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Acoustic evaluation of carbon fiber RF shield structure for a 3T head-only imaging system
Matthew Tarasek1, Tom Foo1, Mark Vermilyea1, Desmond Yeo1, Isabelle Jansen1, Eric Budesheim 1, and Keith Park1
1GE Global Research, Niskayuna, NY, United States

Synopsis

In this work we evaluate the MRI acoustic changes brought about by modifying the RF shield structure in a head-only (MAGNUS) 3T imaging system. Specifically, we use a standardized acoustic measurement protocol to compare sound pressure levels (SPL) in a fiberglass patient-bore structure to a similar newly developed carbon fiber reinforced polymer (CFRP) structure. Results indicate that a modification of the structural properties of the patient-bore structure properties can provide SPL reduction for MR imaging applications, and is likely due to the different modulus of elasticity between the two materials.

Purpose

Sound pressure waves created from the gradient coil moving in a main magnetic field create audible acoustic noise in MRI, which can pose a significant safety risk to the patient and operator. Previous work has focused on sound pressure level (SPL) reduction through various software [1-5] and hardware [6-8] approaches. Here we present data suggesting that the patient-bore structure, a standard component of most clinical MRI scanners, may be useful as an acoustic barrier for acoustic noise reduction. We provide an acoustic evaluation and comparison between a standard fiberglass reinforced polymer (FRP) patient-bore structure and a newly developed carbon fiber reinforced polymer (CFRP) weave patient-bore structure.

Methods

The patient-bore stepped cylindrical structure can be seen in Figure 1 (Left). It has a stepped profile that accommodates the 42-diameter gradient bore, and flairs to meet the 90-cm warm bore of the 3T magnet. As a requirement to eliminate interaction between the transmit/receive RF coil and the gradient coils, a stainless-steel mesh was mounted on the outside of the patient-bore structure to serve as the system RF shield when the structure was constructed from FRP. With the patient-bore structure constructed from CFRP, the stainless-steel mesh was not necessary and was not used. This was because CFRP has high electrical conductivity at high-frequency and can thus serve as an RF shield and also provide structural strength to the patient-bore structure. However, the focus of this work was in reduction of acoustic noise. Acoustic measurements were made using (i) the standard FRP patient-bore structure and (ii) the CFRP structure of the same dimensions as for the FRP version. The acoustic measurement configuration inside the scanner bore is visualized in Figure 1 (Right) and was repeated for both test cases. Multiple scan sequences representing clinically relevant imaging protocols were tested, following the maximum clinical acoustic noise (MCAN) protocol defined by [9]. The gradient waveforms were measured using an oscilloscope to ensure identical duty cycles, gradient amplitudes, and slew rates for the corresponding sequences in both test arrangements. Acoustic measurements were made using a Model G4, Type 2250 handheld analyzer and Type 4189 microphone (Brüel & Kjær, Nærum, Denmark). The microphone was placed at the imaging isocenter, perpendicular to the patient axis, 80mm above the patient table. The A-weighted, equivalent continuous sound level was recorded in decibels (LAeq, dBA), along with the unweighted peak sound level (Lpeak, dB). SPL test results for 11 clinical sequences or MRI pulse sequence variations are summarized in Figures 2 and 3.

Results and Discussion

The CFRP patient-bore structure provided an average of 3.6 dBA, and a maximum 8 dBA reduction in LAeq relative to the FRP patient-bore structure. Similar results were seen for peak sound SPL measurements, Lpeak , with an average of 2.3 dB, and a maximum 7 dB reduction (Figures 2 and 3). The variation of the measured SPL between different pulse sequences is expected since different pulse sequences have different waveform frequency content. We note that every sequence was played identically for each test case, so there was no difference in waveform frequency content for the same pulse sequence in different test arrangements. The results indicate that the use of CFRP for the patient-bore structure is a viable option for MRI scanner hardware noise derating. The reduction in SPL was likely due to the different modulus of elasticity between the materials (240 GPa for CFRP versus 90 GPa for FRP). The findings indicate that material selection for the patient-bore structure and any other structures within the magnet warm bore is an important consideration in SPL management for high-performance MRI scanners, especially at higher field strengths and/or even higher performance gradient coils [10].

Acknowledgements

Eric Fiveland and Charles Seeley at GE Global Research.

References

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Figures

Figure 1. (Left) Elevation view of the head-only RF patient-bore structure. For the structure made from FRP, a stainless-steel mesh RF shield was mounted on the outer-diameter of the structure. In the case of CFRP, no stainless-steel mesh was needed as the CRFP material acts as an RF shield at 128 MHz. (Right) Experimental setup for measuring sound pressure levels. Here the patient-bore structure has been inserted into the gradient, and acoustic measurements are made at imaging isocenter, i.e., the narrow portion of the shield inner bore.

Figure 2. Results of the A-weighted, equivalent continuous sound level (LAeq, dBA) tested for 11 clinical sequences or sequence variations using the Fiberglass RF shield structure (black bars) and Carbon Fiber RF shield structure (gray bars). Shown with the limits assuming hearing protection with noise reduction rating (NRR) = 26 and 33 dB to reach the U.S. Food and Drug Administration (FDA) and IEC 60601-2-33 (3rd Edition) guidelines for nonsignificant risk limit of 99 dBA.

Figure 3. Results of the unweighted, peak sound level (Lpeak, dB) tested for 11 clinical sequences or sequence variations using the Fiberglass RF shield structure (black bars) and Carbon Fiber RF shield structure (gray bars). Shown with the U.S. FDA and IEC 60601-2-33 guidelines for nonsignificant risk limit of 140 dB.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)
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