MR IMAGE QUALITY AND ARTIFACTS

SAHANA KAYASTHA

MSC.MIT 1^ST YR

ROLL NO 26

IOM,MMC

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MR IMAGE QUALITY

• MRI being a digital image greatly depends on image contrast and its spatial characteristics.
• good image quality depends upon making good scanning parameter choices
• The image quality in MRI depends on:
• Spatial resolution : - a)Matrix b)Field of View(FOV) c)Slice thickness
• Signal- to – Noise Ratio (SNR)
• Contrast- to – Noise Ratio
• Scan time
• Artifacts

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Spatial resolution

• ability to distinguish between two points as separate and distinct, and is controlled by the voxel size(3D) or pixel sixe (2D).
• The voxel size is affected by:
• number of pixels or image matrix
• FOV
• slice thickness

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pixel size = field of view / matrix

voxel volume = (field of view / matrix) x slice thickness

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Matrix

• Matrix size is the no. of pixels in the images.
• Increasing the matrix size will increase the number of pixels/voxels in the image but, as they are still within the same field of view, they will be smaller.
• Increasing Matrix size:
• Results in smaller voxels which increses spatial resolution i.e better detail
• But decreases signal i.e decreases SNR ( the no. of spins that contribute to MR signal is less)

High or fine matrix

Low or coarse matrix

• Increases scan time –
• more voxels need to be acquired( only in case of phase encoding direction as each voxel requires a new signal i.e. more voxels means more signals need to be created.

Field of View

• FOV determines the size of the area to be imaged.
• A larger FOV means a larger area imaged
• The matrix size remains the same and so, to fill up a larger area, the voxel becomes larger.
• Increasing FOV:
• Increases the signal ( a larger voxel means more signal received per signal.
• Lower resolution( the voxels become larger)
• Increased viewing area.

• Spatial resolution parameters depend on certain k- space characteristics. Let’s recap:
• Pixel size is determined by the distance traveled in k -space.
• Pixel size in the phase axis of the image is determined by the steepest phase-encoding gradient, both positively and negatively (height of the chest of drawers).
• Pixel size in the frequency axis of the image is determined by the sampling window (width of the chest of drawers).

• Image matrix depends on the number of data points in k –space.
• The phase matrix is the number of k -space lines (or drawers we prefer). It is the number of data points in each column of k -space.
• The frequency matrix is the number of data points in each line of k -space (or pairs of socks we prefer). It is the number of data points in each row of k -space.

Data points in k-space.

• FOV depends on the distance between each data point in k- space.
• The size of the FOV in the phase axis of the image is inversely proportional to the distance between data points in each column of k -space.
• This is the incremental step between each line of k -space (or the depth of each drawer).
• The size of the FOV in the frequency axis of the image is inversely proportional to the distance between data points in each row of k -space.
• This is the sampling interval between each data point.

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Rectangular FoV

• A rectangular FOV may be desired when scanning anatomy that has a smaller dimension in the phase axis than frequency.
• A rectangular or asymmetric FOV maintains spatial resolution because pixel size is unchanged, but reduces the scan time, as only a portion of the total number of phase-encoding steps are performed.
• The dimension of the FOV in the phase direction is reduced compared to that in the frequency direction
• so used when imaging anatomy that fits into a rectangle, for example, a sagittal lumbar spine.

Fig: Square and rectangular FOV. The scan�time is half but the phase resolution�remains unchanged

Slice thickness�

• Increasing the slice thickness:
• Increases the signal
• Decreases the resolution
• Increases the partial volume effect
• Gives larger object coverage

To optimize the spatial resolution

• select Thin slices, Small FOV And High/fine image matrix.
• The slice thickness is determined by the slope of the slice-select gradient.
• Therefore, to achieve thin slices, the slice-select gradient slope is steep

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• The size of the frequency FOV is determined by the sampling interval (and therefore the receive bandwidth) but also by the slope of the frequency-encoding gradient.
• To achieve a small FOV, the frequency-encoding gradient slope is steep.

