Study of the Influence of Implant Material on Magnetocardiography Measurements Using SQUID Sensors
Abstract
Magnetocardiography system is a medical device that diagnoses cardiac disease by measuring magnetic fields generated from electric currents flowing through the myocardium. However, the accuracy of measurement data can be degraded if strong magnetic materials are present or magnetic field changes occur near the MCG system. With the widespread use of implants, the number of patients with metallic implants is increasing, but there is a lack of in-depth research on the potential impact of implant materials on the results of the MCG examination. This study aims to analyze the effect of implant materials on MCG measurements and establish relevant criteria. In this study, a 96-channel MCG system employing Superconducting Quantum Interference Device sensors, specifically first-order gradiometers based on the Double Relaxation Oscillation SQUID method, and a Magnetically Shielded Room were utilized. Ti6Al4V ELI was selected as the representative implant material sample. Experiments were conducted under extreme conditions, where a sample significantly larger than an actual implant was placed as close as possible to the sensors. As a result, when the implant material was at the minimum distance to the sensor, the noise increase was approximately 0.7 fT/$\sqrt{\mathrm{Hz}}$, which satisfies the sensitivity criteria for MCG. Furthermore, since these results were obtained under severely adverse conditions designed to maximize the noise impact, it is anticipated that the effect would be even more negligible in actual clinical settings. In conclusion, it was confirmed that common implant materials have little to no effect on MCG measurements. However, as the experiments were not conducted with the material inserted into the human body, unlike actual clinical environments, the generation of magnetic fields due to micromotion has not been verified, thus requiring further experimentation.
Summary
This paper investigates the impact of common implant materials, specifically Ti-6Al-4V ELI, on magnetocardiography (MCG) measurements obtained using a 96-channel SQUID-based MCG system. The research addresses the concern that metallic implants in patients could degrade the accuracy of MCG data due to magnetic interference. The study utilized a first-order gradiometer based on the Double Relaxation Oscillation SQUID (DROS) method within a Magnetically Shielded Room (MSR). Experiments were conducted under deliberately extreme conditions, placing a large Ti-6Al-4V ELI sample as close as possible to the sensors to maximize potential interference. The key finding was that the presence of the Ti-6Al-4V ELI implant material resulted in a noise increase of approximately 0.7 fT/√Hz at the minimum distance to the sensor. This value still satisfies the sensitivity criteria for MCG measurements as defined by the manufacturer's specified acceptance criteria for the noise level is 10 fT/√Hz @100 Hz. The authors extrapolate and estimate that even at a distance of 38mm from the SQUID sensor (the closest possible proximity), the baseline height would be (8.9, 8.6) fT/√Hz for each trial, respectively, which still satisfies the MCG's sensitivity criteria. The study concludes that common implant materials like Ti-6Al-4V ELI are unlikely to significantly affect MCG measurements. However, the authors acknowledge that the experiments were conducted outside the human body and did not account for potential magnetic field generation due to micromotion of the implant within the body, suggesting a need for further investigation.
Key Insights
- •The study found that Ti-6Al-4V ELI, a common implant material, caused a noise increase of approximately 0.7 fT/√Hz when placed in close proximity to the SQUID sensors of the MCG system.
- •The DROS method used in the SQUID sensors offers an advantage over the DC-SQUID method by achieving an approximate 10-fold increase in the transduction coefficient.
- •The extrapolated baseline height at a minimum proximity distance of 38 mm between the material and the SQUID sensor was estimated to be (8.9, 8.6) fT/√Hz, which still satisfies the MCG's sensitivity criteria.
- •The experimental setup intentionally created "severely adverse conditions" to maximize the noise impact, suggesting the actual effect in clinical settings would be even smaller.
- •The paper highlights the limitations of the study, specifically the absence of in-vivo testing and the failure to account for the generation of magnetic fields due to micromotion of the implant within the human body.
- •The study used a Ti-6Al-4V ELI sample with dimensions of 300mm (W) X 400mm (L) X 7.93mm (H), which is larger than the sensor array and larger than most implants, making it a representative sample size for evaluating implants.
- •The authors found that the baseline height of the noise spectrum increased with decreasing distance between the dewar and the material, and they modeled this relationship using the equation [h]I = α i (d + 38) −3 + β i (i = 1, 2).
Practical Implications
- •The findings provide reassurance to clinicians and patients with metallic implants that MCG measurements are likely to be accurate and unaffected by the presence of these implants.
- •Manufacturers of MCG systems can use this data to refine their system specifications and potentially relax restrictions on patient inclusion criteria for MCG examinations.
- •Future research should focus on in-vivo experiments to assess the impact of implant micromotion on MCG measurements, potentially using motion tracking techniques to correlate movement with noise levels.
- •Further studies could explore a wider range of implant materials and sizes to provide a more comprehensive understanding of their potential impact on MCG measurements.