Nuclear Medicine Thyroid Scan
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Contrast media (sometimes called contrast agents or dye) are chemical substances used in medical X-ray, magnetic resonance imaging (MRI), computed tomography (CT), angiography, and occasionally ultrasound imaging.
Contrast media enhance and improve the quality of images (or pictures), so that the radiologist (a specialist doctor trained to examine the images and provide a written report to the patient’s own doctor or specialist) can more accurately report on how your body is working and whether there is any disease or abnormality present.
The radiologist carrying out the procedure is responsible for deciding if contrast medium is needed and which contrast medium will provide the best images. This decision will be based on the information provided to the radiologist by your doctor or specialist on the referral form, such as medical history and what the doctor or specialist thinks might be wrong, and which part of the body is being scanned.
Contrast medium is not needed for every type of imaging test in order to obtain very high-quality images – whether it is used or not depends on the information provided by your doctor.
X-ray or radiography is the imaging of body structures using X-ray beams that are absorbed by different parts of the body in different ways to create an image or picture. This includes CT scans and plain radiography/X-rays. Different structures or parts of the body look lighter or darker on the images, depending on what these structures are made of; for example, soft tissue shows as dark grey and bone as light grey. This difference in how various structures of the body absorb the X-ray beam and produce an image is called ‘contrast’.
MRI and ultrasound do not use radiation, but still produce images showing the different parts of the body. MRI uses a magnetic field, and ultrasound scans use high-frequency sound waves.
The use of contrast media highlights the differences between various parts of the body, including those parts that have a similar composition. This provides a clearer image of how the body is working, or if there is any disease or abnormality present.
With clearer images, the radiologist can provide your doctor with a more accurate diagnosis of the symptom or condition, to assist in deciding what treatment will be most appropriate.
This information sheet deals with the use of iodine and gadolinium in patients whose kidney function is much less than normal. If you have kidney disease or diabetes, you will need to have a recent kidney function test result before contrast is given, unless the test or procedure is an emergency. If you are unsure whether your kidneys are working normally or you have other medical conditions, then you should contact your referring doctor to see if a kidney blood test is required before the imaging examination.
The radiologist needs to know how your kidneys are functioning before giving contrast. Both gadolinium contrast (used for MRI) and iodinated contrast (used for CT, angiography and other procedures that involve X-rays) leave the body in the urine, which is made by your kidneys.
The InsideRadiology website contains further separate information on both iodine (see Iodine-containing contrast medium) and gadolinium (see Gadolinium Contrast Medium). These information items include details of the risks and benefits involved for those with reduced kidney function and normal kidney function. The items also include information about other side effects when using contrast media.
Contrast media are regarded as generally very safe for patients who do not have severe kidney disease. If you do have severe kidney disease, the radiologist in discussion with your doctor might still decide that you should have contrast, because the benefit of the test or procedure outweighs the risk.
If you have severely reduced kidney function for any reason (in other words, the kidneys are not working as well as would normally be expected), you need to be aware of possible increased risks that might be associated with contrast medium injections (either iodine-based contrast or gadolinium-based contrast).
Each contrast medium has a specific, and important risk associated:
If you are on permanent dialysis, your kidney function will not be made worse by iodinated contrast. Some kinds of gadolinium contrast agents are associated with an increased risk of NSF in people on dialysis.
If you have severely reduced kidney function or actively deteriorating kidney function, the risks associated with iodine contrast agents and gadolinium-containing contrast will be taken into consideration by the radiologist together with your doctor.
If you have severe kidney function impairment (eGFR less than 30 mL/min) and are not on permanent dialysis, this decision is best made through discussion between yourself, your referring doctor and the radiologist at the radiology facility where you are having the test or procedure.
The most important question to be considered is what information your doctor needs from the imaging test or procedure. If the information is to be obtained from either an MRI or a CT, either of which requires contrast (either gadolinium (MRI) or iodine (CT) contrast agents), it is now recommended that MRI using one of the lower-risk gadolinium agents is the preferred option. This is because the risk of NSF in this situation is significantly lower.
The information your doctor needs might only be obtainable from a test or procedure involving iodine contrast media. If this is the case, and your kidney function is very poor, steps will be taken to minimise the risk and this will often involve giving you extra fluids around the time of the iodine contrast injection.
Nephrogenic systemic fibrosis (NSF) (also known as nephrogenic fibrosing dermopathy) This condition is rare and, so far, has occurred only in people with severe kidney disease including patients on dialysis. It causes swelling and tightening of the skin of the arms and legs, and less often the body. It develops over days to weeks after gadolinium contrast is given, and might reduce movement of the joints. In rare cases, it can also cause damage to internal organs.
Certain types of gadolinium contrast are now well known to be associated with a much lower risk of NSF in people with poor kidney function. If you need to have gadolinium contrast, the radiologist will use one of these lower risk types of gadolinium contrast. It is now thought that less than 1 in 100 people with severe kidney function impairment who have these lower-risk types of gadolinium contrast will develop NSF. It is often possible to carry out MRI without using any gadolinium contrast agents, and this is preferred where possible for patients with very poor kidney function or who are on dialysis.
Contrast-induced nephropathy If you already have very poor kidney function, but are not on permanent dialysis due to kidney failure, there is a small risk that iodinated contrast will make your kidney function temporarily worse (20–40% of patients). This occurs mainly in people who have large amounts of iodine-containing contrast medium given into an artery (such as with coronary angiography or complex interventional procedures) or if they have more than one CT scan using intravenous contrast in a short period of time; that is, hours to 1–2 days.
Permanent decrease in kidney function is uncommon (5% of patients), and mainly affects people with the poorest kidney function or rapidly deteriorating kidney function who also receive a large volume of iodine-containing contrast.
If you are on permanent dialysis, you are not at risk of worsened kidney function if you receive iodine contrast agents.
Gadolinium does not have the potential to cause contrast-induced nephropathy.
If you have any questions or concerns, you can discuss the use of contrast media with your own doctor or specialist who is referring you for the radiology test, and about the need for a kidney function test or any special medication that might be required before having the test.
Any concerns can also be discussed with medical staff where you are having the scan or procedure; that is, the radiographer (medical imaging technologist) who will carry out the scan, or to the radiologist (specialist doctor) who interprets the images and provides your referring doctor or specialist with a report.
Page last modified on 26/7/2017.
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RANZCR® intends by this statement to exclude liability for any such opinions, advices or information. The content of this publication is not intended as a substitute for medical advice. It is designed to support, not replace, the relationship that exists between a patient and his/her doctor. Some of the tests and procedures included in this publication may not be available at all radiology providers.
RANZCR® recommends that any specific questions regarding any procedure be discussed with a person's family doctor or medical specialist. Whilst every effort is made to ensure the accuracy of the information contained in this publication, RANZCR®, its Board, officers and employees assume no responsibility for its content, use, or interpretation. Each person should rely on their own inquires before making decisions that touch their own interests.