• The phase matrix size is determined by the number of phase encodings steps.
• To achieve a fine phase matrix, a high proportion of the phase-encoding gradient slopes are steep both positively and negatively.
• The frequency matrix is determined by the number data points in each line of k -space.
• To achieve a fine frequency matrix, for a given digital sampling frequency (as determined by the receive bandwidth) and sampling window, the amplitude of the frequency-encoding gradient is steep.

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• A small FOV, thin slices, and fi ne matrices increase the minimum TE and may result in fewer slices per TR.
• If the TE increases, it takes longer to select and encode each slice, and therefore fewer slices are permissible in a given TR.

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Signal- to – Noise Ratio

• SNR is defined as the ratio of the amplitude of signal received to the average amplitude of the background noise.
• Signal is the voltage induced in the receiver coil by the precession of coherent magnetization in the transverse plane at, or about, time TE.
• Noise represents frequencies that exist randomly in space and time.

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• There are various factors that affects signal amplitude affecting the SNR. These are as follows:
• Magnetic field strength of the system
• Proton density of the area under examination
• Coil type and position
• TR, TE, and flip angle
• Number of signal averages (NSA)
• Receive bandwidth
• Voxel volume

Magnetic field strength

• as field strength increases, so does the energy gap between high- and low-energy nuclei.
• As the energy gap increases, fewer nuclei have enough energy to align their magnetic moments in opposition to B0.
• Therefore, the number of spin-up nuclei increases relative to the number of spin down nuclei.
• The NMV increases at higher field strengths, and there is more available magnetization to image the patient.
• SNR therefore increases

Proton density

• The number of protons in the area under examination determines the amplitude of received signal.
• Areas of low proton density in terms of those that are MR-active (such as the lungs) have low signal and therefore low SNR,
• whereas areas with a high proton density (such as the pelvis) have high signal and therefore high SNR

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Type of coil

• type of coil affects the amount of received signal and therefore the SNR
• Larger coils receive more noise in proportion to signal than smaller coils because noise is received from the entire receiving volume of the coil
• Quadrature coils, Phased array coils and Surface coils increases SNR.
• Large coils, however, increase the likelihood of aliasing, because tissue outside the FOV is more likely to produce signal.
• To induce maximum signal, the coil must be positioned in the transverse plane perpendicular to B0.
• Angling the coil, as sometimes happens when using surface coils, results in a reduction of SNR

TR, TE, and flip angle

• Although TR, TE, and flip angle are usually considered parameters that influence image contrast; they also affect SNR and therefore image quality.
• TR: The TR controls the amount of longitudinal magnetization that recovers before the next RF excitation pulse is applied.
• A long TR allows full recovery of longitudinal magnetization so that more is available to be flipped into the transverse plane in the next TR.
• A short TR does not allow full recovery of longitudinal magnetization so less is available to be flipped.
• The SNR improves as the TR increases

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Fig: Changing TR at 3T

(a) TR 700 ms, (b) TR 500 ms, (c) TR 300 ms, (d) TR 140 ms

• TE : TE controls the amount of coherent transverse magnetization that decays before an echo is collected.
• A long TE allows considerable decay of coherent transverse magnetization before the echo is collected, while a short TE does not
• SNR decreases as the TE increases because there is less transverse magnetization available to rephase and produce an echo.

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Fig: (a) TE 11 ms, (b) TE 20 ms, (c) TE 40 ms, (d) TE 80 ms

• Flip angle: controls the amount of transverse magnetization created by the RF excitation pulse, which induces a signal in the receiver coil.
• If the TR is long, maximum signal amplitude is created with flip angles of 90° because full recovery of longitudinal magnetization occurs with a long TR, and this is fully converted into transverse magnetization by a 90° flip angle.
• SNR significantly decreases in the lower flip angle image.
• If the TR is short, the flip angle required to generate maximum transverse magnetization and therefore signal is less than 90° and is governed by the Ernst angle equation

Number of signal averages (NSA or NEX)

• The NSA controls the amount of data stored in each line of k-space
• is the number of times data are collected with the same amplitude of phase encoding slope and therefore how many times a line of k-space is filled with data.
• Doubling the NSA therefore doubles the amount of data that are stored in each line of k-space, while halving the NSA halves this amount.
• increases by the factor of a square root. doubling the NSA only increases the SNR by √2 (=1.4)
• To double the SNR, the NSA and therefore the scan time are increased by a factor of 4.

