Department of Radiology, University of Massachusetts Medical School, Worcester, MA 01655, USA.
Med Phys. 2012 Mar;39(3):1530-41. doi: 10.1118/1.3688197.
Dedicated breast CT prototypes used in clinical investigations utilize single circular source trajectory and cone-beam geometry with flat-panel detectors that do not satisfy data-sufficiency conditions and could lead to cone beam artifacts. Hence, this work investigated the glandular dose characteristics of a circle-plus-line trajectory that fulfills data-sufficiency conditions for image reconstruction in dedicated breast CT.
Monte Carlo-based computer simulations were performed using the GEANT4 toolkit and was validated with previously reported normalized glandular dose coefficients for one prototype breast CT system. Upon validation, Monte Carlo simulations were performed to determine the normalized glandular dose coefficients as a function of x-ray source position along the line scan. The source-to-axis of rotation distance and the source-to-detector distance were maintained constant at 65 and 100 cm, respectively, in all simulations. The ratio of the normalized glandular dose coefficient at each source position along the line scan to that for the circular scan, defined as relative normalized glandular dose coefficient (RD(g)N), was studied by varying the diameter of the breast at the chest wall, chest-wall to nipple distance, skin thickness, x-ray beam energy, and glandular fraction of the breast.
The RD(g)N metric when stated as a function of source position along the line scan, relative to the maximum length of line scan needed for data sufficiency, was found to be minimally dependent on breast diameter, chest-wall to nipple distance, skin thickness, glandular fraction, and x-ray photon energy. This observation facilitates easy estimation of the average glandular dose of the line scan. Polynomial fit equations for computing the RD(g)N and hence the average glandular dose are provided.
For a breast CT system that acquires 300-500 projections over 2π for the circular scan, the addition of a line trajectory with equal source spacing and constant x-ray beam quality (kVp and HVL) and mAs matched to the circular scan, will result in less than 0.18% increase in average glandular dose to the breast per projection along the line scan.
临床研究中使用的专用乳腺 CT 原型采用单一圆形源轨迹和锥形束几何结构，配备平板探测器，无法满足数据充分条件，可能导致锥形束伪影。因此，这项工作研究了满足专用乳腺 CT 图像重建数据充分条件的圆形加线轨迹的腺体剂量特性。
使用 GEANT4 工具包进行基于蒙特卡罗的计算机模拟，并使用一种原型乳腺 CT 系统的先前报道的归一化腺体剂量系数进行验证。验证后，进行蒙特卡罗模拟以确定作为线扫描中源位置函数的归一化腺体剂量系数。在所有模拟中，源到旋转轴的距离和源到探测器的距离分别保持在 65cm 和 100cm 不变。在线扫描中每个源位置的归一化腺体剂量系数与圆形扫描的归一化腺体剂量系数的比值，定义为相对归一化腺体剂量系数（RD(g)N），通过改变胸廓处乳房的直径、胸廓到乳头的距离、皮肤厚度、X 射线束能量和乳房的腺体分数来研究。
当以相对于线扫描中源位置的函数形式表示 RD(g)N 时，相对于数据充分所需的线扫描最大长度，RD(g)N 被发现最小程度地依赖于乳房直径、胸廓到乳头的距离、皮肤厚度、腺体分数和 X 射线光子能量。这种观察结果便于对线扫描的平均腺体剂量进行简单估计。提供了用于计算 RD(g)N 并因此计算平均腺体剂量的多项式拟合方程。
对于一个在圆形扫描中采集 300-500 个投影的乳腺 CT 系统，在添加具有相等源间距和恒定 X 射线质量（kVp 和 HVL）和与圆形扫描匹配的 mAs 的线轨迹的情况下，在线扫描中每投影的乳房平均腺体剂量将增加不到 0.18%。
