2. Image artifacts
• Discrepancies between the reconstructed
values in an image and the true Attenuation
coefficients of the object.
• Nearly every image produced by a CT scanner
contains an artifact.
• Discrepancies that are clinically significant or
relevant as judged by the radiologist
3. Contd…
• Compared to conventional radiography CT
systems are more prone to artifacts.
• CT images are generated with about 1000
projections and 1000 separate measurements
in each projection (106 independent
measurements )
• Artifacts can seriously degrade the quality of
CT images, making them diagnostically
unusable
4. Appearances of image artifacts
• Streaking
• Shading
• Rings and Bands
• miscellaneous
5. Streaking artifacts
• Appear as intense straight lines across
the image and can be either bright or dark.
• The bright and dark streaks appear in
pairs due to the nature of the
reconstruction process.
• Caused by an inconsistency in isolated
or single measurement.
7. Shading artifacts
• Are due to a group of channels or views
deviating gradually from the true
measurement;
• Appear near objects of high contrast . In the
soft tissue region near bony structures or air
pockets.
• Can be bright or dark depending on the
nature of the problem.
• Not as easy to identify and mimic pathology,
leading to misdiagnosis.
9. Ring and band artifacts
• Appear as full
rings or arcs
superimposed on
the original
image structure.
• Caused by A
miscalibrated or
defective
detector element
in third-
generation
scanner
10.
11. Causes of image artifacts
1) Artifacts related to system design
2) Artifacts related to x-ray tubes
3) Detector induced artifacts
4) Patient induced artifacts
5) Operator induced artifacts
12. 1) Artifacts Related to System Design
• Aliasing (undersampling)
• Partial volume
• Scatter
• Noise
• Beam hardning
13. Aliasing(undersampling)
• The number of projections used to
reconstruct a CT image is one of the
determining factors in image quality
• Too large an interval between projections
(undersampling) can result in misregistration
by the computer of information.
• This leads to an effect known as view
aliasing, where fine stripes appear to be
radiating from the edge of a dense structure.
14.
15. Correction of Aliasing
• Aliasing can be minimized by acquiring
the largest possible number of projections
per rotation (slow rotation speed).
• Using specialized high-resolution
techniques, such as quarter- detector
shift or flying focal spot
16. PARTIAL VOLUME EFFECT
• Mechanism of partial volume artifacts, which
occur when a dense object lying off-center
protrudes part of the way into the x-ray
beam.
17.
18. Partial volume artifact
CT number in each pixel is proportional to
average u in the corresponding voxel,
• some voxels contain mixture of different
tissue types,
• partial volume artifact is most pronounced
for softly rounded structures parallel to the
CT slice. eg, Near top of the head.
19. Correction of partial volume
• The best method to combat partial-volume
artifacts is to use thin slices whenever high
variations in the object attenuation are
expected
21. Scatter…
• Not all x-ray photons that reach the CT
detector are primary photons and portion
of the detected signal is generated from
the scatter.
• Cause either CT number shifts or shading
or streaking artifacts in the reconstructed
images
22. Correction of scatter
• Preventing the scattered radiation from
reaching the detector by a post patient
collimator.
23. Noise induced streaks
• photon starvation
• Insufficient photons reach the detectors
and very noisy projections are produced
• Random thin bright and dark streaks that
appear in the direction of greatest
attenuation
• Low contrast soft tissue boundaries may
25. Noise…
• A result of improper selection of scanning
parameters (low tube current)
• Presence of the patients arm in the
scanning plane
• CT scanner limitations
26. Correction of noise artifact
• Optimizing scanning parameters based on
patient age weight and size.
• Automatic Tube Current Modulation
27. BEAM HARDENING
• Seen in CT images mostly as dark zones
or streaks between bone structures.