Sagittal brain using 1 NSA.

Sagittal brain using 4 NSA.

Receive bandwidth

• is the range of frequencies that are accurately sampled during the sampling window
• Reducing the receive bandwidth results in less noise sampled relative to signal
• By applying a filter, noise frequencies much higher and lower than signal frequencies are filtered out.
• signal frequencies are spread over a wider frequency range in broad bandwidth as the height of signal curve is lower

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• Therefore, as the receive bandwidth decreases, SNR increases as less noise is sampled as a proportion of signal.
• Halving the bandwidth increases the SNR by about 40%, but increases the sampling window .
• As a result, reducing the bandwidth increases the minimum TE and also increases chemical shift artifact

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Voxel volume�

• Voxel dimensions are determined by the pixel area and the slice thickness
• Large voxels consequently have a higher SNR than small voxels
• SNR is proportional to the voxel volume
• Any selection that decreases the size of the voxel decreases the SNR and vice versa.
• There are three ways to achieve this:
• Changing the image matrix
• Changing the FOV.
• Changing the slice thickness

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Changing image matrix

Sagittal brain using 128 phase matrix.

Sagittal brain using 256 phase matrix.

Changing FOV

• The FOV halves, which halves the pixel dimension along both axes.
• Depending on the area under investigation and the receiver coil, it is sometimes necessary to take steps to increase SNR when using a small FOV especially in conjunction with a large coil.

Changing slice thickness

• voxel size is altered by halving the slice thickness from 10 to 5 mm.
• Doing so halves the voxel volume from 1000 to 500 mm3 and hence halves the SNR

To optimize SNR select

• LongTR and short TE
• 90° flip angle in spin-echo or the Ernst angle in gradient-echo pulse sequences
• The correct coil (ensure that it is well tuned and correctly positioned)
• A low or coarse image matrix
• A large FOV
• Thick slices
• High NSA
• Narrow receive bandwidth.

Contrast-to-noise ratio (CNR)

• CNR is the difference in SNR between two adjacent areas
• It is important to maximize the CNR so that pathology is clearly seen as distinct from normal anatomy or so that one structure is clearly seen next to another
• CNR is improved by increasing the signal from pathology or structures that are important to see (e.g. positive contrast agents, T2 weighting, flow techniques)
• CNR is improved by decreasing signal from normal structures (e.g. chemical suppression, MTC)

• T2 weighting: Although T2-weighted images often have a lower SNR than T1-weighted images (due to the longer TE), the ability to distinguish pathology from normal tissue is often much greater
• This is because of the high signal of pathology compared with low signal of surrounding anatomy, i.e. the CNR is higher

Fig: A heavily T2-weighted image of the buttock.

A very long TE was used in this image.

The CNR is optimized showing pathology clearly.

• Contrast agents: The purpose of administering contrast agents is to increase CNR between pathology (which enhances) and normal anatomy (which does not).
• Magnetization transfer contrast (MTC):
• There is always a transfer of magnetization between the bound and the free protons, which causes a change in the T1 recovery times of free spins.
• This is exploited by selectively saturating the bound spins, which reduces the intensity of signal from the free spins due to MTC.

