Imaging Artifacts

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In order to use reconstructed CT volumes for diagnostic and treatment purposes as a proxy for actual patient volumes, it is necessary to assume that all relevant features in the patient appear in the CT image. Likewise, all features which appear in the CT image should correspond with physical features in the patient or imaging object. While this second assumption is usually correct, sometimes an imaging system will produce artificial structures, known as image artifacts. Some major causes of imaging artifacts include patient movement, beam hardening, radiation scatter, partial volume effects, and patient size exceeding the scanner field of view.

Figure 1-14: common CT image artifacts including (a) patient motion, (b) beam hardening, (c) partial volume, (d) metal implant (beam hardening), and (e) exceeding field of view. [5]

Patient Motion and Volume Inconsistency

Patient motion as a source of image distortion is easy to understand intuitively. However, the distortions are not as straightforward as in the case of two-dimensional x-

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ray imaging, when blurs appearing only in the moving regions. Because CT volume reconstruction is achieved by tracing linear x-ray paths backwards from the detector, motion blur in an isolated region of object space can lead to distortions and streaks in surrounding volumes in reconstruction space. Any type of temporal inconsistency in the imaging object space, whether patient motion or contrast agent concentration, can lead to distortion throughout portions of the reconstructed CT volume. We will revisit this topic again as it motivates the need for physiological motion gating in live animal imaging.

Beam Hardening

The artifact known as beam hardening arises from the assumption in CT

reconstruction that the energy spectra of the x-ray beam is constant as it travels through the imaging object, and the fact that this assumption is never strictly true. From the earlier discussion of x-ray energy, we know that certain wavelengths of photons are more strongly attenuated by a given material than others. For example, low-energy photons are strongly attenuated by bone while higher energy photons are not. When a polychromatic x-ray beam passes through bone, more of the low energy photons are attenuated than the high energy photons, leading to a shift in the energy spectrum on either side of the bone. If the x-ray beam then encounters another identically-dense piece of bone along its travel path, it will be less attenuated overall this second time around because there are fewer low-energy photons available to be disproportionally attenuated. What this leads to in reconstruction space are two different types of beam-hardening artifact types. The first is dark streaks of photon starvation on either side of a strongly-attenuating material such as bone or metal. The second is an apparent increase in density toward the center of a bulk of truly uniform material, which is called “cupping” artifact due to the gentle curved shape of this artifact in a line profile drawn across a uniform region in the image.

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Both dark streaks from photon starvation and cupping artifacts are due to the polychromatic nature of the x-ray beam and the lack of accounting for this nature in traditional CT reconstruction. These artifacts could be eliminated entirely if CT imaging was performed with a monochromatic x-ray beam, or if the reconstruction algorithm took into account the energy dependence of attenuation. A common and straightforward fix which reduces the severity of beam hardening artifacts is to pre-filter the x-ray beam with a material, such as steel or aluminum, to reduce the number of low-energy photons in the x-ray beam before they enter the imaging object.

Partial Volume Effect

This class of image artifact applies especially to helical CT image acquisitions, but they are present though less severe in cone-beam CT. In partial volume artifacts, high contrast structures extend partially into adjacent slices. This results in a loss of sharpness in the feature edge along the z-direction and especially the appearance of shadows along the edge of these highly attenuating features. Partial volume effects can also occur within the x-y plane though usually less severely, since spatial resolution in-plane is better than along the z-axis. Partial volume effects, combined with beam hardening dark streaks, create characteristic image artifacts near the interfaces of high- and low-attenuation materials.

Field of View Artifacts

An essential assumption in CT reconstruction is that all portions of the imaging object are contained entirely within the field of view. If some portion of the object is outside of this field of view, then some x-ray paths will pass through it and the

attenuation performed by this excess material will be attributed to some portion of the volume which is contained in the field of view. What this means in practice is that, in the

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reconstructed image, areas at the edge of the field of view and adjacent to the extra material will appear hyperdense compared with their true composition. When the amount of extra material outside the field of view is small, and the outer portions of the

reconstructed volume are not diagnostically crucial to view accurately, FOV artifacts are easy to interpret correctly and they are mostly harmless. However, if there are large amounts exterior material or if they are made of highly attenuating materials such as bone or metal, large streak artifacts can appear which obscure large areas of the reconstructed image.

As with beam hardening artifacts, a robust field of study exists to minimize the effect of field of view artifacts using special algorithms and corrections [Hsieh 2003]. This is an important problem to solve because of the increasing use of flat panel detectors in CT, and especially because the average patient size in the clinic has increased over time.