Programa del congreso

1. Introduction and purpose

Magnetic resonance imaging (MRI) is widely used for many neurological, musculoskeletal, thoracic, and abdominal conditions. However, the growing number of patients with implantable devices raises safety concerns due to possible interactions with the static magnetic field, the gradient magnetic fields, and the radiofrequency (RF) field. In particular, RF-induced heating is a major risk [1], making its assessment essential for defining device contraindications and safe MRI use [2].

We are developing a microsensor as part of a novel platform for remote patient monitoring of heart failure. Implantation of the microsensor, which includes nitinol loops and a Ti6Al4V capsule, has already been assayed in an ovine pulmonary artery, but its MRI safety –and RF-induced heating in particular– remains untested. Here, we report an experimental study conducted at the Comparative Medicine and Bioimage Centre of Catalonia, in accordance with the RF-induced heating standard test method set by the American Society for Testing and Materials (ASTM).

2. Materials and methods

Following ASTM F2182-19 [3], a phantom (42.0 × 65.0 × 9.0 cm) of polyacrilic acid gelled-saline with human-like physical properties was employed (σ_{15 kHz} ∈ [0.423, 0.517] S/m at room temperature). Two fiber optic temperature probes (THR-NS-1164I, Resonetics), one used as a test probe and another used as a reference, were placed inside the phantom near each longitudinal lateral wall; separated 6.0 cm from the wall and 30.0 cm from one another. Beneath each probe, a 3D printed microsensor holder (Figure 1A) was placed to ensure precise positioning and repeatability. The phantom was scanned in a 3T MRI system (Vantage Galan MRT-3020k, Canon) (Figure 1B) using a multi-use RF coil.

We first assessed the temperature increase (ΔT) due to local background RF exposure (i.e., no microsensor) for adapted 15-min 3D-FSE and SSFP; two sequences with dense RF duty cycles and large flip angles, suspected to generate substantial heat. As the adapted 3D-FSE generated the largest ΔT, 0.188 ℃, equivalent to a specific absorption rate (SAR) of 0.830 W/kg, we chose it as our test sequence. The microsensor was then placed in the phantom, with its longitudinal axis parallel to the static magnetic field (i.e., worst-case orientation). RF-induced heating was measured near the capsule and near a loop.

3. Results

The microsensor experienced a ∆T = 0.193 ℃ near the loop (equivalent SAR = 0.870 W/Kg) and ∆T = 0.217 ℃ near the capsule (equivalent SAR = 0.996 W/Kg); both slightly above to the microsensor-free scenario (Figure 2).

4. Conclusions

Although ASTM F2182-19 does not set explicit acceptance limits, regulatory guidance by the FDA indicates that the observed heating is well below safety thresholds, which may permit labelling our microsensors for continuous scanning of up to 1 hour without cooling periods [4].

References

[1] S. Nazarian, R. Beinart, and H. R. Halperin, “Magnetic Resonance Imaging and Implantable Devices,” Circulation: Arrhythmia and Electrophysiology, vol. 6, no. 2, pp. 419–428, Apr. 2013, doi: 10.1161/CIRCEP.113.000116.

[2] D. J. Kotze and C. de Vries, “A quick guide to safety and compatibility of passive implants and devices in an MR environment,” South African Journal of Radiology, vol. 8, no. 2, p. 6, Jun. 2004, doi: 10.4102/sajr.v8i2.126.

[3] ASTM F2182-19. Standard Test Method for Measurement of Radio Frequency Induced Heating On or Near Passive Implants During Magnetic Resonance Imaging, Apr. 2020. doi: 10.1520/F2182-19.

[4] “Testing and Labeling Medical Devices for Safety in the Magnetic Resonance (MR) Environment.” U.S. Department of Health and Human Services. Food and Drug Administration Center for Devices and Radiological Health, Oct. 2023.