Monday, 13 June 2011
Delivery of CT contrast media
IV contrast travels from the veins at the injection site to the right heart -> pulmonary arteries -> pulmonary veins -> left heart -> arterial system (first pass).
After the contrast is distributed throught the body it reenters the right heart (equilibrium phase).
Injection duration affects the cummulative arterial enhancement and peak enhancement. Both will be smaller if the injection duration is smaller. The maximal arterial enhancement response is also directly propotional to the iodine administration rate (iodine flux). and can be controlled by increasing the injection rate and / or the iodine concentration.
The bolus method involves a small test injection and multiple low dose scans performed over the artery of interest until the contrast is visualised.
The bolus triggering method involves eliminating the test injection and scan only begins when contrast density in a preselected artery exceeds a pre-set limit.
USUALLY 200 HU IS THE TRIGGERING DENSITY
There is an additional 4-8 sec delay built in to the system for table movement time in the test bolus method.
Either the Aorta, or the Pulmonary artery is selected for monitoring contrast density.
Flow in the lower limb can be assymetric and very variable. With the current 16-slice scanners it is possible to outrun the contrast bolus to the lower limbs. But it is also possible to over delay and end up with venous contamination. One trick is to use a slightly slower injection rate of 3 ml/sec and to trigger the scanner from the common femoral vein.
At 2 ml/sec injection rate, a 45-50 sec scan delay time for the chest and a 70 sec delay time for the abdomen is used.
At 3 ml/sec injection rate, a 40-4 sec scan delay, and for the abdomen a 55-60 sec scan delay time is applied.
Some studies like dual phase liver scan requires images in both arterial and venous phase, in these cases it is important to add 10-15 sec delay after the contrast appears in the aorta, because our goal is to image the liver in the late arterial phase, which usually occures bw 25-30 sec at 4-5 ml/sec injection rate.
The second scan can be performed at 70 sec after the start of the injection.
25-30 s at 4-5 ml/min OR AO+10 s
Portal phase: 70s
Pancreatic phase occures at approximately 40 sec, or at 20-25 sec after the contrast appears in the aorta.
AO + 20-25 s
Contrast amount and density:
To maximise arterial enhancement for vascular CTA studies, high-density contrast >350 mgI/ml is recommended, and should be paired with rapid injection rate of 4-5ml/sec.
Lower density contrast can be used but then it should be paired with higher injection rate of 5-6 ml/sec.
A good rule of thumb for MDCT is that the contrast time should last for the duration of the scan plus a small delay factor, which is usually 4-10 sec and may correspond to the triggering delay.
Injection duration = scan duration + delay factor
Total scan time = (Total length covered x rotation time)/(Pitch x slice thickness)
Contrast volume (mL) = Injection duration x Injection rate (mL/sec)
Standard injection rate for CTA is 3-6ml/sec. The faster the contrast is injected the higher the peak aortic enhancement. The peak enhancement is also related to the injection duration.
Biphasic contrast injection could produce a more uniform plateau.
25-50 ml of saline is injeted. It will clear the residual contrast from the IV tubing and accelerate washout from the arm veins. This can provide an effective 10mL of additional usable contrast .
This will decrease the risk of contrast nephrotoxicity. Saline flush also reduces the very dense contrast seen in the subclavian vein, brachiocephalice vein, and superior vena cava. This helps to reduce streak artifacts.