Hintenlang David E, Jiang Xia, Little Kevin J
Department of Radiology, Ohio State University Wexner Medical Center, Columbus, OH, USA.
J Appl Clin Med Phys. 2018 Jul;19(4):290-298. doi: 10.1002/acm2.12366. Epub 2018 Jun 15.
To document a study in shielding a high-sensitivity digital mammography system detector from AC magnetic fields of magnitudes great enough to induce imaging artifacts.
METHODS/MATERIALS: Preliminary evaluation of AC magnetic fields at a site designated for a digital breast tomosynthesis (DBT) system raised concerns that the magnetic component of electromagnetic interference (EMI) may be great enough to induce imaging artifacts. Subsequent measurements using digital detector arrays from two separate manufacturers verified this concern, and AC magnetic fields were mapped, spatially and temporally, throughout the area of concern. A simple shielding model was developed to elucidate the physics of extremely low-frequency (ELF) EMI shielding and independently verify a commercial group's proposed shielding design and installation. Postshielding measurements were performed to demonstrate that the EMI fields were reduced to acceptable levels.
Preshielding measurements showed AC magnetic fields significantly exceeding manufacturers' tolerances for artifact-free imaging in DBT. Continuous measurements demonstrated that the EMI fields varied significantly over time. Some locations in the room routinely averaged above 30 mG and occasionally exceeded 100 mG. The source was attributed to an adjacent electrical supply room, and temporal changes of the EMI were attributed to variations of the building electrical loads. The proposed shielding primarily consisted of continuous aluminum (6.35 mm thickness) and was installed by a group specializing in electromagnetic field shielding. Postshielding measurements demonstrated that the EMI fields were significantly reduced, generally to less than 0.5 mG, and that the shielding effectively dampened the large variations due to dynamic building electrical loads. Subsequent installation and evaluation of a DBT system revealed no issues with imaging artifacts.
The successful shielding of ELF EMI involves physical principles that are not commonly encountered by medical physicists. Modern high-sensitivity digital detectors may be successfully shielded against imaging artifacts with careful application of these principles.
记录一项关于对高灵敏度数字乳腺摄影系统探测器进行屏蔽，使其免受强度足以诱发成像伪影的交流磁场影响的研究。
方法/材料：在为数字乳腺断层合成（DBT）系统指定的场地对交流磁场进行初步评估后，人们担心电磁干扰（EMI）的磁场成分可能足以诱发成像伪影。随后使用来自两家不同制造商的数字探测器阵列进行的测量证实了这一担忧，并在整个相关区域对交流磁场进行了空间和时间上的测绘。开发了一个简单的屏蔽模型来阐明极低频（ELF）电磁干扰屏蔽的物理原理，并独立验证一个商业团队提出的屏蔽设计和安装方案。进行屏蔽后测量以证明电磁干扰场已降低到可接受水平。
屏蔽前测量显示，交流磁场显著超过了制造商对DBT中无伪影成像的耐受标准。连续测量表明，电磁干扰场随时间有显著变化。房间内的一些位置通常平均超过30 mG，偶尔超过100 mG。源头归因于相邻的配电室，电磁干扰的时间变化归因于建筑物电气负载的变化。提议的屏蔽主要由连续的铝（厚度6.35毫米）组成，由一个专门从事电磁场屏蔽的团队进行安装。屏蔽后测量表明，电磁干扰场显著降低，一般降至小于0.5 mG，并且屏蔽有效地抑制了由于建筑物动态电气负载引起的大幅变化。随后安装和评估的一个DBT系统未发现成像伪影问题。
成功屏蔽极低频电磁干扰涉及医学物理学家不常遇到的物理原理。通过谨慎应用这些原理，现代高灵敏度数字探测器可以成功地免受成像伪影的影响。
