Home Forums Other Specialities Radiology CT and MRI scan – The facts about Contrast Medium:

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    Not so long ago I needed a CT scan of the abdomen. 6 hours after the scan I developed severe itching of my skin over the back and neck. 4 further hours later I developed periorbital oedema. This was most unexpected as I have had a scan in the past without any side effects. Fortunately symptoms settled during the next 2 days through anthistamine tablets and low dose steroid. This episode prompted me in to wondering how many doctors were aware of the side effects that one may experience through the contrast medium that is used in a scan. This prompted me in to writing this paper.

    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. The contrast medium that is injected for a CT scan usually contains Iodine and the one for MRI contains Gadolinium.

    Why Use Contrast Medium:
    The contrast medium is radio opaque. This enables better definition of the image making 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.
    With the increase in the number of scans being performed, the number of people receiving contrast media is also increasing. 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 (ingested orally) was used with x-rays for investigating the gastrointestinal tract. It is still being used and reasonably safe. It may cause only minor side effects like constipation, nausea, abdominal pain or very rarely wheezing. 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 very small percentage of patients, doctors who prescribe the scan should be aware it. They should also make sure the patient is fully informed about the nature of material that will be injected and 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 absorbs x-rays. The radio opaque iodine concentrates in an organ that is being investigated. The area is therefore clearly visible in the scan making 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 the patient has had a CT scan in the past and developed some allergic reaction, the doctor must be made aware of it. If a scan had never been done 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 requested we should check for other medical problems that a patient may be suffering from. If they suffer from any of the following the scan should be differed or special precautions undertaken.
    The radiologist must be informed if a patient suffers from any of the following.

    • Poor Kidney function
    • diabetes
    • history of asthma and hay fever
    • history of heart disease
    • thyroid problems
    • sickle cell anemia/color]

    Normally base line blood tests would have been done to check for kidney function, diabetes, thyroid disease and sickle cell anaemia before the scan. The patient should also have had a routine check 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
    Gadolinium, a paramagnetic ion that moves differently under a magnetic field is used in a third of MRI scans. Gadolinium when used as a contrast medium in an MRI scan shows the body organs and tissues more clearly. 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 urticaria, facial and laryngeal oedema, broncho spasm and dyspnea.
    • Severe Reaction – life threatening anaphylactic shock – 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 may be undertaken. Some of them
    • Reducing the volume of contrast used
    • Intravenous infusion of isotonic saline (when patient is not in heart failure)
    • Isotonic sodium bicarbonate may sometimes be used
    • If the kidney function is affected significantly hemodialysis as a prophylactic measure may also be considered.

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