Aslund Magnus, Cederström Björn, Lundqvist Mats, Danielsson Mats
Department of Physics, Royal Institute of Technology, AlbaNova, 106 91 Stockholm, Sweden.
Med Phys. 2005 Nov;32(11):3367-74. doi: 10.1118/1.2089487.
A theoretical evaluation of nonuniform x-ray field distributions in mammography was conducted. An automatic exposure control (AEC) is proposed for a scanning full field digital mammography system. It uses information from the leading part of the detector to vary the scan velocity dynamically, thus creating a nonuniform x-ray field in the scan direction. Nonuniform radiation fields were also created by numerically optimizing the scan velocity profile to each breast's transmission distribution, with constraints on velocity and acceleration. The goal of the proposed AEC is to produce constant pixel signal-to-noise ratio throughout the image. The target pixel SNR for each image could be set based on the breast thickness, breast composition, and the beam quality as to achieve the same contrast-to-noise ratio between images for structures of interest. The results are quantified in terms of reduction in entrance surface air kerma (ESAK) and scan time relative to a uniform x-ray field. The theoretical evaluation was performed on a set of 266 mammograms. The performance of the different methods to create nonuniform fields decreased with increased detector width, from 18% to 11% in terms of ESAK reduction and from 30% to 25% in terms of scan time reduction for the proposed AEC and detector widths from 10 to 60 mm. Some correlation was found between compressed breast thickness and the projected breast area onto the image field. This translated into an increase of the ESAK and decrease of the scan time reduction with breast thickness. Ideally a nonuniform field in two dimensions could reduce the entrance dose by 39% on average, whereas a field nonuniform in only the scanning dimension ideally yields a 20% reduction. A benefit with the proposed AEC is that the risk of underexposing the densest region of the breast can be virtually eliminated.
对乳腺摄影中不均匀X射线场分布进行了理论评估。针对扫描全场数字化乳腺摄影系统提出了一种自动曝光控制（AEC）方法。它利用探测器前端部分的信息动态改变扫描速度，从而在扫描方向上产生不均匀的X射线场。通过对每个乳房的透射分布进行数值优化扫描速度剖面，并对速度和加速度加以限制，也能产生不均匀的辐射场。所提出的AEC的目标是在整个图像中产生恒定的像素信噪比。可以根据乳房厚度、乳房组成和射线质来设置每个图像的目标像素信噪比，以便在感兴趣结构的图像之间实现相同的对比度噪声比。结果根据相对于均匀X射线场的入射表面空气比释动能（ESAK）降低和扫描时间进行量化。对一组266幅乳腺造影片进行了理论评估。对于所提出的AEC，随着探测器宽度从10毫米增加到60毫米，产生不均匀场的不同方法的性能下降，ESAK降低从18%降至11%，扫描时间减少从30%降至25%。发现压缩乳房厚度与图像场上的投影乳房面积之间存在一定相关性。这转化为ESAK随乳房厚度增加而增加，扫描时间减少量随乳房厚度减小。理想情况下，二维不均匀场平均可使入射剂量降低39%，而仅在扫描维度上不均匀的场理想情况下可使剂量降低20%。所提出的AEC的一个优点是，几乎可以消除乳房最致密区域曝光不足的风险。
