Contrast Resolution
Background: Contrast resolution, sometimes referred to as functional resolution or low-contrast
detectability, is a test of an imaging system's ability to detect and display the size, shape, and depth of
dark, or anechoic, target structures. This test indicates the smallest diameter target a system can
resolve at a given depth.
When performing this test, anechoic targets should appear circular with sharp, clearly defined edges,
and be free of any observable shading (echo fill-in). Possible sources of echo fill-in include poor
resolution (which causes speckle from the echogenic background to spread out into the anechoic
target), internal system noise, the presence of side lobes in the ultrasound beam, and defects in the
image processing architecture. Figure 1 provides a good example of echo fill-in.
Figure 1 The 8 mm targets in the ATS
539 appear to shrink with depth as
speckle in the background fills in the
target. Other sources of fill-in include
electronic noise and side-lobes in the
ultrasound beam profile.
When imaging anechoic targets, bright echoes may be observed at the top and bottom of anechoic
targets, as shown in figure 2. These echoes are normal specular reflections and do not present a
problem.
Figure 2 The bright echoes at the top and
bottom of these contrast targets in the
54GS phantom are due to specular
reflection at the interface between the
target and the background. These are
normal and do not indicate a problem with
the system.
Contrast resolution testing also provides a good opportunity to check for horizontal banding in the
image, a uniformity artifact caused by problems with the system software or electronic circuitry. While
this problem is rare, horizontal banding can be missed in the Image Uniformity & Artifact Survey due to
the limited depth of field used in that procedure.
Measurement Procedure:
In the Model 40GSE, contrast resolution is measured using anechoic stepped cylinders, whose diameters
change as the transducer moves along the width of the phantom (figure 3).
Figure 3 Layout of the anechoic stepped cylinders in the Model 40GSE
The procedure for measuring contrast resolution in the 40GSE follows:
1. Position the transducer over the anechoic target structures so that a pair of round targets are
visible at each depth.
2. Scan along the length of the cylinder to determine the size of the smallest cylinder visible for the
first pair of targets (1.5 cm deep). While scanning, examine the image for geometric distortion
and areas of non-uniformity, particularly horizontal banding. Freeze and save images for any
detected artifacts.
3. At each depth, save images showing the smallest detectable cylinder at each depth. Record
measurements in QA portal.
4. If artifacts were detected during step 2, repeat the scan at a different location to rule out a
defect in that region of the phantom. If the artifacts are still present, repeat the scan using a
different gain and focal setting. Be sure to save images of any artifacts found. Finally,
document your findings in the “Distortion Assessment” field of the QA portal and make a
pass/fail decision based on the clinical significance of the finding. This may require consultation
with the system manufacturer and the physician in charge.
Figure 4 Three different cross-sections of
the anechoic stepped cylinders as imaged
with an abdominal probe.
In the top image, the largest step running
down the middle of the image are visible
diameters (6.7 mm diameter at a depth of
1.5 and 4.5 cm; 10.0 mm diameter for 7,
10 and 13 cm depths). The smallest step
diameters to the left of the larger steps are
not visible due to limited resolution. These
steps are 1.3 mm in diameter at 1.5 and
4.5 cm and 2.0 mm in diameter at 7, 10
and 13 cm.
In the middle image, the next set of step
diameters (4.5 mm for 1.5 and 4.5 cm
depth and 6.7 mm for 7, 10 and 13 cm) are
visible. The 2.0 mm step to the left is
visible at 4.5 cm but not at 1.5 cm. The 3.0
mm step is visible at 7 and 10 cm, but not
at 13 cm due to fill-in from electronic
noise.
In the bottom image, the diameter of the
two columns of step diameters match (3
mm at 1.5 and 4.5 cm and 4.5 cm at 7, 10
and 13 cm). The cylinders are visible at all
depths (although the left cylinder at 1.5
cm is difficult to see at this resolution).
Thus, on the QA portal the following
values would be entered for the smallest
visible step diameter:
-
1.5 cm: 3 mm
4.5 cm: 2 mm
7 cm: 3 mm
10 cm: 3 mm
13 cm: 4.5 cm
16 cm: skip, since it is outside
the maximum depth of
penetration
Figure 5 Three different cross-sections of
the anechoic stepped cylinders as imaged
with a small parts probe. The only
cylinders in the field of view of this
transducer are those at 1.5 and 4.5 cm.
In the top image, the largest step
diameters (6.7 mm) on the left hand side
are visible at both 1.5 and 4.5 cm depths.
The smallest step diameter (1.3 mm) is
visible at a depth of 1.5 cm (but is difficult
to see here due to the quality of the
image) but is not visible at 4.5 cm due to
signal attenuation / loss of contrast.
In the middle image, the next set of step
diameters on the left side (4.5 mm) are
still visible, while the 2.0 mm diameter
cylinders to the right are also visible..
In the bottom image, both cylinders are 3
mm in diameter and are visible at both
depths.
Thus, on the QA portal the following
values would be entered for the smallest
visible step diameter:
-
1.5 cm: 1.3 mm
4.5 cm: 2 mm
7 cm: skip
10 cm: skip
13 cm: skip
- 16 cm: skip