CT Image Artifact PDF

Artifacts in CT Mohd Hafizi Mahmud Centre for Medical Imaging Studies Faculty of Health Sciences Universiti Teknologi MARA Puncak Alam Campus CT Physics and Instrumentation MRD530 ...

Artifacts in CT Mohd Hafizi Mahmud Centre for Medical Imaging Studies Faculty of Health Sciences Universiti Teknologi MARA Puncak Alam Campus CT Physics and Instrumentation MRD530  Learning Outcomes  Define artifact in CT  Identify the different types of CT artifact  Determine the appropriate steps to reduce the artifact on CT images. Definition: Image Artifacts  Anything appearing on the image that is not present in the object scanned  “Any discrepancy between the CT numbers represented in the image and the expected CT number based on the linear attenuation coefficient”  Have many different presentations and causes  They can be broadly classified (sources) as (causes):  Patient-based  Equipment-induced  Physics-based (eq: radiation)  Appearance of Artifacts 1. STREAKS:  intense straight lines, bright or dark enhanced by recon filter, map by the back projection process.  Caused to unreadable or unreliable images due to inconsistencises during data acquisition resulting false image production during reconstruction 2. SHADING: near high contrast object, within softer tissue region - CT number shifted which can result to misdiagnosed. 3. BANDS: Layering effect, superimposed with original images 4. RINGS: Ring formation on image, superimposed. Artifact Patient induced artifact Patient Induced Artifact 1. Motion 2. Beam hardening 3. Metal artifact 4. Out of field artifact 5. Edge gradient artifact Cause of Motion Artifact  Voluntary motion  Involuntary motion  Produces “ghosting” effect object  CT image appears– as if composed of superimposed images Motion Artifacts  Artifacts from patient motion appear as shading, streaking, blurring, or ghosting Motion Artifacts Solutions  Voluntary motion: explanation of the procedure and good communication with a patient  Involuntary motion: short scan time  Beam-Hardening Artifacts  Caused by the polychromatic nature of the x- ray beam used in CT  As an x-ray beam passes through an object, lower-energy photons are preferentially absorbed, creating a “harder” beam that cannot be adjusted for by the system  CT systems minimize beam hardening in three ways  Filtration (common)  Calibration correction  Beam-hardening correction software  Beam-Hardening Artifacts  Manifest as cupping artifacts or as dark bands or streaks between dense objects in the image  Common Areas of Manifestation  Skull – petrous pyramids  Upper chest and shoulders  Hips  Solution  Increase kVp  Decrease slice thickness  Increase filtration – bowtie filter  Metal Artifact  Manifest itself as “star streaking” artifact.  Metal objects in the SFOV will create streak artifacts  Metallic object absorbs the photons causing an incomplete profile  Best reduced by removing the metal Metal Artifact Solution  Removal of external metallic objects  MAR software (Specific software)  Gantry angulation Out of Field Artifact  Patient is not entirely enclosed in the scanning field of view.  Patients body can obstruct detectors  The patient’s soft tissue outside the SFOV will further harden the x-ray beam.  Artifact appears as streaks and shading Out of Field Artifact Solution  Selection of larger SFOV (concept of umbra penumbra)  Taping patient tissue (Compress technique)  Raising patients arms above their head on the scan of chest and abdomen  Edge Gradient Artifacts  Results in streak artifact or shading arising from irregularly shaped objects that have a significant difference in density from surrounding structures (eq: barium and air lie adjacent to each other)  Largely unavoidable, but can be reduced by thinner slices or using a low HU oral contrast in place of barium or water (neutral CM) Artifact 2. Equipment artifact Equipment and Imaging Process Artifacts 1. Partial volume effect 2. Aliasing 3. Photon starvation (noise) 4. Ring 5. Tube arcing 6. Line in topogram 7. Staircase 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. (when more than one type of tissue in scanning region)  Only seen in some views, create shading Solution  Thinner slice selection and incrementation  Aliasing Insufficient data result to undersampling (eq: helical pitch is greater/extended) – too large in interval Result to inaccuracies – sharp edge and small line Fine line on an image (experiment) Solution Increase scan time by: Reduce pitch Slow gantry rotation Use complete arc scan Photon Starvation : Noise  Occur at highly attenuating area - so insufficient of photon reach detector (eq: at shoulder)  Cause horizontal streak artifact Solution  kVp increase  mAs increase  Slice thickness increase Ring  Malfunction of a detector in a third generation scanner.  A detector row not able to collect data, result to loss information.  Faulty or miscalibration Solution  Detector calibration  Detector replacement Tube Arcing  Tungsten vapor from anode and cathode intercepts the projectile electrons intended for collisions with the target. (common term = short circuit)  May induce to crackling sound.  electrical activity within the x-ray tube  Also called high-voltage arcing  No specific pattern in their appearance  Can be minor or severe  Typically produce an error message  Require a service call Solution  Gas burn-off  Tube replacement Line in Topogram  Bad detector causes continuous line on the topogram Solution  Detector replacement Staircase  Improper selection of slice thickness and slice incrementation when generating the MPR and 3D images.  Increase the slice thickness selected, result to higher staircase artifact Solution  Thinner slice use.  50% overlap on recon slice incrementation.