Methods to Maximize CBCT Image Quality
Adult vs. Child
- Child settings are significantly lower than adults. Children are 2-10 times more sensitive to radiation than adults. With smaller anatomy, less radiation is needed in children to achieve equivalent image quality.
mA (Tube Current)
- Increasing mA will decrease image noise with a proportional increase in dose.
kVp (Tube Voltage)
- Increasing kVp will decrease beam hardening artifact, but with disproportionate increase in dose. Increasing by 10 kV will more than double the dose.
Automatic exposure control (AEC)
- Modulates mA after scout image acquisition, eliminating the need to manually adjust exposure settings based on scout data.
Field of View (FOV) Size
- FOV size limitations via beam collimation improve image quality by reducing scatter radiation, also reducing patient dose.
Number of Basis Images
- Basis image counts are determined by detector frame rates (projections acquired per second), trajectory arcs (180° vs. 360°), and rotation speed of the source and detector.
- Higher frame rates and trajectory arcs and slower scan speeds increase spatial and contrast resolution and signal-to-noise ratio, resulting in decreased noise and metallic artifact but with a proportional increase in patient dose.
- High-resolution modes increase signal-to-noise ratio but also increase dose.
Image Matrix Size
- Matrix size increases (within same-sized FOV) lead to higher spatial resolution & less volume averaging, but with more noise and less contrast-to-noise ratios.
- Minimize unnecessary image retakes by acquiring pre-scan a positioning shot to confirm the area of interest will be included within the FOV.
- Flat Panel Detector (FPD) vs. Image Intensifying Tube (IIT) – FPDs have better image quality with less distortion. IITs generate increased exposure due to the spherical shape & larger volume size.
- Pulsed vs. Continuous – Pulsed beam reduces patient dose without affecting signal-to-noise ratio.
Film scintillator coating
- Cesium iodide (CsI) vs. Gadolinium oxobromide – CsI scintillators convert x-rays to light with a higher image quality and dose efficiency because their columnar structure reduces the light spreading between the scintillators.
- Iterative vs. Filter-back – Iterative generates a higher signal-to-noise ratio with lower dose.
- Optimized filtration decreases dose by removing low-energy x rays from the beam that would otherwise only contribute to image degradation.
- 1 x 1 pixel binning has higher resolution and lower noise, but larger image file size, slower transmission, and increased dose.
Quality Assurance & Machine Calibration
- Quality assurance protocols ensure radiology processes function optimally to maximize diagnostic yield while minimizing patient radiation exposure.
- Process requires routine use of a radiologic phantom where consistent image quality is determined through trend analysis of control charts constructed over time.
- Parameters typically measured: Uniformity (Artifacts), Geometric Precision, Voxel Density Values, Noise, Low Contrast resolution, Contrast-to-Noise Ratios, and Spatial Resolution.
- State Health Departments each have unique Radiation Control Regulations governing the periodicity of testing radiologic equipment.