There are two questions a clinician needs to consider when ordering a contrast CT scan: is intravenous contrast necessary? How long after the injection should the patient be scanned? In this video, we'll explore the three scenarios where IV contrast is necessary, the three phases of scanning with contrast, and the one thing you should never do when scanning.
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Two questions a clinician needs to consider when ordering a contrast CT scan are: is intravenous contrast necessary and how long after the injection should the patient be scanned? Intravenous, or IV contrast may be necessary in three different scenarios. The first scenario is when we want to differentiate between two adjacent structures based on their blood flow.
Without contrast, adjacent structures with the same attenuation cannot be distinguished, but with contrast, the difference in blood flow will enhance the two structures to different degrees, so that they can each be clearly seen. In this example, the blood vessel next to a mass is not seen on the left image, it is clearly seen on the right when contrast is used.
Another example, is when a higher blood vessel is perfused more than a higher lymph node can easily be seen with contrast. A difference enhancement may also be due to the absence of blood flow in one of the structures such as in pleural fluid, or the pulmonary abscess, as seen in this example.
The second scenario which requires IV contrast is when we want to see areas of abnormally increased blood flow, there can be two causes for this. The first is increased blood flow in a normal structure. Examples include hyperemic pleura, in the presence of empyema, or aterial vasculature in the presence of inflammation.
The second cause is increased blood flow in area where there shouldn't be any. An example of this is a hemothorax. In this trauma patient, we can see free blood in the pleural effusion just below the lacerated, collapsed lung. The third scenario which requires IV contrast is when we need to define vascular structures.
We can define morphology, such as an arteriovenous malformation, or we can define their patency, such as atherosclerotic stenosis. Understanding these reasons for giving contrast also helps know when it is not needed. If there is enough natural contrast to see what needs to be seen without injected contrast, then contrast won't be needed. For example, when assessing a lung nodule contrast is not needed, as the nodule is well defined by the adjacent air filled lung.
When evaluating the dilated ascending aorta, the diameter of the vessel can easily be determined without contrast, as it's well defined by the adjacent mediastinal fat. The second question clinician needs to consider when we're going to contrast CT is how long after contrast injection should the patient be scanned.
Contrast is injected into the arm and travels as a bolus from the arm to the right side of the heart, to the lungs, to the left side of the heart to the aorta, and then finally returned by the systemic veins. There are three time points at which scans are done to highlight structures. Early arterial phase is done when the contrast is brightest in the pulmonary arteries.
This is great for looking for pulmonary emboli, but it's too early for most other things which are perfused by the aorta, and which have not yet been reached by the contrast. Standard arterial phase is done when the contrast has already passed through the lungs, and has reached the descending aorta.
Most of the thoracic structures will be well perfused at this point, although, as the bolus peak has passed the pulmonary arteries this phase is somewhat less sensitive for pulmonary emboli. Finally, venous phases is done later when the contrast has reached the veins. This is optimal if there are questions about venous anatomy or patency or to remove artifacts from the initial contrast bolus. Although the radiologist will take care of the details of the timing, the ordering clinician had to communicate clearly about what is needed to be seen, so that the correct timing can be used.