For morphological and compositional characterization of atherosclerotic plaque, multi-contrast MR imaging has been proposed which often involves several separate scans (1, 2). However, the motion caused mis-registration among scans and the long scan time limited its clinical application. Thus, MATCH (3) was proposed to acquire multi-contrast images in a single scan, but suffers from small coverage and larger slice thickness. SNAP (4), containing two acquisition segments after an inversion recovery (IR) pulse, has been demonstrated in luminal stenosis measurement and intraplaque hemorrhage (IPH) detection. But, the fixed inversion time (TI) in SNAP is not favorable for vessel wall depiction. In this study, we aimed to develop an IR prepared 3D golden angle radial sampling sequence enabling flexible posteriori selection of TI time in the acquisition and frame duration for arbitrary-contrast reconstruction, which may provide a solution for morphological and compositional evaluation of plaque in one scan.

Sequence design: As shown in Fig. 1, the proposed sequence consists of the IR preparation pulse, which can provide different contrasts by selecting different TI, and 3D radial sampling for efficient 3D isotropic acquisition. To better achieve uniform distribution of spokes for the following flexible contrast reconstruction, a novel 3D golden angle order was used. Spokes (S1, S1’, Fig. 1) of the same TI in different IRTRs conform to the golden angle distribution defined by the 2D golden angle means (5), while for adjacent spokes (S1, S2, Fig. 1) in one IRTR the azimuthal angle increment and polar angle increment were defined: $$\triangle\beta(S1, S2)=\arccos(\left\{N\phi_1\right\}),\ \triangle\alpha(S1, S2)=2\pi\left\{N\phi_2\right\}$$where N is the total number of IRTRs, $$$\phi_1$$$=0.4656 and $$$\phi_2$$$=0.6823 are the 2D golden angle means, {} means getting the fraction parts.

Reconstruction of Flexible Contrast: The spokes acquired at the same TI in all IRTRs can be grouped for a specific contrast reconstruction using Non-uniform FFT, which may have streaking artifacts due to limited spokes. Thus, KWIC (6) was extended for the 3D radial reconstruction (3D KWIC) by using spokes adjacent to the selected TI frame. After selection of TI frame and frame duration, the spokes acquired within the frame duration from each shot were used in the center most k-space region which dominates the image contrast. Then in the subsequent adjacent regions the number of spokes will be increased by twice until all the spokes were used for the outer-most annulus region, with the transition radius determined by the Nyquist criterion (Fig. 2).

MRI Experiment: The proposed sequence and reconstruction method was tested on phantoms with T1 values ranging from 300ms to 2000ms and then evaluated on 3 healthy volunteers (2 males, mean age: 26.3 years) for carotid artery imaging on a 3.0T Philips scanner with an 8-channel carotid coil. The imaging parameters were: FOV=100x100x100 mm³ (2-fold oversampling); voxel size=0.6x0.6x0.6mm³; TR/TE=12/4.9ms; flip angle=15°; TFE factor=168; scan duration=5min50s. To test the proposed method, firstly the data in each shot was equally divided into 2 parts, with the latter part used for the following phase sensitive reconstruction. Then multi-contrast images were generated with posteriori selection of TI locations and frame durations.

Data acquisitions and reconstructions were successfully performed in phantom and all subjects. The multi-contrast images of phantom by phase sensitive reconstruction at 6 equally spaced TIs with frame duration of about 200ms were shown in Fig. 3. One typical carotid artery reconstructed at 3 TIs are shown in Fig. 4, with negative, near zero, and high positive blood signal shown.In this study, the feasibility of imaging with flexible contrast in one scan using IR prepared 3D golden angle radial sampling was demonstrated in both phantom and carotid artery imaging. The contrast similar to SNAP (4), contrast for vessel wall and bright-blood imaging were generated in one scan by the proposed method. Therefore, the proposed multi-contrast imaging technique may be a one-stop solution for 3D large coverage plaque imaging by providing inherently co-registered multi-contrast images in a single scan within short scan time.