The performance of interventional loopless MRI antennae at higher magnetic field strengths

Abstract

Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study investigates whether loopless antennae offer signal-to-noise ratio (SNR) and field-of-view (FOV) advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to 5 T is investigated both analytically, using electromagnetic (EM) dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low-loss loopless antennae fabricated for 1.5, 3, and 4.7 T, immersed in physiologically comparable saline. Gains of 3.8 +/- 0.2- and 9.7 +/- 0.3-fold in SNR, and approximately 10- and 50-fold gains in the useful FOV area are observed at 3 and 4.7 T, respectively, compared to 1.5 T. Heat testing of a 3 T biocompatible nitinol-antenna fabricated with a redesigned decoupling circuit shows maximum heating of approximately 1 degrees C for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the 3 T antenna versus an equivalent commercial 1.5 T device in vivo. This work is the first to study the performance of experimental internal MRI detectors above 1.5 T. The large SNR and FOV gains realized present a major opportunity for high-resolution imaging of vascular pathology and MRI-guided intervention.

Figure 1

(a) Model of actual loopless antenna and (b) corresponding dipole antenna model. The antenna axis is parallel to the B₀ field of the MRI system, along the z axis. The tuned cable transforms the voltage source from the cable end in (a), to the whip junction in (b), which excites an equivalent dipole.

Figure 2

SNR performance at the antenna junction of a dipole antenna in a lossy medium with of ε=80, σ=0.63 S∕m, as modeled from analytical Eqs. 1, 2, 3, 4, 5 as a function of B₀ for different ρ⩽50 mm. Fitting the B₀ axis results shows that SNR varies as $B_0^{1.9}–B_0^{2.0}$ for ρ⩽50 mm.

Figure 3

Schematic of partially filled coaxial cylindrical transmission line (left) and calibration loads (right) used to measure saline electrical properties with the HP4395 impedance analyzer. The stray capacitance is measured with the device empty.

Figure 4

Measured B₀ dependence of dielectric constant (a) and conductivity (b) of the 0.35% saline used for SNR experiments. Each point derives from impedance measurements at N=13 (diamonds) and N=19 (stars) different L_i values. The mean values (solid lines) are: ε=79±4 and σ=0.63±0.05 S∕m.

Figure 5

(a) Absolute SNR (shaded boxes, in ml⁻¹ Hz^1∕2) computed by EM MoM at ρ=1 cm from the junction for antennae made with λ_c∕4 cable portions tuned at 0.5, 1, 1.5, 2, 3, 4, 4.7, and 5 T in the 0.35% saline (squares). Experimental values measured at 1.5, 3, and 4.7 T (stars) are overlaid. The data are best fit to a curve of the form $B_0^2$ (solid). Experimental data are corrected to eliminate contributions from system noise (NF), but all data include cable losses. The dotted curves show the 95% confidence interval for the fitted curve. (b) Absolute computed SNR performance of the loopless antenna in blood based on the electrical properties in Table 1. This SNR has a 7∕4th power dependence on B₀.

Figure 6

Theoretical (solid, black) and experimental (color, dotted) absolute SNR (ml⁻¹ Hz^1∕2) in the plane of the whip junction of the semi-rigid minimal-loss antennae in the 0.35% saline. (a) The 1.5 T antenna made with λ_c∕4 cable. (b) The 4.7 T antenna made with λ_c∕4 cable. (c) The 3 T antenna made with λ_c∕4 and (d) 3λ_c∕4 cables. The experimental SNR data (blue, 50 000; green, 100 000; red, 200 000; purple, 400 000; cyan, 800 000 ml⁻¹ Hz^1∕2) overlap the continuous circular theoretical contours corresponding to the same values. Contributions from the system noise (NF) are subtracted from the experimental data. Theoretical and mean experimental SNR values at a specific location differ by <10% in all cases.

Figure 7

The computed ratio of the SNR of loopless antennae tuned by adjusting the whip length for minimum resistance, to that obtained using a whip length tuned to λ_m∕4 in the 0.35% saline. As frequency increases both conditions converge, resulting in the same SNR.

Figure 8

(a) Schematic of the loopless antenna with its matching, decoupling and balun circuit. The balun is formed from the coaxial cable tuned with C_b and the center conductor running through to the whip: The outer conductor is grounded at the proximal end. C_d compensates for the balun inductance. (b) Annotated photograph of the 4.7 T experimental setup showing phantom, loopless antenna, and circuit box.

Figure 9

Heat testing of the 3 T biocompatible nitinol loopless antenna. (a) Locations of the fiber optic temperature sensors relative to the cable (thick black section) and whip (thin line). Relative to the whip junction, Ch3 is 2 cm along the whip; Ch2 is 1 cm on the whip; Ch1 is on the junction. Ch5 and Ch4 are at 6 and 12 cm (Ch4) along the cable portion, proximal to the junction. Ch6, located remotely, serves as a control to read the phantom’s normal temperature change. (b) Temperature of the fully decoupled antenna recorded from the sensors over a 10 min period during which MRI is performed at a scanner-estimated 10 W∕kg with a higher, 0.9% saline gel to simulate a worst case heating condition. (c) With the decoupling circuit disconnected, a rapid temperature rise 7 °C is recorded in the first 100 s.

Figure 10

In vivo performance of biocompatible loopless antennae for MRI of the rabbit aorta. (a) The biocompatible, 3λ_c∕4 cable length, 3 T intravascular loopless antenna with its tuning∕matching∕decoupling box adapted from a Surgi-Vision 1.5 T nitinol MRI antenna. The high resolution (156 μm×156 μm×3 mm) axial MRI acquired at the junction of the 1.5 (b) and 3 T (c) MRI antennae (FSE, TR:2.5 s, TE:20 ms, ETL:8, BW:31.25, FOV:4×4 cm², ST:3 mm, matrix=256×256, NEX=2, scan time=16 s) are scaled by ρ to compensate for the ∼1∕ρ drop-off in SNR which amplifies noise at large ρ (Ref. 8). The elliptical dashed contour on (b) and (c) enclose comparable FOV areas with SNR>9.