Exam 3: X-ray Production

Critical Absorption Energy

The threshold energy that an incident electron must have in order to first remove an electron from the atom basically, the incident energy must be equal to or grater than the binding energy of the electron

kVp- Tube Voltage

This affects both the number of x-ray photons emerging and the energy distribution. KVP affects number of photons produced than mA

Interactions between electron and electron produces _________________.

characteristic photons

What is thermionic emission?

"boiling off" electrons from the copper filament wire. The cathode has a heated filament that gets very hot like a toaster wire. The electrons are boiled off this filament and are accelerated towards the anode.

How can we reduce heat at the anode?

- Conduction (using a thick copper anode) - Rotating the anode - Using oil, water, or air to cool anode

Glass Envelope

- Encloses the anode and cathode assemblies - The envelope is evacuated

Bremsstrahlung

- German word that means "braking radiation" - The result of radiative interaction between a high-speed electron and a nucleus

Bremsstrahlung: Low-Voltage X-Ray Tubes

- In lower-voltage x-ray tubes (diagnostic x-ray units), it is technically advantageous to use a target which is oriented at 90° to the electron beam direction - Energy loss per atom by electrons depends on the square of the atomic number (Z^2) - efficiency is defined as the ratio of output energy admitted as x-ray to the input energy deposited by electrons

Reducing heat at the anode can prevent...

- Melting, pitting, and cracking of the target from thermal stress - Decreased output from rough target surface

Cathode

- Negatively charged electrode - Consists of a wire filament and a focusing cup - Potential difference between the electrodes is called tube voltage

Three Types of Generators:

- Single Phase - Three Phase - High Frequency Inverter

Half Value Layer (HVL)

-thickness of an absorber required to attenuate the beam intensity to half its original value -HVL specify the overall penetrating ability of the beam with a single parameter HVL = .693/u

As the incident electron's energy increases, the more likely the Brems photon will be FORWARD peaked:

- The direction of emission of bremsstrahlung photons depends on the energy of the incident electrons - at electron energy is below 100 KEV, x-rays are emitted more or less equally in all directions - as kinetic energy of the electron increases, the direction of x-ray emission becomes increasingly forward

The X-Ray Tube Operation

- The power supply applies a voltage across the anode and cathode - The power supply applies current to the filament - due to the resistance in the filament wire, the temperature increases and electrons are "boiled off" vie thermionic emission. - The negatively charged focusing cup "focuses" electrons into a pencil beam" - The voltage accelerates electrons toward the anode, achieving high velocities before striking the target - X-rays are produced by the interactions between the electrons and the nuclei in the target - The x-ray beam emerges through a thin glass window in the tube envelope (not shown in the diagram to the right)

X-Ray Energy Spectra

- X-Ray photons produced by an x-ray machine are a mix of bremsstrahlung and characteristic - The x-ray spectrum is a combination of the energy spectrum of the bremsstrahlung x-rays with the characteristic radiation of discrete energies super imposed upon it

What is the percentage of characteristic x-rays at diagnostic levels? R.T. levels?

-30% for diagnostic -only a few % for therapy energies (200 kV+) ***R.T. beam is primarily brems photons

X-Ray Tube Major Components

-High Voltage Power Supply -Anode -Cathode -Filament -Rotor & Stator -Glass Envelope -Tube Housing

X-Ray Tube Anode

-Metallic piece that contains the target -Commonly made of copper with embedded tungsten (W) target. -Positively charged electrode

Thermionic emission

-Resistance in the filament wire increases the temperature -Electrons are "boiled off" from the filament wire

Characteristic Radiation

-Result is an ionizing interaction -electron with kinetic energy E interacts with the atoms of the target by ejecting an orbital electron, such as a K, L, or M electron -this interaction leaves the atom ionized - When a vacancy is created in an orbit, an outer orbital electron will fall down to fill that vacancy - In so doing, the energy is radiated in the form of electromagnetic radiation - The energy that radiates from the outer shell of the atom that results from the filling of lower electron shells is called characteristic radiation - note that characteristic radiation is characteristic of the atoms in the target because of the spacing of the electron shells

Heel Effect

-The anode (target) itself attenuates the beam -Beam intensity variation can be as great as ~30% -Effect is particularly pronounced in diagnostic tubes due to low x-ray energy and steep target angles

Tube Housing

-The external structure of the x-ray tube -Provides structural support, supplies electric insulation, and shields the patient and personnel from stray radiation

X-ray production review continued

-The higher the energy of electrons bombarding the target, the more forward is the direction of x-ray emission -efficiency of x-ray production is proportional to the atomic number of target and voltage applied to the two. Efficiency is less than 1% for x-ray tubes operating at 100 KVP (99% of input energy is converted into heat). Efficiency improves considerably for high energy accelerator beams (30% to 95% depends upon energy). -filtration reduces the intensity but usually raises average energies bye preferentially removing low energy x-rays.

