MRI Imaging Failures -- Categories
The Basics of MRI
Chapter 11
IMAGE ARTIFACTS
-
Introduction
-
RF Quadrature
-
Bo Inhomogeneity
-
Gradient
-
RF Inhomogeneity
-
Motion
-
Flow
-
Chemical Shift
-
Partial Volume
-
Wrap Around
-
Gibbs Ringing
Introduction
An image artifact is any feature which appears in an image which is not present in the original imaged object. An image artifact is sometime the result of improper operation of the imager, and other times a consequence of natural processes or properties of the human body. Artifacts are typically classified as to their source. The following table summarizes a few of these.
[Karl Note: Something that is present in the original object being scanned with the MRI but NOT shown in the image would NOT be counted as one of these imaging failures -- but obviously would be a failure. However, the very specifications for the capability of the MRI device to "see" something makes the failure to "see" a cell membrane NOT a failure, but just something to be expected from the inherent specifications of the machine.]
| Artifact | Cause |
|---|---|
| RF Quadrature | Failure of the RF detection circuitry |
| Bo Inhomogeneity | Metal object distorting the Bo field |
| Gradient | Failure in a magnetic field gradient |
| RF Inhomogeneity | Failure of RF coil |
| Motion | Movement of the imaged object during the sequence |
| Flow | Movement of body fluids during the sequence |
| Chemical Shift | Large Bo and chemical shift difference between tissues |
| Partial Volume | Large voxel size |
| Wrap Around | Improperly chosen field of view |
An example of each of the artifacts is presented next. The reader is cautioned that a problem with the imager can manifest itself in a number of ways. Therefore not all artifacts of a given type will appear the same.
RF Quadrature
RF
quadrate artifacts are caused by problems
with the RF detection circuitry. More
specifically, these problems are typically
associated with what was referred to in the
hardware section as the quadrature detector.
These problems arise from improper operation
of the two channels of the detector. For
example if one of the amplifiers has a DC
offset in its output the Fourier transformed
data can display a bright spot in the center
of the image. If one channel of the detector
has a higher gain than the other it will
result in a ghosting of objects diagonally in
the image. This artifact is the result of a
hardware failure and must be addressed by a
service representative.
Bo Inhomogeneity
All magnetic resonance imaging assumes a homogeneous Bo magnetic field. An inhomogeneous Bo magnetic field will cause distorted images. The distortions can be either spatial, intensity, or both. Intensity distortions result from the field homogeneity in a location being greater or less than that in the rest of the imaged object. The T2* in this region is different, and therefore the signal will tend to be different. For example, if the homogeneity is less, the T2* will be smaller and the signal will be less. Spatial distortion results from long-range field gradients in Bo which are constant. They cause spins to resonate at Larmor frequencies other than that prescribed by an imaging sequence.
The
animation window contains an image of four
water filled straight tubes positioned so as
to form a square. The magnetic image shows a
severe bending in one of the tubes due to a
nonuniformity in the Bo magnetic
field.
Gradient
Artifacts arising from problems with the
gradient system are sometimes very similar to
those described as Bo
inhomogeneities. An gradient which is not
constant with respect to the gradient
direction will distort an image. This is
typically only possible if a gradient coil
has been damaged. Other gradient related
artifacts are due to abnormal currents
passing through the gradient coils. In this
image the frequency encoding (left/right
encoding) gradient is operating at half of
its expected value.
RF Inhomogeneity
An RF
inhomogeneity problem is a variation in
intensity across an image. The cause is
either a nonuniform B1 field or an
nonuniform sensitivity in a receive only
coil. Some RF coils, such as surface coils,
naturally have variations in sensitivity and
will always display this artifact. The
presence of this artifact in other coils
represents the failure of an element in the
RF coil or the presence of nonferromagnetic
material in the imaged object. For example a
metal object which prevents the RF field from
passing into a tissue will cause a signal
void in an image.
The
accompanying sagittal image of the head
contains an RF inhomogeneity artifact in the
region of the mouth.
(See arrow.)
The patient has a large amount of non
ferromagnetic metal dental work in the mouth.
The metal shielded the regions near the mouth
from the RF pulses thus producing a signal
void. The Dental work did not significantly
distort the static magnetic field Bo.
Motion
As the name implies, motion artifacts are caused by motion of the imaged object or a part of the imaged object during the imaging sequence. The motion of the entire object during the imaging sequence generally results in a blurring of the entire image with ghost images in the phase encoding direction. Movement of a small portion of the imaged object results in a blurring of that small portion of the object across the image.
To
understand this artifact picture the
following simple example. A single small spin
containing object is imaged.
The central
portion of the MX raw data will
look something like this.
