Infographics / CT Physics

ECG gating & temporal resolution (prospective vs retrospective)

An ECG trace spanning three R-R intervals with the R-R interval marked. Beneath it, two lanes aligned to the same time axis. The prospective (step-and-shoot) lane shows the x-ray beam on only in a short mid-to-late diastole window (about 70 to 75 percent of the R-R interval), giving lower dose. The retrospective (helical) lane shows the beam on throughout the whole cycle with reconstruction windows marked in diastole, allowing any-phase functional data at higher dose. Diastole is the phase of least cardiac motion and the chosen reconstruction window.

ECG-gated cardiac CT: prospective versus retrospective acquisition timing An ECG trace spanning three R-R intervals with the R-R interval marked. Beneath it, two lanes aligned to the same time axis. The prospective (step-and-shoot) lane shows the x-ray beam on only in a short mid-to-late diastole window (about 70 to 75 percent of the R-R interval), giving lower dose. The retrospective (helical) lane shows the beam on throughout the whole cycle with reconstruction windows marked in diastole, allowing any-phase functional data at higher dose. Diastole is the phase of least cardiac motion and the chosen reconstruction window. ECG-gated cardiac CT — acquisition timing R–R interval diastole (least motion) ECG Prospective step-and-shoot lower dose Retrospective helical higher dose any phase Prospective: beam on only in mid–late diastole (~70–75% R–R) — teal window. Retrospective: beam on throughout R–R; reconstruct any phase (functional). Dashed = diastolic recon window. Diastole = least cardiac motion → chosen reconstruction window. ECG tube-current modulation can lower retrospective dose.
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