3D magnetization-prepared imaging using a stack-of-rings trajectory

Abstract

Efficient acquisition strategies for magnetization-prepared imaging based on the three-dimensional (3D) stack-of-rings k-space trajectory are presented in this work. The 3D stack-of-rings can be acquired with centric ordering in all three dimensions for greater efficiency in capturing the desired contrast. In addition, the 3D stack-of-rings naturally supports spherical coverage in k-space for shorter scan times while achieving isotropic spatial resolution. While non-Cartesian trajectories generally suffer from greater sensitivity to system imperfections, the 3D stack-of-rings can enhance magnetization-prepared imaging with a high degree of robustness to timing delays and off-resonance effects. As demonstrated with phantom scans, timing errors and gradient delays only cause a bulk rotation of the 3D stack-of-rings reconstruction. Furthermore, each ring can be acquired with a time-efficient retracing design to resolve field inhomogeneities and enable fat/water separation. To demonstrate its effectiveness, the 3D stack-of-rings are considered for the case of inversion-recovery-prepared structural brain imaging. Experimental results show that the 3D stack-of-rings can achieve higher signal-to-noise ratio and higher contrast-to-noise ratio within a shorter scan time when compared to the standard inversion-recovery-prepared sequence based on 3D Cartesian encoding. The design principles used for this specific case of inversion-recovery-prepared brain imaging can be applied to other magnetization-prepared imaging applications.

(c) 2010 Wiley-Liss, Inc.

Figures

FIG. 1

3D stack-of-rings pulse sequence. 2D k-space is sampled with N uniformly spaced concentric rings (a), where sinusoidal gradients are designed for the outermost ring (b) and then scaled down to acquire one ring after each RF excitation pulse (d). Slice encoding gradients (c) are implemented for 3D spatial coverage. A time-efficient retracing design is used to acquire each ring through multiple revolutions (b). Similar to multi-echo acquisitions, each revolution Revm captures the image at a different time tm to enable fat/water separation (m = 1, 2, 3).

FIG. 2

3D centric ordering and spherical coverage. The 3D stack-of-rings can be acquired with centric ordering in all three spatial dimensions and supports spherical coverage in 3D k-space for additional time-savings while maintaining isotropic spatial resolution. The central region of k-space is covered very rapidly using only a small fraction of the full set of samples, thus ensuring that the intended contrast is captured very effectively at a desired point in time.

FIG. 3

IR-SPGR pulse sequence. This sequence consists of three main blocks. An RF pulse with angle β first inverts the magnetization. After observing an inversion evolution time TI, Q k-space encoding steps are acquired to capture the intended contrast. Parameters for the SPGR acquisition module include excitation angle θ, excitation time TE, and repetition time TR. In this work, the 3D stack-of-rings with spherical coverage is used for readout. Finally, a recovery delay period TD allows the magnetization to return to equilibrium before the next preparation segment. This whole prepare-acquire-recover loop is repeated P times such that P · Q equals the desired set of encoding steps.

FIG. 4

Bloch simulation for IR-SPGR. We calculated the signal evolution for GM, WM, and CSF using typical parameters TI/TD = 900/600 ms and Q/TE/TR/θ = 120/2 ms/12 ms/15° (a). The signal evolution stabilizes after about two full cycles of the prepare-acquire-recover outer loop. During the acquisition segment, WM/GM signal difference starts out high because of the preparation, but decays away with each readout (b). The magnetization levels are normalized such that CSF magnetization at equilibrium equals 1.

FIG. 5

Contour plots of sequence trade-offs. Bloch simulations were performed for TI = 600–1200 ms (in steps of 50 ms) and TD = 0–1200 ms (in steps of 100 ms) with Q/TE/TR/θ = 120/2 ms/12 ms/15°. Signal levels for WM and GM from the first encoding step of the readout segment were recorded. Mxy(WM) is proportional to SNRWM (a, b), while the contrast efficiency η = |Mxy(WM) − Mxy(GM)|/Mxy(WM) is proportional to CNR/SNRWM (c, d). Contour plots (b) and (d) include the scan time Tscan required for whole-brain coverage for each combination of TI and TD. Two protocols were chosen to balance SNR, CNR, and scan time: protocol A (TI/TD = 600/0 ms, Tscan = 4 min 52 s) is a fast scan while protocol B (TI/TD = 900/600 ms, Tscan = 7 min) achieves higher SNR. Both protocols theoretically achieve η on the order of 30%.

FIG. 6

Timing delays cause a rotation. When the default calibrated gradient-to-acquisition delay is used, the reconstruction faithfully depicts the phantom, as expected (a). Relatively large delays of +8 and −8 samples were intentionally introduced in (b) and (c). Since all concentric rings are acquired with a constant angular velocity, timing delays only cause a bulk rotation of the reconstructed image and do not degrade the image quality.

FIG. 7

3D stack-of-rings IR-SPGR fat-water-separated axial head images. The same brain volume was imaged with both protocol A and B. Good WM/GM CNR is observed and uniform fat/water separation is obtained over the whole brain for both protocols. Shown here is the central axial slice from both scans. Results from protocol B have higher SNR, as expected. The actual measured values are listed in Table 1.

FIG. 8

Comparison of 3DFT and 3D stack-of-rings scans for the same volume. In addition to protocols A and B, we also imaged the same brain volume using a product 3DFT IR-SPGR sequence. Only the reconstructed water images were used for comparison for the 3D stack-of-rings scans. Representative axial, coronal, and sagittal slices of the same position are displayed here. All scans show good WM/GM contrast. ROIs were localized in homogeneous areas of white matter and regions of cortical gray matter and the caudate nucleus for quantitative analysis. The 3D stack-of-rings scan using protocol A required roughly half the scan time of the 3DFT sequence, while achieving higher CNR and higher contrast efficiency. Protocol B required a slightly longer scan time than protocol A, but was still faster than the product sequence and achieved the highest SNR and CNR levels of the three. The actual measured values are listed in Table 1.