External Surfaces

This library provides an interface to provide an external surface, which is used for source-surface distance (SSD) and depth calculations. Physically, the external surface denotes the boundary between air and the dose absorbing medium (e.g. the patient's skin or the surface of a phantom).

All external surfaces exposes two methods,

getSSD(surf, p, s): Returns the source-surface distance (SSD)
getdepth(surf, p, s): Returns the depth below the source-surface intersection

external_surface

Both of these methods take an external surface (surf) and two position vectors, p and s. The position p is usually a location where dose is computed, and is a three-element vector. The position s is the position of the radiation source. Both positions are in the IEC Fixed coordinate system.

For example:

julia> surf = PlaneSurface(800.)PlaneSurface{Float64}(800.0)
julia> s = [0., 0., 1000.]3-element Vector{Float64}:
    0.0
    0.0
 1000.0
julia> p = [10., 20., 0.]3-element Vector{Float64}:
 10.0
 20.0
  0.0
julia> getSSD(surf, p, s)800.1999750062481
julia> getdepth(surf, p, s)200.04999375156194

Types of External Surface

To enable source-surface and depth computations, a number of concrete external surface types are defined. These store the surface information and provides the implementation of the getSSD and getdepth methods.

The choice of which type to use is largely dependent on your use case, whether you prefer accuracy or speed.

Where accuracy is preferred over speed, Cylindrical Surface and Mesh Surface provide accurate source-surface and depth computations. Generally Cylindrical Surface is recommended over Mesh Surface.
In the case of beam commissioning or water-tank doses, Plane Surface is recommended.
Where speed is preferred, consider using Linear Surface
For specialised setups, it may be best to implement a custom external surface.

Plane Surface

A simple plane surface (PlaneSurface), positioned at a constant source-surface distance along the central beam axis with normal pointed towards the source. Construct by supplying a source-surface distance, e.g. 800mm:

surf = PlaneSurface(800.)

This surface is best used in two scenarios:

The external surface is a box and the source does not rotate. This setup is common in beam commissioning, where dose in computed in a water tank.
Accuracy is less important than speed: this method computes source-surface distance or depth much faster than other methods.

Cylindrical Surface

The CylindricalSurface stores the external surface on a cylindrical-polar grid. This coordinate system is aligned to the IEC Fixed coordinate system, defined by the following convention:

rho: radial distance from the IEC fixed y axis.
y: axial position along the IEC fixed y axis.
ϕ: azimuthal position, equivalent to the gantry angle.

This a faster way of computing mesh intersections, recommended over using a Mesh Surface.

CylindricalSurface can be constructed by directly supplying the cylindrical-polar surface: y, ϕ and rho,

ϕ = 0:deg2rad(2.):2π
y = -100.:2.:100.
rho = @. (80+20*sin(ϕ))*(1-(y'/200)^2)
surf = CylindricalSurface(ϕ, y, rho)

CylindricalSurface can also be constructed using a mesh,

mesh = load_structure_from_ply("path/to/stl-or-ply")
surf = CylindricalSurface(mesh, Δy)

In this example, the bounds in the y direction will encompass the whole mesh with spacing of Δy. The axial y axis can be provided instead of a spacing, allowing finer control of the extent of the surface. See CylindricalSurface for further details.

Note

CylindricalSurface assumes the mesh can be well defined in a cylindrical-polar coordinate system. It is up to the user to ensure this, which may require rotating the mesh such. See Coordinate Transformations for more information on rotating meshes.

CylindricalSurface can be written to the VTK file format for visualisation:

write_vtk("surface", surf)

Mesh Surface

The MeshSurface is defined by a 3D mesh provided by the user.

This surface can be constructed by supplying a mesh:

mesh = load_structure_from_ply("path/to/stl-or-ply")
surf = MeshSurface(mesh)

Warning

This is considered a slow method - the ray-tracing method is inefficient as it iterates through every face in the mesh (see Roentgen.intersect_mesh) - and should only be used if the mesh is small, computation time is not important, or a better surface is not available. A better alternative would be to create an Cylindrical Surface.

MeshSurface can be written to the VTK file format for visualisation:

write_vtk("surface", surf)

Linear Surface

LinearSurface is a linear approximation to a general surface. It computes a set of planes normal to a general surface at the intersection of the line between the iso-center and source positions.

It is recommended to construct from a mesh,

mesh = load_structure_from_ply("path/to/stl-or-ply")
surf = LinearSurface(mesh)

where it computes the plane position and normal for each degree of gantry angle. See LinearSurface for further details.

Constant Surface

A constant surface (ConstantSurface) returns constant source-surface distance. Depth is computed as the distance from point to source minus the source-surface distance. Generally only used for testing purposes.