Song H K, Wehrli F W
Department of Radiology, University of Pennsylvania Medical Center, Philadelphia 19104, USA.
Magn Reson Med. 1998 Feb;39(2):251-8. doi: 10.1002/mrm.1910390212.
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.
富含胶原蛋白的组织，如皮肤或纤维软骨，具有非常短的T2，因此，为了使其可见，需要相应地缩短回波时间。对于使用当前可用的全身梯度硬件在大视野下对人体进行成像而言，缩短回波时间很简单，但在微观分辨率范围（<100微米）内，这个问题更具挑战性。在这项工作中，针对三维部分翻转角梯度和自旋回波序列，描述了一种简单的方法，即朝着较低空间频率动态缩短回波时间。在1.5T全身扫描仪上以50微米分辨率在体内获得的微观图像显示，在人体皮肤的真皮层中，信噪比提高了100%以上。点扩散函数分析表明，可变回波时间梯度回波序列在相位编码方向上产生了一种以前未报道过的独特的失谐伪影。该伪影是由可变回波时间期间发生的相位调制引起的，可表现为模糊、强度波动以及相位编码方向上边界的偏移。然而，对于共振条件，图像没有这些伪影，并且信噪比显著提高。
