Identify the artifacts produced on the images during CT scans. Describe the methods used to reduce or remove these artifacts. Essay Example
Q: Identify the artifacts produced on images during CT scans. Describe the methods used to reduce or remove these artifacts
Table of Contents
4Reduction of motion artifacts
5Reduction of Metal Artifacts
6Beam hardening artifacts
7Reduction of Beam artifacts
7Partial volume artifacts
8Reduction of Partial volume artifacts
9Reduction of noise-induced artifacts
10Reduction of equipment-induced artifacts
10Photon starvation artifacts
11Reduction of photon starvation artifacts
A computed tomographic (CT) image refers to a display of anatomy of the human body developed from numerous X-ray absorption measurements created around the periphery of the body. The CT image is created mathematically through the data that arises exclusively from the sections of interest (Al-Shakhrah and Al-Obaidi 2003, 25). Generation of such images is restricted to cross-sections of the anatomy that are essentially oriented perpendicular to the body’s axial dimension. On the other hand, artifact refers to any error or distortion in an image that is not related to the subject under examination. Artifacts are relatively prevalent in CT imaging and are typically considered as a type of noise, although their causes may not often be noticeable. In any case, there are a several different effects that may cause artifacts in CT (Barret and Keat 2004, 1679-1682). Given that artifacts in CT image occur resulting from the interaction between the patient and the machine. It is therefore useful to define the artifacts depending on the nature of the error made during the scanning process. This essay discusses the different types of artifacts produced on images during CT scans and the methods used to reduce or remove these artifacts.
Movement of patients during CT examination can either be voluntary or involuntary. Voluntary motion is described as when a patient has the capacity to control the movement (Al-Shakhrah and Al-Obaidi 2003, 26). A typical motion is that resulting from chest activity during expiration and inspiration. Involuntary movements are types of motions that the patient has no control over. A typical one is the cardiac motion. In any case, these movements appear as streaks in the image which result from incapacity to reconstruct algorithm to function with the inconsistent data in vixel attenuation that occurs at the edge of the moving structure. In this case, the computer is unable to track voxel location (Barrett and Keat 2004, 1680-1682).
Fig 1: CT image of head showing motion artifacts (Barrett and Keat 2004).
Reduction of motion artifacts
There are several techniques that can be applied to reduce motional artifacts depending on the type of movement. Involuntary motion can be reduced using an ECG gating technique and a short scan time (Yazdi and Beaulieu 2001, 135-139). Conversely, reduction of voluntary motion is enabled through controlling patient mobilization. In this case, when the patient is positioned comfortably and in a stable position, his breathing can be controlled. Respiratory motion is a primary source of temporal and positional uncertainty in the abdomen and the thorax that can cause a miss due to considerable dosimetric consequences (Han, Bayouth and Bhatia 2011, 2074-2075). In addition, since the movement is under the control of the patient, he can be made to understand impacts of his movement during the scanning. In this way, he can control the body movements in a way that is friendly to the examination. Other methods that could be used to reduce motion artifacts include the use of synchronized motion artifact correction software to reduce the streak artifacts on the CT image (Barrett and Keat 2004, 1680-1682).
Occurrence of metallic substances during the CT examination process can cause metal artifacts to appear as streaks on the CT image (Al-Shakhrah and Al-Obaidi 2003). There are two types of metallic objects known to cause the metal artifacts. Those that are removable and those are non-removable. Typically, removable artifacts are the metallic objects worn by the patients during scanning, such as belt buckles, earrings, necklaces or any other metallic material. Non-removable metallic objects form part of the CT scan equipment, are essential for scanning or constitute metallic materials that form part of the body of the patient such as dental fillings(Barrett and Keat 2004, 1680-1682).
Fig 2: CT image of subject with metal spine implants (Barrett and Keat 2004).
Reduction of Metal Artifacts
The effects of removable and non-removable objects can be significantly reduced by removing or eliminating them during the CT examination. Non-removable objects such as the prosthetic devices, dental filings and surgical clips can be eliminated from the close anatomy through use of thin slice sections and gantry angulations (Barrett and Keat 2004, 1680-1682). In cases where they cannot be excluded, use ofmetal artifact reduction software can be essential in reducing their streak artifacts on the CT image(Barrett and Keat 2004, 1683). Conversely, for removable objects, the patient can be advised to remove them before the CT examination.
Beam hardening artifacts
Beam hardening artifacts entails amplified mean x-ray beam energy when passing through the patient. Given that the size of the object increases, the mean beam energy moves since the lower energy photons become absorbed. However, the rate of absorption is higher than the energy once since the beam passes through the object (Al-Shakhrah and Al-Obaidi 2003, 27).
Additionally, beam hardening artifacts occur in situations where the radiation beams having varying path lengths resulting to beam hardening artifacts. The reason for this is because the CT numbers of particular structure change and appear as streaks and bands. These types of artifacts are known as cupping artifacts (Barrett and Keat 2004, 1680-1684).
