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Technical Terms

Phantom

A phantom is a specially designed reference object used to evaluate the performance, image quality, and calibration of a CT system. Various types of phantoms are available to assess specific performance parameters and ensure measurement accuracy.

The following commercially available phantoms for micro-CT are widely used for calibrating CT numbers, verifying linearity, assessing spatial resolution, checking image uniformity, ensuring geometric accuracy, and calibrating pixel size.

Phantom Type Purpose
Point Spread Function (PSF) Phantom
(Wire Phantom)		 Evaluates 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 resolve fine structures by imaging a point or line source, such as an ultra-fine wire, and analyzing the spread of the resulting image.
Multi-Layer Phantom
(Defrise / Multi-Disk Phantom)		 Features a structure of high-density disks separated by low-density material layers. It is designed to evaluate artifacts (image distortions) originating from reconstruction algorithms, particularly in cone-beam CT. It visualizes issues such as cone-beam artifacts to verify image accuracy.
Low Contrast Phantom Evaluates low-contrast detectability--the ability to distinguish objects with slight differences in CT values from their background. This phantom is critical for measuring performance in imaging subtle density differences, such as in soft tissues, and examining the relationship between image noise and detectability.
Contrast Scale Phantom Contains inserts of materials with known concentrations and CT values, such as calcium hydroxyapatite (CaHA--a bone equivalent) and iodine contrast agents. It is used to calibrate the CT value scale, enabling quantitative evaluation of high-contrast substances.
Slice Sensitivity Phantom Evaluates spatial resolution in the slice thickness direction (z-axis). By imaging a thin metal foil placed inside, the Slice Sensitivity Profile (SSP) is measured. This allows for the verification of slice thickness accuracy and the axial Modulation Transfer Function (MTF).
CT Dose Phantom Measures the radiation dose during micro-CT scans. Constructed from tissue-equivalent material, it accommodates a standard pencil-shaped ionization chamber in its center. It is used to determine metrics such as the CT Dose Index (CTDI) and Dose Length Product (DLP) for dose optimization and management.
Water Phantom A uniform container filled with water, used to evaluate basic image quality parameters such as noise and uniformity. It also serves as the primary reference for calibrating the system, setting the CT value of water to 0 Hounsfield Units (HU).
Hydroxyapatite (HA) Phantom Contains inserts with known concentrations of calcium hydroxyapatite (CaHA). It serves as a reference standard for the quantitative measurement of Bone Mineral Density (BMD) in micro-CT, allowing for the creation of calibration curves that correlate CT numbers to bone density.
Mouse Phantom Mimics the anatomical shape and internal structures of a small mouse. Used for image quality assurance in small-animal micro-CT systems, it allows for assessment using soft tissue and bone-equivalent inserts of varying densities.

Phantoms can be custom-made or purchased from commercial suppliers. Major manufacturers include:

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