Scatter fraction

The scatter fraction (SF) is another parameter that is often used to compare the performances of different PET scanners. It is given by 

Contrast of an image arises from the relative variations in count densities between adjacent areas in the image of an object. Contrast (C) gives a measure of the detectability of an abnormality relative to normal tissue and is expressed as 

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. 

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. 

Film contrast is a component of overall image contrast and depends on the type of film used. The density response characteristics of X-ray films are superior to those of Polaroid films and provide the greatest film contrast, thus adding to the overall contrast. Developing and processing of exposed films may add artifacts to the image and, therefore, should be carried out carefully. 

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Another important property of the specularly scattered fraction of atoms is their great sensitivity to surface disorder. On scattering from a well ordered surface, nearly 15% of the scattered helium atoms appear in the specular helium beam. This fraction decreases to 1 to 5% when the surface is disordered. Thus measurements of the fraction of specularly scattered helium can provide information on the degree of atomic disorder in the solid surface. 

The translational and internal energy dependences of the dissociation probability can yield a great deal of information regarding the PES, but the final state is not fully specified (only given as dissociated or not dissociated) and this leads to some loss of information. Much more detail can be obtained by examining the scattered fraction instead. Diffraction intensities tell us about the surface site dependence of the PES, while comparison of the internal state populations before and after scattering tells us about the changes of vibrational and rotational state, and hence about the curvature of elbow PESs and the molecular orientation dependence of the PES. 

Photon cross-correlation spectroscopy (PCCS) uses a novel three-dimensional cross-correlation technique which completely suppresses the multiple scattered fractions in a special scattering geometry. In this setup two lasers A and B are focused to the same sample volume, creating two sets of scattering patterns, as shown in Fig. 21-14. Two intensities are measured at different positions but with identical scattering vectors. 

The pulse height window cuts off a large fraction of the scattered radiations, which is limited by the width of the window. In 2D acquisition, the use of septa in multiring PET systems removes additional scattered events, whereas in 3D acquisition, they become problematic because of the absence of septa. Typically, the scatter fraction ranges from 15% in 2D mode to more than 40% in 3D mode for modern PET scanners. 

Note that the sensitivity of a PET scanner increases as the square of the detector efficiency, which depends on the scintillation decay time and stopping power of the detector. This is why LSO, LYSO and GSO detectors are preferred to Nal(Tl) or BGO detectors (see Table 2.1). In 2D acquisitions, system sensitivity is compromised because of the use of septa between detector rings, whereas these septa are retracted or absent in 3D acquisition, and hence the sensitivity is increased by a factor of 4-8. However, in 3D mode, random and scatter coincidences increase significantly, the scatter fraction being 30—40% compared to 15-20% in 2D mode. The overall sensitivities of PET scanners for a small-volume source of activity are about 0.2-0.5% for 2D acquisition and about 2-10% for 3D acquisition, compared to 0.01-0.03% for SPECT studies (Cherry et al, 2003). The greater sensitivity of the PET scanner results from the absence of collimators in data acquisition. 

Figure 6.3. NEMA phantoms for PET performance tests, (a) This NEMA body phantom is used for evaluation of the quality of reconstructed images and simulation of whole body imaging using camera-based coincidence imaging technique, (b) This phantom is used for measuring scatter fraction, dead time, and random counts in PET studies using the NEMA NU 2-2007 standard, (c) Close up end of the sensitivity phantom, (d) Set of six concentric aluminum tubes used in phantom (c) to measure the sensitivity of PET scanners. (Courtesy of Data Spectrum Corporation, Hillborough, NC)...

Acceptance tests are a battery of quality control tests performed to verify various parameters specified by the manufacturer for a PET scanner. These are essentially carried out soon after a PET scanner is installed in order to establish the compliance of specifications of the device. The most common and important specifications are transverse radial, transverse tangential, and axial resolutions sensitivity scatter fraction and count rate performance, ft is essential to have a standard for performing these tests so that a meaningful comparison of scanners from different manufacturers can be made. 

The system scatter fraction SF is calculated from the weighted average of the SF, values of all slices. These values are given in Table 6.1 for several PET scanners. 

For Lu-based scanners, there is an intrinsic activity as well as possible intrinsic random events due to176Lu, which need to be subtracted from the total count rate. These can be measured with the plastic tubing in place in the scanner but without any activity in it. Both prompt and delayed acquisitions are made of the tubing as described in section on Scatter Fraction, from which intrinsic prompt rate and intrinsic random rate are calculated. The true intrinsic count rate Rmt is obtained by subtracting the random rate from the prompt rate in all sinograms. Thus, for Lu-based scanners, true count rate Rtme is given as... 

What are acceptance tests Describe the methods of determining sensitivity and scatter fraction for a PET scanner. 

The practical counting efficiency e represents the probability that any particular photon or particle of radiation emitted by the sample source will be recorded by the detector. As explained in Section 8.2, its value may depend on many factors, including the detector, the type and energy of the radiation, the composition of the source, and the geometry of the source-detector configuration. It includes the loss factor in the pulse analysis system and attenuation and scattering fractions associated with the sample-detector system. All of these factors are discussed further in Section 8.2. 

This situation is similar for all frequencies within PAR, as indicated by the spectral variations of the asymmetry parameter (see Section 3.2) and the forward scattering fraction f. 

Haze characterizes the loss of contrast that results when objects are viewed through a scattering mediiun. The deterioration of contrast is due mainly to light scattered forward at large angles to the undeviated transmitted beam and is usually expressed through the forward-scattered fraction. 

The ideal characteristics of a SPECT or PET camera are given in Table 2.3. Key factors include spatial resolution, sensitivity, image contrast, counting rate ability, and scatter fraction. Issues related to spatial resolution and instmment sensitivity have already been discussed, but resolution and sensitivity by themselves do not suffice to fully characterize these imaging systems or to fully compare the different models. In clinical SPECT and PET imaging, the National... 

Another parameter of importance is the scatter fraction, that is, the fraction of events arising from scatter of one of the emitted photons within the animal. Again, no standard methods have yet been adopted for measuring scatter in small animal PET instruments. However, it is reasonable to expect that standards for performance measurements should be published soon. These will allow easier and more in-depth comparisons of the different camera models. 

Figure 3.7 Scattered Cs ion fractions derived by MARLOWE from a Silicon wafer as a function of incoming ion energy and angle of incidence (with respect to surface normal). In the inset is the same data with the scattered fraction plotted on a log scale (ordinate). Both Mollier and Ziegler potentials yield similar results (those using Mollier potentials are shown).

As in the case of light scattering, fractions of low polydispersity are preferred since [rj] is sensitive to the viscosity-average molecular weight My instead of to the usually measured M or M . In utilizing [rj], to evaluate o it is preferable to use M (instead of M ) since this value is closer to My. Correction factors for polydispersity effects [60] should be applied to prior to computing Coo > although the corrections are much smaller for [ti] measurements than for . 

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... 

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