Maximize the density of contrast in the arteries at the time of imaging.
Minimize the contrast density in the veins
These goals are approached by optimizing the injection and the timing of the scanning relative to that injection.
To try and get an understanding of how the study went, all studies should be evaluated by the tech in terms of the timing of the filming (ideally would it have been better to start scanning a few seconds earlier or a few seconds later) and what was the contrast density (Hounsfield measurement) achieved in the arteries of interest. (usually easiest to measure in the internal carotid artery in some place where there is no artifact).
NO contrast in veins. Arterial density is low and seems to improve on later images
Contrast dense in the veins. In extreme cases, it will be denser in the veins than the arteries. Arterial density is low and deteriorates on later images.
In day-to-day work, you will see a wide range of values from about 200 to 800. Higher is always better. If you evaluate every case, over time you will get a “feel” for what makes a good study.
The wide variability in contrast density, even in the face of optimal timing, relates to different degrees of dilution of the contrast by the unopacified blood entering the heart from the IVC. Factors working against optimal opacification are high cardiac outputs which tend to be seen in young patients and agitated patients (trauma victims for example) and obesity, which for some reason, seems to be associated with higher flow from the IVC (also higher cardiac output in general).
If large volumes of contrast are used and are infused at a constant rate, the density of contrast achieved within the pulmonary and systemic arteries will not be constant. This will be seen frequently on pulmonary CTA’s where the contrast density is higher in the aorta than in the pulmonary artery. This is related to variability over time of the relative contribution of SVC and IVC blood to the heart.
The best chance for good opacification is to image early. The injection itself increases the pressure on the SVC side and helps make its flow dominant early. However, with the IVC “backing up” the pressure on that side will start increasing and later in the injection will contribute a higher percentage to the flow to the heart. In old people with lower cardiac outputs, it is easier to dominate the flow from the IVC by the contrast being injected via the arm into the SVC. In rare cases when doing Chest CT, you will see that the injected contrast will reflux down into the IVC and hepatic veins. These are patients with poor cardiac outputs.
When doing imaging of the head and neck, the field of view has to encompass the arteries in the head as well as the arteries in the upper chest and neck. This can generally be accomplished with a field of view of 20cm. In an unusual patient, such as a severely kyphotic patient, this has to be increased. But higher DFOV should not be used routinely for convenience because it decreases the detail on the images.
If this is a head only CTA, then the FOV can be reduced below 20 cm as with any head CT.
The key fact about the GE scanners is that the imaging starts about 6 seconds AFTER the tech pushes the GO button. So you are trying to judge and optomize, from the smart prep images, how things will look 6 seconds after you push the GO button. On the 64 slice scanner, we are able to get fairly consistent results with small volumes of contrast (50 – 70 cc) and judging when to push the GO button by looking at the opacification of the pulmonary artery and always scanning from the neck toward the head. This is harder to do with the 16 slice scanner.
Set the patient up to scan from the top down, with the highest slice about 1-2 cm below the very top of the head (Unless there is a specific reason to be imaging up there. The major arteries that we are interested in are all in the lower 2/3 of the head. Use the aortic arch for the smart prep. When you see ANY contrast in the aortic arch push the GO button. Do NOT wait for denser contrast to appear. The patient is scanned from the top down with the hope that by the time the scans of the lower neck are being obtained, the dense, artifact-causing, contrast in the subclavian veins has been partially washed out by saline. These studies can be done with 100 cc of contrast
We have commonly used a strategy of looking at the skull base and then pushing the GO button when contrast is first visible in the ICA at the skull base. This works pretty well but sometimes it is hard to see the ICA on the non-contrast image so you are not sure exactly where you should be looking and the artery is relatively small in this position. This strategy results in images that are usually a couple seconds too late to be optimal: the veins are a little too opacified.
I would recommend using the aorta for the smart prep. Set the patient up for a head CTA scanning from bottom up. Then image the aortic arch for the smart prep. Start imaging at ~10 seconds, then every two seconds until you see ANY contrast in the aorta. Push the GO button at the first sign of contrast in the aorta. The patient will move in the scanner until positioned at the base of the skull and imaging will start 6 seconds later. Do NOT wait until you see dense contrast in the aorta. You will then be too late. The 6 seconds will elapse whether or not you have the patient’s skull base in view at the start of the study. You do not lose anything by doing the smart prep at the aorta, which is a big vessel, and then having the machine move the patient so the head can be imaged. These studies can probably be done with 80 cc of contrast.
Set the patient up to scan from the top down, with the highest slice a cm above the sella turcica. Use the aortic arch for the smart prep. When you see ANY contrast in the aortic arch push the GO button. The patient is scanned from the top down with the hope that by the time the scans of the lower neck are being obtained, the dense, artifact-causing, contrast in the subclavian veins has been partially washed out by saline. These studies can be done with 90 cc of contrast