As such, the inconsistent nature of motion can induce different phase shifts during the image readout period. While some gradient combinations can compensate for motion (e.g., the flow-compensated gradients see below for more details), many of the imaging gradient pulses cannot. The extensive use of spatial gradients complicates and amplifies motion artifacts, as stronger gradients induce larger phase shifts from motion (see Understanding the Need for Gradients In Image Formation and Its Implication in Motion Artifacts).
In most conventional imaging methods, motion artifact is predominantly manifested in the phase-encoding direction. Motion is often divided into two categories: gross body movement and physiologic motion, such as cardiac and respiratory cycles, or blood or cerebrospinal fluid (CSF) flow. More specifically, when motion is present, tissues excited at a specific location during the radiofrequency (RF) pulse are erroneously mapped to a different location (or often multiple different locations in cases of motion artifacts) during detection. Motion artifacts are the result of movement during the data acquisition period. Motion is a very common cause of artifacts in magnetic resonance imaging (MRI). Figure 1D is from a breast MR image in which the phase-encoding direction is oriented horizontally from right to left. Figures 1A through 1C are from an abdominal MR image, and the phase-encoding direction is oriented vertically therefore, the artifact is also seen along this axis. They are observed only in the phase-encoding direction and can be the result of any periodic motion such as respiration, arterial pulsation, or cerebrospinal fluid pulsation.Ĥ. This artifact is referred to as a “ghosting artifact.” Ghosting artifacts appear in abnormal locations as replicas of the moving structure from which they result. There is a subtle high-signal lesion located equidistant to, but left of the aorta along the same horizontal plane as the low-signal lesion in the right hepatic lobe.ģ. Round low-signal lesion in the right hepatic lobe. However, given its location, oriented vertically just above the aorta, a pseudo-lesion as a result of pulsation artifact from the aorta is also in the differential.Ģ. The differential diagnosis of a subtle, T2 bright lesion in the liver includes hemangioma, metastatic disease, and primary liver tumor. Axial fat-suppressed T1W 3D-GRE sequence from a dedicated breast magnetic resonance (MR) image (different patient) demonstrates a round, low-signal lesion in the right hepatic lobe (black arrow) and a round high-signal lesion in the region of the spleen/abdominal wall (white arrow).ġ.
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