Why is scatter radiation reduced when the size of the exposure field is reduced?

2.1 Morphology Development

SAXS, WAXS and Raman data were collected simultaneously during the in situ polymerization at three temperatures. For the 20 wt.% PE system, the SAXS data are shown as three-dimensional plots for the polymerization temperatures of 110 °C, 115 °C, and 120 °C in Fig. 1(a), 1(b), and 1(c), respectively. The z-axis is the Lorentz-corrected intensity (I(q)q2) versus scattering vector (q) and time. To illustrate the structural changes at the onset of crystallization during polymerization at 120 °C, WAXS data are shown in Fig. 1(d) as a three-dimensional plot of the intensity versus the scattering angle (2θ), and time. The SAXS patterns initially have a low scattering intensity. The SAXS profiles change gradually as the polymerization proceeds. A closer examination of these monotonically decaying patterns as function of time showed that the L-L phase separation proceeded via a nucleation and growth mechanism (Goossens et al. 1998). When the L-S transition sets in, which is apparent from the growth of the 110 and 200 reflections of PE in the WAXS data, the SAXS patterns change from a monotonically decaying curve into a pattern in which a broad maximum, originating from the PE-lamellae, develops in the q-range of 0.01–0.03. This broad maximum intensifies until no further change can be observed, indicating that maximum crystallinity has been reached.

The main difference between the results of the three polymerization temperatures is in the growth of the intensity at low q-values as a function of the time before the onset of crystallization. For the lowest polymerization temperature (110 °C), the evolution of the scattered intensity is the slowest, whereas the highest polymerization temperature shows the fastest increase in intensity.

Excessive scatter radiation

Contrast is reduced by scatter radiation, some of which is unavoidable. The thicker the part of the body, and the denser the soft tissues, the more scatter radiation. It is a particular problem in radiography of the chest, especially in a person with a large antero-posterior diameter and a great deal of soft tissue in the thoracic wall. It is always a problem in radiography of the abdomen. Scatter is greater in fat than thin persons, and in fat persons even the proximal parts of the limbs may show poor contrast from excessive scatter radiation.

The amount of scatter radiation can be reduced in some cases by any of the following methods.

By limiting the incident beam of x-rays to as small an area as possible.

By using a low rather than a high kilovoltage.

By the use of clearing grids which are still referred to as ‘Potter-Bucky’ or ‘Bucky grids’. Scatter may also be reduced by an air gap of 9–12 inches between the subject and the film. To avoid blurring due to the size of the focal spot of the x-ray tube, the tube film distance is increased to 12 feet.

Raman spectroscopy

The Raman effect involves inelastic radiation scattering that provides molecular spectral fingerprints with similar, but often complementary spectral information to mid-IR spectroscopy. Therefore, also a simultaneous reagent-free measurement of multiple analytes is possible. An advantage is the weak water Raman spectrum, contrary to the intensive water absorption bands in the IR spectrum. Recently, glucose measurements in various biofluids such as plasma and even whole blood have been pursued, but interference from strong fluorescence of matrix substances or photo-decomposition of the sample by intensive laser radiation has hampered the studies. Therefore, most recent research has been carried out using near-IR excitation Raman spectroscopy, thus avoiding water and other biosample absorbers. However, tremendous trade-off due to lower Raman intensities is found for longer wavelength excitation, compared with excitation with UV–visible lasers. Despite this, intensive excitation sources such as diode lasers and extremely sensitive detectors, e.g., silicon-based CCDs, are available for miniaturized spectrometer systems. More research will be needed for routine implementation of Raman spectroscopy based assays.

7 Summary

By their nature broad Q x-ray and neutron scattering and NMR procedures emphasize the presence of well-defined distances in the local structural arrangements of glassy polymers. It would seem very reasonable that, on geometric grounds alone, different polymers would generate local structure which reflect the specific nature of the repeating units. Such arrangements, particularly at normal temperatures and pressures, may well represent the most disordered state possible. The presence of short-range order must require some correlations over and above those arising from simple geometric packing for even simple atomic fluids exhibit well-defined structure factors even though the packing units are spherical.

Overall there appears to be little evidence for such short-range order over and above that of the disordered state. However, polymers with well-defined local geometries such as polycarbonates and other aromatic polymers, together with systems exhibiting strong dipoles (for example PVC; Smith et al. 1993) show some characteristics of locally ordered structures. The increasing availability of high quality broad Q neutron scattering data and detailed information from NMR techniques when tightly coupled to atomistic level models may enable more precise information to be extracted. Realistic models of polymer glasses are vitally important for developing an understanding of many physical properties ranging from diffusion to fracture and yield.

4 CONCLUSIONS

In this review two different radiation scattering techniques have been described that can provide information on the microstructure of porous solids and the mechanisms of adsorption processes at surfaces and within pores. Although neutron scattering methods are relatively sophisticated, and not more readily accessible, they can give unique information which complements that available from more classical techniques of pore structure characterisation. This feature is illustrated here by two specialised applications of small angle neutron scattering which are currently under development. The first concerns the characterisation of anisotropic pore structures, where two examples are given, viz. microporous carbon and mesoporous anodic alumina membranes. The second illustrates the use of contrast matching methods to investigate the mechanisms of adsorption in porous networks. This development which is applicable to both micro and mesoporous media, may provide insight into the separation of gas mixtures in microporous membranes where either one or more components is a condensable phase. For example highly efficient separations involving pore “blocking” processes have been reported with microporous membranes (72). Although the mechanisms are still not fully understood, they are of considerable technical interest.

