principle of x-ray systems.pptdarman.tums.ac.ir/content/media/filepool3/2015/11/1451.pdf ·...

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Principle of X-Ray SystemsPrinciple of X-Ray Systems

Hossein Ebrahimi NasabBiomedical Engineer (M.Sc)


X Ray Imaging


History

Wilhelm Conrad ROENTGEN, a German physicist, discovers accidentally unknown rays, able to pass through his hand and image the bones and his ring

November 8, 1895

He called them…. “X-rays”

He was awarded the first Nobel Prize for physics in 1901

• One year later, Elihu Thomson demonstrates use of X-rays for diagnosis of fractures and location of foreign objects and Eddy C. Jerman starts the first school for Radiographers


PHYSICS OF X-RAYS

Nature of X-rays

Energy unitEnergy unit

Interaction with matter



PARTICLES ENERGY (Definition)

When an electron is submitted to an electrical field, it is accelerated and gets a speed (an energy) proportional to the applied voltage.

This energy is expressed in “eV” : Electron Volt

When this electron reaches the positive electrode, it has a kinetic energy of 1000eV or 1 KeV

KeV is a unit of energy used for all kinds of particles


INTERACTION WITH THE MATTER

In vacuum:photon move along a straight line

In materials photon can beIn materials, photon can be- transmitted (no interaction)

- absorbed (they disappear, transferring their

energy to the material),

- scattered: they are deviated, with or without

loss of energy


INTERACTION WITH THE MATTER

Absorption increases with the density of the materialit is what we want to assess, and also what causes biological damage

Transmission is what we can see photographicallyp g p yIt is what we use to assess the amount of radiation absorbed

Scatter is undesirableIt interferes with the measurement of the transmitted radiation

High density tissues (bones) result in low transmissionScattered radiation reduces the quality of the image


PRODUCING AN IMAGE

The X-ray image displays the part of the X-ray beam not absorbed by the tissues. The less the absorption, the darker the film.

The radiograph displays these differences in tissue density

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Technology

The heated filament produces a cloud of free electrons

These electrons are attracted d l t d t d thand accelerated towards the

anode by the electrical field (kV)

The flow of electrons is an electrical current (mA)

The electrical power (100 kV x 100 mA = 10 kW) is converted into heat (99%) and X-ray power (1%)


X-ray Spectrum: Variation with Anode Material

• it changes the emission efficiencyy(continuous spectrum)

• it determines the presence of characteristic radiations and their energy


X-ray Spectrum:Variation with kV

kV changes the emission efficiency

it changes the shape of theit changes the shape of the spectrum : maximum energy of photons in the spectrum (in keV) has the same value as tube HV (in kV)

it has a significant effect on the relative amplitude of characteristic radiations


X-ray Spectrum:Variation with mA

• At a given energy, the rate of photon production is proportional to the mA1 mA10 mA50 mA100 mA500 mA1000 mA

• the shape of the spectrum remains the same when mA is varied


FILTRATIONBeam hardening (low energy filtration)

Any material in the beam path absorbs more of the lower energy radiation (harmful for the patient) than the higher energy

Regulation require a minimum of 2.5 mm Aluminum equivalent filtration


Take Away• 99% of the electrical power is converted into heat

• The X-ray beam is multi-energetic (spectrum)– Continuous spectrum – Specific peaks

• kV control mainly the maximum energy

• mA control the intensity of the beam

• Anode material defines the specific peaks

• Anode/filter combination enhances specific energies


The Main ComponentX-ray tubeProduction of X-rays

CollimatorBeam size control

HV GeneratorPower supply Exposure Exposureparameters control

Image receptor• Static (Radiography)

Film• Dynamic (Fluoroscopy)

Image Intensifier + video camera

• Static & dynamicDigital detector+ Workstation


THE X-RAY TUBE

insert Tube casing (housing)


Tube casing (housing)The x-ray tube insert is mounted inside a protective housing. The

housing is made of aluminum and lined with lead. The housing:

C l d l k di i• Controls scatter and leakage radiation• Isolates high voltage• Cools the tube

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The tube is inserted inside a casing and immersed in oil for electrical insulation and cooling

The casing also shields x-rays emitted in all directions


Insert Compeonent

Tube insert consist of the:

FrameFrame

Cathod

Anode


AnodeFilament and Cathode

Electrons e- beam

X-Rays beam


X-RAY TUBE COMPONENT: Frame

The outer part of the insert (sometimes called the enclosure or frame) can be made of glass, steel, or copper.

