Radiology By: Andy Codding – January 2002 Radiology on the National Board Exam Obtaining and interpreting radiographs- approximately 45 questions

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By: Andy Codding – January 2002

* Radiology on the National Board Exam

Obtaining and interpreting radiographs- approximately 45 questions

  • Principles of radiophysics and biology – 4 questions

  • Principles of radiologic health – 5 questions

  • Technique, Errors – 6 questions

  • Recognition of normal and abnormal – 5 questions

  • Case Study questions – 30 Questions

    • Intraoral Anatomy – 3 questions

    • Panoramic Anatomy – 2 questions

    • Technique Errors – 7 questions

    • Tooth Conditions – 14 questions

    • Restorative Material – 4 questions


  1. Be sure to check the labeling! Sometimes the R & L are on different sides of the films than what we are used to seeing in school!

  2. When there is a question about amalgam tattoos, always double check with the radiograph. If there was a recent extraction- it should tell you.

  3. Check x-rays for missing teeth, restorations, caries classifications

  4. When given an intra-oral picture, always refer back to the radiographs to be sure you are looking on the correct side and teeth numbers. Intra-oral pictures can be a direct shot, or through a mirror. Don’t get confused!

  5. Remember you can double check your Angles Classification with the bitewing radiographs.

Radiology Definition – The flow of energy through space and matter in the form of waves or particles.

Ionizing radiation

Definition - Radiation that is capable of producing ions by removing or adding an electron to an atom.

  1. Characteristics –

      1. Electromagnetic and particulate radiation can interact with atoms and create ion pairs.

      2. Mechanism by which biological systems are altered or damaged

B) Examples

  1. Electromagnetic radiation

  2. Particulate radiation

Electromagnetic radiation

Definition - Transmission of wave energy through space and matter as a combination of electric and magnetic fields.

A)Characteristics –

      1. Have no mass and no electrical charge.

      2. Travels at the speed of light in a straight but oscillating path (like ripples in a pond)

      3. Electromagnetic spectrum is measured according to frequency, energy, and wavelength.


      1. Long wavelengths – radar, television, radio

      2. Short wavelengths- visible light, UV light, x-rays

Particulate radiation-

Definition - Atomic nuclei or subatomic particles that travel at high velocity.

            • Characteristics –

      1. Have mass and energy

      2. All particles have an electrical charge (except neutrons)

      3. Travel in straight lines at high speeds, but can not reach the speed of light
            • Examples

      4. Alpha particles, beta particles, and cathode rays (electrons)

      5. Nucleons – protons, neutrons

Discovery of X-Radiation

        1. Sir William Crookes -1878

          • Crookes tube, also known as “cathode ray tube”

          • Evacuated glass tube with + electrode at one end and – electrode at opposite end.

          • Various gases when injected would glow when electricity was applied.

  1. William Roentgen –1895

          • Discovered and termed “X-Rays”

          • Noticed finger bones glowing on phosphorescent paper

  1. Otto Walkoff – First dentist to attempt x-rays on patient (himself)

  1. W.J. Morton – NY physician made first dental radiograph in U.S. (a skull)

  1. Edmund Kells – First dental radiograph on a live patient in the U.S.

          • Father of dental radiology

          • Numerous cancers of his hands and arms.


Definition – The flow of electrons through a wire or other electrical conductor.


  1. DC – Direct Current – electrons flowing in one direction

  2. AC – Alternating Current – electrons are constantly switching directions

Units of measurement –

  1. Milliamperage (mA) – The quantity of the x-rays produced, regulates the temperature of the cathode filament

    • 1 mA= 1/1000 ampere

    • Typically fixed at a specific mA such as 10 mA

  2. Ampere – the unit of measurement used to describe the number of electrons passing through the cathode

  3. Volt – measures the energy level of electricity (the force of electrons)

    • 1 kVp = 1000 volts

    • Usually fixed at 70 kVp

    • Variable kVp machines range from 50 – 100 kVp

  4. Timer – regulates the length of x-ray exposure, calibrated as fraction of a second

      1. Formats

        1. Standard fractions of a second (1/2 second)

        2. Decimal fraction of a second (.50 second)

        3. Impulses (30 impulses/second)

      2. Conversion

        1. To convert exposure time in seconds to impulses, multiply by 60 (1/2 X 60 = 30 impulses/second)

        2. To convert impulses to exposure time in seconds, divide by 60 (30/60 = ½ or .50)

Parts of the X-Ray unit

    • Generator- supplies electrical DC power to the x-ray tube

    • Transformer- Changes electrical supply to any desired level

  1. Step-down transformer – decreases electrical current from 110 or 220 volts to 8-12 volts to heat tungsten filament

  2. Step-up transformer – increases electrical current from 110 or 220 to 50 to 100 kVp to produce x-rays

  3. Autotransformer- allows selection of variable Kvp settings on x-ray units with a kVp range

    • Rectifier – The process of changing an alternating current into a direct current

