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At least 1,261 records · Page 70Linked to original sources
PubMed2002

Precise real-time correction of anger camera deadtime losses.

An earlier paper dealt with modeling of the camera in terms of the resolving times, tau0 and T, of the paralyzable detector and nonparalyzable computer system, respectively, for the case of a full energy window. A second paper presented a decaying source method for the accurate real-time measurement of these resolving times. The present paper first shows that the detector system can be treated as a single device with a resolving time tau0 dependent on source distribution. It then discusses camera operation with an energy window, window fraction being fw = Rp/Rd < or = 1, where Rd and Rp are the detector and pulse-height-analyzer (PHA) outputs, respectively. The detector resolving time is shown to vary with window fraction according to tau0p = tau0p/f(w), while T is unaffected, so that operation may be paralyzable or nonparalyzable depending on window setting and the ratio kT = T/tau0. Regions of interest are described in terms of the ROI fraction, fr =Rr IR < or = 1, and resolving time, tau0r = tau0p/fr, where R and Rr are the recorded count rates for the field-of-view and the region-of-interest, respectively. As tau0p and tau0r are expected to vary with input rate, it is shown that these can be measured in real-time using the decaying source method. It is then shown that camera operation both with fw < or = 1 and fr < or = 1 can be described by the simple paralyzable equation r = ne(-n), where n=Nwtau0p=Nrtau0r and r=Rptau0p=Rrtau0r, Nw, and Nr being the input rates within the energy window and the region of interest, respectively. An analytical solution to the paralyzable equation is then presented, which enables the input rates Nw = n/tau0p and Nr = n/tau0r to be obtained correct to better than 0.52% all the way up to the peak response point of the camera.

Equipment Design↗
PubMedDate not supplied

Count-based monitoring of Anger-camera spectra--local energy shifts due to rotation.

This study reports on a spectral monitoring method in which (1) a small source fixed to the camera is used, (2) a narrow, offset window is set on the side of the photopeak, and (3) variations in count rate are measured to assess energy shifts in the vicinity of the source. For one camera model, the count rate drops from 100% to 76% over a rotation of 180 degrees, implying a local energy shift of 1.4 keV. Also looked for are local count-rate variations with rotation for (1) wide-symmetric, (2) 20%-symmetric, and (3) 10%-asymmetric windows. The last is in limited use to partially compensate for Compton scattering. The effects of background and time stability are assessed.

Gamma Cameras↗
PubMedDate not supplied

Dead time of an anger camera in dual-energy-window-acquisition mode.

Two side-by-side energy windows are sometimes employed in quantitative single photon emission computed tomography (SPECT) studies. The count-rate losses at high activities for a GE400AT camera were measured in such a dual-window-acquisition mode by imaging a decaying source composed of a hot sphere within a warm cylinder. The data in each window was either kept separate or combined for purposes of dead-time correction and the paralyzable model was assumed. In addition, correction factors were derived from a "monitor" source at the edge of the camera. Finally, energy spectra for only the monitor-source region of interest and for the entire camera were obtained under both low- and high-count conditions and compared. With 99mTc (1) the spectral measurements show no peak shift but reveal pulse-pile-up spectral degradation. (2) The monitor-source corrections do not agree well with those from the model, presumably because of the differential effects of such degradation. For this camera the preferred correction method for patients is one using the model and two, effective, phantom-derived dead times for the separate data from the two windows. The two effective dead times are needed to compensate for pulse pile-up adding more counts to the lower-energy window than to the higher-energy one at high rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Biometry↗