Lung perfusion scan: what it shows, how it is done and what to expect

Perfusion means blood flow. A lung perfusion scan is a nuclear medicine test that creates a picture of how well blood is reaching different parts of the lungs. A hospital leaflet explains that nuclear medicine means the use of radioactive tracers to diagnose, and sometimes treat, medical conditions.

A perfusion scan can be done on its own or together with a ventilation scan, which looks at air reaching the lungs. Together they are often called a V/Q scan. The two sets of pictures are read together by the reporting specialist.

The most common reason is a suspected blood clot in the lungs. The Cambridge University Hospitals leaflet states that the lung ventilation and perfusion scan is used to detect pulmonary embolism, a blood clot in one of the blood vessels supplying the lungs. NICE guidance on diagnosing venous thromboembolic disease says that for people with a likely pulmonary embolism, CT pulmonary angiography is offered, but that for those with an allergy to contrast media, severe renal impairment or a high risk from irradiation, the suitability of a V/Q SPECT scan, or a planar V/Q scan if SPECT is not available, should be assessed as an alternative.

A perfusion scan is also used for a different reason: a lung perfusion quantification scan is described by the same hospital as a test to evaluate the function of your lungs. The leaflet does not go further than that, so ask the referring team what question the scan is answering for you.

The leaflets say no special preparation is needed. You can eat and drink normally and carry on taking medicines. For the scan itself you are asked to remove only the metal objects around your chest, and the quantification scan can usually be done without removing clothing. If you have mobility problems, let the department know beforehand so that arrangements can be made.

Tell the department if you are or might be pregnant, or are breastfeeding. For pregnant patients, the leaflet says it is very common to perform a slightly altered version of the ventilation and perfusion test, and for breastfeeding patients feeds have to be stopped for 13 hours after the test because small amounts of some radioactive substances may appear in breast milk. The lung quantification leaflet says it is usual to avoid radioactive tests during pregnancy.

On arrival you are given a small injection of a radioactive tracer into a vein in your arm. The leaflet says it is no more than a blood test, with no side effects from the tracer injection. You then lie on a bed while a camera takes images of the lungs. You are not put in a tunnel, but you need to lie still, for up to 30 minutes for the ventilation and perfusion scan or up to 5 minutes for the quantification scan.

For the combined test you are also asked to wear a mask and breathe in air that is slightly radioactive, which the leaflet says will not taste of anything. That is the ventilation part. The quantification scan described by the same hospital does not mention a mask.

The leaflet explains that nuclear medicine procedures use ionising radiation, which carries a small risk that, after many years, can contribute to cancer, and that this test carries only a very small chance of this happening. The doses are optimised by a team of clinical scientists and technologists, and your doctor will have considered the benefits and risks before referring you.

The leaflet also notes that the tracer pharmaceutical is prepared from human serum albumin derived from human blood donations, which may not align with certain religious beliefs. If this matters to you, raise it with the department before the appointment.

When the department has checked that the images are good enough, you can leave. There are no restrictions after the test, and you can go home, return to work or go back to your ward. Results are reported and are available from your referring clinician, team or GP within 3 weeks of the appointment, according to the leaflet. The leaflet adds that airports have sensitive radiation monitors, so people travelling within 4 days of the scan are advised to keep their appointment letter, and the department can provide a travel letter.