1 - Radiation Protection and Physics of Diagnostic Radiology

• X-rays were discovered in 1895 by Wilhelm Conrad Roentgen and were immediately used for medical purposes like angiography, diagnosis, and treatment.
• Roentgen was awarded the first Nobel Prize for Physics in 1901 for this discovery.
• Despite advances in diagnostic ultrasound, computed tomography, and magnetic resonance imaging, conventional radiography remains a key element in veterinary imaging.

Basic Properties of X-Rays

• Both x-rays and gamma rays are electromagnetic radiation, differentiated by their source.
• X-rays come from electron interactions outside the nucleus.
• Gamma rays are released from inside the nucleus of unstable atoms that have excess energy.
• The energy level of a gamma ray depends on the amount of energy released by an unstable atom.
• The energy of an x-ray is relative to the energy of the electron interacting with an atom.
• Radiopharmaceuticals in nuclear imaging emit gamma rays with energy similar to x-rays used in diagnostic imaging.

Electromagnetic (EM) Radiation

• EM radiation includes radio waves, radar, microwaves, and visible light in addition to x-rays and gamma rays.
• EM radiation contains electric and magnetic fields oscillating in orthogonal planes described by frequency and wavelength.
• EM radiation velocity matches the speed of light, correlating inversely with frequency per the formula V = fλ (velocity = frequency x wavelength).
• X-rays and gamma rays possess particle-like attributes i.e. photons defined as a discrete bundle of EM radiation.

Properties of X-Rays and Gamma Rays

• Exhibit no charge or mass.
• Travel at the speed of light as invisible entities that cannot be felt
• Follow straight paths, undeterred by magnetic fields
• Able to permeate matter, cause fluorescence in certain substances
• Able to expose photographic emulsions
• Able to ionize atoms

Energy Measurement

• Electromagnetic radiation energy equals Planck's constant times speed of light divided by wavelength, therefore the energy is inversely proportional to wavelength.
• An electron volt (eV) measures electromagnetic radiation energy and is gained when one electron accelerates through a 1V potential difference.
• X-rays of just 15 eV energylevel are capable of ionizing atoms.
• Ionization, induced by x-rays in radiography, is when an electron is ejected from an atom that creates an ion pair.
• X-ray interaction with a person may lead to DNA ionization, which can cause mutations, fetal issues, disease susceptibility, cancer, and cataracts.

Radiation Protection

• Minimizing personnel exposure in radiation environments and minimizing patient ionization during imaging procedures can reduce the risk of side effects.
• Biological injury risk from x-rays or gamma rays is greater because the wavelength of visible light is 10,000 times longer than the wavelength of x-rays
• Radio waves are an even longer wavelength.
• Light and radio waves do not produce tissue ionization or DNA damage due to lower energy levels.
• Microwaves may cause biological damage through tissue heating but do not lead to molecular ionization.

Radiation Diagnostic Goals

• Aim is to obtain maximal diagnostic information while minimizing radiation exposure to the patient, personnel, and public by adhering to safety guidelines and using PPE as necessary to mitigate the risks associated with ionizing radiation.
• Digital radiography improves the image while safe radiation practices are disregarded due to the low cost and minimal time to retake.

Concepts and Units

• Radiation exposure and absorption differ, with some tissues absorbing radiation more effectively.
• Biological effect of absorbed dosage can be influenced by radiation type and energy, estimated by a weighting or quality factor.
• Radiation concepts originally measured in the CGS system, were updated in 1977 with the International System of Units (SI units) for universal adoption.

Radiation Exposure

• Radiation exposure is based on the amount of ionization produced in the air quantified by the electrical charge resulting from the ionization
• Coulombs per kilogram of air (C/kg) in SI units, this is sometimes measured in roentgens
• One roentgen equals a charge of 2.58 C/kg in air.

Absorbed Dose

• The x-ray absorption efficiency varies by material.
• The SI unit is gray (Gy), where 1 Gy is 1 joule/kg of absorbed radiation.
• The obsolete unit, rad, equates to 100 ergs/g of tissue, or 100 rads to one gray.
• Exposure to 1 roentgen equals roughly 0.9 cGy or 0.9 rad of absorption in soft tissue such as muscle while more efficient absorption of x-rays in bone results in 1 roentgen exposure.
• Differential x-ray absorption amongst tissues is the basis for radiographic contrast that decreases as photon energy increases.

