Use of c-arm is now an essential part of orthopaedic practice. Use of C-arm fluoroscope in orthopaedics has improved patient outcomes by improving precision in surgery and by reducing surgical trauma by permitting minimally invasive techniques.
Within the x-ray source an electrically heated filament produces electrons. These electrons are accelerated by a high voltage towards an anode made of high atomic weight elements such as tungsten. When the high energy electrons hit the tungsten and gets decelerated a small number of x-ray photons are emitted and the rest is converted to heat. The number of electrons depends on the strength of the electric current in milliamperes (mA). The maximum kinetic energy of the electrons is expressed as kilovolts peak (kVp). Higher mA produces more x-ray photons and higher kVp produces higher energy x-ray photons with greater penetrability. Higher mA increases the brightness of the image. Higher kVp may reduce the contrast of the image.
As the beam leaves the x-ray tube the rays diverge leading to reduced radiation with increasing distance. As the relationship between radiation and distance is by inverse square law; even small increase in distance can reduce radiation by a large percentage.
Fate of x-ray within the human body
When x-rays are beamed towards the human body they may three outcomes depending on the tissue electron density, tissue thickness and the x-ray beam energy.
- Completely penetrate the body and emerge at the opposite end to be detected by the film or detector. (1% of the beam during fluoroscopy)
- Completely absorbed by the tissue.
- Scattered by the tissue.
After the discovery of xrays by Roentgen in 1895, its potential benefits in the medical field was immediately recognised but the identification of its deleterious effects took a longer time. Radioactivity was discovered in the same year by Becquerel and its usefulness in the treatment of malignancy was recognised early due to its deleterious effects. In the year 1900, Albers Schonberg advised reduced frequency of exposure, use of lead shielding, gap of more than 30 centimetres from source as safety measures when working with radiation. In 1928, roentgen was accepted as the quantitative measurement for radiation exposure. International X-ray and Radium Protection Committee was formed in 1928. It was renamed later as International Commission on Radiological Protection (ICRP). Its aim is to advance the science of radiation protection. It has published several guidelines for radiation protection.
C-arm is an x-ray unit that allows alteration of angle and rotation of X-ray source and detector to permit imaging without changing the position of the patient. It was introduced in 1955. It is comprised of an X-ray generator and a image intensifier. The X-rays strike a fluorescent screen which glows according to the strength of the radiation. C-arms use caesium iodide for the fluorescent screen which converts the X-ray photons into photons in the visual spectrum by its luminescence property. A photocathode made of an antimony caesium compound situated beneath the fluorescent screen captures the glow and amplifies the luminance. In C-arms with flat panel detector the X-Rays are converted digitally into a visible spectrum.
Risks of ionizing radiation
Ionising radiation is potentially hazardous to the personnel and patient. Ionising radiation is classified as a carcinogen by the World Health Organisation. X-ray photons absorbed are a source of injury to the patient and the scattered rays is a potential source of injury to the personnel.
Risk of radiation injury is increased with higher doses and longer exposure times. The harmful effects may be for the individual or his descendants. They may be classified as somatic or genetic. Biological effects of radiation are classified into stochastic effects and deterministic effects. Stochastic effects may be malignancy or genetic defects. Stochastic effects like cancer and genetic defects can occur at any dosage levels. Deterministic effects occur when the threshold level is exceeded and their severity depends on the dosage. Deterministic effects are due to excessive cell death and can be erythema, epilation, skin necrosis or cataract formation.
Radiation Protection Principles
The radiation protection guidelines assume that the health risk of radiation increases with the dose which is called linear no-threshold hypothesis. This has lead to the formulation of ALARA (As Low As Reasonably Achievable) principle as the key to radiation safety guidelines.
As per current laws, the hospital is responsible for the protection of those exposed to ionising radiation within the hospital premises including the patients, personnel and the public. Medical procedures that need use of ionising radiation should be justifiable, safe and should be performed by trained person using appropriate equipments and methods. Any breach of safety regulations prescribed by laws is an offence.
There should be protocols and training of personnel to ensure radiation safety. Dosage restrictions should be stipulated and appropriate monitoring badges should be provided. An audit of the use of ionising radiation, compliance with safety protocols and exposure dosage monitoring is required as per the guidelines. Exposure time should be recorded in the patient case sheet. As the hazards are not immediately evident and also due to ignorance, the compliance with the safety measures is often alarmingly low.
The three basic factors that determine the safety are the exposure time, distance from source and shielding. In simple terms; reduce the exposure time, increase the distance from the source and use appropriate shielding. The exposure to the surgical team is actually greater than in conventional radiography due to the reduced distance, less shielding and exposure time especially during difficult procedures. Lead aprons, thyroid shields and leaded eyewear are a must for personal protection. Though heavier, wraparound aprons are better.