• Eg: Hounsfield bar: between petrous
bones
• As the beam passes through an object its
mean energy increases, because the
lower energy photons are absorbed more
rapidly than the higher-energy photons
28. How it Forms?
• The beam-hardening phenomenon
induces artifacts in CT because rays from
some projection angles are hardened to a
differing extent than rays from other
angles, and this confuses the
reconstruction algorithm
• X-ray beam hardening may cause a
decreases in CT numbers so that the
middle of the image appears darker than the
periphery
32. 2) Artifacts Related to X-ray Tubes
• Off focal radiation
• Tube arching / Tube spit
• Tube rotor wobble
33. Off- focal Radiation
• X-ray photons are emitted from a larger area
on the target.( not from a single point)
• Off-focal radiation is caused mainly by two
effects: secondary electrons and field-
emission electrons
• cause shading artifacts.
34. Correction of off-focal radiation
• It can be partially controlled by placing
collimators outside the x-ray tube.
• Software correction
35. Tube Rotor Wobble
• streaking artifacts are caused by mechanical
failure or imperfection. This can be the result
x-ray tube rotor wobble.
• The actual x ray beam position deviates from
the ideal position assumed by the
reconstruction algorithm.
38. 3) Detector Induced Artifacts
• In the detector manufacturing process, it is
impossible to produce identical detector cells
• The size, roughness and reflective material
placed between detector cells can vary.
• The net effect is that the gains of individual
detector cells cannot be made equal and
detector gains are likely to change over time.
39. Gain…
• Ring or band artifacts will result if a
proper correction is not rendered.
• To compensate for the gain variation
calibration technique are used.
41. Patient Motion
• Motion artifacts occur when the patient
moves during the acquisition. Small
motions cause image blurring, and larger
physical displacements during CT image
acquisition produce artifacts that appear
as double images or image ghosting
42.
43. Correction of motion artifacts
• Various methods can be used to reduce
voluntary motion artifacts
• instructing the patient to hold his or her
breath during the scan
• the patient can be immobilized by sedation
• Another approach is to shorten the scan
time. scan time on the order of 40 ms can
practically freeze any type of patient
motion
44. Metal Artifacts
• With metallic implants: beam hardening and
partial volume artifacts are intensified.
• Can Completely extinguish the image
contents in the vicinity of the metallic object,
leaving only disturbing noise structures.
Eg: Metallic orthopedic hardware inside patient.
Equipment attached to the patient’s body
46. Correction of metal artifacts
• Higher KVp
• Scanning with thin slices
• Avoidance of metal artifacts by the
operator
• Software corrections
47. Incomplete Projection(Out of field)
• Occurs if parts of the patient are infact in
the gantry, but are positioned outside of
the field of measurment
• Artifact appears as streak or shading.
50. Incomplete…
• To avoid artifacts due to incomplete
projections, it is essential to position the
patient so that no parts lie outside the scan
field
• Algorithmic correction
51. 5) Operator Induced Artifacts
• The knowledge and skill of the operators
makes a significant difference in the final
image quality
• the CT operator plays important role in
artifact prevention and avoidance
• proper protocol selection
• Proper patient positioning
52. Helical and multi section artifacts
• Helical Artefacts in Single-Section
Scanning
• Helical Artefacts in Multi-section
Scanning
• Multiplanar and Three dimensional
Reformation
•
53. Helical Artefacts in Single-Section Scanning
• High-pitch helical artifacts
• For single-slice helical CT, the level of helical-
related artifacts increases with the helical pitch
• A more degraded image quality is expected for
data collected with a higher helical pitch
• When the helical pitch approaches zero no
helically induced image artifacts will be present
54.
55. Solutions
• Low pitch
• In typical clinical applications, helical pitches
between 1 and 1.5 are routinely used
• Thin acquisition sections rather than thick.
Use axial rather than helical imaging to avoid
helical artifacts (eg, in brain scanning).
56. Helical Artefacts in Multi-section Scanning
• More complicated form of axial image
distortion on multi section scanners
• Windmill artifacts
• several rows of detectors intersect the plane
of reconstruction during the course of each
rotation
• As helical pitch increases, the number of
detector rows intersecting the image plane
per rotation increases
57.
