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A kind of x-ray that can produce 3D pictures

Creation of a cross-section tomographic

section (slices, cuts) of the body using arotating fan beam (x-ray tube), detector array

and computed (complex computer)

reconstruction.

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A CT (computerised tomography) scanner is a

special kind of X-ray machine. Instead ofsending out a single X-ray through your body

as with ordinary X-rays, several beams aresent simultaneously from different angles.

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CAT Computer-Assisted Tomography

or Computed Axial Tomography

CTAT Computerized Transaxialtomography

CRT Computerized Reconstruction

Tomography DAT Digital Axial Tomography

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WILHELM CONRAD

ROENTGEN

1845-1923

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Alessandro Vallebona

Early 1900s

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A method whereby body-section roentgenography isaccomplished by relative

motion of the tube and filmwith respect to the bodyroentgenographed, the tubeand film meanwhile having no

motion relative to oneanother. This can beaccomplished by rotating thebody between a stationary

tube and film

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1950

developedmathematicalsolutions

involved in CT

scan

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1967 1st successful demonstration of CT was conducted by Hounsfieldfrom Central Research Laboratory of EMI in England

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The first patient brain-scan was

done on 1 October 1971. Themachine takes 5 minute per scan

and can be processed by over 2.5

hours.

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Dr. Robert Ledley atGeorgetown

University MedicalCenter developed the

1st scanner capable of

visualizing anysection.

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1979- Cormack and Hounsfield received a

nobel prize for medicine.

1982 - 1979- Cormack and Hounsfieldreceived a nobel prize for physics.

1989 spiral CT introduced

1991 Multiple CT introduced

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1ST GENERATION

2ND GENERATION

3RD GENERATION 4TH GENERATION

5TH GENERATION

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THE GANTRY

COMPUTER

OPERATING CONSOLE

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XRAY TUBE

DETECTOR ARRAY

HIGH VOLTAGE GENERATOR PATIENT COUCH

COLLIMATOR ASSEMBLY

MECHANICAL SUPPORT

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HARDWARE

SOFTWARE

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FLOPPY DISK

MAGNETIC TAPE

COMPACT DISK

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Film

Paper copy

PACS

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Refers to the systematic collection of

information from the patient to produce the

CT imager.

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IMAGE RECONSTRUCTION IS THE PHASE IN

WHICH THE SCAN DATA SET IS PROCESSED

TO PRODUCE AN IMAGE.

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THE TIME FROM THE END OF IMAGING

(END OF DATA COLLECTION) TO IMAGE

APPEARANCE

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1. scanning phase produces data but not an

image

2. the reconstruction phase processes the

acquired data and forms digital image.

3. digital to analog conversion phaseproduces visible and displayed analog image

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PIXEL

MATRIX

FOV (FIELD OF VIEW) VOXEL

CT NUMBERS

WINDOW LEVEL

WINDOW WIDTH

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TWO DIMENSION REPRESENTATION OF

CORRESPONDING TISSUE VOLUME

PIXEL SIZE = FOV DIVIDED BY MATRIX SIZE

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ARRAY OF NUMBERS IN ROWS AND

COLUMNS OF PIXELS DISPLAYED ON A

DIGITAL IMAGE

256X256

512X512

1024X1024

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DIAMETER OF IMAGE RECONSTRUCTION

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TISSUE VOLUME

VOXEL SIZE (CUBIC mm) = PIXEL SIZE(mmSQUARED) X SLICE THICKNESS (mm)

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SMALL PIXEL = IMPROVED SPATIAL

RESOULTION AND CONTAIN HIGH

FREQUENCY INFO LARGE PIXEL IMAGE HAVE REDUCED

SPATIAL RESOLUTION AND CONTAIN LOWFREQUENCY INFO

SPATIAL RESOLUTION IS DETERMINED BYMATRIX SIZE AND FOV

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Number that represents the attenuation

value for each pixel.

Optical density & Brightness level as

displayed on the monitor.

Numbers level ranges from -1000 to +1000for each pixel.

