MIBG scan: what it looks for, the two-day process and how to prepare

MIBG is the short name for a chemical, metaiodobenzylguanidine, that is attached to a radioactive isotope. Once it is injected into a vein, the isotope travels around the body and sticks to certain abnormal areas. A gamma camera then picks up the signal, and those areas show as bright spots, sometimes called hot spots, on the pictures. Great Ormond Street Hospital describes the scan as a way of looking for uncontrolled or abnormal cell growth across the whole body.

Because the tracer is taken up by neuroendocrine cells, the scan is used for a particular group of tumours. Cambridge University Hospitals names suspected neuroblastoma and phaeochromocytoma as examples, and says the scan is often required for cancer staging. Staging means working out how far a cancer has spread, which in turn helps the specialist team plan treatment.

This is a nuclear medicine test rather than an X-ray or MRI. It shows how tissues behave and take up the tracer, rather than giving a detailed picture of shape and size. That is why the result is usually read together with other imaging.

An MIBG scan is requested by a specialist, not as a general health check. The cases described in the leaflets are children and adults in whom a neuroendocrine tumour is suspected or already known. Great Ormond Street explains that CT, ultrasound and X-rays can all be used to look for abnormal cell growth, but each is specific to one area of the body, so they cannot look at the entire body in one scan.

The scan is rarely used alone. The same hospital says it is usually done in conjunction with an MRI scan and an ultrasound scan to build an accurate and detailed picture, and that the results of all three are used to plan and track treatment. If you are being offered an MIBG scan, it is reasonable to ask the referring team which question the scan is meant to answer and how its result will be combined with your other tests.

Preparation differs between hospitals, so the instructions in your own appointment letter take priority over anything general. The leaflets we read agree on the main themes.

• Thyroid-blocking tablets. The thyroid gland can absorb the iodine in the tracer, so a thyroid-blocking medicine is given first. In the adult leaflet from Cambridge you collect six potassium iodide tablets: two about 30 minutes before the day 1 appointment, two in the evening of day 1 and two on the morning of day 2. Great Ormond Street describes three doses of potassium iodate for children, at lunchtime on the day before the injection, on injection day and on scan day. It stresses that taking all the doses is vital.

• Medicines. The Cambridge leaflet asks you to contact the department if you take tricyclic antidepressants or blood-pressure medicines, naming labetalol, diltiazem, nifedipine, verapamil and amitriptyline as examples. Bring a list of your medicines and your repeat prescription so the team can review your preparation. Do not stop any medicine unless the department tells you to.

• Clothing. Avoid clothes with metal buttons. You may be asked to remove metal objects such as buckles, coins, jewellery, buttons and zips while pictures are taken.

• Mobility. If you have mobility issues, tell the department before you attend so they can make arrangements.

• Children. Great Ormond Street says a child under five, or one who would find it hard to lie still for two hours, will probably need sedation or a general anaesthetic for the scan.

The Cambridge leaflet sets out the adult timeline. On day 1 you take your first tablets, then arrive and check in, and a small injection of radioactive tracer is given into a vein in your arm. You are given a time to return. There is a waiting period of four hours before the first scan, and you can leave the department during that time. When you return, you lie on your back on an imaging bed while the camera moves slowly from your head down your body, imaging the whole body. You need to lie still for up to 30 minutes.

On day 2 you take your tablets in the morning and attend again. Whole-body images are repeated, and an additional SPECT/CT is done. Great Ormond Street describes SPECT-CT as an additional scan that gives images in 3D, which can show structures at different depths in the body that might otherwise be hidden. The Cambridge leaflet says you will need to lie still for up to one and a half hours. Allow time for the team to check that the images are good enough before you leave.

For children, Great Ormond Street describes the injection as taking about five minutes and as potentially uncomfortable, because it is given slowly in case of a reaction such as a sudden drop in blood pressure. If a child has a central venous catheter, it can be used instead of a cannula.

Cambridge University Hospitals explains that a specialist in radiology agrees the test is the best way to answer the clinical question and that the benefit is greater than the risk. The dose is kept as low as practicable. The leaflet adds that ionising radiation can in principle cause cell damage, but that the risk from the examination is considered very low.

As a standard precaution, the trust recommends avoiding radioactive tests in pregnancy and notes that some radioactive substances can pass into breast milk in small quantities. Anyone aged 12 to 55 will be asked about the possibility of pregnancy. If you are pregnant, think you might be, or are breastfeeding, contact the department before the appointment. Great Ormond Street adds that a pregnant parent should let the team know at least two days before a child's injection, because they advise another adult to help look after the child for the first 48 hours afterwards.

Radiation monitors at airports can detect very small residual amounts. Cambridge advises keeping your appointment letter if you are travelling within 14 days of the scan, and says the department can provide a signed travel letter if told in advance.

Companions are limited because of radiation protection. The Cambridge leaflet asks you to bring no more than one adult, and says children and pregnant women cannot accompany you into the department.

Images are reported in order of clinical priority, so the most urgent are reported first. The team that referred you is responsible for passing the report back to you, and a result is not normally given at the scan appointment. If you have an online patient record, a report may appear there before your clinician has discussed it, so it can help to ask in advance how and when you will be told.

A report from an MIBG scan will usually be interpreted alongside your CT, MRI or ultrasound pictures and your blood or urine tests. A single bright area does not on its own establish a diagnosis, and an absence of signal does not always rule one out. That is a question for the specialist team rather than something to interpret from the written report alone.