Positive susceptibility-based contrast imaging with dephased balanced steady-state free precession

Dephasing gradients can be introduced within a variety of gradient-echo pulse sequences to delineate local susceptibility changes ("White-Marker" phenomenon), e.g., for the visualization of metallic interventional devices which are otherwise difficult to display. We investigated dephased balanced steady-state free precession (d-bSSFP) and compared it with similar contrast techniques: dephased RF-spoiled fast low-angle shot (d-FLASH) and dephased steady-state free precession (d-SSFP).

A signal model was formulated to describe the positive contrast in d-bSSFP. For the example of an MR-compatible aspiration needle, the positive contrast artifact appearance was theoretically derived, and the model was verified in a water phantom at B₀ = 0.55 T. Model accuracy was evaluated by comparing the measured artifact size (for TEs between 3.4 ms and 50 ms) and the signal magnitude to the model prediction.

While positive contrast artifacts for d-FLASH and d-SSFP are axisymmetric with respect to the generating object, for d-bSSFP, a point-symmetric susceptibility artifact arises for a cylindrical needle due to the characteristic signal formation. The observed d-bSSFP artifact size was in accordance with the model (error < 1 mm). Measured (predicted) cumulated artifact signal was 1.13 ± 0.07 (1.27) times higher and 5.9 ± 0.4 times higher than the d-SSFP and d-FLASH cumulated artifact signal, respectively. In contrast to d-SSFP, the d-bSSFP artifact was robust against banding artifacts.

d-bSSFP contrast is well described by the introduced model. Positive contrast artifacts show higher cumulated signal magnitude, symmetry, and homogeneity compared with d-FLASH and d-SSFP and can therefore improve device visualization and potentially device localization.