Overview
Radiographic techniques are a core competency tested on the DANB Radiation Health and Safety (RHS) exam. This guide covers film placement, exposure factors, error identification, infection control, digital radiography, and radiation safety. Mastery of these concepts is essential for both the exam and safe clinical practice.
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Film Placement & Technique
The Paralleling Technique
The paralleling technique is the preferred and most widely used intraoral technique in modern dental practice.
• The film/sensor is placed parallel to the long axis of the tooth
• The central ray is directed perpendicular to both the tooth and the film
• Requires a film-holding instrument such as the Rinn XCP (Extension Cone Paralleling) device
• The film is positioned farther from the tooth (often toward the center of the mouth)
Primary Advantages:
• Less image distortion
• More accurate representation of true tooth shape and length
• Reproducible results due to standardized film holder positioning
The Bisecting Angle Technique
The bisecting angle technique is an older method, used when anatomical limitations prevent film holder use.
• An imaginary bisecting line is drawn between the long axis of the tooth and the plane of the film
• The central ray is directed perpendicular to this imaginary bisecting line
• More prone to distortion (elongation or foreshortening) than the paralleling technique
Bitewing Radiographs
Bitewing radiographs image the crowns and interproximal areas of both maxillary and mandibular teeth simultaneously.
• Film is centered to cover both maxillary and mandibular posterior teeth
• The occlusal plane bisects the film vertically
• The bite tab is centered in the patient's bite
• Vertical angulation: +8 to +10 degrees
• Horizontal angulation: directed through the interproximal contact points
• Most useful for detecting interproximal caries and assessing alveolar bone levels
Occlusal Radiographs
Occlusal radiographs use a large film placed on the occlusal surface to image a wide area.
• Images a large area of the maxilla or mandible on a single film
• Useful for detecting:
- Pathology (cysts, tumors)
- Impacted teeth
- Foreign objects
- Cleft palate
- Salivary stones
Full-Mouth Series (FMS)
A full-mouth series (FMS) provides a complete radiographic survey of all teeth and surrounding structures.
| Component | Typical Number |
|---|---|
| Periapical radiographs | 14–18 |
| Bitewing radiographs | 4 |
| Total | ~18–21 images |
Key Terms – Film Placement
• Paralleling technique – Film placed parallel to the long axis of the tooth
• Bisecting angle technique – Central ray aimed at the bisecting angle between tooth and film
• Rinn XCP – Film-holding device used with the paralleling technique
• Bitewing – Radiograph showing crowns of maxillary and mandibular teeth simultaneously
• Occlusal radiograph – Large film placed on occlusal surface for broad area imaging
• Full-mouth series (FMS) – Complete set of periapical and bitewing radiographs
> Watch Out For: The bisecting angle technique is NOT preferred — the paralleling technique is the standard of care. Do not confuse the two when identifying advantages and disadvantages on the exam.
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Exposure Factors & Settings
The Three Primary Exposure Factors
| Factor | Controls | Effect When Increased |
|---|---|---|
| kVp (kilovoltage peak) | Quality/energy of X-ray beam | More penetrating beam; longer scale contrast (more gray tones, less black-and-white) |
| mA (milliamperage) | Quantity of X-rays produced per unit time | Increases density (overall darkness) of the image |
| Exposure Time | Duration of X-ray production | Increases density (overall darkness) of the image |
Radiographic Contrast
• kVp primarily controls contrast
• Low kVp → High (short-scale) contrast → More black and white, fewer gray tones
• High kVp → Low (long-scale) contrast → More shades of gray, better for differentiating subtle density differences
Source-to-Film Distance (SFD)
• Long cone (PID): Recommended distance = 16 inches (~40 cm)
• Short cone (PID): Approximately 8 inches
• A longer SFD reduces magnification and improves image sharpness
• Increasing SFD increases the inverse square law effect — exposure time must be adjusted accordingly
Key Terms – Exposure Factors
• kVp (kilovoltage peak) – Controls the energy/penetrating power of the X-ray beam
• mA (milliamperage) – Controls the number of X-rays produced per unit time
• Radiographic density – The overall darkness of the radiographic image
• Radiographic contrast – The difference in density between adjacent areas of the image
• Short-scale contrast – High contrast; few shades of gray; mostly black and white
• Long-scale contrast – Low contrast; many shades of gray
• PID (position indicating device) – The cone or cylinder that directs the X-ray beam
> Watch Out For: Students frequently confuse density and contrast. Remember: mA and time control density (darkness); kVp controls contrast (gray scale). Both mA and time affect quantity; kVp affects quality.
