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Cardiac BSSFP Cine MRI for MR Guided Radiation Therapy of Centrally Located Thoracic Tumors

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Y Yang

Y Yang*, M Cao , S Rashid , F Han , Y Gao , J Lamb , P Beron , P Lee , D Low , P Hu , UCLA School of Medicine, Los Angeles, CA


TU-FG-FS2-4 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: Four Seasons 2

Purpose: To achieve optimal treatment results with modern technologies, motion compensation techniques are critical. Much research has been conducted to address respiration-induced tumor motion in radiotherapy (RT) but very few studies looked at the impact of cardiac motion. Intra-fractional soft tissue based motion gating, offered by MRI guided RT, has the potential to greatly reduce treatment uncertainty caused by respiratory motions. However, for some tumors, such as central lung, esophagus or cardiac sarcomas/metastasis, additional efforts may be necessary to evaluate cardiac motion.

Methods: Cardiac CINE images were acquired in 6 healthy volunteers for sequence optimization and 1 lymphoma patient, who has a mediastinal mass abutting heart, at 0.35T using a balanced Steady-state free precession (bSSFP) cine sequence. For the lymphoma patient, cardiac CINE MRI was acquired right after pretreatment 3D bSSFP. Physiological Monitoring Unit (PMU) was used to acquire ECG signal to gate cardiac CINE MRI acquisition. Each scan took 10 seconds for 20 cardiac phases and was acquired within one breath hold.

Results: On 0.35T, CINE images could be acquired for a large flip angle (up to 150ยบ) without encountering any specific absorption rate (SAR) restrictions. The volunteer study demonstrated that large flip angle greatly improves blood-soft tissue contrast which compensates for the reduced SNR caused by low field strength. In the patient study, contours were drawn individually on all 20 cardiac phases to generate the internal target volume (ITV) and to compare with the contour drawn on breath-hold 3D MRI. Substantial difference between these two contours, due to cardiac motion, was observed with a Dice similarity index of 0.82.

Conclusion: We demonstrated that cardiac bSSFP cine MRI is feasible at the low-field strength of 0.35T on MR guided RT system. Taking cardiac motion into consideration may bring value to MR guided RT for centrally located thoracic tumors.

Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge research support from ViewRay, Inc.

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