58. Multiplanar and 3D reconstructions
• Stair step artefacts
– Stair step artefacts appear around the edges of
structures in multiplanar and 3D reconstructions
when wide collimations and non-overlapping
reconstruction intervals are used.
– They are less severe with helical scanning, which
permits reconstruction of overlapping slices
without the extra dose to the patient that occurs
when overlapping axial scans are performed.
– Stair-step artifacts are virtually eliminated in
multiplanar and 3D reconstructions of thin slice
data from today’s multi-slice scanners.
59.
60. References
• Computed tomography: Principles,
design, artifacts, and recent advances ,
Hsieh J.
• CT artifacts: Causes and reduction
techniques, F Edward Boas & Dominik
Fleischmann
• Artifacts in CT: Recognition and
Avoidance Julia F. Barrett & Nicholas
Keat, MSc
Theoretically, an image artifact can be defined as any discrepancy between the reconstructed values in an image and the true attenuation coefficients of the object.
Although this definition is broad enough to cover nearly all types of nonideal images, it has little practical value since nearly every image produced by a CT scanner contains an artifact by this definition. In fact, most pixels in a CT image are “artifacts” in some shape or form.
In practice, we have to limit our discussion to the discrepancies that are clinically significant or relevant as judged by the radiologists
Compared to conventional radiography, CT systems are inherently more prone to artifacts. CT image is generated with a larger number of projections (about 1000). In a typical CT system, each projection contains roughly 1000 separate measurements. As a result, nearly 106 independent readings or measurements are used to form an image. Since inaccuracies in the measurements usually manifest themselves as errors in the reconstructed images, the probability of producing an image artifact is much higher for CT.
Artifacts can seriously degrade the quality of computed tomographic (CT) images, sometimes to the point of making them diagnostically unusable. To optimize image quality, it is necessary to understand why artifacts occur and how they can be prevented or suppressed
Generally speaking, CT image artifacts can be classified into four major categories: streaking, shading, rings and bands, and miscellaneous
Streaking artifacts often appear as intense straight lines (not necessarily parallel) across the image. They can be either bright or dark. In many cases, the bright and dark streaks appear in pairs due to the nature of the reconstruction process.a streak is usually caused by an inconsistency in the isolated measurements.
The inconsistency could be the result of an inherent problem associated with the data collection process (e.g., patient cardiac motion), a mechanical malfunction, or abrupt changes between views. Under normal conditions, the FBP process maps each data point in the projection space onto a straight line in the image domain, the positive and negative contributions among neighboring lines are combined, and no straight lines appear in the final image. When an inconsistency occurs in the projection data set, the reconstruction process is no longer able to properly combine the positive and negative contributions, and lines or streaks result.
Illustration of streaking artifact production with a simulated water phantom. Top: parallel projection, filtered projection, and reconstructed images without error; bottom: about 10% error was added to three channels in a single projection at 45 deg.
After the filtering process, the errors appear to be significantly enhanced in terms of their relative magnitude, and large undershoots appear next to the erroneous channels, as depicted by lower middle. This occurs because the ramp filter used in tomographic reconstruction is essentially a derivative operator, so discontinuities in the projection yield overshoots and undershoots at locations of discontinuity after filtering. These patterns are mapped to bright and dark lines by the backprojection process, as illustrated by the reconstructed image shown in lower right.
Under normal conditions, these artifacts are unlikely to cause misdiagnosis, since human pathologies rarely resemble their appearance. However, when they appear in large quantities and large magnitudes, these artifacts can degrade the image quality to such an extent that images become either unreadable or unreliable.
Shading artifacts often appear near objects of high contrast. For example, they usually appear in the soft tissue region near bony structures or air pockets.
