Topics covered in this article include:

• Supported Phantoms
• Phantom Rotation
• Scanning for CBCT
• Varian CBCT
• Elekta CBCT
• Recommended Tests
• Detailed Discussion of Measurements
  • Low Contrast
  • Modulation Transfer Function (MTF)
  • Sensitometry
  • Slice Thickness
  • Spatial Linearity
  • Uniformity
  • Wave Insert

Supported Phantoms

The analysis supports phantoms in the Catphan® line in the 500, 600, and 700 series of phantoms supplied for radiation therapy and diagnostic imaging QA. For radiation therapy, in particular, the following phantoms are supported:

• Varian Linacs: Catphan® 504 , Catphan® 604
• Elekta Linacs: Catphan® 503

Phantom Rotation

Figure 1.1: The orientation of the Catphan® x, y, and z-axes. These axes will be referred to in other sections when discussing how measurements are calculated.

• Center of Phantom - The center of the phantom described is the pixel coordinate in the center of the phantom (in pixels).
• Rotation - Degrees of phantom rotation around the z-axis.
• Tilt - Degrees of phantom rotation around the x-axis.
• Yaw - Degrees of phantom rotation around the y-axis.

The manual drawings for each Catphan® reflect the orientation typically seen when imaged. It is important to note the position of the notch, whether it is at the top or bottom of the image, as the module notch orientation can change between Catphan® models.

Scanning for Cone Beam Computed Tomography (CBCT)

The automatic analysis service performs best if the phantoms are scanned according to the standard protocols laid out by the LINAC manufacturers.

Figure 1.2: A diagram depicting the placement of the Catphan® and phantom case on the scanning table, closest to the gantry. The gantry is the structure that houses the X-ray tube and the detectors (not shown in image above).

• Place the phantom case on the gantry end of the table with the box hinges oriented away from the gantry. It is best to place the box directly on the table and not on the table pads. Open the box, rotating the lid back 180°.
• Remove the phantom from the box and hang the Catphan® from the gantry end of the box. Make sure the box is stable with the weight of the phantom and is adequately counterweighted to prevent tipping.
• Use the lasers (or linac crosshairs) to ensure that the Catphan® is centered on the first lateral height dot and center dot proximal to the gantry.
• Use the level and adjusting thumb screws to level the Catphan®. Then re-check the Catphan® centering on the first lateral height dot and center dot.
• Move the Catphan® in towards the gantry so that it is aligned with the 3rd alignment dots from the front of the phantom.
• Perform CBCT scan.

Varian CBCT

The algorithm supports all CBCT tests suggested by Varian provided in their Installation Product Acceptance guidelines, outlined below. Catphan® 504 and Catphan® 604 versions are automatically identified and analyzed.

Scanning Modes

The following scan modes are used to demonstrate essential CBCT operation, functionality, and conformance.

• Standard Dose Head (full fan)
• Pelvis (half fan)

Phantom Setup

Position the phantom on the front edge of the couch top, using the storage box as the holder. See Figure 1.2 above in the section called "Scanning for CBCT" for a visual reference to this setup.

Align the phantom as follows:

• VRT (vertical, y-axis) Axis: Wall lasers intersecting the side reflective dots on phantom
• LAT (lateral, x-axis) Axis: Overhead or Sagittal laser intersecting the top reflective dots on phantom
• LNG (longitudinal, z-axis) Axis: Middle of the Low Contrast module CTP515 aligned to wall vertical laser

Before continuing, rotate the gantry 360º around the Catphan® phantom to make sure there are no collision possibilities.

Scanning Test Method

Table 1.1: Setup parameters for CBCT scans of Head and Pelvis.

1. With the Clinac in CLINICAL mode, launch Treatment on the 4DITC and verify the PRO values (from the LVI) are displayed in the Treatment application.
2. Open the test patient CATPHAN plan from the Treatment Queue, and mode up the CBCT setup field. Use DICOM RT mode and perform a Machine Override if the patient is not imported into the OIS and/or not available in the Treatment Queue.
3. Close the OBI application and launch the CBCT application.
4. Open a New Patient and enter a New Patient ID, using the ID names shown in the setup table for the applicable scan mode.
5. Select Acquire new scan and follow the on-screen instructions to acquire the desired scan mode.

Verification Tests

The full set of DICOM files from each scan mode can now be uploaded to Catphan® QA for analysis.

Elekta CBCT

Catphan® QA includes all Catphan® tests from the ELEKTA CAT guidelines. Automatic module identification is performed when a full phantom upload (or a set including the CTP763 module) is provided.

For partial phantom uploads (which do NOT include the CTP763), DICOM fields must be set as suggested in the ELEKTA CAT guidelines to provide automatic module identification. Elekta CBCT images can be manually  identified by adding the following ID strings to the Patient ID or Study ID DICOM tags of individual slices.  Multiple manual IDs can be set up in a series, however, if any manual IDs are employed, only slices with manual ID strings will be processed. 

• The system has only been tested on data with 120kV, 40mA, 40ms as suggested in the ELEKTA CAT guidelines.
• For robust results, please use a reconstructed FOV that just covers the phantom, 200-300mm

Recommended Tests

TG142 Summary Data

To include TG142 summary data for CBCT in analysis reports select measurements from the Cone Beam CT reports subsection of the Monthly Imaging QA [Tg142 Table VI] section when building your template. 

If you are using the Catphan® 503 phantom supplied with Elekta Linac's choose the TG142 summary data items from the ELEKTA CAT (CBCT and kV planar) reports subsection of Detailed Catphan Analysis.

Recommended Catphan® 504 Tests

The recommended detail CBCT test selection for the Catphan® 504 is shown below:

Figure 1.3: CBCT Preset Test selection for the Catphan® 504.

Recommended Catphan® 604 Tests

The recommended detail CBCT test selection for the Catphan® 604 is shown below:

Figure 1.4: CBCT Preset Test selection for the Catphan® 604.

Recommended Catphan® 503 Tests

The recommended detail CBCT test selection for the Catphan® 503 is shown below:

Figure 1.5: CBCT Preset Test selection for the Catphan® 503.

Detailed Discussion of Measurements

For the detailed discussion of measurements, please see the individual articles below: