Susceptibility and flow effects in functional MR imaging using tailored RF pulse

Functional imaging based on the susceptibility only is achieved by separation of the susceptibility effect from the mixture with flow effect by use of the tailored RF pulses in conjunction with gradient echo sequence. Use of the tailored RF pulse for the susceptibility enhanced functional imaging appears to be explicitly related to the deoxygenation processes, while in the conventional gradient echo T₂ * effect functional imaging the susceptibility dependent contrast appears to be mixed with a significant fraction of blood flow (inflow) signal of both arterial and venous bloods due to the nature of the fast sequence employed with the gradient echo technique. In addition, to enhance the susceptibility contrast using the conventional gradient echo technique it is necessary to increase the echo time TE, thereby the signal intensity is reduced. On the other hand, the susceptibility contrast using the tailored RF pulse technique is not explicitly dependent on TE. It is, therefore, no longer necessary to increase TE to enhance the susceptibility contrast, thereby one can avoid the possible reduction of signal intensity. Thus, using the tailored RF pulses one can unambiguously separate the susceptibility and flow effects in functional imaging. Due to the increased contrast sensitivity demonstrated in the present study the many functional imagings currently believed possible only at the high field (4.0 T and above) are now possible to perform with commonly employed MR scanner with field strength of 1.5-2.0 Tesla. Since the signal obtained can be made sufficiently high with the tailored RF gradient echo technique and represents oxygenation process more accurately, it seems possible to study quantitative oxygen metabolisms in brain function hitherto difficult to do with other gradient echo techniques