RT A 2009 WK 5 X-RAY TUBE & EXPOSURES INVERSE SQ LAW CIRCUITRY

RTA

2009WK 5

X-RAY TUBE & EXPOSURES

INVERSE SQ LAW

CIRCUITRY

Objectives

X-ray tube review

Properties of X-rays

“Prime Factors”

Inverse Square Law

X-ray Circuit

X-Ray Properties

Are highly penetrating, invisible rays which are a form of electromagnetic radiation.

Are electrically neutral and therefore not affected by either electric or magnetic fields

X-Ray Properties

Can be produced over a wide variety of energies and wavelengths (polyenergetic & heterogeneous).

Release very small amounts of heat upon passing through matter.

X-Ray Properties

Travel in straight lines.

Travel at the speed of light, 3 X 108 meters per second in a vacuum.

Can ionize matter.

X-Ray Properties

Cause fluorescence of certain crystals.

Cannot be focused by a lens.

Affects photographic film.

X-Ray Properties

Produce chemical and biological changes in matter through ionization and excitation.

Produce secondary and scatter radiation.

PRIME FACTORS

How does the “technique”

What is it?

How does it affect the “image”

PRIME FACTORS

• KVP

• MAS

• DISTANCE

• The amount of overall blackening on a radiograph, or of a certain part of the image, is referred to as density.

• Density results from two things: • the amount of radiation that reaches a

particular area of the film, • and the amount of black metallic

silver deposited when the film is developed.

Producing optimal radiographs

How much of the radiation received by the patient

Actually reaches the IR ?

About 1%

Creating the IMAGE

• When x-rays pass through a patient's body, three things can happen:

• (1) the x-ray photon is transmitted, passing through the body, interacting with the film, and producing a dark area on the film;

• (2) the x-ray photon is absorbed in an area of greater tissue density, producing lighter areas on the film; and

• (3) the x-ray photon is scattered and reaches the film causing an overall gray fog.

Radiographic Prime factors

• The factors principally responsible for x-ray quality and quantity.

• These are mAs,

• kVp,

• distance (SID).

PRIME FACTORS

• SID - ADJUSTED IN THE ROOM

• KVP - “intensity of beam”

CONTAST RANGE FOR PART

• MAS – “density of image”

CHANGES WITH PT SIZE

mAs & kVp

Introduction to

Technical Factors to

Create an image

X-ray quantity (mAs)

• is a measure of the number of x-ray photons in the beam.

• Also called x-ray output, intensity or exposure.

X-ray quality (kVp)

• is a measure of the penetrating ability of the x-ray beam

• The quantity of electron flow, or current, in the X-ray tube is described in units of milliamperes (mA).

• The maximum kinetic energy of the accelerated electrons is defined in terms of kilovolts peak potential (kVp).

IMAGES

• DENSITY = THE AMOUNT OF BLACKENING “DARKNESS” ON THE RADIOGRAPH

• CONTRAST – THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES

Film Screen• Overexposed

• Referring to a radiograph that is too dark because too much x-radiation reached the image receptor

• Underexposed

• Referring to a radiograph that is too light because too little x-radiation reached the image receptor

Density on Image• When a radiologist looks at a radiograph, they looks for any anatomic or pathologic

change that causes a change from the normal density.

• That could be, a solid mass in the lung that stops x-rays from reaching the film and decreases the amount of film darkening in that area (appears “light or white on image”

• Or excessive “AIR” – looks BLACK

mAs

mA X s = mAs

Milliamperage

• mA

• One milliampere is equal to one thousandth of an ampere.

• The amount of current supplied to the

x-ray tube

• Range 10 to 1200 mA

Tube current (mA)

• Tube current is equal to the number of electrons flowing from the cathode to the anode per unit time

• Exposure of the beam for a given kVp and filtration is proportional to the tube current

Time

• In seconds

• How long x-rays will be produced

• 0.001 to 6 seconds

mAs Changes

• at least 20 - 30 % mas change needed to see a visible change in density

mAs DOUBLED = DENSITY DOUBLED

+ 25% + 50 % mAs

Kilovoltage Peak

• kVp

• One kilovolt is = to 1000 volts

• The amount of voltage selected for the

x-ray tube

• Range 45 to 120 kVp (diagnostic range)

