Stability and Trapping of Magnetic Resonance Imaging Contrast Agents During High-Intensity Focused Ultrasound Ablation Therapy

Objectives: The purpose of this study was to investigate the use of Gd-DTPA shortly before magnetic resonance guided high-intensity focused ultrasound MR-HIFU thermal ablation therapy with respect to dissociation, trapping, and long-term deposition of gadolinium (Gd) in the body. Materials and Methods: Magnetic resonance-HIFU ablation treatment was conducted in vivo on both rat muscle and subcutaneous tumor (9L glioma) using a clinical 3T MR-HIFU system equipped with a small-animal coil setup. A human equivalent dose of gadopentetate dimeglumine (Gd-DTPA) (0.6 mmol/kg of body weight) was injected via a tail vein catheter just before ablation (<= 5 minutes). Potential trapping of the contrast agent in the ablated area was visualized through the acquisition of R1 maps of the target location before and after therapy. The animals were sacrificed 2 hours or 14 days after the injection (n = 4 per group, a total of 40 animals). Subsequently, the Gd content in the tissue and carcass was determined using inductively coupled plasma techniques to investigate the biodistribution. Results: Temporal trapping of Gd-DTPA in the coagulated tissue was observed on the R1 maps acquired within 2 hours after the ablation, an effect confirmed by the inductively coupled plasma analysis (3 times more Gd3+ was found in the treated muscle volume than in the control muscle tissue). Two weeks after the therapy, the absolute amount of Gd3+ present in the coagulated tissue was low compared with the amount present in the kidneys 14 days after the injection (ablated muscle, 0.009% +/- 0.002% ID/g; kidney, 0.144% +/- 0.165% ID/g). There was no significant increase in Gd content in the principal target organs for translocated Gd3+ ions (liver, spleen, and bone) or in the entire carcasses between the HIFU- and sham-treated animals. Finally, an in vivo relaxivity of 4.6 mmol(-1)s(-1) was found in the HIFU-ablated volume, indicating intact Gd-DTPA. Conclusions: Magnetic resonance-HIFU treatment does not induce the dissociation of Gd-DTPA. In small-tissue volumes, no significant effect on the long-term in vivo Gd retention was found. However, care must be taken with the use of proton resonance frequency shift-based MR thermometry for HIFU guidance in combination with Gd3+ because the susceptibility artifact induced by Gd3+ can severely influence treatment outcome.