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

Abstract

Understanding sources of electromagnetic interference are important in designing any electronic system. This is especially true when combining positron emission tomography (PET) and magnetic resonance imaging (MRI) in a multimodality system as coupling between the subsystems can degrade the performance of either modality. For this reason, eliminating radio frequency (RF) interference and gradient-induced eddy currents have been major challenges in building simultaneous hybrid PET/MRI systems. MRI requires negligible RF interference at the Larmor resonance frequency, while RF interference at almost any frequency may corrupt PET data. Moreover, any scheme that minimizes these interactions would, ideally, not compromise the performance of either subsystem. This paper lays out a plan to resolve these problems. A carbon fiber composite material is found to be a good RF shield at the Larmor frequency (300 MHz in this work) while introducing negligible gradient eddy currents. This carbon fiber composite also provides excellent structural support for the PET detector components. Low frequency electromagnetic radiation (81 kHz here) from the switching power supplies of the gradient amplifiers was also found to interfere with the PET detector. Placing the PET detector module between two carbon fiber tubes and grounding the inner carbon fiber tube to the PET detector module ground reduced this interference. Further reductions were achieved by adding thin copper (Cu) foil on the outer carbon fiber case and electrically grounding the PET detector module so that all 3 components had a common ground, i.e. with the PET detector in an electrostatic cage. Finally, gradient switching typical in MRI sequences can result in count losses in the particular PET detector design studied. Moreover, the magnitude of this effect depends on the location of the detector within the magnet bore and which MRI gradient is being switched. These findings have a bearing on future designs of PET/MRI systems.

Binary polyamide hybrid composites containing carbon allotropes and metal particles with radiofrequency shielding effect

“…This effect is usually achieved by the simultaneous incorporation of conductive, magnetic and/or dielectric fillers into a matrix that can be also polymeric . Novel structural RAM or EMI materials are attracting increasing interest from academia and industry due to their diverse applications in military stealth technologies, in aircraft and ship transports as well as in television for image interference reduction , and in medical diagnostic equipment , among others.…”

From PET/CT to PET/MRI: Advances in Instrumentation and Clinical Applications

“…The major interaction problems of simultaneous PET/MRI are the following: (1) the PMT used in the PET system cannot work properly in the magnetic field of the MRI system; (2) the PET photodetector may interfere with the radiofrequency (RF) and gradient coils, resulting in a nonuniform magnetic field; (3) an eddy current may occur if metallic shielding is used. 24 Many prototype PET/ MRI systems were developed to overcome these problems. In the early designs, researchers used PET detectors based on traditional PMTs.…”

MRI‐compatibility study of a PET‐insert based on a low‐profile detection front‐end with submillimeter spatial resolution

“…8 Regarding this issue, metallic shielding layers such as copper or composite layers such as carbon fiber are commonly used for eliminating the EMIs in a dual-modality PET/MRI scanner. 9,10 Nonetheless, designing PET detection modules with lower electromagnetic emissions and less sensitive to electromagnetic interactions is still an ongoing challenge which is one of the objectives of the new design presented here.…”