Purpose
The merit of magnetic resonance imaging (MRI) for guiding the thermal ablation is its ability to restrict the energy deposition to the target tissue by demonstrating the transient temperature elevation in the surrounding normal tissue and to signal the irreversible phase transition within the target. Our challenge was to propose a dedicated temperature imaging feedback control system for guiding and assisting the thermal liver ablation in a double-donut 0.5T open MR scanner. This system has near-real time feedback capability based on a newly developed “self-referenced” temperature imaging method using “moving-slab” and complex-field-fitting techniques.
Material and methods
To evaluate the ability of the feedback control system as well as comparison of the newly developed self-referenced method with the conventional subtraction method, two phantom validation studies and one ex-vivo experiment were performed in a 0.5T open GE MR scanner. In the first part of the validation studies, microwave heating was applied in an agar phantom using fast spoiled gradient recalled echo in steady state (FSPGR) with TR/TE, 32/12ms; flip angle, 30o; bandwidth, 15.6 kHz; FOV, 200´200 mm2; slice thickness, 5 mm; and spatial matrix, 256 ´ 128. In the second part of the studies, to evaluate the feasibility and utility of the feedback control system, we performed another phantom experiment using the near-real time sequential acquisition of three orthogonal planes by fast spoiled gradient-echo sequence (3pFSPGR) with the same imaging conditions as the first validation study. Preliminary ex-vivo experiment using porcine liver sample placed on a wooden base for easy changing of both location and angle of the sample was performed in the same MRI scanner. Microwave-induced thermal ablative lesions were created in ex-vivo liver sample using an MR compatible 18G Teflon-coated needle type electrode placed into the target volume. After setting the feedback control parameters using the temperature imaging software, the ex-vivo liver was then heated with the microwave power of 10 to 60W. The needle tip tracking calculations were based on the acquired image planes along the needle. The FSPGR sequence parameters for needle tracking were TR/TE, 34/12ms; flip angle, 30°; bandwidth, 15.6 kHz; slice thickness, 20 mm; FOV, 200 ´ 200 mm2; and spatial matrix, 256 ´ 128. The set of complex MR images was acquired at the position of needle tip using near-real time FSPGR for monitoring the treatment with the imaging parameters stated above except for 5 mm slice thickness.
Results
Temperature maps during the heating of the phantom and ex-vivo with both self-referenced and conventional external-referenced methods were demonstrated. The self-referenced method does not require the separate phase image taken before the treatment and the subtraction of the “external” reference image from the current image. The main obstacle of MRI-guided thermal ablation was the quantitative temperature feedback due to organ motion through the treated volume. This study showed that the motion artifacts during imaging of the thermal heating in liver were minimized with the temperature imaging method described in this study.
Conclusion
The temperature imaging of moving organ based on self-referenced method with moving slab traces the markers on the heating needle moving with liver and allows the capability of monitoring online whether the target tissue is receiving a sufficient ablative thermal dose or the surrounding tissue is receiving a safe dose. The proposed system through the distribution of both temperature and thermal dose may offer clinicians precise control of MR-guided thermal liver ablation procedures. The applicability to clinical procedures has to be examined further by experiments on animals and human subjects. The evolution of the proposed system to be in clinical use is greatly expected.