American Registry of Radiologic Technologists Certification (ARRT) Radiation Physics and Imaging Science Syllabus

What is the anode heel effect?

The x-ray beam intensity is greater on the cathode side and lesser on the anode side because x-rays produced deeper in the angled anode are absorbed by the target material. Thicker body parts should be positioned under the cathode side.

How is Bremsstrahlung (braking) radiation produced?

Bremsstrahlung occurs when a projectile electron passes near the positively charged nucleus of a target atom, is slowed and deflected, and loses kinetic energy that is emitted as an x-ray photon. It produces a continuous range of energies.

How is characteristic radiation produced?

A projectile electron ejects an inner-shell (e.g., K-shell) electron; an outer-shell electron drops in to fill the vacancy and emits an x-ray photon whose energy equals the difference in the two shells' binding energies. Energies are discrete (characteristic of the element).

What is the minimum kVp required to produce K-shell characteristic x-rays from a tungsten target?

About 70 kVp, because tungsten's K-shell binding energy is approximately 69.5 keV. Below this energy, projectile electrons cannot eject K-shell electrons, so no K-characteristic radiation is produced.

At a typical diagnostic kVp (e.g., 80-100 kVp), which type of radiation predominates in the x-ray beam?

Bremsstrahlung (brems) radiation predominates. Characteristic radiation makes up only a small fraction, and below 70 kVp the tungsten beam is essentially 100% Bremsstrahlung.

What does the x-ray emission spectrum show, and what determines its maximum energy?

The emission spectrum plots the number of photons versus their energy. It includes a continuous Bremsstrahlung curve plus discrete characteristic lines. The maximum photon energy (in keV) equals the set kVp.

How does increasing mA (or mAs) affect the x-ray emission spectrum?

Increasing mA/mAs increases the amplitude (number of photons) of the entire spectrum proportionally but does NOT change the maximum or average photon energy. It changes quantity, not quality.

How does increasing kVp affect the x-ray emission spectrum?

Increasing kVp increases both the amplitude (quantity) and shifts the spectrum to the right, raising the maximum and average photon energy (quality/penetrability). Higher kVp means a more energetic, penetrating beam.

How does added filtration affect the x-ray emission spectrum?

Filtration removes low-energy (soft) photons, decreasing the overall amplitude (quantity) while raising the average photon energy (beam hardening). Maximum energy stays the same.

State the formula for calculating heat units (HU) in a single-phase x-ray unit.

HU = kVp × mA × time (seconds) × generator factor. For single-phase the factor is 1.0; three-phase six-pulse = 1.35; three-phase twelve-pulse and high-frequency ≈ 1.41.

What is the purpose of a tube rating chart?

A tube rating chart shows the maximum safe combinations of kVp, mA, and exposure time for a single exposure without damaging the tube. Combinations below the curve are safe; those above risk tube overload.

What is an anode cooling chart used for?

An anode cooling chart shows how much heat (in heat units) the anode stores and how long it takes to dissipate, allowing the technologist to determine waiting time before additional exposures to prevent thermal damage.

Describe the photoelectric effect (photoelectric absorption).

An incoming x-ray photon transfers all its energy to an inner-shell electron, ejecting it (photoelectron) and being completely absorbed. A characteristic photon may then be emitted. It produces no scatter to the image and is the source of useful subject contrast.

How does the probability of the photoelectric effect depend on atomic number and photon energy?

Photoelectric absorption is proportional to Z³ (cube of atomic number) and inversely proportional to E³ (photon energy cubed). It dominates at low energies and in high-Z materials like bone and contrast media.

Describe Compton scattering (Compton effect).

An incoming x-ray photon interacts with a loosely bound outer-shell electron, ejecting it (recoil/Compton electron) and continuing in a new direction with reduced energy as a scattered photon. It is the main source of scatter (fog) and occupational exposure.

How does the probability of Compton scattering depend on photon energy and atomic number?

Compton scattering is relatively independent of atomic number and decreases only slightly with increasing photon energy. It depends mainly on tissue (electron) density, becoming the dominant interaction at higher diagnostic energies.

