Earlier chapters in this book assumed the magnetic resonance scanner was functioning exactly as presented in the theory. This section describes what happens when the scanner does not behave as expected and an image artifact is created.
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. It is important to be familiar with the appearance of artifacts because artifacts can obscure, and be mistaken for, pathology. Therefore, image artifacts can result in false negatives and false positives.
Artifacts are typically classified as to their source, and there are dozens of image artifacts. The following table summarizes a few of these.
|RF Offset and
|Failure of the RF detection circuitry
|Failure of the RF shielding
|Metal object distorting the Bo field
|Failure in a magnetic field gradient
|Objects in the FOV with a higher or lower magnetic susceptibility
|Failure or normal operation of RF coil, and metal in the anatomy
|Movement of the imaged object during the sequence
|Movement of body fluids during the sequence
|Large Bo and chemical shift difference between tissues
|Large voxel size
|Improperly chosen field of view
|Small image matrix and sharp signal discontinuities in an image
|Angle between Bo and dipole axis in solids.
The ability to identify the source of an artifact is related to your understanding of the previous material presented in this book. Spin physics, the imaging pulse sequence, Fourier transforms, Fourier pairs, hardware, and signal processing are particularly useful. For example knowledge of the spin echo pulse sequence, Fourier pairs and the signal processing will enable you to predict the affect of motion during a scan. An example of each of the artifacts is presented next. The reader is cautioned that some problems with the imager can manifest themselves in a number of ways. Therefore not all artifacts of a given type will appear the same.
A DC offset artifact is one of two possible artifacts associated with the radio frequency (RF) detector. The RF detector was referred to in the Hardware chapter as the quadrature detector. The DC offset artifact is caused by a DC offset voltage in one or both of the signal amplifiers in the detector. Recall, from the Fourier Transform chapter that the Fourier transform of a time domain DC offset is a peak at zero frequency. The FT of a time domain signal with a DC offset is the FT of the signal, which has the same zero frequency peak. K-space data that has a DC offset gives the same zero frequency peak when Fourier transformed. Therefore, there is a bright spot exactly in the center of the image.
The second type of artifact associated with the RF detector is the quadrature ghost artifact. This artifact is caused by a mismatch in the gain of the real and imaginary channels of the quadrature detector. For the Fourier transform to function properly, the gain of the two sets of doubly balanced mixers, filters, and amplifiers in the real and imaginary channels of the quadrature detector must have identical efficiencies. When this is not the case, the Fourier transform may have a small component at the negative of any frequencies present in the signal. This small negative frequency component causes a ghosting of objects diagonally in the image. Here is an example of this artifact when the signals differ by 50%. Both the DC offset and quadrature ghost artifacts are the result of a hardware failure and must be addressed by a service representative.
A failure of the RF shielding that prevents external noise from getting into the detector is the cause of an RF noise artifact. The form of the artifact in the image depends on the source of noise and where it is introduced into the signal. Much can be gained about the source of RF noise by inverse Fourier transforming the image. For example, a bright spot somewhere in the image can be caused by a single frequency leaking into the signal. The animation window contains an image with two different RF noise artifacts represented by the diagonal lines and the two horizontal lines marked with arrows. To possibly fix the problem before calling a service representative, check to see that the scan room door is closed and sealing properly.
All magnetic resonance imaging assumes a homogeneous Bo magnetic field. An inhomogeneous Bo magnetic field causes 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 in time. They cause spins to resonate at Larmor frequencies other than that prescribed by an imaging sequence. For example, consider the diagram in the animation window representing a perfect linear (black) and distorted (red) one-dimensional magnetic field gradients. Ideally, spins at a single x position should experience a single magnetic field and resonate at a single frequency. With a distorted gradient, there is no linear relationship between position x and frequency ν. Because linearity is assumed in the imaging process, the resultant image is distorted.
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.
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.
