Purpose: We investigate the feasibility of slot-scan dual-energy x-ray absorptiometry (DXA) on robotic x-ray platforms capable of synchronized source and detector translation. This novel approach will enhance the capabilities of such platforms to include quantitative assessment of bone quality using areal bone mineral density (aBMD), normally obtained only with a dedicated DXA scanner. Methods: We performed simulation studies of a robotized x-ray platform that enables fast linear translation of the x-ray source and flat-panel detector (FPD) to execute slot-scan dual-energy (DE) imaging of the entire spine. Two consecutive translations are performed to acquire the low-energy (LE, 80 kVp) and high-energy (HE, 120 kVp) data in <15 sec total time. The slot views are corrected with convolution-based scatter estimation and backprojected to yield tiled long-length LE and HE radiographs. Projection-based DE decomposition is applied to the tiled radiographs to yield (i) aBMD measurements in bone, and (ii) adipose content measurement in bone-free regions. The feasibility of achieving accurate aBMD estimates was assessed using a high-fidelity simulation framework with a digital body phantom and a realistic bone model covering a clinically relevant range of mineral densities. Experiments examined the effects of slot size (1 – 20 cm), scatter correction, and patient size/adipose content (waist circumference: 77 – 95 cm) on the accuracy and reproducibility of aBMD. Results: The proposed combination of backprojection-based tiling of the slot views and DE decomposition yielded bone density maps of the spine that were free of any apparent distortions. The x-ray scatter increased with slot width, leading to aBMD errors ranging from 0.2 g/cm2 for a 5 cm slot to 0.7 g/cm2 for a 20 cm slot when no scatter correction was applied. The convolution-based correction reduced the aBMD error to within 0.02 g/cm2 for slot widths <10 cm. Reproducible aBMD measurements across a range of body sizes (aBMD variability <0.1 g/cm2) were achieved by applying a calibration based on DE adipose thickness estimates from peripheral body sites. Conclusion: The feasibility of accurate and reproducible aBMD measurements on an FPD-based x-ray platform was demonstrated using DE slot scan trajectories, backprojection-domain decomposition, scatter correction, and adipose precorrection.
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.
Purpose: We optimize scan orbits and acquisition protocols for 3D imaging of the weight-bearing spine on a twin-robotic x-ray system (Multitom Rax). An advanced Cone-Beam CT (CBCT) simulation framework is used for systematic optimization and evaluation of protocols in terms of scatter, noise, imaging dose, and task-based performance in 3D image reconstructions. Methods: The x-ray system uses two robotic arms to move an x-ray source and a 43×43 cm2 flat-panel detector around an upright patient. We investigate two classes of candidate scan orbits, both with the same source-axis distance of 690 mm: circular scans with variable axis-detector distance (ADD, air gap) ranging from 400 to 800 mm, and elliptical scans, where the ADD smoothly changes between the anterior-posterior view (ADDAP) and the lateral view (ADDLAT). The study involved elliptical orbits with fixed ADDAP of 400 mm and variable ADDLAT, ranging 400 to 800 mm. Scans of human lumbar spine were simulated using a framework that included accelerated Monte Carlo scatter estimation and realistic models of the x-ray source and detector. In the current work, x-ray fluence was held constant across all imaging configurations, corresponding to 0.5 mAs/frame. Performance of circular and elliptical orbits was compared in terms of scatter and scatter-to-primary ratio (SPR) in projections, and contrast, noise, contrast-to-noise ratio (CNR), and truncation (field of view, FOV) in 3D image reconstructions. Results: The highest mean SPR was found in lateral views, ranging from ~5 at ADD of 300 mm to ~1.2 at ADD of 800 mm. Elliptical scans enabled image acquisition with reduced lateral SPR and almost constant SPR across projection angles. The improvement in contrast across the investigated range of air gaps (due to reduction in scatter) was ~2.3x for circular orbits and ~1.9x for elliptical orbits. The increase in noise associated with increased ADD was more pronounced for circular scans (~2x) compared to elliptical scans (~1.5x). The circular orbit with the best CNR performance (ADD=600 mm) yielded ~10% better CNR than the best elliptical orbit (ADDLAT=600 mm); however, the elliptical orbit increased FOV by ~16%. Conclusion: The flexible imaging geometry of the robotic x-ray system enables development of highly optimized scan orbits. Imaging of the weight-bearing spine could benefit from elliptical detector trajectories to achieve improved tradeoffs in scatter reduction, noise, and truncation.