Hagberg Gisela E, Scheffler Klaus
Biomedical Magnetic Resonance, Department of Radiology, Eberhard-Karls University, University Hospital Tübingen, Germany.
Contrast Media Mol Imaging. 2013 Nov-Dec;8(6):456-65. doi: 10.1002/cmmi.1565.
Most contrast agents for magnetic resonance imaging (MRI) are gadolinium-based T₁ shortening agents. At increasing magnetic field strengths their r1 relaxivity tends to decrease while the r₂ relaxivity increases. In parallel, at high fields the tissue T1 times increase and may mitigate the loss in contrast enhancement in T₁-weighted images owing to improved background suppression. In the present work we explored the MR signal for T₁-weighted spoiled gradient echo MRI sequences by simulations at three magnetic field strengths: 3, 7 and 9.4 T. The maximal available contrast enhancement (maxCE) was evaluated in absolute terms with the purpose of assessing how much of the full, underlying magnetization can be exploited, for a wide range of compound properties (r₁, 2-45 mM⁻¹ s⁻¹ ; r₂ /r₁, 1.2-30). Despite the theoretically predicted loss in r1 relaxivity at high fields, the same maxCE can be obtained as at low fields if the r₂ /r₁ ratio remains unchanged, albeit at the cost of a longer sequence repetition time and 1.5-2 times higher administered doses. For a fixed maximum tissue concentration, there is an optimum field-dependent value for the r1 relaxivity that yields the greatest maxCE. If the upper bound for the gadolinium concentration is 2 mM, the greatest maxCE is found for compounds with a r₂/r₁ ratio of 1.2 and an r₁ relaxivity of 20.5 mM⁻¹ s⁻¹ at 3 T, 18 mM⁻¹ s⁻¹ at 7 T and 16.5 mM⁻¹ s⁻¹ at 9.4 T. For compounds that do not change their r1 relaxivity or r₂ /r₁ ratios, the necessary dose can be reduced by 10-15% owing to the improved background suppression at higher fields.
大多数用于磁共振成像（MRI）的造影剂是基于钆的T₁缩短剂。随着磁场强度增加，它们的r1弛豫率趋于降低，而r₂弛豫率增加。同时，在高场时组织T1时间增加，并且由于背景抑制改善，可能减轻T₁加权图像中对比增强的损失。在本研究中，我们通过在三个磁场强度：3、7和9.4 T下进行模拟，探索了T₁加权扰相梯度回波MRI序列的MR信号。以绝对术语评估了最大可用对比增强（maxCE），目的是评估对于广泛的化合物性质（r₁，2 - 45 mM⁻¹ s⁻¹；r₂ / r₁，1.2 - 30），可以利用多少全部的潜在磁化。尽管理论上预测高场时r1弛豫率会降低，但如果r₂ / r₁比值保持不变，在高场仍可获得与低场相同的maxCE，尽管代价是序列重复时间更长且给药剂量高1.5 - 2倍。对于固定的最大组织浓度，存在一个与场强相关的r1弛豫率最佳值，可产生最大的maxCE。如果钆浓度上限为2 mM，则对于r₂ / r₁比值为1.2且r₁弛豫率在3 T时为20.5 mM⁻¹ s⁻¹、7 T时为18 mM⁻¹ s⁻¹、9.4 T时为16.5 mM⁻¹ s⁻¹的化合物，可发现最大的maxCE。对于r1弛豫率或r₂ / r₁比值不变的化合物，由于高场时背景抑制改善，所需剂量可降低10 - 15%。
