Digital Imaging

Digital Imaging

Moh

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Abd

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CONTENTS Introduction

Formation of digital images

CHARGE-COUPLED DEVICE (CCD)

COMPLEMENTARY METAL OXIDE SEMICONDUCTORS (CMOS)

Digital Subtraction Radiography (DSR)

PHOTOSTIMULABLE PHOSPHOR PLATES (PSP)

Introduction

Digital Imaging refers to the numeric format of the image in terms of pixels

with its different shades.

• Pixel = picture + element

• A digital image consists of a large collection of individual pixels that are organised in a matrix consisting of rows and columns.

Formation of digital images

• The formation of digital images involves several steps, beginning with analog processing.

• Each pixel ---consists of individual electronic detector --- absorb x-ray --- generate small voltage proportional to the amount of x-ray absorbed by each individual pixels within the detector.

• The voltage fluctuation at each pixel is therefore known as “analog signal”

• Conversion of the analog signal into an usefull digital image is done through a process called “Analog to Digital Conversion (ADC)”

ADC --- SAMPLING --- QUANTIZISING

• In order to view the image, the computer organizes the pixels in their proper location and

gives them a grey shade that corresponds to the number that was assinged during

quantization step.

• Various receptors that are used to absorbs the x-ray from the source were developed over the period times which includes…

• Each has their own credits and limitations in producing the final digital images DSR PSP

CMOS CCD

CHARGE-COUPLED DEVICE (CCD)

• Made up of thin layer of silicon.• When exposed to radiation--- covalent

bonds between the silicon atoms are broken--- prduce electron-hole pairs which are proportional to amount of exposure.

• Electrons are then attracted towards the positive site in the device creating a “charge packets”

• The image is read by transferring each rows of pixel charges in a “bucket brigade” fashion.

• Each packet corresponds to one pixel.

• As the charge reaches the end of its row--- transferred to the amplifier--- then transmitted as a voltage to ADC

COMPLEMENTARY METAL OXIDE SEMICONDUCTORS (CMOS)

• As same as that of CCD with few difference such as…

• It is made of silicon based semi-conductor

• Unlike CCD, each pixels areconnected individually to atransistor.

• These are less expensive than CCD

Digital Subtraction Radiography (DSR)

Two images( base line and follow up image) of the same object registered --- image intensities of corresponding pixels are subtracted --- a uniform difference image is produced.

Projection geometry of both two images should be same to avoid errors in distinguishing the actual changes.

Brighter area --- changes represent gainDarker are --- changes represent loss.

PHOTOSTIMULABLEPHOSPHOR PLATES (PSP)

• Made of…. barium + iodine + chlorine/bromine…. Which forms a crystal lattice.

• Europium (Eu+2) when added forms imperfection in the lattice.

• Eu+2 --- when exposed to sufficient radiation --- valence electron in Eu+2 absorbs energy --- move into the near by halogen vacancies (F-Centers) --- trapped there in a metastatic state --- which represents the latent image.

• Number of electrons trapped is proportional to the amount of exposure.

PSP IMAGE PROSESSING:

When stimulated by redlight(600 nm)--- bariumfluorohalide releases thetrapped electron --- electronreturns to Eu+3 ion ---energy is released in greenspectrum (300-500nm)

Lights from PSP --- conducted by fiberoptics to a photomultiplier tube

Digital image obtained

Lights from PSP --- conducted by fiberoptics --- to a photomultiplier tube

Quantified by ADC

Converts light into electrical energy

Is inbuilt with red filter that selectively remove the stimulating red light.

