A study of grid artifacts formation and elimination in computed radiographic images

Abstract

Computed radiography (CR) has many advantages such as filmless operations, efficiency, and convenience. Furthermore, it is easier to integrate with the picture archiving and communication systems. Another important advantage is that CR images generally have a wider dynamic range than conventional screen film. Unfortunately, grid artifacts and moiré pattern artifacts may be present in CR images. These artifacts become a more serious problem when viewing CR images on a computer monitor when a clinic grade monitor is not available. Images produced using a grid with higher frequency or a Potter--Bucky grid (i.e., a moving grid, Bucky for short) can reduce occurrence but cannot guarantee elimination of these artifacts [CR & PACS (2000); Detrick F (2001), pp 7-8]. In this paper, the formation of the artifacts is studied. We show that the grid artifacts occur in a narrow band of frequency in the frequency domain. The frequency can be determined, accurately located, and thus removed from the frequency domain. When comparing the results obtained from the proposed method against the results obtained using previous computer methods, we show that our method can achieve better image quality.

Fig 1

This figure shows the cross section of a grid and some grid specifications related to this work.

Fig 2

There is an angle θ between the sampling signal (UV-coordinate) and the grid (XY-coordinate).

Fig 3

Along a horizontal scan line, the vertical stripe signal is a square wave.

Fig 4

Moiré pattern (the dashed lines) caused by two periodical functions.

Fig 5

A change in an angle θ from 0° to 2° will cause the stripe angle in the moiré pattern to change from 0° (a) to 24° (b). (a) 3,062 × 3,730 pixels; (b) selected area from (a), showing grid artifacts. (c) Selected area from (a), with grid artifacts removed. (d) 1-Dimensional Fourier transform of the image in (a). Grid artifact frequency indicated by the circle. (e) The spectrum after grid artifacts are removed. (f) 540 × 648 pixels; (g) 540 × 648 pixels, with grid artifact removed.

Fig 6

(a) Mammography Quality Control Phantom (Phantom No. C104, Fuji, Japan) image with grid artifacts. (b) A selected region in (a) is shown in the original resolution. The artifacts are easily seen. (c) The same region, with grid artifacts eliminated. Note that many details, such as the vertical stripes, can be clearly distinguished. (d) The spectrum of a 1-dimensional Fourier transform of the image shown in (a). The y-axis is logarithmic. The frequency of the grid artifact is highlighted with a circle. (e) The spectrum after grid artifacts are removed. (f) An image of the Mammography Quality Control Phantom scaled down 17% to a resolution of 540 × 658 pixels. It shows a very serious artifact (the moiré pattern). (g) The moiré pattern was eliminated using the proposed method. (a) 2,048 × 2,494 pixels; (b) selected area from (a), enlarged. It contains fine vertical stripes. (c) Selected area from (a), with grid artifacts removed. (d) 1-Dimensional Fourier transform of the image in (a). Grid artifact frequency indicated by the circle. (e) The spectrum after grid artifacts are removed.

Fig 7

(a) A patient with left lower lobe consolidation due to pneumonia. Grid pattern can be seen on CR chest image. (b) The portion that is highlighted in white in (a). (c) The grid pattern was removed using the proposed method. (d) The spectrum of a 1-dimensional Fourier transform of the image shown in (a). The y-axis is logarithmic. The frequency of the grid artifact is highlighted with a circle. (e) The spectrum after grid artifacts are removed.

Fig 8

(a) The original image with grid textures. (b) The grid textures were removed using the proposed method. (c) The grid textures were removed using the blur kernel proposed by Barski and Wang. (d) The grid pattern was removed using a notch filter proposed by Belykh and Cornelius.