[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Radiology](https://durgical.com/en/radiology/) › [Principles and safety](https://durgical.com/en/radiology/areas/principles/) › How each imaging study makes a picture: X-ray, CT, ultrasound, MRI and nuclear medicine

# How each imaging study makes a picture: X-ray, CT, ultrasound, MRI and nuclear medicine

- Modality

Each scan builds its picture from something different: X-rays passing through, sound echoes, a magnet, or a tracer that glows from inside the body.

Updated Nov 12, 2026

## Key points

- X-ray and CT: attenuation; more absorption is whiter.
- Ultrasound: echoes, no radiation, blocked by gas and bone.
- MRI: protons in a magnetic field, no ionizing radiation, best soft-tissue contrast.
- Nuclear medicine: the tracer is the source; it shows function rather than fine anatomy.

## What it is

Every imaging study turns one physical signal into shades of grey. Plain X-ray and CT both use X-rays: tissue that absorbs (attenuates) more of the beam looks whiter. A radiograph squeezes the whole body part into one flat shadow; CT spins the tube around the patient and a computer rebuilds thin cross-sectional slices. Ultrasound sends sound pulses from a probe and draws the echoes that bounce back. MRI places the patient in a strong magnetic field, tips the hydrogen protons with radiofrequency pulses and records the signal they give off as they relax. Nuclear medicine works the other way round: a radioactive tracer is given and a camera detects the gamma rays coming out of the body, so it shows function more than anatomy. Knowing how each picture is made explains what each study sees well, what it misses and what it costs the patient in radiation, time and risk.

## How the image is made

- Radiograph: an X-ray tube fires through the patient onto a detector; bone and metal absorb most and look white, air absorbs least and looks black.
- CT: the tube and detectors rotate around the patient; a computer reconstructs slices and gives each pixel a density in Hounsfield units (water 0, air about −1000).
- Ultrasound: a probe with piezoelectric crystals sends sound pulses and listens for echoes; fluid is black, strong reflectors (bone, stones, gas) are bright.
- MRI: a strong magnet aligns hydrogen protons, radiofrequency pulses disturb them, and the signal they return as they relax builds T1 and T2 images.
- Nuclear medicine: a tracer taken up by an organ emits gamma rays that a gamma camera records (planar or SPECT); PET detects the paired photons from a positron-emitting tracer.

## What it is good for

- Radiograph: fast, cheap and everywhere; first study for the chest, bones and many abdominal questions.
- CT: detailed anatomy in minutes; the workhorse of trauma, the acute abdomen, stroke and pulmonary embolism.
- Ultrasound: no radiation, at the bedside; gallbladder, pregnancy, pelvis, vessels, free fluid and guiding procedures.
- MRI: the best soft-tissue contrast; brain, spinal cord, joints, soft-tissue tumours and the pelvis.
- Nuclear medicine and PET: physiology (perfusion, bone turnover, thyroid uptake, glucose use in tumours).

| Study | Signal | Ionizing radiation | Strength | Main limit |
| --- | --- | --- | --- | --- |
| Radiograph | X-ray attenuation | Yes, low | Fast, cheap | Overlapping shadows |
| CT | X-ray attenuation in slices | Yes, higher | Detail, speed | Radiation, contrast |
| Ultrasound | Sound echoes | No | Bedside, real time | Gas, bone, operator |
| MRI | Proton signal in a magnet | No | Soft tissue | Time, implants, cost |
| Nuclear medicine / PET | Gamma rays from a tracer | Yes | Function | Low anatomic detail |

## Limitations and risks

- Radiographs and CT use ionizing radiation; a CT gives far more than a plain film, so it must be justified.
- A radiograph is a summation shadow: structures overlap, so two views or a cross-sectional study may be needed.
- Ultrasound cannot see through gas or bone and depends heavily on the operator and the patient's body habitus.
- MRI is slow, noisy and confining; some implants, pacemakers and metal fragments make it unsafe until checked.
- Nuclear studies carry radiation from the tracer and show function at low spatial detail.

## Next step

- Start with the simplest study that can answer the question, often a radiograph or ultrasound.
- Choose ultrasound or MRI first in children and in pregnancy when they can answer the question.
- Before MRI, screen for implants, devices and metal; before contrast, ask about previous reactions and kidney function.
- When unsure which study answers the clinical question, call the radiologist with the question, not just the body part.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does bone look white and air look black on a radiograph?

Bone absorbs (attenuates) most of the X-ray beam, so little reaches the detector; air absorbs almost none, so the detector is fully exposed and appears black.

## Which two common imaging studies use no ionizing radiation?

Ultrasound (sound waves) and MRI (a magnetic field and radiofrequency pulses).

## What is the Hounsfield value of water and of air on CT?

Water is 0 and air is about −1000 Hounsfield units; fat is negative and bone is strongly positive.

## How does nuclear medicine differ from X-ray imaging in where the radiation comes from?

In nuclear medicine the radiation comes from a tracer inside the patient and is detected outside; in X-ray imaging it comes from a tube outside and passes through the patient.

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## Connections

## Related 6

- [The five densities on an X-ray](https://durgical.com/en/radiology/the-five-densities-on-an-x-ray/)
- [Radiation in medical imaging: risk, ALARA and pregnancy](https://durgical.com/en/radiology/radiation-in-medical-imaging-risk-alara-and-pregnancy/)
- [CT: windows, Hounsfield units and contrast phases](https://durgical.com/en/radiology/ct-windows-hounsfield-units-and-contrast-phases/)
- [Ultrasound: echogenicity, probes, Doppler and artefacts](https://durgical.com/en/radiology/ultrasound-echogenicity-probes-doppler-and-artefacts/)
- [MRI: T1, T2, FLAIR and diffusion](https://durgical.com/en/radiology/mri-t1-t2-flair-and-diffusion/)
- [Nuclear medicine and PET: how they work](https://durgical.com/en/radiology/nuclear-medicine-and-pet-how-they-work/)

[← All of radiology](https://durgical.com/en/radiology/)

Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: [X-ray Production Technical Evaluation](https://www.ncbi.nlm.nih.gov/books/NBK564332/) (StatPearls (NCBI Bookshelf), 2022); [Computed Tomography Instrumentation and Physics](https://www.ncbi.nlm.nih.gov/books/NBK597347/) (StatPearls (NCBI Bookshelf), 2026); [Diagnostic Ultrasound Imaging: Physics Principles and Clinical Applications](https://www.ncbi.nlm.nih.gov/books/NBK567710/) (StatPearls (NCBI Bookshelf), 2026); [Nuclear Medicine Instrumentation](https://www.ncbi.nlm.nih.gov/books/NBK597384/) (StatPearls (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.

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