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) ELIMINATES TISSUE SUPERIMPOSITION INCREASES CONTRAST OF LOW SUBJECT CONTRAST TISSUES TOMOGRAPHY TOMOGRAPHY TOMOGRAPHY CT NUMBER LINEAR ATTENUATION COEFFICIENT ( cm-1) BONE BLOOD G MATTER W MATTER CSF WATER FAT AIR 0.528 0.208 0.212 0.213 0.207 0.206 0.185 0.0004 CT # vs BRIGHTNESS LEVEL + 1000 -1000 CT # 1000 CT # - 500 CT # OF CYST CT # OF LIPOMA ( FATTY TUMOR) -100 W 120 L 40 W 80 L 40 DATA FLOW IN CT REFERENCE DETECTOR REFERENCE DETECTOR PREPROCESSOR ADC COMPUTER RAW DATA PROCESSORS RECONSTRUCTED DATA DISK TAPE BACK PROJECTOR CONVOLVED DATA DAC CRT DISPLAY CT ADVANTAGES AND DISADVANTAGES EXCELLENT LOW CONTRAST RESOLUTION WINDOWING- IMAGE MANIPULATION TAILORED TO OBSERVER NEEDS SPIRAL CT-SINGLE BREATH HOLD STUDIES ( CTA, MPR, VIRTUAL REALITY CT, CT ENDOSCOPY) CT ASISST IN RADIATION THERAPY BONE SCAN PACKAGE XENON CT PERFUCION CT DIGITAL PROCESSING ABILITY ... ISOCENTER SCAN FOV SFOV DETECTORS SCAN FOV-SMALL SFOV DETECTORS SFOV - HEAD TOO SMALL OF SFOV – OUT OF FIELD ARTIFACT SCAN FOV-RESOLUTION SFOV RECONSTRUCTION Ц RECONSTRUCTION CT# SCAN FOV-RESOLUTION... TISSUES TOMOGRAPHY TOMOGRAPHY TOMOGRAPHY CT ADVANTAGES LIMITATIONS OF CT UNABLE TO DIFFERENTIATE BETWEEN TISSUES WITH SLIGHT CONTRAST DIFFERENCES < 1% GOALS OF CT MINIMAL SUPERIMPOSITION IMAGE... DIFFERENCE RECORDING CT DATA AQUISITION SLIP RINGS SEQUENTIAL-SLICE BY SLICE SCANNING SCANNING TRANSMISSION RELATIVE TRANSMISSION=Io/I Total # of trans measurements= # of views X # of rays in each