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MRI Artifacts: Mechanism and Control

Chun Ruan

Abstract

A wide variety of artifacts is routinely encountered on MR images. This articlepresents the cause, appearance, diagnostic effect, and available remedies for theartifacts that are most frequently observed on MR images and are of greatest clinicalsignificance. Combined with routine preventive maintenance of imaging equipment,consistent quality control, and appropriate selection of imaging parameters, awarenessof the manifestations of these artifacts will allow image quality and diagnosticinterpretation to be optimized.

I. Introduction

Magnetic resonance imaging (MRI) is widely used in medical diagnosis for its variousadvantageous features, such as high-resolution capability, the ability to produce anarbitrary anatomic cross-sectional image, and high tissue contrast. Unfortunately,there are many potential sources of image artifacts associated with the technology ofMRI. They can potentially degrade images sufficiently to cause inaccurate diagnosis.Many MR artifacts are neither obvious nor understandable from previous experiencewith conventional types of imaging. While some MR artifacts are machine specific,the majority are inherent in the imaging method itself. MR imaging artifacts can be grouped into two general categories. First, there areartifacts that are hardware related. These artifacts are relativelyuncommon—fortunately, because they are often difficult to diagnose and usuallyrequire service personnel to correct. The second category consists of artifacts relatedto the patient or under operator control. This category is encountered much morecommonly and may often be easily prevented or corrected once they are recognized.

II. Technique and Methods

A. Motion Artifacts

Motion is the most prevalent source of MR imaging artifacts. As the name implies,motion artifacts are caused by motion of the imaged object or a part of the imagedobject during the imaging sequence. Motion results in two effects on MR images.View-to-view effects are caused by motion that occurs between the acquisitions ofsuccessive phase-encoding steps. The inconsistent location and signal intensity ofspins that move as phase-encoding data are acquired result in phase errors. Whenmotion is periodic—that is, occurs in a regular pattern—the result is complete orincomplete replication of the moving tissue, commonly referred to as ghostingartifacts. These artifacts are observed along the phase-encoding direction of theimage, regardless of the direction in which the motion actually occurred. Periodicphysiologic motions that commonly result in ghosting artifacts include cardiacmotion, respiratory motion, vascular pulsation, and cerebrospinal fluid (CSF)pulsation.1

Motion occurring between the time of radiofrequency (RF) excitation and echocollection (i.e., within-view) results in a lack of coherent phase among the population

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of moving spins at the time of echo formation.2 This incoherence manifests asblurring and increased image noise. Unlike phase errors encountered in view-to-viewmotion effects, this within-view effect is expressed throughout the image. It is mostfrequently associated with random motion, as can occur with gastrointestinalperistalsis, swallowing, coughing, eye motion, and gross patient movement.3

1. Respiratory motion

Respiratory motion results in ghosting artifacts and blurring that can obscure orsimulate lesions. A variety of methods have been used to reduce the effect ofrespiratory motion artifacts. Mechanical methods, such as use of an abdominal orthoracic binder or taking images with the patient in a prone position, are intended torestrict the amplitude of respiratory motion. However, these maneuvers often producepatient discomfort and may therefore have counterproductive effects; Signalaveraging is the use of multiple data acquisitions to improve the signal-to-noise ratio(SNR) of the image ( Fig.1). In this process, the prominence of ghosting artifacts isreduced by approximately the square root of the number of signal averages obtained.Of course, imaging time increases linearly with the number of signal averages; Withrespiratory triggering, data are collected only during a limited portion of therespiratory cycle, usually near end-expiration, when respiratory movement is minimal.The major drawback of this technique is its marked prolongation of imaging time,because so much of the time is not being used productively for data acquisition;Respiratory ordered phase encoding involves monitoring the patient’s respiratorycycle during imaging using a bellows device. Unlike respiratory triggering, however,with this method data are collected in a continuous fashion. This method does notrestrict the operator’s selection of TR, and it does not significantly extend imagingtime; Gradient moment nulling involves the application of additional gradient pulsesto correct for phase shifts among a population of moving protons at the time of echocollection. This method corrects for constant-velocity motion and helps reduce thesignal loss and ghosting associated with such movement. Use of gradient momentnulling requires a nominal TE, which may preclude its use on T1-weighted pulsesequences. This method does not prolong image acquisition time; Another method toreducing the signal intensity of moving tissue is the use of fat suppression methods.With this method, not only subcutaneous fat but also mediastinal, mesenteric,retroperitoneal, and other stores of internal fat are suppressed and are thus lesscapable of generating ghosting artifacts.

