Читать книгу Magnetic Resonance Microscopy - Группа авторов - Страница 69

Typical MRI scanners are sited inside a passively shielded conductive room (Faraday cage) to prevent external radiofrequency sources from inducing image artifacts. This >US$100 000 siting component is both costly and prevents portability. Passive shielding can be partially achieved by placing copper foil around the magnet. The passive radiofrequency shielding is generally sufficient for phantoms and objects that do not extend outside of the magnet (shielded area) but proves insufficient when a human subject is in the magnet. This is because the conductive human torso and legs act as an antenna which picks up external radiofrequency interference and pipes it into the detection coil. Imaging a cantaloupe and a living human can differ in external interference artifact by as much as 100-fold.

Additional EMI mitigation can be achieved using external interference detectors (additional radiofrequency coils outside of the magnet) and retrospective removal from the images. The interference is picked up from external coils and the artifact is removed from the image using a trained correlation model between that seen in imaging coil and the external detection coil. This approach has a broad history in electromagnetic measurements and in EEG and magnetoencephalography (MEG). Muller-Petke provides some review as well as an application to MR [138], and several methods have emerged specifically for low-field MRI systems [139,140]. Figure 3.9 shows results of an example phantom experiment in an 80-mT portable brain scanner [20,141] using retrospective EMI correction with five external detector coils [140]. Although refinements will continue, it appears that the EMI issue can be managed for POC MRI without a Faraday-shielded room.

Figure 3.9 Retrospective EMI correction of a time-varying EMI using five external coils using a k-space correlation matrix approach in a low-field scanner [140]. Four electromagnetic interference (EMI) sources are shown including a dynamic source (right-hand column). The bottom row shows difference images using “Ground Truth” (GT) images obtained without the noise sources present.