The following are some typical reasons for errors. A list of recommended protocols is found in our help pages. Please contact us directly, firstname.lastname@example.org, if you find you need further assistance.
Please consult the Catphan® Manual for information on positioning here for issues with:
- Slice acquired between modules.
- Slice is badly centered.
- Slice is badly oriented (too high tilt/yaw).
- HU shift in values from normal expected values (eg, Cone Beam or scatter from lack of centering).
- Targets for the given test are not included in the slice image (eg, FOV too small, or significant lack of centering). For reference, here are the module locations for each model:
If targets non-detectable:
- Dose may be too low for very low contrast targets.
- Artifacts in scan obscure targets.
- Results are model dependent (eg, liberal vs. conservative interpretation of detected target).
Slice thickness - beads and wires
If fewer ramps are detected than expected and/or they are incorrect values or there is a large variation in results, here are some possible issues:
- Slice is not centered within the module - particularily for the z-axis where ramps may be missed or truncated.
- Pixel is too large - for best results use a smaller pixel from a smaller FOV (eg, <140mm for slices ≤2mm).
- Large focal spot is used (some scanners switch to large focal spot when mA is increased above 200).
- Low-res kernel is used, this is similar effect as too large a pixel.
If the HU value of water is not what is expected look for these issues:
- Half-filled water plug.
- Poor calibration of scanner or need of updated air calibration.
If the resolution is less than expected check for these issues:
- Pixel is too large from too large FOV- for best results for very high resolution (10LP/cm or higher) use a FOV <140mm and choose a high resolution kernel.
- The focal spot is too large, make sure to use small focal spot which may require mA <200.
- Bead is not included in the input slice, or only partially scanned so that HU peak is too low.
- The image is from slices with visible line pairs (such as the CTP528 or the CTP714). Visible line pairs are not used to calculate MTF. The MTF is calculated using the module that contains the MTF beads and wires. Ensure that either that the entire phantom is analyzed or that a single slice from the correct module is analyzed and that the correct module code tag is in place to identify it. For more information on the module code tag see Manual Identification of RT Planar Images and Individual Catphan Slices through DICOM tags.
If HU values are higher or lower in the center than expected:
- Check calibration and if necessary perform air calibration. If still a problem, contact vendor.