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Figure 1. The image on the left exhibits respiratory motion as blurring of the structuresas well as motion-induced ghosting. By using multiple averages motion can be reduced inthe same way that multiple averages increase the signal to noise ratio. The image on theright was obtained with 16 averages.

2. Cardiac motion

Cardiac motion produces a series of ghost artifacts along the phase-encodingdirection of the image, in addition to blurring and signal loss of cardiac andjuxtacardiac structures.4 The major approach for reducing cardiac motion artifacts iselectrocardiographic triggering, in which data collection is synchronized with cardiacphase ( Fig. 2). This synchronization enables cardiac tissue to be located in aconsistent position as each successive phase-encoding step is acquired, resulting inincreased tissue signal intensity and decreased phase errors. Other approaches includethe use of fast imaging sequences that reduce the opportunity for motion during dataacquisition, gradient moment nulling, and spatial RF presaturation pulses.

Figure 2. The image on the left was acquired without any form of motion compensationtechnique for cardiac motion. The image on the right was obtained using cardiac gating.

3. Vascular Pulsation

Vascular pulsation artifacts are recognized by their alignment with the responsiblevessel along the phase-encoding direction of the image. These artifacts reproduce thecross-sectional size and shape of the responsible vessel, but not necessarily its signalintensity. Spatial RF presaturation pulses applied outside the field of view help reducethe signal intensity of inflowing blood and, hence reduce the resultant pulsationartifact. Other practical methods for reducing the prominence of vascular pulsationartifacts include positioning the section of interest in the middle of a multisectionacquisition, thus reducing any potential entry phenomenon, and maximizing thesaturation of flowing spins.

B. Susceptibility Artifacts

Susceptibility artifacts occur as the result of microscopic gradients or variations inthe magnetic field strength that occurs near the interfaces of substance of differentmagnetic susceptibility ( Fig. 3). Large susceptibility artifacts are commonly seensurrounding ferromagnetic objects inside of diamagnetic materials (such as the humanbody). These gradients cause dephasing of spins and frequency shifts of thesurrounding tissues. The net results are bright and dark areas with spatial distortion ofsurrounding anatomy. These artifacts are worst with long echo times and with

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gradient echo sequences. Susceptibility artifacts can be made less prominent by performing imaging at lowmagnetic field strength, using smaller voxels, decreasing echo time, and increasingreceiver bandwidth. Gradient-echo and echo-planar sequences should be avoided,because they accentuate susceptibility artifacts. The use of spin-echo and particularlyfast spin-echo sequences should be considered.

Figure 3. An axial MRI of the head in a patient with mascara on her eyelids.Susceptibility artifacts from the mascara obscure the front half of theglobes.

C. Chemical Shift Artifacts

A chemical shift artifact is caused by the difference in chemical shift (Larmorfrequency) of fat and water. The artifact manifests itself as a misregistration betweenthe fat and water pixels in an image ( Fig. 4). The effect being that fat and water spinsin the same voxel are encoded as being located in different voxels. The magnitude ofthe effect is proportional on the magnitude of the Bo field and inversely proportionalto the sampling rate in the frequency encoding direction. For a constant sampling rate,the larger Bo, the greater the effect.5 Chemical shift artifacts are typically observedalong the frequency-encoding direction but can also occur along the slice-selectiondirection of the image. Chemical shift artifact can be reduced by performing imaging at low magnetic fieldstrength, by increasing receiver bandwidth, or by decreasing voxel size. The artifactstend to be more prominent on T2-weighted than on T1-weighted images. Fatsuppression methods often eliminate visible artifacts, and gradient reorientation canredirect chemical shift artifacts to another portion of the image.