They can be either bright or dark, depending on the nature of the problem. The cause of shading artifacts is again an inconsistency in the projection measurement. Unlike streaking artifacts, shading artifacts are due to a group of channels or views deviating gradually from the true measurement;
About 10% error was added to 40 channels in a single projection at 45 deg. (a) Profile of the erroneous projection. (b) Profile after filtering operation. (c) Reconstructed image.
Because there is no sharp discontinuity in the signal, the overshoot and undershoot that appear after the filtering step are small(b), and the image produced by these errors (excluding the true attenuation measurements) does not contain clear boundaries(c).
Ring and band artifacts, as their names imply, appear as rings or bands superimposed on the original image structure. They can be either full rings or
arcs. If one of the detectors is out of calibration on a third-generation (rotating x-ray tube and detector assembly) scanner, the detector will give a consistently erroneous reading at each angular position, resulting in a circular artifact
Illustration of the production of a ring artifact. About 1% of error was added to a single channel over all projection angles. (a) Projection profile with 1% added error. (b) Profile after filtering operation. (c) Reconstructed image.
Given the small error magnitude, it is hardly visible in the projection profile, as illustrated in (a). After the filtering step, however, the magnitude of the error is significantly enhanced and can be easily identified in the profile of (b). As discussed previously, the backprojection process maps filtered errors to straight lines in the reconstructed image. These lines are at the same fixed distance to the iso-center, since only rotational motion is present in a third-generation detector during data acquisition. Thus, the tail portions of the lines are canceled, and a ring is formed as shown by the reconstructed image in (c)
Ring artifact. A. Pelvic CT showing severe ring artifact. B. Head CT with subtle ring artifact simulating a pons lesion (arrow).
Image artifacts are caused by many things: the nature of the physics, suboptimal system design, limitations of current and new technologies, patient
characteristics, and suboptimal or inappropriate use of the scanner.
These error patterns lead naturally to the physical sources of error. In particular, we want to link artifacts to major components in a CT system. For the purpose of this discussion, the overall system design is considered to be one of the major components (e.g., artifacts caused by inadequate projection or view sampling). Artifacts related to other components are divided into four major sections: x-ray source, x-ray detector, patient, and operator.
physics-based artifacts, which result from the physical processes involved in the acquisition of CT data
The number of projections used to reconstruct a CT image is one of the determining factors in image quality. Too large an interval between projections
(undersampling) can result in misregistration by the computer of information relating to sharp edges and small objects
CT image of a Teflon block in a water phantom shows aliasing (arrow) due to undersampling of the edge of the block.
View aliasing can be minimized by acquiring the largest possible number of projections per rotation. On some scanners, this can be achieved only by using a slower rotation speed, while on others the number of projections is independent of rotation speed. Ray aliasing can be reduced by using specialized high-resolution techniques, such as quarter- detector shift or flying focal spot, which manufacturers employ to increase the number of samples within a projection.
a partially intruded object is located off the isocenter. When the gantry is rotated so the object is closer to the detector, the x-ray beam profile is quite wide, and a portion of the object is within the FOV. When the CT system rotates to the opposite side, the object is completely outside the xray beam path. This phenomenon obviously causes inconsistencies in the projection data set. The farther the object is from the iso-center, the more significant the problem becomes. an off-axis object can be within the beam, and therefore “seen” by the detectors, when the tube is pointing from left to right but outside the beam, and therefore not seen by the detectors, when the tube is pointing from right to left. The inconsistencies between the views cause
shading artifacts to appear in the image
Streak shaped dark and light artifact.