  1. Half-wave rectifier- where half of the AC current is converted to DC

  2. Full-wave rectifier- where the entire AC current is converted to DC

    • Cathode (-) - Tungsten filament –thin wire (source of electrons)

    • Anode (+) - Tungsten target - copper rod which functions to conduct heat away from target

    • Lead diaphragm/collimator- device that restricts size of x-ray beam

    • PID – device that guides the x-ray beam toward patient

    • X-ray filter – layers of aluminum metal used to filter x-ray beam

A) 1.5 mm added filtration for machines operating at < 70kVp

B) 2.5 mm added filtration for machines operating at > 70 kVp

Steps to X-ray Production

  1. Tungsten filament is heated (controlled by mA settings via the step down transformer)

  2. Electrons are set in motion (+ anode attracts electrons from – cathode)

    1. As electricity flows in the machine, the cathode filament is heated and free electrons are liberated from the filament called thermionic emission.

  3. Anode target bombardment (<1% produce x-rays, >99% produce heat)

  4. Heat is conducted via the copper sheath/rod into the oil surrounding the x-ray tube

  5. X-rays leave the glass tube via the unleaded glass window

  6. Primary X-ray beam

    1. Filtered

    2. Collimated

    3. Guided through the PID

Anode Target Bombardment – 2 ways

Bremsstrahlung (Braking)

              1. As bombarding electron from cathode passes near nucleus of target atom, its direction is changed and it loses speed. Lost speed is transformed into x-rays


              1. Bombarding electron scores a direct hit on target atom nucleus, losing all of its motion. Energy is converted to high energy x-rays

*Vast majority of x-rays are produced this way

Characteristic (Discrete)

  1. Bombarding electron from cathode strips electron rom target atom at the anode, knocking it out of orbit.

  2. An outer shell (higher energy) electron in the target atom drops down to take the empty spot. The drop to the lower level causes a loss of energy in the form of x-rays

Interaction of X-rays with Tissues

Secondary Radiation (Scatter) - Direction of primary radiation is altered by tissues, resulting in fogging of radiographic image
Primary Radiation + Interaction with Tissues = Secondary Radiation (Scatter)
Thompson Scattering (Coherent Scattering)

  • X-ray photon is absorbed by tissue atom electron, but the electron is not displaced.

Photoelectric Effect

  • X-ray photon dislodges inner-shell electron from tissue atom and ceases to exist

  • Outer shell electron then drops into vacancy producing lower energy secondary radiation

  • Tissue ionization due to loss of electron

  • Accounts for 45% of primary radiation

  • Energy is absorbed by the patient

  • Has little effect on film fog

Compton Effect (Compton Scattering)

  • X-ray photon strikes a blow to an outer shell tissue atom electron, dislodging it

  • Deflected photon continues to exist as lower energy scatter radiation

  • Tissue ionization due to loss of electron

  • Accounts for 45% of primary radiation

  • 30% of the scatter escapes the patient tissues

  • Remainder of the scatter contributes to film fog

Ionizing Effect on Cells

  • Direct Effect- x-ray photon strikes critical structure (such as DNA)

  • Indirect Effect- x-ray photon strikes a non-critical structure, the resulting scatter radiation will then strike a critical structure (rebounding)

  • Somatic Effect –results in tissue injury to exposed individual (such as cancer or radiation burn)

  • Genetic Effect – results in damage to gamete of exposed individual, yielding defects in the offspring

  • Acute Effect- injury is immediately manifested, such as radiation burn or radiation sickness

  • Chronic Effect- injury is manifested much later, such as developing cancer

Dose-Response Relationships

    1. Linear – tissue response correlates directly with dose

    2. Non-Linear - tissue response correlates indirectly with dose

    3. Threshold- below a certain radiation dose, there is no tissue response (radio-resistant tissues)

    4. Non-Threshold- any dose results in some tissue response (radio-sensitive tissues)

Sensitivity to Ionizing Radiation

Cells Most Sensitive:

  • Rapidly dividing cells (high mitotic rate)

  • DNA and chromosomes

  • Undifferentiated cells (stem cells, unspecialized cells)

  • Cells with high metabolic activity


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Units of Radiation Measurement

Traditional System

  • Roentgen (R) – unit radiation intensity within a given space

  • Rad (rad) – unit of tissue absorbed radiation dose

  • Rem (rem) – absorbed dose based on type of radiation (quality factor)