Dose Equivalent

• Equivalent doses vary in biological effect when giving the same absorbed dose in Gy from different radiation since damage is greater from particulate radiation.
• Difference in biological damage from equal radiation dosages estimated by a weighting factor is used in cases of high ionizing particles like alpha particles.
• Equivalent dosage unit is the sievert (Sv), with Sv derived from multiplying the absorbed dose in Gy and the weighting factor.
• The earlier unit dose equivalency of radiation equivalent in man, or rem is the product of absorbed dose in rads and the weighting factor, and because 1 Gy = 100 rads, 1 Sv = 100 rem.

Radiation Safety Principles

• Principles are based on preventing unnecessary radiation exposure for workers and the public, with the understanding that some low-level exposure is permissible for workers without leading to major abnormalities or disease.
• Adverse effects can be classified as deterministic (threshold with dose-related severity) or stochastic (random, no threshold, dose-independent severity).
• Maximum permissible dose (MPD) is the safe radiation amount for an individual or organ although radiation will have some effect.

International Commission on Radiological Protection (ICRP)

• The ICRP is an independent organization focusing on safety from ionizing radiation providing recommendations and guidance published quarterly.
• The dose limit for radiation workers that avoids stochastic effects is 20 mSv/year, averaging over 5 years, with no single year exceeding 50 mSv, and quarterly exposure not greater than 12.5 mSv.

National Council on Radiation Protection (NCRP)

• The NCRP chartered by Congress in 1964 develops radiation protection recommendations and cooperates with ICRP
• The whole-body limit avoiding stochastic effects for radiation workers is 50 mSv per year and lifetime accumulation not exceeding 10 mSv x age in years with no occupational exposure below 18 years of age

Nuclear Regulatory Commission (NRC)

• The NRC defines federal exposure standards in the United States adopting a maximum annual radiation dose to workers as 50 mSv per year, mirroring the NCRP guidelines.
• The limit for pregnant radiation workers monthly is 0.5 mSv for embryo or fetus and a notification to the supervison is required.
• General radiation exposure limit for the public excluding medical use is 1 mSv per year.

Radiation Exposure

• The public is exposed to natural and man-made radiation.
• Medical procedures accounted for nearly half of the total radiation exposure of the U.S. population in 2006, with a revised breakdown highlighting changes in medical imaging exposure sources.
• Background radiation exposure levels depends on geographic location.

Practical Considerations

• Technologists acting as radiation workers should understand risks, training in patient positioning and in equipment operation as well as proper PPE use, and ALARA principles regarding distance, time, and shielding.
• Personnel distance from the x-ray tube should expand as much as possible to reduce radiation dosage, although shielding is still important.

Distance

• Exposure dose decreases by the square of the distance, meaning doubling distance reduces exposure by a factor of 4.
• Personnel body parts should avoid the primary beam, even when wearing protective aprons and gloves as well as using chemical restraint and sandbags to vacate prior to exposure.
• Effective beam collimation increases distance and minimizes radiation exposure and also increases the distance of the technologist from the primary x-ray beam.

Time

• Time should be reduced by sedation or anesthesia for uncooperative patients plus rotating technical staff to reduce personal exposure levels.
• Retake examinations increase personnel dose with likelihood increasing with greater digital imaging.

Shielding

• State building codes mandate structural shielding to safeguard personnel and the public against radiation exposure.
• PPE i.e. lead-impregnated aprons, gloves, thyroid shields, and eyeglasses, provide effective radiation worker protection, however not used consistently as protective aprons and gloves are designed for use in the x-ray room are usually 0.5 mm Pb equivalent.
• Aprons and gloves should be placed on racks, and visually inspected annually.

Radiation Supervisor Responsibilities:

• The radiation safety program depends on a well-defined radiation supervisor role
• This is a member of the technical staff that optimizes image integrity while reducing occupational exposure,
• Key qualities for a radiation safety supervisor include establishing/supervising written plans, periodic reviews to ensure regulations, training employees, and overseeing radiation surveys with corrective measures.
• Perform equipment checks, assess unusual exposures, educate personnel about radiation protection.

