Fluke Biomedical 76-823 - Manual (Page 8)

Nuclear Associates 76-823,76-824,76-825
Operators Manual
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B. Resolution - Cold lesions in a Hot Field
In general, for lesions of equal size and contrast, cold lesions in a hot field are less visible to the eye than
hot lesions a cold field. Compare, for example, the visibility of a star in a black sky with that of a high-
flying airplane against the face of the moon.
Procedure
Make a reconstruction of the Cold Lesion Insert (the one with seven rods logarithmically-graded in size).
The image the smaller rods may not be visible in the reconstruction slice. The smallest rod visible is the
measure of the resolution.
C. Plastic Spheres (Installed on Cold Lesion Insert)
The slice images are used to evaluate the cold lesion visibility and resolution with partial volume effects
controlling the contrast. To vary the contrast, move the reconstructed slice position longitudinally so that
the slice contains different-sized portions of the spheres and background activity.
Procedure
The first reconstructed image should be made with the plane of the sphere centers coinciding with the
plane at the center of the slice. Subsequent images at different points along the phantom axis will show
the partial volume effects. The spheres are mounted on screw pins so that their positions can be
interchanged, if desired.
D. Linearity
To check linearity, use the acrylic block with the crosshatch pattern.
Procedure
I) Position the reconstructed slice near the center of the cutout section of the block.
2) Among the common problems are "pincushion" or "barrel" distortion. An elliptical appearance of the
circular pattern is caused by unequal X and Y amplifications. Noticeable distortion requires
adjustment of the apparatus.
E. Uniformity
Examine a slice image of the region of uniform activity. About 3" of such uniform space exists above the
inserts. The purpose is to detect uncompensated radiation self-absorption in the radioactive solution, and
reconstruction artifacts of various kinds.
Procedure
1) Reconstruct a transverse slice of a uniform area.
2) If ring artifacts are seen, the center of rotation offset may be incorrect, or there are uncorrected
nonuniformities in the flood field. If the center of the image is lighter than the rest, self-absorption is
not being compensated.
F. Center of Rotation, Point-Spread Function and MTF
With the tank empty, the center of rotation error and a pointspread function (PSF) can be measured. A
similar measurement in the presence of radiation absorption can be made with the tank filled with water
only.
Procedure
1) Fill a micro-hematocrit capillary tube with a technetium solution (about 20 uCi in 1/15 cc water).
Plug the ends with wax.
2) Insert the tube into the hole in the center of the inner face of the central cover plug. Screw the plug
into the tank cover so that the hematocrit tube extends into the tank, along the tank axis.
3) As in Section E, image and examine a reconstruction of part of the tank, empty of inserts. If the reconstructed
image of the tank is not circular, the center of rotation factor calculated by the computer is wrong and should
be recalculated. A well-centered system will render a PSF that can be used to calculate the system's MTF.