He Wenjun, Huda Walter, Magill Dennise, Tavrides Emily, Yao Hai
Clemson-MUSC Bioengineering program, Department of Bioengineering, Clemson University, Charleston, SC 29425, USA.
Radiat Prot Dosimetry. 2011 Jan;143(1):81-7. doi: 10.1093/rpd/ncq291. Epub 2010 Nov 9.
The aim of the study was to investigate how patient effective doses vary as a function of X-ray tube projection angle, as well as the patient long axis, and quantify how X-ray tube current modulation affects patient doses in chest CT examinations. Chest examinations were simulated for a gantry CT scanner geometry with projections acquired for a beam width of 4 cm. PCXMC 2.0.1 was used to calculate patient effective doses at 15° intervals around the patient's isocentre, and at nine locations along the patient long axis. Idealised tube current modulation schemes were modelled as a function of the X-ray tube angle and the patient long axis. Tube current modulations were characterised by the modulation amplitude R, which was allowed to vary between 1.5 and 5. Effective dose maxima occur for anteroposterior projections at the location of the (radiosensitive) breasts. The maximum to minimum ratio of effective doses as a function of the patient long axis was 4.9, and as a function of the X-ray tube angle was 2.1. Doubling the value of R reduces effective doses from longitudinal modulation alone by ∼4% and from angular modulation alone by ∼2%. In chest CT, tube current modulation schemes currently have longitudinal R values of ∼2.2, and angular R values that range between 1.5 and 3.4. Current X-ray tube current modulation schemes are expected to reduce patient effective doses in chest CT examinations by ∼10%, with longitudinal modulation accounting for two-thirds and angular modulation for the remaining one-third.
本研究的目的是调查患者的有效剂量如何随X射线管投影角度以及患者长轴而变化，并量化X射线管电流调制对胸部CT检查中患者剂量的影响。针对龙门式CT扫描仪的几何结构模拟了胸部检查，获取了束宽为4 cm时的投影。使用PCXMC 2.0.1在患者等中心周围以15°间隔以及沿患者长轴的九个位置计算患者的有效剂量。将理想化的管电流调制方案建模为X射线管角度和患者长轴的函数。管电流调制的特征在于调制幅度R，其取值范围在1.5至5之间。有效剂量最大值出现在（放射敏感的）乳房位置的前后投影中。有效剂量的最大与最小比值，作为患者长轴的函数为4.9，作为X射线管角度的函数为2.1。将R值加倍可使仅纵向调制产生的有效剂量降低约4%，仅角度调制产生的有效剂量降低约2%。在胸部CT中，目前管电流调制方案的纵向R值约为2.2，角度R值在1.5至3.4之间。预计当前的X射线管电流调制方案可使胸部CT检查中的患者有效剂量降低约10%，其中纵向调制占三分之二，角度调制占其余三分之一。