  • Curie (ci) – quantity of radioactive material which is emitting radiation

International System

  • Coulomb (C ) – equal to 3880 roentgens

  • Gray (Gy) – equals 100 rads

  • Sievert (Sv) – equals 100 rems

  • Becquerel (Bq) – similar as Curie

Intraoral radiographic film

  1. Outer plastic or paper cover – protects film from light and moisture

    1. Plain white side directed to x-ray source (“white to the light”)

  2. Lead Foil – reduces film fog by absorbing secondary radiation scattering back from patients tissues

  3. Film base – tint reduces eye fatigue, provides stiffness with some flexibility

  4. Film emulsion- gelatin matrix with suspension of silver halide crystals

    1. Silver halide crystals or grains

      1. 95% silver bromide

      2. Small amount of silver iodide

      3. Sulfur containing, increases sensitivity of silver bromide crystals

  5. Adhesive – aids in adherence of emulsion to film base

  6. Supercoating – gelatin layer over emulsion to help prevent scratching.

Extraoral radiographic film

  1. Intensifying screens- screens fluorescence upon exposure to x-rays

    1. Reduces patient radiation dose

    2. Shorter exposure time

    3. Extended x-ray tube life

Basic components

  1. Base- plastic support material

  2. Reflecting layer- reflect light emitted from phosphor layer back to the film

  3. Phosphor layer- light sensitive phosphorescent crystals suspended in plastic material

    1. Calcium tungstate screens – emit BLUE and blue-violet light

      1. Must be matched with blue light sensitive film

      2. Slower crystals that require more x-ray exposure

    2. Rare earth screens – emit GREEN light

      1. Must be matched with green light sensitive film

      2. Faster crystals that require at least one half the x-ray exposure of calcium tungstate screens.

Types of Extraoral Films

  • Arthrography – joint radiography, good for TMJ

  • Axial (basilar)(submentovertex) projection – “jug handle view”, cranial base

        • Computed tomography (CT) – good for measuring tissue thickness when placing implants

        • Lateral (Cephalometric) head plate- common projection used for orthodontics to assess facial growth

        • Lateral Jaw- records right or left side of the jaws

        • Lateral Oblique mandibular projection- mandibular condyle, coronoid process, ramus and body of


        • Lateral Skull projection – sinuses, nasopharynx, maxilla

        • Panoramic- records entire maxilla, mandible, and immediately adjacent structures, impacted teeth,etc

  • Posterior-anterior (PA) projection- entire mandible except condyles

        • Towne & Reverse Towne projection- condyle, occipital bone, orbital floor, zygomatic arch, middle ear structures

  • Waters projection- view useful for examining maxillary sinuses as well as frontal and ethmoid sinuses

Dark Room

General Principles

      1. Development- transforms latent image into visible image by reducing exposed silver halide crystals

        1. Hydroquinome- (reducting agent) – brings up black tones and contrast

        2. Elon- (reducting agent) brings up gray tones

        3. Sodium cabonate (activator) – soften and swells the emulsion

        4. Potassium bromide (restrainer) – prevents reducing agents from developing the unexposed silver halide crystals

        5. Sodium sulfite (preservative) – prevents rapid oxidation

        6. Water- (solvent) – medium for dissolving chemicals

      1. Rinsing – stops development and removes excess chemicals from the emulsion

      1. Fixation – removes unexposed silver halide crystals from the emulsion

1.Sodium or ammonium thiosulfate (clearing agent) removes unexposed silver bromide crystals from the acidifier

  1. Acetic acid or sulfuric acid (acidifier) – lowers pH which enhances fixing agent

  2. Aluminum chloride or potassium alum (hardener) –shrinks and hardens the emulsion

  3. Sodium sulfite (preservative) – prevents oxidation of the fixer and prolongs its life

  4. Water – (vehicle) – medium for dissolving chemicals

D) Washing – removes all excess chemicals from the emulsion
E) Drying – removes the water from the emulsion

Intraoral and Processing Errors

1. Vertical angulation errors – too much vertical angulation is used.

  • Corrected by decreasing vertical angulation and placing film more parallel to the long axis of the tooth

2. Elongation- not enough vertical angulation is used

  • Corrected by increasing vertical angulation

3. Horizontal angulation errors- overlapping

  • Corrected by directing central ray perpendicular to facial surface of the teeth crowns

4. Cone Cut errors- lack of centering x-ray beam over film

  • Corrected by directing central ray to film center

5. Overexposure- high density or dark films

  • Corrected by reducing exposure time

6. Underexposure - low density or light film

  • Corrected by increasing exposure time, make sure exposure button was not released too soon.

