CT and MR artifacts

CT ARTIFACTS Motion artefact: Patient’s motion can cause misregistration artifacts, which appear as shading in the reconstructed image.

CT ARTIFACTS Streaking artifact: If the body sections contain metal objects, streaking artifact will be produced in the process of image reconstruction. This occurs because the density of the metal is beyond the normal range that can be handled by the computer, resulting in incomplete profiles. Zebra artifacts: Faint stripes may be apparent in multiplanar and 3D reconstructions of helical data, because the helical interpolation process gives rise to some degree of noise inhomogeneity along the z-axis.

CT ARTIFACTS Beam-hardening artifacts: As the beam passes through an object, it becomes harder (this means that the energy increases because the low-energy photons are absorbed faster than the high-energy photons). The harder it becomes, the less the beam is attenuated, so when it reaches the detectors a higher signal is produced. In very heterogeneous cross sections, dark bands or streaks can appear between two dense objects in an image. They occur because the portion of the beam that passes through one of the objects at certain positions of the tube is hardened less than when it passes through both objects at other positions. Axial CT scan shows hyper-hypodensity of the cortex close to the frontal bone that should not be mistaken for small hemorragic contusions (A). Beam-hardening artifacts are typical in the infratentorial space (B), where they can simulate ischemic infarction and areas of demyelination. Normal MR demostrates it is artifactual (C), C B A

MR ARTIFACTS The chemical shift artifact: This artifact is a bright or dark band at the edge of the anatomy where fat and water are in the same location. In neuroradiology, this is commonly seen in the orbits, where fat borders other tissues. Aliasing or wrap-around is a common artifact that occurs when the field of view (FOV) is smaller than the part of the body that is being imaged. The part of the body that lies beyond the edge of the FOV is projected onto the other side of the image.

MR ARTIFACTS Gibbs artifact (also called truncation or ringing artifact) are bright or dark lines that are seen parallel and adjacent to borders of abrupt intensity change, as when going from bright CSF to dark spinal cord on a T2-weighted image or at the brain-calvarium interface. In the spinal cord, this artifact can simulate a small syrinx to the unaware observer. Gibbs artifact near the inner table of calvarium manifests as subtle hypointense lines overlying the cortex (arrows). Motion artefact: These artifacts may be seen derived from arterial pulsations, swallowing, breathing, peristalsis, and any physical movement of a patient. They can be distinguished from Gibbs or truncation artifacts because they extend across the entire FOV, unlike truncation artifacts that diminish quickly away from the boundary that causes them.

MR ARTIFACTS Pulsatility artifact: CSF pulsation artifact is a pitfall of fast fluid-attenuated inversion - recovery (FLAIR) brain MR imaging. CSF enters the brain sections between the inversion pulse and the beginning of the signal sampling, not being properly nulled, and remains hyperintense on axial FLAIR images. The fourth ventricle is the most common site where it can be found, followed by the third and the lateral ventricles.

MR ARTIFACTS Pulsatility artifact: Small hyperintense artifact in the pre-pontine cistern on axial fluid attenuation inversion recovery image does not persist on FSE T2-weighted and SE T1-weighted images.

MR ARTIFACTS The central point artifact appears as a bright spot (or zipper line) at the center of the image. It is caused by a constant offset of the DC voltage in the receiver. After Fourier transformation, this constant offset provokes the bright dot at the center Susceptibility artifacts occur as the result of microscopic gradients or variations in the strength of the magnetic field that take place near the interfaces of substances of different magnetic susceptibility. Large susceptibility artifacts are commonly seen surrounding ferromagnetic objects, for example medical devices in or near the magnetic field or by patient’s implants. In some cases it is even beneficial and it can be used for the detection of hemosiderin deposits.

MR ARTIFACTS Metal artifact: Ferromagnetic metals will cause a magnetic field inhomogeneity. Tissues adjacent to ferromagnetic components become influenced by the induced magnetic field of the metal hardware. Metal artifacts may have several different appearances on MRI scans depending on the type of metal or the shape of the piece of it. Focal shading artifact through the orbits along the line of the eyelids due to metallic property of the patient’s mascara is a good example of what is stated above. It is also common to find artifact from dental hardware. Ferromagnetic artifact from dental fillings and braces is observed due to differences in magnetic susceptibility. Pictures show how braces may cause a signal defect in the orbit and artifactual hyperintensities in the pons (arrow). Acquisitions with thinner slices and in other spatial planes will demonstrate its an artifact

MR ARTIFACTS Diffusion Weighted Imaging (DWI) artifacts: On echo-planar DWI, susceptibility artifacts are prominent in the brain adjacent to the skull base. Susceptibility artifacts are also seen in bilateral medial frontal regions, especially on isotropic DW images, which may be related to narrow bandwidth

MR ARTIFACTS MR venography: On MR venograms performed using a contiguous 2D time-of-flight (TOF) angiographic technique, transverse sinus flow gaps (arrow) can be observed in as many as 31% of patients with no other pathological findings and normal conventional catheter angiograms; these gaps should not be mistaken for dural sinus thrombosis. These limitations are mainly related to artifacts resulting from slow intravascular blood flow, in-plane flow, and complex blood flow patterns. To overcome this problem, it is desirable to set the slice thickness as thin as possible, typically about 1.0 to 1.5 mm, and to position the acquisition plane perpendicular to the long axis of the vessel which is being imaged. 2D TOF GE T1 Gd 2D TOF GE T1 Gd

MR ARTIFACTS Magnetic resonance spectroscopy (MRS) artifacts: MRS is based on the detection of very small variations in resonance frequency, not only by the external field, but also by the small field shift generated by the electron cloud. If the magnetic field is not uniform, the chemical analysis will be artefactual (A, B). Figures show how magnetic field inhomogeneities related to hemosiderin deposits (C) generate an abnormal spectral processing (D). MRS has a low signal-noise ratio, due to the fact that signal source lacks concentration. In order to improve the signal-noise ratio, high number of acquisitions (NEX) are used. A B C D

MR ARTIFACTS Artifacts on perfusion MRI Perfusion MRI has a high susceptibility to artifacts caused by magnetic field heterogeneity (brain-bone-air transition, bleeding, calcium, metals or melanin). Metal or blood artifacts should be considered in postsurgical controls because they may change rCBV values. The images on the left show artefactual perfusion curves due to inhomogeneity magnetic field because of the presence of blood deposits. The images on the right were acquired after the reabsorption of a hematoma and represent a normal perfusion study.