RAD 115 Radiologic Sciences Fundamentals and Physics

SID (Source-to-Image Receptor Distance): The precise linear distance measured from the x-ray tube focal spot (source of radiation) to the surface of the image receptor.
SOD (Source-to-Object Distance): The linear distance measured from the x-ray tube focal spot (source of radiation) to the target structure or patient (object) being imaged.
OID (Object-to-Image Receptor Distance): The linear distance measured from the target structure or patient (object) being imaged to the surface of the image receptor.
kVp (Peak Kilovoltage / Kilovoltage Peak): The maximum electrical potential difference applied across the x-ray tube during an exposure, which accelerates electrons and dictates x-ray beam quality and penetrability.
mAs (Milliampere-seconds): The total quantity of electrical current passing through the cathode filament over a specified exposure duration in seconds, controlling the overall quantity of x-ray photons produced.
IR (Image Receptor): The hardware device (such as a digital flat-panel detector, photostimulable phosphor plate, or film cassette) situated behind the object that intercepts remnant radiation and records the anatomical image.

Discoverer of X-Rays: Wilhelm Conrad Röntgen (Wilhelm Conrad Roentgen).
Year of Discovery: 1895 (specifically November 8, 1895).
First Human Radiograph: Bertha Röntgen (Anna Bertha Ludwig Röntgen). The image captured was of her hand, distinctly demonstrating the skeletal bone structure and her wedding ring.

Cathode: The negative side ( $\text{Negative side}$ `) of the x-ray tube assembly. It contains the tungsten filament and focusing cup responsible for thermionic emission of electrons.
Anode: The positive side ( $\text{Positive side}$ `) of the x-ray tube assembly. It serves as the target structure that decelerates accelerating electrons to produce radiation.
Tungsten Target: A specific high-atomic-number target area on the positive anode constructed of tungsten to withstand extreme thermal heat while promoting efficient photon production.
Flow of Electrons: The accelerated stream of negative electrons traveling across the evacuated tube envelope from the cathode to the target on the anode.
Focal Spot: The exact physical area on the tungsten target of the anode where the electron stream impacts and from which x-ray photons originate and diverge.
Photons (Electromagnetic Energy): Quantum units of high-frequency electromagnetic energy emitted during high-speed electron interactions within the target material.
Primary Radiation: The unattenuated x-ray beam that emerges directly from the tube focal spot and passes through the housing port toward the object.
X-Rays: High-energy electromagnetic waves created at the atomic level when fast-moving electrons undergo abrupt deceleration or inner-shell ionization.

Source: The point of origin for x-ray photon production at the focal spot on the anode target within the x-ray tube.
Object: The body part, patient, or item placed within the radiation beam path between the source and detector.
Image Receptor (IR): The recording medium positioned underneath or behind the object to capture the radiation pattern.
Distance Formula Relationship: $\text{SID} = \text{SOD} + \text{OID}$

Control of X-Ray Quantity:
- Directly controlled and regulated by milliampere-seconds ( $\text{mAs}$ `).
- Increasing $\text{mAs}$ increases the total volume or intensity of photons produced during the exposure without altering their energy profile.
Control of X-Ray Quality:
- Directly controlled and regulated by peak kilovoltage ( $\text{kVp}$ `).
- Increasing $\text{kVp}$ increases the kinetic energy and penetrability of the photon spectrum, yielding a higher-quality, more penetrating beam.

IR Exposure: The total physical amount of x-ray radiation that reaches, strikes, and interacts with the image receptor surface. It dictates the photographic density or digital signal strength across the image field.
Contrast: The visible degree of difference in visual brightness, density, or grey scale levels between adjacent structural regions on a processed radiograph, allowing visual distinction of subject features.

Step 1: Thermionic Emission at the Cathode:
- An electric current passes through the filament on the cathode (negative side).
- Thermal energy causes electrons to break free from the tungsten atoms, forming a concentrated electron cloud (space charge).
Step 2: Electron Acceleration Across the Tube:
- A high potential difference ( $\text{kVp}$ `) is applied between the cathode and anode.
- The cathode strongly repels the free negative electrons, and the positive anode strongly attracts them, accelerating the flow of electrons across the vacuum gap to high kinetic velocities.
Step 3: Collision at the Focal Spot:
- The high-speed electron stream impacts the focal spot of the tungsten target on the positive anode ( $\text{Positive side}$ `).
- Over $99\%$ of the kinetic energy of the striking electrons converts directly into thermal heat.
- Approximately $1\%$ or less converts into x-ray photons through Bremsstrahlung (deceleration) and Characteristic (ionization) atomic interactions.
Step 4: Beam Emergence as Primary Radiation:
- The generated photons leave the target focal spot as electromagnetic energy packets.
- Photons pass through the housing aperture as primary radiation directed at the object and image receptor.