* Chapter 6: The X-ray Imaging System



Anthony’s Fund. Dx Imaging Study Guide – Penguin Points

Chapter 3: The Structure of Matter

- An atom is the smallest particle that has all of the properties of an element

- The fundamental particles of an atom are the electron, the proton, and the neutron

- The atom is essentially empty space

- In their normal state, atoms are electrically neutral; the electric charge on the atom is zero

- Ionization is the removal of an orbital electron from an atom

- Maximum electrons per shell( 2n2

-where n is the shell number

- Electron arrangement: the number of electrons in the outermost shell of an atom is equal to its group in the periodic table. The number of electrons in the outmost shell determines the valence of an atom. The number of the outermost electron shell of an atom is equal to its period in the periodic table

- No outer shell can contain more than eight electrons

- The force that keeps an electron in orbit is the centripetal force

- The atomic mass number and the precise mass of an atom are not equal

- Isotopes: atoms that have the same atomic number but different atomic mass numbers

- Isobar: atomic nuclei that have the same atomic mass number but different atomic numbers

- Isotone: atoms that have the same number of neutrons but different numbers of protons

- Isomer: have the same atomic number and the same atomic mass number

- Molecule: the combination of atoms of various elements

- A chemical compound is any quantity of one type of a molecule

- The smallest particle of an element is an atom; the smallest particle of a compound is a molecule

- Radioactivity is the emission of particles and energy in order to become stable

- Radioactive decay results in emission of alpha particles, beta particles, and usually gamma rays

- Half-life of a radioisotope is the time required for a quantity of radioactivity to be reduced to one-half its original value

- An alpha particle is a helium nucleus that contains two protons and two neutrons

- A beta particle is an electron emitted from the nucleus of a radioactive atom

- X-rays and gamma rays are the only forms of ionizing electromagnetic radiation of radiologic interest

Chapter 4: Electromagnetic Energy

- An x-ray photon is a quantum of electromagnetic energy

- The velocity of all electromagnetic radiation is 3x108m/s (the speed of light)

-the same as 186,000 miles per second

- Amplitude is one-half the range from crest to valley over which the sine wave varies

- Frequency is the number of wavelengths that pass a point of observation per second

- At a given velocity, wavelength and frequency are inversely proportional

- The electromagnetic spectrum includes the entire range of electromagnetic energy

- Diagnostic ultrasound is not a part of the electromagnetic spectrum

- The energy of a photon is directly proportional to its frequency

- The only difference between x-rays and gamma rays is their origin

- Visible light is identified by wavelength, RF is identified by frequency, and x-rays are identified by energy

- Photons interact with matter most easily when the matter is approximately the same size as the photon wavelength

-X-rays behave as though they are particles

- Visible light behaves like a wave

- Electromagnetic energy attenuation is the reduction in intensity that results from scattering and absorption

- Electromagnetic energy (radiation) intensity is inversely related to the square of the distance from the source

I1 = d22

I2 = d12

- Inverse square law: can be applied to distances greater than 7x the longest dimension of the source

- The x-ray photon is a discrete bundle of energy

- The energy of a photon is directly proportional to its frequency

Chapter 6: The X-ray Imaging System

- The autotransformer has a single winding and is designed to supply a precise voltage to the filament circuit and to the high-voltage circuit of the x-ray imaging system

- The kVp determines the quality of the x-ray beam

- Thermionic emission of the release of electrons from a heated filament

- The product of x-ray tube current (mA) and exposure (s) is mAs, which is also electrostatic charge (C)

- Most exposure times are electronic and are controlled by a microprocessor

- mAs timers are used on falling-load and capacitor discharge imaging systems

- The high-voltage generator contains three primary parts: the high-voltage transformer, the filament transformer, and rectifiers

- Radiographers outside the US and Japan may use a frequency of 50 Hz

- Rectification is the process of converting AC to DC

- Voltage rectification is required to ensure that electrons flow from cathode to anode only

- Electron flow is used when medical imaging systems are described

- With three-phase power, the voltage impressed across the x-ray tube is nearly constant, never dropping to zero during exposure

- Full-wave rectification or high-frequency voltage generation is used in almost all stationary x-ray imaging systems

- During capacitor discharge, the voltage falls approximately 1 kV/mAs

- Less voltage ripple results in greater radiation quantity and quality

- Power= current x potential

Watts= amperes x volts

- High-voltage generator power (kW) = maximum x-ray tube current (mA) at 100 kVp and 100 ms

