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PRINCIPLES OF CT
TOMOGRAPHY
TOMOS---SECTION
TOMOGRAPHY
TOMOS---SECTION
RADIOGRAPHY LIMITATIONS
• SUPERIMPOSITION
• DIFFICULTY IN DISTINGUISHING BETWEEN HOMOGENOUS OBJECTS OF NON-UNIFORM THICKNESS.
SUPERIMPOSITION
RADIOGRAPHY LIMITATIONTISUE
DIFFERENCE SENSITIVITY >10%
TOMOGRAPHY (CONVENTIONAL)
• ELIMINATES TISSUE SUPERIMPOSITION
• INCREASES CONTRAST OF LOW SUBJECT CONTRAST TISSUES
TOMOGRAPHY
TOMOGRAPHY
TOMOGRAPHY LIMITATIONS
MOTION BLURR
CT ADVANTAGES
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
CT DATA AQUISITION
TRANSMISSION
RELATIVE TRANSMISSION=Io/I
HISTORY OF CT
Sir Godfrey Newbold Hounsfield CBE (28 August 1919 – 12 August 2004) was an English electrical engineer who shared the 1979 Nobel Prize for Physiology or Medicine with Allan McLeod Cormack for his part in developing the diagnostic technique of X-ray computed tomography (CT).
HOUNSFIELD’S SKETCH
CONSTRUCTION OF FIRST CT
• RADIATION SOURCE – AMERICUM GAMMA SOURCE
• SCAN—9 DAYS• COMPUTER PROCESSING—2.5 HOURS• PICTURE PRODUCTION 1 DAY
HOUNSFIELD’S LATHE BED SCANNER
1972
FIRST CLINICAL PROTOTYPE CT BRAIN SCANNER
1. FIRST SCANS—20 MIN.
2. LATER REDUCED TO 4.5 MIN.
CLINICALLY USEFUL CT SCANNER
1974
DR. ROBERT LEDLEY DEVELOPED THE FIRST WHOLE
BODY CT SCANNER .
SCANNER GENERATIONS
• I
• II
• III
• IV
180 DEG ROTATION
180 DEG ROTATION
360 DEG ROTATION
360 DEG ROTATION
MODERN SCANNER
CT MAIN SYSTEMS
• IMAGING SYSTEM
• COMPUTER SYSTEM
• DISPLAY, RECORDING, STORAGE SYSTEM
• DATA ACQUISITION SYSTEM
IMAGING SYSTEM
• PRODUCTION OF X-RAYS
• SHAPING OF X-RAY BEAM ENERGY
• FILTERING X-RAY BEAM
SCANNER
GANTRY
TABLE/COUCH
GANTRY INSIDE
COMPUTER SYSTEM
• RECONSTRUCTION AND POSTPROCESSING
• CONTROL OF ALL SCANNER COMPONENTS
• CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY.
• DATA FLOW DIRECTION
COMPUTER SYSTEM IN CT
• MINICOMPUTERS
IMAGE DISPLAY, RECORDING, STORAGE
• DISPLAYS IMAGE ( OUTPUT FROM COMPUTER)
• PROVIDES HARD COPY OF THE IMAGE
• FACILITATES THE STORAGE AND RETRIEVAL OF DIGITAL DATA
• COMMUNICATES IMAGES IN THE NETWORK
DATA ACQUISITION SYSTEM (DAS)
• SET OF ELECTRONICS BETWEEN DETECTORS AND HOST COMPUTER.
CT COMPONENTS
GANTRY
TABLE/COUCHCOMPUTER
CONSOLE
ORIGINAL CLINICAL CT SCANS COMPOSED OF
80 X 80 MATRIX
PIXELS
6400
EARLY DAYS vs TODAY80 x 80 512 x 512
COORDINATE SYSTEM IN CT
X
COORDINATE SYSTEM IN CT
Y
COORDINATE SYSTEM IN CT
Z
COORDINATE SYSTEM IN CT
ISO-CENTER
SCAN FOV
SFOV
DETECTORS
DFOV – DISPLAYED FIELD OF VIEW
• SIZE DISPLAYED ON THE MONITOR
PIXEL SIZE
PIXEL SIZE= DFOV (mm)/ MATRIX SIZE
RECONSTRUCTION
Ц CT#RECONSTRUCTION
PIXEL vs VOXEL
PIXEL VOXEL
PIXEL SIZE DEPENDS ON:
• MATRIX SIZE
• FOV
VOXEL SIZE DEPENDS
• FOV• MATRIX SIZE• SLICE THICKNESS
IMAGE DISPLAY
IN CT DIGITAL RECONSTRUCTED IMAGE IS CONVERTED IMAGE IS
CONVERTED INTO A GRAY SCALE IMAGE.
CT # vs BRIGHTNESS LEVEL
+ 1000
-1000
CT #1000
CT # - 500
CT # OF CYST5
CT # OF LIPOMA ( FATTY TUMOR)
-100
SCANNING
TECHNIQUE
• kVp
• mA
• TIME
• SLICE THICKNESS
• SLICE INCREMENTATION
PATIENT ORIENTATION• HEAD FIRST • FEET FIRST
SCANNING
• TOPOGRAM
• REGULAR SCAN
TOPOGRAM (SCOUT)
• TUBE DOES NOT REVOLVE AROUND THE PATIENT
AP SCOUT
• TUBE SUSPENDED ABOVE PATIENT DURING SCOUT GENERATION
TUBE
LAT SCOUT
• TUBE AT THE 90º ANGLE TO PATIENT
AXIAL SCAN
• TABLE STOPS AT THE SCANNING POSITION AND THE TUBE ROTATES AROUND A PATIENT.
SPIRAL
• PATIENT CONTINUOUSLY MOVES IN THE Z-AXIS DIRECTION WHILE THE TUBE ROTATES AROUND.
CONVENTIONAL AND SPIRAL/HELICAL CT
ADVANTAGE OF SPIRAL IMAGING OVER CONVENTIONAL
• SPEED
CT SPECIAL APPLICATIONS
CARDIAC ANGIOGRAPHY
VIRTUAL ENDOSCOPY
RADIATION TREATMENT
3D IMAGING