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50 Cards in this Set
- Front
- Back
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•image manipulation definition
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•other terms for gray level mapping
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Point Processes
windowing results in changes to image brightness and enhancement of displayed image |
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•windowing
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point processing
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•WW and WL
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WW = number of shades of grey (controls image contrast)
WL = midpoint of shades of grey (controls image brightness) |
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•MPR
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multi planar reformation
involved use of comp program to reformat sagittal paraxial control or curve views from a stack of continuous transvers axial images DFOV must be the same slice thickness must be the same |
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•quantitative CT
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most sensitive of all x-ray techniques for the measurement of the mineral content in trabecular bone mineral density
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•xenon CT
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•RTP
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•basic visualization tools
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basic computer programs integrated into the CT system
windowing magnification referencing for biopsies (superimposing coordinates) geometric characteristic eval (distances and angles) highlighting CT histograms ROI stats ROI transfer Split imaging |
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•advanced visualization tools
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require powerful work stations with advanced image processing capabilities and increased memory
3D visualization (surface rendering, MIP's, Volume rendering) CT angiography with 3D 4D CTA (with bone and ST structures) Vessel tracking MPR |
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•advanced workstations for CT
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computer workstations with advanced image processing capabilities and increased memory to handle the vast amount of data used in various visualization techniques
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•connectivity
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PACS, RIS & HIS
filmless system the transfer of data and images to and from CT scanner and workstations DICOM |
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Local Operations
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aka ‘area processes” or ”group processes” because a group of pixels is used in transformation calculation. One type of this
process is ‘spatial frequency filtering’ which can alter image brightness, sharpness, smoothness, blurring, noise reduction and edge enhancement. (algorithimsor reconstruction) One type of spatial location filter is Convolution |
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Spatial frequency processing
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uses the High Pass Filtering aka edge enhancement, sharpens the input image in the spatial domain that
appears blurred |
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Global Operations
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uses the entire input image to calculate pixel value of the output image (Fourier domain processing) to provide edge
enhancement, sharpening and restoration |
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rendering
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aka image reformatting
computer display technique- based approach requires specific hardware and software to deal with millions of points identified in 3D space |
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3D image processing
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a conceptual framework for generating reformatted images
4 major steps |
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wide window
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higher # of grey shades represented
reduced image contrast reduce image noise |
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narrow window
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lower number of grey shades represented
increased image contrast increased image noise |
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Low window level
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bright image
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high window level
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dark image
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multi planar reconstruction criteria
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contiguous transaxial slices (no slice spaces)
DFOV must be the same must be same angle same slice thickness |
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MP reformat
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using raw data
axial, sagittal, coronal |
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MP recon
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changing one of the criteria
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3D image processing
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image synthesis
using image data or raw data separate work station time consuming |
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Perfusion scanning
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same slice scanned over and over
visualises arterial and venous blood shows area being effected by loss of blood flow (function is lost and part of brain can not be saved) penumbra around effected spot is lower blood flow but is able to be operated on and function can be saved |
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Basic Visualization tools
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done at CT workstation
multiview image magnification superimposing coordinates highlighting ROI statistics/transfer windowing measurments |
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advanced workstation
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done at high power dedicated workstation
3D applications vascular 3D for CTA's curved MPR's quantitative measurements multimodality image fusion (PET CT, intra-op surgical assistance) virtual reality visualization tools (virtual colonoscopy) |
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DICOM
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allows for direct digital transfer of digital CT images, which preserves full spatial resolution and image manipulations capabilities
Digital Imaging and Communication in Medicine CT and MRI workstations must be DICOM compliant |
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archiving
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sending images to be stored
online, nearline or offline |
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online archiving
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recent
easy to access |
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nearline archiving
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transferred to "Grid"
takes longer to access |
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offline archiving
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film library
images can get lost |
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Steps of 3D CT image formation
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1 Data acquisition
2 creation of 3D space or scene space (voxel info from sectional images are stored in computer) 3 Processing of 3D image display (function of the workstation) 4 3D image display (simulated 3D image is displayed on 2D screen) |
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Artifacts
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"a distortion of error in an image that us unrelated to the subject being studied"
degrade image quality affect the perceptibility of detail can lead to misdiagnosis |
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Factors affecting Ct resolution
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focal spot size and shape
detector cell size scanner geometry sampling frequency |
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MTF
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Modulation Transfer Function
special frequency is plotted as a function of the image fidelity - a smooth curve is obtained can be used to compare performance of different CT systems ideal MTF curve = FLAT (unity response independent of frequency) |
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Cross- Plane Spatial Resolution
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described by slice sensitivity profile (SSP)
SSP represents the systems response to a Dirac data function |
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Temporal Resolution
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an indication of a CT system's ability to freeze motions of the scanned object
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Ct noise sources
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scanning techniques (voltage, current, slice thickness, scan speed, helical pitch)
scanner efficiency (DQE, detector geometrical efficiency, amber-penumbra ratio patient (pt, size, amount of bone and St in scanning plane) |
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common Artifacts
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Patient Motion Artifacts
Metal Artifacts Beam-Hardening Artifacts Partial Volume Artifacts Aliasing Artifacts Noise-Induced Artifacts Scatter Cone-Beam Artifacts |
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Patient Motion Artifacts
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from pt movement (voluntary or involuntary)
voluntary motion controlled by patient (swallowing or breathing) shorten scan time will reduce temporal resolution |
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Metal Artifacts
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from metal in pt
produce streak artifacts reduced by removal of all removable metal metal artifact reduction (MAR) |
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MAR
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Metal Artifact Reconstruction
1 data acquisition 2 reconstruction of CT image 3 identification of implant 4 automatic definition of the boundaries of the implant within the projection data 5 Linear interpolation of missing projection data 6 reconstruction of the artifact-reduced image from the newly computed projection |
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Beam Hardening Artifact
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increase in the mean energy of x-ray beam as it passes through the patient
can be improved by using more homogeneous beam and/or higher energy x-ray photons |
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Partial Volume Artifacts
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based on linear attenuation coefficients for a voxel of tissue
problem if one voxel contains different tissue types (partial volume averaging) can be reduced with thinner slice acquisitions and computer algorithms can happen even with thin slice acquisitions |
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Aliasing Artifacts
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averaging
to represent a continuous signal the sampling frequency must be at least twice the highest frequency signal content in the signal |
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Noise-Induced Artifacts
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partially by the number of photons that strike the detector
can be affected by poor pt positioning, poor selection of techniques (kV & mAs), scan speed, limitations of the CT scanner (max tube power) |
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Scatter Artifacts
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associated with the interaction between x-ray photons and matter
controlled by placing collimator in front of detector can be corrected with algorithms |
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Cone-Beam Artifacts
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caused by incomplete or insufficient projection samples as a result of the cone beam geometry in multi slice CT
corrected with algorithms (FDK- Feldkamp) |
