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

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 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

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