Early Assessment of MRgFUS Thalamotomy Using a Diffusion Weighted Steady State MRI Sequence in an In-vivo Porcine Model
Juan Camilo Plata1, Sam Fielden2, Bragi Sveinsson3, Brian Hargreaves4, and Craig Meyer2

1Bioengineering, Stanford University, Las Vegas, NV, United States, 2Biomedical Engineering, University of Virginia, Charlottesville, VA, United States, 3Electrical Engineering, Stanford University, Palo Alto, CA, United States, 4Stanford University, Palo Alto, CA, United States

Synopsis

Early detection of thermal lesions generated using MR-guided focused ultrasound systems is critical for treatment feedback. Irreversible changes in the apparent diffusion coefficient (ADC) have been previously shown to be an early indicator for loss of viability in the prostate. Due to poor image quality using standard diffusion weighted imaging strategies inside the focused ultrasound system, radiologists rely on T2-weighted fast spin echoes (FSE) for lesion detection. T2-weighted changes due to lesion formation develop more slowly than ADC changes. We propose using a diffusion-weighted steady state sequence for early detection of thermal lesions inside the focused ultrasound system.

Objective:

Diffusion-weighted imaging has been used to evaluate tissues ablated tissues using MR-guided focused ultrasound (MRgFUS), including uterine fibroids, prostate, and brain tissue (1-4). Quantitative studies in canine prostate found a 36% reduction in the apparent diffusion coefficient (ADC) after either high intensity ultrasound ablation or cryoablation of the prostate, despite differences in histology (4). More recently we studied the evolution of the ADC decrease and found that the time-course for the onset of ADC decrease after ablation of the canine prostate was inversely correlated to the thermal dose achieved (5). As a result, areas that saw high levels of thermal dose saw a more rapid irreversible decrease in ADC, making ADC an early marker for loss of tissue viability in the prostate. Diffusion-weighted EPI inside of the InSightec ExAblate 4000 Neuro System following thalamotomy demonstrates poor image quality. As a result, T2-weighted imaging is the method of choice for lesion detection inside the transducer although it may not be the earliest marker for ablation. We have demonstrated the feasibility of using a diffusion-weighted steady-state sequence for monitoring thermal lesion formation and demonstrated its use in an egg-white phantom model (7). The purpose of this work was to investigate the time course of lesion contrast in a pig model of thalamotomy on a diffusion-weighted steady state sequence in comparison to T2-weighted FSE. In addition, we probe the thermal dose dependence of the contrast by evaluating thermal lesions of two different peak temperatures.

Methods:

MRgFUS thalamotomy was performed in a porcine model (n=2) under MR thermometry guidance. In one lesion in one animal, image collection began approximately 40 minutes after a low peak temperature sonication Tpeak = 52°C in the thalamus. In a second animal, two high peak temperature lesions Tpeak = 60°C were created in the thalamus, and image collection began immediately. In all cases, double-echo in steady-state (DESS) and fast spin echo (FSE) T2-weighted imaging acquisitions were interleaved. The parameters for both sequences are summarized in Table 1. Contrast to surrounding tissue was computed for all time points using regions of interest determined after lesion detection.

Results and Discussion:

Example images demonstrating the lesion on DESS and on FSE after the lower peak temperature sonication are shown in Figure 1. The lesion demonstrates higher conspicuity in DESS than FSE. In the quantitative analysis, in all three lesions, DESS provided superior contrast to T2-weighted FSE images at the early time points (Figure 2), which equilibrated at the later time points. This is presumably due to the mixed diffusion and T2 contrast for the steady state sequence. As edema increases, the steady state sequence loses its advantage over T2-weighted FSE.

Conclusions and Future Work:

DESS provides a higher contrast between the lesion and the surrounding healthy tissue early after treatment is completed. This will allow for an earlier treatment evaluation while the patient is still in the brain transducer. Future work will include an in-depth simulation analysis on how both the diffusion weighting and T2-weighting contribute to the lesion detection time-course in FSE and DESS.

Acknowledgements

PO1 CA159992, RO1 CA111981, FUS Foundation, UVA-Coulter Translational Research Partnership

References

(1) Jacobs MA, et al. JMRI 2009; 29:649-656. (2) MacDannold N, et al. Radiology 2006;240:263–272. (3) Wintermark M, et al. AJNR 2014;35:891–896. (4) Chen J, et al. MRM 2008;59:1365–72. (5) Plata J, et al. 2015. Med Phys 2015:09:5130 (6) Chen L, et al. JMRI 1999;10:146153 (7) Plata J, et al. 2015. ISMRM:1652.

Figures

Figure 1. Thalamotomy thermal lesion detection using fast spin echo (FSE) and double-echo in steady-state (DESS) images. Lesion is not visible with FSE until 60 min while the lesion is detected using DESS at 46min.

Figure 2. Contrast between lesion and surrounding healthy tissue as a function of time for fast spin echo (FSE) and Double Echo Steady State (DESS) images. Contrast for DESS is initially higher for both treatments indicating DESS can serve as an early indicator of lesion formation in brain treatments.

Fast Spin Echo (FSE), Double-Echo in Steady-State (DESS), Echo Time (TE), Repetition Time (TR), Echo Train Length (ETL)



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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