Ensuring safety of implanted devices under MRI using reversed RF polarization

Abstract

Patients with long-wire medical implants are currently prevented from undergoing magnetic resonance imaging (MRI) scans due to the risk of radio frequency (RF) heating. We have developed a simple technique for determining the heating potential for these implants using reversed radio frequency (RF) polarization. This technique could be used on a patient-to-patient basis as a part of the standard prescan procedure to ensure that the subject's device does not pose a heating risk. By using reversed quadrature polarization, the MR scan can be sensitized exclusively to the potentially dangerous currents in the device. Here, we derive the physical principles governing the technique and explore the primary sources of inaccuracy. These principles are verified through finite-difference simulations and through phantom scans of implant leads. These studies demonstrate the potential of the technique for sensitively detecting potentially dangerous coupling conditions before they can do any harm.

Figure 1

Effect of B₁ polarity on excitation. A left circularly polarized B₁ field (a) produces resonant excitation, while a right circularly polarized field (b) has no effect on magnetization. The white vector represents net magnetization, and the white line traces its path over time. The main B₀ field is directed along the positive z axis.

Figure 2

Ideal 32-rung birdcage excitation. A forward-polarized birdcage (a) produces a uniformly large excitation over its central volume. When reverse polarized (b), the excitation becomes uniformly zero over the same volume.

Figure 3

Effect of quadrature imperfection. A perfectly reverse-polarized excitation (a) creates right circularly polarized magnetization from orthogonal components $B_x^I\text{ and }{B}_y^Q$. When quadrature imperfections $\delta B_x^Q\text{ and }\delta B_y^I$ are present (b), elliptical polarization results.

Figure 4

Finite-difference simulations of an elliptical object inside an ideal birdcage with a conductive shield (a). The forward and reversed magnetic fields (b,c) are affected by the object, with the reversed field producing a small amount of undesired excitation. Forward and reversed electric fields (d,e) increase with distance from the coil center, and show an asymmetry that inverts with the polarization change. The absolute difference in these magnitudes is shown in (f). Scaling is identical for (b) and (c), and also for (d), (e), and (f).

Figure 5

Finite-difference simulations of electric field distortions, ‖E_f| − |E_r‖. For circular (a), elliptical (b), square (c), and rectangular (d) loads, distortions are generally proportional to the degree of asymmetry in the load. A centered ellipse pair (e) produces relatively low distortion, but this increases as the object is moved off-center (f). A 10% amplitude imbalance in the coil’s drive current does not add distortion to the circular load (g), but a 10° error in its relative quadrature phase introduces a moderate error (h).

Figure 6

Simulated images of a 60-mA axial current (top) qualitatively agree with acquired images from a coupled guidewire (bottom).

Figure 7

Phantom experiment. Images acquired using forward polarization (top) generate tip errors near unsafe currents, which can be difficult to distinguish from anatomy. When polarization is reversed (bottom), signal is generated only in the vicinity of unsafe currents. When the wire is shortened to eliminate antenna currents (right), reverse-polarization signal disappears.

Figure 8

Forward-polarized (a) and reverse-polarized (b) images from the torso of a healthy volunteer depict the effects of imperfect background suppression. Imperfect background suppression arises both from quadrature miscalibration and from load-induced scattering fields.

Figure 9

Pacemaker lead experiment. Reverse-polarization projection images in coronal (a), sagittal (b), and axial (c) planes clearly show the pacemaker lead, even when integrating over the 32-cm length of this gel phantom. A small ferromagnetic connector is present near the center of the lead (arrows), creating a small blowout artifact.

Figure 10

Pacemaker lead. Projection images of an 85-cm pacing lead in a 36-cm gel phantom. (a): Forward polarization image shows conventional current artifacts. When the receiver polarization is reversed (b) or both transmitter and receiver are reversed in polarization (c), the background signal is well suppressed. Less signal cancellation in the vicinity of the wire is evident in the dually reversed image, along with slightly better background suppression.

Figure 11

Reversed-polarization signal is roughly proportional to the square of induced currents in the wire.