The enclosure:

• Provides a structural base for the cathode and anode• Provides high-voltage insulation• Allows a vacuum to be maintained


X-RAY TUBE COMPONENT: Cathode(1)The cathode provides the electron stream for x-ray production.

The cathode consists of:Filaments: which are coils of tungsten wire that, when

heated, provide the electron stream for x-ray productionheated, provide the electron stream for x ray production

Focusing cup: which surrounds the filament and concentrates, or focuses, the electron beam


X-RAY TUBE COMPONENT: Cathode(2)Dual Focus CupSome focusing cups have two filaments.

Th l fil• The large filament:used for higher power levels needed for dense anatomy. (High mA, high kV, or both)

• The small filament:used for lower dosage levels, which provides better resolution.


X-RAY TUBE COMPONENT: Anode

X-rays are produced by energy conversion when electrons strike the anode.

The anode consists of 3 major components:•A target, which receives a stream of electrons from the cathode

•A rotor of an induction motor, which holds the target and drives its rotation at a rate of approximately 10,000 rpm. This rotation dissipates the heat generated during the production of x-ray

•A bearing assembly, which supports the rotor and target assembly, and provides smooth rotation of the target


Rotore


X-ray Generation

The process by which the x-ray tube produces x-rays is very inefficient.

Only one percent of the energy produced is converted to x-rays; the rest is dissipated as heat.

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Focal Spot Characteristics

Actual Focal Spot:The electron beam from the tube filament lands on the angled target surface and creates the focal spot This areacreates the focal spot. This area(rectangular area) called the actual focal spot.

The actual focal spot is the area on the anode target where electrons strike, and where x-rays and heat are produced


Focal Spot Characteristics

Effective Focal Spot:

The area from which the x-ray beamtappears to come.

Viewed from the perspective of the centralray.

The x-ray tube's effective focal spot is commonly called the focal spot.


Focal Spot CharacteristicsFocal Spot Size and Shape:

Length of the tube filament Diameter of the filament wire coil Width and length of the focusing cup slotg g pDepth of the focusing cup

Focal Spot Size and Target Angle:as the target angle increases, the effective focal spot size increases.

The slope of the anode target allows a larger area to be heated while keeping the apparent area from which x-rays are produced as small as

possible.



Anode Heat Capacity

Energy= Kv*mA*time


KV mAS HU

55 5 275

60 5 30060 5 300

70 16 1100

75 50 3750

)( 80 50 4000

90 100 9000


• 300KHU• 40KHU/min• 1250KHU

• 3000HU 100

• 40KHU/min 4


Take AwayAn X-ray tube is characterized by:

• Focal spot size (single or dual)– The smaller the focus, the better the spatial resolution

• Anode rotation speed

• Maximum heat storage capacity (kHU or kJ)


The Generator

Control kV applied to the x-ray tube, thereby controlling x-ray beam penetration

The main functions of the x-ray generator are:

Control the amount of mA in the tube, thereby controlling the quantity of x-ray beams

Control exposure time and switching

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Types Of Generators(1)• Single phase generator• Three phase generator• High frequency generator• Battery powered generator• Capacitor discharge


Types Of Generators(2)

Single phase generator:


Types Of Generators(3)Three phase generator:


Types Of Generators(4)High frequency generator:


Types Of Generators(5)Battery powered generator:


Types Of Generators(6)

Capacitor discharge:


Generator Control ConsoleThe control console variables include:

– kV– mA– Exposure timeExposure time


Generator Parameters

Importance of High KV & mA

Power of Generator = Kilo Watt (KW) = KV x mA / 1000

mAs = mA x Time (mA x sec)

KV Penetration Good For Thick Patient

mA Resolution High Contrast Image


Power Voltage Tube Current

Exposure Time

Radiation Amount

Unit kW kV mA sec mAs

Clinical M i No Penetration Contrast

Shorter Time avoids Accumulated

D A tMeaning No Penetration Contrast avoidsmotion blur Dose Amount

Correlation W = VxA mAs=mAxsec

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Generator Classes

Middle Power Generators• Around 50kW

High Power Generators• Around 80~100kW• For long time exposure• For high mA Fluoroscopy

Low Power Generators• Example : Less than 20kW• Limited with Radiography

• For General Purpose• Fluoroscopy is possible


General Radiology


System Overview



Collimator(1)The collimator:

• Controls the dimensions of the x-ray beam

• Aligns the x-ray beam, the patient, and the bucky prior g y , p , y pto exposure

• Limits patient and technician exposure to radiation

• Reduces scatter radiation


Collimator(2)Two pairs of shutters control the beam dimensions.

