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Ionising · X-ray

Conventional Radiography

A projection image formed by a single X-ray exposure: radiology's oldest, most common and usually lowest-dose examination.

Modality profile· ~9 min read· With photos & sources
X-ray room dedicated to chest radiography
A room dedicated to chest radiography: a wall-mounted upright Bucky and a ceiling-suspended tube.  Foto: Wikimedia Commons · CC BY-SA 4.0

Overview

In conventional radiography, the beam from the X-ray tube passes through the patient in a single exposure and reaches the detector on the far side. The different attenuation of tissues (bone absorbs much, air little) becomes a shadow pattern on the detector; this projection image forms the basis for diagnosis. Being fast, inexpensive and widespread, it is used as the first-line exam in chest, skeletal and abdominal assessment.1

How it works

Two key parameters govern image quality. kVp (tube voltage) sets the beam energy: higher kVp gives a more penetrating beam and lower contrast (e.g. the high-kV technique preferred in chest radiography), while lower kVp gives high contrast in structures such as bone. mAs (tube current × time) sets the number of photons produced, hence image noise and patient dose.1

Key components

A radiography system includes: the X-ray tube and its generator; a collimator that limits the beam field; an anti-scatter grid that filters scattered radiation to improve contrast; the detector that captures the beam; and automatic exposure control (AEC) that terminates the exposure once the correct exposure is reached. A grid improves contrast but increases dose because it also absorbs part of the primary beam along with scatter; its use is balanced per exam.1

CR and DR detectors

Modern radiography is largely digital, with two main detector families. CR (computed radiography) uses a photostimulable phosphor plate that is scanned in a separate reader to produce a digital image. DR (digital radiography) uses a flat-panel detector that transfers the image directly and immediately to digital form. DR systems generally offer higher detector efficiency, which provides potential to lower dose while preserving image quality.1

Normal PA chest radiograph
A typical output: a normal posteroanterior (PA) chest radiograph.  Image: Wikimedia Commons · CC0 (public domain)

Dose profile

Radiography is a single-shot, usually low-dose exam. The average adult effective dose is about 0.02 mSv for a posteroanterior (PA) chest radiograph and about 0.04 mSv for a lateral chest radiograph.2 This corresponds to a very small fraction of natural background radiation (~3 mSv/year) and is markedly lower than cross-sectional exams such as CT.2 Still, "low dose" does not mean an unnecessary exam; every radiograph should be requested with clinical justification.3

Image type
2D projection
Typical effective dose
~0.02 mSv (PA chest)
Detector
CR / DR
Radiation
Ionizing (X-ray)

References

  1. U.S. FDA. Medical X-ray Imaging (radiography principles, components, detectors). fda.gov · Medical X-ray Imaging
  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. radiologyinfo.org
  4. Images: Dedicated chest x-ray room.jpg (CC BY-SA 4.0); Normal PA chest radiograph (CC0), Wikimedia Commons.
Note: Dose values are typical estimates and vary with technique, patient and device. For clinical decisions, consult a qualified medical physicist and current regulations.

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