• Flow-related techniques:
• There are techniques that are specifically designed to produce signal only from nuclei with certain characteristics.
• Nuclei that do not possess such characteristics do not produce signal, and so there is a good CNR between them and those that do.
• For example, phase-contrast MR angiography is a technique that only images flowing spins.
• Stationary spins produce no signal, and so there is a good CNR between vessels and the tissue around them

Presaturation

• CNR is optimized by using presaturation pulses
• By saturating normal anatomy (which often contains fat), pathology (which is mainly water) is often seen more clearly especially if it has high signal intensity
• Presaturation is also useful to reduce artifacts such as phase mismapping and aliasing

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• Out-of-phase imaging (Dixon technique) is used in gradient-echo sequences to null signal from voxels in which fat and water nuclei coexist.
• This is achieved by selecting a TE when the magnetic moments of nuclei in fat and water are out of phase with each other. As they are incoherent, no signal is received from the voxel.

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To optimize CNR :

• select
• Very long TR and TE
• Saturation techniques to null certain tissues
• Techniques that remove signal from certain nuclei(eg: phase contrast angiography)

Scan time�

• As multiple slices are selected during two- and three-dimensional volumetric acquisitions, movement during these types of scan affects all the slices.
• During a sequential acquisition, movement of the patient only affects those slices acquired while the patient is moving.
• To optimize the scan time :select
• Short TR
• Low phase matrix
• Low NSA
• High turbo factor in TSE
• Use imaging options that reduce scan time, e.g. rectangular FOV

Improving resolution but not increase scan time

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• This is done by the following:
• Changing the frequency matrix only. The frequency matrix does not affect scan time but, if increased, increases resolution.
• Using asymmetric FOV. This maintains the size of the FOV along the frequency axis but reduces the FOV in the phase direction.
• Therefore, the resolution of a square FOV is maintained, but the scan time decreases in proportion to the reduction in the size of the FOV in the phase direction.
• This option is useful when anatomy fits into a rectangle.

Artifacts

• Artifacts are any feature that is not true representation of anatomy.
• Artifacts can cause significant image degradation and can lead to misinterpretation
• The cause of the signal misregistration is variable; artifacts are divided into three main types based on the cause.
• 1. Motion artifacts
• 2. Artifacts related to particular measurement technique or parameters used, e.g. chemical shift, wrap around and truncation artifacts.
• 3. External artifacts results from either a malfunction of the MR scanner or external interference.

Motion Artifacts

• Phase mismapping
• causes: Phase mismapping, ghosting, or motion artifact is caused by periodic motion mainly as a result of spins moving between each phase encode
• It mainly originates from breathing and pulsatile motion of vessels , CSF, swallowing and eye movement
• The tissue is excited at one location but is mapped to a different location during readout due to its motion.
• Also due to time delay between phase encoding and readout

• Ghosting and smearing
• While ghosting results from periodic motion, smearing is a result of aperiodic motion.
• The periodic rise and fall of thorax/abdomen during breathing produces an artifact called ghosting
• Next,that may result from aperiodic motion like that of eyes is smearing

Remedy�

• Solutions to phase mismapping include:
• spatial presaturation bands placed over moving tissues (e.g. over the anterior neck in sagittal cervical spines)
• spatial presaturation bands placed outside the FOV, especially before the entry or after the exit slice for reducing ghosting from vascular flow: arterial and venous
• switching phase and frequency directions

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• For bowel motion, administer antispasmodic agents.
• increasing the number of signal averages as this increases the no. of times the signal is averaged
• shorten the scan time when motion is from patient movement
• Use faster imaging: FSE, HASTE, radial k-space filling technique such as Propellor/ Blade , parallel imaging technique(with several data extrapolation algorithm like GRAPPA, SENSE)
• Cardiac/ respiratory gating/respiratory navigator echoes:
• ROPE(Respiratory Ordered Phase Encoding) can greatly reduce ghosting from respiration

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M-mode display of navigator data showing

motion of liver (lower gray bands) with respiration.

Dark area is lung.

