- Properties of X-rays:
- Electromagnetic Nature: X-rays are transverse electromagnetic waves consisting of rapidly changing electric and magnetic fields that travel at the speed of light .
- Energy and Wavelength: The energy of an x-ray photon is directly proportional to its frequency and inversely proportional to its wavelength. High-energy x-rays have higher frequencies and shorter wavelengths.
- Clinical Range: Diagnostic x-rays typically possess energies between 10 and 150 keV.
- Penetrating Power: X-rays with higher energy (shorter wavelengths) are more penetrating. Highly penetrating beams require fewer incident x-rays to create an image, directly reducing the radiation dose to the patient.
- The X-ray Tube
- Cathode (Negative): Houses the filament wire and focusing cup. Heating the filament causes “thermionic emission,” boiling off electrons that will form the tube current.
- Anode (Positive): Contains the target (commonly tungsten, or molybdenum/rhodium for mammography) and a rotating assembly (rotor/stator) to distribute heat.
- Energy Conversion: X-ray production is highly inefficient; approximately 99% of the bombarding electrons’ kinetic energy is converted into heat, leaving only about 1% converted into x-rays.
- Line Focus Principle: The anode target is beveled (typically a 12.5-degree angle) to distribute heat over a larger actual focal spot while creating a smaller effective focal spot relative to the image receptor, which minimizes image blur and improves spatial resolution.
- Heel Effect: The x-ray beam’s intensity is lower on the anode side of the field because x-rays must travel through more of the target material, causing self-attenuation.
- X-ray Production Mechanisms
- Bremsstrahlung (Braking) Radiation: Accounts for 85% to 100% of the x-ray beam. Produced when bombarding electrons are electrostatically attracted and slowed down by the positively charged nucleus of the target atoms, emitting their lost kinetic energy as x-rays. This produces a continuous, polyenergetic spectrum where the average x-ray energy is approximately one-third to one-half of the set kVp.
- Characteristic Radiation: Accounts for 0% to 15% of the beam. Produced when an incident electron knocks an inner-shell (e.g., K-shell) electron out of orbit, causing an outer-shell electron to drop in and emit an x-ray of a fixed, discrete energy corresponding to the difference in binding energies.
- K-Shell Thresholds: To produce K-characteristic x-rays in a tungsten target, the tube voltage must exceed tungsten’s K-shell binding energy of 69.5 kVp.
- Beam Control: Quantity, Quality, and Filtration
- Milliamperage (mA) and mAs: Controls the tube current (number of electrons boiled off). Doubling the mAs directly doubles the total number (quantity) of x-rays produced without altering the beam’s maximum or average energy.
- Kilovolt Peak (kVp): Determines the maximum accelerating voltage across the tube. Increasing kVp increases the maximum and average x-ray energy (beam quality/penetration) while simultaneously increasing the total number of x-rays produced.
- Beam Quality: Measured by the Half-Value Layer (HVL), which is the thickness of material (usually aluminum) required to reduce the beam’s intensity by 50%. Quality increases with higher kVp and greater added filtration.
- Filtration: Adding filters to the collimator assembly preferentially removes low-energy, non-penetrating x-rays. This “hardens” the beam by increasing its average energy, significantly reducing the radiation dose to the patient’s skin.
Xray Properties, Xray Tube, Xray Production Self-assessment Quiz
Chapter 1: Properties of X-rays
Q1-1: All of the following are electromagnetic waves, except
(a) Heat
(b) Radiation used in magnetic resonance imaging (MRI)
(c) Sunlight
(d) Sound
(e) FM radio signals
- Answer: (d) Sound
- Explanation: Gamma rays, x-rays, ultraviolet waves, infrared waves, light of all types, microwaves, and radio waves are all electromagnetic waves; sound is a longitudinal pressure wave that travels at about 330 meters per second and is not an electromagnetic wave.
