Fast diffusion coefficient mapping using interlaced FM DANTE sequence

Abstract

A DANTE sequence was applied to ultra fast MR imaging in the DUFIS and OUFIS sequence among others. Fast imaging using the DANTE pulse sequence, however, has the advantage that it does not require high speed gradient switching. As an alternative, we have developed a frequency modulated (FM) DANTE fast imaging sequence and its application to diffusion imaging is studied. When a DANTE pulse train is shaped by an FM function, all the spins within each pixel are excited, and thereby improves the signal to noise ratio. In addition, DANTE pulse train modulated by an FM function has a uniform magnitude, thereby resulting in an pulse train with uniform peak magnitudes. An FM DANTE pulse sequence, therefore, could be used to overcome the difficulties of the previous DANTE sequences which generally suffer a poor signal to noise ratio due to the poor excitation profiles and non-uniform echo signal magnitudes. Some of the minor, yet problematic questions, such as the intensity reduction due to the spin phase incoherency in the pixel and its remedy are also discussed. It was found that the sweep velocity can be increased in proportion to the number of interlacings in FM DANTE imaging and can be used to overcome the phase incoherency. The relatively long readout time and strong gradient, however, cause severe diffusion dependent signal attenuation. To reduce diffusion attenuation, an interlacing technique has been incorporated into the FM DANTE sequence and able to reduce the overall readout time by the number of interlacings. Using the interlacing technique in FM DANTE imaging, diffusion coefficients of the object can be obtained by the two or more image data which have undergone different numbers of interlacings. Theoretical study and experimental results are reported and future application potential of the method is discussed.