• Gradient Moment Rephasing: GRE Flow compensation uses additional gradient pulses to correct for phase shift of the moving protons.
• also known as gradient motion rephasing (GMR) or motion artifact suppression technique (MAST)
• Additional gradient pulses are used to eliminate the�phase shifts in moving protons, bringing them back into phase�with no effect on static spins

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Pulsatile Flow Related Artifacts

• Periodic motion due to pulsatile through plane flow (flow�perpendicular to the image plane) results in ghost vessels at�discrete points, frequently seen in axial abdomen gradient�echo images
• The distance between the ghosts depends on the difference between the heart rate and TR.
• GRE images are more prone to these ghost artifacts than SE�sequences.

• Remedy
• Saturation pulse: A common way to reduce the motion�artifact due to through-plane flow is to apply a saturation�band adjacent to the imaging section.
• When applied superior to the acquisition slab in the abdomen, it�eliminates arterial pulsation artifact.
• When placed inferiorly, it eliminates artifacts from venous inflow in the iliac veins.

• Advantage of pulsation artifacts�Pulsation artifacts from vascular lesions can help us reach the�correct diagnosis, when in doubt.
• The vascular nature of the�lesion is confirmed if we see associated pulsation artifacts.

Artifacts Due to Measurement Technique/Parameters

• ALIASING:
• Aka wrap around where anatomy that exists outside the FOV is folded onto the top of anatomy inside the FOV
• Causes:
• Aliasing is caused by undersampling of frequencies
• If frequencies are not sampled often enough, the system cannot accurately represent those frequencies in the image

• Anatomy outside the FOV still experiences the effects of the gradients and produces a signal if it is within the receiving volume of the receiver coil.
• The signal from this anatomy has frequencies that are higher or lower than those within the FOV because nuclei are positioned on parts of the gradient that extend beyond the FOV.
• If the frequency exceeds the Nyquist frequency, it is not accurately digitized and is represented as a lower frequency
• 1. frequency wrap 2.phase wrap

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Remedy�

• Aliasing on MRI can be compensated for by:
• enlarging the field of view (FOV): Increase in FOV increases the SNR but decreases the spatial resolution.
• using pre-saturation bands on areas outside the FOV
• anti-aliasing software/ oversampling/no phase wrap
• switching the phase and frequency directions
• use a surface coil to reduce the signal outside of the area of interest

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Chemical shift artifact

• Fat and water precess at different frequencies. This is 3.5 ppm and is called chemical shift
• Because fat and water protons have different precessional frequencies, these are allocated to different image pixels along the frequency encoding axis.
• Chemical shift causes a displacement of fat and water signals in the frequency direction of the image.( fat-fluid interface) e.g. in imaging of fluid filled structures like bladder, orbits and kidneys which are surrounded by fat( appear as dark band )
• This is dependent upon the field strength, the receive bandwidth, and the FOV

• chemical shift increases with magnetic field strength
• chemical shift increases with decreasing gradient strength
• Remedy :
• chemical shift artifact depends upon the bandwidth; the narrower the bandwidth, the higher the chemical shift.
• Increasing the bandwidth will decrease the artifact but SNR of the image drops

• fat suppressed imaging can be used to eliminate the chemical shift misregistration and the black boundary artifact/ indian ink artifact.
• use of a spin echo sequence instead of a gradient echo can eliminate the black boundary artifact but not chemical shift misregistration
• Advantage :
• Chemical shift along the frequency axis forms the basis of MR spectroscopic imaging

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Out-of-phase signal cancellation

• also called chemical misregistration.
• This artifact is caused when the precession of fat and water are out-of-phase.
• When hydrogen protons in fat and water are in phase within a pixel, signal is produced in these tissues. When fat and water hydrogen protons are out-of-phase within a pixel, no signal is produced.

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• These means that areas where fat and water are found in the same pixel/voxel, the precessional directions of fat and water are opposite and this causes these pixel/voxels to produce no signal. This will be represented as a dark boarder around organs.
• This artifact is similar to chemical shift, but can be seen in both the phase and frequency direction.

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Fig: Out-of-phase signal

cancellation show as a black

line around the abdominal

organs border at the boundaries

between fat and muscle.