Q1-2: If the wavelength of an x-ray is reduced to half, its energy is
(a) Increased by 4
(b) Increased by 2
(c) Unchanged
(d) Decreased to 0.5
(e) Decreased by 0.25
- Answer: (b) Increased by 2
- Explanation: Energy is inversely related to wavelength; therefore, decreasing the wavelength by half makes the energy two times greater.
Q1-3: If the wavelength of an x-ray is reduced to half, its speed is
(a) Increased by 4
(b) Increased by 2
(c) Unchanged
(d) Decreased to 0.5
(e) Decreased by 0.25
- Answer: (c) Unchanged
- Explanation: All electromagnetic waves travel at the speed of light in a vacuum, regardless of their wavelength or frequency.
Q1-4: If the wavelength of an x-ray is reduced by half, its frequency is
(a) Increased by 4
(b) Increased by 2
(c) Unchanged
(d) Decreased to 0.5
(e) Decreased by 0.25
- Answer: (b) Increased by 2
- Explanation: Wavelength and frequency are inversely proportional, meaning if the wavelength is half of a specified value, the frequency is twice as large.
Q1-5: The x-rays that penetrate best through patient tissue have the smallest
(a) Frequency
(b) Energy
(c) Speed
(d) Wavelength
(e) Quanta
- Answer: (d) Wavelength
- Explanation: The most penetrating x-rays have small wavelengths, which correspond to high frequency and high energy.
Q1-6: The wavelength of a 60-keV x-ray is about
(a) 4.5 microns
(b) 0.2 angstroms
(c) 0.001 mm
(d) 1.25 x $10^{-8}$ cm
(e) 6.3 MB
- Answer: (b) 0.2 angstroms
- Explanation: Using the relationship $\lambda = 12.4 / E$, wavelength equals $12.4 / 60 \text{ keV} \approx 0.2$ angstroms.
Q1-7: As the energy of the x-rays is increased, the x-rays have
(a) Faster speed
(b) Lower frequency
(c) Shorter wavelength
(d) Longer wavelength
(e) The same speed, wavelength, and frequency
- Answer: (c) Shorter wavelength
- Explanation: The energy of x-rays is directly related to the frequency and inversely related to the wavelength.
Q1-8: For equal image quality, radiographs have lower radiation doses if the x-rays have
(a) Higher frequency
(b) Longer wavelengths
(c) Faster speeds
(d) More quanta
(e) Longer period
- Answer: (a) Higher frequency
- Explanation: More penetrating (higher energy, higher frequency) x-rays mean fewer incident x-rays are needed to reach the image receptor, which reduces the patient’s radiation dose.
Q1-9: All of the following are transverse waves, except
(a) Gamma rays
(b) Ultraviolet waves
(c) Microwaves
(d) X-rays
(e) Diathermy waves
- Answer: (e) Diathermy waves
- Explanation: Diathermy treatments use high-power ultrasound waves, which are longitudinal waves (force is in the same direction as motion), unlike electromagnetic waves which are transverse.
Q1-10: To make certain that the x-rays have dissipated after clinical radiography, one should wait at least _____ before entering the room.
(a) 50 nanoseconds
(b) 50 microseconds
(c) 50 femtoseconds
(d) 50 milliseconds
(e) 50 deciseconds
- Answer: (a) 50 nanoseconds
- Explanation: X-rays travel at the speed of light and are absorbed nearly instantaneously by the lead walls or after scattering, so there is no need to wait to enter a room after an exposure.
Chapter 2: The X-ray Tube
Q2-1: The x-rays that penetrate through the housing of the x-ray tube are called _____ radiation.
(a) Off focus
(b) Primary
(c) Heel effect
(d) Leakage
(e) Secondary
- Answer: (d) Leakage
- Explanation: By definition, leakage radiation is the amount of x-rays that penetrate through the lead internal lining of the x-ray tube housing.
