Scan protocols

-Two basic types of protocol are used:

  1-high resolution for carotids and visceral vascular beds; and 

  2- high speed for aorta and peripheral work. 

-Pre contrast scans is essential in the following:

1-To detect presence of hemorrhage or mural hematoma.

2-Determination of the field of coverage.

3-deciding an appropriate site to place the cursor for contrast bolus tracking. 

-Contrast injection:

1-The goal of contrast medium injection is to achieve homogeneous “pure arterial” (without venous or tissue contamination) enhancement synchronized with image acquisition. 

2- This is in turn dependent on a number of factors such as rate of injection, contrast medium density, cardiac output and blood volume.

3-Intra venous assess: A good sized antecubital vein.

4-Saline chaser: used to decrease amount of contrast medium and also to prevent streaking artifacts.

The saline chaser can be used by either after loading the contrast medium in a single barrel injector or using one of the double barrel injectors available. 

5-Injection rate:

Monophasic injection: A single static rate of injection is used.

 Biphasic injection: In practical terms this produces a more consistent and reproducible result with less inter patient variation.

 Most modern pump injectors are capable of handling biphasic and even multiphasic injections.

6-Timing: There is no place in CTA for guessing the optimal time of enhancement. We use bolus tracking routinely.

7- Physiological factors:

 (a) Body size: This relates to proportionate blood volume. In general we use 1.5 ml kg⁻¹ body weight of contrast medium.

(b) Cardiac output: In patients with low cardiac output, the peak arterial enhancement is later. Hence, to achieve uniform enhancement it is often useful to place the cursor for bolus tracking at the bottom of the aneurysm. It is important to remember that mean contrast enhancement in patients with high cardiac output is actually less.

(c) High concentration contrast medium: With faster scanners, the use of high concentration contrast medium (having >350 mg I ml⁻¹) is useful to achieve a high concentration of iodine in the vessels. It is possible to reduce both the injection rate and the total volume of contrast medium needed if high concentration contrast medium is used.