Information for the General Public
CT & MRI Scan - The Facts You Must Know: The number of people who are being subjected to a CT or MRI scan is increasing every day. Many of the scans involve the use of a contrast medium ie a chemical that is injected in to the blood stream just before a scan. Why Use Contrast Medium: The contrast medium is radio opaque. This makes it easier for the radiologist to see blood vessels, tumour, or any other abnormal pathology in an organ. The contrast therefore improves the diagnostic accuracy of the scan. Most people assume that a scan is like an x-ray as it is non- invasive and do not think twice about it. Very few patients are aware that a contrast medium will be injected during the procedure. It is therefore necessary to know if the chemicals injected can cause any harm. Before the days of CT and MRI scans Barium Sulphate taken by mouth was used with x-rays for investigating the intestines.  It is still being used and may cause only minor side effects. The contrast media that is injected now for a CT or MRI scan can cause more serious side effects. Although the side effects are seen only in a few, anyone who is listed for a scan should know the possible side effects from the contrast used. CT Scan The CT scan like x-rays emit radiation that is slightly more than during a plain x-ray examination. The contrast medium used during a CT scan contains iodine which concentrates in an organ and makes it easy to see abnormal pathology in and around the region. Although reactions to the contrast material are rare there is a small risk that they can cause allergic reactions or other medical problems. If you have not had contrast injected in the past it will be necessary to have a small test dose of the chemical injected in to the skin prior to the procedure to rule out serious allergic reactions. Most reactions if they occur are mild and result only in a rash or itchiness. In rare instances the reaction can be serious and even life-threatening. Before a scan is done you should inform your doctor of other medical problems that you may be suffering from. If you suffer from any of the following the scan should be differed or special precautions should be undertaken: Poor Kidney function diabetes  history of asthma and hay fever  history of heart disease  thyroid problems  sickle cell anemia Before the scan you should have had blood tests to check for kidney function, diabetes, thyroid disease and sickle cell anaemia. Your doctor should also have checked you for high blood pressure, heart and respiratory function. Of all the medical problems, poorly functioning kidney is the most serious. The iodine based contrast medium can damage the kidney further and may lead to total kidney failure. In a patient suffering from diabetes with poor kidney function the diabetes can actually increase the risk of developing radiocontrast nephropathy. MRI Scan  MRI scan does not require x-rays and hence you are not subjected to radiation. The contrast medium is called Gadolinium. They are safer than iodinated contrast medium and allergic reactions are seen in less than 0.1% of cases. Severe reactions are rare. The only medical condition which can cause a serious problem is a poorly functioning kidney.  The contrast medium in such a kidney can contribute to a condition called Nephrogenic Systemic Fibrosis which affects skin, muscles, joints and internal organs. There is no need to worry if the kidney function is normal. Awareness of the risk factors and early recognition of the reaction would help in prompt and effective treatment. Side effects of Contrast media that you should be aware of: Immediate Effect: Mild Reactions – skin itching, rashes, nausea and vomiting. Moderate Reaction – persistent vomiting, dizziness, spreading urticarial, facial and laryngeal oedema, broncho spasm and dyspnea. Severe Reaction – life threatening anaphylactic reaction - Significant changes in BP (low or high), cardiac arrhythmias, laryngeal/pulmonary oedema, cardiac arrest. Delayed Reaction: Mild – skin pruritus and rash 2 to 12 hours after the injection, nausea and vomitting. Moderate to Severe – Thyroid dysfunction (Hypo or Hyper), Nephropathy ranging from moderate to severe kidney failure. If the Kidney Function is not normal should you avoid a Scan altogether? Being at increased risk for an allergic or adverse reaction to contrast material does not necessarily mean a patient cannot undergo an imaging exam with contrast materials. As we get older the kidney function does suffer a little. Even if the kidney is functioning at 50% capacity you should be able to go ahead with a scan taking a little care to protect the kidney. To help protect the kidney, medications are sometimes given before the contrast material is administered. Generally, it is a good idea to increase the fluid intake before and after an imaging exam to help excrete the contrast material from the body quickly.  During pregnancy both CT and MRI scan should generally be avoided. When the kidney function is impaired and it is still necessary to go ahead with a scan some kidney protecting measures will be undertaken by the doctor. Some of them are: Reducing the volume of contrast used Intravenous saline (when patient is not in heart failure) Intravenous sodium bicarbonate. If the kidney function is affected significantly hemodialysis as a prophylactic measure may also be considered. K.Badrinath