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2.2 Guidelines for adjusting individual exposure parameters by patient
2.2.1 Adjust parameters
The aim of adjusting imaging parameters is to improve the image quality by increasing the
amount of X-ray quantum energy received by detector. We strongly encourage you to work with
your radiologist and medical physicist to determine the appropriate dose for the image quality
desired. Each large patient should be evaluated individually to determine appropriate imaging
parameters.
2.2.2 High kV
It is important to use higher kilovoltage selections on the very large patient. Using 140 kV is
recommended for large patients because it provides higher X-ray penetration of anatomical
structures being imaged. On the other hand, higher kV selections can decrease HU (Hounsfield
Unit) values so the window width for viewing the images may need to be decreased to maintain
a similar appearance.
2.2.3 mA adjustment
The reasonable use of scan technique with SmartmA on a large patient can reduce patient dose
effectively. However, significant differences may exist between the selected noise index and the
image standard deviation when a very large patient provides insufficient detector signal (or
prevents sufficient detector signal). In this case, electronic noise sources can become the
dominant image noise source vs. X-ray noise. Under these conditions, special projection data
dependent filters are applied at various threshold levels to help preserve image quality.
For more information on SmartmA refer to
NOTE:
If the size of the large patient section is beyond 50 cm SFOV, SmartmA technique is
not recommended since noise and mA prediction may not always be reliable. Manual
mA is recommended.
2.2.4 Lower pitch / slow rotation time
Decreasing the pitch and rotation time will allow more mA to be delivered over the area of
interest without compromising the diagnostic quality of the anatomical area scanned; however,
this will be at the expense of extending the exposure time for the acquisition.
2.2.5 Scan Type
Compared to the Helical scan type, the Axial scan type provides a greater opportunity to
increase mAs to achieve improved image quality for large patients but at the compromise of the
total exam scan time.
NOTE:
Using lower pitches, slower rotation times, or axial instead of helical with large
patients to improve image quality, comes at the cost of longer scan times which
means longer breath holds.
Consider the following to offset increasing acquisition times beyond the patient's capability to
hold their breath:
•
If multi-phase studies are needed, use lower dose techniques for the non-contrast series
compared to the contrast series and limit the scan to only the organ or anatomical region
indicated.
Chapter 6 Large Patient
Direction 5480385-1EN, Revision 1
SmartmA in the Scan Parameters section.
Revolution CT User Manual
135
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