Personnel Dose Monitoring

• Monitoring assesses radiation safety, discloses improper habits, and detects exposure incidents.
• Radiation badges are used consististing of a plastic holder containing radiation-sensitive aluminum oxide or lithium fluoride crystals able to trap energized electrons and relate it to the amount of exposure
• Badges should be analyzed at least quarterly, or monthly during pregnancy.
• Wear only in the controlled areas of the workplace not during personal exams, on the upper or lower torso, and exchange if wearing.

Safety Rules

• Only required personnel may be present
• No one under 18 and all pregnant women must leave the x-ray room during the process
• All assistants should operate on a rotational schedule for minimal exposure
• Tools like sandbags and sponges ought to replace manual restraint whenever possible for safety.
• Personnel body parts must stay out of the main path of radiation at all times

Production of X-Rays

• X-rays production requires accelerating high-speed electrons in a controlled environment
• This is completed through an anode with positive chargers, cathode with negatively charged fliaments, within a vacuüm inside a glass tube to prevent interactions along the filament.
• The more current, the more electrons passing through the filament
• Voltage difference is modified with a kilovoltage peak (kVp) command on x-ray control panel affecting output energy.

Energy Production

• X-rays produced by mechanisms: Collisional interactions (hitting orbital electron from target atom) or radiative interaction (electron passes near nucleus slowing and releasing energy.
• The former produces a small fraction of x-rays, with specific energy linked to binding energy which are termed characteristic x-rays and are specific, but are not the most powerful.
• The latter produces bremsstrahlung (braking) radiation, has a large spectrum, happens frequently,
• The highest possible energy x-ray has KeV equals kilovoltage peak voltage,

Key Differences in Concepts

• Kilo electron volts or KeV is a measurement of energy.
• Kilovoltage peak or kVp is a measurement of voltage.
• Alternating current needs transformers for 120 Kilovolts.
• In the former, 1 KeV is the same as 1000 eV
• When the target alternates between positive and negative, rectification removes negative phases.
• The target is always + with respect to the filament

Heat and Focus

• Most of the electron energy striking the target converts to heat, targets should thus have a high melting point and a wide surface to dissipate heat
• Focal spot is the spot on target with electrons and impacts detail, with angling improving radiation
• X-ray tubes tend to have small filaments, that lower mA

M.A.

• Number of electrons impacts the radiation
• The timer impacts the duration that source flows, or voltage to get to the target
• Milliampere, or mAs, seconds quantify radiation and equate to the seconds lightbulb shines
• The generator then increases output from cyclic current by manipulation
• These increase X-ray energy and create a 6 pulse with rectification.

High Frequency Generators

• High Frequency Generators are the best at what they do, voltage is nearly constant.
• X-ray energy is produced highly ineficiently, with lots of it being converted to heat
• As such, the target must be high melting point and able to withstand heat

How Radiation Works

• Five components: direct scatter, photoelectric effect, secondary scatter, pair production, or direct disintegration -
• Each of these requires an understanding of how a proton works with matter and how film is affected.
• Coherent scattering causes a photon to contact an atom and change its direction, but overall the energy output is minimal
• Photoelectric effect describes when the X-ray source interacts with a tissue electron, leading the X-Ray and its energy to being entirely consumed.
• Compton scattering strikes electron of peripheral shell atom and the original proton scatters

Making a Radiograph

• Beam of x-ray must contain filter that is less energetic
• Beam shape must be configured for size, collimated
• Some x-rays must penetrate and go through, to produce a radiograph
• Depending on structure and patient traits, the beam strikes and records output

Measurement of Film

• All depends on how the silver absorbs and reacts to certain light forms, whether through halide or otherwise
• Most films depend on how the mAs affects and strikes the film relative to the kVp for setting, as distances are relevant by inverse squared law
• To measure, use the formula I1/I2 = (d2/d1)^2
• You will need to control motion to allow short times,

Focal Control

• Also consider if there is a penumbral effect from a diffused focal
• More efficient film may lead to enhanced light
• Intensifying screens may also be a help
• Grids are useful and helpful
• Grid depends on rectangular flat plate that is either absorptive, lead, or material
• They depend on lead strips, angle to periphery
• Must work within FFD, range must be accurate
• Film contrast often varies but depends on mA and kVp
• Radiographic contrast is from a wide variation in thickness, density