Look at My Chest!

8/8/2019 Look at My Chest!

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LOOK AT MY CHEST!



CT uses x-rays to produce cross sectional images of an object, X-ray tube and detector rotates 360 deg around patient synchronously

Collimated x-ray beam passes through the patient

The internal structure of the body attenuate the beam according to mass density and atomicnumber

Computed Tomography



X

Y

Z

ISOCENTER



HOW CT WORK

With CT scanning, numerous x-ray beams

and a set of electronic x-ray detectors

rotate around you, measuring the amount

of radiation being absorbed throughout

your body.At the same time, the examination table is

moving through the scanner, so that the x-

ray beam follows a spiral path.

A special computer program processes this

large volume of data to create two-

dimensional cross-sectional images of yourbody, which are then displayed on a

monitor.

This technique is called helical or spiral CT.



COMPONENTS



XENON DETECTORS



Resulting array is aimage of bodyattenuation

FBP or other reconstruction methods -

Attenuation measurements are used to

produce cross sectional array of tissue

co-efficients

Data are converted to CT number or Hounsfield Unit for display



CT NUMBER IS THE VALUE OF THE PIXEL

ASSIGNED IN THE IMAGE

They are computed by calculating the relative difference between the linear attenuation

coefficient of tissue and that of water

The values are normalised to the value of water as

reference



i = different tissue type region along

x-ray trajectory

µi =effective attenuation coefficientfor the different tissue region

Xi = corresponding thickness of the

different tissue region

E = Euler constant

Attenuation Correction factor

´I0 ´ is initial ray &

´Iµ is the measured ray

corrected by attenuationfactor

Attenuation correction

I

THIS ATTENUATON CO-EFFICIENTS ARE

USED TO CORRECT SPECT DATA



The CT number for water

has a value of 0

because:( tissue - water )/ water =

0

For tissue and that of

water the equation is:

CTTissue

= (tissue

-

water )/ water x 1000

Based on CT numbers images are displayed in a 512*512 matrix

HOUNSFIELD SCALE



WINDOW

The process of changing CT image gray scale is known as windowing

Bone window long

gray scale, low

contrast

Soft tissue window

short gray scale,

high contrast.



PIXEL VS VOXEL

PIXEL

VOXEL

PIXEL Based onMATRIX SIZE

FOV

VOXEL based onFOV

MATRIX SIZE

SLICE THICKNESS



1st Generation

Pencil beam Rotate Translate

Single detector

2ND GenerationFan beam

Rotate Translate Detector

Array



3rd GenerationFan beam

Rotate Rotate

Detector Array

4th GenerationFan Beam

Rotate Stationary

Detector Array

Hybrid SPECT/CT camera uses his

3rd & 4th generation design



5th Generation



DATA FLOW IN CTREFERENCE DETECTOR

REFERENCE DETECTOR

ADCPREPROCESSOR

COMPUTER

RAW DATA

CONVOLVED DATABACK

PROJECTORRECONSTRUCTED DATA

PROCESSORS

DISK TAPEDAC CRT DISPLAY



SPATIAL RESOLUTION

Bar patterns(smallest visible repeating pattern)

Edge, wire or bead

Phantom alignment



IMAGED SLICE THICKNESS

Multi-slice look at different detector combinations

Important imaging parameter



FACTORS AFFECT THE CT

Patient dose and image

± X-ray beam parameters

KVp

mA

Filtration

Collimation

Detection efficiency

± Image parameters

Noise

Spatial resolution

Slice thickness



BENEFIT AND RISKBenefits:

-CT scanning is painless, non-invasive and accurate-ability to image bone, soft tissue and blood vessels all at the same time.

-provides very detailed images of many types of tissue

-CT is less sensitive to patient movement than MRI

-No radiation remains in a patient's body after a CT examination

Risks:

-There is always a slight chance of cancer from excessive exposure toradiation

-The effective radiation dose from this procedure ranges from approximatelytwo to 10 mSv, which is about the same as the average person receives frombackground radiation in to 8 months to three years.

-Women should always inform their physician and x-ray or CT technologist if there is any possibility that they are pregnant

-CT scanning is, in general, not recommended for pregnant women unlessmedically necessary because of potential risk to the baby.



HOW WILL THE TUBE CURRENT OF THE

SOURCE IN CT SCANNER AFFECTS THE IMAGE?

Minimal tube current required for good image quality with the least radiation dose



CONCLUSION

CT can visualize small density differences, e.g. grey matter, white matter, and CSF.CT can detect and diagnose disease that cannot be seen with X-ray.

More expensive than X-ray, lower resolution.

Ionizing radiation.

HOW WILL THE TUBE CURRENT OF THE

SOURCE IN CT SCANNER AFFECTS THE IMAGE? Minimal tube current required for good image quality with the least radiation dose

Look at My Chest!

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