Fig 3: CT image showin streak artifacts caused by beam hardening effects of contrast medium (Barrett and Keat 2004).
Reduction of Beam artifacts
Reduction and removal of beam hardening artifacts is effective through the use of bowtie filter that can standardize the beam and detector causing them to be consistent. Additionally, beam hardening correction software can be used to correct the beam hardening effects (Barrett and Keat 2004, 1680-1684).
Partial volume artifacts
Partial volume artifacts (also known as nonlinear partial volume artifacts) happen in cases where high contrast structures widen partially and exclusively into the examined slice (Gormally 2007). This results when individual detector element inexorably averages radiation intensities in the Z-direction for the structure and its adjoining environs. The partial volume that averages the effect causes the partial volume artifacts that lower the quality of image which would appear as rings and streaks (Al-Shakhrah and Al-Obaidi 2003, 27). In this case, the attenuation values are not determined inherently despite the logarithmic correlation between the value of attenuation and the intensities (Sureshbabu and Mawlawi 2005, 157-160).
Fig 4: CT Images showing partial volume effects (Barrett and Keat 2004).
Reduction of Partial volume artifacts
Partial volume artifacts can be reduced through the use of adaptive filtration that can redistribute the date to a lattice through greater sampling density. Adaptive filtration can smoothen the attenuation profile in regions of high attenuation before reconstruction of the image (Barrett and Keat 2004, 1682-1685).
Streaks on the CT image can be caused by noise that results from reduction numerous photons that happen resulting from improper positioning of the patient in the scan field of view, improper scan speed or unsuitable exposure technique factors. Reduction of photons increases the noise which causes streak artifacts (Al-Shakhrah and Al-Obaidi 2003, 26).
Fig 4: CT Images showing noise-induced artifacts due to respiration (Sun and Mok 2012).
Reduction of noise-induced artifacts
The noise-induced artifacts can be minimized by proper position of the patient. Other methods include applying adaptive filtering logarithms, proper scan speed and proper exposure technique factors (Sun and Mok 2012, 50).
Electrical devices used within a clinical setting may induce artifacts through a range of mechanisms (Santosh, P. and M. Souter 2008, 138). The artifacts can caused through CT equipment defects or mechanical failure such as x-ray tube rotation, deficient gantry rigidity, mechanical misalignment and improper detector signals or bad detectors. Bad detectors can cause ring artifacts. Typically, ring factors are features of third generation CT scanners, which is caused by faulty detectors that generate varying signal outputs (Santosh, P. and M. Souter 2008, 139).
Fig 5: CT Images showing artifacts caused by contamination of CT scanners (Rzanny et al 2004).
Reduction of equipment-induced artifacts
The ring artifacts in the third generation CT scanners can be corrected by finding the faulty detectors and recalibrating them. Additionally, the use of balancing algorithm software can effectively reduce ring factors and subsequently correct the raw data during or before the CT examination (Yazdi and Beaulieu 2001, 135-139). Further, an improved understanding of the artifacts induced by equipment and their inherent characteristics is crucial in avoiding misdiagnosis and misinterpretation (Santosh, P. and M. Souter 2008, 138).
Photon starvation artifacts
Photon starvation artifacts may happen due to insufficient photons that pass through the largest parts of the patient thus creating a noisy image projection that cause streaks on the image. Reconstruction of the image using the scanner’s standard algorithms increases noise, which trails the direction of the patient’s widest or largest parts (Barrett and Keat 2004, 1683-1685).
Fig 6: CT image of shoulder phantom showing streaking artifacts caused by photon starvation (Barrett and Keat 2004).
Typically, photon starvation artifacts cause streaks on the CT images nears the heart, hips and shoulders where the patient’s tissue increases volume. The streaks are prevalent in patients with large mass (Barrett and Keat 2004, 1680-1684).
Reduction of photon starvation artifacts
The effects of the streak artifacts caused by photon starvation can be reduced significantly. This causes smoothening of the high values from the image as a result reducing the noise. Multi-dimensional adaptive filtration can further be added to the process of acquiring the image. This can further reduce the noise that has been created by lack of photons to the detectors in particular projections (Al-Shakhrah and Al-Obaidi 2003, 26).
Artifacts can potentially lower the quality of the CT images to the extent of making them to be diagnostically ineffectual. There are several types of artifacts produced on images during CT scans and the methods used to reduce or remove these artifacts. The most common types include motion artifacts, metal artifacts, Beam hardening artifacts, equipment-induced artifacts, Photon starvation artifacts, partial volume artifacts and beam hardening artifacts. To optimize the quality of the CT images, it is critical to understand why these types of artifacts occur and how they can be minimized or eliminated. CT artifacts come about from several sources. Physics-based artifacts originate from the physical processes archetypical in the CT data acquisition. Patient-based artifacts result from factors such as the patient movement or the available metallic materials in or on the patient. However, in most case, these CT artifacts can be removed through proper patient positioning and optimal selection of scanning parameters.
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