Low eddy current RF shielding enclosure designs for 3T MR applications

Watkins

Magnetic Resonance in Med

et al. 2018

Self Cite

Section: Discussionmentioning

confidence: 99%

“…To allow the PET ring operable with EPI, various designs and materials for the shielding cage with sufficiently high shielding effectiveness and low induced eddy current have been investigated. 39 This alternate shielding configuration will allow simultaneous PET/ MR studies during EPI sequence.…”

Section: Discussionmentioning

confidence: 99%

Performance evaluation of RF coils integrated with an RF‐penetrable PET insert for simultaneous PET/MRI

Watkins

Magnetic Resonance in Med

et al. 2018

Self Cite

“…This degradation is mainly due to the eddy currents induced in the Faraday cages when the gradient-intensive EPI sequence was applied, which heats up the detector module and the SiPMs, causing the gain of the SiPMs to drop and the dark counts to increase. A new Faraday cage design made with phosphor bronze mesh is expected to reduce eddy current generation while maintaining high shielding effectiveness [59]. We are also implementing active liquid cooling of the detectors to further mitigate effects of any remaining gradient heating.…”

Section: Discussionmentioning

confidence: 99%

Performance Study of a Radio-Frequency Field-Penetrable PET Insert for Simultaneous PET/MRI

Chang

IEEE Trans. Radiat. Plasma Med. Sci.

Grant

et al. 2018

Self Cite

Hybrid positron emission tomography (PET)/magnetic resonance imaging (MRI) has risen to the cutting edge of medical imaging technology as it allows simultaneous acquisition of structural, functional and molecular information of the patient. A PET insert that can be installed into existing MR systems can in principle reduce the cost barriers for an existing MR site to achieve simultaneous PET/MRI compared to procuring an integrated PET+MRI system. The PET insert systems developed so far for PET/MRI require the RF transmitter coil to reside inside the PET ring as those PET inserts block the RF fields from the MRI system. Here we report for the first time on the performance of a full-ring brain-sized “RF-penetrable” PET insert we have recently completed. This insert allows the RF fields generated by the built-in body coil to penetrate the PET ring. The PET insert comprises a ring of 16 detector modules employing electro-optical coupled signal transmission and a multiplexing framework based on compressed sensing. Energy resolution, coincidence timing resolution (CTR), photopeak position, and coincidence count rate were acquired outside and inside a 3-Tesla MRI system under simultaneous acquisition to evaluate the impact of MRI on the PET performance. Coincidence count rate performance was evaluated by acquiring a cylinder source with high initial activity decaying over time. Tomographic imaging of two phantoms, a custom 6.5-cm diameter resolution phantom with hot rods of four different sizes (2.8 mm, 3.2 mm, 4.2 mm, and 5.2 mm diameter) and a 3D Hoffman brain phantom, were performed to evaluate the imaging capability of the PET insert. The energy resolution at 511 keV and CTR acquired by the PET insert were 16.2±0.1% and 5.3±0.1 ns FWHM, respectively, and remained stable during MRI operation except when the EPI sequence was applied. The PET system starts to show saturation in coincidence count rate at 2.76 million photon counts per second. Most of the 2.8-mm diameter hot rods and main features of the 3D Hoffman brain phantom were resolved by the PET insert, demonstrating its high spatial resolution and capability to image a complex tracer distribution mimicking that seen in the human brain.

Magnetic Resonance in Med

“…To prevent coupling of the RF coil to the gradient coil, a mesh RF shield was used after consideration of different options . An optimal RF shield is necessary to both isolate the RF coil and also minimize the eddy current heating from the high SR, high gradient amplitude switching.…”

Section: Methodsmentioning

confidence: 99%

“…To prevent coupling of the RF coil to the gradient coil, a mesh RF shield was used after consideration of different options. 31,32 An optimal RF shield is necessary to both isolate the RF coil and also minimize the eddy current heating from the high SR, high gradient amplitude switching. The RF shield was mounted on a separate structure to allow easy removal with the RF coil, and to eliminate heat transfer from the shield to the gradient coil.…”

Section: Gradient Coil Designmentioning

confidence: 99%

Highly efficient head‐only magnetic field insert gradient coil for achieving simultaneous high gradient amplitude and slew rate at 3.0T (MAGNUS) for brain microstructure imaging

Foo

Tan

Vermilyea

et al. 2019

Purpose To develop a highly efficient magnetic field gradient coil for head imaging that achieves 200 mT/m and 500 T/m/s on each axis using a standard 1 MVA gradient driver in clinical whole‐body 3.0T MR magnet. Methods A 42‐cm inner diameter head‐gradient used the available 89‐ to 91‐cm warm bore space in a whole‐body 3.0T magnet by increasing the radial separation between the primary and the shield coil windings to 18.6 cm. This required the removal of the standard whole‐body gradient and radiofrequency coils. To achieve a coil efficiency ~4× that of whole‐body gradients, a double‐layer primary coil design with asymmetric x‐y axes, and symmetric z‐axis was used. The use of all‐hollow conductor with direct fluid cooling of the gradient coil enabled ≥50 kW of total heat dissipation. Results This design achieved a coil efficiency of 0.32 mT/m/A, allowing 200 mT/m and 500 T/m/s for a 620 A/1500 V driver. The gradient coil yielded substantially reduced echo spacing, and minimum repetition time and echo time. In high b = 10,000 s/mm2 diffusion, echo time (TE) < 50 ms was achieved (>50% reduction compared with whole‐body gradients). The gradient coil passed the American College of Radiology tests for gradient linearity and distortion, and met acoustic requirements for nonsignificant risk operation. Conclusions Ultra‐high gradient coil performance was achieved for head imaging without substantial increases in gradient driver power in a whole‐body 3.0T magnet after removing the standard gradient coil. As such, any clinical whole‐body 3.0T MR system could be upgraded with 3‐4× improvement in gradient performance for brain imaging.