Three fundamental quantities to set at the generator console:

-Tube Voltage [kVp] -Tube Current [mA] -Exposure Time [s] X-ray "output" = (kVp)^2 x mAs (approximate) Combination of setting is referred to as the technique

To account for heel effect

-Use compensating filters -Orient thicker body parts toward the cathode

Why does the Heel Effect exist?

-X-rays are produced at various depths in the target -A spectrum of x-ray energies are produced -The x-rays' attenuation varies according to depth and energy

X-ray production review

-x-rays are produced by two different mechanisms: bremsstrahlung and characteristic x-ray emission. Useful x-ray beams in imaging and therapy are all brems -bremsstrahlung x-rays have a spectrum of energies. The maximum energy is numerically equal to the peak voltage. Average energy is about 1/3 of the maximum energy. -characteristic x-rays have discrete energies, corresponding to the energy level difference between shells involved in the electron transition. These result when a bombarding electron ejects an inner shell electrons in a target atom.

From the efficiency equation, it can be shown that the efficiency of x-ray production with a tungsten target (Z = 74) for electrons accelerated through 100 kV is less than ______%.

1% The remainder of input energy 99% appears as heat *note that the accuracy of the efficiency equation is limited to a few MV

What are the 2 mechanisms of electromagnetic interaction:

1. Interaction between electron and nucleus 2. Interaction between electron and electron

What 3 things decrease intensity/exposure rate of a beam?

1. Time 2. Distance 3. Sheilding

The average x-ray energy is approximately _______ of the maximum energy or kVp.

1/3

If you double the distance between the x-ray source and object, the intensity of the beam will be ________ the original intensity when it reaches the object.

1/4

X-Ray Generator

A device that supplies electrical power to the x-ray tube

Problem

An x-ray technologist makes an exposure at 120 kVp, 80 mA, 1 s. The film darkness is acceptable, but the image is blurry because the patient is fidgeting. The hnologist decides to reduce the time to 0.5 s. What mA should be chosen to keep the film darkness the same? If you half the time, you must double the mAs to keep the same exposure (darkness). 80mAs x 2 = 160 mAs

Line Focus Principle Equation

Apparent size: a Actual focal spot size: A Angle of target with respect to the electron beam: 0 a = A x Sin0

Focal Spot

Area on the target from which the x-rays are emitted Size depends on the filament size within the cathode

The efficiency of Brems x-ray production depends on what two factors?

Atomic # and Voltage ***efficiency of x-ray production depends on the atomic number (Z) and the voltage (V) applied to the tube

Focal Spot Size is (directly/indirectly) related to Tube Current.

Directly Related -Increases in proportion to tube current -Smaller focal spots generate more heat per unit area of target, thereby limiting currents and exposure -In therapy tubes, relatively larger focal spots are acceptable

Filters

Energy spectrum of a 200-kVp x- ray beam filtered by: A: 1-mm-thick aluminum filter B: Sn +Al C: Sn + Cu + AI (Thoreau's filter) Aluminum filter is typically used for diagnostic Combination filters used for Orthovoltage No added filtration required for Megavoltage

Relationship between x-ray output, filament current, and tube voltage:

Filament current affects the emission of electrons from the filament and, therefore, the tube current. line A

Focal Spot: Line Focus Principle

For Diagnostic tubes: 6°-17° a = 0.1 x 0.1 to 2x2 mm^2 For Therapy tubes: 30° a = 5x5 to 7x7 mm^2

99% of the applied energy manifests as __________.

Heat

X-Ray Tube Summary:

High Voltage Supply -Accelerates electrons towards the anode Anode/Target -Tungsten, positively charged, redu ce heat -focal spot: size, heel effect Cathode -Tungsten filament, focusing cup Tube Housing -Glass Envelope X-Ray Generator -Provides power to filament and high voltage supply Operating Characteristics -Tube voltage [kVp], tube current [mA], Exposure time[s]

Narrow Beam Attenuation of Monoenergetic X-Rays:

I = Io x e^x(-u) I = output intensity Io = Intensity with NO attenuation factor present x = thickness of attenuation factor -u = linear attenuation coefficient

Rule of Thumb:

If the % change in distance from the source is small, the % change in intensity of the radiation beam is 2x the % change.

How is a Bremsstrahlung photon created?

Incident electron passing near a nucleus is deflected from its path by the action of Coulomb forces of attraction and loses energy as bremsstrahlung photon.

As the voltage increases, the likelihood of brems interactions ____________________.

Increases

As the atomic # of a material increases, the likelihood of brems interactions _________________.

Increases - probability of bremsstrahlung production varies with Z^2 of the target material

As the atomic # of a material increases, the likelihood of characteristic x-rays _____________________.

Increases -With higher atomic number targets and the transitions involving inner shells such as K, L, M, and N, the characteristic radiations emitted are of high enough energies to be considered in the x-ray part of the electromagnetic spectrum -unlike bremsstrahlung, characteristic radiation is emitted at discreet energies -if the transition involved in electron filling in the K shell from the L shell, then the photon will have energy ie. hv = Ek - El ; where Ek and El are the electron binding energies of the K shell and the L shell, respectively

What is the difference between kVp and keV?

