PRINCIPLES OF CT

TOMOGRAPHY

TOMOS

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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 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 Ц

RECONSTRUCTION

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

TUBE AP SCOUT • 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