Fluoroscopy Principles
Real-Time Intraoperative Imaging for Orthopaedic Surgery
Fluoroscopy Modes
Continuous: Constant X-ray output — highest dose, smoothest image (rarely needed)
Pulsed: Intermittent X-ray pulses (1-30 per second) — significant dose reduction, adequate image quality
Last Image Hold: Stores the last frame for review — zero additional radiation
Screening (low-dose): Reduced tube output for positioning — image quality reduced but acceptably so
Cine acquisition: High-dose recording for documentation — minimal use in orthopaedics
Key: Pulsed fluoroscopy at low pulse rates with last image hold should be the default mode for all orthopaedic procedures
Critical Must-Knows
- Fluoroscopy is continuous or pulsed X-ray imaging providing real-time visualisation during procedures.
- The C-arm consists of an X-ray tube (below the patient) and an image receptor (above) connected by a C-shaped frame.
- Scatter radiation to the surgeon is greatest on the X-ray tube side — the surgeon should stand on the IMAGE RECEPTOR side.
- Pulsed fluoroscopy reduces radiation dose by up to 90% compared to continuous fluoroscopy.
- The inverse square law: doubling the distance from the X-ray source reduces dose to one-quarter.
Examiner's Pearls
- "The X-ray tube is the source of SCATTER radiation — always position it AWAY from the surgeon (usually under the table).
- "Last image hold (LIH) stores the last fluoroscopic frame, reducing the need for additional exposure.
- "Mini C-arm (extremity fluoroscopy) produces significantly lower radiation than a standard C-arm.
- "Magnification mode increases dose because the X-ray beam is collimated more tightly and tube output increases.
- "Lead thyroid shield reduces thyroid dose by 90% and is mandatory for all fluoroscopy-exposed personnel.
Exam Warning
Fluoroscopy principles are examined extensively in both physics viva stations and clinical/operative scenarios. You must understand: the components and operation of a C-arm, scatter radiation and positioning (tube side vs detector side), dose reduction strategies (ALARA, pulsed mode, collimation, LIH), the inverse square law, magnification effect on dose, and the occupational dose limits for radiation workers. A classic trap is placing the surgeon on the X-ray tube side.
CLAPTDRadiation Dose Reduction
Memory Hook:CLAPTD to reduce dose: Collimate, Last image hold, ALARA, Pulsed mode, Time reduction, Distance.
SAFEC-arm Positioning
Memory Hook:SAFE positioning: Source Under, Away from tube, Face receptor, Entry below — protect yourself from scatter.
MAGWhen Fluoroscopy Dose Increases
Memory Hook:MAG causes dose to go up: Magnification, Automatic brightness control, and Greater patient thickness.
Overview
Fluoroscopy is the most commonly used intraoperative imaging modality in orthopaedic surgery. It provides real-time X-ray imaging that enables accurate fracture reduction, implant placement, joint assessment, and screw trajectory verification during surgical procedures. The mobile C-arm is a fundamental tool in every orthopaedic operating theatre.
Understanding the physics of fluoroscopy and the principles of radiation dose reduction is essential for safe practice. Orthopaedic surgeons are among the highest-exposed medical professionals to occupational radiation, and the cumulative effect of years of fluoroscopy exposure must be actively minimised through proper technique and protective equipment.
Key Components of a C-arm
The C-arm consists of: (1) X-ray tube — generates the X-ray beam (cathode/anode system). (2) Image receptor — either an image intensifier (older) or flat-panel detector (newer). (3) C-shaped gantry — connects tube and receptor, allowing rotation in multiple planes. (4) Monitor — displays the real-time image. (5) Pedal controls — foot-operated for start/stop of exposure. (6) Collimator — adjustable shutters that narrow the beam. A flat-panel detector (FPD) C-arm provides better image quality, less dose, and less distortion compared to an image intensifier (II) C-arm.
ALARA Principle
ALARA (As Low As Reasonably Achievable) is the foundational principle of radiation protection. Every exposure should use the minimum dose necessary to achieve adequate image quality for the clinical task. This applies to both the patient and the operating team. Practical implementation: pulsed mode, collimation, distance, shielding, last image hold, minimising screening time.
