Artifacts in CT Images - Article Example

Artifacts in CT Images Artifacts are structures which are seen on the images by are not part of the actual anatomy (Goldman, 2007, 222).. Artifacts are very common during CT imaging. Infact, of all types of imaging, CT imaging is more prone to artifacts (Goldman, 2007, 222).. This is because image in CT scan is not the original image and is actually a reconstruction one developed from several measurements by independent detectors (Barrett, and Keat, 2004, 1680). Artifacts thus affect the quality of imaging and the value of diagnosis. Radiologists must detect CT artifacts while reporting to enhance the diagnostic value of the imaging technique. Several strategies have been developed to prevent artifacts. In this article, different artifacts in CT imaging and various techniques to prevent then will be elaborated. Different types of artifacts Different researchers have classified artifacts in different manner. While some experts have classified artifacts based on appearance, like ring artifacts, shading artifacts and streak artifacts (Goldman, 2007, 222), others have classified them based on the causes (Yazdi and Beaulieu, 2008, 135). In this articles, classification by Yazdi and Beaulieu (2008, 135) will be used. Figure-1: Types of artifacts (www.googleimages.com) Classification of artifacts (Yazdi and Beaulieu, 2008, 135) Based on the causes, artifacts can be categorized into patient-based artifacts, physics based artifacts, spiral-based artifacts and scanner-based artifacts. Patient-based artifacts Patient-based artifacts occur because of some attributes in the body of the patients. The most common patient-based artifacts are metallic artifacts. These artifacts occur due to presence of irremovable metals in the body of the patient like hip prosthesis, dental filling, fracture fixation rods, cardiac prosthesis, chemotherapy ports and surgical clips. Metallic artifacts appear like streaks on images. They occur because of improper and inaccurate correction of beam hardening within the back projection that is filtered. As such, metals absorb photons heavily and cause overestimation of activity in the metallic region. This is the reason why patients are asked to remove all metallic objects in their body prior to entering the scan room. Several techniques have been developed to prevent on minimize metallic artifacts in CT images (Yazdi and Beaulieu, 2008, 136). One such strategy is to disregard data related to projections from metal objects and reconstruct image only based on projection data from non-corrupted regions. However, this method is very costly for regular scans and convergence problems occur frequently. Another strategy is to reconstruct images by manually identifying the missing projections and replacing them with non-missing projections of the surrounding areas. This method is known as projection-interpolation method. Other strategies to minimize metallic artifacts are based on replacing the missing projections from metal regions by several other techniques like multi-resolution analysis of the wavelets, linear prediction methods (Yazdi and Beaulieu, 2008, 136), visual sinogram concepts (Abdoli, n.d.) polynomial interpolation techniques and summing of not affected projection values withing missing projection centered windows (Yazdi and Beaulieu, 2008, 136). Major disadvantages with some of these techniques are absence of preservation in the continuity of the structures projection and increased noise production (Yazdi and Beaulieu, 2008, 136). Figure-2: Metallic artifacts (www.googleimages.com) Another frequently occurring patient-based artifact is motion artifact. Motion artifacts occur due to voluntary and involuntary movements within the body of the patient. Voluntary movements include breathing and movements of various parts of the body. Involuntary movements are mainly cardiac motions. Respiratory movement artifacts are very common (Gormally et al, 2007, 699) and appear as curvilinear cold areas (Sureshbabu and Mawlaw, 2005, 157-158). Other movement artifacts like movement of hands and legs and other parts of the body are common in children, mentally challenged people and those in severe pain like the injured. Respiratory artifacts can be prevented by asking the patient to hold their breaths in mid- inspiration or mid- expiration phases (Sureshbabu and Mawlaw, 2005, 157-158). In children and others who cannot hold through breaths, final image is constructed from different images taken in different breathing cycles. Cardiac motion artifacts occur obviously in chest CTs because cardiac motion is involuntary and cannot be prevented. One strategy to prevent cardiac motion artifact is by incorporating ECG gating in CT scanners. The gating synchronizes data with cardiac rhythm, thus minimizing the artifact. Most recent scanners are equipped with this technology. Other techniques to prevent cardiac artifact is by pixel-specific reconstruction and introduction of repeated projections which remove movements of the heart in the images. Bowel movements can also result in attenuation defects and artifact appearance (Nakamoto et al, 2004, 221). Figure-3: Motion artifacts (www.googleimages.com) Physics-based artifacts These artifacts occur due to problems based on the physics of the objects. Common examples of physics based artifacts are photon starvation and beam hardening. Photon starvation artifacts occur because of deficient passage of photons through certain parts of the body like wide parts of the body and regions which high tissue volumes like heart and hip. Photon starvation artifacts also appear as streaks and they cause huge noise while imaging. These artifacts can be prevented or minimized by balancing the photon numbers detected by various detectors. Several techniques have been developed to balance photon intake. One such technique is to increase the photon flux while taking projections from wider parts of the body. Another useful technique is to filter projections by using appropriate filters and filter parameters (Yazdi and Beaulieu, 2008, 136- 137). Figure-4: Photon starvation artifacts (www.googleimages.com) Beam-hardening artifacts occur due to defective correction of beam hardening in CT imaging (Goldman, 2007, 222). These non-uniformities occur mostly while the beam is passing through thick bones or contrast media. Beam hardening artifacts can be prevented by viewing through large windows (Goldman, 2007, 222). Figure-5: Beam hardening artifacts (www.googleimages.com) Spiral-based artifacts These occur commonly in spiral CT imaging because of interpolation process of projections from individual slices. the severity and appearance of artifacts basically are dependent on the interpolation algorithm and scanning pitch (Yazdi and Beaulieu, 2008, 137- 138). Figure-6: Spiral-based artifacts (www.googleimages.com) Scanner-based artifacts These artifacts occur because of sensitivities of the detectors and mechanical instability. Truncation artifacts, partial volume artifacts and out-of-view artifacts fall under this category. Truncation artifacts occur mainly due to differences in the sizes of field of view between two different tomographs. Such artifacts are commonly seen either in very large patients or in those who are scanned with hands in down position. Hands-down position scanning may be necessary in patients with evaluation of melanoma and in those with indications for scanning in head and neck region. Hands down-position and large body makes the patient extend beyond the field of scanning and truncation of anatomy beyond the field of view occurs and this results in absent attenuation correction. careful position if hands of the patient and making the field of scanning extend beyond the body of the patient is necessary to avoid truncation artifacts (Goldman, 2007, 222). Partial volume artifacts occur during 360 degree axial scanning. During such scanning, the rays pass through the same anatomy twice, but in opposite directions. However, they may not pass though the same plane, because of dispersion effects. As a result of which, in certain areas like edge of the bone, dispersion can cause missing of the rays while traveling in opposite direction leading to streaking in images (Goldman, 2007, 222). Conclusion Artifacts are very common in CT imaging and can occur due to several reasons. based on the causes of artifacts, they can be divided into physics-based artifacts, patient-based artifacts, spiral-related artifacts and scanner based artifacts. Patient related artifacts like motion artifacts and metallic artifacts are very common and can be prevented by not allowing metal objects to be carried into the scan room, by taking images in mid expiration or mid-inspiration phases and several techniques and methods. Most of these artifacts appear as streaks and some others appear as shading and circles. It is very important to take measures to prevent artifacts because they can cause poor image quality and diminish the value of diagnosis. References Abdoli, M., Mohammed Reza Ay and Alireza Ahmadian. n.d. Metal Artifact Reduction in CT-Based Attenuation Correction of PET Data Using the Virtual Sinogram Concept. 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