Download PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY

PRINCIPLES OF CT
Dr Mohamed El Safwany, MD
Intended learning outcome
• The student should learn at the end of this
lecture 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
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.
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
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
Text Book
• David Sutton’s Radiology
• Clark’s Radiographic positioning and
techniques
Assignment
• Two students will be selected for
assignment.
Question
• Define topogram and tomogram?
•
Thank You