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Radiographic Errors & Corrections
Vertical Angulation Errors
| Error | Cause | Correction |
|---|---|---|
| Elongation | Insufficient vertical angulation (too flat/parallel) | Increase the vertical angulation |
| Foreshortening | Excessive vertical angulation (too steep) | Decrease the vertical angulation |
Memory Tip: Think of it this way — "Not enough angle = elongation" (the image stretches out); "Too much angle = foreshortening" (the image is compressed).
Horizontal Angulation Errors
• Overlapping of interproximal contacts is caused by incorrect horizontal angulation
• The central ray must pass directly through the contact points between teeth
• Correction: Adjust the horizontal angle of the PID to align with the contacts
Other Common Errors
| Error | Cause | Appearance |
|---|---|---|
| Cone cut | PID misalignment; beam misses part of film | Clear (unexposed) area with a curved/rounded border |
| Completely black film | Overexposure, accidental light exposure, or overdevelopment | Entire film is black/opaque |
| Completely clear film | Unexposed film, film not developed, or fixer placed before developer | Entire film is clear/transparent |
| Herringbone (tire-track) pattern | Film placed backwards (lead foil side facing tube) | Embossed geometric pattern across image |
| Blurred/fuzzy image | Patient or tube head movement during exposure | Loss of sharpness throughout image |
| Double image | Film exposed twice | Superimposed double image |
Key Terms – Errors & Corrections
• Elongation – Image appears longer than actual tooth; caused by insufficient vertical angulation
• Foreshortening – Image appears shorter than actual tooth; caused by excessive vertical angulation
• Overlapping – Interproximal contacts appear superimposed; caused by incorrect horizontal angulation
• Cone cut – Unexposed area on film; caused by PID misalignment
• Herringbone pattern – Film reversed/backwards in patient's mouth
> Watch Out For: The herringbone (tire-track) pattern is a classic exam question. Remember: it is always caused by placing the film backwards — with the lead foil side facing the X-ray source rather than the patient's teeth.
> Watch Out For: Do not confuse a completely black film with a completely clear film. Black = overexposed or light-fogged; Clear = unexposed OR processed in fixer first.
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Infection Control in Radiography
Standard Infection Control Protocol
Before Patient Seating:
• Disinfect all surfaces that will be touched (tube head, PID, chair controls, exposure button area)
• Place disposable barriers on equipment
During Radiograph Exposure:
• Dental assistant must wear gloves at minimum
• Mask and protective eyewear are also recommended
• Avoid cross-contamination between patient and equipment
Handling Traditional (Film) Radiographs – The "Dry" Technique
1. Keep gloves on after exposing all radiographs
2. Open the film barrier packet and allow the film to drop into a clean cup or onto a clean paper towel/surface — do not touch the film with contaminated gloves
3. Remove and dispose of gloves
4. Process the clean films with bare (washed) hands
Infection Control for Digital Sensors
| Sensor Type | Sterilization Method |
|---|---|
| Solid-state sensor (CCD/CMOS) | Cannot be heat sterilized; place in a disposable barrier/sleeve before use; disinfect after barrier removal |
| PSP Phosphor Plate | Cannot be heat sterilized; place in a disposable barrier/plastic sleeve; disinfect with approved surface disinfectant after barrier removal |
> Watch Out For: PSP plates and rigid digital sensors CANNOT be heat sterilized (autoclaved). This is a high-yield exam point. They must be protected with barriers and surface disinfected.
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Digital Radiography
Types of Digital Radiography Systems
| Type | Sensor | Process |
|---|---|---|
| Direct Digital Imaging | Solid-state sensor (CCD or CMOS) wired to computer | Image appears immediately on computer screen |
| Indirect Digital Imaging | Photostimulable phosphor (PSP) plate | Plate is scanned by a laser scanner after exposure; then erased with bright light for reuse |
Advantages of Digital Radiography
• Radiation reduction: Requires 50–80% less radiation than conventional film — primary patient safety advantage
• Immediate image display (especially with direct sensors)
• Images can be enhanced, enlarged, and adjusted on-screen
• No chemical processing required
• Easy storage and transfer of images electronically
• Environmentally friendly (no chemical waste)
PSP Plate Special Considerations
• Stores a latent image after exposure
• Must be scanned promptly after exposure
• Ambient light exposure before scanning will erase the latent image → results in a blank or fogged image requiring repeat exposure
• After scanning, the plate is erased with bright light and can be reused
Key Terms – Digital Radiography
• CCD (Charge-Coupled Device) – Type of direct digital sensor
• CMOS (Complementary Metal-Oxide Semiconductor) – Type of direct digital sensor
• PSP (Photostimulable Phosphor) plate – Indirect digital sensor; stores latent image until scanned
• Latent image – Invisible stored image on a PSP plate before scanning
• Direct digital imaging – Sensor connected directly to computer; immediate image
• Indirect digital imaging – PSP plate scanned after exposure to produce image
> Watch Out For: If a PSP plate is exposed to light before scanning, the image is lost. Always transport PSP plates in a light-protected container immediately after exposure.
