the images. The mA value set here is the 100% level for
Acquisition Window Part 2
or
Acquisition Window Part 3
below.
If acquiring data for multiple purposes (coronary, function, et cetera), Coronary Assessment
acquisitions should be prescribed in
Acquisition Window Part 1
.
Widen for SnapShot Freeze (SSF)
Set the Widen for SnapShot Freeze gating parameter to “On” to ensure that the system will
acquire sufficient data to support SnapShot Freeze processing at all of the prescribed phases or
times.
For example, if
Acquisition Window Part 1
is 70%–80% and the Widen for SSF gating setting is
enabled, then as long as the heart rate is within HR Variation Allowance of the predicted heart
rate, the system will be able to generate images to support SnapShot Freeze processing at
70%, 80%, and all phases in between.
Applying this parameter to a part of the Acquisition Window can result in a slightly larger data
acquisition window. To reduce patient dose, this parameter should not be applied to the part or
parts of the Acquisition Window that will not be used for coronary assessment.
3.5.1.3 Acquisition Window Part 2
A second data Acquisition Window can be used to acquire additional cardiac phases during the
same heart cycle used for
Acquisition Window Part 1
.
For example, a value of 45%, or a range of 45%–50%, would acquire scan data near the end-
systole.
A range of 0 –100% would acquire scan data to generate all phases of the cardiac cycle, which
may be useful for functional assessment of the heart. A significantly lower mA value, such as
20%, significantly reduces the tube current except where higher tube current is required by
other acquisition windows. Wide reconstructed slices, such as 2.5 mm instead of the 0.625 mm
typically used to assess coronaries, reduces the amount of increased noise that otherwise
would be seen in functional images.
A range of 60% –150% would acquire data from early-diastole (60%) through the end-diastole
(100%) and continuing to the end-systole in the following beat (150%). Phases reconstructed
every 10% would then show an entire heartbeat and would allow an ejection fraction calculation
based on an end-diastolic measurement followed by the subsequent end-systolic measurement.
Allowed values for the mA % are 10%–100%. A typical value is 100%, indicating that the system
uses the mA value specified in the
Scan
chapter,
kV and mA Control
section.
For additional details, see the
Cardiac Modulated mA
section in this chapter and the
Scan
chapter,
kV and mA Control
section.
3.5.1.4 Acquisition Window Part 3
A third data acquisition window can be used to acquire additional cardiac phases during the
same heart cycle used for
Acquisition Window Part 1
.
For example, if
Acquisition Window, Part 1
is used to acquire 75%, and
Acquisition Window Part
2
is used to acquire 45%, then
Acquisition Window Part 3
can be used to acquire functional
images from 30%–120%. When combined with a lower mA, such as 20%, this allows the
acquisition of functional images for relatively little additional dose.
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3 Cardiac Scan Parameters
