9.4T human MRI: preliminary results

Abstract

This work reports the preliminary results of the first human images at the new high-field benchmark of 9.4T. A 65-cm-diameter bore magnet was used together with an asymmetric 40-cm-diameter head gradient and shim set. A multichannel transmission line (transverse electromagnetic (TEM)) head coil was driven by a programmable parallel transceiver to control the relative phase and magnitude of each channel independently. These new RF field control methods facilitated compensation for RF artifacts attributed to destructive interference patterns, in order to achieve homogeneous 9.4T head images or localize anatomic targets. Prior to FDA investigational device exemptions (IDEs) and internal review board (IRB)-approved human studies, preliminary RF safety studies were performed on porcine models. These data are reported together with exit interview results from the first 44 human volunteers. Although several points for improvement are discussed, the preliminary results demonstrate the feasibility of safe and successful human imaging at 9.4T.

FIG. 1

Hardware developed for 9.4T imaging. a: The Magnex Scientific 65-cm bore magnet. b: A rack of 5, 500-W broadband RF power amplifiers (top) together with an 8-bit digital phase and gain controller (near bottom). Two more racks hold the additional 11 power amplifiers to complete the 16-channel parallel transmitter reported here. c: ADC boards for the 16-channel parallel receiver. d: A capacitively decoupled 16-channel, 400-MHz stripline TEM resonator.

FIG. 2

Schematic of a parallel transceiver. Phase and gain are controlled with 8-bit resolution on multiple independent transmit channels. Transmit and receive functions are separated in time by a transmit-receive switch on each channel. Each receive channel incorporates a decoupling preamp, filters, and receiver gain as needed. Each transmit channel includes a 500-W solid-state power amplifier with feedback for the RF power monitor. The RF power amplifiers are broadbanded to facilitate the addition of a programmable frequency synthesizer for multinuclear control.

FIG. 3

9.4T predictions of B₁ nonuniformities in a head with a homogeneous circularly polarized coil. Each color change represents a 20-dB field gradient.

FIG. 4

Multichannel TEM head-coil design considerations. a: FDTD calculations of B₁ for a head inside a close-fitting, eight-channel TEM coil with a thin dielectric substrate (blue shaded ring) at 400 MHz. b: The head template overlay is removed to better show the RF artifacts. c and d: B₁ calculations for the head inside a roomier coil with a thicker dielectric substrate.

FIG. 5

Modeling SAR in a porcine head. a: Imaged porcine head. b: The anatomy from this 3D image was segmented, assigned frequency-and tissue-specific permittivity and conductivity values, and assembled into a model within a 16-element TEM volume coil. c: SAR contours calculated by the XFTTD method. Such models guide fluoroptic probe placement for experimental studies and further understanding of SAR and heating induced by high-frequency fields.

FIG. 6

RF heating at 9.4T. The temperature vs. time vs. applied continuous-wave RF power is measured by implanted fiber-optic probes. The plot is the average heating recorded from 12 human-sized anesthetized porcine heads in a head coil tuned to 400 MHz.

FIG. 7

B₁ shimming with the parallel transceiver and an eight-channel TEM coil at 9.4T. a: A series of B₁ field shimming steps, left to right, adjusting the field magnitude only. b: B₁ shimming performed by adjusting only the phase angle of the transmit signal. In practice, both B₁ magnitude and phase will be adjusted together to optimize image criteria by automated feedback-driven algorithms.

FIG. 8

Effect of the transmit phase on image homogeneity. a: Scout FLASH image of a head inside a circularly polarized elliptical coil. The loss of signal near the left ear is the result of destructive interference reducing the net B₁⁺. The relative transmit phase for each coil labeled near the two lines representing the conductor and ground planes of each coil is shown. b: By adjusting only the relative transmit phase of the coils, local destructive interference can be reduced.

FIG. 9

B₁-shimmed FLASH images. Using the relative transmit phases shown in Fig. 8b, transverse (a), coronal (b), and sagittal (c) images of the same subject were acquired.

FIG. 10

RF model of B₁ localization in a cylindrical phantom. a: An off-center target B₁ distribution was defined in the center slice of a finite-element model of a head-sized cylindrical phantom placed in a 16-rung TEM coil. b: B₁ magnitude optimization was achieved by iteratively varying both the phase and magnitude of each of 16 line currents in a simulated annealing approach.

FIG. 11

Initial 9.4T FLASH images showing T₂* contrasted venous structure and other features. a: The dark band at the top is an uncorrected B₀ artifact.