Unit 1 - Quia



Unit 1

Creating the Beam

Chapter 6

The X-Ray Tube

Production of X-Rays

Source of electrons

Target

High-voltage

Vacuum

Tube Components

The 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 Arrangements

Thermionic 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 Cup

Grid-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 Anode

Rotating 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 Wear

Warm-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 Effect

The 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 Radiation

Rating Charts and Cooling Curves

Tube Rating Charts

Anode Cooling Curves

Housing Cooling Curves

Anode Cooling Curves

Calculation 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 beam

Electromagnetic (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 (λ)

Angstrom

Frequency (υ)

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 = υ x λ

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 = hυ

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 12

X-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 Between

X-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 MeV

or 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 Technical

Factor Selection

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

Effect on Technical

Factor 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 Technical

Factor 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 Technical

Factor Selection

Compton scatter typically predominates within diagnostic x-ray energy range

Effect on Technical

Factor 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 Technical

Factor Selection

When PE absorption predominates

Resulting image will have short scale contrast

Low kVp, high mAs

Effect on Technical

Factor Selection

When Compton interactions predominate

Resulting image will have long scale contrast

High kVp, low mAs

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