Medical Imaging Part I Lecture Notes

Flashcards based on medical imaging lecture notes.

X-Ray Tube Components

The components of an x-ray tube are enclosed in a glass envelope and protective housing, including the cathode and anode (target or focal spot).

Cathode

The cathode is the negatively charged side of the x-ray tube and consists of the filament and focusing cup; its purpose is to produce electrons and focus and accelerate the electron stream toward the anode.

Filament

A small coil of tungsten wire with an extremely high melting point that heats up when current is passed through it, resulting in electrons boiling off or being emitted from the filament.

Focusing Cup

A metallic shroud, usually made of nickel, containing the dual filaments, which repels electrons and helps direct the electron stream toward the anode.

Anode

Represents the positively charged side of the tube, receives the accelerated electron stream, dissipates heat, and serves as the path for high voltage flow.

Target

The portion of the anode where the electron stream from the cathode strikes and produces x-rays; also known as the focal spot.

Line-Focus Principle

Viewing a sloped surface at an angle reduces its apparent size, used to overcome the disadvantage of creating more heat with a small focal spot.

Vacuum in X-Ray Tube

The removal of air permits the uninterrupted flow of electrons from the cathode to the anode since there are no air molecules to collide with the accelerated electron stream.

Protective Housing

Used to control unwanted radiation leakage and electrical shock and is made of steel and lined with lead, filled with a special oil for insulation and cooling.

Heat Units (HU)

Capacity of the anode and tube housing to store thermal energy.

Thermionic Emission

Heat and the release of ions at the filament when electric current is applied, leading to incandescence and the creation of thermions.

Potential Difference

The amount of kVp needed to move a charged particle from one point to another in a static electric field.

Bremsstrahlung

Produced by the sudden deceleration of the high-speed electron as it is deflected around the nucleus of the tungsten atoms of the anode, accounting for 75-80% of the tube’s output.

Characteristic Radiation

Produced when an inner-shell electron is ejected from orbit and electrons from adjacent shells move in to fill the vacancies, accounting for 15-20% of the tube’s output.

Beam Quality

Refers to the energy of an x-ray beam and its ability to penetrate matter, controlled by kVp.

Attenuation

The gradual reduction in the number of photons or exposure rate as an x-ray beam passes through matter; also known as absorption.

Scattering

When photons in the original or primary beam change direction as they collide with atoms in their path.

Inhomogeneity

Means non-uniform in composition and/or character; high-density structures appear as lighter areas, while low-density structures appear as darker areas on a radiographic image.

Compton Scattering

Produced when an incident x-ray ejects an outer-shell orbital electron, resulting in a scattered x-ray that changes direction.

Coherent Scattering

Produced when the incident x-ray does not have enough energy to eject an orbital electron, resulting in a scattered x-ray that changes direction but with no change in energy.

Photoelectric Effect

Occurs when the incident x-ray ejects an inner-shell orbital electron, with all of the incident x-ray’s energy being absorbed by the ejected electron.

Additive Pathology

Those with increased tissue density, appearing as light regions on the radiograph or CT image (radiopaque).

Destructive Pathology

Those with decreased tissue density, appearing as dark regions on the radiograph or CT image (radiolucent).

Density

Defined as the degree darkening on the image; current (mAs) primarily controls it.

Distance

The gap between the target/focal spot and the image receptor, influencing density according to the inverse square law.

Contrast

Defined as the tonal range of densities on an image, controlled primarily by voltage (kVp).