Download Artifacts on multi-detector computed tomography (MDCT)

Artifacts on multi-detector computed tomography (MDCT)
Poster No.:
C-2106
Congress:
ECR 2012
Type:
Educational Exhibit
Authors:
J. Yoon , A. Eftekhari , D. J. Hou , L. Louis , P. Scheffler , S.
1
2
2 1
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2
Nicolaou ; Montreal, QC/CA, Vancouver, BC/CA
Keywords:
Artifacts, Technical aspects, Physics, CT-High Resolution, CT,
Cardiac, Anatomy, Abdomen
DOI:
10.1594/ecr2012/C-2106
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Learning objectives
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Familiarize themselves with a variety artifacts commonly occuring with
several multidtector computed tomography (MDCT) imaging protocols.
Introduce some of the techniques employed by our centre to reduce these
artifacts.
Background
The wide application of multi-detector computed tomography (MDCT) has led to
substantial improvements in imaging. However, MDCT scanners are still subject to
artifacts. Awareness of these artifacts is of crucial importance to radiologists in order to
differentiate them from true pathologies and prevent errors in clinical decision-making.
Artifact is a systematic discrepancy between CT numbers of reconstructed images and
true attenuation coefficients. CT images are more prone to artifacts than conventional
radiographs due to the process of reconstruction from millions of independent detector
measurements. Any error in individual measurement can result in errors in reconstructed
images. These errors can lead to decrease in overall quality of images and/or decrease
the accuracy of a given scan.
The artifacts can manifest as streaking, shading, rings, and distortion.
In this exhibit, examples of commonly encountered artifacts such as motion artifacts,
metallic artifacts, and beam-hardening artifacts will be examined.
Imaging findings OR Procedure details
Motion artifacts
Patient's motion can cause misregistration and therefore shading or streak artifacts. This
can be caused by lack of cooperation or by involuntary motions such as breathing, and
cardiac motion (Fig 1 - 2).
The motion artifacts can mimic true pathologies at times, making image interpretation.
In Fig 3, motion artifact is creating illusion of aortic dissection. In another set of studies,
motion artifacts is mimicking renal lasceration in a trauma patient (Fig 4).
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Solution for motion artifacts
Motion artifacts can be reduced by reducing patient movements or by shortening the scan
time. Strategies to reduce motion can be categorized into operator-dependent and builtin feature of the scanner.
The operator can achieve immobilization of the patient through positioning aids or
sedation. Also, when scanning the chest, patient can hold the breath to reduce respiratory
motion.
The built-in manufacturer solutions include overscan and underscan mode, software
correction, cardiac gating, and ultra high pitch protocols.
1) Overscan and underscan mode: In overscan mode, X-ray gantry rotates more than
standard 360° to acquire additional projections. These additional images are averaged in
order to reduce motion artifacts. Conversely, motion artifacts can be reduced by having
partial scans by virtue of reduced scan time at the expense of resolution.
2) Software correction: Iterative Reconstruction in Image Space (IRIS) by Siemens is an
image reconstruction algorithm that has the potential to reduce image noise and artifact
whilst increasing the sharpness. (Fig 5, 6)
3) Cardiac gating: In order to image the rapidly moving heart, electrocardiogram-gated
(ECG-gated) acquisition has been developed. The porjections are acquired only at a
certain phase of cardiac cycle (e.g. diastole) capturing relatively constant image of the
heart.
4) Ultra high pitch protocols: FLASH by Siemens is a method of increasing scanning
speed by utilizing both X-ray tubes in Dual Source CT scan. It allows dramatic reduction
in scanning time. Motion artifacts in non-sedated pediatric phantom is reduced by using
FLASH (Fig 7, 8). In this abdominal CT scan, by using ultra high pitch protocol, motion
artifact was significantly reduced (Fig 9, 10).
Metallic artifacts
Metallic material causes severe streak artifacts in multiple ways. The density of the
metal is beyond the normal range of computer resulting in incomplete attenuation profile.
Other artifacts such as beam-hardening, partial volume and aliasing artifacts contribute
to the severity of degradation in image quality. In the following figures, dental amalgam
is causing streak artifacts making assessment of the carotid artery more difficult (Fig 11)
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Solution for metallic artifacts
When feasible, all the metallic objects on the patient should be removed prior to the
scan. In case of irremovable objects such as dental fillings, surgical clips, prosthesis or
orthopedic appliances, the X-ray tube gantry angulations can be set in order to exclude
these objects. Alternatively, increasing kVp will allow some penetration, and using thinner
sections will reduce artifacts due to partial volume.
As for software corrections, interpolation techniques to replace over-range values in
attenuation profiles can be used. However, the use of this technique may be limited in
metal artifact reduction because loss of detail around metal-tissue interface.