Most critical region: Posterior cranial fossa
Can occur in Z direction and scan plane(sampling artifacts)
The best method to combat partial-volume artifacts is to use thin slices whenever high variations in the object attenuation are expected, for example in the posterior fossa
In many situations, the reason for scanning a patient with thicker slices is to reduce photon noise. To reduce the partial volume artifact and suppress photon
noise, we can simply sum up several thin slices to produce a thicker slice
Computer algorithms can be used to reduce partial-volume artifacts An image space correction scheme has been shown to be quite effective
The most important interaction mechanism between x-ray photons and tissue-like materials is incoherent scattering or Compton scattering
When an x-ray photon collides with an electron, part of the energy is transferred to the electron to free it from the atom, and the rest of the energy is
carried away by a photon. Because momentum is conserved in this process, the scattered photon generally deviates from the path of the original photon
Compton scatter causes X-ray photons to change direction (and energy), and thus end up in a different detector [10]. This creates the greatest error when the scattered photon ends up in a detector that otherwise would have very few photons. Scatter also becomes more significant with an increased number of detector rows, because a larger volume of tissue is irradiated.
Because of Compton scatter, not all of the x-ray photons that reach the detector are primary photons. Depending on the CT system design, a portion of
the detected signals is generated from the scatter. These scattered photons make the detected signals deviate from the true measurement of the x-ray intensities and cause either CT number shifts or shading (or streaking) artifacts in the reconstructed images
Most CT designers combat scatter by preventing the scattered radiation from reaching the detector. Much of the scattered radiation can be effectively
eliminated by a postpatient collimator. because most of the scattered photons travel along paths that are significantly different from the fan-beam paths. Therefore, by limiting each detector cell’s FOV to the vicinity of the x-ray focal spot, a majority of the scattered x-ray photons can be rejected.
This can be easily accomplished in third-generation CT scanners by placing a collimator near the detector surface that focuses on the x-ray focal spot
Poisson noise is due to the statistical error of low photon counts, and results in random thin bright and dark streaks that appear preferentially in the direction of greatest attenuation. With increased noise, high contrast objects such as bone may still be visible, but low contrast
soft tissue boundaries may be obscured.
photon starvation, which can occur in highly attenuating areas such as the shoulders. When the x-ray beam is traveling horizontally, the attenuation is greatest and insufficient photons reach the detectors. The result is that very noisy projections are produced at these tube angulations. The reconstruction process has the effect of greatly magnifying the noise, resulting in horizontal streaks in the image.
CT image of a shoulder phantom shows streaking artifacts caused by photon starvation
This often occurs as the result of inadequate patient handling (e.g., the patient arm is inside the scanning plane), or improper selection of scanning parameters (e.g., low tube current). In many cases, however, it is simply the result of CT scanner limitations.
Careful planning can be quite useful in combating artifacts. For example, care can be taken to instruct the patient to keep his or her arms outside the scan
field of view. Various scanning parameters (e.g., kV, mA, scan speed, and aperture) also can be optimized based on the age, weight, and size of the patient.
bowtie filters, which compared to the periphery. Increasing slice thickness.
Automatic Tube Current Modulation.—On some scanner models, the tube current is automaticallyvaried during the course of each rotation, a process known as milliamperage modulation. This allows sufficient photons to pass through the widest parts of the patient without unnecessary dose to the narrower parts
Adaptive Filtration.—Some manufacturers use a type of adaptive filtration to reduce the streaking in photon-starved images. This software correction
smooths the attenuation profile in areas of high attenuation before the image is reconstructed
It occurs because the broad polychromatic spectrum of the x-radiation is attenuated differently, depending on the energy of radiation,the object type and projection direction
The beam-hardening phenomenon induces artifacts in CT because rays from some projection angles are hardened to a differing extent than rays from other angles, and this confuses the reconstruction algorithm
–X-ray beam hardening may cause a decreases in CT numbers so that the middle of the image appears darker than the periphery
Linear attenuation coefficeint is determined by mean energy, object type
Manufacturers minimize beam hardening by using filtration, calibration correction, and beam hardening correction software.
Filtration: A flat piece of attenuating, usually metallic material is used to “pre-harden” the beam by filtering out the lower-energy components before it passes through the patient. An additional “bowtie” filter further hardens the edges of the beam, which will pass through the thinner parts of the patient
Calibration correction: Manufacturers calibrate their scanners using phantoms in a range of sizes. This allows the detectors to be calibrated with compensation tailored for the beam hardening effects of different parts of the patient.
