SE consists of a 90* RF followed by 180* RF pulse before echo, with each iteration ie. TR period filling a single line of k-space.FSE allow reduction in TA by filling the time between acquisition of echo and end of given TR with additional 180* refocusing pulses and signal echoes to fill additional lines in k-space. The no. of additional echoes obtained is known as echo train length.
Figure 5. Axial T2- weighted images of the brain depict a ghosting artifact caused by patient motion when a standard fast spin-echo sequence was used (a) and the absence of ghosting when a radial k-space sampling method (syngoBLADE) was used (b)
Figure 6. Oblique axial (a) and sagittal (b) cine SSFP images obtained with prospective cardiac gating (triggering) depict hypertrophic cardiomyopathy in a 47-year-old man. Four chamber (a) and two-chamber (b) views show a decreased left ventricular volume during systole, with significant thickening of the lateral wall of the left ventricle and the interventricular septum. Cardiac triggering allowed the acquisition of diagnostic MR images with a high SNR, high spatial resolution, and absence of artifact caused by motion of the beating heart.
Placement of the navigator section for respiratory motion compensation. (a) Image with aqua overlay shows the navigator section from which the displacement information is obtained to determine the diaphragmatic position. (b) Graph shows diagphragmatic movement, indicated by the white wave and green line. The yellow boxes represent the best time to image (“window of opportunity”).
Schematics show the MR signal effects of flow compensation with gradient moment nulling. Top: Typical readout gradient waveform. Bottom: Phase of stationary spins (solid line) and constant-velocity spins (dotted and dashed lines). (a) During imaging without flow compensation, the moving spins are not refocused at the desired echo time (TE), and this leads to the loss of signal from flowing spins. (b) During imaging with gradient moment nulling, all the spins are refocused, and flow velocity is compensated for by the 1:2:1 ratio of the gradient lobe areas
(a) Sagittal T1-weighted image of the cervical spine shows areas of low signal intensity (arrows) that represent ghosting artifacts caused by esophageal motion related to swallowing. (b) Sagittal T1-weighted image obtained with a preparatory saturation pulse applied in a plane anterior to the spine shows the absence of the artifacts seen in a.
Fig: coronal T1W GRE images of posterior abdomen acquired on a 1.5 T system. Left image was taken with TE of 2.8 ms whereas right image has TE of 4.2 ms. The arrow shows chemical misregistration artifact.
Axial T1WI of the brain at exactly the same level. Second image shows 7th and 8th cranial nerves (arrow) but the first one merely depicts them. The reason for this is the partial volume averaging. The first slice was taken at thickness of 10 mm while second slice was taken at 3 mm.
Sudil Paudyal(51)BSc.MIT Final yearARTIFACTS IN MRI
• Image Artifact (brit. Artefact) is something observed in ascientific investigation that is not naturally present but occursas a result of the investigative procedure. (oxford dictionary)• A structure not normally present, but visible as a result ofmalfunction in the hardware or software of the device, or aconsequence of environmental influences as heat or humidityor can be caused by the human body itself.5/5/2013 MRI artifacts-sudil 3Introduction
• All MRI images have artifacts to some degree.• Some are irreversible and may only be reduced while otherscan be totally eliminated.• Knowledge of artifacts a must for technologists to maintainoptimum image quality.• Causes should be understood and compensated for if possibleto avoid being misjudged as pathology.• Classified as to their basic principles, Physiologic (motion, flow) Hardware (electromagnetic spikes, ringing) Inherent physics (chemical shift, susceptibility, metal)5/5/2013 MRI artifacts-sudil 4
Artifacts caused by pt. motion1. Ghosting and smearing: Most common artifacts produced by motion of pt.(voluntary /involuntary)– From oesophageal contraction and vascular pulsation during headand neck imaging,– From respiration and cardiac activity during thoracic and abdominalimaging,– From bowel peristalsis during abdominal and pelvic imaging. Occurs when there is magnitude and/or phase deviation fromoptimal k-space encoding.5/5/2013 MRI artifacts-sudil 5
Magnitude error - when a signal producing voxel changesposition during application of RF pulse. Phase errors - Patient motion whenever a transverse componentof magnetization exists and motion is perp. to the appliedmagnetic field Bo.5/5/2013 MRI artifacts-sudil 6
