Non-Cartesian balanced steady-state free precession pulse sequences for real-time cardiac MRI

Abstract

Purpose: To develop a new spiral-in/out balanced steady-state free precession (bSSFP) pulse sequence for real-time cardiac MRI and compare it with radial and spiral-out techniques.

Methods: Non-Cartesian sampling strategies are efficient and robust to motion and thus have important advantages for real-time bSSFP cine imaging. This study describes a new symmetric spiral-in/out sequence with intrinsic gradient moment compensation and SSFP refocusing at TE = TR/2. In vivo real-time cardiac imaging studies were performed to compare radial, spiral-out, and spiral-in/out bSSFP pulse sequences. Furthermore, phase-based fat/water separation taking advantage of the refocusing mechanism of the spiral-in/out bSSFP sequence was also studied.

Results: The image quality of the spiral-out and spiral-in/out bSSFP sequences was improved with off-resonance and k-space trajectory correction. The spiral-in/out bSSFP sequence had the highest signal-to-noise ratio (SNR), contrast-to-noise ratio, and image quality ratings, with spiral-out bSSFP sequence second in each category and the radial bSSFP sequence third. The spiral-in/out bSSFP sequence provides separated fat and water images with no additional scan time.

Conclusions: In this study, a new spiral-in/out bSSFP sequence was developed and tested. The superiority of spiral bSSFP sequences over the radial bSSFP sequence in terms of SNR and reduced artifacts was demonstrated in real-time MRI of cardiac function without image acceleration.

Keywords: bSSFP; fat-water separation; real-time cardiac imaging; spiral; spiral-in/out.

FIG. 1

Spiral-out bSSFP gradients (left) and the corresponding k-space trajectory (right). The spiral readout gradients are 1.44 ms in length with 720 readout samples. The rephaser gradients that simultaneously null the 0^th and 1^st order gradient moments are 1.07 ms in length. The rephaser can overlap with the next slice-select prephaser to reduce TR. On the right, the solid and dotted lines correspond to the k-space trajectories of the spiral readout gradients and the rephaser, respectively.

FIG. 2

Spiral-in/out bSSFP gradients (left) and the corresponding k-space trajectory (right). The spiral readout gradients are 1.60 ms in length with 800 readout samples. The prephaser and rephaser gradients that null the 0^th order gradient moments are each 0.68 ms in length. Both the prephaser and the rephaser can overlap with the slice-selection gradient rephaser and prephaser to reduce TR. On the right, the solid and dotted lines correspond to the k-space trajectories of the spiral readout gradients and the prephaser/rephaser, respectively.

FIG. 3

Comparison of the single-delay trajectory, the estimated trajectory and the measured trajectory for a spiral-in/out bSSFP sequence. Only one interleaf is shown for clarity. The arrows indicate the k-space traversal direction. The estimated trajectory is much closer to the measured trajectory than the single-delay trajectory, especially towards the end of the readout, as predicted by Eq. [2].

FIG. 4

Images reconstructed with (a) the single-delay trajectory, (b) the estimated trajectory and (c) the measured trajectory using the spiral-in/out bSSFP sequence and the difference images between the measured trajectory and the single-delay trajectory (d) and the estimated trajectory (e).

FIG. 5

Separated water and fat images in a short-axis breath-held experiment using the spiral-in/out bSSFP sequence.

FIG. 6

Theoretical PSFs at the center line for radial, spiral-out and spiral-in/out bSSFP sequences with matched spatial and temporal resolutions. The increased side lobes of the radial sequence lead to prominent streak artifacts in the radial images.

FIG. 7

Short-axis and long-axis free-breathing cardiac images with radial (top row), spiral-out (medium row) and spiral-in/out (bottom row) bSSFP sequences. Along each row from left to right are the short axis systolic image, short axis diastolic image, long axis systolic image, and long axis diastolic image.

FIG. 8

SNR_blood (top row) and CNR_{blood/myocardium} (bottom row) of short-axis (left column) and long-axis (right column) cardiac images with radial, spiral-out and spiral-in/out bSSFP sequences. The different bars for each method represent the values computed for six different volunteers. The asterisks indicate statistically significant differences between sequences (p < 0.05). The spiral-in/out sequence had the highest SNR and CNR for each volunteer for both short-axis and long-axis views.

FIG. 9

Average image ratings of short-axis (left) and long-axis (right) views with radial, spiral-out and spiral-in/out bSSFP sequences. The different bars for each method represent the ratings for six normal volunteers and one patient (yellow). The asterisks indicate statistically significant differences between sequences (p < 0.05).