T2-Mapping as a Predictor for Non-Perfused Volume in MRgFUS Treatments of Desmoid Tumors
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

1. Peng PD, Hyder O, Mavros MN, Turley R, Groeschl R, Firoozmand A, Lidsky M, Herman JM, Choti M, Ahuja N, Anders R, Blazer DG, 3rd, Gamblin TC, Pawlik TM. Management and recurrence patterns of desmoids tumors: a multi-institutional analysis of 211 patients. Annals of surgical oncology 2012;19(13):4036-4042.

2. Wang Y, Wang W, Tang J. Ultrasound-guided high intensity focused ultrasound treatment for extra-abdominal desmoid tumours: preliminary results. International journal of hyperthermia : the official journal of European Society for Hyperthermic Oncology, North American Hyperthermia Group 2011;27(7):648-653.

3. Graham SJ, Bronskill MJ, Henkelman RM. Time and temperature dependence of MR parameters during thermal coagulation of ex vivo rabbit muscle. Magn Reson Med 1998;39(2):198-203.

4. Gandhi S, Daniel B, Butts K. Temperature dependence of relaxation times in bovine adipose tissue. Proceedings 6th Scientific Meeting, International Society for Magnetic Resonance in Medicine; 1998; Sydney, Australia. p 701.

5. Baron P, Ries M, Deckers R, de Greef M, Tanttu J, Kohler M, Viergever MA, Moonen CT, Bartels LW. In vivo T2 -based MR thermometry in adipose tissue layers for high-intensity focused ultrasound near-field monitoring. Magn Reson Med 2014;72(4):1057-1064.

6. Ozhinsky E, Kohi MP, Ghanouni P, Rieke V. T2-based temperature monitoring in abdominal fat during MR-guided focused ultrasound treatment of patients with uterine fibroids. Journal of therapeutic ultrasound 2015;3:15.

Figures

Fig. 1 (a) Axial, sagittal and coronal localizer images for patient 2 showing the transducer, gel pad and cooling water bag; (b) coronal T2 maps of two slices acquired during the treatment over time (ROI shown in black on slice 2); (c) plot of T2 values within the ROI over time.

Fig. 2. Comparison between T2 maps (a) and post-contrast images (b) for the same location for patient 2. The non-perfused volume (NPV) appears dark on the post-contrast images. Arrows show gaps in the NPV, which correspond to areas with low T2 in the T2 maps.

Fig. 3. (a) Localizer images of patient 1 in axial and sagittal plane, showing the FUS setup with cooling bag; (b) post-contrast images showing the NPV (patient was repositioned before contrast-enhanced imaging); (c) four T2 maps of the same area, acquired over the course of the treatment.



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