Appearance of ghosting on final clinical image depends onwhere in k-space such phase errors occur:– If along the x-axis of k-space : frequency encoding direction– If along y-axis : phase encoding direction– If in middle of k-space : smearing appearance– If phase errors are periodic (as in pulsatile motion), periodic ghosting. Relatively more common in the phase encoding direction.5/5/2013 MRI artifacts-sudil 7
RemedyTotal elimination impossible unless imaging a cadaver!!!!• Various methodsa) Variants of rectilinear k-space filling techniques Fast spin echo Multisection imaging Single shot single section imaging techniques Parallel imagingb) Radial k-space filling techniquesc) Cardiac gating and triggeringd) Respiratory gatinge) Navigator echo5/5/2013 MRI artifacts-sudil 8
a) Variants of rectilinear k-space filling techniques CSE : dominant because of relative insensitivity to fieldinhomogeneity. FSE/TSE : reduction in TA5/5/2013 MRI artifacts-sudil 9
Multisection imaging Several interleaved sections are imaged simultaneously Helps further decrease acquisition time Single shot single section imaging Sequences such as the HASTE are more resistant to motion Allow more rapid acquisition by filling only half of k-space.5/5/2013 MRI artifacts-sudil 10
Parallel imaging : involves filling of selected lines in k-space at a predetermined interval. Requires use of multichannel, multicoil technology, with each coilpossessing a distinct, known sensitivity profile over the FOV and withat least two coils placed in the phase encoding direction. The no. of phase encoding steps can be reduced by a factor of 1/X,where X is the parallel imaging factor (2 or 3). Thus image acquisition proceed X times faster by filling only one ofevery X lines(2nd or 3rd ) of k-space in phase encoding direction andby known coil sensitivity profiles to synthesize other lines.Eg: GRAPPA (siemens), SENSE (philips), SMASH5/5/2013 MRI artifacts-sudil 11
b) Radial k-space filling techniques Standard line by line filling of k-space replaced by radial k-space fillingwith the use of a multishot radial acquisition technique (eg.syngoBLADE, Siemens healthcare; PROPELLER, GE healthcare) MR imaging datasets acquired in multiple overlapping radial sections,each of which includes data sampled from the center to the peripheryof k- space. A series of low resolution images is first reconstructed from eachradial section and combined to produce a high resolution image. As the phase encoding direction varies with each radial section,ghosting is not propagated in PE direction but is dispersed throughoutradial sections and thus has a lesser effect on final image. Also provide correction for in-plane rotation and bulk translationalmovement.5/5/2013 MRI artifacts-sudil 13
c) Cardiac gating and triggeringCardiac motion periodic, both static and cine imagesCine imaging performed by using a balanced SSFP thatproduces tissue contrast based on ratio of T1 to T2, lendingblood a high signal intensity (so called bright bloodimaging).Cine imaging performed with retrospective cardiac gatingi.e.a series of MR images of a single anatomic section are acquiredduring cardiac cycle, monitored with ECG.Data within k-space are linked with specific time point in cardiaccycle.The acquired datasets are then sorted acc to time stamp toproduce sequential images allowing a dynamic evaluation ofmyocardial function.5/5/2013 MRI artifacts-sudil 15
For static images to evaluate cardiac structure and not cardiacfunc., prospective triggering is used and image acquisition isusually timed to occur during the end diastole. In triggering a certain no. of k-space lines is acquired duringpt. breath holding for a given heartbeat.5/5/2013 MRI artifacts-sudil 16
d) Respiratory gating Infrequently used in clinical imaging as compared to cardiac gating. Breath holding method most often used, but may not be feasible forinfants, children, and pt.s with respiratory or cognitive impairment. Sequences with short acquisition time may be used, or if breathholding possible for limited time, exam may be divided into severalbrief acquisitions. If none is possible, respiratory gating. in which respiration related motion is monitored, using bellows orbreathing belt, image acquisition is timed to take place at endexpiration, when there is little or no motion. Phase reordering with either COPE or ROPE.5/5/2013 MRI artifacts-sudil 17
e) Navigator echo Most commonly used in abdominal imaging at the interface of lungand diaphragm Application of small, one dimensional spatial encoding gradient in aplane perpendicular to diaphragm. Echo measured at this location allows correction of imaging dataset toensure that, only the imaging data acquired, when diaphragm is at itspeak (end expiration), are used in image reconstruction.5/5/2013 MRI artifacts-sudil 18
