EXAM 2 Study Guide

A

Power source

What type of phase power is illustrated?

Single phase power

• uses a single AC waveform

• pulsating x-ray beam

• drops to 0 per cycle = 100% voltage ripple

• low energy & low penetrability = little diagnostic value

What type of phase power is illustrated?

Three phase power:

• combines three overlapping AC waveforms

• multiple waveforms that maintain nearly constant high voltage

• voltage never drops during exposure

What are the types of three-phase power?

1. 6-pulse three phase: reduces voltage ripple to 3.5%-13%; maintains 87%-100% of selected kVp

2. 12-pulse three phase: reduces voltage ripple down to ~3.5%; maintains 96.5%-100% of selected kVp

Blue highlighted

line voltage compensator

• usually wired to the autotransformer and automatically adjusts the power supplied to the x-ray machine to 220 volts

C

Circuit breaker

K

Autotransformer

Pink and Green highlighted

Pink: Major kVp selector

• large adjustments, increments of 10 kVp

Green: Minor kVp selector

• fine tune adjustments, increments of 1-2 kVp

E

Exposure timer / circuit timer

What is the AEC

automatic exposure control

• placed between patient and IR

• automatically terminates x-ray exposure the exact moment the IR receives the necessary amount of radiation to take a diagnostic image

• x-ray passes through the patient and strike the ionization chamber to allow free electrons to create an electrical charge that triggers the AEC

Where is the AEC located within the x-ray circuit?

in the primary circuit where the exposure timer sits

F

Step-up transformer

Purple highlight

Filament circuit

I

Step-down transformer

J

Rheostat (mA selector)

G

Rectifier

Explain the figure

Half-wave rectification

• blocks the negative half of the AC cycle, preventing voltage from swinging negatively

• wastes 50% of available electric power

• doubles exposure time = undesirable

Explain the figure

Full-wave rectification

• inverses the negative half-cycle into a positive pulse to remain continuously relative to the cathode

• utilizes 100% of electrical power

• cuts exposure time in half

Define voltage ripple

the energy that fluctuates from zero (on the line) to the maximum voltage (peak of wave)

• single phase power = 100% ripple (huge dips)

• three-phase power = low ripple (minimum dips)

• high frequency generators = nearly 0% ripple (nearly no dips)

What are the external components of the x-ray tube?

• Glass envelope

• Protective housing

• Support structures

7

Glass envelope

• inside the heavy protective housing

• maintains a vacuum to ensure accelerating electrons do not collide with air/gas molecules which can cause burnout

• usually made of Pyrex glass or metal

9

Target window (tube port)

• thinned area on bottom side of envelope below the anode target

• allows useful x-ray beam to exit the glass enclosure with minimal absorption before entering the collimator

Describe the protective housing

• lead-lined casing that surrounds the envelope with an oil bath

• provides mechanical support, thermal cushion, and isotropic radiation control (leakage/secondary radiation)

Describe the support structures

• external track, floor, or ceiling mounting assemblies attached directly to the protective housing

• ceiling support system (longitudinal/transverse), floor-to-ceiling (column), c-arm

6

Cathode

• x-ray beam intense

8

Filament in focusing cup

Explain small focal spot

• smaller AFS and EFS

• high spatial resolution

• lower heat dissipation

• lower mA techniques only (hands, wrists, feet)

Explain large focal spot

• larger AFS and EFS

• reduced spatial resolution (blur)

• higher heat dissipation

• high mAs techniques (chest/abd/spine)

What is the line-focus principle

angling the face of the anode target maintains a large AFS size and creates a small EFS and increases spatial resolution

Describe thermionic emission

literal boiling off of electrons from a filament by a flow of electrical current

What is the electron cloud

a gathered cluster of free electrons that are attracted to the anode target and hovering around the cathode waiting to be pushed across the x-ray tube

What is the space-charge effect

self-limiting factor caused by the space charge reaching a size commensurate with the current used and making it difficult for additional electrons to be emitted

1

Anode

• x-ray beam weaker

• two types: stationary overheats, rotating dissipates heat as it rotates

5

Anode target (tungsten target)

What is the anode heel effect

angling the target face causes the intensity of the x-ray beam to be less on the anode side because the “heel” of the target is in the path of the beam

What causes target failure

• high heat exposures (99% of projectile electron KE is converted to thermal heat)

• lacking warm-up procedures (cold anode)

• physical or structural damage to the tungsten surface (exceeding melting point)

4

Anode stem/neck

2

Rotor

3

Stator

• part of an induction motor made of electromagnets arranges in pairs around the rotor

• rotates through mutual induction

What are the properties of x-ray?

