kV vs MV imaging in radiation therapy

A guide for radiation therapy students · ~7 min read

Every image-guided treatment starts with an X-ray, but not all treatment-machine X-rays are the same. Kilovoltage (kV) and megavoltage (MV) imaging use beams that differ by a factor of a thousand in energy, and that single difference shapes their image quality, dose, and the jobs they're best at. This guide explains how kV and MV imaging differ, what hardware produces each, and when a therapist reaches for one over the other.

Two beams, a thousandfold apart

The energy of an X-ray beam decides how it interacts with the body, and that drives image contrast. Kilovoltage beams (tens to ~150 kV, like a diagnostic X-ray tube) are low enough in energy that dense tissue such as bone absorbs them far more than soft tissue does — so kV images show high contrast between bone, soft tissue, and air. Megavoltage beams (the multi-million-volt treatment beam, typically 6 MV and up) are so energetic that bone and soft tissue absorb them almost equally, which flattens contrast and makes MV images look washed-out by comparison.

That contrast difference is the single fact that explains most of what follows.

kV imaging and the on-board imager

Modern linacs carry a separate kV imaging system — an X-ray tube and a flat-panel detector on retractable arms, mounted at right angles to the treatment beam (commonly called on-board kV imaging). Because it's an independent diagnostic-quality source, it can take:

• kV planar (2D) radiographs — a quick orthogonal pair (for example AP + lateral) for bony or fiducial setup verification at very low dose.
• kV cone-beam CT — rotating the source and panel around the patient to reconstruct a 3D volume for soft-tissue matching.
• Fluoroscopy / live tracking — real-time imaging for motion management and gating.

kV imaging is the default for most daily IGRT because it gives the best image quality for the lowest imaging dose.

MV imaging and the EPID

Megavoltage imaging uses the treatment beam itself. Opposite the treatment head sits an EPID (electronic portal imaging device) — a flat-panel detector that captures the beam after it passes through the patient. The resulting portal image is what the treatment beam "sees".

MV has real strengths despite its lower contrast:

• It verifies the actual treatment beam — the field shape and aperture, exactly as delivered.
• It cuts through metal. High-density implants (hip prostheses, dense fillers) cause severe streak artifacts on kV but are far less disruptive at megavoltage energies.
• It needs no extra hardware energy source — it reuses the treatment beam, and the EPID also supports in-vivo dosimetry and machine QA.

kV vs MV at a glance

Planar images vs cone-beam CT

It helps to keep two distinctions separate: energy (kV vs MV) and dimensionality (2D planar vs 3D volumetric).

• Planar (2D) imaging takes flat projection radiographs. An orthogonal pair — usually AP/PA plus lateral — is enough to localise in all three translation directions. This is the basis of 2D/2D matching, where each portal image is registered to a digitally reconstructed radiograph (DRR) from the plan.
• Cone-beam CT (3D) reconstructs a volume so you can match on soft tissue and assess all six degrees of freedom. See CBCT 6DOF registration for how that works.

Both kV and MV can do planar imaging; kV is the standard source for cone-beam CT on most linacs (MV-CBCT exists but is less common).

When each is used

• Routine daily setup — usually kV: low dose, good contrast, fast planar pairs or CBCT depending on the site.
• Fiducial-based localisation (e.g. prostate gold seeds) — kV planar pairs show the seeds crisply at low dose.
• Patients with metal implants — MV portal imaging can verify position where kV would streak out.
• Beam aperture / field verification — MV, because it images the treatment beam itself.
• Soft-tissue targets — kV-CBCT for volumetric soft-tissue matching.

How this connects to image matching

Whatever the source, the therapist's job is the same: register the daily setup image to the planning reference and read off the couch correction. With kV or MV planar pairs that's a 2D/2D match to DRRs — a 5DOF correction on bony landmarks or fiducials. With kV-CBCT it's a 3D registration with full 6DOF assessment. The imaging modality changes the picture's contrast and what artifacts you'll fight, but the matching skill — lining up anatomy and judging when it's "good enough" against tolerance — carries across all of them.

Common pitfalls

• Expecting CT-like contrast from MV. Portal images are inherently flat — match on the strong bony edges, not subtle soft-tissue detail.
• Forgetting kV metal artifact. Streaks from implants can mimic or hide anatomy; this is a classic reason to switch to MV.
• Treating imaging dose as free. Every setup image adds dose; protocols balance imaging frequency and modality against benefit.
• Confusing the source with the dimensionality. kV isn't synonymous with 2D, and CBCT isn't a separate "energy" — keep the two axes straight.

How to practice

The fastest way to get comfortable with setup imaging is to do the matching itself, repeatedly, with feedback. The RT Image Matching Trainer lets you practice 2D/2D portal-to-DRR matching (including a kV fiducial case on the prostate) and CBCT 6DOF registration on real 3D volumes — with fusion, window/level, contrast tools and a live residual-error readout graded against tolerance. Working both planar and volumetric cases builds the judgement that transfers regardless of which imaging source your clinic uses.

Frequently asked questions