![]() ![]() Compensation for the phase changes induced by the changing Gd presence is difficult as the magnetic field changes are arising nonlocally in the surroundings of the susceptibility change. Longer waiting time between contrast agent injection and treatment, as well as shortening the ablation duration by increasing the sonication power, will minimize the Gd influence. Depending on the treatment workflow, Gd-induced temperature errors ranging between -4☌ and +3☌ can be expected. The use of a paramagnetic MR contrast agent prior to MR-HIFU treatment may influence the accuracy of the PRFS MR thermometry. In vivo, intravenous Gd-DTPA injection resulted in a perceived temperature change of 2.0☌ ± 0.1☌ at the center of the hind leg muscle. No additional susceptibility effect was measured upon Gd release from paramagnetic liposomes. The temperature dependence of the susceptibility shift showed dΔχ Gd-DTPA/d T = -0.00038 ± 0.00008 ppm/mM/☌. A Gd-DTPA-induced magnetic susceptibility shift of Δχ Gd-DTPA = 0.109 ppm/mM was measured in a cylinder parallel to the main magnetic field at 37☌. The phantom study showed a significant phase shift inside the phantom of 0.6 ± 0.2 radians (mean ± standard deviation) upon Gd-DTPA injection (1.0 mM, clinically relevant amount). In vivo studies were carried out to measure the temperature error induced in a rat hind leg muscle upon intravenous Gd-DTPA injection. By proton nuclear magnetic resonance spectroscopy, the temperature dependence of the Gd-DTPA susceptibility was measured, as well as the effect of liposomal encapsulation and release on the bulk magnetic susceptibility of Gd-DTPA. Phantom measurements were performed to demonstrate the effect of sudden gadopentetate dimeglumine (Gd-DTPA) inflow on the phase shift measured using a PRFS thermometry sequence on a clinical 3 T magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) system. Here, we investigated the susceptibility changes induced by the inflow and presence of a paramagnetic MR contrast agent and their implications on PRFS thermometry. Any other local magnetic field changes will therefore translate into incorrect temperature readings and need to be considered accordingly. Proton resonance frequency shift (PRFS) magnetic resonance (MR) thermometry exploits the local magnetic field changes induced by the temperature dependence of the electron screening constant of water protons. ![]()
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