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This Information has been written by the Lysholm Department of Neuroradiology at The National Hospital for Neurology and Neurosurgery (NHNN). The aim of this page is to provide information about examinations using contrast in x-ray, fluoroscopy, CT (Computed Tomography) and angiography. 

It is intended for use by patients (or their families or carers) who have been referred to our service for imaging requiring contrast agents. If you have any questions about this a member of the Neuroradiology team will be happy to answer them for you.

Contrast agents are used to enhance specific blood vessels, tissues or body cavities during your scan or procedure.

Contrast agents can come in many different forms; in neuroradiology these can be injected, or on occasion swallowed. Contrast agents are made of different compounds depending on what they are being used for. They are not dyes that permanently discolour internal organs. Contrast agents are substances that temporarily change the way x-rays or other imaging tools interact with the body.

In x-ray, CT, fluoroscopy and angiography we use iodine-based contrast agent.

Iodine is a naturally occurring chemical element. Iodine-based contrast agent is a colourless liquid that can be injected into veins or arteries, the fluid spaces of the spine, and into other body cavities. Iodine based contrast can also be taken orally.

Contrast agents are used to enhance the images produced when you are having a scan or procedure, helping to distinguish selected areas of the body from surrounding tissues. This helps the radiologist diagnose medical conditions or plan and carry out treatments.

The scans and procedures using contrast agents have helped in the diagnosis and treatment of many patients with various diseases and injuries.

It is important however, that the benefits of having a contrast enhanced scan or procedure outweigh the risks associated with the use of contrast. The radiographers performing your examination will ask you some questions and/or review your medical records to ensure it is appropriate for you to have contrast. In neuroradiology we follow current published guidance and use contrast agents with lowest possible associated risks.

If you have known hyperthyroidism, previous kidney problems or kidney failure or are currently taking a medication called metformin, please contact neuroradiology prior to your appointment.

Problems that may happen straight away

At the time of injection, it is usual to experience a warm feeling passing around your body. You may also experience a metallic taste. These side effects are temporary and will soon pass.

Contrast agents are safe drugs; however as with all drugs, they have the potential to cause an allergic reaction. Mild reactions include:

  • nausea and/or vomiting
  • warmth or flushing
  • mild rash
  • mild swelling of the skin/tissues
  • perspiration
  • altered taste
  • anxiety
  • nasal congestion
  • slight itchy throat
  • sneezing.

Mild reactions are common and occur in 1-10 in 100 patients.

Moderate reactions happen occasionally and include:

  • wide-spread rash
  • wide-spread skin redness
  • facial swelling
  • throat tightness or hoarseness
  • wheezing
  • chest pain.

Moderate reactions are uncommon and occur in 1-10 in 1,000 patients.

Severe life-threatening reactions are extremely rare and include:

  • wide-spread swelling
  • severe swelling of the throat
  • difficulty breathing or swallowing
  • irregular heartbeat
  • anaphylaxis (low blood pressure and fast heartbeat)
  • death.

Severe reactions are rare and occur in 1-10 in 10,000 patients. The department is equipped to deal with reactions in the rare event of this happening.


Extravasation is when a drug has accidentally leaked from the vein (injection site) into the surrounding skin or tissue. Extravasation is very rare but if left untreated it could lead to serious damage to the skin. It is important that you follow the advice given to you by the radiographer.

If this happens you would have reddening and a stinging sensation in your limb, a possible swelling of the arm or hand and some pain which usually wears off after about 30 minutes.

Sometimes you may need another injection of contrast and if necessary, this will be discussed with you.

We will give you an information sheet to take home with you should this occur.

Problems that may happen later

Some patients may experience delayed reactions in the hours following exposure to contrast, this is usually associated with milder reactions. Patients should seek medical advice from NHS 111, their GP, or local emergency department if necessary. Patients should also notify neuroradiology if they experience any reactions on 020 3448 3440.

Problems that are rare, but serious

In people whose kidneys are working normally contrast very rarely causes kidney problems.

If you have a history of kidney diseases or diabetes you should contact Neuroradiology before attending for your scan as you may require a blood test to assess your kidney function. We may not be able to complete your scan if the required blood test results are not available.