Motion artifact contd..2. Pulsatile flow-related artifacts Periodic pulsation of vascular structures leads to ghosting in phaseencoding direction at constant intervals. Distance between ghosts depends on diff. between heart rate and TR;if synched, no ghosting. Macroscopic motion eg. Blood in large vessels or CSF in spinal canalalso contribute to ghosting.5/5/2013 MRI artifacts-sudil 19
Remedya) ECG gating :– time consuming, seldom used except for cardiac imagingb) Gradient moment nulling:– Application of additional gradient pulses to correct for phase shiftsamong a population of moving protons at the time of echo collection.– Corrects for constant-velocity motion and helps reduce the signal lossand ghosting associated with such movement.5/5/2013 MRI artifacts-sudil 20
c) Saturation pulse:– Additional RF pulses applied before the sequence in a planeperpendicular or parallel to imaging plane– As applied at the beginning of sequence, their use may reduce the no.of imaging secitons that can be obtained per TR in multisectionacquisitions.– Also when SAR is already high, use of saturation pulse may result inexcessive heat deposition.5/5/2013 MRI artifacts-sudil 21
Artifacts caused by field distortions1. Eddy currents source of spatial and temporal distortions in Bo. most frequently encountered in clinical DWI because of the highamplitude and long duration of diffusion- sensitizing gradients. When diffusion gradient applied, change in magnetic field createselectrical currents in neighboring conductive surfaces. Such currentscreate smaller magnetic fields (i.e. eddy currents) that modify Bo. modern gradient coils equipped with active shielding to avoid theseeffects of electrical conduction, eddy currents are not as obtrusive inroutine clinical imaging as they were in the past.5/5/2013 MRI artifacts-sudil 22
5/5/2013 MRI artifacts-sudil 23Remedy Pre-compensation- A “distorted” gradient waveform is used whichcorrects to normal with the eddy current effects. Shielded gradients – Active shielding coils between gradient coils and maingradients.
2. Gradient field nonlinearity Occurs in all MR imaging systems and primarily related to gradientfalloff due to the finite size of the gradient coils. Predictable. Easily corrected by system software, with corrections applied beforethe final images are constructed. Operator unaware of occurrence of mapping errors due to gradientfield nonlinearity. Although post processing techniques can correct distortions resultingfrom gradient field nonlinearity, cannot compensate for losses inspatial resolution that are related to gradient field nonlinearity.5/5/2013 MRI artifacts-sudil 24
3. RF field inhomogeneities Do not cause geometric distortions but contribute to signalnonuniformity. May arise from problems in RF coil construction or standing wave(dielectric) effects. Because the Larmor frequency of protons increases with increasingfield strength, a high-frequency pulse is needed to excite signalproducing protons at MR imaging with high field strengths such as 3 T. When the RF wavelength is shorter than the dimensions of theanatomic structures examined at clinical MR imaging, standing wavesmay result. Leads to inhomogeneous fat suppression.5/5/2013 MRI artifacts-sudil 25
5/5/2013 MRI artifacts-sudil 26Remedy Shimming- allows precise control of overall homogeneity of RF field. Use STIR for Fat sat than spectral tech. like CHESS. Dielectric – use phased array coils, software compensation
Aliasing artifacts• Field of view:– dimensions of the anatomic region to be included in imaging.– mathematic product of the acquisition matrix and pixel dimensions.– Chosen considering size of structure to be evaluated and trade-offsbetween SNR and spatial resolution.– With a constant imaging matrix, a smaller FOV results in higher spatialresolution but lower SNR.• Choosing an FOV that is smaller than the area imaged leads towraparound or aliasing artifacts.5/5/2013 MRI artifacts-sudil 27
• Echoes are sampled along the FE direction at the sampling bandwidthrate, with higher rates resulting in a greater range of frequency sampling.• Nyquist frequency - highest frequency that may be clearly sampled, withhigher frequencies corresponding to tissues outside the specified FOV.• High frequencies outside the FOV falsely detected as lower frequencieswhich leads to a wraparound artifact on the reconstructed images.5/5/2013 MRI artifacts-sudil 28
5/5/2013 MRI artifacts-sudil 30Remedy• Increase the FOV (decreases resolution).• Oversampling the data in the frequency direction (standard)and increasing phase steps in the phase-encoded direction –phase compensation (time or SNR penalty).• Swapping phase and frequency direction so phase is in thenarrower direction.• Use surface coil so no signal detected outside of FOV.• Saturation pulses may also be applied to structures in thenonimaged portion of the FOV to reduce signal and, thus,signal overlap.
