Imaging in the RT workflow

Medical imaging

The imaging modalities, accessory equipment, body planes, and digital-imaging concepts that drive radiation therapy simulation, planning, and verification. Tap a card to expand the detail.

Exposure parameters: mA, kV & mAs

How changing each factor affects the CT image, the DRR, and patient dose

kV (tube voltage)

Effect on CT image
Higher kV → more penetrating (higher-energy) beam, more Compton / less photoelectric, so image contrast drops slightly and beam-hardening artifacts lessen; HU stays calibrated. Lower kV raises contrast (bone vs soft tissue) but increases noise and dose for the same mAs.
Effect on DRR
DRR contrast follows the CT: a lower-kV planning CT gives stronger bone–soft-tissue contrast in the DRR for portal matching, while a higher-kV CT is smoother and more penetrating but flatter.
Effect on patient dose
Higher kV lowers surface dose and improves penetration for the same mAs; raising kV without adjusting mAs usually reduces patient dose.

mA (tube current)

Effect on CT image
Higher mA delivers more photons per rotation, reducing quantum mottle/noise and improving SNR and low-contrast detectability; spatial resolution and contrast are otherwise unchanged.
Effect on DRR
A higher-mA CT produces a less noisy DRR with crisper bony edges, improving the accuracy of DRR-to-portal/CBCT registration.
Effect on patient dose
Patient dose scales linearly with mA — doubling mA doubles dose while reducing noise by ~√2.

mAs (mA × exposure time)

Effect on CT image
mAs sets the total photons per slice/projection. Image noise ∝ 1/√mAs, so increasing mAs reduces noise and improves image quality; lowering mAs increases noise and streak artifacts.
Effect on DRR
A low-mAs (or dose-sparing) CT yields a grainy DRR that is harder to match; a higher-mAs CT yields a sharper, cleaner DRR reference.
Effect on patient dose
Dose is directly proportional to mAs — the main lever for CT dose (CTDIvol/DLP) without changing contrast.

A DRR is a synthetic 2D image computed from the planning CT, so its quality and contrast are inherited from the CT acquisition settings — it is not re-acquired at the linac.

14 topics

CT is the backbone of RT — it provides the 3D anatomy and the density map used for treatment planning and dose calculation.

A cone-beam CT mounted on the linac gantry produces a 3D volume of the patient in treatment position for daily setup verification.

MRI offers superior soft-tissue contrast without ionizing radiation — key for brain, head & neck, prostate, and gynecologic targets.

PET images metabolism (commonly FDG uptake) to support staging, targeting, and response assessment.

Other modalities fill niches from real-time localization to motion-resolved planning and planar verification.

Contrast improves target visualization; fiducials provide a fixed, imageable reference inside the patient for setup and gating.

Devices hold the patient reproducibly and manage internal motion so the image and the treatment match.

Detectors and anti-scatter devices determine image quality; surface imaging adds real-time, marker-less monitoring.

Three orthogonal planes define how cross-sectional images are described and reformatted in RT.

Standard terms and the DICOM coordinate system describe location and orientation unambiguously.

DICOM is the medical-imaging data standard; DICOM-RT extends it for the objects that flow between scanner, TPS, and record system.

Storage systems and review tools manage the digital image lifecycle from acquisition to contouring.

Registration aligns datasets; segmentation defines the volumes used for planning.

The recurring terms that connect imaging to planning, delivery, and verification.