Nuclear Medicine Thyroid Scan
A nuclear medicine thyroid scan uses a radioactive medication (radiopharmaceutical) to take pictures or images of the thyroid gland. The…Read more
Iodine-containing contrast media (ICCM) are widely used in medical imaging to make hollow structures, such as blood vessels, the gastrointestinal tract, the subarachnoid space around the brain and spinal cord, and the interior of joints, visible on imaging studies like CT scans, angiograms, myelograms and arthrograms. ICCM can also be used to assess organ perfusion (e.g. the brain) and to reveal disease in solid organs, such as the liver. For example, liver tissue affected by metastatic disease has differential ICCM uptake when compared with normal liver parenchyma.
The main prerequisite for ICCM injection involves risk assessment for:
A simple patient questionnaire created by the Royal Australasian and New Zealand College of Radiologists will be given to the patient before the procedure. Referrers can download, print and refer to this questionnaire, enabling them to note any relevant clinical information reflecting these questions on the referral form. The practice or hospital should be made aware of any patient risks at the time of scheduling of elective procedures/tests.
These questions are:
There are no absolute contraindications. For each patient, the risks of ICCM administration by any route must be weighed against the perceived clinical benefit to the patient of the diagnostic or therapeutic procedure. See risk factor discussion above.
Asthma, multiple allergies, severe (eGFR <30 mL/min) renal function impairment and current hyperthyroidism (clinical or biochemical) increase the risk of ICCM administration. Beta-blockade can reduce or eliminate the effectiveness of adrenaline in the event of an anaphylactic reaction.
These relative contraindications to ICCM administration need to be notified to the medical imaging practice/hospital because of the increased risk for the patient and to identify the patient as potentially requiring additional preparation or monitoring after ICCM administration.
Allergic-type/anaphylactic reactions occur in less than 3% of patients. These almost always occur within minutes to 1 hour after administration of ICCM given intravenously or intra-arterially, and are less common after ICCM given by other routes (orally or intra-articularly, for example). Mild reactions (itching, a mild rash, sneezing or vomiting) require no treatment or antihistamines only. Moderate reactions can occur in 1 in 3000 people, and present with generalised rash and anaphylactic oedema. This requires intravenous adrenaline and steroids. Severe reactions occur in 1 in 25,000 patients, requiring adrenaline administration and hospital admission. Approximately 1 in 170,000 people die after a severe anaphylactic reaction that fails to respond to medical treatment.
Extravasation of ICCM at the injection site is usually minor and treated with warm or cold compresses as soon as it is noted. Simple analgesia is usually all that is required in the following few days and symptoms usually settle promptly. Increasing swelling and pain at the injection site or limb paraesthesias are rare and usually the result of major extravasation. They might indicate thrombophlebitis and/or compartment syndrome, and require emergency medical treatment.
Aspiration of orally administered ICCM is a risk for patients with altered conscious state and/or impaired gastric emptying for any reason. Aspiration of Gastrografin can produce acute pulmonary edema. Non-ionic ICCM (iopamidol, for example) is often given instead of Gastrografin in this situation, as it is not associated with pulmonary oedema in the event of aspiration.
Delayed (hours to weeks post-ICCM administration):
Decrement in renal function, usually temporary and asymptomatic, is seen in some patients with pre-existing chronic severe or acutely deteriorating renal function (AKI) before ICCM, particularly if they have large volumes of ICCM given intra-arterially or receive multiple ICCM doses over a few hours or days. The only intervention that has been shown to reduce the incidence of AKI after ICCM in such ‘at risk’ patients is intravenous normal saline administered for a few hours before and after ICCM. Administration regimes vary, but 1–2 L of N saline for 4 hours before and 4 hours after ICCM is a typical protocol. This might be modified based on the patient’s cardiac function in order to avoid any worsening or potential precipitation of heart failure. There is no conclusive evidence that the addition of bicarbonate or N–acetyl cysteine to this protocol results in further reduction in the incidence of AKI.
Delayed allergic-type reactions can occur up to 1 week after ICCM, and include salivary gland swelling, jaw or joint pain, skin rash/pruritus or facial swelling. They are almost never reactions of the moderate or severe type that occur within minutes after ICCM administration.
Hyperthyroidism is an uncommon, but recognised, delayed complication of ICCM, and typically occurs 2–8 weeks after ICCM administered intravenously or intra-arterially in patients who have clinical, biochemical hyperthyroidism, an autonomous hyperfunctioning thyroid nodule or less commonly euthyroid patients with a multinodular goitre. This generally responds to conventional medical treatment for the condition.
Rarely, hypothyroidism can be a late complication of ICCM administration, but hyperthyroidism is much more common.
For patients who have thyroid cancer and are to have radioactive iodine treatment, the iodine in ICCM will block uptake of radioactive iodine into primary and metastatic thyroid cancer for up to 8 weeks after ICCM administration. Therefore, patients who are planned for radioactive iodine treatment should not be given ICCM before radioactive iodine treatment unless the benefits outweigh the risks of delaying their thyroid cancer treatment. This might include situations such as patients with thyroid cancer who require emergency coronary angiography, CT perfusion for acute stroke, or CT angiography of the chest or abdomen in the setting of major trauma or suspected aortic dissection.
Lactic acidosis in patients taking metformin-containing medication is rarely seen. It occurs in patients with severely reduced renal function or acute kidney injury, and is a drug-related side effect rather than being precipitated by ICCM.
Is there any specific post procedural care required following an ICCM injection?
Pre- and post-procedural hydration with intravenous normal saline is indicated in patients with chronic stable severe renal impairment (eGFR <30 mL/min) if it is anticipated that they will receive a large volume of contrast over a period of hours to a couple of days as a result of one or more imaging procedures.
Cessation of metformin from the time of ICCM administration is recommended if the patient has acutely deteriorating renal function or chronic severe renal impairment (eGFR <30 mL/min). After 48 hours, renal function should be re-tested before metformin being recommenced if eGFR is acceptable. It is not necessary for metformin to be ceased in patients who have stable renal function with eGFR greater than 30 mL/min, as the risk of lactic acidosis due to post-ICCM renal function deterioration is considered to be extremely low to non-existent.
Non-contrast imaging might be appropriate depending on the clinical indication. Ultrasound or magnetic resonance imaging might also be appropriate alternatives. Contrast might be indicated with magnetic resonance imaging, but this contains gadolinium rather than iodine (See InsideRadiology: Gadolinium Contrast).
The relatively small risks associated with ICCM must be weighed against the considerable benefits to diagnosis in some situations. Risk assessment of your patient for contrast-induced renal impairment allows patients at increased risk to have renal function testing before they make their appointment. Not everyone needs renal function testing: if your patient has no risk factors on questioning, there is no reason to carry out renal function testing.
It is critical that you provide detailed information about the patient’s signs/symptoms and your clinical question on the referral for CT scanning, as this will strongly influence the radiologist’s decision to give ICCM or not. This in turn will influence whether the appropriate scanning technique is carried out to answer the question. CT scanning can be carried out in many different ways depending on the reason for the scan, so accurate information and, in particular, your main clinical questions need to be provided in the referral information.
Page last modified on 26/7/2017.
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