Magnetic susceptibility artifacts Inaccurate spatial encoding from susceptibility gradients within tissueleads to distortion of anatomic structures. Artifact resulting from the presence of metallic objects, not only distortsnearby structures but also may result in signal dropout, depending on thesequence used. The effects are field strength dependent, and the potential for metallicobject–related artifacts is greater at 3 T than at lower magnetic fieldstrengths. However, the increase in magnetic field strength from 1.5 to 3 T results ina significant improvement in SNR, allowing the use of a higher bandwidthsampling rate and parallel imaging to reduce susceptibility artifacts. Worst with long TE and gradient echo sequences.5/5/2013 MRI artifacts-sudil 31
5/5/2013 MRI artifacts-sudil 33Remedy Imaging at low magnetic field strength, using smaller voxels, decreasingecho time, and increasing receiver bandwidth. Gradient-echo and echo-planar sequences should be avoided, becausethey increase susceptibility artifacts. The use of spin-echo and particularly fast spin-echo sequences should beconsidered.
Chemical shift artifacts Protons in fat and water precess at different frequencies in an appliedmagnetic field. The separation between their resonance frequencies increases withincreasing field strength. Eg. At 1.5 T , freqency diff. is 220 Hz, but at 1 T the diff. is 147 Hz and atlower field strengths (0.5 T or less ) usually insignificant. The chemical shift differences lead to spatial misregistration of the MRsignal; i.e. differences in the Larmor frequency are mistaken fordifferences in spatial position along the frequency encoding axis. The resultant chemical shift artifacts on images manifest mostprominently at fat-water interfaces.5/5/2013 MRI artifacts-sudil 34
Amount of chemical shift is expressed in arbitrary units known as parts permillion (ppm) of Bo. It’s value is always independent of Bo and equals 3.5 ppm. Occurs in the frequency encoding direction only.5/5/2013 MRI artifacts-sudil 35
Remedy Imaging at low magnetic field strength, by increasing receiver bandwidth, or by decreasing voxel size. T1-weighted- The artifacts tend to be more prominent on T2-weighted than on T1-weighted images. Fat suppression methods often eliminate.5/5/2013 36MRI artifacts-sudil
Chemical misregistration artifact Also produced as a result of the precessional frequency diff.between fat and water. But this is caused because fat and water are in phase atcertain times and out of phase at others, due to difference intheir precessional frequency. When they are in phase their signals add constructively andwhen out of phase the signals cancel each other out. This cancellation effect is known as chemical misregistrationartifact.5/5/2013 MRI artifacts-sudil 37
Causes a ring of dark signal around certain organs where fat andwater interfaces occur within the same voxel. Eg. Kidneys Mainly occurs in PE direction as it is produced due to phasedifference between fat and water. Most degrading to the image in gradient echo pulse sequenceswhere gradient reversal is very ineffective.5/5/2013 MRI artifacts-sudil 38
Remedy Use SE sequence In GRE, select a TE that corresponds to the periodicity of fat andwater ie. Select a TE that generates an echo when fat and water are inphase.5/5/2013 MRI artifacts-sudil 39
Partial volume artifacts Partial volume occurs if slice thickness > thickness of tissue ofinterest Occurs when multiple tissue types are encompassed within a singlevoxel If small structure is entirely contained within the slice thicknessalong with other tissue of differing signal intensities, the resulting signal displayed on the image is a combination ofthese two intensities. This reduces contrast of the small structure. If the slice is the same thickness or thinner than the small structure,only that structures signal intensity is displayed on the image. Volume averaging is most likely to occur in the slice-selectiondirection of the image.5/5/2013 MRI artifacts-sudil 40
5/5/2013 MRI artifacts-sudil 41Remedy Decreasing voxel size, particularly reducing section thickness. Three-dimensional Fourier transform imaging is particularly useful,because it provides thin sections with no intervening gaps and isconductive to reformatting in alternate imaging planes. Multiplanar imaging option helps to clarify.
