A fast, detailed X-ray-based scan reserved for the jobs it does best — reading the lungs and measuring plaque in the heart.
Last updated October 2026 · New Dawn Health editorial team
What it images
Depends on the body region and protocol
Radiation
Ionizing radiation; dose varies
Contrast
Depends on the examination
Where it fits
A targeted test for a defined question
What is a CT scan?
A CT (computed tomography) scan takes X-rays from many angles and rebuilds them into a stack of
thin slices. A plain X-ray is one
flattened shadow; a CT is a loaf you can look through slice by slice. That is why CT can find a
lung nodule a few millimeters across that a chest film often misses.
You lie on your back and the table glides through a short, open ring. Nothing encloses your head, and you can see out of both ends.
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The ring spins, taking shadows
An X-ray tube and a bank of detectors face each other and spin around you. Every moment of that spin captures one flat shadow from one angle.
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One shadow is ambiguous — hundreds are not
In a single image, things at different depths overlap. Given shadows from all the way around, a computer works out which arrangement of tissue would cast every one of them. That calculation is the computed in computed tomography.
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Breathe in and hold
A recorded voice asks for a breath-hold. With the lungs still, nothing moves mid-scan, so the slices stay sharp.
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Back on your feet
Most of the visit is positioning rather than scanning, and the X-rays themselves are silent and painless. Your coordinator confirms the total visit time when you book.
Before your scan: what to tell the team
A screening low-dose lung CT or a calcium score involves no injection; a diagnostic chest CT may use contrast. A coronary CT angiogram, and some other CT
exams, use contrast dye:
What CT screens for
Of the three CT exams on this page, only one is a recommended screen, and only for defined groups.
(CT colonography, not covered here, is another recognized screening option.) The others answer a
question a clinician has already asked:
Three CT exams on this site, and when each is used.
Images the artery channels directly, including soft plaque a calcium score misses
A contrast exam for a defined clinical question, not a routine screen
Who qualifies for lung CT screening
In Taiwan, the Health Promotion Administration funds a scan every 2 years for people with a parent, sibling or child with lung cancer (men 45–74, women 40–74).
Also in Taiwan, smokers aged 50–74 with at least 20 pack-years (packs per day × years smoked) who currently smoke and are willing to quit, or who quit less than 15 years ago. Funded screening is for people with National Health Insurance who meet the criteria.
In the US, the US Preventive Services Task Force (2021) recommends yearly screening at 50–80 with at least 20 pack-years, still smoking or quit within 15 years.
Radiation, and when another scan does the job
CT uses ionizing radiation — enough energy to break chemical bonds inside a cell.
Dose is counted in millisieverts (mSv), a unit that means little on its own, so read it next to the
exposure you already live with:
Radiation dose, compared
Chest X-rayAbout 0.1 mSv
About 10 days of average US background radiation.
Low-dose lung CTAbout 1.5 mSv
About 6 months of background.
Heart calcium scoreAbout 1.7 mSv
About 6 months of background.
Natural background, one yearAbout 3 mSv
The average yearly US dose from natural sources — the yardstick.
Coronary CT angiogramAbout 8.7 mSv
About 3 years of background.
Background is the US average; your dose varies (RadiologyInfo figures, checked October 2026).
Those doses are small. They are not zero. That is the reason CT is aimed at a question rather than
sprayed across the body: a whole-body CT survey would stack dose on organs nobody had asked about.
✓
Where CT earns its dose
The lungs, which no radiation-free scan reads well
Coronary calcium, and the coronary arteries when a clinician needs them imaged
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Where another test may answer instead
A question about the brain, spine or soft organs — an MRI or an ultrasound often answers it with no radiation.
A general whole-body survey — CT is not paired with MRI into a universal whole-body screen. The full-body MRI page explains why the American College of Radiology says evidence is insufficient to recommend total-body screening for people without symptoms.
The doses are small but not zero. RadiologyInfo puts a lung screening CT at about 1.5 mSv and a calcium score at about 1.7 mSv — each roughly six months of average US background radiation. That is why CT is used for a defined question rather than as a routine whole-body scan.
What is the difference between a CT scan and an MRI?
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CT uses X-rays, is fast, and reads the lungs, bone and calcium well. MRI uses a magnet and radio waves, with no radiation, and shows soft tissue such as the brain and spine in more detail. They answer different questions; neither is a universal whole-body screen.
Where does CT fit in a screening package?
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A package may include a low-dose lung CT, a heart calcium score, or both. Lung CT is a screen for people who meet the eligibility criteria; a calcium score helps when heart risk is uncertain. Whether either suits you depends on your age, smoking, family history and other risk factors.
Is CT safe if I am pregnant?
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Tell the team if you are or could be pregnant. CT is generally avoided in pregnancy unless it is medically necessary, and a radiation-free test may be chosen instead or the scan deferred.
