Now that's a good looking CTPA! What you should be looking for in regards to timing and image quality are:
(1) good enhancement of the pulmonary trunk and branches with HU > 300;
(2) little or no contrast in the left atrium and aorta - you've scanned too late if there is;
(3) minimal beam hardening artefact from the SVC - you've injected too much contrast if seen.
This scan used 30ml contrast at 5ml/sec with a saline flush, scan delay of 8 seconds and scan time 4 seconds.
Timing Bolus should be favoured over Bolus Tracking for two reasons:
1) Bolus Tracking has a delay between initiating the scan and the actual scan beginning (table moves, breathing instructions are delivered, etc). This is usually 3-5 seconds, so if the scan is initiated when contrast is in the pulmonary trunk, it will most likely be in the aorta at the time of the scan and therefore too late.
2) You cannot determine how long it will take for contrast to reach the pulmonary trunk with Bolus Tracking, therefore you need to inject a higher volume of contrast as a safety net. This also leads to higher levels of beam hardening artefact from the SVC because contrast is still flowing in while the scan is being taken.
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