Common Image Artifacts in Cone Beam CT
Dr. Ryan D Lee
Oral and Maxillofacial Radiology
University of Texas Health Science Center
San Antonio, TX
From the Summer 2008 AADMRT Newsletter
Present state-of-the-art cone beam computed tomography
(CBCT) units produce excellent high resolution, three dimensional
images of oral bony anatomy making dental implant planning
and surgical placement simple and reliable. CBCT has
been a valuable asset in dentistry because of its low cost, high
resolution and relatively low radiation burden.
However there are many imaging artifacts that are inherent in
CBCT units due to the nature of the image acquisition and
reconstruction process. These artifacts contribute to image
degradation and can lead to inaccurate or false diagnoses.
Some of these artifacts are more pronounced in CBCT units
than their CT counterparts because of the different processes
in which the images are acquired.
This article serves to introduce and explain
some common artifacts that are found in CBCT images. Many of these
artifacts are also found in conventional CT images because they share
similar reconstruction algorithms. Artifacts relating to CBCT will be
divided into three main categories, physics-based, patient- based and
Physics-based artifacts result from the physical processes involved
in the acquisition of CT data. Patient- based artifacts are caused by
factors related to the patient's form or function. Scanner-based
artifacts result from imperfections in scanner function.
In this article, we will cover some common
physics-based and patient-based artifacts.
Physics based artifacts
Noise is defined as an unwanted, randomly
and/or non-randomly distributed disturbance of a signal that tends to
obscure the signal's information content from the observer. Noise
affects images produced by cone beam CT units by reducing low contrast
resolution, making it difficult to differentiate low density tissues
thereby reducing the ability to segment effectively.
The noise in traditional projection radiography
is primarily from quantum mottle which is
defined as a variation in image density due to
statistical fluctuation of photon fluency in the
radiation field. In well-designed x-ray systems,
the quantum noise is governed by the number of
x-ray photons absorbed in the detector, the
higher the number of photons absorbed, the lower
the quantum mottle. The number of x-ray photons
emitted is directly related to the mA of the
x-ray unit. Another source of noise in computed
tomography is scattered radiation. Scattered
radiation arises from interactions of the
primary radiation beam with the atoms in the
object being imaged and its magnitude is largely
dependent on patient size, shape, and position
in the scan field. It is a major source of image
degradation in x-ray imaging techniques. When
x-ray radiation passes through a patient, three
types of interactions can occur, including
coherent scattering, photoelectric absorption
and Compton scattering. Compton scattering is
the type most seen in diagnostic radiology. In
Compton scattering, the interaction is a
collision between a high energy x-ray photon and
one of the outer shell electrons of an
atom. This outer shell electron is bound with
very little energy to the atom so when the x-ray
photon collides with it, the electron is ejected
from the atom. Because energy and momentum are
both conserved in this collision, the energy and
direction of the scattered x-ray photon depend
on the energy transferred to the electron. If
the initial x-ray energy is high, the relative
amount of energy lost is small, and the
scattering angle is small relative to the
initial direction. If the initial x-ray energy
is small, the scatter angle is large and the
ejected electron disperses in all
There is very little noise in conventional CT
machines because of the high mA used and due to
effective pre- and postpatient collimation which
reduces the scattered radiation to a negligible
amount. However, in CBCT machines the noise is
high due to the lower mA used and because of the
high amount of scattered radiation since there
is no post-patient collimation.
Example of Beam Hardening
An x-ray beam in medical radiography machines are composed
of many x-rays with a wide spectrum of energies. Since all substances
attenuate low-energy x-rays more strongly than high energy
ones, primarily because of photoelectric absorption, a heterogeneous
beam traversing an absorbing medium becomes proportionately richer in high-energy photons, and hence
more penetrating, or 'harder'. Beam hardening manifests as two different artifacts within the reconstructed image, a
cupping artifact and the appearance of dark bands or streaks.
Cupping artifacts from beam hardening occur when x-rays passing through the center of a large object become harder
than those passing through the edges of the object due to the greater amount of material the beam has to penetrate.
Because the beam becomes harder in the center of the object, the resultant profile of the linear attenuation coefficients
appears as a "cup".
The second type of artifact relating to beam hardening are dark streaks and bands between dense objects in an image.
In dental imaging, this type of artifact can be seen between two implants located in the same jaw that are in close
proximity to each other. This occurs because the portion of the beam that passes through both objects at certain tube
positions becomes harder than when it passes through only one of the objects at other tube positions.
Partial Volume Artifacts
The algorithms used in CT data reconstruction assume that the object is completely covered by the detector at all view
angles, and that the attenuation is caused by the object only. When this situation does not occur, reconstructed CT
images can contain a truncated-view artifact. In conventional CT units, this is not a problem as the entire object is
always within the field of view of the unit, however it does affect CBCT units due to their limited FOV. This occurs
because some of the cone beam data penetrating portions of the object other than the region-of-interest (ROI) are
missing because of the insufficient size of the detector.
When the entire volume is not covered by the detector, shading artifacts can be visualized. Another consequence of the
partial volume artifact is that the true linear attenuation coefficients cannot be calculated because some of the x-ray
paths penetrate other portions of the object as well as the region of interest and the data collected no longer represent
this area exclusively but are corrupted by structures outside of the FOV. This issue has a greater affect in machines that
have smaller FOVs as opposed to those that have larger FOVs.
Currently, algorithms attempt to counter this issue by estimating the remaining linear attenuation coefficients for the
areas that are not completely imaged. Although there is improvement in HU precision, this still has not enabled
accurate calculation of Hounsfield units. Many methods are currently being developed and tested to alleviate this issue.
Example of Metal Artifacts
A common problem in CT images is streak artifacts caused by the presence of high-attenuation
objects in the field of view. Metallic objects such as dental restorations, surgical plates
and pins and radiographic markers can cause this type of the artifact. Since the metal in these
materials highly attenuate the x-ray beam, the attenuation values of objects behind the object
are incorrectly high. Due to the reconstruction of the cone beam image, the metal causes the effect of bright and dark
streaks in CT images which significantly degrade the image quality.
In conventional CT images metallic artifacts traverse the object in the direction of the gantry and only at the level of the
high attenuation object. In CBCT, the metallic streak artifacts occur in all directions from the high attenuation object
because of the cone-shaped beam.
Example of Patient Movement Artifact
Patient motion can cause misregistration artifacts within the image. Because of the relatively
long acquisition times (compared to conventional radiography) and volumetric image
acquisition, motion artifacts are common in CBCT. These artifacts can be attributed to
improper patient stabilization.
Small motions cause image blurring and larger physical displacements produce artifacts
that appear as double images or ghost images. This results in poor overall image quality.
Since the resolutions of the present CBCT are very high, ranging from 0.08mm-0.4mm,
even small motions can have a detrimental effect on image quality.
The artifacts presented are some of the common artifacts seen in CBCT images. Care should when acquiring CBCT
images to keep image artifacts to a minimum by selection of optimum scanning parameters and careful patient positioning
and stabilization. When interpreting the images it is also important to recognize imaging artifacts to prevent inaccurate
I'd like to thank Dr. Bruno Azevedo and Dr. Pirkka Nummidoski for their contributions to this article.
- Julia F. Barrett and Nicholas Keat: Artifacts in CT: Recognition and Avoidance. RadioGraphics, Nov 2004; 24: 1679 - 1691.
- Mehran Yazdi, and Luc Beaulieu: Artifacts in Spiral X-ray CT Scanners: Problems and Solutions. International Journal of Biological
and Medical Sciences, 2008; 3: 135-139.