Lesson4 Chapter8
Fundamental Principles of Radiographic Exposure
kVp, which stands for kilovolt peak, is a critical factor in radiography that controls the quality, energy, and contrast of the x-ray beam. It is essential to understand that kVp does not describe or control the quantity of radiation; that function is reserved for mAs. As the primary controller of contrast, kVp determines the scale of grays in an image. A higher kVp produces lower contrast, also referred to as a long scale of contrast. Conversely, low kVp results in high contrast or a short scale. The \"p\" in kVp specifically stands for \"peak,\" representing the maximum voltage applied across the x-ray tube.
mAs (milliampere-seconds) serves as the primary controller of radiographic density, which is the overall darkness or blackness of an image. It also controls the quantity of x-rays produced. A fundamental concept in technique adjustment is the reciprocity law, which states that different combinations of mA (milliamperage) and time can equal the same mAs, and therefore, the same total exposure to the image receptor. When making adjustments to mAs, a general rule of thumb is to either half or double the value. Furthermore, a minimum change of at least in mAs is required to see a visible difference in the image density. While mAs is the primary factor for density and quantity, it does not describe or control image contrast.
Physics of the Atom and Energy Systems
Physics is defined as the study of matter, energy, and how they interact. In the context of radiography, all matter is composed of atoms, which are the smallest units of matter. Atoms themselves consist of protons, neutrons, and electrons. The periodic table of elements is arranged in ascending order by atomic number, which is the number of protons in the nucleus. When an atom loses or gains an electron, it becomes an ion. These concepts are vital for understanding radiation because x-rays are a form of ionizing electromagnetic radiation, meaning they possess enough energy to remove electrons from atoms.
Energy is defined as the ability to do work. Kinetic energy refers specifically to the energy of motion, such as the moving electrons within an x-ray tube. Binding energy is the force required to remove an electron from its shell, particularly an inner shell. Inner-shell electrons have stronger binding energy than outer-shell electrons because they are situated closer to the nucleus. During x-ray production, electrons must have enough energy to overcome this binding energy to cause interactions. In radiography, electrical energy is converted into x-ray energy and heat energy.
The X-ray Tube and Components of X-ray Production
The x-ray tube is a complex piece of equipment where electrons are converted into radiation. The source of electrons is the filament, located at the cathode, which is the negative side of the tube. The filament is typically made of tungsten due to its high melting point and efficient electron emission. In some cases, tungsten-thoriated material is used. The process of boiling off electrons from a heated filament is called thermionic emission. The focusing cup, which carries a negative charge, repels the released electrons to focus the electron cloud into a narrow stream toward the anode.
The anode is the positive side of the x-ray tube. X-rays are produced when the electron beam strikes the angled anode target, a concept known as the line focus principle. The area where the electrons actually strike is the actual focal spot, while the smaller beam projected toward the patient is the effective focal spot. Most x-ray tubes contain two filaments, a small filament and a large filament, which correspond to the small and large focal spots. The small focal spot is preferred for small body parts like extremities because it provides better recorded detail with less geometric blur or penumbra. The large focal spot is used for larger body parts that require higher tube currents. X-ray production is highly inefficient, resulting in approximately heat and only x-rays.
X-ray Circuitry and Timing Systems
The x-ray circuit is composed of three main parts: the primary circuit, the secondary circuit, and the filament circuit. The autotransformer is located in the primary circuit (also known as the low-voltage or control side) and is used to control input voltage. The kVp meter is also found here to measure the voltage from the autotransformer. The filament circuit delivers low voltage and high current to the filament to facilitate thermionic emission, utilizing a step-down transformer. Rectification is the process of converting alternating current (AC) into direct current (DC). This is necessary because electrons must flow in only one direction (cathode to anode). While the United States uses AC, x-ray production requires DC. High-frequency generators are the most efficient because they produce the least voltage ripple, meaning the variation between maximum and minimum voltage is minimal.
