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Diagnosis and tests

Ultrasound, CT, MRI, PET

Four abbreviations, four different methods. One uses sound waves, one X-rays, one a magnet and one a small amount of radioactive sugar. This topic calmly explains how each scan works, what you will feel, how to prepare and what to look out for.

Last updated: ~5 min read
Four translucent glowing symbols in a row on a dark navy background: an ultrasound probe sending sound waves, a CT ring, an MRI tunnel and a PET ring filled with glowing dots, each giving off light of a different colour.
Figure 1.Concept: four methods, four different kinds of light.

01Short answer

Ultrasound creates images of organs with high-frequency sound waves; there is no radiation, it is painless, takes 15–30 minutes and is used for the spleen, liver, kidneys, lymph nodes, veins and heart (echocardiogram). CT (computed tomography) uses X-rays to take thin slices through the body; it is very quick (a few minutes) and shows the lungs and bones very well, but the radiation dose is higher than for an X-ray and iodine contrast is sometimes given. MRI (magnetic resonance imaging) works with a strong magnet and radio waves; there is no radiation and it shows the brain, spinal cord and soft tissues best, but it takes 20–60 minutes, is noisy and may not be compatible with metal devices. PET (positron emission tomography) is usually done together with CT: a small amount of radioactive sugar (FDG) is injected into a vein and shows how active cells are; in leukemia it is used only in selected cases, for example when transformation of CLL is suspected. Which scan is chosen depends on the question, safety and your condition, and is decided by your doctor.

02Simple explanation

Ultrasound works like a ship’s sonar: the device sends out sound and builds a picture from the echo. Gel is put on the skin; it is painless and radiation-free.

CT is a table that moves through a large ring. X-rays capture the body in thin “slices” and a computer joins them up. It takes a few minutes.

MRI is a strong magnet shaped like a long “tunnel”. There is no radiation, but the machine is loud; you are given headphones. You need to lie still for a long time.

PET involves injecting a little “labelled” sugar; more active cells take up more of it and glow on the image. It is rarely needed in leukemia.

Each method has strengths and weaknesses. Your doctor chooses the one best suited and safest for your question.

03Scientific explanation

Ultrasound. According to MedlinePlus, ultrasound uses high-frequency sound waves and involves no ionising radiation. Doppler mode shows blood flow — useful when choosing a vein for a line or when a clot is suspected. An echocardiogram measures how well the heart pumps (ejection fraction) and is used before and during treatment with some drugs, such as anthracyclines.

CT. According to the NCI, CT uses a computer to combine many X-ray images into cross-sections. It shows small areas of infection in the lungs, the sinuses and lymph nodes well. The radiation dose depends on the type of scan; doctors try to avoid unnecessary repeats. Iodine contrast occasionally causes allergy or affects kidney function, so kidney blood tests may be checked first.

MRI. The NHS explains that MRI uses a strong magnetic field and radio waves. It shows the brain, spinal cord, bone marrow and the soft tissues of joints in great detail. Some pacemakers, cochlear implants and metal fragments can be dangerous; modern “MRI-conditional” devices may allow scanning under special settings. Gadolinium contrast is used with caution in kidney failure.

PET-CT. According to the NHS, a small amount of a radioactive tracer, usually FDG (a glucose analogue), is injected for PET; active cells take up more of it. The tracer decays over a few hours. Fasting beforehand is important, as is blood sugar control in diabetes. PET is not routine in leukemia; it is used when a change from CLL to a more aggressive lymphoma is suspected and in some other specific situations.

04What happens in the body?

Ultrasound: what happens?

You lie on a couch, cool gel is spread on your skin and the operator moves a small probe over it. Before an abdominal ultrasound you sometimes need to fast for a few hours, and for the bladder to drink water. It is painless; if the spleen is enlarged, the pressure can feel slightly uncomfortable.

CT: what happens?

You lie on a table that moves through a ring-shaped machine. You may be asked to hold your breath briefly. When contrast is injected into a vein you may feel a brief warm sensation and a metallic taste — this usually passes quickly.

A translucent ring-shaped scanner on a dark navy background; thin slices of light are lined up one after another over a translucent human silhouette passing through it; no face.
Figure 2.CT captures the body in thin slices.

MRI: what happens?

You lie still inside a tunnel for 20–60 minutes. The machine makes knocking and humming noises; you are given headphones or earplugs. If you are afraid of enclosed spaces, say so beforehand — a mild sedative or an open scanner may be possible. All metal objects are removed.

