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Most vascular computed tomography (CT) and magnetic imaging resonance (MRI), the vast majority of angiographic procedures, and many nonvascular interventions rely on contrast media to reveal the anatomy. Contrast media can be broadly classified according to their use and also their chemical structure. X-ray contrast affects tissue X-ray attenuation, ultrasound contrast affects tissue and blood reflectivity and MRI contrast affects tissue relaxation times. The Royal College of Radiologists has issued pragmatic guidance on the use of intravascular contrast agents. Discussion in this chapter is confined to contrast media used for angiography and vascular diagnosis.
Intravascular X-ray contrast agents
The two principal categories of X-ray contrast both affect X-ray attenuation. Details of the chemical and physical properties of these agents are extensively discussed in many texts. It helps to be familiar with the different options and their indications, this will be most relevant in cases where there is kidney disease or a history of adverse reaction.
Positive contrast agents
These are liquids containing iodine or gadolinium that have greater attenuation than the patient's soft tissues.
Negative contrast
This has lower attenuation than the patient's tissues; at present, carbon dioxide gas is the only available option.
Basic principles for using intravascular contrast
Optimal demonstration of anatomy and pathology requires sufficient difference in attenuation between the target tissue and the surroundings, as well as X-ray equipment parameters, e.g. kV and mAs.
The aim is to adequately opacify the vessel but allow a level of grey-scale that allows branches/filling defects to be seen through the contrast. This requires the correct strength contrast in sufficient quantity (volume of contrast) delivered in the right place. If the contrast is too diluted, there will be insufficient change in attenuation, conversely too concentrated contrast can obscure lesions.
As a general principle, the contrast column should opacify the entire vessel segment. To achieve this, the total contrast dose and the duration of the bolus must be correct. Appropriate catheter positions, contrast volumes and flow rates are indicated throughout the diagnostic angiography sections. When the blood flow is slow, it may take several seconds for the opacified blood to pass through the vessel. Hence, a long contrast bolus is necessary. This is one of the reasons for increasing the volume of contrast to image the more distal vessels. Modern angiography equipment allows integration of multiple images, which has the same effect as increasing the length of the bolus, but it can reduce image quality due to minor degrees of patient movement between frames.
Iodinated contrast media
These are the most frequently used agents. Non-ionic contrast media are recommended in high-risk patients (see below).
Most diagnostic and therapeutic intervention is performed using ‘300 strength’ contrast (300 mg/mL iodine). This density of contrast is fine for pump injections into large vessels where the contrast is diluted by rapid blood flow. For selective hand injections in the vascular system and for non-vascular examinations, 300 strength contrast is diluted with saline to ‘two-thirds’ or ‘half strength’.
Contrast reactions with iodinated contrast media
There are two forms of contrast reaction: direct effects and idiosyncratic responses; these are more common in certain groups of patients and emphasis should be on identifying them, reducing risk and preventing reactions. Up to 2% of patients require treatment for adverse reactions to intravascular iodinated contrast agents. Fortunately, the majority of cases require only observation and minor supportive treatment. Less than 1% are severe but these require prompt recognition and immediate treatment.
Alarm
Contrast reactions should be discussed during the consent procedure.
Direct effects
Direct effects are secondary to the osmolality and direct chemotoxicity of the contrast, and they include heat, nausea and pain. More important are the effects on organ systems.
Renal
Contrast-induced acute kidney injury (CI-AKI), defined as a rise in creatinine of 0.5–1 mg/dL or 44–88 µmol/L, is common and most likely in patients with chronic kidney disease. Strategies for preventing CI-AKI are discussed in Chapter 1 .
Cardiac
Cardiac problems are most likely to occur during coronary angiography and are usually manifest as arrhythmias or ischaemia. It is prudent to use non-ionic contrast in patients with ischaemic heart disease or heart failure.
Haematological
Significant haematological interactions are uncommon. Non-ionic iodinated contrast can induce clotting if mixed with blood; hence, scrupulous attention to catheter flushing and avoidance of contaminating syringes with blood are essential.
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