Chapter 28

Magnetic Resonance Imaging Artifacts

Objectives

At the completion of this chapter, the student should be able to do the following:

• Identify the three principal causes of magnetic resonance imaging (MRI) artifacts.

• Define the difference between patient-related and system-related artifacts.

• Recognize image aliasing, chemical shift, and motion artifacts.

• Describe the partial volume artifact and how to reduce it.

Key Terms

Ferromagnetic material

Image artifact

Magnetic susceptibility

Misregistration artifacts

Off-resonance artifact

Truncation

Zipper artifact

An image artifact is a pattern or structure in the image caused by a signal distortion related to the technique of producing an image. Therefore it is an unwanted pattern or structure that does not represent the actual anatomy. Image artifacts can be misleading and interfere with diagnosis.

The complexity of magnetic resonance imaging (MRI) has unfortunately brought with it many confusing imaging artifacts. Luckily, many can be interpreted easily and do not interfere with diagnosis. However, the addition of each new imaging technique or pulse sequence brings the possibility of new artifacts.

The MRI artifacts illustrated here fall into several categories. The outline in Box 28-1 adopts a scheme of three principal classes of MRI artifacts, each of which can be identified as patient related or system related. None of these artifacts are characteristic of a particular manufacturer or model of MRI system.

Box 28-1

Classification of Magnetic Resonance Imaging Artifacts

I. Magnetic and RF Field Distortion Artifacts

A. Patient related

1. Foreign materials

2. Magnetic susceptibility

3. Body shape, conductivity, and extension

4. Chemical shift

B. System related

1. Primary static magnetic field inhomogeneity

2. Gradient magnetic field inhomogeneity

3. RF coil inhomogeneity

4. Gradient coil switching/timing inaccuracy

II. Reconstruction Artifacts

A. Patient related

1. Aliasing (wraparound)

2. Partial volume averaging

B. System related

1. Truncation

2. Zero frequency

III. Noise-Induced Artifacts

A. Patient related

1. Voluntary motion

2. Involuntary motion

a. Bowel peristalsis

b. Respiration

c. Cardiac and vessel pulsation

d. Swallowing

3. Fluid motion

a. Blood flow

b. Cerebrospinal fluid flow

c. Urine movement

4. Misregistration

B. System related

1. Frequency line/point

2. Phase line/star

3. Extraneous RF

4. Off-resonance turning

Magnetic and Radiofrequency Field Distortion Artifacts

MRI requires an exceptionally uniform, homogeneous, primary magnetic field. Magnetic field homogeneity of ±1 ppm (parts per million) over a 20 cm spherical volume can be achieved by many current MRI systems.

A homogeneity of ±1 ppm per 20 cm volume means that a 1 T magnetic field does not vary by more than ±1 µT throughout a 20 cm diameter sphere, centered at magnet isocenter.

Distortion of the magnetic field can be caused by the patient or problems with the MRI system. A poorly shimmed magnet, gradient magnetic field miscalibration, chemical shift of the Larmor frequency, tissue magnetic susceptibility, and foreign materials within or on the patient can all result in distortion of the magnetic field.

Patient Related

Foreign Materials.

Metals have different effects on the local magnetic field depending on their shape and location. Any metal is a potential conductor of electricity, which can support current induced by the pulsing of the gradient and RF magnetic fields. The currents produced by gradient coils will change the local magnetic field, leading to image distortions. The currents produced by the RF coils can lead to tissue heat and patient burns.

Ferromagnetic materials, in particular, should be avoided since they can establish magnetic fields that persist independent of the pulsing MRI secondary magnetic fields. Ferromagnetic materials can produce not only a local signal loss but also a warping distortion of the surrounding areas. An example of such field distortion is provided by the friendly “cone-head” seen in Figure 28-1.

FIGURE 28-1 A small band of metal in this patient’s ponytail produced this “cone-head” artifact. Although the region of interest was not affected by the artifact, the images were incinerated to avoid publicity about unanticipated adverse bioeffects of magnetic resonance imaging.

The typical metal material artifact has a partial or complete loss of signal at the site of the metal. Such metal can distort the local magnetic field sufficiently so that the Larmor frequency for local spins is outside the frequency range of the imaging system. Furthermore, metal contains no hydrogen; the result is signal void at that location.

Sometimes a partial rim of high signal intensity may be seen at the periphery of the signal void, which allows differentiation of metal from other causes of focal signal loss. Usually, the degree of anatomical information loss due to such an artifact is less than that seen on computed tomography (CT) images, because the loss is local and not streaked across the image as in CT.

However, ferromagnetic material artifacts can actually cause very subtle, yet significant artifacts. The classic example is orthodontic braces, which cause severe signal dropout from the mouth and jaw. However, they can also cause artifacts in the remote regions in the brain by distorting the slice selection process.

Distortion of the magnetic field results in a susceptibility artifact.

Exceptions to this are usually caused by the screws in some metallic orthopedic devices, but occasionally distortion is produced by small metallic objects such as buttons, snaps, zippers, or barrettes (Figure 28-2). The distortion of the surrounding tissue is the result of the metal-induced change in the local magnetic field. The magnetic field lines are distorted, resulting in a change in the local Larmor frequency (Figures 28-3 to 28-5).

FIGURE 28-2 The metallic zipper in this patient’s shirt produced the noticeable loss of signal, as seen on this image. The high signal rim at the edges of the void is typical.

FIGURE 28-3 An electrocardiographic lead makes a small, localized distortion of the anterior chest wall.

FIGURE 28-4 A metallic stent, which was temporarily placed in this patient’s ureter, caused image degradation along the entire path from the kidney to the bladder.

FIGURE 28-5 Multiple surgical clips and a Gianturco immobilization coil were in place at the time of this examination. A, The immobilization coil causes a relatively large signal void within the liver. B, Some clips show only small amounts of signal dropout, as seen in the lower abdomen of the same patient.

More mundane but relatively common metallic materials that may be encountered include belt buckles, keys, mascara and other makeup, foreign bodies in the eye or elsewhere, and even certain types of nylon found in clothing, such as gym shorts and warm-up suits (Figures 28-6 to 28-9).

FIGURE 28-6 Gradient echo imaging causes accentuation of local magnetic field inhomogeneities produced by ferromagnetic materials. A, The large area of signal dropout results from two small metallic bra clips that were pulled up between this patient’s scapulae. B, A spin echo image obtained through the same level shows a more typical and less prominent metallic artifact. A wraparound aliasing artifact of the arms onto the abdomen is also present.

FIGURE 28-7 A, A computed tomography image of the abdomen that is nondiagnostic because of the artifacts produced by surgical clips in the postoperative bed. B, The magnetic resonance image of this same patient reveals the recurrent pheochromocytoma. A small signal void immediately above the mass resulted from one of the clips.

FIGURE 28-8 Metallic orthopedic devices can produce severe distortion and signal loss. The screws in the Harrington rods caused this image to be nondiagnostic.

FIGURE 28-9 Ferrometals may cause significant artifacts, such as distortion of the orbital globes produced by eye shadow.

Magnetic Susceptibility Artifacts.

Magnetic susceptibility is a dimensionless quantity that indicates the relative degree of magnetization occuring in a material that is placed in a magnetic field. Materials with magnetic properties that are different from the tissues in which they are embedded can cause a “susceptibility artifact.”

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