Studies of the interactions of an MRI system with the shielding in a combined PET/MRI scanner

Abstract

A positron emission tomography (PET) system or ‘insert’ has been constructed for placement and operation in the bore of a small animal magnetic resonance imaging (MRI) scanner to allow simultaneous MR and PET imaging. The insert contains electronics, components with a variety of magnetic properties, and large continuous sheets of metal— all characteristics of an object that should, by conventional wisdom, never be placed in the bore of an MR scanner, especially near the imaging volume. There are a variety of ways the two systems might be expected to interact that could negatively impact the performance of either or both. In this article, the interaction mechanisms, particularly the impacts of the PET insert and shielding on MR imaging, are defined and explored. Additionally, some of the difficulties in quantifying errors introduced into the MR images as a result of the presence of the PET components are demonstrated. Several different approaches are used to characterize image artifacts and determine optimal placement of the shielding. Data are also presented that suggest ways the shielding could be modified to reduce errors and enable placement closer to the isocenter of the magnet.

New shielding configurations for a simultaneous PET/MRI scanner at 7T

“…These additions of more detector material and electronics into the magnet bore will require a larger supporting structure and more metallic shielding. However, introducing additional metallic shielding into the system has the potential to increase gradient-induced eddy current effects, which could degrade MRI image quality [12, 13]. The purpose of the current paper is to find new ways to shield RF interference while reducing gradient-induced eddy current effects and at the same time maintaining the integrity of both systems’ data acquisition.…”

“…In order to minimize RF interference at 300 MHz while minimizing gradient-induced eddy currents, a careful study of RF shielding design is essential. Inserting breaks or slots in the metallic shielding or using a meshed shielding can help to reduce eddy currents while still providing sufficient electromagnetic shielding [12, 14]. However, epoxy impregnated carbon fiber composite materials have also been proposed as an effective RF shielding material [15-17].…”

“…Eddy current effects were also studied to further validate the reduction of eddy currents with the new configuration when compared to an early prototype PET insert [9] and other shielding configurations [12]. Gradient switching interference was studied as well to identify potential interference in PET data acquisition.…”

Principles of PET/MR Imaging

“…Shielding. PET detector systems require sensitive preamplifiers and front-end electronics, which are also highly sensitive to electromagnetic interferences, such as the radiofrequency pulses generated by MR. To prevent MR sequences or gradient switching from degrading the PET signal quality or from generating false events, the electronic components should be placed at a distance or should be shielded (23). The integration of a PET system in an MR scanner requires a careful design of the radiofrequency shielding to avoid introducing eddy currents.…”

“…These currents are introduced in conductors by changing magnetic fields, for example, applied gradients. Eddy currents create an additional magnetic field that can induce local magnetic field inhomogeneities as well as affect the gradient speed (slew rate) and the spatial encoding ability of the system (23). Furthermore, the PET components should consist of materials with a low magnetic susceptibility.…”