Q2-2: Having fewer x-rays on the anode side of the image receptor in comparison with the cathode side is called
(a) Off focus
(b) Heel effect
(c) Line focus principle
(d) Effective focal spot
(e) Positive beam limitation
- Answer: (b) Heel effect
- Explanation: The heel effect is the reduction in the number of x-rays on the anode side due to self-attenuation of the x-rays within the anode target material.
Q2-3: _____ produces images of reduced intensity outside the edges of the collimated field of view.
(a) Off focus
(b) Heel effect
(c) Line focus principle
(d) Effective focal spot
(e) Positive beam limitation
- Answer: (a) Off focus
- Explanation: Off-focus radiation consists of x-rays produced outside the true focal spot, resulting in poorly collimated, low-intensity shadow images outside the main field.
Q2-4: That the effective focal spot size is smaller than the area where the x-rays are actually produced because of the beveled angle of the x-ray tube target is called
(a) Off focus
(b) Heel effect
(c) Line focus principle
(d) Collimation
(e) Positive beam limitation
- Answer: (c) Line focus principle
- Explanation: The line focus principle uses a beveled anode target angle to make the effective size of the focal spot appear smaller from the perspective of the image receptor.
Q2-5: _____ is attributed to attenuation within the target of the x-ray tube.
(a) Off focus
(b) Heel effect
(c) Line focus principle
(d) Effective focal spot
(e) Positive beam limitation
- Answer: (b) Heel effect
- Explanation: X-rays traveling toward the anode side have longer path lengths through the target material as they emerge, which causes greater attenuation.
Q2-12: One of the two key purposes of x-ray beam collimation is to
(a) Limit heel effect
(b) Reduce the scattered radiation
(c) Control off-focus radiation
(d) Reduce anode heating
(e) Reduce leakage radiation
- Answer: (b) Reduce the scattered radiation
- Explanation: Collimation restricts the beam to prevent exposing non-imaged tissue and utilizes a smaller field of view, which reduces scattered radiation.
Q2-13: _____ percent of the bombarding electrons’ energy is converted in x-ray production.
(a) 1
(b) 10
(c) 25
(d) 50
(e) 99
- Answer: (a) 1
- Explanation: Approximately 99% of the bombarding electrons’ energy is converted to heat, while only 1% results in x-ray production.
Q2-16: The main x-ray tube factor that affects the spatial resolution in the image is the
(a) Anode angle
(b) KVp
(c) Filament size
(d) Effective focal spot size
(e) Stator
- Answer: (d) Effective focal spot size
- Explanation: The effective focal spot size influences focal spot blur, directly impacting the spatial resolution of the image.
Chapter 3: X-ray Tube Heat Loading Characteristics
Q3-1: The kW rating of an x-ray tube depends on all the following factors, except
(a) Focal spot size
(b) Exposure time
(c) Anode angle
(d) Rotation speed
(e) Anode material
- Answer: (b) Exposure time
- Explanation: The kW rating is the instantaneous heat rating for exposures of 0.10 seconds or shorter, meaning it does not depend on exposure duration.
Q3-7: Cooling of a very hot anode primarily occurs by
(a) Conduction
(b) Convection
(c) Radiation
(d) Bremsstrahlung
(e) Cascade
- Answer: (c) Radiation
- Explanation: At very high temperatures, radiative cooling (emission of infrared waves) is the most important process because it is proportional to the fourth power of temperature; convection does not occur inside the tube’s vacuum.
Q3-8: For a constant effective focal spot size, the kW x-ray tube rating can be increased by using
(a) Large anode angle
(b) High kVp values
(c) Low rotation speeds
(d) Small focal spot sizes
(e) Three-phase rather than single-phase generators
- Answer: (e) Three-phase rather than single-phase generators
- Explanation: Three-phase generators maintain a relatively constant mA, whereas single-phase generators have a high peak mA that requires a reduced kW rating to prevent anode damage.