Axial gradient-echo images when the magnetic

moments of fat and water nuclei are out of phase

(below) and in phase (above)

Remedy

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• Echo Time
• The echo time selected will determine what phase fat and water are in during data acquisition.
• In a 1.5 T MR unit, a TE of 4.2, 8.4, 12.6 and so on will create images where fat and water are in-phase.
• By selecting one of these TE values, this artifact can be reduced.
• Spin echo
• The spin echo uses a 180 degree refocusing pulse to rephase hydrogen. This means that we can place hydrogen nuclei back in-phase and reduce this artifact.

Using this artifact

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• Dixon Method
• typically involves a T1 weighted image (in-phase image) and an out-of-phase image.
• Sometimes a water saturation and fat saturation is also created with this technique. This is done by selecting specific TE values.
• For example in a 1.5 T MR unit, we know that a TE of 4.2 will place fat and water in-phase and so will 8.4. If we also collect an echo with a TE between them (2.1, 6.3, 10.5...) we would be collecting an out-of-phase image.

• Advantage
• Opposed- phase images are used to detect intracellular fat and can be used in assessment of fatty infiltration of liver, adrenal masses and in assessment of marrow infiltration.

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Moiré/Fringe artifact

• The first is caused by multiple noise spikes, the second one by field inhomogeneity.
• In heavy patients, a special artifact may be seen in coronal imaging when a large field of view which is still smaller than the object is used.
• Remedy :
• use spin-echo sequences rather than gradient-echo, and use antialiasing remedies
• ensure that the arms of the patient are by the side of his/her body while positioning so that they are within the FOV while imaging chest/abdomen

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Various appearances of multiple noise spikes

Magnetic susceptibility artifact

• Appearance: produces distortion of the image together with large signal voids
• Magnetic susceptibility is caused by different tissues being magnetized differently.
• Sometimes it is a good artifact in that it increases the conspicuity of hemorrhage when using gradient echo sequences

• Causes:
• Different tissue magnetize to different degrees, which results in a difference in precessional frequency and phase.
• This causes dephasing at the interface of there tissues and a signal loss.
• Main causes are metal within the imaging volume.

Remedy

• Remove all metal objects
• Use spin-echo sequences instead of gradient-echo
• Decrease the TE
• Use a wide receive bandwidth when scanning a metal implant
• Although these susceptibility artifacts are more pronounced at higher field strengths, inherent improvement in SNR in 3T systems allows the use of higher bandwidth and parallel imaging to reduce these artifacts

Sagittal gradient-echo images of the knee

with pins in the tibia. Magnetic susceptibility

has produced a large distortion of the image

Sagittal spin-echo images of the same

patient. The artifact is reduced.

Truncation artifact/Gibbs/Ringing artifact �

• Appearance
• Truncation artifact produces a banding artifact at the interfaces of high and low signal .
• It creates a low-intensity band running through a high-intensity area.

• Cause
• This artifact results from undersampling of data (too few k-space lines are filled) so that interfaces of high and low signal are incorrectly represented on the image.
• It is most common when tissue is still producing a high signal at the end of data collection or when the peak of the echo is not centered in the middle of the sampling window.
• The latter is common when using a very short TE.

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Remedy

• It is remedied by filling more of k-space
• increasing the matrix size (i.e. sampling frequency for the frequency direction and number of phase-encoding steps for the phase direction)
• use of smoothing filters (2-D exponential filtering, Gegenbauer reconstruction etc.)
• if fat is one of the boundaries, use fat suppression

Cross-excitation/cross-talk

• Appearance
• This artifact causes a reduction in SNR in adjacent slices in the slice stack
• Cause
• Ideally, the profile of a slice should be square, or rather rectangular, when viewed from the edge.
• Nuclie in slices adjacent to RF excitation pulse may become excited by it.
• Adjacent slices receive energy from the RF excitation pulse of their neighbors.

• This energy pushes NMV of nuclei towards transverse plane, so that they become saturated when they themselves are excited.
• This effect is called cross excitation and affects image contrast.
• The same effect is produced by energy dissipation to adjacent slices, as nuclei within the selected slice relax to Bo.
• There nuclei lose their energy due to spin lattice relaxation and may dissipate this energy to nuclei in neighbouring slices and k/a cross talk.

