The fully magnetized protons yield a high signal in comparison with the rest of the surroundings. Flow enhancement, also known as inflow effect, is caused by fully magnetised protons entering the imaged slice while the stationary protons have not fully regained their magnetization. įlow can manifest as either an altered intravascular signal (flow enhancement or flow-related signal loss), or as flow-related artifacts (ghost images or spatial misregistration). Flow-related signal loss in the carotid and basillary arteries (T2 axial study of the brain). Several methods can be used to reduce motion artifacts, including patient immobilisation, cardiac and respiratory gating, signal suppression of the tissue causing the artifact, choosing the shorter dimension of the matrix as the phase-encoding direction, view-ordering or phase-reordering methods and swapping phase and frequency-encoding directions to move the artifact out of the field of interest. Ghost image intensity increases with amplitude of movement and the signal intensity from the moving tissue. Periodic movements such as cardiac movement and blood vessel or CSF pulsation cause ghost images, while non-periodic movement causes diffuse image noise (Fig. Major physiological movements are of millisecond to seconds duration and thus too slow to affect frequency-encoded sampling, but they have a pronounced effect in the phase-encoding direction. Phase-encoded sampling takes several seconds, or even minutes, owing to the collection of all the k-space lines to enable Fourier analysis. Frequency-encoding sampling in all the rows of the matrix (128, 256 or 512) takes place during a single echo (milliseconds). The reason for mainly affecting data sampling in the phase-encoding direction is the significant difference in the time of acquisition in the frequency- and phase-encoding directions. Motion can cause either ghost images or diffuse image noise in the phase-encoding direction. Ī motion artifact is one of the most common artifacts in MR imaging. Motion artifact (T1 coronal study of cervical vertebrae). Patient-related MR artifacts Motion artifacts įig.
2.6.5.1.3 Dual Refocusing Echo Acquisition Mode (DREAM).2.6.5.1 B1 mapping techniques for image post-processing corrections.2.5.1.4 Optimizing the Optimization Procedure.2.5.1.3 SENSE model parameter optimization.2.4.3 TAMER: Advantages and Disadvantages.1.9 External magnetic field (B0) inhomogeneity.
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