Clinical Imaging
Imaging Gallery
Systematic Approach
Systematic Dose Reduction in the Operating Theatre
Intraoperative Dose Reduction Framework
| Strategy | Implementation | Expected Dose Reduction |
|---|---|---|
| Pulsed fluoroscopy | Use lowest acceptable pulse rate (1-4 pulses/second for positioning; 8-15 for dynamic assessment) | 50-90% reduction compared to continuous mode |
| Last image hold | Review stored images instead of firing additional exposures | Eliminates unnecessary repeat exposures — cumulative benefit |
| Collimation | Narrow the beam to the exact region of interest using the shutters | Reduces irradiated volume AND scatter by 30-50% |
| Distance maximisation | Step back from the beam when not actively operating; extend hands from beam path | Doubling distance = 75% dose reduction (inverse square law) |
| Minimise screening time | Use brief exposures (less than 2 sec each); release pedal between checks | Direct proportional reduction — halving time halves dose |
| Avoid magnification | Use magnification mode only when absolutely necessary for detail | Eliminates 2-4x dose increase associated with magnification |
Critical Exam Pearl: Scatter Radiation
Scatter radiation is the PRIMARY source of occupational dose to the operating team. The X-ray tube is the main source of scatter — scatter is produced when the primary beam enters the patient. Therefore: (1) The surgeon should ALWAYS stand on the IMAGE RECEPTOR (detector) side, not the X-ray tube side. (2) With the standard C-arm orientation (tube below, receptor above), the surgeon stands on the side AWAY from under the table. (3) Scatter intensity decreases rapidly with distance from the patient (approximately inversely with the square of the distance). (4) Lead aprons attenuate approximately 90-95% of scatter radiation at orthopaedic energies.
Physics and Equipment
X-ray Production in Fluoroscopy
The X-ray tube contains a cathode (tungsten filament) heated by electrical current, which produces electrons by thermionic emission. These electrons are accelerated by a high voltage (kV) across a vacuum toward the anode (tungsten target). When the electrons strike the anode, approximately 99% of their kinetic energy is converted to heat and only 1% produces X-rays.
Two processes produce X-rays:
- Bremsstrahlung (braking) radiation: Electrons decelerated by the nuclear electric field of tungsten atoms produce a continuous spectrum of X-ray energies. This is the dominant process.
- Characteristic radiation: Electrons knock out inner-shell electrons from tungsten atoms; the resulting cascade produces X-rays at specific energies (characteristic of tungsten).
Key tube parameters:
- kV (kilovoltage): Controls the ENERGY (penetrating power) of the X-ray beam. Higher kV = more penetrating beam. Typical fluoroscopy: 60-120kV.
- mA (milliamperage): Controls the NUMBER of X-rays produced per unit time. Higher mA = more X-rays = brighter image but more dose.
- Automatic Brightness Control (ABC): The system automatically adjusts kV and mA to maintain consistent image brightness as patient thickness varies. This is why obese patients receive significantly more radiation — the system increases output to compensate for greater tissue attenuation.
Intraoperative Applications
Common Fluoroscopy Applications in Orthopaedic Surgery
| Application | Typical Views | Key Technical Points |
|---|---|---|
| Fracture fixation (long bones) | AP and lateral of fracture site; inlet/outlet for pelvis | Verify reduction, alignment, and implant position. Use traction views for length assessment |
| Intramedullary nailing | AP and lateral at fracture and at proximal/distal locking sites | Freehand interlocking screw technique relies on 'perfect circle' fluoroscopic alignment of the screw hole |
| Hip fracture fixation (DHS/IM nail) | AP and lateral hip views | Ensure tip-apex distance (TAD) is less than 25mm on combined AP and lateral views |
| Pedicle screw insertion | AP, lateral, and oblique views of the spine | AP view confirms medial/lateral position; lateral confirms depth and angulation |
| Joint replacement | AP and lateral views of the joint | Verify component position, alignment, and cement distribution in cemented arthroplasty |
| K-wire and pin placement | AP and lateral of the target area | Real-time guidance for percutaneous fixation. Use mini C-arm for distal extremity |
The Perfect Circle Technique
For freehand interlocking screw placement in intramedullary nailing, the C-arm is rotated until the nail hole appears as a 'perfect circle' on the fluoroscopic image. This means the X-ray beam is perfectly aligned with the axis of the hole. A drill or screw inserted toward the centre of this circle will be perfectly aligned with the hole axis. This technique eliminates the need for jig-based targeting systems and reduces radiation exposure because it minimises the number of screening attempts needed.
