HighlY constrained backPRojection (HYPR) for Ultrafast Undersampled MRI

描述(申请人提供)：在过去的九年中，我们的团队研究了与奈奎斯特定理相关的各种形式的k空间和时间欠采样，以便在具有挑战性的磁共振成像应用中进行更快的采集，特别是时间分辨的高分辨率对比度增强血管成像。使用3D技巧，时间欠采样系数达到3，以提供时间分辨率3D MR血管成像。后来，面内径向欠采样采集被开发用于采集角度欠采样因子为6的血管成像。这两种技术在PR技巧中的结合导致了18的欠采样系数。利用VIPR将欠采样扩展到真正的3D径向轨迹，在某些应用中，典型的欠采样因子50可以在没有明显伪影的情况下使用。此外，VIPR还与相衬成像相结合，可以在大体积上进行3D流动测量，从而能够推导出小容器中的压力梯度。 最近，我们提出了一种近似非迭代重建技术的概念，称为HYPR(High Constraint Back Projection)，以进一步增加时间演化采集的允许欠采样，这取决于数据集的稀疏性和序列图像之间的时空相关性程度。初步结果显示，对比度增强血管造影术的欠采样系数在100到300之间。在使用传统笛卡尔相位编码方法需要39小时的4分钟3843时间分辨相衬检查中，在足够的SNR的情况下实现了近1000倍的信噪比。该技术还被用于在X射线CT灌注中实现10倍的剂量减少，并可以推广到其他图像系列应用，例如MR扩散张量成像，其中该技术已经使用具有不同扩散编码方向的一系列图像模拟了2D实现中的15倍欠采样。这里没有提出的大量其他潜在应用将受益于对所提议的算法及其潜在限制的更好理解。 HYPR通过将长合成图像中获得的SNR转换为单独的时间帧来提供独特的SNR行为。这种复合图像的持续时间必须针对每个临床应用进行调整，以便可以最大限度地减少空间模糊和波形失真。这取决于稀疏性和时空相关性条件。 我们建议使用计算机模拟和模体来研究HYPR的基本特性，并进行三个初步的患者研究，以建立用于未来验证研究的方案。这项建议侧重于使用PR HYPR技巧和HYPR VIPR来评估大脑AVM，并测量颈动脉狭窄处的压力梯度。

DESCRIPTION (provided by applicant): During the past nine years our group has investigated various forms of k-space and temporal undersampling relative to the Nyquist Theorem for faster acquisitions in challenging MR imaging applications, particularly time resolved, high resolution contrast-enhanced angiography. Using 3D TRICKS, a temporal undersampling factor of 3 was achieved to provide time resolved 3D MR angiograms. Later, in-plane radially undersampled acquisitions were developed for acquiring angiograms with angular undersampling factors of 6. The combination of these two techniques in PR TRICKS resulted in undersampling factors of 18. Undersampling was extended to a truly 3D radial trajectory with VIPR, where typical undersampling factors of 50 can be used without significant artifacts in certain applications. In addition, VIPR has been combined with phase contrast imaging to permit 3D flow measurements across a large volume enabling the derivation of pressure gradients in small vessels. Recently, we developed the concept for an approximate non-iterative reconstruction technique called HYPR (HighlY constrained back PRojection) to further increase the permissible undersampling of temporally evolving acquisitions by another large factor that depends on the sparsity of the data set and the degree of spatio- temporal correlation between the serial images. Preliminary results have produced undersampling factors between 100 and 300 for contrast-enhanced angiography. A factor of nearly 1000 was achieved with adequate SNR in a 4 minute 3843 time-resolved phase contrast examination that would have required 39 hours using conventional Cartesian phase encoding methods. The technique was also used to achieve a factor of ten dose reduction in X-ray CT perfusion and can be generalized to other image series applications such as MR diffusion tensor imaging where the technique has simulated a factor of 15 undersampling in a 2D implementation using a series of images having different diffusion encoding directions. A large number of other potential applications that are not proposed here will benefit from a better understanding of the proposed algorithm and its potential limitations. HYPR provides unique SNR behavior by transferring the SNR obtained in a long composite image into individual time frames. The duration of this composite image must be adjusted for each clinical application so that spatial blurring and waveform distortion can be minimized. This depends on sparsity and spatio-temporal correlation conditions. We propose to investigate the basic properties of HYPR using computer simulations and phantoms and to conduct three preliminary patient studies to establish protocols to be used in future validation studies. This proposal focuses on the evaluation of cerebral AVMs using PR HYPR TRICKS and HYPR VIPR and the measurement of pressure gradients across stenotic carotid arteries.