A phantom is a specially designed object used to evaluate the performance of a CT system. Various types of phantoms are available for assessing different performance parameters. They are also used as standards for calibration.
The following phantoms are commercially available for micro-CT and are used for CT number calibration, linearity, spatial resolution, image uniformity, geometric accuracy, and pixel size calibration.
| Phantom name | Purpose |
|---|---|
| Point Spread Function (PSF) Phantom (Wire Phantom) |
This phantom is used to evaluate the spatial resolution of a micro-CT system by measuring the Point Spread Function (PSF) and Modulation Transfer Function (MTF). It enables quantitative analysis of the system's ability to render fine structures sharply by imaging a point or line source, such as an ultra-fine wire placed inside the phantom, and analyzing the spread of its image. |
| Multi-Layer Phantom (Defrise, Multi Disk Phantom) |
Featuring a structure of high-density disks separated by layers of low-density material, it is designed to evaluate artifacts (image distortions) that originate from reconstruction algorithms, especially in cone-beam CT. It visualizes issues such as cone-beam artifacts to allow for the verification of image inaccuracies. |
| Low Contrast Phantom | This phantom evaluates low-contrast detectability, which is the ability to distinguish objects with small differences in CT value from their surroundings. It is important for measuring a system's performance in imaging subtle density differences, such as those in soft tissues, and helps to examine the relationship between image noise and detectability. |
| Contrast Scale Phantom | This phantom contains inserts of materials with known concentrations and CT values, such as calcium hydroxyapatite (CaHA), a main component of bone, and iodine, used as a contrast agent. It is used to calibrate the CT value scale with these reference materials, enabling the quantitative evaluation of high-contrast substances. |
| Slice Sensivirity Phantom | This phantom is used to evaluate spatial resolution in the slice thickness direction (z-axis). By imaging a thin metal foil placed inside, the Slice Sensitivity Profile (SSP) is measured from the resulting image. This allows for the evaluation of the selected slice thickness accuracy and the axial Modulation Transfer Function (MTF). |
| CT Dose Phantom | This is a phantom for measuring the radiation dose during micro-CT scans. Made of a material equivalent to human soft tissue, it is used by inserting a standard pencil-shaped ionization chamber into its central bore. It measures metrics such as the CT Dose Index (CTDI) and Dose Length Product (DLP) to help manage and optimize the radiation dose. |
| Water Phantom | This phantom is a uniform container filled with water, used to evaluate basic image quality parameters such as image noise and uniformity. It also serves as a reference for calibrating the system to set the CT value of water to 0 Hounsfield Units (HU). |
| Hydroxyapatite(HA) Phantom | This phantom contains inserts with various known concentrations of calcium hydroxyapatite (CaHA), the main component of bone. It is used as a reference standard for the quantitative measurement of Bone Mineral Density (BMD) in micro-CT. It is essential for creating a calibration curve that correlates CT numbers to bone density values. |
| Mouse phantom | This phantom mimics the body shape and internal structures of a small mouse. It is used for image quality tests of small-animal micro-CT systems. It allows for the assessment of image quality with different soft-bone-tissue inserts of varying densities. |
Phantoms can be custom-made or purchased from commercial suppliers. Phantoms are available from the following manufacturers:
QRM GmbH, Germany (https://www.qrm.de/): Micro-CT HA Phantoms
QRM GmbH, Germany (https://www.qrm.de/): Micro-CT Bar Pattern Phantoms
QRM GmbH, Germany (https://www.qrm.de/): Micro-CT Low Contrast Phantoms
PTW Freiburg GmbH, Germany (https://www.ptwdosimetry.com/)
Phantom Laboratory, USA (https://www.phantomlab.com/)
Sun Nuclear Corporation, USA (https://www.sunnuclear.com/): Micro-CT Phantom
Pure Imaging Phantoms, UK (https://www.pureimagingphantoms.com/): Micro CT Phantom Kit
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