Post on 15-Jan-2016
IMAGE QUALITY
SPATIAL RESOLUTION CONTRAST RESOLUTION
NOISE
IMAGE ARTIFACTS
RADIATIONDOSE
Resolution in CT
Spatial Resolution Contrast Resolution
SPATIAL RESOLUTION
• DEGREE OF BLURRING IN THE IMAGE
MEASURE OF THE ABILITY TO DISCRIMINATE OBJECTS OF VARYING DENSITY A SMALL DISTANCE APART
AGAINST A UNIFORM BACKGROUND
MTFMODULATION TRANSFER
FUNCTION
MOST COMMONLY USED TO DESCRIBE SPATIAL RESOLUTION
IN CT
MTF
• MTF OF 1 – 100% TRANSFER OF OBJECT TO IMAGE
• MTF OF 0 – 0% TRANSFER OF OBJECT TO IMAGE
HIGH CONTRAST PHANTOM
FACTORS AFFECTING SPATIAL RESOLUTION
• SLICE THICKNESS
• FOCAL SPOT SIZE
• DETECTOR WIDTH
• TYPE OF CONVOLUTION FILTER (KERNEL)
• MATRIX SIZE
• FOV
SLICE THICKNESS
SLICE THICKNESS S. RESOLUTION
5 MM 1 MM
FOCAL SPOT SIZE
FOCAL SPOT SIZE S. RESOLUTION
LARGE FOCAL SPOT = LARGE PENUMBRA (BLURR)
DETECTOR WIDTH
DETECTOR WIDTH S. RESOLUTION
TYPE OF CONVOLUTION FILTER
SHARP KERNEL S. RESOLUTION
SMOOTH SHARP
MATRIX SIZE
MATRIX SIZE S. RESOLUTION
80 X 80 512 X 512
DFOV SIZE
DFOV S. RESOLUTION
20CM 10CM
CONTRAST RESOLUTION
• SENSITIVITY OF THE SYSTEM
• LOW CONTRAST RESOLUTION
• LOW CONTRAST DETECTIBILITY
LOW CONTRAST RESOLUTION
( TISSUE RESOLUTION)
• THE ABILITY OF AN IMAGING SYSTEM TO DEMONSTRATE SMALL CHANGES IN TISSUE CONTRAST
CONTRAST RESOLUTION
• SENSITIVITY OF THE SYSTEM
• LOW CONTRAST RESOLUTION
• LOW CONTRAST DETECTIBILITY
CONTRAST RESOLUTION PHANTOM
FACTORS AFFECTING LOW CONTRAST RESOLUTION
• PHOTON FLUX
• SLICE THICKNESS
• PATIENT SIZE
• DETECTOR SENSITIVITY
• RECONSTRUCTION ALGORITHM
• IMAGE DISPLAY
• QUANTUM NOISE
PHOTON FLUX (INTENSITY)
PHOTON FLUX C. RESOLUTION
mAs P. FLUXkVp P. FLUX
LOW TECHNIQUE ADEQUATE TECHNIQUE
SLICE THICKNESS
SLICE THICKNESS C. RESOLUTION
8 mm 5 mm
10 mm – 100 PHOTONS 5 mm – 50 PHOTONS
PATIENT SIZE
PATIENT SIZE C. RESOLUTION
NORMAL LARGE
DETECTOR SENSITIVITY
DETECTOR SENSITIVITY C. RESOLUTION
TYPE OF CONVOLUTION FILTER
SMOOTH KERNEL C. RESOLUTION
SMOOTH SHARP
IMAGE DISPLAY
MONITOR RESOLUTION C. RESOLUTION
QUANTUM NOISE
QUANTUM NOISE C. RESOLUTION
HIGH NOISE LOW NOISE
NOISE IN CT
• FLUCTUATION OF CT# BETWEEN POINTS IN THE IMAGE FOR A SCAN OF UNIFORM MATERIAL SUCH WATER.
NOISE SOURCES IN CT
• QUANTUM NOISE – X-RAY INTENSITY AT THE DETECTORS
• MATRIX SIZE
• DFOV
• SLICE THICKNESS
• CONVOLUTION FILTER TYPE
• PITCH
QUANTUM NOISE – CONTROLLED BY THE
TECHNOLOGIST
• mA
• Time
• kVp
NOISE
NOISE
• QUANTUM (STATISTICAL) –CONTROLLED BY THE TECHNOLOGIST
• ELECTRONIC – INHERENT IN THE EQUIPMENT
QUANTUM NOISE – CONTROLLED BY THE
TECHNOLOGIST
• DFOV
• MATRIX
• COLLIMATION
• FILTER SPATIAL FREQUENCY
• PITCH
NOISE
DOSE vs
IMAGE QUALITY
• RADIATION DOSE IMAGE QUALITY