A dedicated cone-beam CT system for musculoskeletal extremities imaging: design, optimization, and initial performance characterization

Abstract

Purpose: This paper reports on the design and initial imaging performance of a dedicated cone-beam CT (CBCT) system for musculoskeletal (MSK) extremities. The system complements conventional CT and MR and offers a variety of potential clinical and logistical advantages that are likely to be of benefit to diagnosis, treatment planning, and assessment of therapy response in MSK radiology, orthopaedic surgery, and rheumatology.

Methods: The scanner design incorporated a host of clinical requirements (e.g., ability to scan the weight-bearing knee in a natural stance) and was guided by theoretical and experimental analysis of image quality and dose. Such criteria identified the following basic scanner components and system configuration: a flat-panel detector (FPD, Varian 3030+, 0.194 mm pixels); and a low-power, fixed anode x-ray source with 0.5 mm focal spot (SourceRay XRS-125-7K-P, 0.875 kW) mounted on a retractable C-arm allowing for two scanning orientations with the capability for side entry, viz. a standing configuration for imaging of weight-bearing lower extremities and a sitting configuration for imaging of tensioned upper extremity and unloaded lower extremity. Theoretical modeling employed cascaded systems analysis of modulation transfer function (MTF) and detective quantum efficiency (DQE) computed as a function of system geometry, kVp and filtration, dose, source power, etc. Physical experimentation utilized an imaging bench simulating the scanner geometry for verification of theoretical results and investigation of other factors, such as antiscatter grid selection and 3D image quality in phantom and cadaver, including qualitative comparison to conventional CT.

Results: Theoretical modeling and benchtop experimentation confirmed the basic suitability of the FPD and x-ray source mentioned above. Clinical requirements combined with analysis of MTF and DQE yielded the following system geometry: a -55 cm source-to-detector distance; 1.3 magnification; a 20 cm diameter bore (20 x 20 x 20 cm3 field of view); total acquisition arc of -240 degrees. The system MTF declines to 50% at -1.3 mm(-1) and to 10% at -2.7 mm(-1), consistent with sub-millimeter spatial resolution. Analysis of DQE suggested a nominal technique of 90 kVp (+0.3 mm Cu added filtration) to provide high imaging performance from -500 projections at less than -0.5 kW power, implying -6.4 mGy (0.064 mSv) for low-dose protocols and -15 mGy (0.15 mSv) for high-quality protocols. The experimental studies show improved image uniformity and contrast-to-noise ratio (without increase in dose) through incorporation of a custom 10:1 GR antiscatter grid. Cadaver images demonstrate exquisite bone detail, visualization of articular morphology, and soft-tissue visibility comparable to diagnostic CT (10-20 HU contrast resolution).

Conclusions: The results indicate that the proposed system will deliver volumetric images of the extremities with soft-tissue contrast resolution comparable to diagnostic CT and improved spatial resolution at potentially reduced dose. Cascaded systems analysis provided a useful basis for system design and optimization without costly repeated experimentation. A combined process of design specification, image quality analysis, clinical feedback, and revision yielded a prototype that is now awaiting clinical pilot studies. Potential advantages of the proposed system include reduced space and cost, imaging of load-bearing extremities, and combined volumetric imaging with real-time fluoroscopy and digital radiography.

Figure 1

Illustration of the extremities scanner. (a) Internal components, including the FPD and x-ray source mounted on a motor-driven sickle arm. A magnified view is shown in the inset. The gantry allows two scanning configurations: (b) a standing configuration for imaging of weight-bearing lower extremities and (c-d) a sitting configuration for imaging of (c) nonweight-bearing lower extremities and (d) tensioned or nontensioned upper extremities.

Figure 2

Experimental CBCT imaging bench. The flat-panel detector (FPD) and the x-ray source (X) with a collimator (C) are mounted on three translation stages each (T₁–T₃ and T₄–T₆, respectively), allowing for precise motion along the three major axes of the imaging system. The object (Obj) is placed on a rotation stage (θ), which is mounted on another motorized translational axis (T₇) and can thus be moved laterally. The bench is depicted here in a geometric configuration emulating the prototype extremities scanner. The coordinate systems of the detector (u,v) and reconstructed volume (x,y,z) are also illustrated.

