Computational human phantoms

MIRD dosimetry • computational anatomy

Computational human phantoms

A computational human phantom is a digital representation of human anatomy
used in radiation-transport simulations. In MIRD dosimetry, phantoms provide
the anatomical framework for estimating absorbed dose to organs, tissues,
tumors, and other regions of interest.

Every phantom has two independent descriptions
Axis 1
Phantom format
How anatomy is represented computationally:
stylized, voxel, or mesh-based.
≠
Axis 2
Morphometric category
How the phantom relates to a population or person:
reference, patient-dependent, patient-sculpted, or patient-specific.
The key clarification:
A voxel phantom is not automatically patient-specific, and a mesh phantom is
not automatically patient-dependent. A complete phantom description should
report both its format and its morphometric category.

Axis 1

Phantom format types

Format describes the computational or mathematical technique used to represent
the outer body contour and internal anatomy. Computational human phantoms are
commonly grouped into three format types.

geometric surfaces and equations
Mathematical

Stylized phantoms

Organs and body regions are represented using three-dimensional geometric
equations, such as spheres, ellipsoids, cylinders, toroids, and truncated cones.

  • Efficient for repeated calculations.
  • Can be adjusted for organ size and body shape.
  • Usually less anatomically realistic than image-based models.

labeled three-dimensional volume elements
Tomographic

Voxel phantoms

The body is represented as a three-dimensional array of voxels. Each voxel
can have an organ or tissue identity, elemental composition, and density.

  • Often created by segmenting CT or MR image sets.
  • Provides high anatomical realism.
  • Can be difficult to resize or reshape for another person.

polygon, NURBS, or tetrahedral surfaces
Surface-based

Mesh-based phantoms

Body contours and organ boundaries are represented by polygon meshes,
NURBS surfaces, or—in some models—tetrahedral mesh volumes.

  • Combines realism with geometric flexibility.
  • Can be reshaped, resized, and repositioned.
  • Can be used directly by modern Monte Carlo codes.

Where do hybrid phantoms fit?

Hybrid phantoms commonly combine polygon-mesh and NURBS surface
representations. Historically, mesh-based hybrid phantoms were often
converted to voxels before radiation transport calculations. A
hybrid-voxel phantom is a voxel model derived from mesh or
NURBS surfaces; it is distinct from a traditional voxel phantom created
directly from segmented CT or MR image slices.

Axis 2 • the main reason for this page

Phantom morphometric categories

Morphometric category describes how the phantom’s body size, body shape,
external contour, and internal anatomy relate to a reference population or to
a particular patient. These categories are independent of phantom format.

Increasing use of individual patient information
Reference
population standard
→
Patient-dependent
matched library member
→
Patient-sculpted
external contour matched
→
Patient-specific
individual anatomy matched
Category 1

Reference

A standardized individual defined using reference values for body height,
total body mass, and organ masses, commonly near 50th-percentile values.

Category 2

Patient-dependent

A member of a phantom library selected to match a patient’s body
morphometry, commonly by standing height and total body weight.

Category 3

Patient-sculpted

A library phantom that is further deformed to closely match the patient’s
external body contour. Internal organs may still differ in size, shape,
and position.

Category 4

Patient-specific

A model intended to match the body morphometry and internal anatomy of one
individual patient, commonly created from that patient’s CT or MR images.

Important limitation of patient-sculpted phantoms:
Matching the exterior body contour does not ensure that internal organs have
the same sizes, shapes, or positions as those of the patient.

Combining both axes

Formats and categories form a matrix

The same morphometric category can be implemented using different phantom
formats. This is why the format and category should be stated separately in a
dosimetry study or software description.

Format Reference Patient-dependent Patient-sculpted Patient-specific
Stylized Standard mathematical anatomy. Library of models with varying body dimensions. Possible through geometric resizing and reshaping. Limited for reproducing detailed image-defined anatomy.
Voxel Reference anatomy represented as labeled voxels. Library of image-derived or rescaled voxel models. Possible, but comparatively difficult to modify. Patient CT or MR anatomy segmented into labeled voxels.
Mesh-based Reference anatomy represented with meshes or NURBS surfaces. Scalable library across body sizes and shapes. Surfaces deformed to match external patient measurements or contours. Patient-matched mesh derived from imaging, segmentation, or registration.

MIRD dosimetry applications

Choosing an appropriate phantom

01

Reference dosimetry

Reference phantoms support standardized calculations, comparisons, and
reference dose coefficients.

02

Diagnostic imaging

Patient-dependent or patient-sculpted phantoms can improve estimates when
detailed patient images are unavailable.

03

Radiopharmaceutical therapy

Patient-specific anatomy is generally preferred when dose estimates to
tumors and organs at risk inform treatment decisions.

Key takeaway
A computational human phantom should be described by both its
format—stylized, voxel, or mesh-based—and
its morphometric category—reference,
patient-dependent, patient-sculpted, or patient-specific.
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