To utilize trajectory rewinds to create B₀ maps that are automatically registered to the imaging data and require no additional scan time: B₀ Mapping using Rewind Trajectories (BMART).

BMART B₀ mapping requires that the rewind portions of the trajectory, as used for example in bSSFP, are evenly distributed throughout k-space. This criterion can be satisfied for center-out trajectories, where the sampling density of the rewinds often resembles that of undersampled projection imaging.

The proposed B₀ map reconstruction technique is illustrated in Figure 1a. The k-space data from the outgoing part of the trajectory (k_OUT) and from the rewinds (k_REW) have delayed effective TE with respect to each other, and a B₀ estimate can be estimated from their difference in phase. However, different portions of k-space have different time delays between k_OUT and k_REW (Figure 1b). This is accounted for by splitting the k-space data into N bins, where each bin has an effective delay Δt_i. The outgoing and rewind data is then rebuilt sequentially from the longest Δt (i.e., near k=0) to the shortest (near k-space edge). At the end of each step, the current estimate of B₀ (B_0i) is used to advance the phase of the images (outgoing and rewind) rebuilt so far to Δt_i+1. Gridding and low-pass filtering (Hanning window) of the k-space data is performed before applying BMART, and the average sampling time for each gridded sample is used to determine Δt_i.

To demonstrate the method, we have applied BMART to data acquired using 3D cones ¹ in a phantom, human brain, and heart on a 1.5 T GE Signa Excite. The 3D cones parameters were: TR = 5.6 ms, TE = 0.6 ms, flip 70°, BW = 250 kHz, FOV = 28 x 28 x 14 cm³, 1.2 mm isotropic resolution, 9137 readouts, readout duration = 2.8 ms, 18 cones per heartbeat (for heart scan). The only sequence timing change required was to collect data during the rewinds, which necessitated a 100 μs longer TR for RF transmit/receive switching delays (normally accounted for during the rewinds). The phantom and heart scans used ATR-bSSFP, while the brain scan used spoiled gradient echo (SPGR) to achieve more gray-white matter contrast. Images reconstructed from k_REW (direct gridding of all rewind data, with no Δt correction) were compared to those from k_OUT to demonstrate sufficient sampling density of the rewind portion of the trajectory (Figure 2).

For validation, the BMART B₀ maps in the phantom and brain were compared to B₀ maps obtained using dual echo gradient echo (DE-GRE) (Figure 3). The DE-GRE parameters were: TR = 134 ms, TE₁/TE₂ = 4/6.5 ms, flip 30°, BW = 62.5 kHz, 36 axial slices (3 mm), FOV = 24 x 24 cm², 1.9 mm isotropic in-plane resolution. B₀ correction was performed on the main outgoing trajectories with the BMART B₀ maps (phantom and brain: Figure 4, heart: Figure 5). In the cardiac scan, BMART was applied after rigid-body motion correction.

Excellent agreement between the two methods was observed (Figure 3), and deblurring using the BMART B₀ map improved image sharpness (Figure 4, 5). The primary disadvantage of BMART over traditional B₀ mapping techniques is the lack of freedom in choosing the TE difference. The advantages of BMART are that the B₀ map is automatically registered to the data to be corrected, and that for projection-like trajectories no additional scan time, nor appreciable compromises of TE or TR, are required. In addition, BMART may enable highly efficient trajectory design for dedicated B₀ mapping, since it requires no “fly-back” time between echoes.