• kVp controls contrast

Tube voltage (kVp)

• Determines the maximum energy in the beam

• spectrum and affects the quality of the output spectrum

• Efficiency of x-ray production is directly related to tube voltage

Contrast-the differences

between blacks to whites

• Kilovolts to anode side – kVp (40 -120)

• Kilovolts controls how fast the electrons are sent across the tube

• kVp – controls CONTRAST on images

• Low kVp – more absorbed – black - white

• High kVp - more grays on image

Influencing factors: kVp

15% rule: 15% kVp = doubling of exposure to the film

15% kVp = halving of exposure to the film

15% rule will always change the contrast of the image because kV is the primary method of changing image contrast.

Remember : 15% change ( ) KVP has the same effect as

doubling or ½ the MAS on density

+ 15% kvp - 15% kvp

CONTROL PANEL CONTROLS

• kVp SELECTION

• mA SELECTION

• TIME (sec.)

The Control Console• The control console is

device that allows the technologist to set technical factors (mAs & kVp) and to make an exposure.

• Only a legally licensed individual is authorized to energize the console.

Control panel

X-Ray Machine

• Purpose:– provide a specific current (mA) & voltage

(kV) to the x-ray tube– convert electrical energy to electromagnetic

energy (x rays) in a controlled manner• control the energy of the x-ray photons• control the number of photons

kVp & mAs• kVp = quality of

beam – the “intensity of how much tissue it can penetrate

• mAs – the amount of time the beam is left on

THE X-RAY TUBE

• The ANODE (+) attracts the electrons that are “boiled off

• From the ---

• Negative CATHODE (--)

kVp & mAs

kVp = energy mAs = amount

• HIGH VOLTAGE TO ANODE – ATTRACTS – ELECTRONS FROM CATHODE

• CURRENT TO STATOR CAUSES ROTATION OF ANODE

Rotating Anode

INVERSE SQAURE LAW

• Applies basic rules of geometry

• The intensity of radiation at a given distance from the point source is inversely proportional to the square of the distance.

• Doubling the distance decreases intensity by a factor of four.

INVERSE SQUARE LAW

• The inverse square law –

• Used for RADIATION PROTECTION

• When you change your distance from the “radiation source”

• The intensity of radiation will be reduced by a square of the distance (MOVING AWAY FROM THE SOURCE)

• OR INCREASED – CLOSER TO SOURCE

DISTANCE

• Distance from the radiation source should be kept as great as possible

• Physical Law:

– Inverse Square Law

Application of inverse square law principles can yield significant reductions in patient and

operator radiation exposure.

Inverse square law

INTENSITY IS SPREAD OUT…

INVERSE SQUARE LAW FORMULA

Inverse Square Law Formula

Intensity #1

Intensity #2

Distance #2 - Squared

Distance #1 - Squared

INVERSE SQAURE LAW

• YOUR TURN

• 10 QUESTIONS IN CLASS

X-RAY CIRCUITY

Introduction to Circuitry

Contributions by Mosby, Thompson Publisher, Carlton, Bushberg, and the WWW.

How the current gets to the TUBE

Current from the outlet

Generator+ Transformers(where the power comes from)

Review Handouts

•Circuit Board

•Symbols

•Function

AUTOTRANSFORMER

• RAISES OR LOWERS THE VOLTAGE

• KVP CONTROL TAPS LOCATED

• 220 VOLTS INCOMING CONVERTED FROM 100 T0 300 VOLTS

AUTOTRANSFORMER

TIMER SWITCH

• Timer switch ends exposure

• Timer – length of exposure set at control panel

© UW and Brent K. Stewart PhD, DABMP© UW and Brent K. Stewart PhD, DABMP

7979 Bushberg, et al., The Essential Physics Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 126.of Medical Imaging, 2nd ed., p. 126.

low voltage, high currentlow voltage, high current

high voltage, high voltage, low currentlow current

3 Divisions of Circuit Board

• PRIMARYPRIMARY (CONTROL PANEL) yellow

• SECONDARY

(HIGH VOLTAGE) blue

• FILAMENT

(LOW CURRENT) purple

TRANSFORMERS(Step Up or Step Down)

• Increases the VOLTAGE going to the ANODE side of the tube OR

• 110 volts to 110,000 volts• Decreases the CURRENT

going to the CATHODE side of the tube

• 5 Amps to 50 milliamps

STEP UP TRANSFORMER

X-Ray Tube Circuit

Functional Position

Control Console Transformers Tube

Additional practice Additional practice & preview of next & preview of next

week week

Additional practice Additional practice & preview of next & preview of next

week week Circuitry, Inverse square & Circuitry, Inverse square &

InteractionsInteractions

Circuitry: Source: Carlton & Adler (1996). Principles of radiographic imaging: An art and a science. (96-99).