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Figure 4. This artifact is shown in an axial image of a kidney where thebright border along the top of the kidney and the dark border along thebottom of the kidney represent the artifact.

D. Wrap Around Artifacts

A wrap around artifact is the occurrence of a part of the imaged anatomy, which islocated outside of the field of view, inside of the field of view ( Fig. 5). This artifact iscaused by the selected field of view being smaller than the size of the imaged object.Or more specifically the digitization rate is less than the range of frequencies in theFID or echo.6 The solution to a wrap around artifact is to choose a larger field ofview, adjust the position of the image center, or select an imaging coil which will notexcite or detect spins from tissues outside of the desired field of view.

Figure 5. The first image shows wrap-around of the back of the head on to the front of thehead, where the phase-encoded direction is anterior-posterior. The second image has the phaseand frequency directions reversed resulting in absence of the aliasing artifact. Oversamplingwas used in the frequency direction to eliminate the aliasing.

E. Partial Volume Artifacts

A partial volume artifact is any artifact which is caused by the size of the imagevoxel. It occurs when multiple tissue types are encompassed within a single voxel.For example, if a small voxel contains only fat or water signal, and a larger voxelmight contain a combination of the two, the large voxel possess a signal intensityequal to the weighted average of the quantity of water and fat present in the voxel.Volume averaging is most likely to occur in the slice-selection direction of the image,which has the largest voxel dimension. It also occurs when structures are orientedobliquely to the imaging plane and when structures move in and out of a given sectionduring image acquisition. Volume averaging can simulate abnormalities, decrease thevisualization of low-contrast abnormalities, and blur or distort affected structures. Partial volume averaging is usually recognized by careful analysis of adjacentimages. Decreasing voxel size, particularly reducing section thickness, can be usefulif further confirmation is required ( Fig.6). Three-dimensional Fourier transformimaging is particularly useful, because it provides thin sections with no interveninggaps and is conductive to reformatting in alternate imaging planes; Simple acquisitionof additional two-dimensional images in alternate imaging planes is very helpful forresolving issues relating to partial volume averaging. Retrospective reformatting oftwo-dimensional data can also be performed using an interpolation algorithm and canbe of further assistance.

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Figure 6. These two axial T1-weighted images of the head were obtained at exactly the same location,yet the second image shows the VII and VIII cranial nerves while the first does not. The reasonfor the vanishing nerve is explained by partial volume averaging. The first slice was obtainedwith a thickness of 10 mm while the second was at a thickness of 3 mm.

F. Gibbs Ringing Artifacts

Gibbs ringing artifacts are bright or dark lines that are seen parallel and adjacent toborders of abrupt intensity change. The ringing is caused by incomplete digitization ofthe echo. This means the signal has not decayed to zero by the end of the acquisitionwindow, and the echo is not fully digitized. This artifact is seen in images when asmall acquisition matrix is used. Solutions include use of a higher resolution imagingmatrix and filtration methods ( Fig.7). Gradient reorientation will displace the artifactsto another portion of the image.

Figure 7. The fine lines visible in the image on the left are due to undersampling of the high spatialfrequencies. This results in a "ringing" type of artifact following these borders in the phase direction (Rto L in this image). This problem can be easily fixed by taking more samples such as the image on theright with 256 phase encodes.

G. Zebra Stripes

Zebra stripes can be observed along the periphery of gradient-echo images wherethere is an abrupt transition in magnetization at the air-tissue interface. They areaccentuated by aliasing that results from the use of a relatively small field of view.Solutions include expanding the field of view, using spin-echo pulse sequences, orusing oversampling techniques to reduce aliasing.

