Factors influencing fast low angle positive contrast steady-state free precession (FLAPS) magnetic resonance imaging

The presence of susceptibility-shifting media can lead to signal voids in magnetic resonance images. While signal voids have been traditionally used to detect such magnetic perturbers, selective magnetic resonance imaging of off-resonant spins surrounding susceptibility-shifted media allows for them to be visualized as hyper-intense (positive contrast) regions. These positive contrast methods can potentially improve the detection conspicuity of magnetic perturbers against regions that appear dark due to the absence of protons, such as air. Recently, a fast low angle positive contrast steady-state free precession (FLAPS) technique has been proposed as a positive contrast imaging method. This work systematically evaluates the contrast characteristics and acquisition strategies of FLAPS-based imaging from the standpoint of imaging parameters and physical properties of the magnetic perturbers. Results show that scan parameters (T(R), flip angle, B0), physical properties of the perturber (size and concentration of shift reagent) and the ratio of the relaxation constants (T1/T2) of the medium are significant factors influencing the FLAPS-based positive contrast.