Exposure time and X-ray field size should reduced to the the maximal extent possible. X-ray beam should be well collimated. Simulated skin entrance and exit exposure levels and the scatter radiation levels should be measured by a qualified physicist at all occupied areas around the c-arm to determine the type, number and location of the personal radiation monitors to be used. Ideally a whole body monitor badge should be worn under the lead apron and a badge should be worn outside the thyroid shield. A wrist badge should be worn on the hand closest to the beam to monitor the extremity exposure.
The exposure to the patient is determined by distance from the source, thickness of the patient, kV, mA and the exposure time. Thicker the patient more is the exposure. The closer to the source greater is the exposure. Patient exposure can be reduced by reducing the duration of exposure, increasing the distance from the source and reducing the field size.
Practical Steps to improve radiation safety
- The X-ray source should be kept as far away from the patient as possible. If the source is closer to the patient the beam is concentrated on a small area increasing the chance of injury.
- The source should be kept below the operation table whenever possible. The main source of radiation to the personnel is scattering of beam by the patient. When the source is kept below the radiation is scattered on to the ground.
- When taking lateral or oblique view keep the source away from the personnel. The image intensifier should be towards the personnel.
- Collimate down to the area of interest. This will decrease the amount of tissue irradiated and the scattering.
- Keep it as close as possible to reduce the scattering, to reduce the patient dosage and to obtain a larger field of view.
- Personnel should stand on the side of image intensifier to reduce exposure to scatter rays.
- Use the lowest mA possible as the higher tube current increases the dosage.
- Larger kVp increases the penetrability of beam allowing the use of a lower mA. But large kVp may reduce contrast.
- Reduce the exposure time to the minimum. Normal mode fluoroscopy produces 1 to 10 R/min (0.01 to 0.1 Gy/min). HI or boost mode produces 10 to 20 R/min (0.1 to 0.2 Gy/min).
- Avoid pulse mode and continuous mode.
- If needed. use pulse mode than continuous mode. Continuous mode increases the dose exponentially. Radiation is 10-20 times more during continuous mode.
- When using pulse mode, use a lowest frequency possible.
- Reduce the magnification to the minimum as both digital and geometric magnification increases the dosage. Dose increases at the rate of square of magnification.
- Radiation is higher in larger patient as a bigger mA increasing the dosage and scatter.
Remember that scatter rays are the main source of radiation to the personnel. Injury from scatter rays can be reduced by use of shields and by increasing the distance from the source. Remember that the lens of the eye and the thyroid are most vulnerable to radiation injury.
- Use protective aprons, thyroid shields and lead goggles.
- Exposure from a radiation source decreases by the inverse of the distance squared. Hence stay as far away as possible from the X-ray source.
- Stand on the side of image intensifier as far as possible.
- Use dose monitors.
- Use portable shields if available.
- Preoperative planning an careful checking of the previous images can help to reduce the number of exposures.
- Annual dosage limit for hospital workers is 500 mrem for the whole body, 1500 mrem for the eyes and 5000 mrem for all other organs. Dosage limit for pregnant women is no more than 500 mrem (5 mSv) during the entire gestational period and no more than 500 mrem in a month.
- Full wrap around type protective gowns are recommended.
- It should have 0.50 mm Pb in the front panels and 0.25 mm Pb in the back panels.
- Use protective thyroid shields with an equivalent of 0.50 mm Pb.
- Use of leaded glasses to protect the eyes.
- Protective gloves should have at least a 0.25 mm Pb equivalency. But remember that these gloves do not protect the hands if placed within the primary beam.
- They should be checked yearly for efficiency.
- After use the protective aprons and thyroid shields should be stored properly to prevent damage.
- Lead aprons and thyroid shields with 0.5mm lead thickness provide 85%–95% attenuation of scattered x-rays.
Absorbed dose- The total amount of radiation energy absorbed per volume of tissue exposed.
Effective dose- Depends on the proclivity of tissue or organs exposed to develop stochastic effects and the type of radiation involved.
Tissue-weighting factors is high for breast tissue and ovaries as they are more prone for stochastic effects.
Entrance surface dose
Background effective dose (BRE) is the radiation from natural sources in the general population. In the United States is approximately 3.1 mSv per year. It is up to 70 mSv per year in Kerala, India due to the naturally occurring thorium coated monazite sand. A pelvic radiograph has an effective dose of ~0.6 mSv hence the BRE = 71 days.
kVP – Kilovolt peak
As per the newer guidelines gray (Gy) replaces roentgen (R) for exposure. The gray (Gy) replaces the rad (rad) as the unit of absorbed dose. And the sievert (Sv) replaces the rem (rem) as the unit of equivalent dose.