Ring artifact

Fig. 5.6a-d. CE-MRA failed bolus timing resulted not only in a weak artery signal, but also artificial enhancement of vessel walls (ringing artifacts), similar to intraluminal thrombus. Examination was repeated and revealed normal vessel enhancement... 

Ring artifacts caused by defects or impurities of the scintillator crystals or imperfect detector elements. 

Ring artifacts are caused by detector gain inhomogeneities or defective detector pixels. Fortunately, elaborate detector calibration and built-in FD defect correction can usually correct detector pixel problems to a large extent, thus preventing most serious ring artifacts right at the detector. 

Ring artifacts in the images can appear because of defective detectors or nonlinearities of the detector system. Another reason can be drift of the detector sensitivity between white field calibrations and small misalignments in the... 

Additional processing of the shadow images before the tomographic image reconstruction process can help to reduce ring artifacts (36). Examples of such artifacts are shown in Fig. 6.10. 

Figure 6.10 Example of ring artifacts in an image of a water phantom (a). During the reconstruction process, most of these artifacts can he reduced, (h) Mouse lung scan reconstructed without ring artifact reduction, (c) Mouse lung scan reconstructed with ring artifact reduction which results in significant improvement in image quality.

The 2-D filter function to be used in the rectangular Cartesian coordinates has slope discontinuities at the boundary (near cutoff frequency), resulting in a ring artifact. By the incorporation of proper digital signal processing techniques, however, these problems can easily be resolved. 

The selection and formulation of the 2-D filter function are also important factors in determining the image quality in 2-D BF image reconstruction because the slope discontinuity at the cutoff frequency causes ring artifact. The overall reconstruction procedures of the BF algorithm are as follows ... 

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