• MAIN CIRCUIT

Modifies incoming current to produce x-rays

Boosts voltage to range necessary produce

x-rays.

Modifies incoming line power to produce thermionic emission from the filament wire.

FILAMENT CIRCUIT• Filament circuit adjusts to

mA ratings (50, 100, 200, etc.).

• After mA selection, current sent to step down transformer to modify amps that reach filament on x-ray tube

Important Parts Of The Circuit Board TO ID

1. MAINBREAKER2. EXPSOURE SWITCH3. AUTOTRANSFORMER4. TIMER CIRCUIT5. HIGH VOLTAGE STEP UP TRANSFORMER6. RECTIFIER7. FILAMENT CIRCUIT VARIABLE SELECTPR8. FILAMENT STEP DOWN TRANSFORMER9. X-RAY TUBE10. ROTOR / STATOR

Now it is your turn

Name the parts of the

labeled Circuit board

#1

2

3

4

mA selector

56

78

9

10

See Circuitry Review handout and chart for numbers

Important Parts Of The Circuit Board TO ID

• 1 Incoming Line Voltage• 2 Autotransformer• 3 KVP Selector• 4 Timer• No # Ma Selector• 5 Primary Side (Low Voltage)• 6 Secondary Side (High Voltage)• 7 X-ray Tube• 8 Rectifier• 9 STEP – Up Transformer• 10 STEP – Down Transformer

© UW and Brent K. Stewart PhD, DABMP© UW and Brent K. Stewart PhD, DABMP

9797 Bushberg, et al., The Essential Physics Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 126.of Medical Imaging, 2nd ed., p. 126.

high voltage, high voltage, low currentlow current

TRANSFORMERS(Step Up or Step Down)

• Increases the VOLTAGE going to the ANODE side of the tube OR

• 110 volts to 110,000 volts• Decreases the CURRENT

going to the CATHODE side of the tube

• 5 Amps to 50 milliamps

Filament Current

• Current comes from Autotransformer

• Controls the Ma selection

• Focal Spot Selector Switch located here

FILAMENT CURRENT

MA METER

• MEASURE THE MA GOING TO THE XRAY TUBE

QUESTIONS ?

• A few days after Roentgen's initial public announcement of his discoveries, a doctor in America took X-ray photographs of a person with gunshot wounds in his hands

• Why can you see the bullet fragments?

X-Ray Tube Circuit

• they are electromagnetic waves of shorter wavelength and higher energy than normal light. But the debates over the nature of the rays – waves or particles? –

• Photons can be described both as waves and particles.

The Electromagnetic Spectrum• X-rays have wavelengths much shorter than

visible light, but longer than high energy gamma rays.

IMAGE CREATION

• ATOMS

• INTERACTION WITH “MATTER”

• ATOMIC NUMBER

Why you see what you see

• The films or images have different levels of density – different shades of gray

• X-rays show different features of the body in various shades of gray.

• The gray is darkest in those areas that do not absorb X-rays well – and allow it to pass through

• the images are lighter in dense areas (like bones) that absorb more of the X-rays.

Kinetic energy

• Energy of motion

• The electrons KINETIC energy is converted to PHOTON energy

Tube Interactions

• Heat = 99%

• X-ray = 1%

• Bremsstrahlung

(Brems) = 80%

• Characteristic = 20%

Brems

BREMS RADIATION

• Electron• Passes by

nucleus• Changes

direction• Energy

released as a PHOTON

Characteristic

CHARACTERISTIC (in tube)• Electron hits inner

shell e in orbit – knocked out & creates a hole

• Other E’s want to jump in

• Energy released as PHOTONS

Heat

QUESTIONS ?