The Artifacts Produced on the Images during CT Scans - Case Study Example

Introduction Artifacts, in computed tomography (CT), pertain to “any systematic discrepancy between the CT numbers in the reconstructed imageand the true attenuation coefficients of the object” (Barrett and Keat 2004). Artifacts compromise the quality of the CT image, and therefore, impair the diagnostic capability of the physician. CT artifacts include patient-based artifacts, physics-based artifacts, helical and multisection technique artifacts and scanner-based artifacts. This essay will briefly describe artifacts in CT and the methods to reduce or remove these artifacts. Type of artifacts The following are the types of artifacts that can occur in a CT scan: streaking, rings, distortion, and shading. Streaking is usually due to a single measurement inconsistency; rings are due to individual detector calibration errors; distortion is due to helical reconstruction, and shading, which is due to gradual deviation of a group of channels or views from the actual measurement (Barrett and Keat 2004). The causes for the origin of artifacts can be based on the following causes: physics-based artifacts, patient-based artifacts, helical and multisection artifacts, scanner-based artifacts and artifacts due to other causes. 1. Physics-based artifacts are related to the processes involved in CT data acquisition. a. Beam hardening is the preferential absorption of lower energy photons leaving only the higher energy photons during passage of photons through a dense object (Anschel, Romanelli, and Mazumdar 2007). It is seen as a dark area immediately near an object of high attenuation (Gay and Woodcock 2007). Beam hardening gives rise to two types of artifacts: cupping artifacts and dark bands or streaks (Barrett and Keat 2004) b. Partial volume occurs when adjacent structures of different attenuation are averaged to appear as one intermediate attenuation (Gay and Woodcock 2007.) c. Photon starvation-as the x-ray beam travels horizontally, a great deal of attenuation occurs and only few photons reach the detectors. The image, therefore, has horizontal streaks (Barrett and Keat 2004.) c. Undersampling is the presence of a large interval between projections. View aliasing is radiating fine stripes from a dense structure. Ray aliasing is stripes occurring close to the structure (Barrett and Keat 2004.) 2. Patient-based artifacts occur when the patient moves during the CT scan or if the patient has any metallic objects (removable or non removable) in their body. Metal objects cause streaking artifacts (Barrett and Keat 2004) because x-ray absorption results in incomplete projection profiles (Bushong 2000). Patient movement results in shading or streaking. One of the most prevalent artifacts in CT scanning is due to respiratory motion. This artifact occurs when there is a discrepancy between the chest position on the CT image. As a result, curvilinear cold areas can occur, and this is the most common type (Sureshbabu and Mawlawi, 2005). Streaking or shading artifacts can also result if any part of the patient lies outside the scanning field (incomplete projection) (Barrett and Keat 2004.) 3. Scanner-based artifacts are related to problems in the scanner. Ring artifacts result due to erroneous detector calibration on a third-generation scanner (Barrett and Keat 2004). Nonlinearities in the CT measurement system can also induce artifacts. Noise-streaks is prominent in low-signal regions (Issei and Masahiro 2003) 4. Helical and multisection CT artifacts are due to a rapid change in the anatomic structures in the z direction (Barrett and Keat 2004) 5. Artifacts due to other causes Contrast Media-iodine and barium sulfate are contrast agents, which are given intravenously or orally. These agents are given to patients in order to enhance CT images by delineating vessels and soft tissues. However, these agents, though they improves the CT image quality, can give rise to streaking artifacts due to photon absorption (Sureshbabu and Mawlawi, 2005) Truncation- truncation artifacts are avoided by scanning with the patient’s arms held above their head. However, truncation artifacts can occur if the scan is done with the arms held down or in large patients (Sureshbabu and Mawlawi, 2005) Pictures of artifacts Fig1. Streaking artifacts due to beam hardening From: Barrett, JF and N. Keat. 2004. Artifacts in CT: Recognition and avoidance. RadioGraphics; 24:1679–1691. Fig 2. Metal artifact in a patient with metal spine implants From: Barrett, JF and N. Keat. 2004. Artifacts in CT: Recognition and avoidance. RadioGraphics; 24:1679–1691. Fig 3. CT image of the head with motion artifacts From: Barrett, JF and N. Keat. 2004. Artifacts in CT: Recognition and avoidance. RadioGraphics; 24:1679–1691. Fig 4. Helical artifacts (arrow) From: Barrett, JF and N. Keat. 2004. Artifacts in CT: Recognition and avoidance. RadioGraphics; 24:1679–1691. Methods to resolve artifacts Hardening-in