Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA.
Biomed Res Int. 2013;2013:680292. doi: 10.1155/2013/680292. Epub 2013 Aug 6.
MRI tissue contrast is not well preserved at high field. In this work, we used a phantom with known, intrinsic contrast (3.6%) for model tissue pairs to test the effects of low angle refocusing pulses and magnetization transfer from adjacent slices on intrinsic contrast at 1.5 and 3 Tesla. Only T1-weighted spin echo sequences were tested since for such sequences the contrast loss, tissue heating, and image quality degradation at high fields seem to present significant diagnostic and quality issues. We hypothesized that the sources of contrast loss could be attributed to low refocusing angles that do not fulfill the Hahn spin echo conditions or to magnetization transfer effects from adjacent slices in multislice imaging. At 1.5 T the measured contrast was 3.6% for 180° refocusing pulses and 2% for 120° pulses, while at 3 T, it was 4% for 180° and only 1% for 120° refocusing pulses. There was no significant difference between single slice and multislice imaging suggesting little or no role played by magnetization transfer in the phantom chosen. Hence, one may conclude that low angle refocusing pulses not fulfilling the Hahn spin echo conditions are primarily responsible for significant deterioration of T1-weighted spin echo image contrast in high-field MRI.
MRI 组织对比在高场下不能很好地保持。在这项工作中，我们使用具有已知内在对比（3.6%）的模型组织对来测试低角度重聚焦脉冲和来自相邻切片的磁化转移对 1.5 和 3 Tesla 固有对比的影响。仅测试了 T1 加权自旋回波序列，因为对于这些序列，高场下的对比度损失、组织加热和图像质量恶化似乎存在显著的诊断和质量问题。我们假设对比度损失的原因可能归因于不满足哈恩自旋回波条件的低重聚焦角度，或者归因于多切片成像中来自相邻切片的磁化转移效应。在 1.5 T 下，180°重聚焦脉冲的测量对比度为 3.6%，120°脉冲的对比度为 2%，而在 3 T 下，180°脉冲的对比度为 4%，120°脉冲的对比度仅为 1%。单切片和多切片成像之间没有显著差异，表明在所选的模型中，磁化转移的作用很小或可以忽略不计。因此，可以得出结论，不满足哈恩自旋回波条件的低角度重聚焦脉冲是导致高场 MRI 中 T1 加权自旋回波图像对比度显著恶化的主要原因。