Remedy

• Cross-excitation can be reduced by ensuring that there is at least a 30% gap between the slices.
• This is 30% of the slice thickness itself and reduces the likelihood of RF exciting adjacent slices.
• Multiple sections imaged in separate batches (interleaving).
• Cross- talk can never be eliminated.

Artifacts caused by field distortions�

• Distortions artifacts may be caused by main magnetic field inhomogeneities or non linearity of the gradient field or RF field inhomogeneities.
• ZIPPER ARTIFACT
• appears as a dense broken line across the image perpendicular to the frequency encoding direction
• Cause
• This is caused by extraneous RF entering the room at a frequency that matches the frequencies expected in the echo.
• Frequencies that are close to the Larmor frequency can be generated by broadcast radio, by wireless computer networks, or from “unintentional emitters”

Zipper artifact

• The frequency of the interference tends to be of a much higher amplitude compared to the spin-echo and therefore appears in the image as a high-intensity line representing that particular frequency after FFT.
• caused by operating the scanner with the magnet room door open or by a breach in the RF cage.
• Remedy
• Always close the magnet room door during data acquisition.
• Call the engineer to locate any breaches in the RF shielding and repair them.

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Spike (Herringbone) Artifact

• Spikes are noise bursts of short duration that can occur randomly during data collection.
• Causes:
• are a result of loose electricalconnections that produce arcs or because of breakage of interconnections in an RF coil.
• are more frequently when gradients are applied at high duty cycle, e.g. in echoplanar imaging.
• Fourier transformation of this spike of noise results in a specific ‘crisscross’ or ‘herringbone’ pattern seen as dark stripes overlaid on the image

• Advantage
• This pattern of stripes produced by spiking can be used for tagging, an important technique in cardiac imaging.

Magic angle

• Appearance
• Magic angle artifact produces abnormally high signal intensity in tissues that contain collagen (such as tendons).
• seen in the patellar tendon and may mimic pathology.
• Cause
• Although collagen is rich in hydrogen, it normally exhibits a very low signal on MRI images.
• Magic angle artifact occurs when collagen structures lie at an angle of 55° to the main field

• The anisotropic shape of the molecules in collagen causes reduction to zero of spin–spin interactions.
• The T2 decay time increases in collagen structures when they lie at this angle to B0.
• This causes increased signal intensity in the structure when a short TE is used

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• Remedy:
• Alter the angle of the patient’s anatomy relative to the B0 field or increase the TE.
• If the signal remains hyperintense, it is likely to be fluid (ligament tear) rather than magic angle artifact

Summary

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• Pixel area = field of view / matrix
• Voxel volume = (field of view / matrix) x slice thickness

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• Increasing the matrix size:
• Increases spatial resolution
• Decreases signal
• Increases scan time

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• Increasing the FOV:
• Increases the signal
• Lower resolution
• Increased viewing area

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• Increasing the slice thickness:
• Increases the signal
• Decreases the resolution
• Increases the partial volume effect
• Gives larger object coverage

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Summary

• To improve the SNR:
• Increase NEX
• Lower resolution
• Thicker slices
• Larger FOV
• Use surface coils

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• To improve the resolution:
• Increase the matrix
• Decrease the FOV
• Decrease the slice thickness

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• Increasing NEX:
• Increases signal
• Less noise
• Fewer artefacts due to signal averaging
• Increased scan time

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Summary

References

• Gupta AK, Chowdhury V, Khandelwal N. Diagnostic radiology: recent advances and applied physics in imaging.
• Westbrook C, Roth CK, Talbot J. MRI in practice 4th ed. And 5th ed.
• Chavhan GB. MRI made easy. JP Medical Ltd; 2013 Jan 30.
• http/mriquestions.com
• Radeopaedia.org
• www.radiologycafe.com

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Thank you