Tissues photon distribution

The emitted positron cannot penetrate far into tissue its range is only a few millimeters. After slowing down, it is annihilated with an electron. In most cases two photons (y radiation) are emitted with an energy of 511 keV each under 180. PET exploits the collinear emission by putting a coincidence requirement on detectors opposed to each other, an event detected simultaneously in the two detectors implies that the annihilation took place somewhere along the line between the two points of detection (Fig. 9.2). From the coincidence data, images are constructed which present the three-dimensional distribution of the radioactivity. The images are corrected for... 

A typical image obtained by nondescanned detection and two-photon excitation is shown in Fig. 5.78. The autofluorescence of aortic tissue was excited at 800 nm. The figure shows the intensity image, an image of the average lifetime, and the lifetime distribution over the pixels. The fluorescence decay displayed for a selected pixel is multiexponential, as is typical for autofluorescence. 

Fig. 5.78 Two-photon autofluorescence lifetime image of aortic tissue. Top Intensity image, lifetime image of average lifetime, lifetime distribution. Bottom Fluorescence decay in indicated pixel and double exponential fit with 81% of 294 ns and 18.7% of 2.26 ns. From [39]...

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