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3.5.1.1 Gating Based On
This setting shows the heart rate calculations based on the most recent ECG recording and the
associated average heart rate.
When no ECG recording exists, the values for parameters normally specified by the Auto Gating
feature come directly from the scan protocol definition.
You can change the Auto Gating Profile at this point in the workflow, if needed. If a different
Auto Gating Profile is desired, you must change it from the
Auto Gating
window.
3.5.1.2 Acquisition Window Part 1
This setting directs the system to acquire data so that an image may be generated at specified
cardiac phases or times. For background information about R-wave, R-wave to R-wave, and %
phase, see Section 5.1.8 .
While preparing to acquire scan data, after each R-wave is detected, the system predicts the
heart rate of the upcoming beat to be the same as the last previously measured regular beat.
The system begins to acquire scan data from about half a rotation before the first required
phase or time, continuing to acquire scan data until about half a rotation after the last required
phase or time. For example, at a gantry rotation time of 280 ms and a real-time predicted heart
rate of 60, corresponding to an R-wave to R-wave time of 1000 ms, an Acquisition Window of
75% would result in scan data acquisition from about 610 ms (= 1000 * 75% - 280/2) to about
890 ms (= 1000 * 75% + 280/2).
The system sets the minimum data acquisition so that SnapShot Freeze images can be
generated for at least one cardiac phase.
Although scan data is acquired for a duration of at least one complete rotation, the temporal
resolution is, on average, half of the gantry rotation time (for example, 140 ms when the gantry
rotation time is 280 ms). The additional data is used for the outer few slices, and for image
quality improvements throughout the volume. The tube current (mA) is typically modulated to
20% for the outer views so the additional image information can be provided with minimal
increase in patient dose.
A range can be specified (for example, 70% – 80%), indicating that the system should acquire
data so that all phases in the range can be reconstructed, assuming that the actual heart rate
variation is within the limits specified by the HR Variation Allowance parameter (described
below).
A time value is used to specify an absolute time after the R-wave. For example, a value of 250
ms will acquire scan data centered 250 ms after an R-wave, which may be useful, for example,
for imaging patients prior to electrophysiology procedures.
The phases or times to be acquired are shown graphically in the
Acquisition Window and Heart
Rate
widget.
Allowed values for the Acquisition Window are from 0 – 300%, or from 0 –1000 ms. A typical
value is 75%, typically corresponding to mid-diastole. Values greater than 100% are for phases
to be acquired from the following beat, most commonly used for
Acquisition Window Part 2
or
Acquisition Window Part 3
.
Set the mA value for this Acquisition Window to a percentage of the value specified in the
kV
and mA Control
setting. Lower values reduce dose, but will also increase the amount of noise in
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
Chapter 13 Cardiac 349