RT 131 Unit 3: Ch 8+9+10

In order for xrays to be produced, there must be three things:

1. source of electrons (filament & mAs)

2. means of accelerating electrons to anode target (kVp)

3. deceleration of electrons

When the electrons in the filament are boiled off, what is used to propel them to the anode?

kVp (voltage)

T/F: Xrays are produced when projectile electrons gain a proportional amount of energy

False

How is energy being lost from the projectile electrons?

Projectile electrons are either decelerating or transitioning between the inner shells of the tungsten atoms

Which exposure factor determines the total quantity of electrons boiled off the filament via thermionic emission?

mA/mAs

What is the specific term for the process where the filament heats up and releases electrons?

thermionic emission

What exposure factor provides the potential difference required to accelerate projectile electrons from the cathode to the anode?

kVp

The high-speed projectile electrons interact with the ____ or ____ electrons of tungsten atoms in the anode to produce xrays.

orbital or inner

When projectile electrons bombard into the anode target and lose kinetic energy, what two primary target interactions occur?

Bremsstrahlung and characteristic xrays

From what area of the anode target are xrays projected out towards the patient?

The effective focal spot

Briefly describe production of xrays

1. mA/mAs is selected to tell how many electrons are being boiled off in filament

2. filament heats up and boils off electrons

3. kVp is applied to increase kinetic energy and accelerate electrons from cathode to anode

4. projectile electrons bombard with tungsten target and interact with its atoms

5. projectile electrons lose energy when interacting with tungsten atoms in target

6. xrays are projected out of effective focal spot

During the process of X-ray production at the anode target, what percentage of the kinetic energy of the projectile electrons is converted into X-ray photons, and what happens to the remaining energy?

Less than 1% of the kinetic energy is converted into X-ray photons. The remaining 99% is converted into thermal energy (heat) via excitation of outer-shell electrons.

What minimum threshold energy is required to produce useful Characteristic X-rays in a tungsten target?

A minimum of 69.5 keV (or a set technical factor of at least 70 kVp) is required.

The projectile electrons travel from the cathode to anode at ___.

half the speed of light

T/F: The voltage across the tube is not constant, causing all projectile electrons to gain different amounts of kinetic energy and produce a heterogenous beam

True

The interactions between the projectile electrons from the cathode and the orbital shell electrons in the tungsten target are determined by the ____.

binding energies of orbital shells

The binding energy can vary between different elements because of what?

The shell #, atomic #, and the element itself

What are the three interactions that can happen between the projectile electrons from cathode to tungsten atoms in anode?

1. Heat is produced

2. Characteristic xrays

3. Bremsstrahlung xrays

The excitation of outer-shell electrons from projectile electrons produces ___.

heat

Replacing inner-shell electrons with cascading outer-shell electrons emits energy in the form of _____.

characteristic xrays

Because of the binding energy of a K-shell in the tungsten atom, the kinetic energy of a projectile electron needs to be ____ to replace an inner shell electron.

69 kVp or higher

T/F: Bremsstrahlung xrays are more penetrating because they occur at discrete energy levels

False

Characteristic xrays contribute less than ___ of an xray beam. The majority of xray production results from Bremsstrahlung radiation.

15%

Define cascade effect

Outer shell electrons fill inner-shell vacency

T/F: If a kVp is lower than 69, no characteristic xrays will be produced

True

The energy of the characteristic xray is dependent on the ____.

difference between binding energies of inner and outer shells

The energy of the characteristic xray is dependent on the ____.

difference between binding energies of inner and outer shells

What specific atomic event creates the characteristic x-ray photon?

1. An incident electron possesses enough energy to ionize a tungsten atom by ejecting a K-shell electron

2. An outer-shell electron (such as from the L-shell or M-shell) then drops down to fill the vacancy via cascade effect, emitting a discrete photon equal to the difference in binding energies between the two shells.

What specific atomic events create Bremsstrahlung radiation?

