(peaking), uniformity, and spatial resolution of the camera. These tests can
be carried out with the collimator attached to the camera (extrinsic) or
without the collimator (intrinsic), and should be performed for each
radionuclide used in a specific clinical study.
In the intrinsic method, the source of a particular radionuclide contain-
ing approximately 100 to 200mCi (3.7 to 7.4 MBq) in a syringe is normally
placed at a distance of four to five times the detector field of view to ensure
uniform irradiation of the detector. Because the collimator is removed, the
integrity of the collimator cannot be assessed by this method.
In the extrinsic method, a sheet source is used made of plastic contain-
ing the radionuclide of interest. Because 99mTc is most commonly used in
nuclear medicine studies, a 99mTc sheet source is prepared by adding several
millicuries of 99mTc activity to a water-filled plastic sheet container. The
source should be thoroughly mixed and free of air bubbles. Due to the
inconvenience of daily preparation of the 99mTc sheet source and radiation
exposure to the technologist during preparation, an alternative solid^57 Co
sheet source is used, which is commercially available in rectangular or
circular forms.^57 Co has a longer half-life (~270 d) and emits photons of
122 keV and 136 keV, which are equivalent to the 140 keV photons of
99mTc. These sources, typically made with 10 mCi (370 MBq) of (^57) Co, are also
called floodsources and most commonly used for over a period of one to
two years. The source is placed on the collimator and an image is taken. The
use of these sources in the extrinsic method provides information on PM
tubes as well as any structural imperfections in the collimator. Because^57 Co
activity decays over time, counting time increases with time to accumulate
sufficient counts for the image.
Daily Checks
Positioning of Photopeak
Positioning of the photopeak must be done daily or as needed for each
radionuclide used in the clinical study to center the PHA window on the
center of the photopeak. In older analog cameras, a source of radioactivity
of interest is placed on the collimator attached to the detector (extrinsic)
and the high voltage on the PM tube is adjusted to center the energy
window on the photopeak. For 99mTc, typically 1 mCi (37 MBq) of the activ-
ity in a syringe is used as a source for peaking and a 20% window is set
around 140 keV. Peaking for^111 In,^67 Ga,^123 I,^201 Tl, and so on must be done
separately, as needed.
In modern cameras, peaking is performed automatically by menu-driven
protocol-based software provided by the manufacturer. Initially at the time
of the camera set-up, the photopeak window is set with a 99mTc source using
the intrinsic method. Subsequently the daily check of the position of the
photopeak is performed with a^57 Co flood source by the extrinsic method
134 10. Performance Parameters of Gamma Cameras