Scatter reduction in mammography with air gap

Abstract

Scatter reduction by air gaps in mammography was investigated. We have experimentally demonstrated that, independently of the imaging geometry, scatter in air-gap mammography can be well described by a virtual source of scatter (VSS) model. This model postulates that scatter radiation originates from a virtual point source of scatter placed on the central axis between the x-ray source and the exit surface of a patient at distance delta and utilizes only two parameters: delta and (S/P)0. The (S/P)0 parameter represents scatter-to-primary ratio without an air gap and delta is the distance from the exit surface of a patient to the virtual source of scatter. We have experimentally determined the analytical form of the two independent parameters of the VSS model; delta exhibits a linear increase proportional to the radiation field size, does not depend on patient thickness, and is in the 10-30 cm range, while (S/P)0 increases with the field size as a power function and is in the 0.4-1.3 range. In the framework of the VSS model the selectivity, the contrast improvement factor, and the signal-to-noise improvement factor were employed to evaluate performance of air-gap mammography systems. We have demonstrated that selectivity of an air gap rapidly deteriorates at some well-defined critical value of scatter fraction that has profound consequences on air-gap performance. Assuming fixed patient exposure, the results shows that, if a contrast limited detection system (such as film/screen mammography) is used, an air gap system can outperform a grid system only if a very large source-to-patient (SPD) distance is utilized, which might be possible with new laser-based x-ray sources. For the noise limited detection systems (such as digital mammography) even a small SPD (70 cm) and a small air-gap (20 cm) system will outperform a grid system.

Generalized performance evaluation of x-ray image intensifier compared with a microangiographic system

“…Figure 6 shows a comparison of fit to equation (2) to the measured data points for spatial frequencies of 1, 2, 3 and 4cycles/mm. This interpolated function allows us to evaluate the NNPS at any intermediate detector entrance exposures.…”

“…Assuming the linearity of the detector along with flat-field and offset corrected images, the NNPS D can be expressed as 5 , 19 , 20 (2) where a(f ) and b(f) are constants dependent on frequency, and are obtained by fitting the measured NNPS D at different detector entrance exposures X. Figure 6 shows a comparison of fit to equation (2) to the measured data points for spatial frequencies of 1, 2, 3 and 4cycles/mm.…”

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

“…Therefore the NNPS D in Eq. (3) as a function of the entrance exposure X takes the form (5) where a(f) and b(f) are constants dependent on the frequency and are obtained by fitting the measured NNPS D at different entrance exposures X, as seen in Fig. 1(b).…”

Scatter reduction for grid-less mammography using the convolution-based image post-processing technique

“…The CNR was calculated following equation (5). The area analysed corresponds to the area surrounding the step phantom, the equation was applied to the 5 different regions of interest (ROIs), see Figure 5-left.…”

“…Although this method helps to improve the image quality, it is not a complete solution. The grids do not block all the scattered radiation and attenuate a proportion of primary radiation, leading to an increase in the patient dose delivered, and add complexity and cost to the overall system 1,4,5 .…”