Free-breathing high-resolution respiratory-gated radial stack-of-stars magnetic resonance imaging of the upper abdomen at 7 T

Abstract

Ultrahigh field magnetic resonance imaging (MRI) (≥ 7 T) has the potential to provide superior spatial resolution and unique image contrast. Apart from radiofrequency transmit inhomogeneities in the body at this field strength, imaging of the upper abdomen faces additional challenges associated with motion-induced ghosting artifacts. To address these challenges, the goal of this work was to develop a technique for high-resolution free-breathing upper abdominal MRI at 7 T with a large field of view. Free-breathing 3D gradient-recalled echo (GRE) water-excited radial stack-of-stars data were acquired in seven healthy volunteers (five males/two females, body mass index: 19.6-24.8 kg/m²) at 7 T using an eight-channel transceive array coil. Two volunteers were also examined at 3 T. In each volunteer, the liver and kidney regions were scanned in two separate acquisitions. To homogenize signal excitation, the time-interleaved acquisition of modes (TIAMO) method was used with personalized pairs of B₁ shims, based on a 23-s Cartesian fast low angle shot (FLASH) acquisition. Utilizing free-induction decay navigator signals, respiratory-gated images were reconstructed at a spatial resolution of 0.8 × 0.8 × 1.0 mm³. Two experienced radiologists rated the image quality and the impact of B₁ inhomogeneity and motion-related artifacts on multipoint scales. The images of all volunteers showcased effective water excitation and were accurately corrected for respiratory motion. The impact of B₁ inhomogeneity on image quality was minimal, underscoring the efficacy of the multitransmit TIAMO shim. The high spatial resolution allowed excellent depiction of small structures such as the adrenal glands, the proximal ureter, the diaphragm, and small blood vessels, although some streaking artifacts persisted in liver image data. In direct comparisons with 3 T performed for two volunteers, 7-T acquisitions demonstrated increases in signal-to-noise ratio of 77% and 58%. Overall, this work demonstrates the feasibility of free-breathing MRI in the upper abdomen at submillimeter spatial resolution at a magnetic field strength of 7 T.

Keywords: TIAMO; radial sampling; respiratory gating; ultrahigh field MRI.

FIGURE 1

Diagram of the golden-angle radial stack-of-stars spoiled gradient-recalled echo pulse sequence with TIAMO $B_1^{+}$ shim. The phases of the two TIAMO modes for a single transmit channel are indicated in blue and red. The complementary excitation modes were identical in spatial encoding, differing only in the phases used for the RF pulses. Slab-selective water excitation was performed using a (121) binomial pulse with bipolar gradients. A nonlocalized FID acquisition between slab-selection rephasing and partition encoding acted as a respiratory navigator. ADC, analog-to-digital converter; FID, free induction decay; RF, radiofrequency; TIAMO, time-interleaved acquisition of modes.

FIGURE 2

Acquisition scheme of the radial stack-of-stars sequence (image inspired by Wang et al.). Using TIAMO, each readout was acquired with two complementary RF shim modes (blue and red lines). All phase-encode steps were scanned twice, once per spoke direction and TIAMO mode, while using a randomized partition acquisition order according to the single-spoke binning method. After scanning an entire spoke stack, the readout angle was incremented by the golden angle and the process was repeated for the updated projection angle. RF, radiofrequency; TIAMO, time-interleaved acquisition of modes.

FIGURE 3

Representative example of a radial stack-of-stars liver acquisition with the TIAMO $B_1^{+}$ shim. The top row contains motion-averaged reconstructions of the radial data for both TIAMO modes, the right column shows the sum of both. The bottom row shows the magnitude of the complex sums of the $B_1^{+}$ maps, modulated by the phase factors per mode obtained from the calibration (according to Equation 2). TIAMO, time-interleaved acquisition of modes.

FIGURE 4

Estimated flip angle distributions of the radial stack-of-stars sequence across two slices in a single volunteer. Flip angle maps were generated by scaling 3DREAM flip angle maps according to the integral of the radial sequence’s binomial pulse RF envelope. 3DREAM, 3D dual refocusing echo acquisition mode; RF, radiofrequency.

FIGURE 5

(A–D) Axial views of kidney MRI data of four volunteers. The images illustrate the effectiveness of the TIAMO shim and water excitation. The images show a sharp depiction of the kidneys, vessels, and lymph nodes (green arrow in (A)), as well as clear corticomedullary distinction (green arrow in (C)). Some anterior subcutaneous fat signal remained (red arrows in (A)), likely due to B_o inhomogeneities in areas strongly affected by respiratory motion. Magnetic susceptibility differences led to signal loss at the intestines (red arrow in (C)), obscuring the intestinal wall. Residual motion caused streaking artifacts on the anterior side (red arrow in (D)). MRI, magnetic resonance imaging; TIAMO, time-interleaved acquisition of modes.

FIGURE 6

Comparison of (A and C) Motion-averaged, and (B and D) Respiratory-gated reconstructions of liver scan data. Respiratory gating resulted in sharp depictions of the diaphragm (arrow in (B)) and the liver dome (arrow in (D)). The uncorrected images (A) and (C) show large losses of signal near the diaphragm (arrows in (A) and (C)), which was attributed to phase errors due to translational motion.

FIGURE 7

(A–F) Axial and coronal views of the liver images of three volunteers. The images show enhancement of the vessels without the use of an external contrast agent. The reconstructions demonstrated an effective ${\text{B}}^{+}$ shim with only minor fluctuations in signal strength at the liver in axial views (arrow in (A)). The coronal views display sharp depictions of the adrenal glands (arrow in (B)) and the vessels (arrow in (F)). Some streaking artifacts were visible and originated from the abdominal cavity (arrow in (C)).

FIGURE 8

(A–D) Axial views of SNR maps averaged across 20 liver slices (or 2-cm slice thickness), generated through the “pseudo multiple replica” method. Each row corresponds to data from a different volunteer, with dotted outlines delineating regions of interest for indicative mean SNR calculations. In general, SNRs were superior in 7-T acquisitions, except in areas near the anterior side, where the dense body coil receiver arrays of the 3-T system exhibited better performance compared with the fractionated dipole antennas of the 7-T system. SNR, signal-to-noise ratio.