Quantifying perfusion-related energy losses during magnetic resonance-guided focused ultrasound
Journal of Therapeutic Ultrasound volume 3, Article number: O103 (2015)
The focused ultrasound power required for successful ablation of uterine fibroid tissue varies substantially between patients and within single treatments.[1, 2] Fibroids with high signal intensity in pretreatment T2-weighted MR images have been shown to require increased power to achieve adequate temperature elevation for ablation;[2, 4] thus, T2-weighted signal intensity has been suggested as a predictor of MRgFUS treatment response.[2, 3] Physiologically, the high intensity of T2-weighted MR images of uterine fibroids may represent vascularization, fluid-rich tissues, or degeneration.[4, 6] By quantifying perfusion-related energy losses (Qb) during MRgFUS treatments, this study is the first step in linking perfusion-related energy losses with MR perfusion imaging. This knowledge could be used to improve biothermal modeling of MRgFUS fibroid treatments and as a potential independent predictor of treatment response and outcome.
Experiments were performed in ex vivo porcine kidneys perfused with a heparin- H2O solution in variable flow (0, 20, 40 mL/min) situations and embedded in a gelatin phantom (Figure 1). Heating was achieved by electronically steering a phased-array ultrasound transducer (256 elements, f=1 MHz) in an 8 mm-radius circle for 120 s (Figure 2). MR temperature data (Figure 3) were acquired with a 3T Siemens Trio MRI (3D segmented-EPI, TR/TE=30/11 ms, FA=15°, EPI factor=9, 2x2x3 mm3, 3.3 s acquisition, ZFI to 0.5-mm isotropic spacing). Based on conservation of energy principles, deviation of a thermal model that excludes perfusion effects from the experimental temperatures was used to quantify Qb. Estimates of Qb were obtained at the time of each MR acquisition during cooling, transformed into perfusion values via the Pennes bioheat transfer equation, and averaged to mitigate the effects of noise.
Results and conclusions
High perfusion values (Figure 4) correspond to regions of increased cooling (Figure 3) and likely indicate locations of discrete vasculature. Constant, uniform perfusion values ranged from -0.7–0.1, 1.6–3.9, and 3.4–4.4 kg/m3/s for 0, 20, and 40 mL/min flow rates, respectively, following anticipated trends with perfusion approximately zero for the no flow case and increasing with flow rate. Future work will relate MR perfusion imaging to Qb, which should eliminate the need for tissue heating for improved biothermal modeling. This study demonstrates that obtaining perfusion estimates from 3D MR temperature data during MRgFUS is feasible and has the potential to improve biothermal models of MRgFUS fibroid treatments.
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This work was funded by Siemens Healthcare AG, the FUS Foundation, the Ben and Iris Margolis Foundation, and by NIH grants R01 CA87785 and R01 EB013433.
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Dillon, C., Roemer, R. & Payne, A. Quantifying perfusion-related energy losses during magnetic resonance-guided focused ultrasound. J Ther Ultrasound 3 (Suppl 1), O103 (2015). https://doi.org/10.1186/2050-5736-3-S1-O103