photon characteristics

• no mass, no charge

• lots of energy, travel at speed of light

electromagnetic spectrum

• share the same fundamental nature as visible light, gamma rays, and radio waves

wavelength and frequency relationship

• wavelength is inversely proportional to frequency and energy but the velocity stays the same

polyenergetic beam

• consist of million of photons across a wide range of different energy levels

What are target interactions

• different from x-ray interactions

• happens within the tube when projectile electrons interact with the target x-ray produced

What are the types of target interactions

Characteristics

• projectile electron ejects an inner shell electron from a target atom

• outer shell electron drops to fill vacancy

• release of photon with energy equal to exact difference of BE between two shells

• requires > 69.5 kVp to knock out tungsten inner shell

Bremsstrahlung

• projectile electron is attracted to the nucleus

• slows down and changes direction

• energy lost is released as an x-ray photon

• requires any # kVp to create x-rays

What is required to make x-ray photons

Source of electrons

• current (mA) passes through tungsten cathode filament causing thermionic emission

Acceleration (kVp)

• applying high voltage across the x-ray tube to create a strong potential difference

Deceleration

• energy is transformed into x-ray photons via Bremsstrahlung and characteristic interactions

What is a characteristic cascade

process of outer-shell electron filling inner-shell vacancies which creates a cascading effect

Know how technique changes the production of X-Rays (or type of X-Rays produced)

mAs

• controls the volume of projectile electrons hitting the target per second

• changing mAs = changes radiation quantity/intensity but no effect on beam quality
  
  

kVp

• controls the kinetic energy of projectile electrons

• changing kVp = changes beam quality + quantity

What are the properties of the x-ray beam

quality, quantity, emission spectrum, and filtration

Explain beam quality

refers to the penetrating power of the beam and varies directly with changes in kVp and filtration

What is the half-value layer

thickness of absorbing material (3-5 mm aluminum) necessary to reduce the energy of the x-ray beam to ½ of its original intensity (mAs)

Describe beam quantity

refers to the total number of x-ray photons in a beam and is affected by mAs, kVp, distance, and filtration

What is the x-ray emission spectrum

graphic representation of the x-ray beam as a whole, combining relevant parts of the discrete and continuous emission spectra

Describe discrete x-ray emission spectrum

• photon energy depends strictly on the specific difference in electron BE of target atom

• produced by characteristic x-rays compose of predictable photon energies

Describe continuous emission spectrum

• incoming projectile electrons lose amounts of KE depends how close they pass to the target nucleus

• produced by Bremsstrahlung radiation containing continuous range of photon energies extending from the maximum selected kVp to 0

Describe heterogeneous/polyenergetic emission

• describes the primary x-ray beam as consisting of many different photon energies

• caused by both continuous Bremsstrahlung interactions and discrete characteristic interactions simultaneously at the anode target

Describe characteristic peak

• a distinct vertical spike on the spectrum graph

• represents useful K-shell characteristic x-rays for a tungsten target

• only appears on graph when technique is set to > 69.5 kVp

Factors that affect the spectrum

beam quantity (mAs) - intensity

• controls volume of projectile electrons flowing from cathode to anode per second

• graph: changing mAs raises amplitude height

beam quality and quantity (kVp) - avg photon energy

• controls potential difference across the tube for projectile electrons

• graph: changing kVp also raises almplitude height and shift the curve up→right

filtration

• aluminum absorption of undesirable low-energy photons

• hardens beam = decreases quantity + increasing quality

generator type

• three-phase and high-frequency generators maintain voltage near peak levels

• decreasing voltage = increases intensity (quantity) + avg photon energy (quality)

target atomic number

• targets with higher atomic # = higher BE

• higher Z targets = increase x-ray quantity + quality

Describe filtration

any material (usually aluminum) designed to effectively absorb photons from the x-ray beam

What are the types of filtration

inherent filtration

• glass or metal enclosure of an x-ray tube

added filtration

• thin sheet of aluminum positioned between protective x-ray tube housing and the x-ray beam collimator

compensating filter

• additional filter added by the radiographer to compensate in the differences in subject radiopacity

What are the minimum requirements for each type of filter

standard material

• aluminum or Al/eq or HVL

total filtration formula

• inherent + added = total filtration

regulatory standard

• general diagnostic x-ray equipment requires a minimum total filtration of ~2.5mm Al/eq

MATH

Transformer Law

MATH

Heat Units

MATH

Energy of a characteristic photon

MATH

Energy of a bremsstrahlung photon

MATH

Energy of a photon after passing through a HVL

MATH

Half value layer required to reduce the beam by a certain amount