Patient Centering

A light and mirror are built into theA light and mirror are built into the collimator to pre-position the x-ray beam.

The simulation light allows the technologist to pre-center the beam on anatomical landmarks.

Notice the cross hair on the patient's shoulder.



Cassette Tray• The cassette tray is a mechanical

component of the x-ray system.

Purpose :Purpose :– Holds the cassette – Centers the cassette


Bucky A bucky provides the housing for three assemblies:

– Cassette tray– Grid– Automatic exposure control

A b k b i h i t l– A bucky can be in a horizontal(table) or vertical (chest stand) orientation.

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Automatic Exposure Controller (A.E.C)

Automatic Exposure Control measures the dose ofradiation that strikes the X-ray film behind the patient, andturns the X-ray system off when the predetermined dosefor that screen-film combination has been reached. Thisassures that only the smallest required dose isadministered. The resulting images all show a uniformblackening, and the danger is reduced that the X-rayexamination might have to be repeated owing to an error inthe image. In this way, automatic exposure timing alsoindirectly reduces the dose.


GridIn 1913, Gustav Bucky introducedthe radiographic grid, a deviceplaced between the patient andthe cassette.

The purpose of the grid is to:

Reduce the amount of scatterradiation reaching the filmIncrease contrast in the x-ray imageScatter radiation is a noise factor. Itcauses a fogging which impairsradiographic quality, lesseningimage contrast. Scatter providesno useful information.


• GRID TYPES

• GRID SPECIFACTIONS


Grid Design

• A radiographic grid is composed ofstrips of lead, separated by aluminum, amaterial that is relatively transparent to x-rays. The graphic shows how gridremoves scatter radiation.

• Because the scatter radiation,represented by dotted lines, moves invarious directions, it is largely absorbedby the lead strips.

• The transmitted x-rays pass untouchedthrough the grid and reach the film.


Grid Pattern(1)X-ray grids are available with either focused or parallel lead strips.

Focused Grid:The lead strips of this grid are angled.A line drawn through each lead strip intersects at apoint a specified distance from the grid, as shownhere.here.

Two important characteristics ofthe focused grid are:

• The distance from the grid to the point of intersection is called the focal distance of the Grid

• The density of the x-ray image is uniformfrom edge to edge, provided the grid is properlycentered and leveled relative to the


Grid Pattern(2)

Parallel GridWhen the lead strips are not angled, they are all perpendicular to the face of the grid. This type of grid is termed parallelis termed parallel.

Parallel grids have limited use because:• The uniform density of an x-ray exposure is limited to a width of about 10 cm

The advantage of a parallel grid is:• significant only for an extremely long focal distance


A E C


The X-Ray FilmTHE RADIOGRAPHIC FILM CASSETTE:


Latent Image Formation

When the radiation interacts with the silver halide crystals in the film emulsion, the image on the film is produced.

The image, which is not visible before processing, is called g , p g,the latent image.

An example of another type of latent image is fingerprints. If you touch an item, you leave your fingerprints even though you cannot see them on that item. When that item is treated, your fingerprints become visible.

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Film Speed

• “Film speed” refers to the amount of radiation required to produce a radiograph of standard density (darkness).y ( )

• Film speed is determined by the following factors: – The size of the silver halide crystals – The thickness of the emulsion– The presence of special radiosensitive dyes

(Cont’d)


(Cont’d)

• The film speed determines how much exposure time is required to produce the image on the film.

• A fast film requires less radiation; the film responds more

Film Speed

q ; pquickly because the silver halide crystals in the emulsion are larger.

• The larger the crystals, the faster the film speed. This is the same principle as film speed on photographic film.

• F-speed film, the newest and fastest film on the market today, reduces radiation exposure to the patient by 20% to 60% compared with E-speed and D-speed film.


• An intensifying screen intensifies or increases the effect of the radiation and thus decreases the amount of exposure time needed.

Intensifying Screen

• The intensifying screen is coated with a material called phosphor that gives off light when struck by x-radiation.

• The film inside the cassette is sandwiched between the intensifying screens and is affected by both the light produced by the phosphor and the x-radiation.


X-Ray Film Processing• Processing is a series of steps that changes the latent image

on the exposed film into a radiograph by producing a visible image on the film.

• Proper processing is just as important as exposure technique in producing diagnostic-quality radiographs.

• Radiographs that are nondiagnostic because of poor processing techniques must be retaken, exposing the patient to unnecessary radiation.


The Five Steps in Processing

• Development • Rinsing • Fixation• Fixation• Washing • Drying


Developing• Developing is the first step in processing films.