Beam hardening correction software: An iterative correction algorithm may be applied when images of bony regions are being reconstructed. This helps minimize blurring of the bone–soft tissue interface in brain scans and also reduces the appearance of dark bands in nonhomogeneous cross sections
One method is to optimize the time interval between the contrast intake and the scanning. Another method is to select a contrast agent with a lower beam-hardening effect
CT images of a skull phantom reconstructed without bone correction (a) and with bone correction (b).
CT images of the posterior fossa show the dark banding that occurs between dense objects when only calibration correction is applied (a) and the reduction in artifacts when iterative beam hardening correctionis also applied (b).
we treated the x-ray source as a single point known as the x-ray focal spot. All photons emitted from the x-ray tube were assumed to come from that location. In reality, however, photons are emitted from a larger area on the target. This is referred to as off-focal radiation, or more precisely, extra-focal radiation.
Off-focal radiation is caused mainly by two effects: secondary electrons and field-emission electrons
Off-focal radiation can be partially controlled by placing collimators outside the x-ray tube. For example, a lead diaphragm with a small port can be placed
outside the tube glass envelope to stop some of the off-focus beam spread. Although this approach can limit the amount of offfocal radiation, it cannot completely eliminate it.
Artifacts caused by off-focus radiation can be reduced or eliminated by software correction, though this is by no means simple or straightforward
In recent years, new technologies have been introduced into the x-ray tube design to control off-focal radiation. One such approach uses an electron
collection cup, as shown in Fig. 7.38. The collection cup is biased to a higher electrical potential to absorb the backscattered electrons and prevent them from re-entering the target. This approach controls off-focal radiation with little need for additional compensation techniques.
In some cases, streaking artifacts are caused by mechanical failure or imperfection. This can be the result of a lack of rigidity in the gantry, a mechanical misalignment, or x-ray tube rotor wobble. In all cases, the actual xray beam position deviates from the ideal position assumed by the reconstruction
algorithm.
(a) shows a patient scan with severe x-ray tube rotor wobble. The same patient was later scanned after the problem was resolved, as shown in(b).
The best approach to fix this type of problem is to replace the faulty component
In the detector manufacturing process, it is impossible to produce absolutely identical detector cells. For example, the size of the detector cells can vary
slightly when they are diced in different batches. The roughness of the detector cell surface may change from cell to cell because of the slight variation in the treatment process. The reflective material placed between detector cells cannot be guaranteed to be identical. The photodiode connected to each scintillator cell can exhibit a slightly different spectral response and conversion efficiency. The data acquisition electronics are inherently different from channel to channel. The net effect is that the gains of individual detector cells cannot be made equal. In addition, detector gains are likely to change over
time.
Similar to the case with detector offset, ring or band artifacts will result if a proper correction is not rendered. To compensate for the gain variation from
channel to channel and over time, most CT manufacturers use a calibration technique called air scans. In this process, a set of scans is acquired without any
object inside the scanning FOV once per day.
patient motion during the scan causes data inconsistencies.
voluntary (respiratory motion) or involuntary (peristalsis and cardiac motion)
Various methods can be used to reduce motion artifacts. For respiratory motion, one could instruct the patient to hold his or her breath during the scan.
However, in many cases a patient may be unable to hold his or her breath or is simply unconscious or uncooperative, so this approach becomes ineffective.
Alternat-ively, the patient can be immobilized by sedation, but this is obviously not comfortable for the patient and may be prohibitive. Another approach is to shorten the scan time. For example, scan time on the order of 40 ms can practically freeze any type of patient motion
Depending on the shape and density of the metal objects, the appearance of this type of artifact can vary significantly. In medical applications, metal objects can be metallic orthopedic hardware inside the patient (e.g., surgical pins and clips) or equipment attached to the patient’s body (e.g., biopsy needles).
produce beam hardening, partial volume, aliasing, under-range in the data acquisition electronics, or overflow of the dynamic range in the reconstruction process
a higher kVp setting (e.g., 140 kVp instead of 120 kVp) can help to reduce the beamhardening effect, and scanning with thin slices (e.g., 2.5 mm instead of 5 mm) can reduce the partial-volume impact.
