Eugene Ozhinsky1, Matthew D. Bucknor1, and Viola Rieke1
1Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, United States
Synopsis
Desmoid tumors are benign but locally aggressive
soft tissue tumors that arise from fibroblast cells. Focused ultrasound has
shown promising results in reduction of tumor volume without significant side
effects. Post-treatment contrast enhanced MR imaging allows assessment of the
non-perfused volume (NPV), the gold standard assessment of the quantity of
tumor ablation. However, safety concerns regarding heating of tissue after
gadolinium injection prevent further treatment following the NPV assessment. We
have shown that T2 mapping can be used to visualize the extent of ablation with
focused ultrasound and be used as a predictor of NPV without the need for
contrast injections.Introduction
MR-guided focused ultrasound (MRgFUS) is a noninvasive ablation
technique that has been successfully used for the treatment of uterine fibroids
and bone metastases. More recently, focused ultrasound has been investigated for
novel indications such as treatment of essential tremor, focal breast lesions,
osteoid osteomas, and desmoid tumors.
Desmoid tumors are benign but locally aggressive soft tissue
tumors that arise from fibroblast cells. Conventional therapies include
surgical resection, radiation and chemotherapy. Despite these treatments, the
tumors have a high recurrence rate of up to 50% in 5 years (1). As an
alternative treatment, focused ultrasound has shown promising results in
reduction of tumor volume without significant side effects (2).
Visualization of the ablated volume remains a large problem
in focused ultrasound therapy. Thermal dose maps are used during the treatment
to visualize the treated volume, but fail to reliably predict the extent of
ablation. Post-treatment contrast enhanced MR imaging allows assessment of the
non-perfused volume (NPV), the gold standard assessment of the quantity of
tumor ablation. However, safety concerns regarding heating of tissue after
gadolinium injection prevent further treatment following the NPV assessment.
In this study we investigated rapid T2 mapping as a way to
visualize tissue changes during MRgFUS treatment of patients with desmoid
tumors.
Methods
MR-guided focused ultrasound ablation was performed in two
patients with pediatric desmoid tumors in lower extremities using an ExAblate 2100
system (InSightec, Haifa, Israel) integrated with a 3.0 Tesla MR scanner (GE
Healthcare, Waukesha, WI, USA). Sonications were performed with duration of 20-30
sec and acoustic power between 49 and 107 W. The patients received 68 (patient
1) and 65 (patient 2) treatment sonications with 2-4 verifications sonications over
the course of about 3 hours.
Over the course of the treatment 4 (pt. 1) and 6 (pt. 2)
double echo Fast Spin Echo images were acquired (TE = 35/186 ms, TR = 1500 ms,
echo train length = 40, FOV = 24-28 cm, 128 x 128 matrix size, BW = 15.6 kHz, 10mm slice
thickness, 1-2 slices, 15 sec acquisition time) before, during and after the
treatment. T2 maps were generated with an exponential fit for two data points.
At the end of the treatment pre- and post-contrast 3D FSPGR images were
acquired. T2-maps were compared to post-contrast images.
Results and Discussion
Our results in Figs. 1-3 show that T2 mapping could be used
to visualize the changes in tissue during focused ultrasound treatments of
desmoid tumors. The areas of T2 elevation showed an excellent agreement with
the non-perfused volumes in the post-contrast images.
T2 values of tissues such as muscle and fat have been shown
to increase with temperature (3-6). Although some of the observed T2 elevation
could be caused by the heat from the previous sonications, the elevated T2 values were observed
throughout the interior of the tumor. Future work will study the T2 values in
the tumor after it returned to the baseline temperature.
In
conclusion, we have shown that T2 mapping can be used to visualize the extent of
ablation with focused ultrasound and be used as a predictor of NPV without the
need for contrast injections. This could be used by physicians to ensure
complete ablation of the tissue within the region of treatment.
Acknowledgements
We would like to thank Misung Han for the help
with protocol optimization. This study has been funded by NIH R00 HL097030 and
GE Healthcare.References
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