Unit 1Creating the Beam

Chapter 6The X-Ray Tube

Production of X-Rays• Source of electrons• Target• High-voltage• Vacuum

Tube ComponentsThe Cathode Assembly

• Filament• Focusing cup• Associated wiring

The Filament• Coil of thoriated tungsten– 0.1: 0.2 mm thick– 1: 2 mm wide– 7: 15 mm long• Filament length and width impact recorded detail

Filament Material• Tungsten selected due to:– High melting point– Difficult to vaporize• Rhenium and molybdenum– Also good choices

Dual Focus ArrangementsThermionic Emission

• Filament is heated

• Causes electrons to be released from filament

Tube Failure• Tube arcing– Vaporized tungsten collection on the envelope• Filament breakage

The Focusing Cup• Composed of nickel• Low negative potential applied• Compresses the thermionic cloud• Biased focusing cup• Space charge effect• Saturation current

The Focusing CupGrid-Biased Tubes

• Precise control of thermionic cloud• Instead of having the same potential of the filament, Grid-biased tubes can increase their negative charge therefore making the electron stream more narrow.• Used predominantly in mammo.

Space charge effect• As more and more electrons build up in the area around the filament, their negative charges begin to oppose the emission of additional electrons.• There is just not enough room• Limits x-ray tubes to a maximum of 1,000 to 1,200 mA range.

Saturation current• At this point an increase in kVp will not increase the tube mA.

• It is another filament phenomenon that affects the efficiency of the x-ray tube.• The filament amperage curve flattens out when there are no further thermionic electrons to be driven toward the anode.

The Anode Assembly• Three functions– Target surface– Conducts high voltage– Primary thermal conductor

The Anode Assembly• Components– Anode– Stator– Rotor

Stationary vs. Rotating AnodeRotating Anode

• Tungsten-rhenium alloy– High atomic number– High melting point– Heat-conducting ability

Anode Layering• Assists with heat loading• Backed with molybdenum and/or graphite

Mammographic Equipment• Molybdenum target material– Creates needed lower energy photons

Normal Anode WearWarm-Up Procedure

• Gradually warms the anode

– Prevents cracking• Helps maintain the vacuum• Stress relieved anode

The Target Area• Portion of anode that electron stream contacts– Target– Focus– Focal point– Focal spot– Focal track• Point source of x-ray photons (SID)

Anode Heat Loading• Rotating anode– RPM– Diameter of disk• Target material• Actual vs. effective focal spot

Line Focus Principle• Used to reduce the effective area of the focal spot. This permits the best resolution of detail• Effective focal spot– Controlled by:• Actual focal spot (length of filament)• Target angle

Line Focus Principle

• Angle– When the target angle is less than 45 degrees, the effective focal spot is smaller than the actual focal spot.– The most common diagnostic radiography target angle is 12 degrees.– Geometry of angle can limit the size of the beam. An 14 x 17 can use no less than 12 degrees.

Anode Heel EffectThe Stator

• Located outside the envelope• Bank of electromagnets• Stator failure

The Rotor• Copper cylinder connected to anode disk by molybdenum stem• Turns when stator is energized• Ball bearings– Bearing failure

The Envelope• Pyrex glass or metal– 10” long– 6” central diameter– 2” peripheral diameter• The window• The vacuum

Protective Housing• Controls leakage and scatter radiation• Isolates high voltages• Provides means to cool the tube

Control of Leakage Radiation and Scatter Radiation

• Housing made of lead-lined cast steel• Leakage radiation limit– 100 mR/hr at 1 meter

High-Voltage Isolation and Tube Cooling• Dielectric oil– Insulates– Promotes cooling

– Sometimes circulated through heat exchanger• Air fan

Off-Focus RadiationRating Charts and Cooling Curves

• Tube Rating Charts• Anode Cooling Curves• Housing Cooling Curves

Anode Cooling CurvesCalculation of Heat Units

• kVp x mA x time x rectification constant

X-Ray Production

Conditions• X-rays vs. gamma rays• Gap between filament and target• Velocity of accelerated electrons• Incoming electrons = incident electrons

(Solid arrow)• Departing photons

(Wave arrow)

Target Interactions• All occur within 0.25 to 0.5 mm of target surface– Heat production– Bremsstrahlung interactions– Characteristic interactions

Heat Production• 99+% of the incident electrons’ kinetic energy is converted to heat

• Incident electrons transfer kinetic energy to outer shell electrons of the target atoms– Causes them to emit infrared radiation (heat)

Target Materials• Tungsten and rhenium– High Z#’s– High melting points– Similar electron binding energies• Mammography– Molybdenum

Bremsstrahlung “Brems” Interactions• German word for braking• Incident electron interacts with electrostatic force field of the nucleus– Mutual attraction - slows electron– Strong nuclear force - keeps them apart and deflects incident electron

Brems Interactions• Result is x-ray photon production– Accounts for 85-100% of the beam– Photon energy dependent on how close electron comes to nucleus

