Biomedical subjects

T Woldeselassie

Publications and source records attributed to T Woldeselassie.

5 recordsLinked to original sources

PubMed2001

Decaying source method for scintillation camera resolving times.

An earlier paper by the author showed that scintillation camera systems can be described effectively using the resolving times T and tau0 of the dominant nonparalyzable and paralyzable components, that is, the detector system and the computer interface, respectively. When used with a full spectrum window, the camera has a lower nonparalyzable and an upper paralyzable operating range with normalized threshold input rate n(t)= N(t)tau(0) = ln(1 +k(T)n(t)), where kT= T/(tau)0. Correct determination of T and tau(0) requires that both r12 and r in the normalized two-source equations kT=(2/r12- 1/r) and k0=(r12/2r2)ln(2r/r12) come from the nonparalyzable (n< or =n(t)) and paralyzable (n> or =n(t)) ranges, respectively. A serious constraint of the two-source method, therefore, is that the large ratio a = n12 ln = 2 can lead to an input rate range (n,n12), which includes the threshold point n,, and in which neither T nor tau0 can be measured correctly. The decaying source method constitutes a refinement of the two-source method, which enables smaller ratios 1<a<2 to be used, and also includes the two-source method as a special case (a=2). This new method requires just two consecutive readings on a single decaying source, as opposed to three measurements on two sources of activity, for each determination of T or tau0, thus also minimizing staff exposure. The fact that only count rates and time intervals need now be recorded greatly simplifies computerization of the data acquisition and analysis activities, and the potential for real-time applications is obvious. The method enables T and tau0 to be measured accurately and with sufficient resolution to reveal possible variations with input rate. Long measurement times using a decaying source can be avoided, if required, by using a set of decaying sources simultaneously to cover different portions of the count rate range. The application of the measurement procedure in real-time and the use of the resolving times in the accurate correction of deadtime losses, also in real-time, are treated separately in another paper.

Gamma Cameras↗

PubMed1999

Modeling of scintillation camera systems.

Despite their widespread use, the satisfactory modeling of scintillation camera systems has remained difficult. Although the resolving time and deadtime T of a nonparalyzable counter are identical and also invariable, a distinction needs to be made between the fixed resolving time tau0 and the variable deadtime tau of a paralyzable counter. It is shown here that tau = tau0(e(n) - 1)/n, where n = Ntau0 = N/Nmax is the normalized input rate and N the absolute input rate. The normalized output rate, r = Rtau0, where R is the absolute output rate, has a maximum value r(max) = 1/e approximately 0.368 at the input rate n(max) = 1, where tau = tau0(e - 1) approximately 1.718tau0. It is also shown that the response of a system of nonparalyzable and paralyzable components at all input rates is determined by just the dominant nonparalyzable and paralyzable components in the system, the response at any particular input rate being that of the component with the higher of the two deadtimes T or tau. A system can be purely paralyzable (kT = T/tau0 < or = 1), combined paralyzable/nonparalyzable (1 < kT < or = 1.718), or essentially nonparalyzable (kT > 1.718), the combined paralyzable/nonparalyzable system having a lower nonparalyzable (T > tau) and an upper paralyzable (tau > T) operating range separated by a threshold input rate n(t) = ln(1 + kTn(t)) at which tau = T. A highly accurate and explicit expression for n(t) has also been derived. In the essentially nonparalyzable case, the system operates as nonparalyzable all the way up to the system's peak response point, which may occur at or above n(max) = 1. A two-component system with kT > 1 can also be described mathematically as nonparalyzable using r = n/(1 + k(tau)n), where k(tau) = tau/tau0 = kT for n < or = n(t), and k(tau) = (e(n) - 1)/n for n > or = n(t), or as paralyzable using r = ne(-nk0) with k0 = [ln(1 + kTn)]/n for n < or = n(t) and k0 = 1 for n > or = n(t). These alternative descriptions will be of considerable importance in the measurement of T and tau0 for such systems. The model described is able to account fully for the three different operating modes possible with scintillation camera systems.

Models, Theoretical↗

PubMed1989

Improved photomultiplier tube for positron emission tomography.

The paper describes an investigation in which it is shown that small positive voltage pulses applied to an external conductor placed against the photocathode of a photomultiplier tube can be used to switch the photocathode completely off for the duration of the pulses. This suggests that a photomultiplier tube with a multisegment photocathode can be constructed, the individual cathode segments of which can be switched off independently by means of such pulses. A theoretical explanation for the effect is provided with the aid of a simple circuit model for the photocathode. Analysis of the model also shows that it is possible to identify the particular cathode segment in which a photon is detected when a pulse is recorded at the phototube's anode. A phototube with these characteristics can have important implications for positron emission tomography, as it can provide improved spatial resolution, simultaneous multislice capability and the ability to eliminate distortion due to dead-time effects at high count rates.

Biomedical Engineering↗

PubMed1989

Grey level linearising circuit for computer-interfaced display devices.

An earlier paper by the authors on the linearisation of computer-interfaced display devices has described a radiometric procedure for quantifying device non-linearity and a software technique for using the data to linearise screen luminance with respect to display grey level. As the display non-linearity varies with the particular settings of the brightness and contrast controls, any change in these settings requires that the data used with the software also be modified. While it is possible to store data for a number of selected control settings, it would be more convenient if linearisation could be performed using hardware for all relevant settings of these controls. The electronic circuit described in this paper makes use of the same non-linearity data to achieve device linearity with the added advantage of increased flexibility of use. The circuit is suitable for connection immediately following the digital-to-analogue converter or video processor and provides two useful new controls. One of these determines the amount of non-linearity correction to be applied to the video signal, while the second enables screen contrast to be varied without affecting the linearity achieved. The standard brightness and contrast controls of the display device are left as initially preset and require no subsequent adjustment.

Amplifiers, Electronic↗

PubMed1988

Radiometric procedure for linearising computer-interfaced display systems.

Procedures for the adjustment of computer-interfaced display devices have been based on a subjective approach, generally relying on the visual examination of digital test patterns. The lack of an objective adjustment procedure means that the best image quality may not always be achieved. This paper describes how a simple home-made radiometer can be used to make screen luminance as well as density measurements on transparency film. The measurements are first used to determine the brightness and contrast settings for the display which enable the film to be used in the linear range of its optical densities. The display device characteristics, showing how luminance varies with grey level, are then determined. Using this information a simple display-linearising mapping can be produced to ensure that screen luminance (and hence film density) is a linear function of display grey-level value. Film-density measurements are also used to investigate the luminance and hence phosphor uniformity characteristics of the screen. The linearisation of the display device is an essential step in optimising the quality of recorded images. The measurements described are reproducible and sufficiently simple to provide the basis for a reliable quality-control procedure for the periodic assessment of the performance and settings of the display device.

Image Processing, Computer-Assisted↗