- For three-phase and high-frequency:

Power rating (kW)= mA x kVp

1000

- For single-phase:

Power rating (kW)= (0.7) mA x kVp

1000

Chapter 7: The X-ray Tube

- Protective housing guards against excessive radiation exposure and electric shock

- X-ray tubes are designed w/a glass or a metal enclosure

- The cathode is the negative side of the x-ray tube, it has primary parts: a filament and a focusing cup

- Tungsten vaporization w/deposition on the inside of the glass enclosure is the most common cause of tube failure

- The x-ray tube current is adjusted by controlling the filament current

- Thermionic emission at low kVp and high mA can be space charge limited

- The anode is the positive side of the x-ray tube; it conducts electricity and radiates heat and contains the target

- Higher tube currents and shorter exposure times are possible w/the rotating anode

- The rotating anode is powered by an electromagnetic induction motor

- The focal spot is the actual x-ray source

- The line-focus principle results in an effective focal spot size much less than the actual focus spot size

- The smaller the anode angle, the larger is the heel effect

- The heel effect results in smaller effective focal spot and less radiation intensity on the anode side of the x-ray beam

- Excessive heat results in reduced x-ray tube life

- Maximum radiographic techniques should never be applied to a cold anode

- The most frequent cause of abrupt tube failure is electron arcing from filament to enclosure due to vaporized tungsten

- Single phase:

Heat Units (HU)= kVp x mA x s

- Three-phase and high-frequency:

HU= 1.4 x kVp x mA x s

Chapter 8: X-ray Production

- Kinetic energy is the energy of motion

- Approximately 99% of the kinetic energy of projectile electrons is converted to heat

- Characteristic x-rays are emitted when an outer shell electron fills an inner shell electron

- Only the K-characteristic x-rays of Tungsten are useful for imaging

- This type of x-radiation is called characteristic because it is characteristic of the target element

- Bremsstrahlung x-rays are produced when a projectile electron is slowed by the electric filed of a target atom nucleus

- In the diagnostic range, most x-rays are Bremmstrahlung x-rays

- A discrete spectrum contains only specific values

- A continuous spectrum contains all possible values

- Characteristic x-rays have precisely fixed (dsicrete) energies and form a discrete emission spectrum

- Bremmstrahlung x-rays have a range of energies and form a continuous emission spectrum

- Maximum x-ray energy is associated w/ the minimum energy wavelength

- A change in mA or mAs results in a proportional change in the amplitude of the x-ray emission spectrum at all energies

- A change in kVp affects both the amplitude and the position of the x-ray emission spectrum

- A change in kVp has no effect on the position of the discrete x-ray emission spectrum

- In the diagnostic range, a 15% increase in kVp is equivalent to doubling the mAs

- The result of added filtration is an increase in the average energy of the x-ray beam w/an accompanying reduction in x-ray quanity

- Increasing target atomic number enhances the efficiency of x-ray production and the energy of characteristic and bremmstrahlung x-rays

- Because of reduced ripple, operation w/three-phase power or high-frequency is equivalent to an approximate 12% increase in kVp, or almost a doubling effect of mAs over single phase power

Chapter 9: X-ray Emission

- X-ray quantity is the number of x-rays in the useful beam

- X-ray quantity is proportional to mAs

- X-ray quantity is proportional to the kVp2

- X-ray quantity is inversely proportional to the square of the distance from the source

- Adding filtration to the useful x-ray beam reduces patient dose

- Penetrability is one description of the ability of an x-ray beam to pass through tissue

- Attenuation is the reduction in x-ray intensity that results from absorption and scattering

- The HVL of an x-ray beam is the thickness of absorbing material necessary to reduce the x-ray intensity by half of its original value

- HVL is the best method for specifying x-ray quality

- X-ray beam quality can be identified by voltage or filtration, but HVL is most appropriate

- Increasing the kVp peak increases the quality of an x-ray beam

- Increasing filtration increases the quality of an x-ray beam

- Added filtration results in increased HVL

Chapter 10: X-ray Interaction With Matter

- Coherent scattering is of little importance to diagnostic imaging

- The probability of the Compton effect is inversely proportional to x-ray energy (1/E) and independent of atomic number

- Compton scattering reduces image contrast

- The photoelectric effect is total x-ray absorption

- Pair production does not occur during x-ray imaging

- Photodisintegration does not occur in diagnostic radiology

- Differential absorption occurs b/c of Compton scattering, photoelectric effect, and x-rays transmitted through the patient