1. Incoming electron passes very close to the nucleus of target atom

2. Sudden deceleration causing loss of kinetic energy and deviation from course stimulating projectile electrons to release energy

T/F: Bremsstrahlung radiation has a heterogenous beam because the radiation ranges from zero to max kVp set

True

The quality or penetrating power of an xray beam is controlled by ___.

kilovoltage peak (kVp)

Useful characteristic x-rays are produced in tungsten by:

removing a K-shell electron

Characteristic x-rays are called characteristic because they are characteristic of the ____.

target element/atomic #/Z#

In a tungsten-targeted x-ray tube operated at 90 kvp, the most abundant x-ray would be a ___.

30 keV bremsstrahlung x-ray

If mA/mAs/time is increased, then the intensity of the beam will ___.

increase

If kilovoltage is decreased, then the penetrating power and intensity of the beam will ___.

decrease

The x-ray emission spectrum is a visualization of the x-ray tube output demonstrating the ____ of x-rays vs. the ___ of the x-rays.

intensity; energy

The x-ray emission spectrum is heavily affected by these factors:

mA/mAs, kVp, added filtration, target material, and voltage waveform

The majority of the emission spectrum contains ____.

bremsstralung x-rays

The only time where characteristic x-rays appear on the emission spectrum is when ___.

Energy is greater than 69 kVp

The amplitude or highest # of XRs in the bremsstrahlung x-ray emission spectrum are created at about ___.

1/3 of the maximum energy

According to the emission spectrum, how would the curve change if mA/mAs decreased?

Intensity (amplitude) decreases, energy stays the same

According to the emission spectrum, how would the curve change if kVp decreased?

Intensity (amplitude) decreases, energy decreases (shifts to left)

According to the emission spectrum, how would the curve change if filtration is added?

Intensity (amplitude) decreases, energy increases (shifts to right)

According to the emission spectrum, how would the curve change if target material decreased from tungsten to molybdenum?

Intensity (amplitude) decreases, energy decreases (shifts to left), and characteristic line shifts left

According to the emission spectrum, how would the curve change if voltage generator increased from single phase waveform to high-frequency waveform?

Intensity (amplitude) increases, energy increases (shifts to right)

Increasing mA/mAs/time or tube current causes a ___.

proportional increase in intensity/# of XRs/quantity/amount of XRs (amplitude increases)

If mA/mAs doubles, then the intensity ___

doubles

If mA/mAs is halved, then intensity is ___

halved

T/F: In diagnostic range, a 15% decrease in kVp is equivalent to doubling the mAs in order to maintain receptor exposure

False

Define the 15% rule

In order to maintain receptor exposure, we can increase kVp by 15% and decrease mAs by ½

Increasing the tube voltage (kVp) would cause what?

Increase in penetrability of beam (energy/quality)

Increase in intensity of beam (quantity/# of XRs)

What does adding filtration to the intensity of the beam? What about the energy?

Decreases number of XRs since the lower energy photons are absorbed

Increase energy because the beam quality is higher

If the filtration in the beam went from 4 mL of Al to 2mL, that means the energy of the beam ___ while the intensity ___.

decreases; increases

Going from a less dense to more dense target material not only increases the binding energy of the inner shell electrons, but also shifts the ___.

characteristic x-rays line

Increasing the target atomic number (also the same as changing to a heavier material) causes what?

Increase in intensity

Increase in energy

Shifts characteristic x-ray line to the right

Having a more efficient generator will increase the # of waveforms, which in turn increases ___.

both kVp (energy) and mAs (intensity)

If a radiographic technique calls for 85 kVp on single-phase equipment (generator), then on three-phase equipment, a ___ will produce similar results.

12% reduction in kVp

Several factors impact the intensity of the xray beam including:

mAs, kVp, distance, and filtration

X-ray intensity is measured in ___.

mGya/air kerma/# of photons produced in air

What does air kerma stand for?