Avoidance of Metal Artifacts by the Operator.— Patients are normally asked to take off removable metal objects such as jewelry before scanning commences. For nonremovable items, such as dental fillings, prosthetic devices, and surgical clips, it is sometimes possible to use gantry angulation to exclude the metal inserts from scans of nearby anatomy
Software Corrections for Metal Artifacts.— Streaking caused by overranging can be greatly reduced by means of special software corrections
An incomplete projection occurs when a portion of a projection is not available for reconstruction. Incomplete projections occur more often than one would
expect. For example, a projection can be truncated when the scanned object is partially outside the scan FOV. For most commercially available CT scanners,
the size of the gantry opening and the size of the scan FOV are different. In most cases, the gantry opening is significantly larger than the scan FOV. For example, while the average gantry opening of a CT scanner is about 70 cm, the scan FOV is limited to 50 cm.
The purpose of this design is mainly ease of patient handling and placement of accessories
PATIENT IS NOT ENTIRELY ENCLOSED IN THE SCANNING FIELD OF VIEW. PATIENTS BODY CAN OBSTRUCT DETECTORS. IN ADDITION, PATIENT TISSUE OUTSIDE THE SFOV WILL FURTHER HARDEN THE X-RAY BEAM. ARTIFACT APPEARS AS STREAKS AND SHADING.
Schematic diagram of scan field-of-view and gantry opening
To avoid artifacts due to incomplete projections, it is essential to position the patient so that no parts lie outside the scan field
One approach to combat projection truncation is prevention. If a patient can be carefully centered in the scanning FOV, most truncation artifacts can be
avoided. Occasionally, however, projection truncation is unavoidable. For example, if the largest dimension of a patient cross-section exceeds the limiting
FOV of the scanner, projection truncation occurs regardless of patient centering. For these cases, the only remedy is to use algorithmic correction
The knowledge and skill of the operators makes a significant difference in the final image quality
the CT operator playsimportant role in artifact prevention and avoidance.
proper training and instruction can be given to a patient prior to the CT scan to hold his or her breath during the data acquisition to minimize voluntery motion.using restrainers.
Another example of the operator’s impact on image quality is proper protocol selection. CT scanning protocols need to be not only organ- or disease-specific,
but also patient-dependent
the patient’s position.For example, the patient needs to be properly centered. Positioning of the arms.
In general, the same artifacts are seen in helical scanning as in sequential scanning. However, there are additional artifacts that can occur in helical
scanning due to the helical interpolation and reconstruction process
High-pitch helical artifacts
For single-slice helical CT, the level of helical-related image artifacts increases with the helical pitch; that is, a more degraded image quality is expected for data collected with a higher helical pitch. A higher helical pitch means that the patient table travels at a greater distance in one gantry rotation
When the helical pitch approaches zero, we can revert to the step-andshoot mode of data collection and no helically induced image artifacts will be present
In typical clinical applications, helical pitches between 1 and 1.5 are routinely used. The selection of helical pitch depends largely on the tradeoffs between coverage, slice thickness, and image artifacts.
The helical interpolation process leads to a more complicated form of axial image distortion on multisection scanners than is seen on single-section scanners
The typical windmill-like appearance of such artifacts is due to the fact that several rows of detectors intersect the plane of reconstruction during the course of each rotation. As helical pitch increases, the number of detector rows intersecting the image plane per rotation increases and the number of “vanes” in the windmill artifact increases.
CT imag. e of a 12-mm-diameter acrylic sphere supported in air, obtained with 0.6-mm section acquisition and beam pitch of 1.75, shows windmill artifact.