The
frequency of the waves will be related to the
position in the frequency encoding direction
and the variation in phase of the waves will
be related to the position in the phase
encoding direction. Fourier transforming
first in the frequency encoding direction
yields a single oscillating peak.
Viewing the
data as a function of a phase shows this more
clearly.
Fourier
transforming last in the phase encoding
direction yields a single peak at the
location of the original object.
Now
picture the same example except that midway
through the acquisition of phase encoding
steps the object moves to a new location in
the frequency encoding direction.
The central
part of the MX raw data looks like
this.
Fourier
transforming first in the frequency direction
gives two oscillating peaks which abruptly
stop oscillating.
Viewing the
data as a function of a phase shows this more
clearly.
Fourier
transforming in the phase encoding direction
gives several repeating peaks at the two
frequencies. This is because the Fourier pair
of an abruptly truncated sine wave is a sinc
function. The magnitude representation of the
data makes all the peaks positive.
The
solution to a motion artifact is to
immobilize the patient or imaged object.
Often times the motion is caused by the heart
beating or the patient breathing. Both of
which can not legally be eliminated. The
solution in these cases is to gate the
imaging sequence to the cardiac or
respiratory cycle of the patient. For example
if the motion is caused by pulsing artery,
one could trigger the acquisition of phase
encoding steps to occur at a fixed delay time
after the R-wave in the cardiac cycle. By
doing this the artery is always in the same
position.
Similar gating could be done to the respiratory cycle. A disadvantage of this technique is that the choice of TR is often determined by the heart rate or respiration rate. Imaging techniques designed to remove motion artifacts are given different names by the various manufacturers of magnetic resonance imagers. For example, a few names of sequences designed to remove respiratory motion artifacts are respiratory gating, respiratory compensation, and respiratory triggering.
The
accompanying axial image of the head shows a
motion artifact.
A blood
vessel in the posterior side of the head
moved in a pulsating motion during the
acquisition. This motion caused a ghosting
across the image.
Flow
Flow
artifacts are caused by flowing blood or
fluids in the body. A liquid flowing through
a slice can experience an RF pulse and then
flow out of the slice by the time the signal
is recorded. Picture the following example.
We are using a spin-echo sequence to image a
slice. Here the timing diagram and side view
of the slice are shown.
During the
slice selective 90o pulse blood in
the slice is rotated by 90o.
Before the 180o pulse can be
applied, the blood which experienced the 90o
pulse has flown out of the slice. The slice
selective 180o pulse rotates spins
in the slice by 180o. However the
blood in the slice has its magnetization
along +Z before the pulse and along -Z after
the pulse. It therefore yields no signal. By
the time the echo is recorded the slice has
only blood in it which has not experienced
the 90o or the 180o
pulse. The result is that the blood vessel
which we know to contain a high concentration
of hydrogen nuclei yields no signal.
Here is
an example from an axial slice through the
legs. Notice that the blood vessels appear
black even though they contain a large amount
of water.
In a multislice sequence, the slices could be positioned such that blood experiencing a 90o pulse in one slice can flow into another slice and experience a 180o rotation and into a third and contribute to the echo. In this case the vessel will have a high signal intensity. The effect is usually that some slices have low signal intensity blood vessels and others have high signal blood vessels.
Chemical Shift
A chemical shift artifact is caused by the difference in chemical shift (Larmor frequency) of fat and water. The artifact manifests itself as a misregistration between the fat and water pixels in an image.
The
difference in chemical shift is approximately
3.5 ppm which at 1.5 Tesla corresponds to a
frequency difference between that of fat and
water is approximately 220 Hz. During the
slice selection process there is a slight
offset between the location of the fat and
water spins which have been rotated by an RF
pulse. This difference is exaggerated in this
animation.
During the
phase encoding gradient the fat and water
spins acquire phase at different rates. The
effect being that fat and water spins in the
same voxel are encoded as being located in
different voxels. In this example all nine
voxels have a red water vector. The center
voxel has some fat magnetization in addition
to the water.
In a uniform
magnetic field the vectors precess at their
own Larmor frequency.
When a
gradient in the magnetic field is applied,
such as the phase encoding gradient, spins at
different x positions precess at a frequency
dependent on their Larmore frequency and
field. In this example the fat vector has the
same frequency as the water vector in the
voxel to its right.
When the
phase encoding gradient is turned off each
vector has acquired a unique phase dependent
on its x position.
During the
frequency encoding gradient, fat and water
spins located in the same voxel precess at
rates differing by 3.5 ppm. The net effect is
that the fat and water located in the same
voxel are encoded as being located in
different voxels. In this example the fat
vector in the center voxel possesses a phase
and precessional frequency as if it was
located in the upper right voxel.
The resultant
image places the fat in the upper right voxel
rather than in the center.
