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PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY. RADIOGRAPHY LIMITATIONS. SUPERIMPOSITION DIFFICULTY IN DISTINGUISHING BETWEEN HOMOGENOUS OBJECTS OF NON-UNIFORM THICKNESS. SUPERIMPOSITION. TISUE DIFFERENCE SENSITIVITY 5%-10%. TOMOGRAPHY ( CONVENTIONAL).
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RADIOGRAPHY LIMITATIONS • SUPERIMPOSITION • DIFFICULTY IN DISTINGUISHING BETWEEN HOMOGENOUS OBJECTS OF NON-UNIFORM THICKNESS.
TOMOGRAPHY ( CONVENTIONAL) • ELIMINATES TISSUE SUPERIMPOSITION • INCREASES CONTRAST OF LOW SUBJECT CONTRAST TISSUES
LIMITATIONS OF CT • UNABLE TO DIFFERENTIATE BETWEEN TISSUES WITH SLIGHT CONTRAST DIFFERENCES < 1%.
GOALS OF CT • MINIMAL SUPERIMPOSITION • IMAGE CONTRAST IMPROVEMENT • SMALL TISSUE DIFFERENCE RECORDING
TRANSMISSION RELATIVE TRANSMISSION=Io/I
Total # of trans. measurements=# of views X # of rays in each view
DATA AQUSITION GEOMETRIES • CONTINUOUS • STATIONARY
CT 120-140 KVP • REDUCED DEPENDENCY ON ATTENUATION COEFFICIENT • REDUCED CONTRAST • INCREASED PHOTON FLUX
ORIGINAL CLINICAL CT SCANS COMPOSED OF 80 X 80 MATRIX PIXELS 6400
SCAN FOV SFOV DETECTORS
SCAN FOV-SMALL SFOV DETECTORS
SCAN FOV-RESOLUTION SFOV
RECONSTRUCTION RECONSTRUCTION
SCAN FOV-RESOLUTION SFOV
DISPLAY FOV vs SCANNING FOV • DFOV CAN BE EQUAL OR LESS OF SFOV • SFOV – AREA OF MEASUREMENT DURING SCAN • DFOV - DISPLAYED IMAGE
PIXEL SIZE PIXEL SIZE= FOV (mm)/ MATRIX SIZE
EXAMPLE: • FOV= 40 CM= 40 X 10 MM=400 mm • MATRIX= 512 X 512 = 5122 400/512 = 0.78 mm 0.8 mm
EACH PIXEL IN CT HAS RANGE OF GRAY SHADES • 2 8 = 256 SHADES • 2 12 = 4096 SHADES = -100 TO 3095 SHADES OF GRAY
PIXEL vs VOXEL PIXEL VOXEL
PIXEL SIZE DEPENDS ON: • MATRIX SIZE • FOV