Topics covered in this article include: 

• Introduction

• Measurement Summary

• Analysis Method

• Available Tests

• Image Acquistion Suggestions

• Detailed Discussion of Measurements

Introduction

4D CT is crucial for radiotherapy planning, as it captures the impact of respiratory motion on tumor and normal tissue positions. Unlike static 3D CT, 4D CT acquires images across breathing phases, visualizing and quantifying tumor and organ movement, especially in the thoracic and upper abdominal regions.

Several publications,, and Canadian national guidelines have emphasized the importance of performing QA on 4D CT systems to ensure image quality and motion accuracy in 4D reconstructions. However, this QA can be time-consuming and involve the purchase of specialized equipment. A recent study (Yang Y, Lee D et al) indicated that analysis time could be shortened fourfold by using automated software that utilizes the ubiquitous Catphan Phantom from The Phantom Laboratory.

Image Owl supported this study by modifying the Catphan analysis within TotalQA and has recently further streamlined the process to automatically perform analysis across multiple acquisitions of the Catphan phantom during the QA procedure.

The study may be downloaded from the JACMP site.

All Catphan models feature a sensitometry module with multiple materials of known characteristics, enabling the analysis of image quality parameters such as HU constancy and linearity. All of the models contain a Teflon TM plug and air voids. These features are cylindrical, with known X, Y, and Z dimensions, and tight manufacturing tolerances. By mounting the Catphan on a motion platform, the reconstructed dimensions of these features can be measured in static image sets and from various phase reconstructions during the 4D CT process to quantify the degree of distortion and motion accuracy.

Measurement Summary

The analysis displays the following test results:

• Teflon Plug Dimensions
• Teflon Plug Profiles Plot
• Air Void Dimensions
• Air Void Profiles Plot

Available Tests

There are two options for selecting tests to run on 4D image acquisitions.  In the manual setup, specific tests can be added to a Catphan template.  Alternatively, the automated setup uses a preset 4D CT Analysis template that is now a standard option in the software.  The table below details the available tests in the manual setup.

The manual analysis reflects the original analysis done by Yang, Lee, et al. It remains useful for researchers or clinical medical physicists who wish to include measurements of the plugs at rest and during various phases or amplitude bins during motion in a custom test template, along with other built-in or custom tests.

Image Acquistion Suggestions

MIP and MinIP reconstructions are also recommended during the testing process.

Detailed Discussion of Measurements

Teflon Plug Dimensions

The dimensions of the Catphan’s Teflon sensitometry plug. This is used to determine if the reconstructed image is distorting the shape of the plug.

• Phase: If phase information is included in the DICOM tags, it will be displayed.
• Teflon Value: The median HU value of a circular ROI of 10mm diameter centered on the Teflon plug in the axial direction at the center of the module.
• Threshold: The mean HU of the sensitometry measurements of the Acrylic and Teflon plugs. This threshold value is where the widths of the x, y, and z profiles of the Teflon plug are determined.
• X Width: Width in mm of a horizontal profile through the Teflon plug axial center at the z center of the phantom’s sensitometry module. The width is measured at the threshold level above.
• Y Width: Width in mm of a vertical profile through the Teflon plug axial center at the z center of the phantom’s sensitometry module. The width is measured at the threshold level above.
• Z Width: Width in mm of a profile through the z slices containing the Teflon plug in the phantom’s sensitometry module. The profile is calculated as the mean of all voxels within 5mm of the plug’s xy center on each slice. The width is measured at the threshold level above.
• Z Center  (IPP): The interpolated center in mm of the Teflon plug’s z profile using the Image Position Patient DICOM tags of the slices.

Teflon Plug Profiles Plot

The Teflon Plug Profiles plot provides a visualization of the Teflon Plug Dimensions statistics. Note that the phase notation will only appear if the phase information is contained in the DICOM tags.

Air Void Dimensions

This test displays dimensions of the Catphan’s sensitometry air void. This is used to determine if the reconstructed image is distorting the shape of the void.

• Phase: If phase information is included in the DICOM tags, it will be displayed.
• Air Value: The median HU value of a circular ROI of 10mm diameter centered on the air void in the axial direction at the center of the module.
• Threshold: The mean HU of the sensitometry measurements of the air void and LDPE plug. This threshold value is where the widths of the x, y, and z profiles of the air void are determined.
• X Width: Width in mm of a horizontal profile through the air void axial center at the z center of the phantom’s sensitometry module. The width is measured at the threshold level above.
• Y Width: Width in mm of a vertical profile through the air void axial center at the z center of the phantom’s sensitometry module. The width is measured at the threshold level above.
• Z Width: Width in mm of a profile through the z slices containing the air void in the phantom’s sensitometry module. The profile is calculated as the mean of all voxels within 5mm of the void’s xy center on each slice. The width is measured at the threshold level above.
• Z Center  (IPP): The interpolated center in mm of their void’s z profile using the Image Position Patient DICOM tags of the slices.

Air Void Profiles Plot

The Air Void Profiles Plot provides a visualization of the Air Void Dimensions statistics. Note that the phase notation will only appear if the phase information is contained in the DICOM tags.