Gating
Retrospective
Prospective
Helical
Axial (Step-and-shoot)
Full volume
High pitch Helical (“Flash”)
- Low radiation
- Disadvantage: Wind up with 1 volume of data, no padding (cannot shift to earlier/later phase of cardiac cycle)
Methods to reduce radiation ☢️ exposure
- Retro to pro → 70-90% ↓
- Retrospective to dose-modulated retrospective → 30-50% ↓
- Filtered backprojection to Iterative reconstruction → 40-60% ↓
- ↓ kVp from 120 to 100 → 40% ↓
- Wide array/high pitch → varies
Scan Timing: Test vs Tracking Bolus
- Test bolus - determine bolus timing, i.e. see when it is at its peak → use this timing for the “real” scan
- Test bolus injection for calculation of contrast arrival
- 20 cc (15-20% total) of extra contrast
- ~1 mSv
- ALTERNATIVE: bolus tracking
- Tracking bolus
- Real time imaging of contrast arrival
- Triggers the scan after a threshold reached
- Table movement after threshold delays scan
- ~1 mSv
Figure source
Cardiac output influences timing of peak contrast
Figure source: 1
Footnotes
-
Bae KT, Heiken JP, Brink JA. Aortic and hepatic contrast medium enhancement at CT. Part II. Effect of reduced cardiac output in a porcine model. Radiology. 1998 Jun;207(3):657-62. doi: 10.1148/radiology.207.3.9609887. PMID: 9609887. ↩