Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Erlangen, Germany.
Siemens Healthcare GmbH, Erlangen, Germany; Max Schaldach-Stiftungsprofessur für Biomedizinische Technik (MSBT), Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Erlangen, Germany.
Magn Reson Imaging. 2019 Nov;63:193-204. doi: 10.1016/j.mri.2019.08.014. Epub 2019 Aug 18.
3D Time-of-Flight (TOF) MR-angiography (MRA) substantially benefits from ultra-high magnetic field strengths (≥7 T) due to increased Signal-to-Noise ratio and improved contrast. However, high-resolution TOF-MRA usually requires long acquisition times. In addition, specific absorption rate constraints limit the choice of optimal pulse sequence parameters, especially if venous saturation is employed.
To implement and evaluate an arterial TOF-MRA for accelerated high-resolution angiography at ultra-high magnetic field strength.
FIELD STRENGTHS/SEQUENCE: 7 T modified gradient-echo TOF sequence including venous saturation using Variable-Rate Selective Excitation (VERSE), Compressed Sensing (CS) and sparse application of saturation pulses, called segmentation, were included for acceleration.
To analyze the acceleration techniques all volunteers were examined with the same protocols. CS with different sampling patterns and regularization factors as well as segmentation were applied for acceleration. For comparison, conventional acceleration techniques were applied (GRAPPA PAT 3 and Partial Fourier (6/8 in slice/phase encoding)). Images were co-registered and 40 mm thick transversal maximum intensity projections were created to calculate the relative number of vessels. To analyze the visibility of small vessels, the lenticulostriate arteries (LSA) were examined. This was done via multiscale vessel enhancement filtering in a VOI and quantification via Fiji ImageJ as well as qualitatively evaluation by two radiologists. Additionally, the venous/arterial vessel-to-background ratios (vVBR/aVBR) were calculated for chosen protocols.
For the acceleration of a high resolution TOF-MRA (0.31 mm isotropic), under-sampling of 9.6 showed aliasing artifacts, whereas 7.2 showed no aliasing. The regularization factor R had a strong impact on the image quality according to smoothing (R = 0.01 to R = 0.005) and noise (R = 0.0005 to R = 0.00005). With the alternating sampling patterns it was shown that the k-space center should not be under-sampled too much. Additionally segmentation could be verified to be feasible for stronger acceleration with sufficient venous suppression.
The combination of several independent techniques (VERSE, CS with acceleration factor 7.2, R = 0.001, Poisson disc radius of 80%, 3 segments) enables the application of high-resolution (0.31 mm isotropic) TOF-MRA with venous saturation at 7 T in clinical time settings (TA ≈ 5 min) and within the SAR limits.
3D 时间飞跃（TOF）磁共振血管造影（MRA）由于信噪比提高和对比度改善，从超高磁场强度（≥7T）中受益匪浅。然而，高分辨率 TOF-MRA 通常需要较长的采集时间。此外，特定吸收率限制了最佳脉冲序列参数的选择，尤其是如果使用静脉饱和。
在超高磁场强度下实现并评估用于加速高分辨率血管造影的动脉 TOF-MRA。
磁场强度/序列：7T 改良梯度回波 TOF 序列，包括使用可变速率选择性激发（VERSE）、压缩感知（CS）和稀疏应用饱和脉冲（称为分段）的静脉饱和，用于加速。
为了分析加速技术，所有志愿者均使用相同的方案进行检查。CS 采用不同的采样模式和正则化因子以及分段进行加速。为了比较，还应用了传统的加速技术（GRAPPA PAT 3 和部分傅里叶（切片/相位编码中的 6/8））。对图像进行配准，并创建 40mm 厚的横断位最大强度投影，以计算血管的相对数量。为了分析小血管的可视性，检查了纹状体动脉（LSA）。这是通过在感兴趣区域中进行多尺度血管增强滤波以及通过 Fiji ImageJ 进行定量以及由两位放射科医生进行定性评估来完成的。此外，还为选定的方案计算了静脉/动脉血管与背景的比值（vVBR/aVBR）。
对于高分辨率 TOF-MRA（0.31mm 各向同性）的加速，9.6 的欠采样显示了混叠伪影，而 7.2 则没有。根据平滑度（R=0.01 至 R=0.005）和噪声（R=0.0005 至 R=0.00005），正则化因子 R 对图像质量有很大影响。通过交替采样模式表明，不应该对 K 空间中心进行过多的欠采样。此外，分段可以验证在具有足够静脉抑制的情况下，对于更强的加速是可行的。
几种独立技术（VERSE、CS 加速因子 7.2、R=0.001、泊松圆盘半径 80%、3 段）的组合使得在临床时间设置（TA≈5min）和 SAR 限制内应用高分辨率（0.31mm 各向同性）TOF-MRA 成为可能，同时还具有静脉饱和功能。
