Department of Medical Physics (G68), University of the Free State, PO Box 339, 9300 Bloemfontein, South Africa.
Phys Med. 2011 Jan;27(1):11-20. doi: 10.1016/j.ejmp.2010.01.004. Epub 2010 Mar 4.
Transmission sources used for image attenuation correction, allowing image quantification, are collimated to reduce scatter. We propose the same effect can be achieved for an uncollimated source by increasing source to patient distance. The aim was to compare planar image performance characteristics and absorbed doses of uncollimated and collimated radioactive printed paper transmission sources. The scatter contribution to the uncollimated (⁹⁹m)Tc source data was evaluated for different combinations of detector phantom distance, detector source distance and phantom source distance. Measurements were performed by increasing the Lucite phantom thickness in 1cm steps to 20 cm. Spatial resolution, detection efficiency and entrance absorbed dose rate were measured for the uncollimated and collimated transmission source images. Results derived from the energy spectra, obtained with the uncollimated transmission source indicate that scatter contribution increases with decreasing detector source distance. The scatter component in the uncollimated transmission images (detector source distances ≥ 60 cm; phantom source distances ≥ 40 cm) was comparable to that obtained with collimated transmission images. Attenuation coefficients obtained compared well (0.168 cm⁻¹ vs. 0.171 cm⁻¹). The full widths at half maxima differed by less than 0.9 mm. The detection efficiency of the uncollimated source was 2.5 times higher than obtained with the collimated source. The entrance absorbed dose obtained from an uncollimated source was 3.75 times larger than that obtained from the collimated source. An uncollimated transmission source (detector source distance ≥ 60 cm) results in acceptable image characteristics and presents a low cost, low dose, high efficiency option for transmission imaging.
用于衰减校正的传输源可使图像量化，其准直用于减少散射。我们提出，通过增加源到患者的距离，可以为非准直源实现相同的效果。目的是比较非准直和准直放射性打印纸传输源的平面图像性能特征和吸收剂量。对于不同的探测器-幻影距离、探测器-源距离和幻影-源距离组合，评估了非准直（⁹⁹m）Tc 源数据的散射贡献。通过在 Lucite 幻影中以 1cm 的步长增加厚度来进行测量，直至达到 20cm。为非准直和准直传输源图像测量了空间分辨率、检测效率和入口吸收剂量率。从非准直传输源获得的能谱得出的结果表明，随着探测器-源距离的减小，散射贡献增加。在非准直传输图像（探测器-源距离≥60cm；幻影-源距离≥40cm）中，散射分量与准直传输图像相当。获得的衰减系数相当（0.168cm⁻¹ 与 0.171cm⁻¹）。半最大值全宽相差小于 0.9mm。非准直源的检测效率比准直源高 2.5 倍。从非准直源获得的入口吸收剂量比从准直源获得的大 3.75 倍。非准直传输源（探测器-源距离≥60cm）产生可接受的图像特性，并为传输成像提供了低成本、低剂量、高效率的选择。
