Huang Yuexi, Wright Graham A
Department of Medical Biophysics, University of Toronto, Toronto, Canada.
Magn Reson Med. 2007 Aug;58(2):316-25. doi: 10.1002/mrm.21312.
A method for reconstruction of time-resolved MRI called highly-constrained backprojection (HYPR) has been developed. To evaluate the HYPR reconstruction in relation to data sparsity and temporal dynamics, computer simulations were performed, investigating signal modulations under different situations that reflect dynamic contrast-enhanced MR angiography (MRA). In vivo studies were also performed with gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) for abdominal MRA in a canine model to demonstrate the application of HYPR for three-dimensional (3D) time-resolved MRA. When contrast dynamics vary over space, large vessels (e.g., veins) tend to introduce signal interference to small vessels (e.g., arteries) in HYPR, particularly when the vessels are in close proximity. The enhancement of background tissue signals may also alter the arterial and venous temporal profiles in HYPR. However, the artifacts are manifest as intensity modulation rather than structural interference, and therefore have little impact on structural diagnosis. Increasing the number of projections per time point increases temporal blur while reducing corruption of temporal behavior from adjacent tissues. Uniformly interleaved acquisition order, such as the bit-reversed order, is important to reduce artifacts. With high signal-to-noise ratio (SNR) and limited artifacts, HYPR reconstruction has potential to greatly improve time-resolved MRA in clinical practice.
一种名为高约束反投影（HYPR）的时间分辨磁共振成像重建方法已经被开发出来。为了评估HYPR重建与数据稀疏性和时间动态性的关系，进行了计算机模拟，研究了在反映动态对比增强磁共振血管造影（MRA）的不同情况下的信号调制。还使用钆二乙烯三胺五乙酸（Gd-DTPA）在犬模型中进行了腹部MRA的体内研究，以证明HYPR在三维（3D）时间分辨MRA中的应用。当对比动态在空间上变化时，大血管（如静脉）在HYPR中往往会对小血管（如动脉）产生信号干扰，特别是当血管彼此靠近时。背景组织信号的增强也可能改变HYPR中的动脉和静脉时间轮廓。然而，伪影表现为强度调制而非结构干扰，因此对结构诊断影响很小。增加每个时间点的投影数量会增加时间模糊，同时减少相邻组织对时间行为的破坏。均匀交错的采集顺序，如位反转顺序，对于减少伪影很重要。由于具有高信噪比（SNR）和有限的伪影，HYPR重建在临床实践中有潜力极大地改善时间分辨MRA。
