A DRR is a line integral of attenuation from a point source through the CT to a flat detector - the simulated radiograph image guidance compares a portal or kV image against. Tools ▶ ☢ Digitally reconstructed radiograph… renders one with two independent forward projectors.
drr -i exact, after Siddon (Med. Phys. 1985) with Jacobs' incremental
formulation: the ray walks voxel to voxel and each voxel contributes
exactly the length of ray inside it. For a piecewise-constant volume the
result is the integral, the reference for the other projector; edges
come out hard, because in the voxel model they are.
itk::RayCastInterpolateImageFunction, the projector behind ITK's 2-D/3-D
registration metrics: the ray is marched at a fixed step, trilinearly
interpolated values accumulated with a midpoint rule - the volume as a
smooth field, so edges are softer and step size a real accuracy/speed knob.
The difference image of the two on the same geometry and its statistics (max, mean absolute, RMS, relative, Pearson correlation) measure the interpolation error; on a uniform phantom at 0.5 mm step the two agree to r > 0.999 and a few percent mean difference, concentrated on the edges.
[Geometry] is a cone-beam geometry in IEC 61217 terms - how a linac
states it and an RTPLAN beam stores it:
- SAD / SID - source-to-axis and source-to-imager distances, mm.
- Gantry angle - 0° source above the patient, 90° at the patient's left.
- Couch angle - patient-support rotation about the vertical axis.
- Isocentre - in patient coordinates; ⌖ takes the workspace's crosshair.
- Panel size and pixel count - the window reports the resolution projected back to the isocentre plane.
The IEC fixed frame maps to the DICOM patient frame for a head-first supine
patient: Xf (patient left) = +x, Yf (the gantry rotation axis,
towards the head) = +z, Zf (vertical, up) = −y. Unit tests assert the
source position at 0° and 90° and the detector axes orthonormal and
perpendicular to the beam.
From beam takes gantry angle, couch angle and isocentre from a beam of the loaded plan.
- Attenuation (μ from HU) -
μ = μ_water · (1 + HU/1000), clamped at zero, withμ_water = 0.0206 mm⁻¹(≈ 60 keV, the effective energy plastimatch's DRR preprocessing assumes); the integral is a real optical depth - 40 mm of water on the central axis integrates to0.0206 × 40, unit-tested for both projectors. - Raw line integral - the values as they are (plastimatch
-h none); for comparing against another tool's raw output. - Threshold - voxels below it contribute nothing, keeping air and the couch out.
The two renderings sit side by side with a shared display window (black/white points as fractions of the value range), an invert toggle, and a Difference view mapping signed difference blue↔red about a grey zero.
➕ Add to workspace A/B files the rendering (or both, when run together) under Planar images in the workspace's tree as an RT Image, with its own viewer (window/level, correct physical aspect ratio), renaming, and travel with the workspace when copied or moved.
The producing geometry rides along as the planar viewer's info rows -
engine, SAD/SID, gantry and couch angles, isocentre, panel size, HU model,
threshold, sampling step (ray-cast only) and render time. Labels are
DRR Siddon · G 90° C 0°, made unique on the way in.
Whichever greyscale the window shows is what gets stored: with Invert on (the default) values are mirrored about the middle of the range so dark is high attenuation, as on a radiograph; the range itself is unchanged, and the info rows say which convention was used.
Planar images are viewer-side objects: they are not written by File ▶ Export DICOM, which covers CT, RTSTRUCT, SEG, RTDOSE and RTPLAN.
DRR generation is a simulation feature sharing no code with registration.md; it is, however, the natural input to 2-D/3-D registration, whose ITK metrics use the interpolating projector.