Phantom Overview: CT (Revised 1-3-2024)

Revision History

The intent of the CT Accreditation Program (CTAP) is to use the information obtained from the review of both clinical and phantom images to assess overall image quality. Your facility will need to submit up to four specified phantom scans and corresponding dosimetric data using your facility’s routine clinical adult head, adult abdomen, pediatric head and pediatric abdomen protocols. In order to be accredited with the ACR, a CT unit must pass both the clinical and phantom image quality tests. The phantom will fail (and the unit will fail ACR accreditation) if the dose from the CT unit exceeds the pass/fail criteria. The facility must repeat the entire ACR phantom and dosimetry portion of the accreditation testing, and pass, for the unit to be granted accreditation.

Materials submitted for accreditation review are sent to 2 physicist reviewers in active practice. Phantom images are reviewed for image quality and radiation dosimetry.

An ACR phantom (Gammex 464) must be used to accredit all units used to produce diagnostic clinical images for patients at the facility. A complete set of phantom images—along with dose measurements taken with the appropriate dose phantoms—from every CT unit must be submitted. Using the Gammex 464 phantom and Computed Tomography Dose Index (CTDI) phantoms, your medical physicist will be able to calculate various descriptors of dose for your adult head, adult abdomen, pediatric head (1 y.o.) and pediatric abdomen (5 y.o., approx. 40 lbs) examinations in correlation to your use of the scanner and your application.

The phantom must be ordered from the manufacturer (see Gammex 464 phantom order form attached below). Please make sure that you order your phantom as soon as possible after you receive your CTAP number (assigned to you with your initial application) to allow ample time for shipping and completion of all testing materials within the allotted time frame provided by the ACR.

The ACR CT accreditation phantom has been designed to examine a broad range of scanner parameters, including:

You will not have to assess all of the above measurements as part of your phantom submission for accreditation. You will, however, use all of them for your routine Quality Control (QC).

Please be aware that the requirement for CT accreditation is that facilities use the same protocols for the Gammex 464 phantom that the facility uses for its clinical imaging whether Single Energy/Single Tube, Single Energy/Two Tubes (Dual Source) or Dual Energy (DE). 

Failure to comply with this requirement could result in failure to achieve accreditation. The only exceptions to this are that automatic mA modulation must be turned off, the scan field of view (SFOV) may be adjusted to a size appropriate to the size of the phantom and a display field of view (DFOV) of 21 cm is to be used.

It is essential that the scan acquisition parameters match your facility’s clinical use in all portions of your phantom submission:

1. Phantom Data Form

2. Gammex 464 phantom images

3. Dose Data Forms

4. CTDI phantom images

The Phantom Site Scanning Data Form and the Dose Calculation Forms (see attachment below) are part of the online testing package that you must submit in the ACR Accreditation database, ACRedit. These will allow you to gather the data, but you must enter the data into the online testing package and submit the data to the ACR. Please note that the CT Phantom Data/Dose Forms have SE and DE tabs. You only need to complete the tabs relevant to your submission type.

The ACR CT accreditation phantom (Gammex 464) is a solid phantom containing four modules and is constructed primarily from a water-equivalent material. Each module is 4 cm in depth and 20 cm in diameter. There are external alignment markings scribed and painted white (to reflect alignment lights) on EACH module to allow centering of the phantom in the axial (z-axis, cranial/caudal), coronal (y-axis, anterior/posterior), and sagittal (x-axis, left/right) directions. There are also “HEAD”, “FOOT” and “TOP” markings on the phantom to assist with positioning.

Module 1 is used to assess positioning and alignment, CT number accuracy, and slice thickness. The background material is water equivalent. For positioning, the module has 1-mm diameter steel BBs embedded at the longitudinal (z-axis) center of the module, with the outer surface of the BB at the phantom surface at 3, 6, 9, and 12 o’clock positions within the field of view (19.9cm center to center). To assess CT number accuracy, there are cylinders of different materials: bone-mimicking material (“Bone”), polyethylene, water equivalent material, acrylic and air. Each cylinder, except the water cylinder, has a diameter of 25 mm and a depth of 4 cm. The water cylinder has a diameter of 50 mm and a depth of 4 cm. To assess image width, two ramps are included that consist of a series of wires that are visible in 0.5-mm z-axis increments. 

Module 2 is used to assess low contrast resolution. This module consists of a series of cylinders of different diameters, all at 0.6% (6 HU) difference from a background material having a mean CT number of approximately 90 HU. The cylinder-to-background contrast is energy-independent. There are four cylinders for each of the following diameters: 2 mm, 3 mm, 4 mm, 5 mm and 6 mm. The space between each cylinder is equal to the diameter of the cylinder. A 25-mm cylinder is included to verify the cylinder-to-background contrast level and to assess the contrast-to-noise ratio.

Module 3 consists of a uniform, tissue-equivalent material to assess CT number uniformity. Two very small BBs (0.28 mm each) are included for optional use in assessing the accuracy of in-plane distance measurements. They may also be used to assess section sensitivity profiles.

Module 4 is used to assess high contrast (spatial) resolution. It contains eight bar resolution patterns: 4, 5, 6, 7, 8, 9, 10 and 12 lp/cm, each fitting into a 15-mm x 15-mm square region. The z-axis depth of each bar pattern is 3.8 cm, beginning at the Module 3 interface. The aluminum bar patterns provide very high object contrast relative to the background material. Module 4 also has four 1mm steel beads, as described for Module 1.

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