Figure 3

MTF analyzed as a function of system geometry and focal spot size. (a) Frequency at which MTF_system = 0.5 (f₅₀) is shown as a function of focal spot size and system magnification. The dashed black line indicates the boundary between the region dominated by detector MTF and the region dominated by focal spot MTF. (b) System MTF plotted for three values of focal spot size at a fixed magnification of 1.3. Reducing the focal spot size below 0.5 mm yields only marginal improvements in MTF_system, where this combination of magnification factor and focal spot size (marked with white dashed lines in (a)) corresponds to the optimum between regions dominated by MTF_FPD and MTF_spot.

Figure 4

DQE analyzed as a function of kVp, filtration, and dose. (a) Zero-frequency DQE as a function of x-ray spectrum (kVp) and added Cu filtration for a fixed detector signal level of 100 × electronic noise floor. The dose and source power required to achieve detector signal of 100 × electronic noise are indicated by the pink isodose contours and black iso-power contours. Operating at 90 kVp and 0.2–0.4 mm Cu gives relatively high DQE (∼0.7) while maintaining the dose below 5 mGy and the source power below 0.5 kW. (b) Zero-frequency DQE as a function of kVp and the detector signal level for a fixed filtration of 0.3 mm Cu. The system appears to be quantum limited for doses above ∼2 mGy. This is further corroborated in (c) where DQE(f) is plotted for various dose levels and a fixed beam of 90 kVp + 0.3 mm Cu. The frequency axis covers the range up to the Nyquist frequency of the detector. There is little improvement in DQE above ∼1.5 mGy, indicating quantum-limited operation of the system.

Figure 5

Dose to the center of a 16 cm CTDI phantom placed at the iso-center of the proposed extremities CBCT system as a function of tube output (mAs) for a scan of 480 projections. Solid line: 80 kVp beam, dash-dotted line: 90 kVp beam, dashed line: 110 kVp beam. The two imaging protocols used throughout this study are marked on the graph: the bone protocol at 0.1 mAs/projection and the soft-tissue protocol at 0.25 mAs.

Figure 6

Comparison of reconstructions of a cylindrical phantom acquired under the following conditions: (a) collimated beam and an antiscatter grid (low scatter conditions), (b) grid with full FOV collimation, (c) no grid and full FOV collimation (high scatter conditions). (d) Image profiles along central the horizontal row of the reconstructions shown in (a), (b), and (c). (e) Signal-difference-to-noise ratio as a function of dose for imaging with and without the grid. The SDNR is measured using the two ROIs marked with black squares in (a). The use of the grid improves SDNR without increase in dose.

Figure 7

A cadaveric knee imaged (a) without and (b) with an antiscatter grid. Comparison of central vertical image profiles through (a) and (b). The grid improves image uniformity and soft-tissue contrast resolution.

Figure 8

Reconstructions of a cadaveric knee obtained on the CBCT test-bench emulating the proposed extremities scanner and on a conventional CT system. CBCT images are shown for protocols corresponding to high-contrast bone visualization (0.1 mAs per projection and 2 × 2 detector binning) and soft-tissue visualization (0.25 mAs per projection and 2 × 2 detector binning). Axial, coronal, and sagittal slices through the reconstructions are presented (top to bottom row); in each case several slices perpendicular to the display direction were averaged to reduce noise. Voxel dimensions are listed beneath each image, with x, y, and z directions indicated by the axes at left.

Figure 9

Comparison of reconstructions of a cadaveric hand obtained on the CBCT test-bench emulating the proposed extremities scanner and conventional CT system. Similar to Fig. 8 CBCT images obtained with the “Standard” and “HighQ” protocols are shown. Voxel sizes are roughly matched between the CBCT and conventional CT for sake of comparison. The CBCT images exhibit similar contrast resolution and slightly improved spatial resolution in comparison to conventional CT.

Figure 10

Coronal and axial slice through the reconstruction of a cadaveric hand obtained from a test-bench scan using the ”Sharp” protocol, as defined in the paper (1 × 1 detector binning, voxel size of 0.15 mm and 0.25 mAs per projection). Volume rendering of this data (performed along a curved surface) is shown in the rightmost column, demonstrating the fine level of structural detail provided by the CBCT system.