Ct Artifacts

 Artifacts are defined as anything appearing on the image that

is not present in the object scanned. Artifacts have many
different presentations and can be attributed to many causes.

• They can be broadly classified as *physics-based (resulting

from the physical processes associated with data acquisition),
*patient-based, or *equipment-induced.

• Artifacts can seriously degrade the quality of CT images.

• Recognizing various artifacts and understanding why they

occur and how they can be prevented or reduced is an
important aspect of image quality assurance.
 Beam Hardening
• as an x-ray beam passes through an object, lower-energy
photons are preferentially absorbed, creating a “harder” beam.
Individual rays are hardened to differing degrees, and this
variation cannot be adjusted for by the reconstruction
algorithm.

• Two types of artifact can result from this effect, cupping

artifacts (the periphery of the image is lighter) and the
appearance of dark bands or streaks between dense
objects in the image (Fig. 7-5)
CT systems use three features to minimize beam hardening: *
*filtration, *calibration correction, and *beam hardening
.correction software
 Partial Volume Artifact
• the partial volume effect occurs when more than one type of
tissue is contained within a voxel.

• To illustrate, Figure 7-6 exaggerates the geometry of the x-

ray beam to demonstrate how an object that lies to the
periphery may not appear on all views.

• The best method of reducing partial volume artifacts is to

*use thinner slices.
 Aliasing
• Insufficient projection data (for instance, when the helical
pitch is greatly extended) is known as under sampling.
• Under sampling causes inaccuracies related to reproducing
sharp edges and small objects and results in an artifact known
as aliasing, in which fine stripes appear to be radiating
from a dense structure.

• Because aliasing artifacts consist of evenly spaced lines they

are easy to distinguish from anatomic structures and,
therefore, seldom do they render an image undiagnostic.
However, when resolution of fine detail is important, aliasing
artifacts should be avoided to the degree possible.

• Aliasing artifacts can be combated by *slowing gantry

rotation speed (i.e., increasing scan time) or *by reducing
the helical pitch.
 Edge Gradient Effect
• results in streak artifact or shading (both light and dark)
arising from irregularly shaped objects that have a pronounced
difference in density from surrounding structures. A common
clinical example is artifacts that result when barium and air lie
adjacent to each other in the stomach (Fig. 7-7).
• Artifacts from the edge gradient effect are largely unavoidable,
but are somewhat *reduced by thinner slices.

• Using *a low HU-value oral contrast, such as Volumen, or

water (a neutral HU contrast agent) in place of a barium
suspension can eliminate the streak artifacts from the
gastrointestinal tract.
 Motion
• Artifacts from patient motion typically appear as shading,
ghosting (objects appear to have a shadow), streaking or
blurring (Fig. 7-8).
• Manufacturers have built features into the CT systems to
reduce motion artifacts such as *overscan and partial scan
modes, *software correction, and *cardiac gating.

• Voluntary motion can be reduced or eliminated by adequately

*preparing the patient for the examination.

• Scanning the chest and abdomen using the *shortest scan

time possible also helps to minimize artifact from involuntary
motion.
* Metallic Artifacts
•Metal objects in the SFOV will create streak artifacts(Fig. 7- 9).

•The best way to reduce metallic artifact is to *minimize the

metal present in the SFOV.

•Patients are asked to take off any removable metal objects such
as jewelry before scanning begins.

•For non removable items, such as dental fillings, prosthetic

devices, and surgical clips, it is sometimes *possible to angle
the gantry to exclude the metal objects.
Out-of-Field Artifacts
•Out-of-field artifacts are caused by anatomy that extends
outside of the selected SFOV.

•These artifacts occur because the anatomy outside the SFOV

attenuates and hardens the x-ray beam.

•A common clinical example is when imaging of the body must

be done with the patient’s arms down by their side, rather than
raised out of the way of the scan (Fig. 7-10).
* Ring Artifacts
•Ring artifacts appear on the image as a ring or concentric rings
centered on the rotational axis (Fig. 7-11).

• They are caused by imperfect detector elements—either

faulty or simply out of calibration.

• In some instances technologists may eliminate circular

artifacts by *recalibrating the scanner.
* Tube Arcing
•A common cause of equipment-induced artifact occurs when
there is an undesired surge of electrical current (i.e., a short-
circuit) within the x-ray tube. This is referred to as either high-
voltage arcing or tube arcing.
•There is no specific pattern in the appearance of tube arc
artifacts
•Their effect on the image will vary depending on the severity
and frequency of arching. Artifacts from tube arcing can range
from a single slight streak to multiple streaks
•will produce an error message that can help the technologist
identify the problem
•A service engineer should be called when tube arcing occurs
CT Radiation Dosimetry
Why ??
•At installation, Commissioning ,Annually QC and a system
service which might impact on dose, including, but not limited to,
replacement of an X ray tube or filtration component.
•Check constancy and performance
• Compare output between different vendors (GE, Siemens,
Canon ,Philips)
• Estimate the patient dose (not calculation)
• Dose optimization Techniques
Patient Dose determination is very hard ,Why ???
•1-Unequal distribution the dose around the slice and along the
patient .
•2-Variation in The shape and size of the body and internal
composition
•3-Radiation that scatters out of the slice and produces some
dose in the adjacent tissues.
* This scattered radiation complicated the process of determining
the dose when more that one slice is imaged as in the usual
procedure.
•4-we cannot place instruments, or dosimeters,directly into the
body to make measurements.
 To solve the scatter issue:
•The first step is to measure the dose in a phantom that
represents a patient body.

•The typical phantom is an acrylic cylinder.

•1-A smaller phantom is used to represent a head(16 cm) and a

larger one (32 cm ) for the abdominal section.

•2-A dosimeter, typically an ionization chamber, is placed in the

phantom and a one-slice scan is performed and the dose
measured.
• It has been demonstrated that if a
dose measurement is made for
just one scanned slice the
scattered radiation out of the slice
and measured by the dosimeter
will give a good approximation to
the dose within a slice including
the scattered radiation from
adjacent multiple slices.
2-To solve the variation in the dose distribution through the
slice:
To account for this the usual
procedure is to use a phantom
with five (5) dosimeters. The
five measured values are then
used to calculate a "weighted"
value using the formula shown
here (CTDIW) mGy
• Dose is the concentration of radiation
absorbed in tissue, increasing the pitch
value with all other factors remaining
unchanged "spreads" the radiation and
makes it less concentrated. The dose
becomes inversely proportional to the
pitch.
CTDIvol(mGy) =CTDlw/Pitch
• DLP values when combined with other factors
can be used to calculate the effective dose to
the patient.
DLP(mGy.Cm) = CTDlvol (mGy) x L
(cm)
• Effective dose is the
quantity that is used to
estimate relative risk to a
patient taking into
account the distribution
of dose values to the
various organs and
anatomical regions and
the relative sensitivity of
the different tissues.

Effective Dose(msv) =
DLP (mGy.cm) xf(conversion factor)(uSv/mGy.cm)