Neutron diffraction may also provide information on the nature of the sorbed phase confined in micropores, as also discussed here, with reference to water in different microporous systems.

The other technique described - resonant ion beam backscattering, has received little development since its first application in pore structure characterisation. The method has the same advantages as SANS, by being an in-situ and non-destructive technique, which is equally appropriate for measurements on micro and meso porous materials. It does however have a unique advantage over other techniques, which could be very useful in the characterisation of microporous membranes. This results from the use of a microfocussed beam, which allows measurements as a function both of sampling depth (from ∼3 μm to 100 μm), and as a function of distance across the sample, as is illustrated here.

2.05.4.1 Basic Principles of the Antiscatter Grid

The antiscatter grid is the most common technology to reduce scatter radiation in mammography. Basically, it is aimed at improving image contrast and it is located between the breast and the detector. Other methods of scatter control in mammography include breast compression to minimize tissue thickness and air-gap geometry (with magnification) to reduce the number of scattered photons incident on the detector. After the 1970s, the influence of scattered radiation was shown to reduce greatly the visibility of microcalcifications, and subsequent measurement of the scatter-to-primary ratio (S/P) demonstrated its dependence on breast thickness and radiation field size (Barnes, 1992; Barnes and Brezovich, 1978).

In particular, Figure 20 shows that the amount of scatter radiation is greater than the primary one for compressed breast thicknesses of 6 cm and more. Screen/film mammography was particularly affected by such degradation of image contrast and a contrast degradation factor (CDF) has been defined as follows:

[2]CDF=11+S/P

In a mammography unit, the grid system is made up of the grid, a cassette holder (for nondedicated systems), a breast support table, and a mechanism for grid translation. Antiscatter grids of most mammography systems consist of lead absorbers and carbon fiber interspace material (see Figure 21). Typical grid ratios (the height of the lead strips divided by the interspace thickness) are 4:1 and 5:1 and the number of line pairs per centimeter is within the range 30–40 lp cm− 1. One commercial system makes use of cellular grid technology providing a honeycomb structure without any interspace material (air-core grid) (Boone et al., 2002b). Technology has evolved also with linear grids since modern microcontrolled sawing processes allow high uniformity and flexibility in modifying grid specifications (see Figure 22).

There are several parameters that characterize the grid, and the procedures to check its performance are defined in the IEC 60627 international standard (IEC 60627, 2013). The most important parameters are the grid selectivity ∑ = Tp/Ts and the contrast improvement ratio K = Tp/Tt, where Tp is the transmission of primary radiation, Ts is the transmission of scattered radiation, and Tt is the transmission of total radiation.

Tp, Ts, and Tt are calculated, by using a phantom, as the ratio of the radiation transmitted with and without grid under specific measuring conditions (see the IEC document for details). Finally, B is also called the Bucky factor and is measured as the ratio between the incident radiation and the total transmitted radiation. The ideal grid would transmit all the primary x-rays and it would absorb all the scattered ones. In practice, the Bucky factor is about 2 in mammography and this value corresponds to the increase of glandular dose, which is needed to guarantee image quality. An example of x-ray performance data of linear grids used in mammography can be found in the following weblink (JPI grid).

Scanning systems do not incorporate antiscatter grids because the x-ray beam is collimated and the amount of detected scatter radiation is negligible. Since the primary photons traversing the breast are not attenuated, we obtain an almost perfect Bucky factor and a reduction of glandular dose. Of course, scatter cleanup performance depends on slot width that has to be optimized according to other practical issues (Boone et al., 2002a).

With the advent of digital mammography, several investigators have proposed to record a mammogram without the grid and to reduce the effect of scattered radiation from it via IP (Ducote and Molloi, 2010; Nykanen and Siltanen, 2003). Reducing the scattered field from the digital image implies the use of convolution techniques, based on the observation that the scattering of x-rays in conventional mammography is a low-frequency phenomenon in the spatial domain. A scatter algorithm based on Monte Carlo simulation has been also recently presented. The normalized measure of tissue radiodensity is computed through the use of a detailed model of the physics of image acquisition by considering both primary and scattered photons; scatter kernels arising in the neighborhood of each primary ray are summed up to generate the scatter image (Highnam et al., 1997). Finally, it is worth noting that studies applied to full-field digital mammography (FFDM) have also shown that below 5 cm, grid removal allows lowering the dose to the patient without losing image quality (Gennaro et al., 2007; Veldkamp et al., 2003).

In order to reduce entrance dose in mammography screening, Siemens Healthcare has very recently launched a grid-less system that carries out software-based scatter correction (PRIME). The correction algorithm estimates the scatter field using kernels generated via Monte Carlo simulations (Fieselmann et al., 2013). A simplified illustration of this algorithm is shown in Figure 23.