7. Developer Stain – dark stains on film

8. Fixer Stain – light stains on film
9. Fog - visible light that exposes film, unsafe darkroom illumination, safe light too close to working area
10. Green Film – unprocessed film which has been opened and exposed to light
11. Static Electricity - causes black lightening bolts or “naked tree” pattern

  1. Reticulation – causes cracks in emulsion that look like “cracked ice,” due to extreme temperature differences in processing chemicals and the wash water

13. Fluoride Stain – causes dark staining on films

14. Air Bubbles – cause light spots on film where develop failed to contact the emulsion
Panoramic Errors
1. Smile image- patient chin is too low
2. Frown image- patients chin is too high
3. Blurred maxillary anterior teeth – patients chin is tilted upward

  1. Blurred mandibular anterior teeth – patients chin is tilted downward

  1. Cervical spine error- spinal column is slumped

6. Radiolucent artifact is created in maxillary apical region – patient is NOT pressing tongue against palate

        1. Density- overall degree of blackness on the x-ray

  1. Factors affecting density – exposure factors (mA, time, kVp) , source-film distance

    1. May be increased by increasing exposure time

    2. May be increased by increasing mA

    3. May be increased by increasing kVp

        1. Contrast- difference in densities among various regions on a x-ray

  1. Factors affecting contrast – kVp, subject contrast, film contrast

    1. May be increased by decreasing kVp

        1. Sharpness- ability of a radiograph to define an edge

  1. Factors affecting sharpness- focal spot size, movement, film emulsion grain size, intensifying screen fluorescence

D) Resolution – ability of a radiograph to record and demonstrate separate structures that are close . . together.

1. Factors affecting resolution – focal spot size, movement, film emulsion grain size, intensifying screen fluorescence
Buccal Object Rule

-SLOB Rule

- Same lingual opposite buccal

            • MBD Rule

            • If object moves in same direction as x-ray head, object is lingual in location

            • If object moves in opposite direction to x-ray head, object is buccal in location

Inverse Square Law

  1. The intensity of radiation is inversely proportional to the square of the distance from the source of radiation

  2. i1= d22 I = Intensity

i2 d12 D= Distance

Sample Problem:

If a person standing 3 feet from an x-ray source receives 4 Rads of exposure, how much would they receive at 6 feet?

I1= 4 Rads D1=3’ D2=6’

X= 162 = .40 sec


If the distance is doubled = then you multiply by 4 --- If the distance is halved = then you divide by 4

Radiation Safety
AlARA Principle- “As Low As Reasonably Achievable”

    • Methods to minimize patient exposure

      • Minimize exposure time, use fast film

      • Maintain long source-object distance

      • Utilize proper added filtration

      • Utilize appropriate lead shielding

      • Use rectangular vs. round collimation


        • 1.5 mm added filtration for machines operating at < 70kVp
        • 2.5 mm added filtration for machines operating at > 70 kVp

        • All lead aprons should have at least .25mm lead thickness

        • Inherent filtration- unleaded glass window and oil bath that x-ray beam passes through

        • Added filtration- metal filter, usually aluminum, installed by manufacturer

Total Filtration = Sum of inherent and added filtration

            • Thyroid collar – 50% exposure reduction to thyroid

            • Cover front from neck to thighs


            • No Thyroid collar

            • Cover front and back from shoulders to waist

MPD – Maximum Accumulated Dose

-Specific for job description

            • Assumes linear, non-threshold response

            • Is a cumulative measure of exposure

            • Limits for general public are 10% of limits for occupationally exposed workers

            • Formula implies that no one under age 18 may be occupationally exposed

Formula N=Age

MPD = 5 (N-18) Rem


MPD = .05(N-18)Sv

MAD – Maximum Permissible Dose

- Accumulated lifetime dose limit for occupationally exposed workers

            • MAD for general public is 10% of limits for occupationally exposed workers

Formula N= Age

MAD = 5Rem



Example: MAD for a 35 year old radiation worker: MAD = (35-18) X 5 = 85 rems or 850 mSV
Half Value Layer – Defines the thickness of material required to block 50% of x-rays

Periapical Radiolucencies

  • Periapical granuloma – a localized mass of chronically inflamed granulation tissue at the apex of a non-vital tooth.

  • Periapical cysts (radicular cyst) – a lesion that develops over a prolonged period of time, cystic degeneration takes place within a periapical granuloma and results in a periapical cyst.

            • Results from pulpal death and necrosis

  • Periapical abscess - a localized collection of pus in the periapical region of a tooth that results from pulpal death.

    • Acute periapical abcess- painful

    • Chronic periapical abcess- usually asymptomatic

Periapical Radiopacities

  • Condensing osteitis – a well-defined radiopacity that is seen below the apex of a non vital tooth with a history of long-standing pulpitis

  • Hypercemetosis – excess deposition of cementum on root surfaces

- Results from super-eruption, inflammation, or trauma, sometimes no obvious cause

Anatomical Landmarks


Quizzes on Radiology to

Radiology Case Studies
UCLA Oral Radiology Online Course


Tooth development on panoramics

Dental Radiographic Interpretation Test

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