VARIABLE TE GRADIENT AND SPIN-ECHO SEQUENCES FOR IN-VIVO MR MICROSCOPY OF SHORT T-2 SPECIES

Collagen-rich tissues such as skin or fibrous cartilage have very shor t T-2 ,and thus, in order to be visible, demand a commensurate reducti on in echo time. Whereas short echo time for imaging of humans is stra ightforward at large fields of view with currently available whole bod y gradient hardware, the problem is more challenging in the microscopi c resolution regime (<100 mu m). In this work a simple approach consis ting of shortening the echo time dynamically toward the lower spatial frequencies is described for three-dimensional partial flip-angle grad ient and spin-echo sequences. Microimages obtained in vivo at 50 mu m resolution on a 1.5 T whole body scanner are shown to afford a signal- to-noise gain of over 100% in the dermis of the human skin. A point-sp read function analysis indicates that the variable echo time gradient- echo sequence produces a unique not previously reported off-resonance artifact in the phase-encoding direction. The artifact results from th e phase modulation occurring during the variable echo time and can man ifest as both blurring and intensity fluctuations, as well as shifts o f boundaries in the phase-encoding direction. However, for the on-reso nance condition, the images are free from these artifacts and exhibit significantly improved signal-to-noise ratio.