Variable TE gradient and spin echo sequences for in vivo MR microscopy of short T2 species.

Collagen-rich tissues such as skin or fibrous cartilage have very short T2 and thus, in order to be visible, demand a commensurate reduction in echo time. Whereas short echo time for imaging of humans is straightforward at large fields of view with currently available whole body gradient hardware, the problem is more challenging in the microscopic resolution regime (<100 microm). In this work a simple approach consisting of shortening the echo time dynamically toward the lower spatial frequencies is described for three-dimensional partial flip-angle gradient and spin-echo sequences. Microimages obtained in vivo at 50 microm 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-spread 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 the phase modulation occurring during the variable echo time and can manifest as both blurring and intensity fluctuations, as well as shifts of boundaries in the phase-encoding direction. However, for the on-resonance condition, the images are free from these artifacts and exhibit significantly improved signal-to-noise ratio.