Evidence Base
Radiation Dose to Orthopaedic Surgeons During Fluoroscopy
- Orthopaedic surgeons receive a mean effective dose of approximately 0.05 mSv per procedure, well below annual occupational limits.
- The hands receive the highest dose, followed by the eyes and thyroid.
- Cumulative career dose can become significant for high-volume trauma surgeons performing hundreds of fluoroscopy-guided procedures.
Pulsed vs Continuous Fluoroscopy Dose Reduction
- Pulsed fluoroscopy at 4 pulses/second reduced dose by 75-90% compared to continuous fluoroscopy.
- Image quality was clinically acceptable for orthopaedic applications at 4-8 pulses/second.
- Motion blur was more pronounced at very low pulse rates (1-2 pulses/s) but acceptable for static fracture assessment.
Dose reduction strategies are well-supported by evidence.
Australian Context
In Australia, radiation safety for fluoroscopy is regulated by ARPANSA (Australian Radiation Protection and Nuclear Safety Agency) and state radiation safety authorities. Orthopaedic surgeons using fluoroscopy must hold a radiation use licence (RUL) or work under the supervision of a licence holder, depending on the state or territory.
Australian occupational dose limits for radiation workers are: 20 mSv per year averaged over 5 consecutive years, with no single year exceeding 50 mSv. The lens of the eye limit is 20 mSv per year averaged over 5 years. Personal radiation monitoring (dosimetry badges) is mandatory for all staff regularly exposed to fluoroscopy.
RANZCR and the Australian Orthopaedic Association support the ALARA principle and recommend pulsed fluoroscopy as the default mode for clinical practice. Australian operating theatre standards require appropriate lead shielding availability, radiation warning signage, and regular equipment safety testing by qualified medical physicists.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
"An examiner asks: 'Where should you stand relative to the C-arm during fluoroscopy and why?'"
"You are performing an intramedullary nail for a tibial shaft fracture and need to place the distal interlocking screws using fluoroscopy."
"An examiner asks you to discuss the physiological effects of ionising radiation and the annual dose limits for radiation workers."
Fluoroscopy Principles — Exam Day Reference
High-Yield Exam Summary
C-arm Basics
- •X-ray tube below, image receptor (detector) above = standard orientation
- •Stand on the IMAGE RECEPTOR side (away from the tube)
- •Flat-panel detector (FPD) is superior to image intensifier (II) — less dose, no distortion
- •Mini C-arm: approximately 1/10th dose of standard C-arm for extremity procedures
Dose Reduction (CLAPTD)
- •Collimate — narrow beam to region of interest
- •Last image hold — review stored frames, not new exposures
- •ALARA — use minimum technique factors for adequate image quality
- •Pulsed — 50-90% dose reduction vs continuous
- •Time — brief exposures, release pedal between checks
- •Distance — inverse square law: 2x distance = 1/4 dose
Radiation Safety
- •Lead apron: 0.5mm Pb equivalent attenuates 90-95% of scatter
- •Thyroid shield: 90% thyroid dose reduction — mandatory for all
- •Lead glasses: 85-90% lens dose reduction
- •Personal dosimetry badge is mandatory for radiation workers
Dose Limits (Australian)
- •Occupational: 20 mSv/year averaged over 5 years (max 50 mSv any single year)
- •Eye lens: 20 mSv/year (recently reduced from 150 mSv)
- •Skin: 500 mSv/year
- •General public: 1 mSv/year
Dose Increases With
- •Magnification mode (2-4x increase — use sparingly)
- •Continuous vs pulsed fluoroscopy
- •Greater patient thickness (ABC compensates with higher output)
- •Closer distance to the X-ray source