Describe coherent (classical/Rayleigh/Thompson) scattering.

A low-energy photon (typically <10 keV) interacts with an atom, causing it to vibrate and re-emit a photon of equal energy in a different direction, with no ionization. It is a minor contributor to image fog in diagnostic radiology.

Compare the three main x-ray interactions by whether they cause ionization and produce scatter.

Photoelectric: ionizes, fully absorbs photon, no scatter (builds contrast). Compton: ionizes, scatters photon (degrades image, hazard). Coherent: no ionization, scatters a small fraction. Photoelectric and Compton both eject electrons.

What is attenuation of the x-ray beam?

Attenuation is the reduction in x-ray beam intensity as it passes through matter, due to both absorption (photoelectric) and scattering (Compton, coherent). It increases with tissue thickness, density, and atomic number, and decreases with higher kVp.

What is differential absorption and why is it essential for the radiographic image?

Differential absorption is the difference in x-ray attenuation among various tissues (e.g., bone vs. soft tissue vs. air). These differences in absorbed versus transmitted photons create the contrast that forms the radiographic image.

How does kVp affect differential absorption and image contrast?

Lower kVp increases differential absorption (more photoelectric effect), producing higher (short-scale) contrast. Higher kVp decreases differential absorption (more Compton/transmission), producing lower (long-scale) contrast and more scatter.

What is the role of the high-voltage (step-up) transformer in the x-ray generator?

The high-voltage step-up transformer increases the incoming voltage (hundreds of volts) to the kilovoltage (kVp) range needed to accelerate electrons across the tube. Its turns ratio determines the voltage increase.

Why must the high-voltage circuit be rectified, and what does rectification do?

The x-ray tube requires direct current (electrons flow only cathode to anode). Rectification, using solid-state diodes, converts the transformer's alternating current into direct current, preventing reverse (anode-to-cathode) electron flow that could damage the tube.

Compare single-phase, three-phase, and high-frequency generators by voltage ripple and efficiency.

Single-phase has ~100% ripple (least efficient). Three-phase six-pulse has ~13% ripple, twelve-pulse ~4%. High-frequency has the lowest ripple (<1%), giving the most constant, efficient, and higher average-energy output.

What does the autotransformer (variable transformer) control in the x-ray circuit?

The autotransformer, located in the primary (low-voltage) side, selects the voltage supplied to the high-voltage transformer, thereby controlling the kVp. It works on self-induction using a single winding with variable taps.

What is the purpose of automatic exposure control (AEC)?

AEC automatically terminates the exposure once a preset amount of radiation has reached the image receptor, producing consistent receptor exposure (density) regardless of patient thickness and reducing repeat exposures.

What are the two main types of AEC detectors and where are they located?

Ionization chambers (most common, placed between the patient and image receptor) and phototimers (photomultiplier tubes placed behind the receptor). Both measure radiation and signal the timer to stop the exposure.

What is the backup timer (and density/exposure adjustment) in an AEC system?

The backup timer is a safety maximum exposure time that ends the exposure if the AEC fails, protecting the patient and tube. The density (±) control lets the operator increase or decrease receptor exposure in preset increments from the default.

What is the image intensifier in fluoroscopy and what is its main function?

The image intensifier is an electronic vacuum tube that converts the remnant x-ray beam into a brighter visible-light image. Its components include the input phosphor, photocathode, electrostatic focusing lenses, accelerating anode, and output phosphor.

Define brightness gain in an image intensifier (and its two components).

Brightness gain is the increase in image brightness, equal to minification gain × flux gain. Minification gain results from compressing the image from the large input phosphor to the small output phosphor; flux gain results from accelerating the electrons.

What are the key radiation safety features and characteristics of mobile/portable x-ray units?

Mobile units are battery-powered (capacitor discharge or battery) or capacitor-discharge generators with limited output; they require a dead-man (continuous-pressure) exposure switch, an exposure cord at least 6 feet (1.8 m) long, and proper operator distance and shielding because they are used outside fixed protected rooms.