Download PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY

PRINCIPLES OF CT
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 Xray 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
COMPUTER
TABLE/COUCH
CONSOLE
ORIGINAL CLINICAL CT
SCANS COMPOSED OF
80 X 80 MATRIX
PIXELS
6400
EARLY DAYS vs TODAY
80 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
RECONSTRUCTION
Ц
CT
#
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 CYST
5
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
• TUBE SUSPENDED ABOVE PATIENT
DURING SCOUT GENERATION
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