Module I Computed Tomography Physics, Instrumentation, and Imaging

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CT Artifacts and Quality control Seeram’s definition: “a distortion or error in (on) an image that is unrelated to the subject being studied”. Seeram’s definition: “a distortion or error in (on) an image that is unrelated to the subject being studied”. CT Artifacts include: CT Artifacts include: Foreign objects Foreign objects Breathing motion Breathing motion Peristalsis Peristalsis Cardiac motion Cardiac motion Inaccurate Hounsfield numbers Inaccurate Hounsfield numbers Equipment artifacts (electronics and mechanics or algorithms) Equipment artifacts (electronics and mechanics or algorithms) Production artifacts Production artifacts

Data Correction Processes Calibration procedures Calibration procedures Pre and post patient operations Pre and post patient operations …………have significantly reduced the occurrence of artifacts related to the CT imaging process. …………have significantly reduced the occurrence of artifacts related to the CT imaging process.

CT Artifacts Classification is related to the cause and appearance of the artifact. Classification is related to the cause and appearance of the artifact. Streaks Streaks Shading Shading Rings Rings Bands Bands cone cone

Streak Artifacts Most common classification of artifacts manifested in CT. Most common classification of artifacts manifested in CT. Causes: Causes: Improper sampling of data because of beam hardening Improper sampling of data because of beam hardening Edge-gradient effect Edge-gradient effect Equipment malfunction Equipment malfunction Metal objects Metal objects Partial volume averaging Partial volume averaging Patient motion Patient motion

Beam Hardening Occurs when there is an increase in the mean energy of the x-ray beam as it attenuates through the patient. Occurs when there is an increase in the mean energy of the x-ray beam as it attenuates through the patient. Low energy photons are absorbed, leaving high energy photons striking the detectors. Low energy photons are absorbed, leaving high energy photons striking the detectors. Also called “cupping” Also called “cupping”

Image taken from Seeram’s text page 192

Edge-gradient Artifacts CT system is unable to transfer information from the area of interest accurately due to large differences in atomic numbers of adjacent structures. CT system is unable to transfer information from the area of interest accurately due to large differences in atomic numbers of adjacent structures. ie: barium / air ie: barium / air Appear as straight thin black streaks Appear as straight thin black streaks

Equipment-induced artifacts Poor gantry rigidity Poor gantry rigidity Mechanical misalignment Mechanical misalignment Radiographic tube rotor wobble Radiographic tube rotor wobble Inadequate sampling of the detector signals Inadequate sampling of the detector signals Corrupt detectors can produce: Corrupt detectors can produce: Streaks Streaks Ring artifacts Ring artifacts

Streak Artifacts Metal inside scan area Metal inside scan area In some instances the gantry can be tilted to avoid streak artifacts which can not be removed. In some instances the gantry can be tilted to avoid streak artifacts which can not be removed. MAR – (Metal Artifact Reduction) software program which assists in the reconstruction through interpolation. MAR – (Metal Artifact Reduction) software program which assists in the reconstruction through interpolation.

MAR – (Metal Artifact Reduction) The raw data is acquired and stored The raw data is acquired and stored The CT image is reconstructed The CT image is reconstructed Rough tracing of the prosthesis (metal implant) with a light pen is performed Rough tracing of the prosthesis (metal implant) with a light pen is performed Automatic definition of the borders of the prosthesis within the projection data is performed. Automatic definition of the borders of the prosthesis within the projection data is performed. Using ROI threshold values Using ROI threshold values Linear Interpolation is performed Linear Interpolation is performed Reconstruction of the artifact reduced image from the new computed projection data is accomplished Reconstruction of the artifact reduced image from the new computed projection data is accomplished

Partial Volume Averaging Occurs when a voxel is occupied by more than one type of tissue. Occurs when a voxel is occupied by more than one type of tissue. The different tissues therefore can cause the area to be averaged. The different tissues therefore can cause the area to be averaged. Partial volume averaging can cause partial volume artifacts (bands and streaks) Partial volume averaging can cause partial volume artifacts (bands and streaks) –Can be reduced by selecting thinner slices and using (VAR) volume artifact reduction technique software.

VAR Works by taking the different types of tissue within a voxel and dividing the tissues into thinner slices with a table incrimination between each. Works by taking the different types of tissue within a voxel and dividing the tissues into thinner slices with a table incrimination between each. Data sets of the raw data are averaged to produce a composite thicker slice that will be free of the partial volume artifact. Data sets of the raw data are averaged to produce a composite thicker slice that will be free of the partial volume artifact. Illustration and more information on beam hardening and partial volume artifact can be found on pages in the Seeram’s 2 nd edition text. Illustration and more information on beam hardening and partial volume artifact can be found on pages in the Seeram’s 2 nd edition text.

Patient Motion Voluntary and Involuntary Voluntary and Involuntary –Use positioning aids such as pillows, sponges, myelogram straps. –Make your patient comfortable before beginning the exam. –Shorter scan times –Explain the exam to the patient –Motion software –Illustration for MAC (Motion Artifact Correction) can be found on page 189, Figure 11.22, Seeram’s 2 nd edition text.

Sampling Errors Sampling errors cause streak artifacts. Sampling errors cause streak artifacts. Sampling errors are called Aliasing. Sampling errors are called Aliasing. Nyquist Theorem Sampling must occur at least twice the spatial frequency of the object scanned. Nyquist Theorem Sampling must occur at least twice the spatial frequency of the object scanned.

Shading –Can be the result of: Partial volume averaging Partial volume averaging Beam hardening Beam hardening Spiral scanning Spiral scanning Scatter radiation Scatter radiation Off focal radiation Off focal radiation Incomplete projection profiles Incomplete projection profiles

Cone Artifact –Results when interpolation is not successful. –Image appears elliptical instead of round –The cone artifact can represent as two crescent-shaped bands of increased density

Figure 2b Medicamundi, Volume 43, Issue 1, March 1999

Figure 2c Medicamundi, Volume 43, Issue 1, March 1999

Noise Produced from an insufficient number of x-ray photons striking the detectors. Produced from an insufficient number of x-ray photons striking the detectors. Can appear as graininess or streaks Can appear as graininess or streaks Can be caused from: Can be caused from: Poor centering of the patient within the gantry Poor centering of the patient within the gantry Incorrect selection of exposure factors Incorrect selection of exposure factors Aperture size Aperture size

Quality Control Tests Daily, Monthly, or yearly depending on the type of test. Daily, Monthly, or yearly depending on the type of test. Must be done to produce high quality diagnostic scans. Must be done to produce high quality diagnostic scans. The scanner must be performing at optimal levels to insure the accuracy of the images and Hounsfield numbers. The scanner must be performing at optimal levels to insure the accuracy of the images and Hounsfield numbers. Some QC tests are done by the technologist and some are performed by engineers during PM (preventive maintenance). Some QC tests are done by the technologist and some are performed by engineers during PM (preventive maintenance).

3 Basic Components of Quality Control 1. Quality control must be performed on a regular, periodic basis. 2. Prompt interpretation of QC test measurements. 3. Faithful and meticulous record keeping.

Daily QC Tests 1. Average CT number in water 2. Standard deviation of CT numbers in water

CT Testing Phantoms CT scanners should never be sold without providing the QC phantoms for regular quality control measures. CT scanners should never be sold without providing the QC phantoms for regular quality control measures. FDA – Department of Public Health – Office of Radiation Control FDA – Department of Public Health – Office of Radiation Control