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要旨:
Most contrast agents for magnetic respnance 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.