Q3-10: In general, x-ray tube overheating can be prevented by
(a) Using high kVp
(b) Using high mA settings
(c) Using large anode angles
(d) Using small focal spots
(e) Allowing time between exposures
- Answer: (e) Allowing time between exposures
- Explanation: Spreading x-ray exposures over time gives the anode the opportunity to cool down between heat inputs, preventing overheating.
Chapter 4: X-ray Production
Q4-1: Bremsstrahlung x-ray production accounts for _____ % of all the x-rays.
(a) 85-100
(b) 70-85
(c) 55-70
(d) 30-45
(e) 15-30
- Answer: (a) 85-100
- Explanation: In tungsten targets, characteristic x-rays are not produced below 69.5 kVp, making the beam 100% bremsstrahlung; at a maximum of 150 kVp, characteristic x-rays only make up about 15% of the total.
Q4-2: The energy spectrum of bremsstrahlung x-rays is
(a) Monoenergetic
(b) Polychromatic
(c) Composed of multiple discrete peaks
(d) Isotropic
(e) Homogeneous
- Answer: (b) Polychromatic
- Explanation: Bremsstrahlung x-rays possess a continuous distribution of varying x-ray energies, meaning they are polyenergetic or polychromatic.
Q4-3: The shortest wavelength x-ray photons are dependent on
(a) Characteristic x-rays
(b) K-edge x-rays
(c) kVp
(d) X-ray beam filtration
(e) mAs settings
- Answer: (c) kVp
- Explanation: The shortest wavelength x-rays correspond to the highest energy x-rays in the beam, which can never exceed the applied kVp.
Q4-5: The K-characteristic x-rays for a tungsten x-ray tube target have energies of about _____ keV.
(a) 17-20
(b) 20-23
(c) 30-50
(d) 50-70
(e) 70-88
- Answer: (d) 50-70
- Explanation: For a tungsten anode, K-alpha x-rays appear between 57 and 59 keV, and K-beta x-rays appear between 67 and 69.5 keV.
Q4-11: To be able to produce K-characteristic x-rays in a tungsten target x-ray tube, the x-ray tube voltage must be no lower than _____ kVp.
(a) 101.5
(b) 98.5
(c) 88.5
(d) 76.5
(e) 69.5
- Answer: (e) 69.5
- Explanation: The bombarding electron must have kinetic energy equal to or greater than tungsten’s K-shell binding energy (69.5 keV) to knock a K-shell electron out of orbit.
Q4-12: The x-ray quantity increases when the _____ is larger in magnitude.
(a) kVp
(b) mAs
(c) Filtration
(d) Both a and b
(e) Both a and c
(f) a, b, and c
- Answer: (d) Both a and b
- Explanation: Increasing the kVp increases both the number and energy of x-rays, while increasing the mAs strictly increases the total number (quantity) of x-rays.
Q4-13: The quality (ability of x-rays to penetrate through the patient’s body) improves when the _____ is larger in magnitude.
(a) kVp
(b) mAs
(c) Filtration
(d) Both a and b
(e) Both a and c
(f) a, b, and c
- Answer: (e) Both a and c
- Explanation: Raising the kVp increases maximum and average x-ray energy, and adding filtration preferentially removes low-energy x-rays, which increases the average energy; both actions make the beam more penetrating.
Q4-18: If the x-ray tube voltage is increased from 60 to 120 kVp, the quantity is changed by _____ the previous value.
(a) 4.0 times
(b) 2.0 times
(c) 1 (same)
(d) 0.5 times
(e) 0.25 times
- Answer: (a) 4.0 times
- Explanation: X-ray production quantity is related to the square of the kVp; therefore, doubling the kVp (from 60 to 120) quadruples the quantity.
Q4-21: Quality is usually measured in units of
(a) mm of aluminum
(b) keV
(c) kVp
(d) mR
(e) Calories
- Answer: (a) mm of aluminum
- Explanation: Quality measures penetration capability using the half-value layer (HVL), which for diagnostic x-rays is the thickness in millimeters of aluminum required to reduce the beam quantity to 50%.