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A Potential Approach of Real-Time Motion Tracking of MR-Guided-Radiotherapy Using Ultra-Fast Volumetric 4D-MRI Generated VOI Positional Probability Distribution

O Wong*, J Yuan , Y Zhou , S Yu , K Cheung , Hong Kong Sanatorium & HospitalHong Kong


(Thursday, 7/18/2019) 10:00 AM - 12:00 PM

Room: 303

Purpose: The current MR-guided-radiotherapy(MRgRT) motion tracking was mostly based on the limited number of representative 2D slices while lacking volumetric information due to the insufficient MRI acquisition speed. We propose a real-time motion tracking solution for MRgRT using an ultra-fast volumetric 4D-MRI generated VOI positional probability distribution(VOI-PPD).

Methods: A free-breathing volumetric 4D-MRI (CAIPIRINHA-accelerated 3D gradient-echo-sequence with1.63 volume/sec, 56 axial-slice/volume, voxel size=2.7x2.7x4mm³, 144 dynamics) was developed and applied on 9 healthy subjects (34.33±5.77 years) using a 1.5T MR-simulator. Left kidney(LK) and right kidney(RK) motion tracking were retrospectively conducted. Time-resolved motion was expressed as a transformation matrix (Tvoi) by linearly registering the dynamic target VOI to a reference volume such as planning GTV. A binary VOI mask was created on the reference volume and transformed by Tvoi for the generation of dynamic mask (M(j), j: time-frame index). The dynamic VOI-PPD at overlapping probability ≥ i%(Vᵢ) was estimated and updated at each time-frame by calculating the overlapping volume, (M(1)+...+M(j))/j. A baseline VOI-PPD was calculated using the first 10 time-frames. The dynamic VOI-PPD with positional probability≧i%(Vi%) was then updated at every time frame. To evaluate the accuracy of the VOI-PDD approach, the V10%at the last time-frame 144 was also compared to the MIP volume at 40-60% respiratory phase(Vʳᵉˢᵖ₄₀₋₆₀) using signed-rank test.

Results: The dynamic V10% stabilized after the first 10 time-frame (i.e.~10s). For both kidneys, similar V�₀ were observed at time-frame of 44 (LK=190±36mm³, RK=181±33mm³), 64 (LK=192±35mm³, RK=181±33mm³),84 (LK=191±35mm³, RK=181±33mm³), 104 (LK=193±35mm³, RK=180±32mm³) and 144 (LK=192±35mm³, RK=180±33mm³). Significantly smaller V�₀ than Vʳᵉˢᵖ₄₀₋₆₀ (V�₀ vs Vʳᵉˢᵖ₄₀₋₆₀: LK=192±35 vs 207±35cm³, p<0.05; RK=180±33 vs 190±29cm³, p<0.05) was observed in both kidneys, indicating a more accurate motion tracking using the proposed VOI-PPD approach.

Conclusion: A motion tracking approach using an ultra-fast volumetric 4D-MRI generated VOI-PPD was proposed and might benefit motion tracking accuracy for real-time MRgRT.


MRI, Organ Motion


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined- IGRT and tracking

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