Image Quality - Computed Tomography - Lecture Slides, Slides of Computed Tomography

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Image Quality

CT Image Quality Parameters

Spatial Resolution

Image Noise

Contrast Resolution

Artifacts

Spatial Resolution

 Quantifies image blurring

 “Ability to discriminate objects of varying density a

small distance apart against a uniform

background”

 Minimum separation required between two high

contrast objects for them to be resolved as two

objects

Spatial Resolution

Resolvable Object Size:

Limiting Resolution

 Smallest resolvable high contrast object is half the

reciprocal of spatial frequency

 Example:

 Limited resolution = 15 line pairs per cm  Pair is 1/15th^ cm  Object is half of pair  1/15th^ / 2  1/30th^ cm  .033 cm  0.33 mm

1/15th^ cm

1/30th^ cm

Geometric Factors affecting

Spatial Resolution

Focal spot size detector aperture width slice thickness or collimation  Less variation likely for thinner slices  attenuation variations within a voxel are averaged  partial volume effect

Geometric Unsharpness & CT

Decreased spatial

resolution if object blurred over several detectors

Detector aperture size

 must be < object for object to be resolved

Focal Spot

Detectors

Small Object to be Imaged

Non-geometric Factors

affecting Spatial Resolution

 # of projections

 Display matrix size

 512 X 512 pixels standard

 Reconstruction algorithms

 smoothing or enhancing of edges

Hi-Resolution CT Technique

 Very small slice thicknesses

 1-2 mm

 High spatial frequency algorithms

 increases resolution  increases noise  Noise can be offset by using higher doses

 Optimized window / level settings

 Small field of view (FOV)

 Known as “targeting”

Contrast Resolution

 Ability of an imaging system to demonstrate small

changes in tissue contrast

 The difference in contrast necessary to resolve 2

large areas in image as separate structures

CT Contrast Resolution Depends

Upon

 reconstruction algorithm

 low spatial frequency algorithm smooths image  Loss of spatial resolution  Reduces noise  enhances perceptibility of low contrast lesions

 image display

CT Contrast Resolution Depends on Noise

# of Photons Detected

Depends Upon

 photon flux (x-ray technique)

 slice thickness

 patient size

 Detector efficiency

 Note:

 Good contrast resolution requires that detector sensitivity be capable of discriminating small differences in intensity