Department of Experimental Physics 5, University of Wuerzbug, Wuerzbug, Germany.
Magn Reson Med. 2012 Feb;67(2):510-8. doi: 10.1002/mrm.23017. Epub 2011 Jun 30.
Sequences with ultrashort echo times enable new applications of MRI, including bone, tendon, ligament, and dental imaging. In this article, a sequence is presented that achieves the shortest possible encoding time for each k-space point, limited by pulse length, hardware switching times, and gradient performance of the scanner. In pointwise encoding time reduction with radial acquisition (PETRA), outer k-space is filled with radial half-projections, whereas the centre is measured single pointwise on a Cartesian trajectory. This hybrid sequence combines the features of single point imaging with radial projection imaging. No hardware changes are required. Using this method, 3D images with an isotropic resolution of 1 mm can be obtained in less than 3 minutes. The differences between PETRA and the ultrashort echo time (UTE) sequence are evaluated by simulation and phantom measurements. Advantages of pointwise encoding time reduction with radial acquisition are shown for tissue with a T(2) below 1 ms. The signal to noise ratio and Contrast-to-noise ratio (CNR) performance, as well as possible limitations of the approach, are investigated. In-vivo head, knee, ankle, and wrist examples are presented to prove the feasibility of the sequence. In summary, fast imaging with ultrashort echo time is enabled by PETRA and may help to establish new routine clinical applications of ultrashort echo time sequences.
短回波时间序列可实现 MRI 的新应用，包括骨骼、肌腱、韧带和牙齿成像。本文提出了一种序列，通过限制脉冲长度、硬件切换时间和扫描仪的梯度性能，实现了每个 k 空间点尽可能短的编码时间。在径向采集的点编码时间减少（PETRA）中，外 k 空间用径向半投影填充，而中心则在笛卡尔轨迹上单点测量。这种混合序列结合了单点成像和径向投影成像的特点。不需要硬件更改。使用这种方法，不到 3 分钟即可获得各向同性分辨率为 1 毫米的 3D 图像。通过模拟和仿体测量评估了 PETRA 和超短回波时间（UTE）序列之间的差异。对于 T2 低于 1ms 的组织，展示了径向采集的点编码时间减少的优势。研究了信号噪声比和对比噪声比（CNR）性能以及该方法的可能限制。呈现了头部、膝盖、脚踝和手腕的体内示例，以证明该序列的可行性。总之，通过 PETRA 实现了超短回波时间的快速成像，这可能有助于建立超短回波时间序列的新常规临床应用。
