Contrast CT


Contrast CT is X-ray computed tomography using radiocontrast. Radiocontrasts for X-ray CT are, in general, iodine-based types. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues. Often, images are taken both with and without radiocontrast. CT images are called precontrast or native-phase images before any radiocontrast has been administrated, and postcontrast after radiocontrast administration.

Bolus tracking

Bolus tracking is a technique to optimize timing of the imaging. A small bolus of radio-opaque contrast media is injected into a patient via a peripheral intravenous cannula. Depending on the vessel being imaged, the volume of contrast is tracked using a region of interest at a certain level and then followed by the CT scanner once it reaches this level. Images are acquired at a rate as fast as the contrast moving through the blood vessels.
This method of imaging is used primarily to produce images of arteries, such as the aorta, pulmonary artery, cerebral, carotid and hepatic arteries.

Washout

"Washout" is where tissue loads radiocontrast during arterial phase, but then returns to a rather hypodense state in venous or later phases. This is a property of for example hepatocellular carcinoma as compared to the rest of the liver parenchyma.

Phases

Depending on the purpose of the investigation, there are standardized protocols for time intervals between intravenous radiocontrast administration and image acquisition, in order to visualize the dynamics of contrast enhancements in different organs and tissues. The main phases thereof are as follows:
PhaseTime from injectionTime from bolus trackingTargeted structures and findings
Non-enhanced CT --
Pulmonary arterial phase6-13 sec-
  • Pulmonary embolism
    • Pulmonary venous phase17-24 sec-
    Early systemic arterial phase15-20 secimmediately
  • Arteries, without enhancement of organs and other soft tissues.
    • Late systemicarterial phaseSometimes also called "arterial phase" or "early venous portal phase"35-40 sec15-20 sec
    • All structures that get their blood supply from the arteries have optimal enhancement.
    • Some enhancement of the portal vein
    Pancreatic phase30 or 40 - 50 sec20-30 sec
  • Pancreatic cancers become hypodense compared to the parenchyma.
    • Hepatic or late portal phase70-80 sec50-60 sec
  • Liver parenchyma enhances through portal vein supply, normally with some enhancement of the hepatic veins.
    • Nephrogenic phase100 sec80 sec
  • All of the renal parenchyma enhances, including the medulla, allowing detection of small renal cell carcinomas
    • Systemic venous phase180 sec160 sec
  • Detect venous thrombosis
    • Delayed phaseSometimes called "wash out phase" or "equilibrium phase"6-15 minutes6-15 minutes

    Angiography

    is a contrast CT taken at the location and corresponding phase of the blood vessels of interest, in order to detect vascular diseases. For example, an abdominal aortic angiography is taken in the arterial phase in the abdominal level, and is useful to detect for example aortic dissection.

    Amount

    Adults

    The following table shows the preferable volume in normal weight adults. However, dosages may need to be adjusted or even withheld in patients with risks of iodinated contrast, such as hypersensitivity reactions, contrast-induced nephropathy, effects on thyroid function or adverse drug interactions.
    The dose should be adjusted in those not having normal body weight, and in such cases the adjustment should be proportional to the lean body mass of the person. In obese patients, the Boer formula is the method of choice :
    For men: Lean body mass = + − 19.2
    For women: Lean body mass = + − 48.3

    Children

    Standard doses in children:

    Adverse effects

    Iodinated contrast agents may cause allergic reactions, contrast-induced nephropathy, hyperthyroidism and possibly metformin accumulation. However, there are no absolute contraindications to iodinated contrast, so the benefits needs to be weighted against the risks.
    As with CT scans in general, the radiation dose can potentially increase the risk of radiation-induced cancer.
    The injection of iodinated contrast agents may sometimes lead to its extravasation