Contrast Media in Medical Imaging Flashcards
Why are contrast agents used in medical imaging?
What needs to be considered before administering contrast and why?
- they are used as they enhance images and give additional information about certain pathologies
- they are associated with some adverse effects / complications
- it is important to question whether the required information could be sought via a different imaging modality / without contrast
What are the 5 most commonly used contrast agents?
- iodine-based agents
- barium salts
- gadolinium
- air
- water
What is the difference between a “positive” and a “negative” contrast agent?
What are double contrast techniques and why are they used?
“Positive” agents:
- these block the transmission of x-rays through tissues
- pathologies often appear as filling defects
“Negative” agents:
- these include air and water and they distend structures, allowing for greater X-ray transmission
Double contrast techniques:
- this uses both types of agent and gives excellent detail of the gut mucosa
How can iodine-based agents be administered?
What are the 3 most common uses?
- they can be administered IV, for purposes such as:
- CTPA and CT head
- imaging the vasculature (e.g. coronary angiogram)
- imaging the urinary tract
- they can also be administered orally for CT scanning the abdomen
- preparations are taken PO approx 1 hour before imaging
When might water soluble iodine-based contrast be used for GI studies?
- water soluble iodine-based contrast can be used for GI studies instead of barium if there is a risk of peritoneal contamination
- e.g. fistula, suspected colonic obstruction
What is shown in this image?
Why is timing important?
CT pulmonary angiogram (CTPA) using iodine contrast
- timing is important as images need to be acquired at the point that the contrast is in the arterial circulation (arterial phase)
What is the technique used in a coronary angiogram?
What type of contrast is used?
- coronary angiography uses iodinated contrast
- the technique used is fluoroscopy, which is real time imaging using X-rays
- the image can be processed to make the vessels clearer - digital subtraction angiograhy
What other types of investigation use iodinated contrast?
- micturating cystourethrogram
- hysterosalpingogram
- intravenous pyelogram / urography
- sialogram
- cholangiography
What is the imaging modality of choice for imaging the urinary tract for renal calculi?
non-contrast CT
- CT is more sensitive for renal calculi
- the radiation dose between CT and X-ray is similar, and other pathologies may be seen on CT
What are the adverse effects of iodinated agents?
- hypersensitivity / allergic reactions
- anaphylaxis
- bronchospasm
- angio-oedema
- urticarial reactions
- minor side effects, such as nausea
- contrast-induced nephropathy (CIN)
- caution should be taken in thyroid disease
What is contrast-induced nephropathy (CIN)?
When does it occur and why?
- acute renal impairment that occurs within 3 days of contrast administration, in the absence of other precipitating causes
- it occurs secondary to the toxic effects of contrast on the renal tubular epithelium
- renal ischaemia is also implicated
Who is more at risk of the side effects of iodinated contrast?
- there are some conditions that increase the risk of an allergic / adverse reaction to iodine-based contrast materials
- previous adverse reactions to iodine-based contrast
- history of asthma
- history of allergy
- heart disease
- renal disease
- dehydration
What conditions are associated with an increased risk for an adverse reaction to iodinated contrast?
- history of a previous reaction to iodinated contrast (200x increased risk)
- hyperthyroidism
- family history of hypersensitivity reaction to iodinated contrast
- allergic diseases, including asthma
- previous history of multiple allergies
- allergic diseases and allergies are not contraindications to the use of contrast, but the risk of severe contrast reactions is elevated
- a more detailed history about the allergies should be obtained
What are some other medical conditions are associated with putting someone at increased risk of contrast administration?
- asthma
- renal problems
- diabetes mellitus
- metformin therapy
If someone has had a previous allergic reaction to contrast, what steps should be taken prior to administration?
- need to determine the exact nature of the previous reaction and the specific compound used on that occasion
- re-examine the need for contrast and consider an unenhanced study or other methods of investigation
- assess the risk-benefit ratio of the procedure, as a non-diagnostic examination may be more detrimental to the patient than the risk from contrast exposure
- if the injection is deemed necessary, use a different contrast compound to the one previously used
What factors increase the risk of AKI in patients receiving iodinated contrast?
- chronic kidney disease (eGFR < 40 ml/min/1.73m3
- heart failure
- renal transplant
- age 75 or over
- hypovolaemia
- increasing dose of contrast or repeated administration of contrast
- intra-arterial adminsitration of contrast
In what groups of people should precaution with iodinated agents be taken?
