Fan Shu-Juan, Ma Yajun, Chang Eric Y, Bydder Graeme M, Du Jiang
Department of Radiology, University of California, San Diego, CA, USA.
Department of Radiology, VA San Diego, Healthcare System, San Diego, CA, USA.
NMR Biomed. 2017 Oct;30(10). doi: 10.1002/nbm.3767. Epub 2017 Jul 21.
Inversion recovery ultrashort echo time (IR-UTE) imaging holds the potential to directly characterize MR signals from ultrashort T tissue components (STCs), such as collagen in cartilage and myelin in brain. The application of IR-UTE for myelin imaging has been challenging because of the high water content in brain and the possibility that the ultrashort T * signals are contaminated by water protons, including those associated with myelin sheaths. This study investigated such a possibility in an ovine brain D O exchange model and explored the potential of IR-UTE imaging for the quantification of ultrashort T * signals in both white and gray matter at 3 T. Six specimens were examined before and after sequential immersion in 99.9% D O. Long T MR signals were measured using a clinical proton density-weighted fast spin echo (PD-FSE) sequence. IR-UTE images were first acquired with different inversion times to determine the optimal inversion time to null the long T signals (TI ). Then, at this TI , images with echo times (TEs) of 0.01-4 ms were acquired to measure the T * values of STCs. The PD-FSE signal dropped to near zero after 24 h of immersion in D O. A wide range of TI values were used at different time points (240-330 ms for white matter and 320-350 ms for gray matter at TR = 1000 ms) because the T values of the long T tissue components changed significantly. The T * values of STCs were 200-300 μs in both white and gray matter (comparable with the values obtained from myelin powder and its mixture with D O or H O), and showed minimal changes after sequential immersion. The ultrashort T * signals seen on IR-UTE images are unlikely to be from water protons as they are exchangeable with deuterons in D O. The source is more likely to be myelin itself in white matter, and might also be associated with other membranous structures in gray matter.
反转恢复超短回波时间（IR-UTE）成像具有直接表征来自超短T组织成分（STC）的磁共振信号的潜力，例如软骨中的胶原蛋白和脑中的髓磷脂。由于脑内高含水量以及超短T信号可能被水质子污染，包括与髓鞘相关的质子，IR-UTE在髓磷脂成像中的应用一直具有挑战性。本研究在绵羊脑D₂O交换模型中研究了这种可能性，并探讨了IR-UTE成像在3 T下定量白质和灰质中超短T信号的潜力。在顺序浸入99.9% D₂O之前和之后检查了六个标本。使用临床质子密度加权快速自旋回波（PD-FSE）序列测量长T磁共振信号。首先以不同的反转时间采集IR-UTE图像，以确定使长T信号归零的最佳反转时间（TI）。然后，在这个TI下，采集回波时间（TE）为0.01 - 4 ms的图像以测量STC的T值。浸入D₂O 24小时后，PD-FSE信号降至接近零。在不同时间点使用了广泛的TI值（在TR = 1000 ms时，白质为240 - 330 ms，灰质为320 - 350 ms），因为长T组织成分的T值发生了显著变化。白质和灰质中STC的T值均为200 - 300 μs（与从髓磷脂粉末及其与D₂O或H₂O的混合物获得的值相当），并且在顺序浸入后变化最小。在IR-UTE图像上看到的超短T*信号不太可能来自水质子，因为它们可与D₂O中的氘核交换。其来源更可能是白质中的髓磷脂本身，也可能与灰质中的其他膜状结构有关。
