4
GENERAL GUIDELINES FOR PERFORMING MEASUREMENTS
It is recommended that all measurements be performed at the most frequently used
imaging arrangements. The importance of these tests is to make sure the system
performance remains constant over an extended period of time. Measurements
may also be used to compare the performance of various setups of the same ma-
chine or to compare different machines in a quantitative manner.
The following are general steps for imaging all targets:
•
Some wires will appear as short lines rather than dots. When using the
electronic calipers, always take measurements from a point on one echo
to the same point on the next, i.e., center to center. Otherwise, errors may
be introduced.
•
If a convex probe is used, center the target within the scan plane in order to
minimize degradation and distortion introduced on the outer edges of the
probe.
•
When assessing vertical distance measurements, DO NOT press on the
scanning surface. Pressure on the scanning surface causes the wires to
become temporarily displaced, making vertical distance measurements inac-
curate.
•
When assessing horizontal distance accuracy, ensure the scan plane is
perpendicular to the horizontal target group. Rotation of the probe will result
in inaccurate distances.
•
Always be sure the phantom is scanned while at room temperature. A
phantom just received may be colder or hotter than room temperature de-
pending on where it was stored during shipping. Temperature affects the
speed of sound and, ultimately, the perceived measurements. The phantom
should be stored at room temperature for at least 24 hours before use to
ensure its core temperature is correct.
•
The most accurate measurements will be made with the phantom 22˚C ±
1˚C (70˚F–73˚F).
ESTABLISHING A BASELINE
Before performing routine quality assurance measurements, establish:
1. System settings for each measurement:
System setup can have a dramatic impact on the results obtained from quality as-
surance measurements. You must establish and record what system settings
should be used for each of the quality assurance tests. These same settings
should be used each time the test is performed. If not, then the conclusions
drawn may not be valid. CIRS recommends that you use the most commonly set-
tings for the type of probe tested- i.e. the liver preset values for an abdominal
probe- which are called a "normal" technique in the sections that follow.