Nondestructive measurement of the grid ratio using a single image

Abstract

The antiscatter grid is an essential part of modern radiographic systems. Since the introduction of the antiscatter grid, however, there have been few methods proposed for acceptance testing and verification of manufacturer-supplied grid specifications. The grid ratio (r) is an important parameter describing the antiscatter grid because it affects many other grid quality metrics, such as the contrast improvement ratio (K), primary transmission (Tp), and scatter transmission (Ts). Also, the grid ratio in large part determines the primary clinical use of the grid. To this end, the authors present a technique for the nondestructive measurement of the grid ratio of antiscatter grids. They derived an equation that can be used to calculate the grid ratio from a single off-focus flat field image by exploiting the relationship between grid cutoff and off-focus distance. The calculation can be performed by hand or with included analysis software. They calculated the grid ratios of several different grids throughout the institution, and afterward they destructively measured the grid ratio of a nominal r8 grid previously evaluated with the method. They also studied the sensitivity of the method to technical factors and choice of parameters. With one exception, the results for the grids found in the institution were in agreement with the manufacturer's specifications and international standards. The nondestructive evaluation of the r8 grid indicated a ratio of 7.3, while the destructive measurement indicated a ratio of 7.53 +/- 0.28. Repeated evaluations of the same grid yielded consistent results. The technique provides the medical physicist with a new tool for quantitative evaluation of the grid ratio, an important grid performance criterion. The method is robust and repeatable when appropriate choices of technical factors and other parameters are made.

Figure 1

C is the off-axis distance from the centerline of the grid to a point on the flat field image with noticeable grid cutoff. $f_1$ or $f_2$ is the distance at which the off-focus image is acquired. Finally, h is the height of the grid septa and D is the width of the interspace.

Figure 2

Examples of variation in calculated grid ratio with changes in f or kVp. (a) Calculated r with changes in kVp for grid 3 imaged at an $85\mathrm{in.}$ SGD. (b) Calculated r versus $f_1$ for grid 2 measured at $40\mathrm{kVp}$. (c) Calculated r versus $f_2$ for grid 3 measured at $40\mathrm{kVp}$.

Figure 3

Analysis of an off-focus flat field image made at $f_2=203\mathrm{cm}$ with grid 3. (a) Change in the calculated r as a function of C. (b) $U_C∕U_0$ versus C in the off-focus flat field image.