Contrast, image scatter radiations

Background in the image increases with scattered radiations and thus adds to degradation of the image contrast. Maximum scatter radiations arise from the patient. Narrow PHA window settings can reduce the scatter radiations, but at the same time the counting efficiency is reduced. 

A major goal of the PET studies is to obtain a good quality and detailed image of an object by the PET scanner, and so it depends on how well the scanner performs in image formation. Several parameters associated with the scanner are critical to good quality image formation, which include spatial resolution, sensitivity, noise, scattered radiations, and contrast. These parameters are interdependent, and if one parameter is improved, one or more of the others are compromised. A description of these parameters is given below. 

Several factors affect the contrast of an image, namely count density, scattered radiations, type of film, size of the lesion, and patient motion. Each contributes to the contrast to a varying degree. These factors are briefly discussed here. 

Barnes GT, Brezovich IA (1977) Contrast effect of scattered radiation. In Logan WW (ed) Breast carcinoma the radiologist s expanded role. Wiley, New York, pp 73-81 Barnes GT, Brezovich lA (1978). The intensity of scattered radiation in mammography. Radiology 126 243-247 Barrett HH, Myers KJ (2001) Foundations of image science. Wiley, New York... 

Figure 4.4 demonstrates the effect of scatter radiation for image reconstruction for a physical test object in a large collimation CT system. The strongly reduced contrast due to large amounts of scatter is obvious (Schlattl and Hoeschen 2008)... 

If the scattered beam is a sharp spot diffracted from a single crystal, the phase contrast image when it is recombined is an image of the crystal lattice. This specialized phase contrast technique is applied to the study of atomic scale structure in crystalline specimens of metals and ceramics. It has only rarely been applied to the study of polymer materials due primarily to their instability in the electron beam. Lattice images have been obtained from radiation stable aromatic molecules, such as the liquid crystalline polymers (Section 5.6). They have shown important information regarding the ordered structure. 

Phase contrast means that scattered radiation has a phase shift applied to it, and this results in intensity changes that form an image. The basic reason for this can be seen in Fig. 3.7. Normally, scattered radiation is 90° out of phase with the unscattered radiation. The result of recombining them is a phase shift in the transmitted wave but no change in amplitude (Fig. 3.7A). This phase shift is normally described in terms of a refractive index not equal to 1. If the sample is homogenous and all the scatter is in the forward direction, there is no way to affect the phase of one wave and not the other. But if inhomogeneities make the scattered... 

Although the raw efficiency, or number of true coincidences acquired, is a basic determinant of the quality of a PET scanner, the complicating factors of scattered and random events and dead time have to be brought into the analysis. Details of the distribution of scattered events and correction methods can be found elsewhere, but for the present purposes it can be stated that scattered radiation (or scatter for short) produces a relatively flat background on the projection and image data, impairing contrast and reducing quantitative accuracy. The quantity of scatter detected, the scatter fraction (SF), is expressed simply in terms of the total true (unscattered -i- scattered) events (Tj j) and the scattered events S) by... 

Fig. 2 Laser written micro-optical components, a Optical microscopic image of a Y coupler drawn in pure fused silica, which guides 514.5-nm light from an argon-ion laser. The scattered radiation from the coupled argon-ion light is observed in the photograph. The vertical direction is magnified with respect to the horizontal direction for clarity, b Phase contrast microscopic image of one of the two X-couplers that make up a Mach-Zehnder interferometer, c Optical microscopic image of a Fresnel zone plate...

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