In pulmonary CT angiography, different protocols are applied with different contrast volumes. However, if we know the physiological average amount of CM that fills the pulmonary artery down to the tertiary branches, we can do good exam.
In timing the scan, we need to exclude the CM-filled SVC and the left side of the heart in order to avoid artifacts. So knowing the physiological CM volume on average patient and adjusting the scan time given the size of the chest, and heart rate, a good timing of pulmonary angiography can be performed.
One more technical factor is calculating the scan time in testing bolus technique. This method ensures a maximum concentration of CM in the pulmonary trunk. But giving that we start the scan in the caudo-cranial direction, the CM may be not maximum by the time PA is scanned. It is said that 5 seconds should be added to the time to peak, I don't understand why 5 second? In fact, rationally some seconds should be deducted from the time to peak because this time is consumed by scanning the chest from the base till it reaches the pulmonary artery.
1) What are those extra 2 seconds for?
2) Will SVC appear filled with contrast in this technique? How many slices does that Flash scan have? ( 64, 128?)
3) Is 30 ml enough to fill the pulmonary trunk, secondary and tertiary branches?
Regarding the third point, there may be a smart way to trace the contrast in different locations by adjusted the scan parameters to satisfy the prerequisites of excluding all contrast filled structures but PA and its branches.
I am thinking of adjusting the scan parameters to get the contrast in PA only. This requires knowledge of the hemodynamics of the blood flow from SVC to the tertiary branches. A mathematical diffusion function of independent variables like: patient size, heart rate and the contrast volume is then created followed by solving for the time (t) that satisfies the aforementioned prerequisites. Then the scan parameters like pitch, coverage area are applied to match with that time solution.
I've founded that the artifact from SVC or left sided heart is not as significant in interpretation as poor/ incomplete filling of the pulmonary artery branches. Therefore I usually use higher contrast volume but rapid scan time.
1) 30 ml is enough, 2) 2 secs is because of test bolus ROI analysis in pulmonary trunk, so as interval for "filling" of peripheral branches (sometimes 5 secs are recommended). 3) using test bolus and small amount of CA means that left heart is "empty" and of course saline flush follows to limit SVC artifacts 4) 128
As Sornsupha Limchareon said empty pulmonary veins and IVC are not necessities in pulmonary CTA, but complete filing of the pulmonary arteries is mandatory. It can always be achieved by a 15 second delay after the start of injection. I mean that without a test bolus you can simply wait for 15 seconds after the start of injection and then start scanning. Usually 2.5-3ml/S is sufficient and the amount of contrast used depends on how fast the scanner can cover the thorax because the injection should be continued during data acquisition. Therefore, for a flash scanner with a thoracic coverage time of 3 seconds 30--40ml may suffice.
The other acceptable method is bolus tracking which takes a slice every 2sec from main pulmonary artery after the start of injection and after reaching a density threshold of 130-150 in the artery, main expose begins. This way you can avoid pulmonary venous and systemic arterial filing.
In addition, using a dual injector may reduce the volume of the injected contrast by injecting saline instead of contrast in the last few seconds to propel the contrast in the tubing, systemic veins and right atrium, but this is only possible when you use a test bolus which is not recommended for pulmonary CTA.
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