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 PartnershipReferences
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