(RFOV) A different field of view (the scanned region) in the frequency and phase encoding directions that means the data acquisition with fewer measurement lines. Because there are fewer rows than columns, a rectangular image is obtained. To reduce the FOV in phase encoding direction (foldover direction) saves scan time by decreasing signal but invariable spatial resolution.
Also called HFI or undersampling.

If the scanned object is oval, e.g. head or abdomen, a rectangular FOV is an easy to use scan parameter to reduce the scan time without loss of resolution.

Quick Overview

Artifacts either by distorting the k-space trajectory (i.e. due to imperfect shimming) or as a consequence of the reduced bandwidth in the phase encode direction, commonly with EPI sequences.
While a standard spin warp-based sequence has an infinitely large bandwidth in the phase encode direction (about 1 or 2 kH), the bandwidth in EPI is related to the time between the gradient echoes (about a millisecond).
Hence even small frequency offsets can result in significant shifts of the signal in the phase encoding direction. Segmentation can introduce ghosting if there are significant difference in the amplitude and phase of the signal. This can be a particular problem when trying to acquire the segments in rapid succession.

Suitable choices of excitation schemes and/or subsequent correction can help to reduce this artifact. The signal from fat can easily be offset by a large fraction of the FOV, and must be suppressed. The effect of frequency offsets can be reduced by collecting data with more than one excitation, which effectively increases the bandwidth in the phase encoding direction.

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Uneven intensity or brightness may occasionally be noted on high field MRI e.g. of the brain. There are various causes of localized inhomogeneous brightness across the MRI images such as improper tuning of the RF transceiver, unbalanced deposition of the RF energy due to incorrect geometry and localized attenuation due to positioning or anatomical variants.

Tuning of the RF transceiver, a homogeneity correction filter and better positioning of the scanned object and/or coil in the scanner.

Quick Overview

The slice overlap artifact is another name for crosstalk artifact. If slices of multislice acquisitions are overlapping, the spinning nuclei belonging to more than one slice getting multiple times saturated, which leads to signal loss in this areas.

This problem occurs often in cervical or lumbar spine MRI, when scanning each disc with multi angle oblique technique. If prevention of overlapping is not possible, try to position the saturated region posterior to the spinal canal, outside the region of interest.

See also Crosstalk (Crosstalk), and Multiple Slice Imaging.

(THK) The thickness of an imaging slice. As the slice profile may not be sharp edged, a criterion such as the distance between the points at half the sensitivity of the maximum (FWHM) or the equivalent rectangular width (the width of a rectangular slice profile with the same maximum height and same area) is used to determine thickness.

For the image quality its important to choose the best fitting slice thickness for an examination. When a small item is entirely contained within the slice thickness with other tissue of differing signal intensity then the resulting signal displayed on the image is a combination of these two intensities. If the slice is the same thickness or thinner than the small structure, only that structures signal intensity is displayed on the image. This partial volume averaging effect explains the vanishing of fine details by choosing slices too large for the scanned object.

See also Partial Volume Artifact.