ABSTRACT
Accurate equations for calculating the inversion time of the null point (TInull) in inversion recovery (IR) sequences are required for adequate suppression of fat or cerebrospinal fluid (CSF) but are not widely known. The purpose of this study is to elucidate the process of deriving accurate TInull equations using schematic diagrams that allow the equations to be easily understood, and to devise a convenient online tool for instant calculation of TInull. We investigated various IR sequences in which a 180° inversion pulse is followed by spin echo (SE) type sequences, termed IR-SE-type sequences, including FLAIR (fluid attenuated inversion recovery), STIR (short inversion time inversion recovery), and SPAIR (spectral adiabatic inversion recovery, spectral attenuated inversion recovery). We classified these sequences into three types according to the behavior of the longitudinal magnetization before the next IR pulse: having a train of multiple spin echoes, a single spin echo, or a train of multiple inversions by SPAIR pulses (with no spin echo). For each sequence type, we produced a precise diagram of the behavior of the longitudinal magnetization and clarified the process of deriving the equation for TInull. Three accurate TInull equations were derived. We created an online tool that calculates TInull using these three equations. The validity of the resulting TInull was evaluated on pelvic SPAIR diffusion-weighted (DW) images at 3T in 21 volunteers, using various inversion times (TI) around the calculated TInull. The tool displays the calculated TInull value instantly, after inputting imaging parameters and the T1 values of fat or CSF. The TInull values calculated using the tool achieved sufficient suppression in all subjects. When the actual TI value differed by more than 5% from the calculated TInull value, the fat suppression effect was significantly less on pelvic SPAIR DW images (P<0.01). In conclusion, this online tool is easily available and enables adequate suppression of fat or CSF according to the imaging parameters.