The
magnitude of the effect is proportional on
the magnitude of the Bo field and
inversely proportional to the sampling rate
in the frequency encoding direction. For a
constant sampling rate, the larger Bo,
the greater the effect. At 1.5 T and a 16 kHz
sampling rate, the effect is approximately
3.5 pixels. At 0.5 T and a 16 kHz sampling
rate, the effect is approximately one pixel.
In this axial slice image through the legs
there is a chemical shift artifact between
the fat and muscle in the legs.
Partial Volume
A partial volume artifact is any artifact which is caused by the size of the image voxel. For example, if a small voxel contains only fat or water signal, and a larger voxel might contain a combination of the two, the large voxel possess a signal intensity equal to the weighted average of the quantity of water and fat present in the voxel. Another manifestation of this type of artifact is a loss of resolution caused by multiple features present in the image voxel.
Here is
a comparison of two axial slices through the
same location of the head. One is taken with
a 3 mm slice thickness and the other with a
10 mm thickness. Notice the loss of
resolution in the 10 mm Thk image.
The solution
to a partial volume artifact is a smaller
voxel, however this may result in poorer
signal-to-noise ratios in the image.
Wrap Around
A wrap around artifact is the occurrence of a part of the imaged anatomy, which is located outside of the field of view, inside of the field of view. This artifact is caused 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 the FID or echo. The solution to a wrap around artifact is to choose a larger field of view, adjust the position of the image center, or select an imaging coil which will not excite or detect spins from tissues outside of the desired field of view.
In the
accompanying sagittal image of a breast, the
portion of the image below the arrow should
appear on the top of the image.
This portion
was located at a position that had a greater
resonance frequency than the digitization
rate. As a consequence, it was wrapped around
and appears at the bottom end of the image.
Many
newer imagers employ a combination of
oversampling, digital filtering, and
decimation to eliminate the wrap around
artifact. Oversampling creates a larger FOV,
but generates too much data to be
conveniently stored. Digital filtering
eliminates the high frequency components from
the data, and decimation reduces the size of
the data set. The following flowchart
summarizes the effects of the three steps by
showing the result of performing an FT after
each step.
Let's examine oversampling, digital filtering, and decimation in more detail to see how this combination of steps can be used to reduce the wrap around problem.
Oversampling is the digitization of a time domain signal at a frequency much greater than necessary to record the desired field of view. For example, if the sampling frequency, fs, is increased by a factor of 10, the field of view will be 10 times greater, thus eliminating wrap around. Unfortunately digitizing at 10 times the speed also increases the amount of raw data by a factor of 10, thus increasing storage requirements and processing time.
Filtering is the removal of a select band of
frequencies from a signal. For an example of
filtering, consider the following frequency
domain signal.
Frequencies
above fo could be removed from this frequency
domain signal by multipling the signal by
this rectangular function.
In MRI, this
step would be equivalent to taking a large
FOV image and setting to zero intensity those
pixels greater than some distance from the
isocenter.
Digital
filtering is the removal of these frequencies
using the time domain signal. Recall from
Chapter 5 that if two functions are
multiplied in one domain (i.e.
frequency), we must convolve the FT of the
two functions together in the other domain (i.e.
time). To filter out frequencies above fo
from the time domain signal it must be
convolved with the Fourier transform of the
rectangular function, a sinc function. (See
Chapter 5.)
This process
eliminates frequencies greater than fo from
the time domain signal. Fourier transforming
the resultant time domain signal yields a
frequency domain signal without the higher
frequencies. In MRI, this step will remove
image components fo / 2 g Gf
away from the center of the image.
Decimation is the elimination of data points
from a data set. A decimation ratio of 4/5
means that 4 out of every 5 data points are
deleted, or every fifth data point is saved.
Decimating the digitally filtered data above,
followed by a Fourier transform, will reduce
the data set by a factor of five.
High speed digitizers, capable of digitizing at 2 MHz, and dedicated high speed integrated circuits, capable of performing the convolution on the time domain data as it is being recorded, are used to realize this procedure.
Gibbs Ringing
Gibbs ringing is a series of lines parallel to a sharp intensity edge in an image. The ringing is caused by incomplete digitization of the echo. This means the signal has not decayed to zero by the end of the acquisition window, and the echo is not fully digitized. (The reader is encouraged to prove this using the convolution theorem.) This artifact is seen in images when a small acquisition matrix is used. Therefore, the artifact is more pronounced in the 128 point dimension of a 512x128 acquisition matrix.
In the
following example, a rectangular object with
a spatially uniform signal is imaged. An
inadequate number of points are collected in
the horizontal (x) direction. The resultant
image displays a ringing in the intensity at
the edge. The animation window displays the
upper right hand corner of this image and a
plot of signal intensity.
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