced: more than four out of five detections in this photopeak window originate from contaminants. Measurements indicate that cold lesion contrast in myocardial perfusion SPECT imaging is at maximum close to ART. In addition to a higher noise level, relative contrast decreases 15% 2 days early to ART, which is explained by an increase in 200Tl contamination. After ART, contrast decreased by 16% when the 202Tl contamination increased to the maximal allowed limit.
Contra-intuitive to the low-level percentages in which they are typically present, penetration and downscatter of high-energy photons from 200Tl and 202Tl significantly contribute to thallous (201Tl) chloride images, thereby, reducing contrast and adding noise. These findings may prompt for improved production methods, for updated policies with regard to timing of usage, and they also render the usefulness of adding the high photopeak window (167 keV) questionable. A window-based correction method for this contamination is advisable.
氯化铊（201Tl）是一种单光子发射计算机断层扫描（SPECT）示踪剂，主要用于评估心肌组织的灌注和存活情况。201Tl发射能量约为72 keV的X射线和167 keV的γ射线，半衰期为73小时。相关规定允许的固有污染率高达3 - 5%，这主要是由200Tl（368 keV；26小时）和202Tl（439 keV；12.2天）造成的。与这些污染物低含量的情况相悖的是，由于γ相机对这些高能光子发射的灵敏度高得多，它们的影响可能很大。因此，我们从投影中检测到的光子分数以及重建图像中的对比度降低方面研究了这些污染物的影响。
使用经过验证的蒙特卡罗工具模拟了填充有氯化铊（201Tl）的数字胸部模型的采集情况，从而对每种200Tl和202Tl的1%污染进行建模。此外，还在双头γ相机上对胸部模型进行了测量。所使用的产品在活性参考时间（ART）时被0.17%的200Tl和0.24%的202Tl污染。这个ART由制造商指定，从而考虑到201Tl及其污染物半衰期的差异。在与不同污染水平相关的不同日期重复进行了这些测量。
模拟表明，当存在1%的200Tl和202Tl时，72 keV窗口中的总污染可能上升到每三个检测到的光子中有一个。对于167 keV窗口，污染更为明显：在这个光电峰窗口中，每五次检测中就有超过四次来自污染物。测量表明，心肌灌注SPECT成像中的冷区对比度在接近ART时最大。除了更高的噪声水平外，相对对比度在比ART提前2天时降低了15%，这是由200Tl污染增加所解释的。在ART之后，当202Tl污染增加到最大允许限度时，对比度降低了16%。
与它们通常存在的低水平百分比情况相悖的是，来自200Tl和202Tl的高能光子的穿透和散射对氯化铊（201Tl）图像有显著贡献，从而降低了对比度并增加了噪声。这些发现可能促使改进生产方法、更新使用时间的政策，并且它们还使得添加高光电峰窗口（167 keV）的有用性受到质疑。对于这种污染，基于窗口的校正方法是可取的。
