Sensitivity and Depth of Interaction

The sensitivity of a PET scanner represents its ability to detect the coincident photons emitted from inside the scanner FOV. It is determined by two parameters of the scanner design; its geometry and the stopping efficiency of the detectors for 511 keV photons. Scanner geometry defines the fraction of the total solid angle covered by it over the imaging field. Small-diameter and large axial FOV typically leads to high-sensitivity scanners. The stopping efficiency of the PET detector is related to the type of detector being used. As we have seen, scintillation detectors provide the highest stopping power for PET imaging with good energy resolution. The stopping power of the scintillation detector is in turn dependent upon the density and Zeff of the crystal used. Hence, a majority of commercially produced PET scanners today use BGO as the scintillator due to its high stopping power (see Table 2.4). A high-sensitivity scanner collects more coincident events in a fixed amount of time and with a fixed amount of radioactivity present in the scanner FOV. This generally translates into improved SNR for the reconstructed image due to a reduction in the effect of statistical fluctuations.

A high stopping power for the crystal is also desirable for the reduction of parallax error in the acquired images. After a photon enters a detector, it travels a short distance (determined by the mean attenuation length of the crystal) before depositing all its energy. Typically, PET detectors do not measure this point, known as the depth of interaction (DOI) within the crystal. As a result, the measured position of energy deposition is projected to the entrance surface of the detector (Fig. 2.26). For photons that enter the detector at oblique angles, this projected position can produce significant deviations from the real position, leading to a

Flight path of photons Assigned line of response

Figure 2.26. Schematic representation of parallax error introduced in the measured position due to the unknown depth-of-interaction of the photons within the detectors for a flat detector (left) and ring-based system (right).

blurring of the reconstructed image. Typically, annihilation points located at large radial distances from the scanner's central axis suffer from this parallax blurring. For a BGO whole-body scanner, measurements show that the spatial resolution worsens from 4.5 mm near the centre of the scanner to about 8.9 mm at a radial distance of 20 cm [15]. A thin crystal with high stopping power will help reduce the distance travelled by the photon in the detector and so reduce parallax effects. However, a thin crystal reduces the scanner sensitivity. Thus, to separate this inter-dependence of sensitivity and parallax error, an accurate measurement of the photon depth-of-interaction within the crystal is required.

Development of PET detectors with depth-of-inter-action measurement capabilities is an ongoing research interest. Currently there are two practically feasible techniques that can be used for depth-of-inter-action measurement. The first is the phoswich detector [16] method that involves stacking thin layers of different scintillators on top of each other, instead of using a thick layer of one crystal type. The depth-of-interac-

tion measurement in a phoswich detector depends on the identification of interaction layer through an examination of the different signal decay times for the scin-tillators. As a result, the scintillators used in a phoswich detector need to have significantly different decay times in order to successfully distinguish them via pulse shape discrimination techniques. Another potential problem in its implementation is the optical coupling between the individual layers of crystals. Good optical coupling is necessary for the successful transmission of scintillation photons from the crystals into the photo-detectors, thereby achieving good spatial and energy resolution as well.

Another technique for determining the depth of interaction involves the use of photo-detectors at both the ends of a thick (or long) scintillator. This technique is based upon the physical principle according to which the relative number of scintillation photons reaching either of the end photo-detectors is a function of the photons depth of interaction in the crystal. Figure 2.27 shows a single-channel implementation of this technique. For a

Scintillation Detector

Scintillation Detector

Figure 2.27. A single channel of one layer detector for DOI determination through the use of two photo-detectors at the crystal ends. In this schematic conventional light collection by PMTs are used at one end and an array of avalanche photodiodes are used on the incident face of the detector.

Array of Photodetectors

Photomultipliers

Array of Photodetectors

Photomultipliers

Figure 2.27. A single channel of one layer detector for DOI determination through the use of two photo-detectors at the crystal ends. In this schematic conventional light collection by PMTs are used at one end and an array of avalanche photodiodes are used on the incident face of the detector.

practical implementation in a scanner design, the use of regular photo-multiplier tubes at both ends is not feasible. As result, at least one such detector design has considered using a different type of photo-detector, such as PIN photodiodes or Avalanche photodiodes, on the crystal end that enters the scanner field of view [17].

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