Medical imaging: CT, MRI, PET scan or ultrasound — which test for which situation?

There is no “best test” for every situation

Imaging answers a clinical question. Ultrasound may be ideal for the gallbladder or a pregnancy, MRI for many neurological or joint structures, CT for an injury or certain emergencies, and PET/CT for certain metabolic questions in oncology. The right test is the one that provides the useful information with the least unnecessary risk and delay.

CT: fast and very useful in emergencies

CT uses X-rays and reconstructs cross-sectional images. It is fast and can examine the chest, abdomen, bones, brain and blood vessels. An iodine-based contrast agent may be used to show certain structures better. The doctor then checks relevant allergies, kidney function in certain situations and any possible pregnancy.

As it uses X-rays, the indication and protocols aim to obtain the necessary information with an appropriate dose.

MRI: a magnetic field, not X-rays

MRI uses a powerful magnetic field and radio waves. It is particularly effective for the brain, spinal cord, many joints, the liver, the pelvis and other indications. A pacemaker, an implant, metal fragments or a prosthesis must be reported: many modern devices are compatible under certain conditions, but this must be checked.

The test takes longer; claustrophobia, an inability to keep still or kidney failure when a specific contrast agent is planned require precautions.

Ultrasound: dynamic and radiation-free

Ultrasound uses sound waves. It allows organs to be observed in real time, blood flow to be studied with Doppler and procedures to be guided. However, its quality depends on the area examined, the patient’s body build and the operator’s experience.

It is very useful for many abdominal organs, the thyroid, superficial tissues, obstetrics and the study of blood vessels.

PET/CT: metabolic and anatomical information

PET uses a radiotracer and is often combined with CT. It is used in particular in certain cancers for staging, recurrence or response to treatment, but it is not suitable for all tumours.

It does not replace pathology and should not be used as untargeted general screening.

Relying on appropriateness criteria

Reference frameworks such as the American College of Radiology Appropriateness Criteria (ACR Appropriateness Criteria) link symptoms, context and possible tests. This approach avoids redundant tests and helps choose the modality most likely to answer the medical question.

Why is contrast sometimes necessary?

Contrast agents improve the visibility of blood vessels or tissues. Iodine-based contrast is mainly used in CT; gadolinium is used in some MRI scans. The radiologist chooses according to the question and checks the specific precautions.

Frequently asked questions