Schöfer F H, Hoeschen C
Institute of Radiation Protection, Helmholtz Zentrum München, Ingolstaedter Landstr 1, 85764 Neuherberg, Germany.
Radiat Prot Dosimetry. 2010 Apr-May;139(1-3):81-5. doi: 10.1093/rpd/ncq053. Epub 2010 Feb 25.
In contrary to conventional screen film radiography digital radiographic methods allow the flexible adaptation of the visualisation of an image to a clinical question even after its generation. Nevertheless, the information content of an image is in addition to covering effects like anatomical noise ultimately limited by the applied exposure, its energy distribution and the dose to the detector. This limitation needs to be analysed quantitatively and in connection with efficiency properties of the image generation process. The random variation of pixel values in plane digital radiography is in general attributed to the limited number of imaging quanta. This allows determining a minimal number of applied quanta from requirements to the image information. The number of applied quanta is closely related to the entrance dose. It can be calculated by understanding the imaging process as the sum of many binomial sampling processes. This approach is useful for the separation and examination of the influences of the transmission, absorption and scattering properties of an imaging setup, including the used radiation quality. The model imaging task examined here is the detection of a thin contrast layer of one material behind a homogeneous main absorber of a second material by projection radiography. As the physical properties of the setup are dependent on the energy of the applied radiation, the energy leading to a minimal number of applied photons to achieve the required information can be calculated. It turns out to depend on the materials of both but on the thickness of only the main absorber. The efficiency of the exposure by other energies can be determined as the ratio between the minimal number and the number of quanta needed to achieve the same information. For monoenergetic exposures it is shown that changing the optimal energy by a fixed factor leads to the same loss of efficiency independent of the thicknesses of contrast layer and main absorber. The efficiency of the detection process can shift the optimal position. It directly follows that the optimal range of photon energy becomes smaller for thinner specimens. This clearly stresses the need for an adaptation of the applied energies to the physical properties of the patient especially when thin objects are examined.
与传统的屏-片摄影不同，数字射线照相方法允许在图像生成后，根据临床问题灵活调整图像的可视化效果。然而，图像的信息内容除了受解剖噪声等覆盖效应影响外，最终还受所施加的曝光量、其能量分布以及探测器所接受的剂量限制。这种限制需要进行定量分析，并结合图像生成过程的效率特性来分析。平面数字射线照相中像素值的随机变化通常归因于成像量子数量有限。这使得可以根据对图像信息的要求确定所施加量子的最小数量。所施加量子的数量与入射剂量密切相关。通过将成像过程理解为许多二项式采样过程的总和，可以计算出该数量。这种方法对于分离和研究成像装置（包括所使用的辐射质量）的透射、吸收和散射特性的影响很有用。这里研究的模型成像任务是通过投影射线照相检测一种材料的均匀主吸收体后面的另一种材料的薄对比层。由于成像装置的物理特性取决于所施加辐射的能量，因此可以计算出导致达到所需信息所需的施加光子数量最少的能量。结果表明，它取决于两种材料，但仅取决于主吸收体的厚度。其他能量曝光的效率可以通过最小数量与获得相同信息所需的量子数量之比来确定。对于单能曝光，结果表明，将最佳能量改变一个固定因子会导致相同的效率损失，而与对比层和主吸收体的厚度无关。检测过程的效率会改变最佳位置。由此直接得出，对于更薄的样本，光子能量的最佳范围会变小。这清楚地强调了需要根据患者的物理特性调整所施加的能量，特别是在检查薄物体时。
