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Sequences

A variety of acronyms describe MRI sequences. Each manufacturer adds their own names to an already extensive list of generic sequences. In general, basic MRI sequences are as follows:

Spin Echo
Conventional Spin Echo
Fast or Turbo Spin Echo
Gradient Echo
Inversion Recovery
Phase Contrast
Echo Planar Imaging

MRI Artifacts

The quality of final MR image is dependent on equipment used, choices made by radiologists and technologists, surrounding environment, several other factors and interplay between all of these variables. In addition, there are artifacts inherent in physics of image acquisition and in biology of imaged tissues. The following is a list of MRI artifacts and brief description of each:

55 Degree Artifact "Magic Angle" Phenomenon
55 degree artifact results from high order of collagen fibers oriented at approximately 55 degrees to the magnetic field. This artifact is the increased signal, which in many cases is a desirable effect. However, when imaging structures of normally low signal intensity such as tendons of the rotator cuff or tendons in the ankle, 55 degree artifact, also known as "magic angle" phenomenon produces high signal, which may be mistaken for pathology in these structures. The artifact is a problem on short TE sequences (T1, PD, some GRE sequences). This artifact disappears on long TE sequences (T2), where high signal intensity in tendons may represent pathology. Magic angle artifact is most troublesome in the joints where tendons demonstrate angulation, such as in the shoulder and ankle.

Wrap-Around Artifact
Wrap-around artifact

Motion Artifact
motion artifactMotion artifact results from patient motion. Keeping the patient calm and comfortable and avoiding long acquisition times are strategies that help reduce this artifact. Respiratory motion artifact can be diminished by placing a patient in prone position when imaging sternum or sternoclavicular joints. Of course, patient has to be able to tolerate such position in the MRI scanner.

Pulsation Artifact
pulsation artifactThis artifact is very similar to motion artifact. Physiologic motion of the heart, vessels and CSF results results in pulsation artifact. Ghost images of the vessels or vessel walls are seen along the phase encoding direction. Changing the orientation of the phase encoding may result in better delineation of particular anatomic structure.

Susceptibility Artifact
Susceptibility artifact results from inhomogeneities in the magnetic field. Gradient echo imaging is sensitive to this artifact. Lower flip angles and higher TE exagerate this artifact.

Optimization of MRI Sequences

Various parameters of MR sequences can affect the final quality of the acquired image. These include:


TR

TE

Bandwidth

Slice thickness

Slice spacing

NEX

Post-processing filters
Various filters maybe available on individual MR units.