SPECT-CT scan: what it is, how it works and what to expect

SPECT-CT belongs to the family of nuclear medicine scans. Unlike an X-ray or standard CT, which show structure, a nuclear medicine scan shows function: how an organ or tissue is taking up a tracer. The tracer is a radiopharmaceutical, a medicine that carries a small amount of radioactive material and is taken up by particular parts of the body depending on the test.

A trust leaflet explains that SPECT images are taken with a gamma camera, which rotates around you and detects the radioactivity inside your body, displaying it as a picture. The CT images are taken to show exactly where in the body the tracer has been taken up. Combining the two lets specialists pinpoint an area of activity on a detailed anatomical map.

Nuclear medicine imaging is used in many parts of the body. A King's College Hospital cancer patient page lists bone, kidney, lung, thyroid and other nuclear scans, and says the tests can show where a tumour started, its size and structure, or whether disease has spread. The Newcastle leaflet says the combined SPECT-CT scan can give precise information about how different parts of the body are working and clearly identify problems. The tracer is chosen for the question being asked.

NICE guidance on diagnosing pulmonary embolism also names ventilation/perfusion SPECT as something to assess as an alternative to CT pulmonary angiography for people with an allergy to contrast media, severe renal impairment or a high risk from irradiation. That is an example of a SPECT technique being used for a specific clinical question.

Most people do not need to change anything. A trust leaflet advises eating and drinking as normal and continuing prescribed medicines unless told otherwise, wearing clothing without metal zips or clips if possible, and leaving valuables at home. A gown and changing facilities are provided if needed. Your appointment letter will contain any specific instructions.

People with diabetes are asked to read their letter carefully, because for some tests staff need to speak to you beforehand. Tell the team if you are or could be pregnant, or are breastfeeding. The Newcastle leaflet says SPECT-CT scanning of pregnant women should be avoided whenever possible, and that people aged 11 to 55 are routinely asked the date of their last period so staff can decide whether the test should be postponed.

You are met by a technologist or nurse, who explains what will happen. The tracer is injected into a vein in your arm or hand. Depending on the test you may be scanned straight away, or asked to wait for a few hours or even days so the tracer can reach the target area; your letter explains which applies.

For the scan you lie on a table while the gamma camera moves slowly around you, followed by the CT scan on the same scanner. You need to lie still because movement blurs the pictures. Leaflets describe the test as not uncomfortable. If an X-ray dye is given for the CT part, you may feel a warm sensation all over and a feeling of passing water, which the leaflet says pass within 30 seconds.

The additional amount of radiation from a SPECT-CT scan is small, according to the Newcastle leaflet, and modern equipment is designed to keep doses as low as possible. A King's College Hospital page says almost all of the tracer leaves the body within a few days. Your doctor will have judged that the benefit of the information outweighs the small risk.

King's College Hospital says you can have a nuclear scan if you are breastfeeding but should contact the department for advice beforehand. Departments may also give short-term precautions after your injection, which vary by test and trust, so follow the written advice you receive. The Newcastle leaflet says you can drive home afterwards.

A radiologist or physicist studies the images carefully, so results are not available on the day. The report is sent to the doctor who referred you. The Newcastle leaflet says this takes approximately 2 weeks, so unless other arrangements have been made you should expect to hear after that. Timescales differ between hospitals, so check your appointment letter.