Medical Imaging in Practice

Practice vocabulary flashcards covering medical imaging principles, X-ray production, tube components, and radiation interactions with matter based on the lecture notes.

Computed Tomography (CT)

An imaging modality used to visualize the tumor and surrounding structures, often combined with PET for enhanced imaging.

Image Fusion

A multidisciplinary approach and complex process of incorporating and integrating different imaging modalities into the simulation planning process.

Wilhelm Conrad Rontgen

The German mechanical engineer and physicist who discovered X-rays in 1895.

Roentgen (R)

The conventional unit of measurement for X-ray and gamma rays for exposure in air.

Air Kerma ($Gy_a$)

The standard international unit that now corresponds to the conventional unit Roentgen for radiation exposure in air.

Rad

The historical conventional unit designated for absorbed dose in tissue.

Gray ($Gy$)

The standard international metric system equivalent to the rad for absorbed dose.

Rem

The historical conventional unit designated for measuring occupational dose.

Sievert ($Sv$)

The standard international metric system equivalent to the rem for occupational dose.

Cathode

The negative side of the X-ray tube composed of the filament and focusing cup, which functions to produce and focus the electron stream.

Filament

A small coil of wire made of tungsten, which has an extremely high melting point of $(3380^{\circ}C)$, that emits electrons when heated.

Focusing Cup

A small oval depression in the cathode assembly that uses a negative charge to direct electrons toward the anode in a straighter, less divergent path.

Anode

The positive side of the X-ray tube that receives electrons from the cathode, dissipates heat, and serves as the path for the flow of high voltage.

Rotor

A device that allows most anodes to reach speeds of $3400\,rpm$ to help dispel the great amounts of heat created during X-ray production.

Target

The portion of the anode hit by electrons from the cathode; it contains the focal spot where X-ray photons are produced.

Focal Spot

The specific area within the target where interactions occur to produce X-rays and from which photons begin to fan out in a divergent path.

Line Focus Principle

A principle achieved by angling the target (typically $7$ to $20$ degrees) so that the actual focal spot size is larger than the effective focal spot size, allowing for heat dissipation while maintaining image detail.

Glass Envelope

A vacuum tube (measuring $20$ to $30\,cm$ in length) that permits the uninterrupted flow of electrons from the cathode to the anode by removing air molecules.

Protective Housing

A lead-lined mounting for the X-ray tube that prevents radiation leakage, prevents electrical shock, and uses oil for insulation and cooling.

Speed of Light

The velocity at which all radiant energies travel, expressed as $3 \times 10^{10}\,cm/s$.

Wavelength

The distance between the crests in a wave.

Frequency

The number of complete wave cycles per second.

Ionizing Radiation

Electromagnetic radiation (specifically X-rays and gamma rays) that carries enough energy to remove tightly bound electrons from atoms, creating ions.

Thermionic Emission

The process of "boiling off" electrons from the tungsten filament atoms as a result of being heated by an electrical current (mA).

Potential Difference

The electrical force created by applied voltage ($kVp$) that repels electrons from the negative cathode and draws them toward the positive anode.

Bremsstrahlung Radiation

A term meaning "braking" radiation; it accounts for approximately $75\%-80\%$ of the tube's output and is produced by the sudden deceleration and deflection of electrons near the tungsten nucleus.

Characteristic Radiation

X-rays created by the direct interaction of cathode electrons with inner-shell electrons of the target, where the photon energy is dependent on the binding energy of the target atom's electrons.

Penetration

The ability of a radiation photon to pass through matter, such as human body tissues, which increases as photon energy increases.

Beam Quality

A description of the photon beam's energy and penetrating ability, primarily controlled by the kilovoltage peak ($kVp$) setting.

Beam Quantity

The number of photons in the radiation beam, which is directly proportional to the milliampere second ($mAs$) applied to the filament.

Attenuation

The reduction in the number of photons or exposure rate in the primary beam as it passes through a given thickness of material.

Differential Absorption

The basis of radiographic imaging where variations in tissue density and atomic number (air, fat, water, muscle, and bone) cause different rates of radiation absorption.

Classical Scatter

Also known as coherent, Rayleigh, Thompson, or unmodified scatter; it involves low-energy X-rays (< 10\,keV) interacting with atoms without causing ionization.

Compton Scatter

The predominant interaction in the diagnostic energy range ($40$ to $150\,kVp$) where an incident X-ray interacts with and ejects an outer-shell electron, causing ionization and a change in photon direction.

Photoelectric Effect

An interaction described as "total absorption" where an incident photon ejects an inner-shell (K-shell) electron and is completely absorbed by the atom.

Image Density

The degree of darkening on a radiographic image, which is light when few photons reach the receptor and dark when many reach the receptor.

15% Rule

A guideline stating that an increase in $kVp$ of only $15\%$ doubles the image density.

Secular Equilibrium

A condition that occurs when the half-life of a parent isotope is much greater than that of the daughter (t_{hparent} > t_{hdaughter}), causing the daughter activity to eventually equal the parent activity.

Transient Equilibrium

A condition where the parent half-life is longer than the daughter's but not dramatically so, causing the daughter activity to grow slightly larger than the parent before decaying at the same rate.

Half-Value Layer (HVL)

The thickness of an absorber required to reduce the intensity of a photon beam to one-half of its original value, used to describe beam quality.

Beam Hardening

The process of increasing the overall HVL of a poly-energetic beam by passing it through an attenuator that absorbs softer, low-energy X-rays.

Auger Electron

An orbital electron that is ejected from an atom after absorbing a characteristic X-ray photon produced within the same atom.

Backscatter

The effect produced during a direct hit Compton interaction where the scattered photon travels backward from the atom carrying minimum energy.

Pair Production

A high-energy interaction ($\ge 1.022\,MeV$) where an incident photon is absorbed near the nucleus and instantly re-emitted as an electron-positron pair.

Annihilation Reaction

The result of a positron colliding with a free electron, where both are destroyed and their energy is emitted as two photons of $0.511\,MeV$ each traveling in opposite directions.

Photodisintegration

A high-energy interaction (> 7\,MeV) where a photon strikes the nucleus directly, causing it to emit neutrons and gamma rays to maintain stability.