Brems Interactions• As incident electrons get closer to the nucleus the following occurs:– Photon energy increases– Larger deflection of the incident electron

Brems Interactions

• Direct interaction between nucleus and incident electron– Possible, but not probable

– Maximum energy photon

Characteristic Interactions• Incident electron interacts with K-shell electron– Incident electron continues in slightly different direction• Kinetic energy must overcome binding energy– Occurs in techniques using 70 kVp or higher

Characteristic Interactions• Characteristic cascade– Hole in inner shell and must be filled by an electron from outer shell– Electron energy difference– Secondary photons produced• Only electron that drops into K-shell will contribute to the beam

Emission Spectrum• Brems and characteristic emissions combined• Selected kVp will determine the maximum keV possible for any photon

Emission Spectrum• Average keV is approximately 30-40% of the selected kVp• Characteristic peaks at 69 and 59 keV– Increased output due to tube potential change to 69 or 70

Summary• Conversion to x-ray photon energy in the x-ray tube• Bremsstrahlung target interaction• Characteristic target interaction• Characteristic K-shell photon production• X-ray photon emission spectrum curve

The x-ray beamElectromagnetic (EM) Energy

• Combination of electric and magnetic fields traveling through space

Electromagnetic Energy• Results from acceleration of a charge• EM Radiation can travel through a medium or vacuum• Wave/particle duality• Excitation/ionization

Characteristics of EM Radiation• Wavelength• Energy• Frequency

Wave Theory• Waves are disturbances in a medium– Ocean, sound, etc.• Wavelength ()– Angstrom• Frequency ()– Cycles per second (Hz)

Wave Equation• Frequency and wavelength are inversely related• Velocity = frequency x wavelength• Velocity of all EM radiation is c– c = 3 x 108 m/sec• c = x

Particle Theory• High frequency, high energy EM radiation– Interacts like a particle when contacting matter• Photon energy and frequency are directly related

• If frequency is doubled, energy doubles• E = h

X-Ray Properties• Penetrating and invisible form of EM radiation• Electrically neutral• Can be produced over a wide variety of energies and wavelengths.• Release heat when passing through matter

X-Ray Properties• Travel in straight lines• Travel at the speed of light• Can ionize matter• Cause fluorescence in certain crystals• Cannot be focused by a lens

X-Ray Properties• Affect photographic film• Produce chemical and biological changes in matter through ionization and excitation• Produce secondary and scatter radiation

Prime Exposure Factors• -Kilovoltage peak (kVp)• -Milliamperage second-(mAs)– mA– Exposure time• -Distance (d)– SID

Quantity and Quality• Quantity of the beam – Intensity of the beam– How many photons are within the beam– Measured in Roentgen (R)

• Quality refers to beam penetrability – How many of the photons will penetrate the anatomy– Numerically represented by HVL

mA• Determines beam quantity or intensity• Change the mA station on equipment– Change current delivered to filament• Change current to filament– Change how many electrons are released through thermionic emission

mAs• mA x seconds = mAs – Controls

-Quantity-Radiographic film density-Patient dose

kVp• Controls beam quality• Energy and penetrability– Influences Scatter• Dramatic effect on radiographic contrast• Influences beam quantity– Increased target interactions with increased kVp• Directly squared relationship to change in kVp selected

Density Relationship• How will changing kVp affect beam quality and quantity?– Increasing kVp• Increases beam penetrability• In addition it increases beam quantity– Decreasing kVp• Vice versa

15% Rule• Because kVp affects both quality and quantity, a change of only 15% will demonstrate a doubling of film density• In order to obtain on overall image quality, when kVp is increased or decreased by 15% mAs must either be halved or doubled.

Inverse Square Law• Intensity of radiation at a given distance from point source is inversely related to the square of the distance between the object and the source

Exposure (Film Density) Maintenance Formula• As SID increases, beam intensity decreases– And vice versa• Provides technique correction for change in SID– Maintains the same film density

Chapter 12X-Ray Interactions

Attenuation• Definition– Reduction in the number of x-ray photons in the beam

Attenuation• Definition– Result of x-ray photons interacting with matter, and therefore giving up their energy to the matter they interact with

Interaction Basics• X-rays can: – Be transmitted without interaction– Or interact with:• Entire atom• Orbital electron• Nucleus of an atom

Photon Energy Dependent Interactions• Low energy photons interact with whole atom• Moderate energy photons interact with orbital electrons• High energy photons interact with nucleus

Atomic Structure• Nucleus• Orbital electrons– Electrons close to nucleus are “bound”– Electrons further away are “loose” or “free”

Five Basic Interactions BetweenX-rays and Matter

• Coherent scattering• Photoelectric (PE) absorption• Compton scattering• Pair production• Photodisintegration• Photon energy range– Low – Moderate – High

Photoelectric Absorption• Incident photon energy is completely absorbed by an inner shell electron– Most likely to occur when x-ray photon has just slightly more energy than Eb of a K or L-shell electron

