Signal to Noise Ratio Increases with Miniature Surface Coil Position as Demonstrated by Both Numerical Simulation and Experimental Validation

Objective: Cushing's disease, characterized by abnormal secretion of adrenocorticotropic hormone from a pituitary adenoma, can lead to significant multi-organ morbidity if left untreated. These functional adenomas are often too small for current magnetic resonance imaging (MRI) technology to identify. We designed a miniature surface coil for placement in direct proximity to the pituitary gland to increase signal to noise ratio (SNR) and provide sufficient spatial resolution for more accurate identification of microadenomas on peri-operative MRI. The prostate cancer literature has demonstrated that introducing surface coils in a body cavity can successfully improve MRI resolution for specific regions of interest (ROI). Various factors can impact SNR including field strength, ROI depth, tissue characteristics, repetition time, time to echo, flip angle, slice thickness and gap, matrix size, and field of view. We aim to study the impact on increased SNR of two characteristics related to coil position (ROI depth and coil rotation angle) using numerical simulation and MRI experimental validation.

Methods: A 20-mm diameter circular loop surface coil was developed with flexible polychlorinated biphenyl and connected to a preamplifier box ([Fig. 1]). In the simulation model, the coil is placed on a 15 cm × 30 cm × 15 cm phantom block. The relative permittivity of the phantom is εr = 57, the relative permeability is μr = 1, and the electrical conductivity is σ = 0.48 S/m. The coil was then tested experimentally with a pork meat slab phantom of similar dimensions ([Fig. 1]). MRI scans were performed for different ROI depths and angles of the coil. SNR was evaluated by simulating the effective B1 magnetic field from the coil based on Ocegueda and Rodriguez's Method.

Results: Though field strength, and maximum SNR, decreases with increasing ROI depth, the field in the ROI is more uniform at a larger depth according to our experimental scan experiment ([Fig. 2]). Different MRI protocols have different optimal target depths ([Fig. 3]). In the simulation experiment, the maximum B field decreases as the coil rotation angle increases. Larger dead spots are observed in both simulation and phantom experiments with increase in angle. However, in the phantom experiment results, the maximum SNR increases initially prior to decreasing as the angle increases ([Fig. 4]).

Conclusion: SNR remains increased with use of the miniature coil regardless of the various coil positions compared with standard MR imaging techniques. As the target depth increases, there is a trade-off between field intensity, SNR and uniformity. There is discordance between our predicted numerical simulation of field strength with coil angle and the measured SNR on a few experimental MRI measurements for different angles. More data points are required to better understand this discrepancy in results. Based on our preliminary results in both numerical simulation and MRI experimental validation, the position of a surface coil is predicted to significantly alter the SNR in sellar MR imaging. Thus, identifying the ideal coil position can further improve the already increased SNR when using a miniature surface coil.

No conflict of interest has been declared by the author(s).

Publication History

Article published online:
12 February 2021