- Differential absorption increases as the kVp is reduced

- To image small differences in soft tissue, one must use low kVp to get maximal differential absorption

- The interaction of x-rays w/tissue is proportional to the mass density of the tissue regardless of the type of interaction

-Attenuation is the product of absorption and scattering

Chapter 14: Control of Scatter Radiation

- Collimation reduces patient dose and improves contrast resolution

- Approximately 1% of x-rays incident on the patient reach the image receptor

- Scatter radiation increases as the field size of the x-ray beam increases

- Compression of anatomy improves spatial resolution and contrast resolution and lowers patient dose

- Reduced image contrast results from scattered x-rays

- Collimation reduces patient does and improves contrast resolution

- Under no circumstances should the x-ray beam exceed the size of the image receptor

- Total filtration= inherent filtration + added filtration

- Grid ration= grid height

width of interspace material

- High ratio grids increase the patient radiation dose

- The use of high-frequency grids requires high radiographic technique and results in higher patient radiation dose

- The principle function of a grid is to improve image contrast

- The contrast improvement factor is higher for high-ratio grids

- As the Bucky factor increases, radiographic technique and patient dose increases proportionately

- The main disadvantage of parallel and crossed grids is grid cutoff

- High-ratio grids have less positioning latitude than low-ratio grids

- In general, grid ratios up to 8:1 are satisfactory at tube potentials below 90 kVp. Grid ratios above 8:1 are used when kVp exceeds 90 kVp

- Grid selection factors:

1. patient dose increases with increasing grid ratio

2. high-ratio grids are used for high kVp examinations

3. patient dose at high kVp is less than that at low kVp

- One disadvantage of the air-gap technique is image magnification with associated focal-spot blur

Chapter 11: Radiographic Film

- Image forming x-rays are those that exit the patient and interact w/the image recptor

- The base of radiographic film is 150 to 300 (m thick , semirigid, lucent, and made of polyester

- The latent image is the invisible change that is induced in the silver halide crystal

- The ion is an atom that has too many or too few electrons and therefore has an electric charge

- The result is the same whether the interaction involves visible light from intensifying screens or direct exposure by x-rays

- Large-grain emulsions are more sensitive than small-grain emulsions

- Crossover is the exposure of an emulsion caused by light from the opposite radiographic intensifying screen

- Rare Earth screens are made with rare Earth elements- those w/ atomic numbers of 57-71

- Exposure= Intensity x Time

= Constant Optical Density

- The fog level for unprocessed film is approximately 0.2 mR

- It is bad practice to store film and boxes of chemistry in the same cupboard

Chapter 13: Radigraphic Intensifying Screens

- The radiographic intensifying screen amplifies the effect of image-forming x-rays that reach the screen film cassette

- The phosphor converts the x-ray beam into the light

- Isotropic emission refers to radiation emitted w/equal intensity in all directions

- Higher conversion efficiency results in increased noise (quantum mottle)

- Generally, those conditions that increase the IF, reduce special resolution

- In mammography, the screen is positioned in contact with the emulsion on the side of the film away from the x-ray source, to reduce screen blur and improve special resolution

- Screen-film compatibility is essential; use only those films for which the screens are designed

- Rare Earth radiographic intensifying screens have the principle advantage of speed

- The combination of improved CE (conversion efficiency) and higher DQE (detective quantum efficiency) results in the increased speed of rare earth radiographic intensifying screens

Chapter 12: Processing the Latent Image

- Developing is the stage of processing during which the latent image is converted to a visible image

- Fixing the silver halide that was not exposed to radiation is the process of clearing it from the emulsion and hardening the emulsion to preserve the image

- Synergism occurs when the action of two agents working together is greater that the sum of the action of each agent working independently

- Lack of sufficient glutaraldehyde may be the biggest cause of problems with automatic processing

- Archival quality refers to the permanence of the radiograph: the image does not deteriorate with age but remains in its original state

- Silver halide stain is the most common cause of poor archival quality

- The shorter dimension of the film should always be against the side rail, so the proper replenishment rate is maintained

- The speed of the transport system is controlled by the speed of the motor and the gear reduction system used. The tolerance on this mechanical assembly is rigid

- Cleaning the tanks and the transport system should be a part of the routine maintenance of any processor

- Most processing faults leading to damp film are due to depletion of glutaraldehyde, the hardener in the developer

- A finished radiograph that is damp easily picks up dust particles that could result in artifacts

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