where “high” and “low” are the maximum and minimum differences in
counts over five contiguous pixels in all rows and columns of the matrix.
The computer program provides these values for both central field of view
(CFOV) and the useful field of view (UFOV). These values should be in
the range of 1 to 2%, otherwise, the camera needs to be tuned. The UFOV
is a circular area with the largest diameter inscribed within the collimated
field of view. The CFOV is the circular area with the diameter equal to 75%
of the diameter of the UFOV. These definitions may not apply to rectan-
gular FOV scanners.
Weekly Checks
Spatial Resolution and Linearity
The spatial resolution and linearity of the gamma camera is checked weekly
by using a bar phantom (Fig. 10.3). The bar phantom is placed on the detec-
tor head with a low-energy high-resolution collimator attached (extrinsic),
and a flood source of ~10 mCi (370 MBq) of^57 Co is placed on the top of the
bar phantom. An image is taken with approximately 10 million counts and
visually inspected to check the linearity and separation of the smallest bars.
This is a qualitative method. For a quantitative method, a line spread func-
tion must be determined and then FWHM measured as discussed earlier.
Although extrinsic tests are done for convenience, intrinsic tests are prefer-
able for better accuracy.
Annual or As-Needed Checks
Tuning of the camera is performed monthly or quarterly by the protocols
described earlier. Other essential parameters such as energy resolution,
high count rate response, multiwindow registration (e.g.,^111 In and^67 Ga with
multiple photons) and sensitivity should be evaluated at least annually or
as needed after extensive modification or repair of the camera.
In addition, tests on accessories such as computers, multiformat cameras,
scanning tables, rotation of gantry, and so on should be performed period-
ically. Furthermore, all tests must be documented in a record book with per-
tinent information, such as the date, time, total counts, window settings, the
type of radioactive source, the type of camera, and initials of the technolo-
gist performing the tests.
Questions
- (a) Define the spatial resolution of a gamma camera.
(b) What are the different components of the spatial resolution?
differential uniformity
high low
high low=
−
+
× 100
136 10. Performance Parameters of Gamma Cameras