Nakonechny K D, Fallone B G, Rathee S
Department of Medical Physics, Cross Cancer Institute, University of Alberta, Edmonton, Alberta T6G 1Z2, Canada.
Med Phys. 2005 Jan;32(1):98-109. doi: 10.1118/1.1835571.
Computed tomography dose index (CTDI) is a conventional indicator of the patient dose in CT studies. It is measured as the integration of the longitudinal single scan dose profile (SSDP) by using a 100-mm-long pencil ionization chamber and a single axial scan. However, the assumption that most of the SSDP is contained within the chamber length may not be valid even for thin slices. We have measured the SSDPs for several slice widths on two CT scanners using a PTW diamond detector placed in a 300 mm x 200 mm x 300 mm water-equivalent plastic phantom. One SSDP was also measured using lithium fluoride (LiF) TLDs and an IC-10 small volume ion chamber, verifying the general shape of the SSDP measured using the diamond detector. Standard cylindrical PMMA CT phantoms (140 mm length) were also used to qualitatively study the effects of phantom shape, length, and composition on the measured SSDP. The SSDPs measured with the diamond detector in the water-equivalent phantom were numerically integrated to calculate the relative accumulated dose D(L)(0)calc at the center of various scan lengths L. D(L)(0)calc reached an equilibrium value for L > 300 mm, suggesting the need for phantoms longer than standard CT dose phantoms. We have also measured the absolute accumulated dose using an IC-10 small volume ion chamber, D(L)(0)SV, at three points in the phantom cross section for several beamwidths and scan lengths. For one CT system, these measurements were made in both axial and helical scanning modes. The absolute CTDI100, measured with a 102 mm active length pencil chamber, were within 4% of D(L)(0)SV measured with the small volume ion chamber for L approximately 100 mm suggesting that nonpencil chambers can be successfully used for CT dosimetry. For nominal beam widths ranging from 3 to 20 mm and for L approximately 250 mm, D(L)(0)SV values at the center of the water-equivalent phantom's elliptic cross section were approximately 25%-30% higher than the measured CTDI100. For small beamwidths, the difference in D(L)(0)SV for L approximately 250 mm and L approximately 14 x beamwidth (CTDI14nT) reached up to 50%. Peripheral point doses at 70 mm depth along the major axis of the phantom for L approximately 250 mm were up to 22% higher than for L approximately 100 mm. The differences between CTDI100 and D(L)(0)SV for L approximately 250 mm were in good agreement with the predictions made from the numerical integration of the measured SSDPs. Due to the considerable dose measured beyond the length of standard CT phantoms, CT dosimetry for longer body scan series should be performed in longer phantoms. Measurements could be made as we have shown, using a small volume chamber translating through the beam using multiple scans.
计算机断层扫描剂量指数（CTDI）是CT检查中患者剂量的传统指标。它通过使用100毫米长的笔形电离室和单次轴向扫描对纵向单扫描剂量分布（SSDP）进行积分来测量。然而，即使对于薄层扫描，假设大部分SSDP包含在电离室长度内可能也不成立。我们使用置于300毫米×200毫米×300毫米水等效塑料模体中的PTW金刚石探测器，在两台CT扫描仪上测量了几种层厚的SSDP。还使用氟化锂（LiF）热释光剂量计和IC - 10小体积电离室测量了一个SSDP，验证了使用金刚石探测器测量的SSDP的大致形状。还使用标准圆柱形PMMA CT模体（140毫米长）定性研究模体形状、长度和组成对测量的SSDP的影响。对在水等效模体中用金刚石探测器测量的SSDP进行数值积分，以计算不同扫描长度L中心处的相对累积剂量D(L)(0)calc。当L > 300毫米时，D(L)(0)calc达到平衡值，这表明需要比标准CT剂量模体更长的模体。我们还使用IC - 10小体积电离室在模体横截面的三个点测量了几种束宽和扫描长度下的绝对累积剂量D(L)(0)SV。对于一个CT系统，这些测量在轴向和螺旋扫描模式下均进行。用102毫米有效长度笔形电离室测量的绝对CTDI100，与用小体积电离室在L约为100毫米时测量的D(L)(0)SV相差在4%以内，这表明非笔形电离室可成功用于CT剂量测定。对于标称束宽范围为3至20毫米且L约为250毫米的情况，水等效模体椭圆形横截面中心处的D(L)(0)SV值比测量的CTDI100高约25% - 30%。对于小束宽，L约为250毫米和L约为14×束宽（CTDI14nT）时的D(L)(0)SV差异高达50%。对于L约为250毫米的情况，沿模体主轴70毫米深度处的周边点剂量比L约为100毫米时高22%。L约为250毫米时CTDI100与D(L)(0)SV之间的差异与根据测量的SSDP数值积分所做的预测高度吻合。由于在标准CT模体长度之外测量到了相当可观的剂量，对于更长的身体扫描序列的CT剂量测定应在更长的模体中进行。测量可以按照我们所展示的那样进行，使用小体积电离室通过多次扫描在射束中平移。
