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Purpose: We investigate cone-beam CT (CBCT) imaging protocols and scan orbits for 3D cervical spine imaging on a twin-robotic x-ray imaging system (Multitom Rax). Tilted circular scan orbits are studied to assess potential benefits in visualization of lower cervical vertebrae, in particular in low-dose imaging scenarios. Methods: The Multitom Rax system enables flexible scan orbit design by using two robotic arms to independently move the x-ray source and detector. We investigated horizontal and tilted circular scan orbits (up to 45° tilt) for 3D imaging of the cervical spine. The studies were performed using an advanced CBCT simulation framework involving GPU accelerated x-ray scatter estimation and accurate modeling of x-ray source, detector and noise. For each orbit, the x-ray scatter and scatter-to-primary ratio (SPR) were evaluated; cervical spine image quality was characterized by analyzing the contrast-to-noise ratio (CNR) for each vertebrae. Performance evaluation was performed for a range of scan exposures (263 mAs/scan – 2.63 mAs/scan) and standard and dedicated low dose reconstruction protocols. Results: The tilted orbit reduces scatter and increases primary detector signal for lower cervical vertebrae because it avoids ray paths crossing through both shoulders. Orbit tilt angle of 35° was found to achieve a balanced performance in visualization of upper and lower cervical spine. Compared with a flat orbit, using the optimized 35° tilted orbit reduces lateral projection SPR at the C7 vertebra by <40%, and increases CNR by 220% for C6 and 76% for C7. Adequate visualization of the vertebrae with CNR <1 was achieved for scan exposures as low as 13.2 mAs / scan, corresponding to ~3 mGy absorbed spine dose. Conclusion: Optimized tilted scan orbits are advantageous for CBCT imaging of the cervical spine. The simulation studies presented here indicate that CBCT image quality sufficient for evaluation of spine alignment and intervertebral joint spaces might be achievable at spine doses below 5 mGy.
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