Contrast CT is X-ray computed tomography (CT) using radiocontrast. Radiocontrasts for X-ray CT are, in general, iodine-based. 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 is a technique used in computed tomography imaging, to visualise vessels more clearly. 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 (abbreviated "R.O.I.") 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. The image shown illustrates this technique on a sagittal MPR (multi planar reformat). The image is demonstrating the blood flow through an abdominal aortic aneurysm or AAA. The bright white on the image is the contrast. You can see the lumen of the aorta in which the contrast is contained, surrounded by a grey 'sack', which is the aneurysm. Images acquired from a bolus track, can be manipulated into a MIP (maximum intensity projection) or a volume rendered image.
"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.
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:
|Phase||Time from injection||Time from bolus tracking||Targeted structures and findings|
|Non-enhanced CT (NECT)||-||-|
|Early arterial phase||15-20 sec||immediately||
|Late arterial phase
Sometimes also called "arterial phase" or "early venous portal phase"
|35-40 sec||15-20 sec||
|Pancreatic phase||40-50 sec||20-30 sec|
|Hepatic (most accurate) or late portal phase||70-80 sec||50-60 sec||
|Nephrogenic phase||100 sec||80 sec||
|Systemic venous phase||180 sec||160 sec|
Sometimes called "wash out phase" or "equilibrium phase"
|6-15 minutes||6-15 minutes|
CT 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.
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