Cardiac CT (retrospective acquisition)

A retrospective ECG-gated cardiac CT is usually conducted in cases in which adequate control of heart rate cannot be achieved or in which additional information on ventricular or valvular function is required.

Indications

Please refer to our coronary CT angiography article for general indications. Additional indications for a retrospective acquisition in a cardiac CT include the following ^1,2:

• valvular heart disease (requiring systolic and diastolic image information)
• selected indications where information on cardiac function is needed

Purpose

In a retrospective ECG-gated cardiac CT imaging data is constantly obtained throughout the whole cardiac cycle while the patient moves slowly through the scanner ^1,2.

Retrospectively ECG gated helical or spiral acquisitions are obtained in situations in which information on ventricular or valvular function is needed or in cases where control of heart rate is not sufficient for a diagnostic prospective ECG gated acquisition (e.g. high heart rate or irregular rhythm) ^1-3.

Practical points

• a retrospective acquisition should be conducted with ECG-based tube current modulation if possible, which helps to reduce radiation dose up 30-50% as compared without ^2,3
• in acquisitions with ECG-based tube current modulation the acquisition window should be adjusted to the following:
  • heart rate <65 bpm: 65-75%
  • heart rate 65-72 bpm: 60-80%  
  • heart rate >72 bpm: 35-80%
• ECG-based dose modulation might be omitted in highly variable heart rates or atrial fibrillation 
• each artery will suffer from motion artifact at different phases, the following intervals are the best ‘windows’ to prevent movement for different arteries
  • right coronary artery: 40% of R-R interval
  • left anterior descending: 60-70% of R-R interval
  • left circumflex artery: 50-60% of R-R interval