In people whose kidneys are not working normally and who are given a contrast injection, a temporary kidney injury may occur. This is known as CI- AKI (Contrast Induced – Acute Kidney Injury) or PC-AKI (Post Contrast – Acute Kidney Injury). If this were to happen, the majority of people would not feel any different than usual, and the injury would only be apparent if a kidney function blood test was performed. If you were to experience symptoms, they would be similar to those of kidney disease, which include feeling more tired, poor appetite, swelling in the feet and ankles, puffiness around the eyes, or dry and itchy skin.

In the vast majority of cases the kidney function returns to a normal value after a few days but very occasionally the kidney injury may affect your health. Although very rare, there are occasions where it may be necessary to support your kidneys on a dialysis machine.

If your kidneys are not working normally the referring doctor may advise precautions such as extra fluids or avoiding certain medications around the time of your scan. If your kidney function is particularly low, the risk of significant kidney injury is higher and we may not give you a contrast injection. We may perform, where possible, an alternative scan where iodine-based contrast is not needed.

If you wish to discuss why you need to have contrast or if you have any concerns about possible risk factors, you can do so with your referring doctor or clinical nurse specialist. If you do need to have contrast then make sure you follow any instructions before, during or after the procedure.

If you do not wish to have contrast, this is your choice and you will not be treated any differently.

Contrast is given to help distinguish between normal and abnormal tissues and organs; this will give your doctors the best opportunity to make a diagnosis and to plan your future management and care.

Your doctor or clinical nurse specialist will have taken into account the potential risks and benefits of having a scan or procedure involving contrast when referring you to neuroradiology.

There are some alternative scan methods in MRI or ultrasound that do not use contrast agents, the possibility of these should be discussed with your referring doctor or clinical nurse specialist first.

Please contact neuroradiology prior to your scan should you have:

  • known or suspected hyperthyroidism
  • hyper-functioning thyroid nodule with or without associated multinodular goitre
  • are expecting to undergo radioisotope scanning of your thyroid.

We may need to refer you to an endocrinologist for review. If you have untreated hyperthyroidism or a hyper-functioning thyroid nodule as you may be at an increased risk of developing thyrotoxicosis following the use of iodinated contrast. 

Eating and drinking guidance

  • Before your scan or procedure please see your specific appointment letter.
  • Gently increase your intake of fluids starting the day before your scan and continuing for another 24 hours after the scan to ensure you are well hydrated.

Medication guidance 

  • Please contact us if you take metformin.
  • Take other medications as normal, before and after contrast, unless advised otherwise.
  • Contact neuroradiology if you are unsure about any of your medications.

If possible, aim to arrive shortly before your appointment to give staff time to prepare you for your scan or procedure.

The length of your scan or procedure will vary depending on which type of examination you are having, please see your appointment letter or associated information leaflet for further information.

We want to involve you in all the decisions about your care and treatment. If you decide to go ahead with a contrast scan or procedure, by law we must ask for your consent. This confirms that you agree to have the procedure and understand what it involves.

Staff will explain all the risks, benefits and alternatives before they ask you to verbally consent to contrast. If you are unsure about any aspect of your proposed scan or procedure, please don’t hesitate to speak with a senior member of staff again.

You will be given contrast before or during your scan or procedure. If we give you oral contrast, we will ask you to drink the liquid before or during the examination.

If we are injecting contrast before or during your scan, then we will give the contrast through a cannula or small plastic tube, placed in a vein in your limbs or through an artery in your groin, or wrist, depending on the type of scan/procedure you have. This cannula will remain in the vein or artery for the duration of your scan.

The radiographer and/or radiologist will observe you throughout the duration of your scan and inform you when they are about to inject the contrast through the cannula, this may be done by hand or by an automatic pump injector.

If you have had an injection of iodine-based contrast, we will ask you to remain in the neuroradiology department for up to 30 mins after your scan or procedure unless you are an inpatient where you will return to the ward.

We will keep the cannula in place during this time, in case we need to give you any further medication. After this, our staff will check you feel well enough to leave the department.

Lysholm Department of Neuroradiology

National Hospital of Neurology and Neurosurgery

Queen Square




Direct line: 020 3448 3440

Switchboard: 020 3456 7890

Extension: 83440 / 83103

Fax: 020 3448 4723

The Lysholm Department of Neuroradiology reception is located in Chandler wing, on the lower ground floor of the National Hospital for Neurology & Neurosurgery, Queen Square.

Please turn left when you exit the Chandler wing lifts on the lower ground floor to find our main Neuroradiology reception. 

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Page last updated: 28 May 2024

Review due: 01 May 2026