Prevent distortion in the final image

Specialized Radiographic Techniques

COMPUTED TOMOGRAPHY (CT)

CONE-BEAM COMPUTED TOMOGRAPHY (CBCT)

MAGNETIC RESONANCE IMAGING (MRI)

COMPUTED TOMOGRAPHY (CT)

Invented by Godfrey Hounsfield and Allan Cormack(Noble Price in medicine = 1979)

Tomography --- which meansthin layer radiography

Principles of CT

X-ray beams are collected in a grid like pattern called “Matrix”

Made of array of pixels

Each pixel represent a calculation that denote the actual attenuation of the X-ray beam by the

constituents of the body

A number (CT number / Hounsfield unit) is designated for each pixel corresponding to the

degree of attenuation to the x-ray beam

• Calibration of the Hounsfield units are inbuilt within the CT machine which differs according to the manufacturer’s.

• Most moniter may display only 256 gray-scales to which the human eyes can percieve only 64 shades of gray.

ADVANTAGES OF CT

• Multiplanar imaging

• Greater geometric precision

• Manipulation of the acquired image (able to change contrast and brightness)

• Soft tissue imaging

• Ability to distinguish objects of different density (such as air, fat, water, CSF, muscle and cortical bones)

DISADVANTAGES OF CT• Expensive• High radiation dosage to patient• Chances of artifacts

ENDODONTIC USES OF CT• Evaluate the presence and extent of the lesion• Detection in displacement of the fracture segments

CONE-BEAM COMPUTED TOMOGRAPHY (CBCT)

Since CT scanners are expensive and offers a high dosage of radiation, CBCT were invented as it costs

lesser as well as offers lower radiation exposure to the patient than that of CT scanners.

FUNDAMENTALS OF CBCT

Uses cone-shaped X-ray beam

This serious of 2D images are Combined to generate an accurate 3D images (Projection

Data)

Produces a “serious of 2D images (Basis Projection)” using 2D detectors

ADVANTAGES OF CBCT• Lower radiation than CT• Lower scanning time, space and cost than CT• Greater hard tissue definition than CT• Interactive display module (zooming is able)

DISADVANTAGES OF CBCT• Lesser soft tissue discrimination than CT• Scattering is more than CT• Chances of artifacts

Pre, Intra and Post-operative assessment

Diagnosis of root resorption and identification between its type

Diagnosis of root fracture

Diagnosis of dentoalveolarfracture

ENDODONTIC USES OF CBCT

Root canal morphology:Precisely determine root curvatureDefinitely identify accessory canals

Periapical pathosis:Uses in cases were the lesions are superimposed

by anatomical structuresCan support diagnosis of ‘lesions of non-endodontic

origin’Determines the extent of the lesion and its effect on

surrounding structures

MAGNETIC RESONANCE IMAGING (MRI)

MRI --- uses strong magnetic fields + radio waves --- form images of the body.

Human body consists of nearly 70% water (H2O)

Water has hydrogen molecules in it

Hydrogen atom has only a single proton within its nucleus

Proton has a positive charge and also spin on its own

Hence has a magnetic property i.e. has both north and south poles

Now an external magnetic field is applied

All hapazaradly aligned proton align them self towards the direction of

external magnetic field

Then a radiofrequency wave is applied

This enforces the protons to turn away from the direction of the external

magnetic field

Sudden stoppage of the radiofrequency wave

Protons tends to return back again to the direction of the external magnetic

field

This results in the release of energy in the form of photons

Photons detected by the specialized detectors

“MRI image” constructed on computer

The amount of photons emitted is proportional to the concentration of the hydrogen atoms

within the tissue

The images can be taken in all the planes by altering the direction of the external magnetic

field towards the plane of interest

ADVANTAGES OF MRI• Absolutely no radiation• Provide information about blood circulation

DISADVANTAGES OF MRI• Contraindicated in patients with pace maker

support• This procedure cannot be carried out on patients

with any kind of fixed metals prosthesis• Creates loud noises during the procedure• Highly expensive

References

• Textbook of white and pharoah, oral radiology

• Textbook of oral medicine, oral diagnosis and oral radiology – 2nd Edition – Ravikumar ongole, Praveen BN – Pg no: 760 - 779

Digital Imaging

Health & Medicine

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