A magnetic susceptibility artifact is caused by the presence of an object in the FOV with a higher or lower magnetic susceptibility. The magnetic susceptibility of a material is a measure of whether an applied magnetic field creates a larger or smaller field within the material. Materials that are diamagnetic have a slightly lesser field than in a vacuum, while paramagnetic materials have a slightly greater field. Ferromagnetic materials have a much higher field. The image in the animation window depicts a region with a homogeneous magnetic field into which an object with a higher magnetic susceptibility has been placed. As a result, the magnetic field lines bend into the object. Consequently, the fields are stronger and weaker at various locations around the object. This distortion is seen in the applied static magnetic field Bo, the radio frequency magnetic field B1, and the gradients in the magnetic field. Often, the susceptibility artifact is caused by metal, such as a titanium or stainless steel object inside the body. These objects cause additional artifacts, such as the RF inhomogeneity artifact described next, that make it difficult to present an example image.
An RF inhomogeneity artifact is the presence of an undesired variation in signal 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 animation window contains an image from a surface coil with its characteristic intensity fall off as you go away from the coil. The presence of this artifact in other coils represents the failure of an element in the RF coil or the presence of metal 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. The metal does not significantly distort the static magnetic field Bo at greater distances; therefore, the image of the brain is not significantly distorted.
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 animation window contains a magnetic resonance image of the head in which the head moved in the superior/inferior direction midway during the acquisition.
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 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 90° pulse blood in the slice is rotated by 90°. Before the 180° pulse can be applied, the blood which experienced the 90° pulse has flown out of the slice. The slice selective 180° pulse rotates spins in the slice by 180°. 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 90° or the 180° 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 90° pulse in one slice can flow into another slice and experience a 180° 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.
In an image, the chemical shift artifact is a misregistration between the relative positions of two tissues with different chemical shifts. Most common is the misregistration between fat and water. The chemical shift artifact is caused by the difference in chemical shift (Larmor frequency) of fat and water.
Recall from the NMR Spectroscopy chapter that the definition of chemical shift, δ, is
where ν is the resonance frequency of a nucleus and νREF the resonance frequency of a reference nucleus. The difference in chemical shift between two nuclei referred to as 1 and 2 is
The difference in chemical shift of water and adipose or fat-like hydrogens 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 voxel to the top rather than in the center. Even though the phase is different, the fat is not encoded as being in a different phase encoding direction voxel. What matters in phase encoding is the difference in phase between the steps and this is not changing.
The chemical shift artifact in the distance units of the FOV is
In general, the term partial-volume artifact describes any artifact that occurs when the size of the image voxel is larger than the size of the feature to be imaged. 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. For example, a small blood vessel passing diagonally through a slice may appear sharp in a 3 mm thick slice, but distorted and blurred in a 5 mm or 10 mm slice.
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.
A wraparound artifact is the appearance of a part of the imaged anatomy, which is located outside of the field of view, inside of the field of view. For example, an image of the human head may have a part of the nose outside the field of view. The nose, however, appears in the image, but at the back of the head. In this artifact, objects located outside the field of view appear at the opposite side of the image, as if one took the image and wrapped it around a cylinder.
This artifact occurs when the selected field of view is smaller than the size of the imaged object, or, more specifically, when the digitization rate is less than the range of frequencies in the FID or echo. The origin of this problem was first presented in the chapter on Fourier Transforms. The solution to a wraparound artifact is to choose a larger field of view, adjust the position of the image center, or select an imaging coil that does not excite or detect spins from tissues outside the desired field of view.
The accompanying sagittal images of the head and breast contain wraparound artifacts. In the image of the head, the nose extends beyond the field of view on the left, and its imaged position is wrapped around and appears on the right of the image. In terms of frequency and digitization rate, the nose is located at a position that has a greater resonance frequency than the digitization rate. Consequently, it is wrapped around, and it appears at the right end of the image.In the sagittal breast image, 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 in the frequency encoding direction. This point was discussed in the detector section of the Hardware chapter. Wraparound in the phase encoding direction can be minimized using a no phase wrap option which applies a saturation pulse to spins outside of the field of view in the phase encoding direction. Hence, minimal signal is us present in tissue which are wrapped around into the phase encoding direction FOV.
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. The solution to Gibbs-ringing artifact is to use a larger image matrix.
All of magnetic resonance imaging requires the spins to be free to rotate and tumble freely in the tissue. In solids this does not happen. As a consequence, the chemical shift and the spin-spin coupling are dependent on the orientation of the molecule. The dipole interaction
in such cases is zero when the angle θ between Bo and dipole axis in solids is 54.7°. This interaction causes dark regions in cartilage where θ = 54.7°.
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