Principles of CT Operation: What determines the Voxel Resolution?
In this article:
- Voxel Resolution Defines CT Image Detail: Voxel resolution, or the size of each 3D pixel in a CT scan, directly determines the spatial resolution and clarity of internal features in industrial computed tomography
- Voxel Size Depends on Detector Pixel Pitch and Magnification: The voxel size (V) is calculated as the detector pixel pitch (P) divided by the geometric magnification (M), where M = FDD/FOD (focus-detector distance / focus-object distance)
- Object Size and Detector Field of View Limit Magnification: To keep the entire object within the CT field of view during rotation, the maximum magnification is constrained by the ratio of detector size (D) to object diameter (d), limiting the achievable resolution
- Smaller Objects Allow Higher Resolution Scans: For small or partially scanned objects, higher magnification is possible, enabling finer voxel resolution—often only limited by the X-ray source’s focal spot size
- Optimizing Voxel Resolution Enhances Inspection Accuracy: Understanding and controlling voxel resolution is critical for high-precision applications in aerospace, electronics, and additive manufacturing where internal defect detection is essential
The spatial resolution in a CT image is limited by the voxel (= volume pixel) size V of the reconstructed volume data set given by the detector pixel pitch P divided by the geometric magnification M, i.e. V=P/M. M is equal to the ratio of focus-detector distance and focus-object distance: M=FDD/FOD.
If the entire object is to be contained in the CT volume, M is limited by the widest diameter d of the object and the detector size D because the full object must remain in the field of view whilst the object is rotated during data acquisition: Mmax = D/d. So the highest achievable resolution or minimum voxel size is V=P * d/D. For small objects, or partial scans very high magnifications can be achieved and the resolution is only limited by the focal spot size of the X-ray source.
CT Operation: How Does a Digital Detector Array Work?