Manufacturers of many implantable active devices provide conditions for safe MRI scanning that exclude use of the transmit radiofrequency (RF) body coil. This limits the types of MRI exams available to patients with such devices, possibly prohibiting access to life-saving diagnoses. However, the stray RF field rapidly decreases outside of the body coil¹, and the typical 50-55 cm electrical length of the body coil is significantly shorter than the 105-130 cm MR scanner bore length. A conservative MR exam using body coil transmit might have low theoretical risk of implant heating if centering the imaged anatomy places the implant sufficiently far from isocenter, beyond the location of an approximate 10 dB reduction in B1-power². IEC60601-2-33 requires manufacturers to provide locations along the z-axis where the RF transmit field is reduced by 3 and 10 dB¹ . Here we set out to measure the RF B1-field on a specific MRI scanner for comparison against the manufacturer-provided data, as shown in Figure 1³. The ultimate goal is to better inform clinical risk/benefit assessment for patients with MR-conditional implanted devices that exclude use of the body transmit coil, but otherwise allow for MR exams with volume head and extremity transmit coils.

MRI measurements were performed on a 1.5T Optima 450W scanner (GE Healthcare, Milwaukee, WI). Transmit gain (TG) for a 90° pulse was set to 150, a value commonly seen for normal BMI patients on this particular scanner. To map changes in the transverse RF B1-field relative to isocenter, induced voltages in a single-loop RF pick-up coil tuned to 64 MHz (1.5T) were measured using an oscilloscope; the voltages induced by an unloaded body coil were measured with the pick-up coil oriented in a sagittal and coronal plane at 10 cm increments along the z-axis, as described in IEC60601-2-33¹. B1-field was also mapped along a line shifted 15 cm lateral to the z-axis to correspond to a typical position of an Implantable Pulse Generator (IPG) and associated leads. B1-field measurements were repeated with a body TLT sphere and body-coil loader phantom within the RF coil.

Reduction in the RF field should correspond to reduction in RF-induced heating. To demonstrate this, the temperature rise was measured at the lead tips of insulated copper leads placed in a head-and-torso ASTM phantom (ASTM F2182-11) filled with gelled saline⁴ . Two 18-gauge, 15 cm long wires, parallel to the scanner z-axis, were submerged in the phantom, as depicted in Figure 2 A. Fluoroptic temperature probes (STF-2, model 750; LumaSense Inc., Santa Clara, CA, USA) were positioned at both ends of each of the wires. An RF spin echo with 4-echoes, TR/TE1/TE2/TE3/TE4 100/17/34/51/68 ms, with an estimated SAR of 1.2 W/kg (input weight=150 lbs) was scanned for 259 seconds to generate lead heating. The phantom was initially landmarked and scanned with the middle of the wires at the center of the RF transmit coil, and subsequently at 10 cm increments as the phantom was advanced out of the RF transmit coil. The maximum temperature increase for each wire was recorded as a function of landmark location.

B1-field power and max temperature elevations as a function of distance along the z-axis from isocenter are shown in Figure 2B. Along the Z-axis, B1-field power falls below 10 dB at a distance of 30 cm from isocenter. At the 40 cm lateral location from isocenter, the B1-field power has a small, reproducible, local peak, at 13%/9 dB reduction relative to isocenter. This power increase closer to the bore wall is consistent with B1-field modeling of a high-pass birdcage transmit coil.

Temperature measurements show a marked reduction in RF-induced heating: temperature elevations were measured to be 3% of those at isocenter when the wires were positioned at a landmark location 40cm from isocenter. Again, these temperature measurements were for demonstration purposes only. Actual temperature increases would be dependent on device, patient, lead path, scan location and scan technique.

RF power over an implanted device, and the resulting heating of said device, are expected to drop dramatically when the implanted device is located far away from magnet isocenter. Our scanner specific measurements demonstrated localized B1-field power increases near the end-rings of the body coil, information that would not be immediately apparent from the manufacturer’s provided data. The scanner specific measurements provide information that can help with conservatively scanning and positioning a patient for whom risk/benefit analysis favors proceeding with MRI despite the implanted device’s exclusion of body coil transmit from its MR conditions for use.