Signal truncation artifacts Occur in regions of boundary between high and low signal intensity Caused by approximation errors in Fourier transform analysis. As the signal is sampled over a limited period of time, some dataare necessarily omitted (truncated) in k-space, causing the signalintensity of a given pixel on the final image to vary from its idealsignal intensity. Commonly seen at the interface of the low-signal intensity spinalcord with high-signal-intensity CSF on T2WI of the spine, mimicspinal canal dilatation (ie. Hydromyelia,syrinx). Appears as a periodic “ringing” at high contrast interfaces.5/5/2013 MRI artifacts-sudil 42
5/5/2013 MRI artifacts-sudil 43128x256256x256Remedy• Use of higher resolution imaging matrix and filtration methods.(undersampling avoided)• Gradient reorientation will displace the artifacts to another portion of theimage.
Slice-overlap (cross-slice) artifacts• Loss of signal seen in an image from a multi-angle, multi-sliceacquisition.• If the slices obtained at different disk spaces are not parallel, thenthe slices may overlap when two levels are done at the same time,e.g., L4-5 and L5-S1.• The level acquired second will include spins that have already beensaturated.• This causes a band of signal loss crossing horizontally in image,usually worst posteriorly.• Therefore, overlap of sections within areas of diagnostic interestshould be carefully avoided.5/5/2013 MRI artifacts-sudil 44
5/5/2013 MRI artifacts-sudil 45Remedy Avoid steep change in angle between slice groups. Use separate acquisitions. Use small flip angle.
Cross-excitation artifacts• The imperfect shape of RF slice profiles leads to the unintendedexcitation of adjacent tissue.• This excitation results in the saturation of such tissue• Manifest as decreased signal intensity and decreased contrast thatcan hinder lesion detection.5/5/2013 46MRI artifacts-sudil
Remedy• Introduce an intersection gap that is 10% to 50% of theprescribed section thickness.• Interleaved image acquisition, in which odd-numberedsections are initially acquired,followed by acquisition of even-numbered sections.• optimized RF pulses that have a more rectangular slice profilecan be implemented.5/5/2013 MRI artifacts-sudil 47
Cross-talk Artifact Perfect RF pulse is a sinc function (FT = ‘top hat’) Real RF pulse is a truncated sinc (FT = ‘top hat with roundededges’) Result of imperfect slice excitation of adjacent slices causingreduction in signal over entire image.5/5/2013 MRI artifacts-sudil 48
5/5/2013 MRI artifacts-sudil 49• Inter-slice cross talk could cause increased T1 weighting andreduced SNR.• May be reduced by using gap, interleaving slices andoptimized (but longer) RF pulses.
Magic Angle Effects Produced by the particular physical properties of fibrillary tissues and theirinteraction with the static magnetic field. Seen most frequently in tendons and ligaments that are oriented at a 55oangle to the main magnetic field. Due to dipolar interactions that reduce their T2 relaxation time. Normal dipolar interactions between the H+’s in water molecule aligned intendons shortens T2, causing loss of signal. T2 relaxation time is lengthened and maximal when these fibrillarystructures are at a 55° angle to B0. Maximal for short TE.5/5/2013 MRI artifacts-sudil 50
Zipper Artifacts• Most are related to hardware or software problems• May occur in either frequency or phase direction.• Zipper artifacts from RF entering room during imageacquisition are oriented perpendicular to the frequencydirection and easily controlled.5/5/2013 MRI artifacts-sudil 52
Spike artifact• Caused by one ‘bad’ data point in k-space.• Fig. shows one data point in k-space, which is out of theordinary.• The resulting image show diagonal lines throughout theimage.Remedy - Repeat the scan.5/5/2013 MRI artifacts-sudil 53
Zebra stripes• Observed along the periphery of gradient-echo images (abrupttransition in magnetization at the air-tissue interface)• Increased by aliasing that results from the use of a relatively smallfield of view.• May also occur when pt. touches the coil or a result of phase wrap.Remedy• expanding the FOV, using SE pulse sequences.• using oversampling techniques to reduce aliasing.5/5/2013 MRI artifacts-sudil 54
RF Overflow Artifacts (Clipping) Causes a nonuniform, washed-out appearance to an image. Occurs when the signal received from the amplifier exceedsthe dynamic range the analog-to-digital converter causingclipping. Autoprescanning usually adjusts the receiver gain to preventthis from occurring.5/5/2013 55MRI artifacts-sudil