A long softly lit translucent tunnel on a dark navy background, surrounded by gently waving magnetic field lines, with an empty comfortable bed inside; no people.
Figure 3.MRI: a calm magnetic tunnel.

PET-CT: what happens?

You arrive having fasted. The tracer is injected into a vein, then you rest quietly for about an hour while it spreads through the body. Then you lie in the scanner for 20–40 minutes. Afterwards you may be advised to drink plenty of water and to avoid close contact with small children and pregnant women for a short time that day.

Children and sedation

Young children find it hard to lie still for long. Light sedation or a general anaesthetic may be used for MRI and sometimes CT. Children’s nurses and play specialists can help prepare a child beforehand.

05Four methods, four physical principles

Sound, X-rays, a magnet and labelled sugar.

Ultrasound — Sound waves; no radiation.1UltrasoundSound waves; no radiation.
CT — X-ray slices; fast.2CTX-ray slices; fast.
MRI — Magnet and radio waves; detailed.3MRIMagnet and radio waves; detailed.
PET — Labelled sugar shows active cells.4PETLabelled sugar shows active cells.
The right choiceA method that fits the question.
SafetyRadiation and contrast kept in check.
Calm preparationYou know what to expect.

06The four scans compared

ScanHow it worksDurationRadiationShows best
UltrasoundSound waves15–30 minNoneSpleen, liver, veins, heart
CTX-raysA few minutesYesLungs, sinuses, bones, lymph nodes
MRIMagnet and radio waves20–60 minNoneBrain, spinal cord, soft tissues
PET-CTRadioactive tracer + CT2–3 hours with preparationYesCell activity (in selected cases)

* Durations and preparation rules depend on the hospital. Get your own instructions from the place where you will have the scan.

07Key terms

Ultrasound
Imaging with sound waves.
Doppler
An ultrasound mode that shows blood flow.
CT
Computed tomography — cross-sections made with X-rays.
MRI
Magnetic resonance imaging — images made with a magnet and radio waves.
PET
Positron emission tomography — images of cell activity using a tracer.
FDG
A radioactive glucose analogue used in PET.
Contrast dye
A liquid that makes vessels and organs clearer (iodine- or gadolinium-based).
Sedation
Mild drowsiness and calm brought on by medicine.

08Key points

  • Ultrasound and MRI involve no radiation.
  • CT is fast and shows the lungs very well, but uses radiation.
  • Before MRI, mention every metal device and implant.
  • PET is used in leukemia only in selected cases.
  • Before contrast, mention allergies and kidney problems.
  • Check the preparation rules with your hospital.
  • If enclosed spaces worry you, say so in advance.

09Frequently asked questions

I have a central line (PICC or port). Can I have an MRI?

Most modern lines and ports are MRI-compatible, but the team should check this against the device documents. If you have a card showing the type of line or port, bring it.

Will I be radioactive after a CT scan?

No. In CT, X-rays are used only during the scan and nothing stays in the body. After PET, the tracer gradually decays and leaves the body over a few hours.

My platelets are low. Is a contrast injection dangerous?

Contrast is given through an ordinary IV needle or an existing line. With low platelets, the needle site may need to be pressed a little longer. Procedures such as biopsies are planned separately.

What if I might be pregnant?

If pregnancy is possible, always say so. Methods without radiation (ultrasound, sometimes MRI without contrast) may then be preferred.

Who will explain the result?

A radiologist reads the images and writes the report, but it is your hematologist who links it to your treatment. Discuss the result with your own doctor.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. I have a central line (PICC or port). Can I have an MRI?

  2. Will I be radioactive after a CT scan?

  3. My platelets are low. Is a contrast injection dangerous?

  4. What if I might be pregnant?

  5. Who will explain the result?

My questions

12Sources

  1. American Cancer Society. Imaging (Radiology) Tests for Cancer
  2. National Cancer Institute (NCI). Computed Tomography (CT) Scans and Cancer
  3. MedlinePlus (U.S. National Library of Medicine). CT Scans
  4. MedlinePlus (U.S. National Library of Medicine). MRI Scans
  5. MedlinePlus (U.S. National Library of Medicine). Ultrasound
  6. MedlinePlus (U.S. National Library of Medicine). Nuclear Scans
  7. NHS. CT scan
  8. NHS. MRI scan
  9. NHS. Ultrasound scan
  10. NHS. PET scan
  11. RadiologyInfo.org (RSNA and ACR). Radiation Dose in X-Ray and CT Exams
  12. American Cancer Society. Tests for CLL
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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