Photoelectric Absorption• Ion pair is formed when: – An electron is ejected from the atom – It becomes known as the photoelectron– Remaining atom has a vacancy in its inner electron shell

The Photoelectron• Photoelectron characteristics:

– Kinetic energy (Eke)– Mass– Reabsorbs quickly• Within 1-2mm of tissue

Ionized Atom• Inner shell electron vacancy makes atom electrically unstable• Characteristic cascade– Vacancy filled by an outer shell electron– Electron undergoes change in energy level– Emits characteristic photon

Secondary Radiation• Radiation that originates from irradiated material outside of x-ray tube• Production similar to characteristic x-rays production within target• Characteristic photons emitted from atoms of patient after PE absorption interaction has occurred

Secondary Radiation Energy• Low Z# in tissue– Low energy secondary radiation• Higher Z# with contrast agents– Higher energy secondary radiation

Photoelectric Absorption Condition #1

• Incident photon energy (Ei) must be greater than or equal to binding energy (Eb) of inner-shell electron

Photoelectric Absorption • PE absorption interaction is more likely to occur if:

– Incident photon energy (Ei) and inner-shell electron binding energy (Eb) are close to each other

Photoelectric Absorption • As photon energy increases, chance of PE interaction decreases dramatically

Photoelectric Absorption

• PE absorption interaction is more likely to occur in elements with a higher Z#, and therefore higher binding energy (Eb) of inner-shell electrons

Photoelectric Absorption

•Increased Z# has a dramatic impact on the amount of PE absorption– Direct cubed relationship

• Double Z#– Increase chance of PE absorption interaction by a factor of 8

Photoelectric Absorption

• Low Z# atoms experience PE absorption interaction with the K-shell• Higher Z# atoms experience PE absorption interaction in the K, L, or M-shell• Example– Bone vs. soft tissue

Coherent Scatter• Involves low energy photons (below 10keV)• Two types with same result

– Thompson (single outer-shell electron)– Rayleigh (all electrons of the atom)

Coherent Scatter• Electrons are excited and vibrate at photon frequency • No electrons are ejected• No ionization takes place

Coherent Scatter• Atom stabilizes itself by releasing a photon equal in energy to incident photon (Ei), but in a different direction

Compton Scatter• Incident photon (Ei) interacts with outer-shell, loosely bound electron and ejects it• Ion pair is formed

Compton Scatter• Photon transfers some of its kinetic energy to the recoil (Compton) electron and continues on in a different direction

Compton Scatter• Energy transferred to recoil electron (Eke) affects angle and energy of scattered photon (Es)– And therefore, the frequency and wavelength of the scattered photon

Compton Scatter• Recoil electron travels until it fills a vacancy in another atom• Scattered photon continue to interact until absorbed photoelectrically

Compton Scatter• Source of occupational exposure and radiation fog• Most scatter travels in forward direction

• Backscatter

Pair Production• Incident photon energy must be 1.02 MeVor higher• Photon energy absorbed by nucleus

Pair Production• The nucleus becomes unstable • Nucleus releases a positron and a negatron to stabilize itself

Pair Production• Both have mass equal to an electron but with opposite charges– Negatron - negative– Positron - positive• Negatron acts like a free electron and will combine with a nearby atom

Pair Production• Positron is unstable antimatter• Combines with nearest electron • Annihilation reaction occurs • Matter of particles is converted to energy– Results in two photons of .511 MeV traveling at 180o to each other

Pair Production• Does not occur in diagnostic range of energies• More significant in radiation therapy• Not a significant interaction until energies of 10 MeV are being used

Photodisintegration• Extremely high energy photon (10 MeV or greater)• Absorption of photon by nucleus

• Excited nucleus releases alpha particle• Not significant in diagnostic imaging range

Effect on TechnicalFactor Selection

• Most of the x-ray beam is attenuated while some of the beam is transmitted

Effect on TechnicalFactor Selection

• As kVp increases the number of photons transmitted without interaction increases– Decreased probability of PE absorption and Compton interactions– Vice versa is true, too

Effect on TechnicalFactor Selection

• Within the attenuated beam…– As kVp increases• PE absorption decreases• Compton effect increases– Increases percentage of scatter and decreases percentage of absorption

Effect on TechnicalFactor Selection

• Compton scatter typically predominates within diagnostic x-ray energy range

Effect on TechnicalFactor Selection

• PE absorption interactions predominate in two circumstances:

– Lower energy ranges (25-45 keV produced by 40-70 kVp techniques)– In elements with higher Z#’s– Introduction of contrast agents results in increase PE absorption

Effect on TechnicalFactor Selection

• When PE absorption predominates– Resulting image will have short scale contrast– Low kVp, high mAs

Effect on TechnicalFactor Selection

• When Compton interactions predominate– Resulting image will have long scale contrast– High kVp, low mAs