order to reduce beam hardening, various methods like correcting the calibration (by the use of phantoms in a range of sizes), filtration (flat piece of attenuating material is used to pre-harden the beam) and beam hardening correction software (use of correction algorithm) are used. The operator can also avoid beam hardening by proper positioning of the patient or by tilting the gantry, in order to avoid scanning of the bony regions (Barrett and Keat 2004). A head tilt technique has been described to decrease beam-hardening artifacts in patients having aneurysm clips (Brown et al. 1999) The use of a thin acquisition section width can avoid the development of partial volume artifacts. The use of automatic tube current modulation and adaptive filtration can minimize photon starvation. Obtaining the largest possible number of projections per rotation can minimize view aliasing, while ray aliasing can be reduced by quarter-detector shift or flying focal spot (specialized high-resolution techniques) (Barrett and Keat 2004). Metal artifacts can be avoided by asking the patient to remove any metals like jewelry before the start of scanning. For metals like prosthetic devices, surgical clips and dental fillings, which cannot be removed for the scanning, gantry angulation can be used. If the metal object cannot be excluded by any of the above technique, increasing the kilovoltage and the use of thin sections can reduce the partial volume artifact. The use of special software corrections can reduce streaking due to overranging (Barrett and Keat 2004.) Patient-based artifacts due to movement can be prevented by appropriate means. For most patients, the use of positioning aids can prevent any voluntary movement. However, in some patients, especially in the pediatric age-group, sedation may be required to immobilize them. In order to minimize artifacts in areas where natural movement is present, a short scan time can be used. The patient can be told to hold their breath in midexpiration or mid-inspiration during scanning in order to minimize any respiratory motion. (Barrett and Keat 2004; Sureshbabu and Mawlawi, 2005). The use of overscan and underscan modes, cardiac gating and software correction are some of the methods used by manufacturers to minimize motion artifacts. It is better to align the start and end position of the tube to the primary direction of motion (Barrett and Keat 2004.) By positioning the patient in such a manner that no body parts lie beyond the scan field, it is possible to prevent artifacts due to incomplete projections. Ring artifacts can be prevented by using software that characterizes and corrects detector variations (Barrett and Keat 2004). A post-processing method has been described to reduce ring artifacts (Kyriakou, Prell, and Kalender 2009.) Helical artifacts can be minimized by using a low pitch, a 180° helical interpolator and thin acquisition sections. All this reduce the effects of variation along the z axis, and thus reduce helical artifacts (Barrett and Keat 2004.) In order to avoid artifacts due to contrast agents, it is preferable not to schedule a patient for a CT scan if they have undergone a scan with oral contrast agent on the previous day. Truncation artifacts can be avoided by positioning the patient at the center of the field of view and with their arms above the head (Sureshbabu and Mawlawi, 2005). Conclusion The types of artifacts that can occur in a CT scan include streaking, rings, distortion, and shading. CT artifacts can be classified as patient-based artifacts, physics-based artifacts, helical and multisection technique artifacts and scanner-based artifacts. Physics-based artifacts are related to the processes involved in CT data acquisition, and include: beam hardening, partial volume, photon starvation, and undersampling. Patient-based artifacts occur when the patient moves during the CT scan or if the patient has any metallic objects in their body. Scanner-based artifacts are related to problems in the scanner, and include ring artifacts. Helical and multisection CT artifacts are due to a rapid change in the anatomic structures in the z direction. There are various methods to reduce or prevent artifacts in CT imaging like: correcting the calibration, filtration, use of a thin acquisition section, removal of jewelry from the patient, making the patient to hold their breath during scanning etc. Artifacts in a CT scan can be due to various causes and can impair the image quality of a CT scan. While most modern scanners are designed in such a manner to prevent or avoid artifacts, it is also important for the operator to practice proper techniques like patient positioning and the optimum selection of scan parameters so that CT artifacts can be avoided. Reference Anschel, DJ, Romanelli, P and A. Mazumdar. 2007. Clinical Neuroimaging. McGraw-Hill Barrett, JF and N. Keat. 2004. Artifacts in CT: Recognition and avoidance. 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