Different types of timers are used to control exposure duration. A synchronous timer uses AC cycles but is inaccurate for exposure times less than . Electronic timers are more precise and can time exposures in milliseconds. An mAs timer terminates the exposure once a pre-selected milliampere-second value is reached. The Automatic Exposure Control (AEC) terminates the exposure automatically once a sufficient amount of radiation reaches the detector to prevent over- or under-exposure.
X-ray Interactions with Matter and Anode Interplay
There are two primary ways x-rays are produced at the anode. Bremsstrahlung, or \"braking radiation,\". occurs when a fast-moving electron is slowed or deflected by the nucleus of a target atom, converting lost kinetic energy into x-ray photons. Characteristic radiation occurs when an inner-shell electron is ejected from an anode target atom, and an outer-shell electron drops into the vacancy, releasing the difference in binding energy as an x-ray photon.
When x-rays interact with the patient, several effects may occur. The Compton effect involves an interaction with an outer-shell electron, resulting in scatter radiation that increases patient dose and occupational exposure while lowering image quality. The Photoelectric effect involves the total absorption of the x-ray photon by an inner-shell electron in the patient, which is essential for creating image contrast. Coherent scattering (also known as classical or Thompson scattering) involves excitation of the atom without ionization. High-energy interactions like Pair Production (creating a positron and electron pair) and Photodisintegration occur at levels used in radiation therapy, not standard diagnostic radiography.
Image Quality, Geometry, and Technical Factors
Image quality is defined by photographic and geometric properties. Photographic properties include radiographic density (controlled by mAs) and contrast (controlled by kVp). Geometric properties include recorded detail (sharpness), magnification, and distortion. Recorded detail is the sharpness of structures, and its most important factor is focal spot size. Blur at the edges of an image is called penumbra. Distortion, or the misrepresentation of the object's size or shape, is caused by improper alignment of the tube, the body part, or the image receptor.
Distance significantly impacts the beam. According to the inverse square law, radiation intensity is inversely proportional to the square of the distance; thus, increasing distance (such as moving from to SID) decreases intensity and density while making the image sharper and less magnified. The density maintenance formula (involving , , , and ) helps adjust technique for distance changes. The Anode Heel Effect describes how radiation intensity is greater on the cathode side because x-rays on the anode side must pass through more target material. Consequently, thicker anatomy should be placed toward the cathode side. Grids are used to absorb scatter radiation before it reaches the receptor, which improves image contrast and prevents \"fogging.\"
Safety, History, and Clinical Practice
The history of radiography began in when Wilhelm Conrad Röntgen discovered x-rays. The earliest tubes used were gas-filled tubes, which were less efficient than modern vacuum tubes. X-rays travel at the speed of light, approximately or . Radiation is categorized as ionizing (capable of removing electrons, like x-rays and gamma rays) or non-ionizing. Gamma rays differ from x-rays because they originate from radioactive decay in the nucleus, whereas x-rays are produced electronically outside the nucleus. Everyone is exposed to natural background radiation from the sun, stars, and earth.
In clinical practice, the American Society of Radiologic Technologists is the organization for technologists. Positioning terms include the Trendelenburg position, where the head is lower than the feet. Calipers are tools used to measure the thickness of a body part to determine the correct exposure technique. Cassettes should be stored upright in a clean, dry place. Above all, the most important person in the imaging department is the patient; all decisions regarding safety, comfort, and diagnostic quality must focus on their care.
Questions & Discussion
1. What is the difference between characteristic radiation and photoelectric effect?
Answer: Characteristic radiation happens at the anode target during x-ray production. Photoelectric effect happens in the patient when the x-ray is absorbed by an inner-shell electron.
2. Why does a small focal spot improve detail?
Answer: Because it reduces penumbra and geometric blur.
3. Why is tungsten used in the filament?
Answer: Because it withstands high temperatures and releases electrons efficiently.
4. Why do we use grids?
Answer: To absorb scatter radiation and improve image contrast.
5. Why is the cathode side more intense?
Answer: Because of the anode heel effect.
6. When does the AEC terminate the exposure?
Answer: When enough radiation reaches the detector, it stops exposure automatically to prevent over- or under-exposure.
7. What is the disadvantage of a synchronous timer?
Answer: It has the disadvantage of inaccuracy for exposure times less than .