In Dr. Foley's practice, using a mini-bolus technique, aortic
peak is determined from a time attenuation curve. Then, the time
from the beginning of injection to aortic peak is the circulation
time to the mid-ascending aorta. Four seconds are then allowed from
aortic peak on the mini-bolus until acquisition begins. Dr. Foley
stated that there are two reasons for doing this, "Number one, we
know that aortic attenuation will continue to rise with the longer
duration injection until it reaches a relative plateau at about 4
seconds after the peak from the mini-bolus. The second reason is if
there is any cardiac reactivity to contrast, it's likely to occur in
those first 4 seconds after the aortic arrival. So it's essential to
understand that the bolus that's injected is not a square wave front
bolus, it actually is a sloping front bolus. So that the reason for
delaying for those 4 seconds is to capture the plateau peak of
contrast reached about 4 seconds after the peak from the
mini-bolus."
"We do inject saline rapidly at 6 cc per second. We inject it
for 8 seconds before we acquire data. The reason for doing that is
to clear the superior vena cava. Just to return to the issue of the
second phase of the injection, we inject deliberately at a slow
rate, that's 2 cc per second, in order to achieve this partial
opacification of the right heart at reproducible attenuation values
of about 100-150 Hounsfield units. So, some opacity of the right
heart is necessary in order that the computer algorithms can
separate out the coronary arterial tree from the background
ventricle." |
