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Ionising · CT

Computed Tomography (CT)

Imaging thin cross-sections of the body with a rotating X-ray tube — modern radiology's most powerful modality, and the one demanding the most dose awareness.

Modality profile· ~10 min read· With photos & sources
Siemens Somatom Sensation 64 CT scanner in a hospital
A 64-slice CT scanner (Siemens Somatom Sensation 64): gantry, patient table and console.  Foto: Wikimedia Commons · CC BY-SA 4.0

Overview

Computed tomography (CT) is based on an X-ray tube rotating around the patient to collect attenuation data from many angles, which a computer reconstructs into cross-sectional images. By removing the overlapping shadows of conventional radiography, it shows organs in millimetric slices with high contrast. This diagnostic power makes CT the backbone of emergency and oncologic imaging — and the same power makes careful dose management essential.1

How it works

Modern CT performs helical (spiral) scanning: the patient table moves through the gantry at constant speed while the X-ray tube and the opposing detector array rotate continuously. This continuous rotation is enabled by slip-ring technology. The fan-shaped beam from the tube passes through the patient, thousands of detector elements on the far side measure the remaining radiation, and the "projection" at each angle is recorded.1

The collected raw projection data is turned into slice images by a reconstruction algorithm. Today, filtered back projection has largely given way to iterative and deep-learning–based methods, which make it possible to achieve the same image quality at lower dose.4

Key components

A CT system has several main parts: the gantry housing the rotating hardware; the X-ray tube fed by a high-frequency generator; the multi-row detector array (MDCT) that measures the beam; the table that positions the patient; and the workstation that performs reconstruction and image processing. Multidetector systems acquire several slices per rotation, shortening scan time and reducing motion artifact.1

Image-formation parameters

The balance between image quality and dose is set by a few key parameters. Understanding them correctly is the key to optimization:

kVp
Tube voltageSets beam energy and penetration; affects contrast.
mAs
Tube current × timeDrives photon count, hence noise and dose directly.
Pitch
Table travel / beam width>1 speeds up and lowers dose; <1 overlaps and raises dose.
Slice / Kernel
Thickness & reconstruction filterSets the spatial-resolution–noise trade-off.

Modern scanners include tools that set these parameters automatically: automatic tube current modulation (ATCM) varies mAs in real time according to the patient's anatomy; automatic kV selection picks the most suitable voltage for the exam type. These automations can substantially reduce dose while preserving image quality.1

Dose profile

CT delivers a markedly higher dose per exam than conventional radiography, which is why dose metrics (CTDIvol, DLP, SSDE) matter especially in CT. Typical effective doses vary by exam: roughly 2 mSv on average for head CT and about 8 mSv for abdominal CT.2 RadiologyInfo.org reports about 7.7 mSv effective dose for abdomen–pelvis CT.3 For comparison, the average dose from natural background radiation is about 3 mSv per year.2

Go deeper
Our detailed article on what CT dose metrics (CTDIvol, DLP, SSDE) mean: Understanding CTDIvol, DLP & SSDE. For reconstruction methods: FBP, Iterative & Deep Learning.
Strengths and cautions
Strengths: high spatial and contrast resolution, fast scanning, broad clinical use. Cautions: higher dose than radiography; justification and protocol optimization are critical.

References

  1. U.S. FDA. Computed Tomography (CT) & Medical X-ray Imaging. fda.gov · Computed Tomography
  2. U.S. FDA. Initiative to Reduce Unnecessary Radiation Exposure from Medical Imaging (typical effective doses; background ≈ 3 mSv/yr). fda.gov · White Paper
  3. RadiologyInfo.org (RSNA/ACR). Radiation Dose in X-Ray and CT Exams (abdomen–pelvis CT ≈ 7.7 mSv). radiologyinfo.org
  4. DoseSave. FBP, Iterative and Deep-Learning Reconstruction (reconstruction and dose).
  5. Image: File:CT Scan Siemens Somatom Sensation 64.jpg, Wikimedia Commons, CC BY-SA 4.0.
Note: The dose values given are typical/average estimates and vary markedly with device, protocol and patient size. For clinical decisions, consult a qualified medical physicist and current regulations.

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