H. Slice-overlap Artifacts

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The slice-overlap artifact is the loss of signal seen in an image from a multi-angle,multi-slice acquisition, as is obtained commonly in the lumbar spine. If the slicesobtained at different disk spaces are not parallel, then the slices may overlap. If twolevels are done at the same time, e.g., L4-5 and L5-S1, then the level acquired secondwill include spins that have already been saturated. This causes a band of signal losscrossing horizontally in image, usually worst posteriorly. Therefore, overlap ofsections within areas of diagnostic interest should be carefully avoided.

I. RF Overflow Artifacts

RF overflow artifacts cause a non-uniform, washed-out appearance to an image. Thisartifact occurs when the signal received by the scanner from the patient is too intenseto be accurately digitized by the analog-to-digital converter. Autoprescanning usuallyadjusts the receiver gain to prevent this from occurring but if the artifact still occurs,the receiver gain can be decreased manually.

J. Entry Slice Phenomenon

Entry slice phenomenon occurs when unsaturated spins in blood first enter into aslice or slices. It is characterized by bright signal in a blood vessel (artery or vein) atthe first slice that the vessel enters. Usually the signal is seen on more than one slice,fading with distance. This artifact has been confused with thrombosis with disastrousresults. The characteristic location and if necessary, the use of gradient echo flowtechniques can be used to differentiate entry slice artifacts from occlusions.

K. Zipper Artifacts

There are various causes for zipper artifacts in images. Most of them are related tohardware or software problems beyond the physicist immediate control. The zipperartifacts that can be controlled easily are those due to RF entering the scanning roomwhen the door is open during acquisition of images.7 RF from some radio transmitterswill cause zipper artifacts that are oriented perpendicular to the frequency axis ofimage. Broad-band noise degrades the entire image, whereas narrow frequency noiseproduces linear bands that transverse the phase-encoding direction of the image.Solutions include identifying and removing external RF sources, ensuring that thedoor to the imaging room remains closed, and verifying the integrity of the magnetroom enclosure and associated seals.

L. Cross-Excitation

Cross-excitation is caused by the imperfect shape of RF slice profiles, which leads tothe unintended excitation of adjacent tissue. This excitation results in the saturation ofsuch tissue, manifest as decreased signal intensity and decreased contrast that canhinder lesion detection. One way to avoid this artifact is to introduce an intersectiongap that is 10% to 50% of the prescribed section thickness. Another method isinterleaved image acquisition, in which odd-numbered sections are initially acquired,followed by acquisition of even-numbered sections.8 Also, optimized RF pulses thathave a more rectangular slice profile can be implemented.

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Figure 8.The dark bands visible on the T1-weighted Spin Echo axial image are due to theintersection of other axial images through the image. These are acquired in a multi-planarfashion and thus cause pre-excitation (saturation) of the protons in the area where the slicesintersect. The sagittal image illustrates where the slices were obtained and how they intersectposteriorly.

M. Shading

Shading artifacts manifest as foci of relatively reduced signal intensity involving aportion of the image. Abnormalities contained in the shaded portion of the MR imagemay be obscured.9 There are many potential causes for this artifact, including partialvolume averaging malfunction of the RF transmitter, amplifier, or receiver, excessiveRF absorption, etc.10, 11 To minimize shading artifacts, the anatomy of interest shouldbe centered within the magnet, within the coil, and within the group of sections to beacquired.

III. Conclusions

Artifacts are common in magnetic resonance imaging. Most occur as a result ofinteractions of multiple factors, especially motion. MR artifacts primarily cause imagedegradation, although they can occasionally mimic pathological lesions. Some MRartifacts, such as those caused by periodic respiratory or vascular motion, mayobscure important diagnostic findings. These and other MR artifacts may beminimized in some instances by proper selection of the directions of the phase andfrequency encoding gradients. The ability to select and vary the direction of thesegradients is a useful option in MR imaging. Investigations in the origins of MRIartifacts can not only lead to further understanding of the imaging process itself, butcan also improve the quality and diagnostic accuracy of MR examination.

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