K - kinetic

E - energy

R- released (in)

M/A - matter, mass, material

Measuring x-ray intensity is the same as measuring the ___.

exposure in air before penetrating tissue

number of x-ray photons

amount of x-ray photons

When increasing mAs, the x-ray quantity ___.

increases proportionally

Which of the factors is the main driving force for patient dose?

mAs

When the kVp is increased, the x-ray quantity ___.

increases proportionally to kVp²

Increasing the kilovoltage will do what to the x-ray quality and quantity?

Increases quality (energy)

Increases quantity (intensity)

T/F: According to inverse square law, the intensity of the x-ray is inversely proportional to the square of the distance.

True

As the distance from the beam doubles, the x-ray intensity is reduced by ___

¼

Adding filtration to the useful x-ray beam does what?

Reduce patient dose (ESD - entrance skin dosage)

How does adding filtration decrease the intensity of the x-ray beam?

It takes away lower energy photons that only introduce the patient to more dose

Since filtration will take away lower energy photons, the quality of the beam ____.

increases

How many mL of Al (aluminum) filtration is required for most general x-ray machines?

1-5 mL

Measuring the energy of an x-ray beam is the same as measuring ___.

penetrability of beams

quality of x-ray photons in beam

Define penetrability

The ability of x-rays to penetrate deeper in tissue and transmit through the patient to the IR

As the energy of the x-ray beam increases, the penetrability of the beam through the patient or shielding material is ___.

increased

Main factors that affect x-ray beam energy include:

kVp and filtration

T/F: The penetrating power of an x-ray beam is increased when filtration is added

True

T/F: Increasing the kilovoltage will increase the penetrating power in the x-ray beam

True

The energy or quality of an x-ray beam is measured by __.

half-value layer (HVL)

The half-value layer (HVL) of an x-ray beam is what?

The thickness or density of absorbing material (filtration) used to reduce the x-ray intensity to half of its original value

A diagnostic x-ray beam has an HVL of ____. The HVL of soft tissue is ___.

3-5 mm AL; 3-6 cm

Increasing the mAs will increase x-ray intensity, but not ___.

energy (it stays put)

When kVp is increased, the number of x-ray photons in the beam ___, which means the penetrating power of the beam ___.

increases; increases

T/F: An increase in distance will increase the x-ray intensity and energy

False

Is the energy of the x-ray beam influenced by the changes in distance?

No

For diagnostic equipment to keep operating at 100 kVp, a minimum of ____ is FDA required.

2.5 mm of Al

The glass or metal enclosure in the tube is ___ filtration.

inherent

The thing sheets of aluminum placed between the tube housing and collimator mirror is ____ filtration.

added

Inherent filtration makes up about ____ of filtration of the tube.

0.5 mm of Al

Added filtration makes up about ___ of filtration of the tube.

2.0 mm of Al

A _____ is used when there are differences in thickness or opacity in a subject that need uniform exposure.

compensating filter

List the different types of compensating filters

wedge, bow-tie, trough, and conic/convex/concave

Which compensating filter is used during radiography of a body part that varies in thickness, such as a foot?

wedge filter

How is a wedge filter positioned to obtain uniform exposure?

The thick portion is placed over thinner part of the body

The thinner portion is placed over thicker part of the body

Which compensating filter can be used during chest radiography?

trough or bilateral wedge filter

How is a trough filter positioned to obtain uniform exposure of the chest?

Thinner central region is placed over middle of the chest (mediasteum)

Thicker lateral regions are placed over lung fields

Which compensating filters are mainly used in computed tomography imaging systems?

bow-tie

Which compensating filter can used in digital fluoroscopy or certain facial bone imaging?

conic/concave/convex

There are a few interactions that can occur between matter and x-rays. Name the interacts that are important to radiography.

Coherent scattering

Compton scattering

Photoelectric effect

Which two x-ray interactions are most important to creating an x-ray image?

compton scattering and photoelectric effect