Moire Fringes An interference pattern most commonly seen when doing gradientecho images. One cause is aliasing of one side of the body to the other results insuperimposition of signals of different phases that add and cancel. Can also be caused by receiver picking up a stimulated echo. Similar to the effect of looking though two window screens.5/5/2013 56MRI artifacts-sudil
Central Point Artifact• A focal dot of increased or decreased signal in the center of animage.• Caused by a constant offset of the DC voltage in theamplifiers.Remedy Requires recalibration by engineer Maintain a constant temperature in equipmentroom for amplifiers.5/5/2013 57MRI artifacts-sudil
Quadrature ghost artifact• Another amplifier artifact caused by unbalanced gain in the twochannels of a quadrature coil.• Combining two signals of different intensity causes somefrequencies to become less than zero causing 180 degree “ghost.”5/5/2013 58MRI artifacts-sudil
Entry slice (Inflow) phenomenon• Unsaturated spins in blood or CSF entering the initial slices results ingreater signal then reduces on subsequent slices.• Characterized by bright signal in a blood vessel (artery or vein) at entrysite.• May be confused with thrombus.• The use of gradient echo flow techniques can be used to differentiateentry slice artifacts from occlusions.• Can cause spatial saturation to reduce.• Mechanism for TOF angiography.5/5/2013 MRI artifacts-sudil 59
Shading artifacts Produces a loss of signal intensity in one part of image. Main cause is uneven excitation of nuclei within the pt. due to RFpulses applied at flip angles other than 90* and 180* Also caused by abnormal loading on coil or by coupling of coil atone point. This may occur with large pt. who touches one side ofthe body coil and couples it at that point. Appear as foci of relatively reduced signal intensity involving aportion of the image. Abnormalities contained in the shaded portion of the MR imagemay be obscured. Also be caused by inhomogeneities in the main magnetic field thatcan be improved by shimming.5/5/2013 MRI artifacts-sudil 60
Remedy• Always ensure that coil is loaded correctly i.e correct size of coil foranatomy under examination, and pt. is not touching the coil at any point.• Use of foam pads or water bags bet.n coil and patient• Ensure that appropriate pre-scan parameters have been obtained beforethe scan, as these determine the correct excitation frequency andamplitude of applied RF pulses.5/5/2013 MRI artifacts-sudil 61
Conclusion…• Artifacts in MR images are an inevitable truth.• MRI arifacts occur because one or more of the assumptions underlying theimaging principles have been violated.• Some can be reduced while others can be totally eliminated.• Artifact correction methods usually involve one or more of the following: Hardware calibration Scanning parameter optimization Special pulse sequence design Signal and image postprocessing• By understanding the mechanism of their production and their effects on the finalimage, technologists should considerably try to minimize these artifacts with theuse of reduction techniques.• Ideally, we want all image artifacts to be below the level of users perception.• Artifact correction is an active area of research today, and will continue to be inthe future as advances in MRI technology reveal new image information and newkinds of artifacts.5/5/2013 MRI artifacts-sudil 62
Lastly…Observer artifact:Self explanatoryOtherwise known as “Upside down Error”Apply for time off from duty.5/5/2013 MRI artifacts-sudil 63
Find more at….1. Morelli JN, Runge VM, Ai F, et.al. An Image-based Approachto Understanding the Physics of MR Artifacts. RadioGraphics2011; 31:849–8662. MRI in practice, 2nd edn. By Catherine Westbrook andCarolyn Kaut3. MRI artifacts, PPT presentation by Ray Ballinger4. MRI physics course; Artifacts and suppression technique byJerry Allisson5. www.mr-tip.com6. www.mritutor.org5/5/2013 MRI artifacts-sudil 64
On examining a knee with prosthesis in situ, what artifact isexpected?Difference between chemical shift and chemicalmisregistrationTruncation artifact and reductionList different motion compensation options.Why is motion artifact seen only in PE direction?What are the sources of zipper artifact?Why does aliasing occur?Difference between cross talk and cross excitation.5/5/2013 MRI artifacts-sudil 65