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. 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.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. 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.