MRI Artifacts: Mechanism and Control

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gradient echo sequences. Susceptibility artifacts can be made less prominent by performing imaging at low magnetic field strength, using smaller voxels, decreasing echo time, and increasing receiver bandwidth. Gradient-echo and echo-planar sequences should be avoided, because they accentuate susceptibility artifacts. The use of spin-echo and particularly fast spin-echo sequences should be considered. C. Chemical Shift Artifacts Figure 3. An axial MRI of the head in a patient with mascara on her eyelids. Susceptibility artifacts from the mascara obscure the front half of the globes. A chemical shift artifact is caused by the difference in chemical shift (Larmor frequency) of fat and water. The artifact manifests itself as a misregistration between the fat and water pixels in an image ( Fig. 4). The effect being that fat and water spins in the same voxel are encoded as being located in different voxels. The magnitude of the effect is proportional on the magnitude of the Bo field and inversely proportional to the sampling rate in the frequency encoding direction. For a constant sampling rate, the larger Bo, the greater the effect. 5 Chemical shift artifacts are typically observed along the frequency-encoding direction but can also occur along the slice-selection direction of the image. Chemical shift artifact can be reduced by performing imaging at low magnetic field strength, by increasing receiver bandwidth, or by decreasing voxel size. The artifacts tend to be more prominent on T2-weighted than on T1-weighted images. Fat suppression methods often eliminate visible artifacts, and gradient reorientation can redirect chemical shift artifacts to another portion of the image. 4
D. Wrap Around Artifacts Figure 4. This artifact is shown in an axial image of a kidney where the bright border along the top of the kidney and the dark border along the bottom of the kidney represent the artifact. A wrap around artifact is the occurrence of a part of the imaged anatomy, which is located outside of the field of view, inside of the field of view ( Fig. 5). This artifact is caused by the selected field of view being smaller than the size of the imaged object. Or more specifically the digitization rate is less than the range of frequencies in the FID or echo. 6 The solution to a wrap around artifact is to choose a larger field of view, adjust the position of the image center, or select an imaging coil which will not excite or detect spins from tissues outside of the desired field of view. Figure 5. The first image shows wrap-around of the back of the head on to the front of the head, where the phase-encoded direction is anterior-posterior. The second image has the phase and frequency directions reversed resulting in absence of the aliasing artifact. Oversampling was used in the frequency direction to eliminate the aliasing. E. Partial Volume Artifacts A partial volume artifact is any artifact which is caused by the size of the image voxel. It occurs when multiple tissue types are encompassed within a single voxel. For example, if a small voxel contains only fat or water signal, and a larger voxel might contain a combination of the two, the large voxel possess a signal intensity equal to the weighted average of the quantity of water and fat present in the voxel. Volume averaging is most likely to occur in the slice-selection direction of the image, which has the largest voxel dimension. It also occurs when structures are oriented obliquely to the imaging plane and when structures move in and out of a given section during image acquisition. Volume averaging can simulate abnormalities, decrease the visualization of low-contrast abnormalities, and blur or distort affected structures. Partial volumeaveraging is usually recognized by careful analysis of adjacent images. Decreasing voxel size, particularly reducing section thickness, can be useful if further confirmation is required ( Fig.6). Three-dimensional Fourier transform imaging is particularly useful, because it provides thin sections with no intervening gaps and is conductive to reformatting in alternate imaging planes; Simple acquisition of additional two-dimensional images in alternate imaging planes is very helpful for resolving issues relating to partial volume averaging. Retrospective reformatting of two-dimensional data can also be performed using an interpolation algorithm and can be of further assistance. 5
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MRI Artifacts: Mechanism and Control

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