Generalised use of contrast degradation and contrast improvement factors in diagnostic radiology. Application to vanishing contrast

Abstract

Although field area and object thickness are important parameters in comparisons of techniques for optimal reduction of scattered radiation to the image, they are in practice seldom varied. For this reason, we suggest that contrast degradation (CDF) and contrast improvement (CIF) factors be more frequently used and appropriately defined to make the dependence of CDF and CIF on field area (collimation) and object thickness (compression) explicit. Definitions are formulated and the results of experiments and Monte Carlo calculations (comprising effects of collimation, compression, air gap, antiscatter grid, detector thickness) cited to illustrate their usefulness. Currently used expressions for CIF (derived assuming monoenergetic radiation) lack a factor to account for the change in primary contrast caused by the antiscatter method when this affects the energy distribution of the transmitted primary photons (grids and compression) or the fractions of photon energy imparted to the detector (when comparing different detectors). Values of this factor are calculated for some cases. Also, the appropriate choice of physical quantity to be used in the formulae for CDF and CIF is discussed. The energy imparted to the detector is advocated since this is directly related to the detector signals forming the image on, e.g. the x-ray film.

Study of the generalized MTF and DQE for a new microangiographic system

“…From Eqs. (4) and (5) we can write the GNNPS as: (6) where the exposure to the detector X is measured with the scattering material near the detector. a and b are the constants previously determined by Eq.…”

<title>Performance characteristics of a flexible antiscatter grid used in computed radiography</title>

“…(1) where i; is the total transmittance through the grid. Carlsson et al 15 have pointed out that the commonly used definition of CIF must be multiplied by a factor that properly accounts for the effect of the grid on subject contrast measured with only the primary (unscattered)…”

Simulação de mamografia digital por dupla energia para imagens quantitativas

“…As grandezas q 1 e q 2 representam as fluências de energia absorvidas no receptor de imagem, relacionadas ao tecido circundante e ao objeto contrastante, respectivamente [44] e incluem as contribuições dos fótons primários e espalhados.…”

“…Embora as equações 2.12 e 2.13 forneçam resultados similares, a primeira é comumente utilizada para descrever o contraste em um sistema tela-filme, enquanto a segunda é empregada em receptores digitais [44].…”

“…Segundo Carlsson et al [44], a grandeza física mais apropriada para q, é a fluência de energia absorvida no receptor de imagem, embora existam outras definições apresentadas na literatura [45,46,43,47].…”