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Radiation Safety
Patient Protection
• Lead apron with thyroid collar must be placed on every patient before exposure
- Protects radiosensitive organs: gonads, bone marrow, and thyroid gland from scatter radiation
• Use the ALARA principle (As Low As Reasonably Achievable)
• Use proper technique to minimize retakes
• Perform radiographs only when clinically indicated
Operator Protection
• Never hold films or sensors in the patient's mouth during exposure
• Stand behind a lead barrier or at least 6 feet away from the tube head at a 90–135 degree angle from the primary beam
• Wear a dosimeter badge (radiation monitoring device) to track cumulative exposure
Maximum Permissible Dose (MPD)
| Category | MPD Per Year |
|---|---|
| Occupationally exposed workers | 50 mSv (5 rem) per year – NCRP Report No. 116 |
| General public (non-occupational) | 1 mSv (0.1 rem) per year |
> Note: The trend in radiation protection guidelines is moving toward lower limits; always reference NCRP Report No. 116 for the DANB exam.
Radiation Units
| Unit | Measures |
|---|---|
| Roentgen (R) | Ionization in air |
| Rad | Absorbed dose of radiation |
| Rem/Sievert (Sv) | Biological effect (dose equivalent) — most relevant for MPD |
Key Terms – Radiation Safety
• ALARA – As Low As Reasonably Achievable; guiding principle of radiation protection
• MPD (Maximum Permissible Dose) – 50 mSv/5 rem per year for occupationally exposed workers
• Lead apron with thyroid collar – Patient protective device against scatter radiation
• Dosimeter badge – Monitors cumulative radiation exposure for dental personnel
• NCRP – National Council on Radiation Protection and Measurements; sets MPD guidelines
• Scatter radiation – Secondary radiation deflected from the primary beam
> Watch Out For: The thyroid collar is essential and should never be omitted — the thyroid gland is highly radiosensitive. Some outdated sources omit the thyroid collar; the current standard includes it for all patients.
> Watch Out For: Know the MPD in both rem AND mSv (5 rem = 50 mSv). DANB questions may use either unit.
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Quick Review Checklist
Use this checklist to confirm exam readiness. Check off each item as you master it:
• [ ] Explain the difference between the paralleling and bisecting angle techniques and identify which is preferred
• [ ] Describe correct film/sensor placement for bitewing radiographs (vertical position, angulation)
• [ ] State the number of radiographs in a full-mouth series (~18–21 total)
• [ ] Explain the purpose of occlusal radiographs
• [ ] Identify what kVp, mA, and exposure time each control (quality vs. quantity; contrast vs. density)
• [ ] Distinguish between short-scale and long-scale contrast and their relationship to kVp
• [ ] State the recommended source-to-film distance for the long cone: 16 inches
• [ ] Identify the causes of elongation (insufficient angle) and foreshortening (excessive angle)
• [ ] Identify the cause of overlapping (incorrect horizontal angulation)
• [ ] Recognize the herringbone pattern and its cause (film placed backwards)
• [ ] Recognize the cause of a cone cut (PID misalignment)
• [ ] Describe the dry technique for aseptic film processing
• [ ] State that PSP plates and digital sensors cannot be autoclaved — barrier protection + surface disinfection only
• [ ] Identify the two types of digital systems (direct: CCD/CMOS; indirect: PSP)
• [ ] State the radiation reduction of digital radiography: 50–80% less than conventional film
• [ ] Explain what happens when a PSP plate is exposed to ambient light (latent image erased)
• [ ] State the MPD for occupationally exposed workers: 50 mSv (5 rem) per year (NCRP Report No. 116)
• [ ] Explain the purpose of the lead apron with thyroid collar (protects gonads, bone marrow, thyroid)
• [ ] Apply the ALARA principle to clinical decision-making
• [ ] Identify proper operator positioning during X-ray exposure (6 feet away, 90–135° from beam)
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Good luck on your DANB RHS exam! Focus on understanding the why behind each technique and safety measure — this will help you apply knowledge to scenario-based questions.