Helical & Multidetector CT (Pitch)
- Helical (spiral) CT moves the patient through the gantry while the tube spins, so the X-ray beam traces a continuous corkscrew instead of taking one ring-shaped slice at a time.
- Multidetector CT (MDCT) stacks many rows of detectors side by side, so each rotation captures a whole slab of the body at once — that's how a chest scan finishes in a single breath-hold.
- Pitch is one tidy ratio: how far the table travels per rotation divided by the beam's width. It's the dial that trades speed against image quality and dose.
- Pitch ≈ 1 means the corkscrew turns are touching nicely. Pitch > 1 spreads them out (faster, less dose, gaps to fill in). Pitch < 1 overlaps them (slower, more dose, often smoother images).
Old CT scanners had a frankly ridiculous workflow. The tube would spin once, grab a single slice, then stop — and the cables coiled around the gantry literally had to unwind before it could spin again. Move the table a notch, re-coil, spin, stop, unwind, repeat. Scanning a chest this way was like photographing a parade one frame at a time while the band keeps marching off without you. Helical CT is the engineering fix for all of that, and pitch is the single number that describes how aggressively you're allowed to use it.
The corkscrew: helical scanning
The breakthrough was the slip ring — a clever electrical contact that lets power and data flow into a gantry that spins forever without tangling cables. Free the tube to rotate continuously, then glide the table through the doughnut at a steady pace at the same time, and the beam no longer carves flat parallel slices. From the patient's point of view, the focal spot spirals around them in a smooth helix, like the stripe on a candy cane or the thread on a screw.
The catch: those raw data points are now smeared along a corkscrew, not lined up in neat planes. So the scanner does a bit of mathematical interpolation — it estimates what a flat slice would have looked like at any chosen level along the spiral. The payoff is enormous: you can reconstruct slices anywhere you like, as thin or as overlapping as you want, all from one continuous sweep done in a single breath-hold.
Many rows at once: multidetector CT
Helical scanning solved the spinning problem. Multidetector CT (MDCT) solved the width problem. Instead of one ring of detector elements, MDCT lines up many rows along the patient's long axis — so a single rotation no longer collects a wafer-thin slice but a whole slab. Think of mowing a lawn: a single-row detector is a push mower one blade wide, while a 64-row detector is a riding mower that flattens a meter-wide strip in one pass. Same yard, a fraction of the time.
This is why modern CT is so fast. More rows means more anatomy per rotation, which means you cover an entire abdomen before the patient even thinks about breathing — and that speed is exactly what makes CT angiography catch a bolus of contrast at its peak.
Pitch: the one ratio to remember
Here's the star of the page. Pitch ties the table speed to the beam width in a single number:
Pitch = table travel per rotation ÷ total width of the X-ray beam
That's it. It answers one question: relative to how wide my beam is, how far did I scoot the patient during one full spin? Picture wrapping a ribbon around a cardboard tube. If each wrap snugs right up against the last, that's a pitch of about 1. Wind it loosely with gaps showing between turns — pitch above 1. Overlap each wrap on the one before — pitch below 1.
| Pitch | What the helix does | Speed | Dose (all else equal) | Trade-off |
|---|---|---|---|---|
| = 1 | Turns just touching | Standard | Standard | The comfortable default |
| > 1 | Turns spread apart | Faster | Lower | Risk of gaps the math must fill; can hurt detail |
| < 1 | Turns overlapping | Slower | Higher | Redundant data, often smoother, less motion |
Why does higher pitch lower dose? You're stretching the same number of rotations across more anatomy, so (all else held equal) any given sliver of the patient sits in the beam for less time. Speed and dose savings come bundled together — but hold that thought, because real scanners don't always leave "all else equal" (see the pitfall below).
When you'd reach for each
A trauma patient who can't hold still, or a coronary scan racing a beating heart? Crank the pitch up — you want the whole region frozen before motion ruins it, and you'll happily accept a touch less fine detail. Hunting for a tiny lung nodule or a subtle fracture line where every detail counts? Dial the pitch down, let the helix overlap, and feed the reconstruction richer, redundant data.
Pitch is not a standalone dose number. A high pitch lowers dose only if the scanner doesn't compensate — many modern systems automatically boost tube current as pitch rises to keep image quality constant, which can erase the dose savings. If you actually want to know patient dose, look at the reported CTDI and DLP, not the pitch alone.
The one-sentence version
Helical scanning lets the tube spin without stopping, multidetector arrays let it grab a thick slab each spin, and pitch is the dial — table speed over beam width — that you turn to trade speed and dose against image quality. Master that single ratio and you understand the personality of nearly every modern CT protocol.