It's important to understand that KVP is not the same as KEV. The maximum voltage across the tube is given by the KVP. Electrons cross the tube almost instantaneously. At the instant that the voltage is at its maximum, those electrons accelerated across the tube will have the maximum possible energy. The maximum energy of the electrons keV(max), is numerically equal to the KVP. Even though they do have the same numerical value, they are not equal because they do not have the same units.

For diagnostic beams, the attenuation is specified with ________, _____________, and __________.

KVP, filtration, and HVL

For megavoltage beams, the attenuation is specified with ________ and ______________________.

MVE and percent depth dose in water

How does filtration change the shape of the x-ray spectrum?

Shape of x-ray energy spectrum is the result of: - Alternating voltage waveform applied to the tube - Multiple bremsstrahlung interactions with in the target filtration in the beam Difficult to characterize the being quality in terms of energy, penetrating power, degree of beam hardening.

Added Filtration

Placed externally to tube Purpose is to enrich the beam with higher-energy photons by absorbing the lower-energy components of the spectrum

Inherent Filtration

Preexisting in the X-ray tube: - Absorption in target - Glass walls of the tube - Thin beryllium window Usually equivalent to 0.5 mm to 1.0 mm aluminum

Angle of the target determines the ______ of the x-ray field and is generally angled between ____ and _____ degrees for diagnostic units.

Size ; 6-17 degrees orthovoltage uses 30 degree angle

High Frequency Inverter

State of the art generator Benefits: -Very low ripple (4-15%) -Low cost -Small size -Modular (easy to repair)

What three things make up a technique?

Technique: combination of KVP, mA, and time or mA multiplied by time equals mAs.

Why is that material used?

The choice of tungsten for filament (cathode) and target (anode) is based on it's having a high melting 3,370°C and a high atomic number (Z equals 74), which is needed to boost efficiency a factor of production.

What is the filament made of and why?

The filament is usually made of tungsten because of its high electrical resistance and high melting point

mA- Tube Current

The number of x-ray photons emitted is directly proportional to the tube current. The tube current has no effect on the energy distribution of the x-rays, only the intensity. REMEMBER: x-ray output is directly proportional to kVp^2 x mAs

What is Heel Effect?

The phenomenon that the x-ray beam intensity is greater toward the cathode side

What is Line Focus Principle?

The relationship of the angle of the anode (target) to the effective focal spot

What energy will a Bremsstrahlung photon posses?

The resulting bremsstrahlung photon may have any energy up to the initial energy of the electron. ie. if the incident e- has an energy of 120kV, the brems photon can posses energy levels from 0-120kV. - The direction of emission of bremsstrahlung photons depends on the energy of the incident electrons - at electron energy is below 100 KEV, x-rays are emitted more or less equally in all directions - as kinetic energy of the electron increases, the direction of x-ray emission becomes increasingly forward - The forward peaked nature of the bremsstrahlung x-rays for high energy beams is the reason transmission type targets are used in megavoltage x-ray tubes (accelerators)

X-Ray Generator

Total number of electrons striking the target is directly proportional to the product of the tube current [mA] and the time of the exposure [s]. mA x sec = "mAs" Assume that kVp stays the same

What happens to the beam with added filtration?

Transmitted beam hardens (achieves higher average energy and thus greater penetrating power)

T or F: A Bremsstrahlung interaction may result in partial or complete loss of electron energy.

True

T or F: An electron may have one or more bremsstrahlung interactions in the material

True

What material is the target and filament?

Tungsten

Anode Target is typically made of _______________.

Tungsten (which has a Z value of 74) or other high-Z material with a high melting point

X-Ray Production Problem

What is the ratio between the apparent size and actual size of the focal spot in a tube with target angle of 6°? a = A x Sin0 ——-> a/A = Sin0 a/A = Sin0 = Sin(6) = 0.1

Interactions between electrons and nucleus produces ________________ photons.

bremsstrahlung photons

As voltage is applied to the focusing cup, the size of the focal spot _____________.

decreases

As filtration increases, the intensity (# of photons) in the beam ___________________.

decreases -low energy photons are attenuated

As kVp increases, the size of the focal spot _______________.

decreases slightly

Exposure rate and tube current have a(n) (indirect/direct) relationship.

direct ; LINEAR

What is the job of the focusing cup?

directs the electrons toward the focal spot of the anode

A small increase in filament current will produce a large change in ______________________.

exposure rate line A

As the angle of the anode increases, the apparent focal spot size ________________.

increases

As the focal spot size decreases, the sharpness of the image ____________.

increases

As voltage increases, the intensity (# of photons) in the beam _________________.

increases -more electrons boiled off the filament and accelerated towards the anode

What is attenuation?

penetrating ability if the radiation

The output of the X-ray machine varies approximately as a ...

square of kilovoltage line C

The forward peaked nature of the bremsstrahlung x-rays for high energy beams is the reason _________________________________ are used in megavoltage x-ray tubes (accelerators)

transmission type targets

Constance of filament current is critical to the constance of the ___________________.

x-ray output