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). Offending artifacts may obscure, distort, or completely misrepresent the true underlying electrophysiological signal sought.Fig. These artifact signals may stem from, but are not limited to: light sources monitoring equipment issues utility frequency (50 Hz and 60 Hz) or undesired electrophysiological signals such as EMG presenting on an EEG-, EP-, ECG-, or EOG- signal. In medical electrophysiological monitoring, artifacts are anomalous (interfering) signals that originate from some source other than the electrophysiological structure being studied. When these assumptions are not maintained, artifacts occur. and acoustic energy of an echo is uniformly attenuated. These are: echoes originate only from the main ultrasound beam (while there are side lobes and grating lobes apart from the main ultrasound beam) echoes returns to transducer after a single reflection (while an echo can be reflected several times before reaching the transducer) depth of an object relates directly to the amount of time for an echo to reach the transducer (while an echo may reflect several times, delaying the time for the echo return to the transducer) speed of ultrasound in human tissue is constant, echoes travel in a straight path. In ultrasound imaging, several assumptions are made from the computer system to interpret the returning echoes. Physicians typically learn to recognize some of these artifacts to avoid mistaking them for actual pathology. These artifacts may be caused by a variety of phenomena such as the underlying physics of the energy-tissue interaction as between ultrasound and air, susceptibility artifacts, data acquisition errors (such as patient motion), or a reconstruction algorithm's inability to represent the anatomy. In medical imaging, artifacts are misrepresentations of tissue structures produced by imaging techniques such as ultrasound, X-ray, CT scan, and magnetic resonance imaging (MRI). This prediction is a statistical artifact, since it is spurious to use the model when the percentage of citizens making over $50,000 is so high, and gross error to predict an approval rating greater than 100%. For instance, imagine a hypothetical finding that presidential approval rating is approximately equal to twice the percentage of citizens making more than $50,000 annually if 60% of citizens make more than $50,000 annually, this would predict that the approval rating will be 120%. Such an artifact may be called a statistical artifact. In econometrics, which trades on computing relationships between related variables, an artifact is a spurious finding, such as one based on either a faulty choice of variables or an over-extension of the computed relationship. In microscopy, visual artifacts are sometimes introduced during the processing of samples into slide form. In computer science, digital artifacts are anomalies introduced into digital signals as a result of digital signal processing. In natural science and signal processing, an artifact or artefact is any error in the perception or representation of any information introduced by the involved equipment or technique(s). Diffraction spikes and the Airy disk are optical artifacts caused by the diffraction of light through the aperture of an optical system.
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