2D Imaging Based on the Anger Principle

The method of position-sensitive and energy-selective detection of radioactive gamma radiation was described in the previous chapter. Relying on the physical basis explained there, now we discuss the most effective and most prevalent imaging device used in diagnostic nuclear medicine, the gamma camera that operates based on the Anger principle. This device creates the two dimensional – 2D – projection image of the radioactivity distribution of a given organ in a given direction. The 2D imaging technique of the spatial distribution of radioactivity used in noninvasive medical diagnostics, which is to be discussed below, was first created by Hal Anger in 1956. The structure and the main elements of the Anger gamma camera - where the detector system assembly didn’t change over time - is shown in Figures 1. and 2. of the next chapter (3.3.3 The centroid method and the Anger camera).

The distribution image of the radioactive gamma-radiating object – the organ being examined – is projected on the Nal(Tl) single crystal (which can be circular, hexagonal or rectangular) by the collimator, to which the PMT block is fitted by optical couplings through glass windows and light guides in a way that the PMTs optimally cover the surface of the scintillation detector.

The PMT block of today’s modern Anger gamma cameras made for general diagnostic purposes contains 37÷90 PMTs, depending on the arising economical and technical demands. The outputs of the PMTs are connected directly to the inputs of the charge-sensitive preamplifiers that are attached to the PMTs. The task of the preamplifiers is to convert the signals provided by the PMTs (charge quantity) into voltage signals and to fit them into impedance for further signal processing. (Henceforth, when the signals created by the PMTs are mentioned, we always mean the signals produced by the preamplifiers and fitted into impedance, because the preamplifier is considered to be an integral part of the PMT at this functional level).

The outputs of the PMTs are connected to the Position and Energy Decoder unit, where the position decoding – X coordinate signal, Y coordinate signal – of the individual nuclear events is performed using the centroid method (see the previous chapter and Figure 10.).

Parallel to this, the analysis of the energy signal E is also carried out within the Pulse-height Analyzer. In practice it is a differential discriminator, which gives a logic signal when the maximum value of the energy signal E is within a pre-chosen window. If this condition is met, the Pulse-height Analyzer sets off the Trigger Circuit, the output signal Z of which will permit the flash of the gamma event in the point corresponding to its position on the screen, depending on the values of the X and Y coordinates (Figure 10.).

 
If a Polaroid camera or an X-ray film is placed in front of the screen of the oscilloscope and the flashes of light are collected for a sufficiently long time, the activity distribution image of the organ being examined will appear in the photo or on the X-ray film. Therefore, the image is created based on the flashes of light according to the position of the detected events, which flashes need to be collected on the applied photosensitive material (film) for a sufficiently long time. The Trigger Circuit ensures that unless the processing of an event has been performed, the system does not permit the processing – position and energy decoding – of another event. Naturally, it causes loss of time (dead time), which results in a certain loss of information. In today’s state-of-the-art cameras the dead time is not longer than 5 µs, but devices with a dead time of 2 µs exist as well. Taking into consideration the random nature of the process, this rate of signal processing is sufficiently rapid, because in most cases in clinical practice there rarely are impulse rates around 50000 imp/sec ÷ 100000 imp/sec, which is just a minimal loss compared to the aforementioned dead time.

Based on all the above mentioned conditions - Figure 1. -, the theoretical fundaments of Anger camera - Anger criteria’s - can be described as follows:

1. The activity distribution of the radioactive material is imaged by a collimator mounted in front of the surface of 2D position sensitive and energy selective detector, where the radioactive source is located face to the collimator.
2. The 2D position sensitive and energy selective detector consists of a NaI(Tl) single kernel scintillation crystal with a large surface, upon which an optically coupled PMT block is placed in a way to optimally cover said surface.
3. The position estimation of the emitted nuclear events is performed by the centroid method, where the linear high gradient range of MDRF is under consideration along the surface of a detector, as well as in that range the energy function E(x,y) can be considered location independent i.e. constant.
4. If a new non-noise event arrives during the processing of an event, then the processing of the new events won’t begin, while the current event processing is in progress. This is the “dead time” of the system (“Queuing Theory” of random events).

 

---