A Monte Carlo Study to Investigate the Feasibility of an On‐Board SPECT/Spectral‐CT/CBCT Imager for Medical Linear Accelerator

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Medical Physics

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Purpose: The on-board flat-panel cone-beam computed tomography (CBCT) lacks molecular/func-tional information for current online image-guided radiation therapy (IGRT). It might not be adequatefor adaptive radiation therapy (ART), particularly for biologically guided tumor delineation and tar-geting which might be shifted and/or distorted during the course of RT. A linear accelerator (Linac)gantry-mounted on-board imager (OBI) was proposed using a single photon counting detector (PCD)panel to achieve single photon emission computed tomography (SPECT), energy-resolved spectralCT, and conventional CBCT triple on-board imaging, which might facilitate online ART with anaddition of volumetric molecular/functional ima ging information.Methods: The system was designed and evaluated in the GATE Monte Carlo platform. The OBI sys-tem including a kV-beam source and a pixelated cadmium zinc telluride (CZT) detector panelmounted on a medical Linac orthogonally to the MV beam direction was designed to obtain onlineCBCT, spectral CT, and SPECT tri-modal imaging of patients in the treatment room. The spatial res-olutions of the OBI system were determined by imaging simulated phantoms. The CBCT imagingwas evaluated by a simulated contrast phantom. A PMMA phantom containing gadolinium wasimaged to demonstrate quantitative imaging of spectral-CT/CBCT of the system. The capability oftri-modal imaging of the OBI was demonstrated using three different spectral CT imaging methodsto differentiate gadolinium, gold, calcium within simulated PMMA and the SPECT to image radioac-tive99mTc distribution. The dual-isotope SPECT imaging of the system was also evaluated by imag-ing a phantom containing99mTc and123I. The radiotherapy-related parameters of iodine contrastfraction and virtual non-contrast (VNC) tissue electron density in the Kidney1 inserts of a simulatedphantom were decomposed using the Bayesian eigentissue decomposition method for contrast-enhanced CBCT/spectral-CT of the OBI in a single scan.Results: The spatial resolutions of CBCT and SPECT of the OBI were determined to be 15.1 lp/cmat 10% MTF and 4.8–12 mm for radii of rotation of 10–40 cm, respectively. In CBCT image of thecontrast phantom, most of the soft-tissue inserts were visible with sufficient spatial structure details.As compared to the CBCT image of gadolinium, the spectral CT image provided higher image con-trasts. Calcium, gadolinium, and gold were separated well by using the spectral CT material imagingmethods. The reconstructed distribution of99mTc agreed with the spatial position within the phanto m.The two isotopes were separated from each other in dual-isotope SPECT imaging of the OBI. Theiodine fractions and the VNC electron densities were estimated in the iodine-enhanced Kidney1 tis-sue inserts with reasonable RMS errors. The main procedures of the tri-modal imaging guided onlineART workflow were presented with new functional features included.Conclusions: Using a single photon counting CZT detector panel, an on-board SPECT, spectral CT,and CBCT tri-modal imaging could be realized in Linacs. With the added online mol ecular/func-tional imaging obtained from the new OBI for the online ART proposed, the accuracy of radiationtreatment delivery could be further improved.


Cone Beam CT; Molecular Imaging; On-Board Imager; SPECT; Spectral CT; Tr I-Modality


Health and Medical Physics | Medicine and Health Sciences | Public Health



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