Department of Radiology, University of Alabama at Birmingham, Birmingham, AL 35249, USA.
Med Phys. 2012 Jun;39(6):3142-53. doi: 10.1118/1.4711751.
Grids are often not used in mobile chest radiography, and when used, they have a low ratio and are often inaccurately aligned. Recently, a mobile radiography automatic grid alignment system (MRAGA) was developed that accurately and automatically aligns the focal spot with the grid. The objective of this study is to investigate high-ratio grid tradeoffs in mobile chest radiography at fixed patient dose when the focal spot lies on the focal axis of the grid.
The chest phantoms (medium and large) used in this study were modifications of the ANSI (American National Standards Institute) chest phantom and consisted of layers of Lucite™, aluminum, and air. For the large chest phantom, the amount of Lucite and aluminum was increased by 50% over the medium phantom. Further modifications included a mediastinum insert and the addition of contrast targets in the lung and mediastinum regions. Five high-ratio grids were evaluated and compared to the nongrid results at x-ray tube potentials of 80, 90, 100, and 110 kVp for both phantoms. The grids investigated were from two manufacturers: 12:1 and 15:1 aluminum interspace grids from one and 10:1, 13:1, and 15:1 fiber interspace grids from another. MRAGA was employed to align the focal spot with the grid. All exposures for a given kVp and phantom size were made using the same current-time product (CTP). The phantom images were acquired using computed radiography, and contrast-to-noise ratios (CNR) and CNR improvement factors (k(CNR)) were determined from the resultant images. The noise in the targets and the contrast between the targets and their backgrounds were calculated using a local detrending correction, and the CNR was calculated as the ratio of the target contrast to the background noise. k(CNR) was defined as the ratio of the CNR imaged with the grid divided by the CNR imaged without a grid.
The CNR values obtained with a high-ratio grid were 4%-65% higher than those obtained without a grid at the same phantom dose. The improvement was greater for the large chest phantom than the medium chest phantom and greater for the mediastinum targets than for the lung targets. In general, the fiber interspace grids performed better than the aluminum interspace grids. In the lung, k(CNR) for both types of grids exhibited little dependence on kVp or grid ratio. In the mediastinum, k(CNR) decreased 4%-10% with increasing kVp, and varied up to 5.3% with grid ratio.
When the focal spot is accurately aligned with the grid, the use of a high-ratio grid in mobile chest radiography improves image quality with no increase in dose to the phantom. For the grids studied, the performance of the fiber interspace grids was superior to the performance of the aluminum interspace grids, with the fiber interspace 13:1 grid producing the best overall results for the medium chest phantom and the fiber interspace 15:1 producing the best overall results for the large chest phantom.
移动胸部放射摄影中通常不使用栅格，如果使用，其比例也很低，且经常不准确地对准。最近，开发了一种移动放射摄影自动栅格对准系统（MRAGA），可准确且自动地将焦点与栅格对准。本研究的目的是在固定患者剂量的情况下，当焦点位于栅格的焦轴上时，研究移动胸部放射摄影中的高栅比权衡。
本研究中使用的胸部体模（中号和大号）是对美国国家标准协会（ANSI）胸部体模的修改，由层压的有机玻璃、铝和空气组成。对于大号体模，有机玻璃和铝的数量比中号体模增加了 50%。进一步的修改包括纵隔插件以及在肺和纵隔区域添加对比目标。在 X 射线管电压为 80、90、100 和 110 kVp 时，评估了五个高栅比栅格，并将其结果与非栅格结果进行了比较，体模分别为中号和大号。研究的栅格来自两个制造商：一个是 12:1 和 15:1 铝间隔栅格，另一个是 10:1、13:1 和 15:1 纤维间隔栅格。MRAGA 用于将焦点与栅格对准。对于给定的 kVp 和体模尺寸，所有曝光均使用相同的电流时间乘积（CTP）进行。使用计算机射线照相术获取体模图像，并从所得图像中确定对比度噪声比（CNR）和 CNR 改善因子（k（CNR））。使用局部去趋势校正计算目标和目标与其背景之间的噪声和对比度，CNR 定义为目标对比度与背景噪声的比值。k（CNR）定义为用栅格成像的 CNR 与不用栅格成像的 CNR 的比值。
在相同的体模剂量下，使用高栅比栅格获得的 CNR 值比没有栅格时高 4%-65%。对于大型胸部体模，这种改善大于中型胸部体模，对于纵隔目标，这种改善大于肺目标。一般来说，纤维间隔栅格的性能优于铝间隔栅格。在肺部，两种类型的栅格的 k（CNR）几乎不随 kVp 或栅格比而变化。在纵隔中，k（CNR）随 kVp 增加而降低 4%-10%，随栅格比变化高达 5.3%。
当焦点准确地与栅格对准时，在移动胸部放射摄影中使用高栅比栅格可在不增加体模剂量的情况下提高图像质量。对于所研究的栅格，纤维间隔栅格的性能优于铝间隔栅格，纤维间隔 13:1 栅格对中型胸部体模的整体效果最好，纤维间隔 15:1 栅格对大型胸部体模的整体效果最好。