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TISSUE CT NUMBERS

DENSE BONE 1000+

MUSCLE 50

WHITE MATTER 45GRAY MATTER 40

BLOOD 20

CSF 15

WATER 0

FAT -100

LUNGS

AIR

-200

-1000

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REFERS TO THE MANIPULATION OF WL

AND WW TO OPTIMIZE IMAGE CONTRAST

WINDOW WIDTH = RANGE OF CT NUMBERS

DISPLAYED

WINDOW LEVEL = CENTER OF RANGE ANDCENTRAL VALUE OF WINDOW WIDTH

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PART WW WL

BRAIN 80 40

CHEST SOFT TISSUE 400 40

CHEST LUNG 1500 -400

ABDOMEN400 50

SPINE 1600 300

BONE 3000 500

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Spatial resolution

contrast resolution

linearity

noise

artifacts

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Spatial resolution is the CT

system's ability todifferentiate small objects

that are adjacent to oneanother.

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SLICE THICKNESS DECREASE SLICE

THICKNESS TO IMPROVE SPATIAL

RESOLUTION

PIXEL SIZE SMALL PIXEL SIZE improvesspatial resolution

INFLUENCED BY FOV,MATRIX, WIDTH OF THE DETECTORS,

INTERSPACING BETWEEN DETECTOR

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Contrast Resolution is the

ability of a CT scanner todifferentiate small

attenuation differences on the

CT image.

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AS IMAGE NOISE INCREASES CONTRAST

RESOLUTION DECREASES

COLLIMATION DETERMINESSECTION

THICKNESS

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" Property of a detector

characterized by an outputelectrical current that is exactly

linearly proportional to the input

radiation incident on thedetector." (Morgan 1983)

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contains no information. Noise

is characterized by a grainyappearance of the image. Many

authors describe noise as a salt

and pepper pattern on the CTimage.

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RESULT OF LOW

RADIATION;CHARACTERIZED

BY GRAINY IMAGE

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INTERPOLATION = EXTRACTION OF VALUESBETWEEN KNOWN VALUES

EXTRAPOLATION = EXTRACTION OF VALUESOUTSIDE THE KNOWN VALUES

Spiral/helical CT data uses a mathematical

technique called interpolation along with areconstruction algorithm to produce axial imagedata from a volume of data.

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1. QUANTUM NOISE - is a result

of too few photons reaching adetector after being attenuated

by the body.

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2. COMPUTATIONAL NOISE - is primarily

caused by all the statistical fluctuations

that occur from the reconstructionmathematics that are essential to

produce a CT image.

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3. ELECTRONIC NOISE caused by electrical

fluctuations

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"An Artifact is any distortion or error in the

image that is unrelated to the subject being

studied (Morgan 1983).

Wolbarst (1993) describes artifacts asaberrations that arise at the interfaces of

materials significantly different from theradiologic properties of the structures being

scanned.

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SYSTEMATIC DISCREPANCIES BETWEEN

THE CT NUMBERS IN THE

RECONSTRUCTED IMAGE AND THE TRUEATTENUATION COEFFICIENTS OF THE

OBJECT.

NON RANDOM, OR STRUCTURED IMAGENOISE

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Motion artifacts occur primarily because

during reconstruction the mathematical

algorithm is unable to solve for theinconsistencies in attenuation.

Appears as streaks or step-like patterns at

high contrast edges.

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Beam hardening artifacts occurwhen the low energy photons are

absorbed leaving the high-energyphotons to strike the detectors

Appears as a dark ring inside cranialbone and cupping at the center ofthe image

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Ring artifacts appear on a CT image as a

ring or a number of rings superimposed

on the structures being scanned.

The artifact is commonly associated

with third generation CT systems.

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The partial volume effect orvolume averaging is when two or

more different tissue typesoccupy the same pixel and areaveraged together.

Blurring over sharp edges

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GENERALLY, PATIENT RADIATION DOSE IS

HIGHER DURING CT THAN DURINGRADIOGRAPHY OR FLOUROSCOPY.

PATIENT RADIATION DOSE DURING CT ISAPPROXIMATELY 5000 mrad per

EXAMINATION

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REPEAT SCAN

LOW PITCH

THIN SLICE

OVERLAPPING SLICE

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PATIENT DOSE IS DESCRIBED BY CTDI AND

DLP.

CTDI CT DOSE INDEX

DLP DOSE LENGTH PRODUCT

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Lowest area radiation exposure plane of the

gantry and outside patient aperture.

Highest area radiation exposure - near thepatient and is due to scatter radiation

produced in the patient.

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