• A chemical solution called the developer is used.

• The purpose of the developer is to chemically reduce the exposed silver halide crystals to black metallic silver.

• The developer solution also softens the film emulsion during this process.


• Rinsing of the films is necessary to remove the developer from the film so that the development process stops.

• Usually agitating the film rack for 20 seconds is sufficient

Rinsing

• Usually, agitating the film rack for 20 seconds is sufficient.

• This must be done under safelight conditions.


• The acidic fixing solution removes the unexposed silver halide crystals from the film emulsion.

• The fixer also hardens the film emulsion during this process. • For permanent fixation the film is kept in the fixer for a

Fixing

• For permanent fixation, the film is kept in the fixer for aminimum of 10 minutes.

• However, films may be removed from the fixing solution after 3 minutes for viewing.

• Films that are not properly fixed will fade and turn brown in a short time.

• Leaving films in the fixer for a long time (e.g., over a weekend) can remove the image from the film.


Washing• After fixation, a water bath is used to wash the film.

• The washing step requires about 20 minutes to thoroughly ll h i l f th l iremove all excess chemicals from the emulsion.

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• The final step in film processing is the drying of the films.

• Films may be air-dried at room temperature in a dust-free area or placed in a heated drying cabinet

Drying

area or placed in a heated drying cabinet.

• Films must be completely dried before they can be handled for mounting and viewing.


The Automatic Processor Automatic film processing is a fast and simple method used to process x-ray films.

Other than opening the film packet, all steps of film processingp g p , p p gare handled by the automatic processor.

Automatic film processing requires only 4 to 6 minutes for the development, fixing, washing, and drying of a film, whereas manual processing techniques require approximately 1 hour.(Cont’d)


The Automatic Processor(Cont’d)

The automatic processor maintains the correct temperature of the solutions and adjusts the processing time.

Proper maintenance of the automatic processor reduces the chance of errors during film processing.


Components of the Automatic Processor

• The processor housing covers all of the component parts. • The film feed slot is for the unwrapped films to be inserted

into the automatic processor. • The roller film transporter is a system of rollers that• The roller film transporter is a system of rollers that

rapidly moves the film through the compartments. • The developer and fixer compartments holds the

solutions.The film is transported directly from the developer into the fixer without a rinsing step.

• The water compartment holds circulating water. • The drying chamber holds heated air and dries the wet

film.



CHARACTERISTICS OF X-RAY FILM

• In general radiography, the X-ray image is first converted to a light image using intensifying screens, which in turn produce a visible pattern of metallic black silver on the X-ray film. Ultimately, the degree of blackening is related to the intensity of the radiation reaching the intensifying screen. The amount of blackness on the film is called the optical density, D, which is defined in Figure 5. For example, if 100 light photons are incident on a film and only one is transmitted the film density would be log10(100) or 2. Useful densities in diagnostic radiology range from about 0.2 to about 2.5. High density means black films.


Characteristic Curve• If the relationship between the logarithm of the radiation exposure and the

optical density is plotted we obtain a curve known as the Characteristic Curve. For film exposed with an intensifying screen, this curve is essentially sigmoidal in shape It is characterised by:

• The important part of the curve diagnostically is the approximately linear p p g y pp yregion between the toe and the shoulder where the density is proportional to the logarithm of the exposure.


• The information content resulting from the radiograph arises from differences in the film density, which we can define as radiographic contrast. Radiographic contrast depends on subject contrast and film contrast. For the moment you should recall that subject contrast depends on the differential attenuation of the X-ray flux as it passes through the patient and is affected by thickness, density and atomic number of the irradiated parts of the subject, the kVp, the presence of contrast medium and scattered radiation. For example, relatively few X-ray photons pass through bone compared with soft tissue but care must be taken in selecting the correct kVp in order to produce an X-ray image of high information content for the screen-film to record. That is, the kVp influences the magnitude of the subject contrast.

• Film contrast depends on four factors:– the characteristic curve of the film,– the film density,– use of intensifying screens or direct exposure and– the film processing.

• The slope of the straight line portion of the characteristic curve tells us how much change in film density will occur as exposure changes. The slope or gradient of the curve may be measured and the maximum gradient is called the film gamma,which tells us how well the film will amplify the subject contrast.


Image Quality(1)INFLUENCE OF ENERGY (kV) The higher the energy,

the higher the transmission ratio,the better the penetration

• High energy may be necessary in dense or thick areas (abdomen, pelvis)• Very low energies are rarely of use and are harmful to the patient

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