Beam-hardening artifacts
An X-ray beam is made of photons with a range of energy levels. As these photons go
through a material, photons of relatively lower energy becomes preferentially attenuated
resulting in "Beam Hardening". This phenomenon can cause two types of artifacts,
streaking and cupping artifacts.
Cupping artifacts manifest as apparent hypo-attenuation in the centre of reconstructed
images. Because at the centre, there is more material to go through, there is a higher
degree of beam hardening therefore decreased level of attenuation.
Streaking artifacts occur between two dense objects in an image. They occur because
when the X-ray goes through two dense objects, higher degree of beam hardening occurs
compared to when the X-ray goes through one dense object depending on the tube
position.
In this intubated trauma patient, his vertebral arteries are obscured by beam-hardening
artifact due to the endotracheal (ET) tube (Fig 12).
Dense bony structures of the shoulders can also cause streak artifacts through beamhardening. In this patient with cervical spine fractures, beam-hardening artifacts from ET
tube and the shoulder are obscuring the fracture (Fig 13)
Solution for beam-hardening artifacts
A flat piece of metal filter can be used to pre-harden the beam to filter out the lower energy
component of the X-ray beam before passing through the patient.
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CT scanner can be calibrated using phantoms to compensate the cupping artifact.
Beam hardening correction software can be applied to bony regions.
Avoid scanning bony regions by altering position or tilting gantry.
Select appropriate scan field of view so that the scanner uses the correct calibration data
and other measures to decrease the artifacts.
Images for this section:
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Fig. 1: Sagittal section of the chest CT showing motion artifact due to cardiac motion.
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Fig. 2: Coronal reformat of an abdominal CT scan in which the quality is significantly
degraded by motion artifact.
Fig. 3: Axial image of chest CT showing apparent intimal flap and false lumen in the
aorta. This finding was a motion artifact due to aortic pulsation.
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Fig. 4: Coronal reformat of abdominal CT in a trauma patient. Motion artifact mimics renal
lasceration (green arrow).
Fig. 5: Sagittal reformat of a head CT. Streak artifacts above the level of the foramen
magnum can be appreciated.
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Fig. 6: Same image as Fig 5. reconstructed using IRIS. In comparison, there is lesser
degree of streak artifacts, and noise levels.
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Fig. 7: Pediatric phantom, simulating a non-sedated baby. Coronal reformat of standard
spiral CT and Volume-rendered technique (VRT) showing poor image quality due to
motion artifacts.
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Fig. 8: Pediatric phantom, simulating a non-sedated baby as in Fig. 7. Coronal reformat
of Flash spiral CT and Volume-rendered technique (VRT) showing significantly reduced
level of motion artifacts due to rapid scanning.
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Fig. 9: Coronal reformats of abdominal CT scan. The quality of the scan is reduced by
motion artifacts.
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Fig. 10: Abdominal CT scan obtained with a ultra high pitch protocol showing lack of
motion artifacts in the same patient.
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Fig. 11: CT angiogram of the carotid artery, sagittal reformat. Due to the streak artifacts
from dental amalgam.
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Fig. 12: Axial cut of CT angiogram. Beam-hardening artifacts from endotracheal tube is
obscuring the left vertebral artery.
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Fig. 13: Sagittal reformat of cervical spine CT showing vertebral fracture in C7 (green
arrow) is obscured by beam-hardening artifacts from ET tube and the shoulders.
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Conclusion
Despite the continuing advances in CT imaging technology, MDCT remains susceptible
to a variety of artifacts. A fundamental understanding of these artifacts is of critical
importance to radiologists in order to ensure accurate diagnoses and prevent missing
potentially critical injuries.
Some of the measures to reduce artifacts include:
1) Optimizing arm positioning, preferably above head.
2) Minimizing metallic objects outside patient.
3) Ultra high pitch - reduce motion artifacts (bowel).
4) Software correction - post-processing reconstruction to reduce noise at constant dose.
Personal Information
References
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a review publication of the Radiological Society of North America, Inc, 2004. 24(6): p.
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2. Choi, S.I., et al., Recent developments in wide-detector cardiac computed tomography.
The international journal of cardiovascular imaging, 2009. 25 Suppl 1: p. 23-9.
3. Fishman, E.K., K.M. Horton, and P.T. Johnson, Multidetector CT and threedimensional CT angiography for suspected vascular trauma of the extremities.
Radiographics : a review publication of the Radiological Society of North America, Inc,
2008. 28(3): p. 653-65; discussion 665-6.
4. Kataoka, M.L., et al., A review of factors that affect artifact from metallic hardware
on multi-row detector computed tomography. Current problems in diagnostic radiology,
2010. 39(4): p. 125-36.
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