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A Novel method to generate on‐board 4D MRI using prior 4D MRI and on‐board kV projections from a conventional LINAC for target localization in liver SBRT
Author(s) -
Harris Wendy,
Wang Chunhao,
Yin FangFang,
Cai Jing,
Ren Lei
Publication year - 2018
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.12998
Subject(s) - magnetic resonance imaging , medicine , radiology
Purpose On‐board MRI can provide superb soft tissue contrast for improving liver SBRT localization. However, the availability of on‐board MRI in clinics is extremely limited. On the contrary, on‐board kV imaging systems are widely available on radiotherapy machines, but its capability to localize tumors in soft tissue is limited due to its poor soft tissue contrast. This study aims to explore the feasibility of using an on‐board kV imaging system and patient prior knowledge to generate on‐board four‐dimensional (4D)‐ MRI for target localization in liver SBRT. Methods Prior 4D MRI volumes were separated into end of expiration ( EOE ) phase ( MRI prior ) and all other phases. MRI prior was used to generate a synthetic CT at EOE phase ( sCT prior ). On‐board 4D MRI at each respiratory phase was considered a deformation of MRI prior . The deformation field map ( DFM ) was estimated by matching DRR s of the deformed sCT prior to on‐board kV projections using a motion modeling and free‐form deformation optimization algorithm. The on‐board 4D MRI method was evaluated using both XCAT simulation and real patient data. The accuracy of the estimated on‐board 4D MRI was quantitatively evaluated using Volume Percent Difference ( VPD ), Volume Dice Coefficient ( VDC ), and Center of Mass Shift ( COMS ). Effects of scan angle and number of projections were also evaluated. Results In the XCAT study, VPD / VDC / COMS among all XCAT scenarios were 10.16 ± 1.31%/0.95 ± 0.01/0.88 ± 0.15 mm using orthogonal‐view 30° scan angles with 102 projections. The on‐board 4D MRI method was robust against the various scan angles and projection numbers evaluated. In the patient study, estimated on‐board 4D MRI was generated successfully when compared to the “reference on‐board 4D MRI ” for the liver patient case. Conclusions A method was developed to generate on‐board 4D MRI using prior 4D MRI and on‐board limited kV projections. Preliminary results demonstrated the potential for MRI ‐based image guidance for liver SBRT using only a kV imaging system on a conventional LINAC.