- people with a history of allergic reactions / anaphylaxis
- people with pre-existing renal disease - this often includes people with diabetes
- people who take nephrotoxic drugs (there are lots - check the BNF in doubt)
- preventative measures should be taken in those with moderate-severe renal disease
What preventative measures can be taken to reduce risk of CIN in those with moderate to severe renal failure?
- the dose of contrast medium should be minimised, taking into account the indication and body weight
- iodine-based contrast should be maximised by using a saline flush for intravenous volume expansion
- consider stopping ACEi and angiotensin II receptor blockers when eGFR < 40
- maintain a hydration regime post contrast administration
What decisions need to be made before contrast is given?
How much contrast is given?
- how much contrast should be given?
- how fast should the contrast be given?
- when should the scan be performed?
- a standard dose is given in adults, unless they are very small/large but it is weight-dependent in children
- how fast the contrast is given depends on the bore of the cannula used
- when the scan is performed depends on what is needed to be seen in the image
What are the 3 contrast phases and when are they used?
Arterial phase:
- used to look at arteries, including the pulmonary arteries
- useful for identifying GI bleeds, ?PE and in CT angiograms
Portal venous phase:
- used to look at abdominopelvic viscera and venous structures
Delayed phase:
- this allows tissues to enhance, which is good for many cancers
- it can be used to see if contrast “washes out” quickly or not, which can identify adrenal lesions, parathyroid adenoma and liver lesions
- used to look at the urinary tract as it needs time to be excreted
When is barium used as a contrast agent?
- it is a “positive” contrast agent that is used widely in fluoroscopic imaging of the GI tract
- barium contrast agent coats the luminal surface of the gut
How is barium administered?
How are images taken and what pathologies is this good for visualising?
- barium is administered PO or PR
- real-time X-ray images are then taken (fluoroscopy)
- this can give functional, dynamic information as well as structural / anatomical information
- it is good for visualising pathologies such as malignancy and polyps**, as well as visualising the **terminal ileum in Crohn’s disease
What are the drawbacks of using barium salts?
What are the potential complications?
- patients must have adequate bowel preparation for these studies
- barium contrast is dense, so CT abdomen is contraindicated for several days after barium enema
- barium studies deliver a large radiation dose to the patient
- potential complications are chemical pneumonitis or peritonitis
What is the radiation dose associated with barium studies?
- 4 mSv
- this is compared to 1 mSv for plain AXR
When might air and water be used as contrast media?
- they are negative contrast agents, meaning that they distend structures and allow XRs to pass through
- they can be used together with barium agents to improve visualisation of the bowel mucosa in fluoroscopy
When is carbon dioxide used as a contrast agent?
When should it not be used?
- it is a negative contrast agent
- it can be used with barium (instead of air) for a double contrast barium enema
- it should not be used in any patient with risk of perforation (air should not be used either)
What is shown in these images?
- these are examples of upper GI filling defects on barium studies
What is demonstrated in these images?
barium swallow and meal
Barium swallow:
- involves drinking barium liquid, and X-ray images are taken as the patient swallows
- used to look for problems of the oesophagus, such as stricture, hiatus hernia, tumours, reflux from the stomach
Barium meal:
- involves drinking barium liquid, followed by the patient lying down whilst images are taken over the abdomen
- this looks for problems in the stomach and duodenum, such as ulcers, polyps and tumours
What is the term given to this feature and what pathology does it suggest?
“apple core lesion”
- the presence of a carcinoma growing into the colonic lumen creates a filling defect that resembles an apple core
When is gadolinium contrast used?
What are the minor side effects associated with it?
- it is used in MRI scanning and enhances signalling on T1 weighted images
- it may be used as an alternative if the use of iodinated agents is contraindicated
- it is associated with some minor side effects:
- headache
- nausea
- local irritation at the site of injection
What serious side effect is gadolinium contrast associated with?
What must be checked before giving this?
- U&Es need to be checked beforehand as it is renally excreted
- it is associated with the development of nephrogenic systemic fibrosis (NSF) in patients with pre-existing renal impairment
- this is a serious condition characterised by fibrosis in widespread tissues including the skin, muscles, eyes, liver and heart
- there is no effective treatment
What is meant by “inhaled contrast” and when is it used?
- inhaled contrast agents are technetium** and **xenon
- they are used in ventilation / perfusion scintigraphy (“V/Q scans”), which is used to look for PE
- these agents are radioactive, but a very minimal / low dose exposure
