KAHIBARO
8.3. Common Medical Materials
Table of Contents
Air
Air is the simplest and most frequently used non-tissue material in GATE medical simulations. It is usually present as the world material and in cavities such as lungs or air gaps in equipment. In medical physics, air is typically modeled as a low density mixture of nitrogen, oxygen, argon, and a small fraction of other gases.
In GATE, you will usually rely on the Geant4 NIST definition called G4_AIR. This standard material provides a realistic elemental composition and a typical density at standard temperature and pressure around $1.205 \times 10^{-3} \,\text{g/cm}^3$. For most imaging and dose simulations, using G4_AIR without modification is both sufficient and recommended.
The main role of air in simulations is to provide an environment where radiation can travel with relatively few interactions. This is important for modeling the path between the patient and detectors, and for representing low density regions in the body such as lungs. Because its density is small, air contributes very little to attenuation and dose compared with tissues or bone.
If you create a custom material, it is important to keep the density and composition physically reasonable. However, beginners should strongly prefer NIST predefined materials for air over hand defined mixtures. You can set the world material to air directly in your simulation configuration by referring to its name in the Geant4 database.
Important rule: For most GATE simulations, always use the NIST material G4_AIR for the world and air regions. Avoid redefining air unless you have a specific and documented experimental condition.
A concise view of typical air properties used in medical simulations is:
| Property | Typical value |
|---|---|
| Material name | G4_AIR |
| Density | $\approx 1.2 \times 10^{-3}$ g/cm³ |
| Main elements | N, O, Ar (trace gases) |
Water
Water is the reference material in medical physics. It is often used to approximate soft tissues and is the basis for many calibration and validation procedures. In GATE, you will almost always use the NIST material G4_WATER, which is a standard composition of H and O with density close to $1 \,\text{g/cm}^3$.
Many important quantities, such as absorbed dose and stopping powers, are tabulated in water. Because human tissues are mostly water, water phantoms are frequently used in simulations to represent simplified patients, to study basic physics, or to validate dose calculations. For example, a simple water box can mimic a water tank used for beam commissioning in radiotherapy.
When you set up geometries such as water phantoms or water-equivalent slabs, you simply assign G4_WATER as the material. For more advanced work, different types of water, such as liquid water or ICRU standard water, may appear in the Geant4 database, but for beginners, G4_WATER is the correct default.
Key point: Use G4_WATER as the standard reference material for basic dose and imaging simulations. It is the default comparison material for many benchmark depth dose and attenuation studies.
Typical water parameters useful in medical simulations are:
| Property | Typical value |
|---|---|
| Material name | G4_WATER |
| Density | $\approx 1.0$ g/cm³ |
| Composition | H, O (H₂O) |
| Role | Reference tissue / medium |
Bone
Bone is much denser than soft tissue or water and contains heavier elements such as calcium and phosphorus. Because of this, it has very different attenuation and scattering properties, especially for X rays and gammas. Correct modeling of bone is essential in CT simulations, dose calculations near the skeleton, and internal dosimetry for bone seeking radionuclides.
In GATE, you typically use NIST predefined bone materials. Examples include cortical bone and trabecular bone, each with its own density and composition. They differ mainly in density and in the fraction of mineral content. Cortical bone is the compact outer layer of bones and has higher density, while trabecular bone is the spongy inner part with lower density.
Since this course focuses on GATE usage and not on detailed elemental composition, you will usually choose a standard NIST bone material and apply it to bone regions in a phantom or to high density voxels in image based geometries. This provides realistic attenuation for X ray imaging and correct scattering behavior for nuclear medicine and radiotherapy simulations.
Important rule: Use NIST bone materials instead of inventing your own. Bone has significantly higher density than water, so assigning water where bone should be will underestimate attenuation and dose.
A qualitative comparison of water and bone is shown here:
| Property | Water | Typical bone (cortical) |
|---|---|---|
| Density | ≈ 1.0 g/cm³ | ≈ 1.8–1.9 g/cm³ |
| Main elements | H, O | H, O, C, N, Ca, P, others |
| Role | Reference medium | High attenuation regions |
In voxelized phantoms or CT based patient models, bone regions are usually detected via higher CT numbers and then mapped to appropriate bone materials using CT to material conversion tables.
Lung
Lung tissue is characterized by its very low effective density due to the presence of air spaces in the alveoli. This low density has a major impact on photon and charged particle transport. For example, in radiotherapy and proton therapy, beams passing through lung experience reduced attenuation and altered range, which you must capture properly in simulations.
In GATE, lung is usually represented by dedicated NIST materials that correspond to averaged lung compositions at different inflation states or densities. These materials have compositions similar to soft tissue but with much lower density. Unlike air, lung still contains a large fraction of water and biological elements, so it does not behave like an empty cavity.
When working with voxelized geometries derived from CT images, intermediate CT values between soft tissue and air are typically mapped to lung materials. This is done through a CT to material conversion, which links CT number ranges to specific lung materials with corresponding densities.
Key point: Lung is not air. Even though the density is low, it still has tissue composition. Using air instead of lung will strongly distort attenuation and dose in the thorax.
An illustrative table of densities is:
| Material | Approximate density (g/cm³) |
|---|---|
| Air | ≈ 0.0012 |
| Lung | ≈ 0.2–0.4 |
| Water | ≈ 1.0 |
| Bone | ≈ 1.8–1.9 |
In imaging simulations such as SPECT or PET, realistic lung materials are important to correctly reproduce photon attenuation and scatter in the thoracic region, which influences quantitative imaging and internal dosimetry.
Soft tissue
Soft tissue refers to the bulk of non bone, non lung biological tissues such as muscles, organs, and fat. For many medical physics applications, these tissues are approximated by water or by standardized soft tissue materials. While water is often a good first approximation, dedicated soft tissue materials can capture subtle differences in composition, especially the presence of carbon and other elements.
In GATE, you will most commonly use NIST soft tissue materials that represent average human tissue, sometimes specified for particular anatomical regions such as muscle or adipose tissue. These materials have densities close to 1 g/cm³ but differ slightly according to their fat and water content. As a beginner, using a single standard soft tissue material for most non lung, non bone tissues is usually acceptable unless your study requires detailed tissue specificity.
Soft tissue plays a central role in simulations of dose deposition from external beams, internal radionuclides, and diagnostic X ray beams. Because much of the body volume is soft tissue, any dose or imaging study must represent it correctly to obtain meaningful results.
In patient specific voxelized simulations, CT to material conversion tables usually assign most mid range CT values to one or more soft tissue materials with slightly different densities. This allows more accurate modeling of attenuation and dose without requiring a separate material for every organ.
Practical rule: When in doubt, model generic organs and body regions as water or a standard soft tissue material from the NIST database. Only refine to specific tissues if your application explicitly requires it.
A simple comparison of typical medical materials often used together is:
| Material | Approx. density (g/cm³) | Typical use in simulations |
|---|---|---|
| Air | ≈ 0.0012 | World, air cavities |
| Lung | ≈ 0.2–0.4 | Thoracic regions, lungs |
| Soft tissue | ≈ 1.0 | Organs, muscles, generic body tissues |
| Water | ≈ 1.0 | Reference phantoms, water tanks |
| Bone | ≈ 1.8–1.9 | Skeleton, high attenuation structures |
These common medical materials form the basic set you will use in most GATE simulations before moving on to more specialized or custom materials.
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