Lasers and MRI safety

Lasers and MRI scanners use non-ionizing energy, so they do not cause the cancer risk of X-rays, and nobody needs a dosimeter to work near them. They are still among the most dangerous pieces of equipment in a hospital, because their hazards are immediate and severe. A surgical laser beam reflected off a steel instrument can burn a retina in a fraction of a second. An MRI magnet can pull an oxygen cylinder across a room with enough force to kill. Both hazards depend on strict access control and training for everyone who enters the area, including porters, cleaners, maintenance workers and firefighters. This lesson covers both.

How medical lasers are used

A laser produces a narrow, intense beam of light of a single wavelength. Different wavelengths are absorbed by different tissues, which lets surgeons cut, vaporize, coagulate or reshape tissue very precisely. Lasers are used in many specialties: carbon dioxide lasers in ear, nose and throat surgery and gynecology; holmium lasers to break up kidney stones; argon and other lasers in eye clinics; diode and Nd:YAG lasers in many procedures; and a range of lasers and intense pulsed light devices in dermatology and cosmetic clinics. Low-power lasers are also used in physiotherapy and in some measuring and alignment devices.

Laser classes

Lasers are classified by how much harm their beam can do. The classes, from the international standard used in both Canada and the United States, are:

Most surgical and many dermatology lasers are Class 4, the most hazardous class, and these are the focus of a laser safety program.

The hazards of Class 3B and 4 lasers

• Eye injury: the eye is the organ most at risk. Visible and near-infrared lasers are focused by the lens onto the retina, where they can burn a permanent blind spot in an instant. Far-infrared lasers, such as carbon dioxide lasers, and ultraviolet lasers injure the cornea and lens at the front of the eye. Many surgical laser beams are invisible, so a worker may not know a beam has hit their eye.
• Skin burns from the direct beam or close reflections.
• Fire: a Class 4 beam can ignite drapes, gauze, alcohol-based skin preparations and hair. In the airway, where oxygen-rich gas flows, a laser can ignite an endotracheal tube, causing a devastating airway fire.
• Laser plume or surgical smoke: when a laser vaporizes tissue, it produces smoke containing chemicals such as benzene and formaldehyde, fine particles, and sometimes viral DNA and viable cells. Electrosurgery (cautery) produces similar smoke.
• Electrical hazards: lasers use high voltages internally, a hazard mainly for service technicians.

Laser safety controls

The CSA standard Z386, Safe use of lasers in health care, sets out a laser safety program for Canadian facilities, and the CSA standard Z305.13 covers plume scavenging. Regulation 67/93 procedures and the general duty under the OHSA make these standards the benchmark for Ontario hospitals.

A laser safety program, in either country, includes:

• A laser safety officer (LSO), with authority to oversee all laser use, approve procedures, train staff and investigate incidents, and a laser safety committee in larger hospitals.
• A nominal hazard zone (NHZ): the area around the laser within which the beam or its reflections could exceed the safe exposure level. For most surgical lasers in an operating room, the whole room is the NHZ.
• Controlled access: warning signs at every entrance showing the laser type and the eyewear needed, doors closed during use, windows covered if the beam could pass through them, and a person assigned to control the laser.
• Laser eyewear for everyone in the NHZ, matched to the wavelength of the laser in use, with the right optical density (OD), labelled on the frame. Patients' eyes are protected too, with eyewear or moist pads.
• Key control, so that only trained users can switch a laser on, and standby mode whenever the laser is not being fired.
• Fire prevention: non-reflective or matte instruments, moist drapes and sponges near the site, letting alcohol skin preparations dry fully, laser-resistant airway tubes, low oxygen concentrations near the airway, and water and a fire extinguisher at hand.
• Smoke evacuation: a smoke evacuator with its capture device within a few centimetres of the site, with filters handled as contaminated waste, plus general room ventilation.
• Training and audits for surgeons, nurses, technicians and anyone who works in laser areas.

Eyewear is often the weakest point. An operating room may have several lasers with different wavelengths, and eyewear for one gives no protection against another. The optical density needed depends on the laser's power and the safe exposure level at that wavelength: each unit of OD reduces the light by a factor of ten. The LSO calculates the OD needed for each laser and buys eyewear to match.

# Optical density (OD) needed for laser eyewear
# OD = log10(worst-case exposure / maximum permissible exposure)
import math

lasers = [
    # (name, wavelength in nm, worst-case irradiance at the eye W/cm2, MPE W/cm2)
    ("CO2 surgical", 10600, 1000.0, 0.1),
    ("Nd:YAG", 1064, 500.0, 0.005),
    ("holmium", 2100, 400.0, 0.1),
    ("diode", 980, 200.0, 0.004),
]

for name, wl, exposure, mpe in lasers:
    od = math.log10(exposure / mpe)
    print(f"{name:14}{wl:6} nm   OD needed: {od:4.1f}   buy OD {math.ceil(od)} or higher at {wl} nm")

CO2 surgical   10600 nm   OD needed:  4.0   buy OD 4 or higher at 10600 nm
Nd:YAG          1064 nm   OD needed:  5.0   buy OD 5 or higher at 1064 nm
holmium         2100 nm   OD needed:  3.6   buy OD 4 or higher at 2100 nm
diode            980 nm   OD needed:  4.7   buy OD 5 or higher at 980 nm

The numbers in the script are illustrative, not values to use; the LSO works from the laser's specifications and the standard. The point is the pattern: each laser needs eyewear rated for its own wavelength, and an OD 4 rating at one wavelength may be worthless at another. At Northbridge, eyewear is stored on a rack outside each laser room, labelled with the laser it goes with, and the circulating nurse checks that everyone entering is wearing the right pair.

MRI: how the hazards arise

Magnetic resonance imaging uses a very strong magnetic field, radio waves and rapidly switching magnetic fields to produce detailed images without ionizing radiation. The magnet in most hospital scanners is superconducting, kept extremely cold by liquid helium, and its field is always on, day and night, whether or not a patient is being scanned. Typical clinical scanners have fields of 1.5 or 3 tesla, tens of thousands of times stronger than the Earth's field.

The main hazards are:

• The projectile effect: ferromagnetic objects, such as oxygen cylinders, IV poles, wheelchairs, floor polishers, scissors, pens, keys and tools, are pulled toward the magnet with great force and speed. People have been seriously injured and killed by objects flying into a scanner. The force increases very steeply close to the magnet, so an object that feels only a slight tug at the door can become uncontrollable a few steps later.
• Implants and devices: some pacemakers, defibrillators, aneurysm clips, neurostimulators, cochlear implants and drug pumps can be moved, heated or made to malfunction. Many modern devices are labelled MR conditional, safe only under specific conditions.
• Heating and burns from the radio waves, especially where wires, monitoring leads or skin-to-skin contact form loops.
• Noise: the switching gradient fields make loud knocking sounds, so patients and anyone in the room during scanning need hearing protection.
• Quench: if the magnet suddenly loses its superconductivity, the liquid helium boils off rapidly. The helium is meant to vent outside through a quench pipe; if it leaks into the room, it can displace oxygen and cause asphyxiation and cold burns.
• Effects on staff moving near the magnet, such as dizziness, nausea and a metallic taste, which are temporary.

MRI safety zones and screening

The most widely used approach to MRI safety divides the MRI area into four zones, from the guidance of the American College of Radiology, which is used in Canada as well as the United States:

• Zone I: public areas outside the MRI suite.
• Zone II: the reception and changing area, where patients are screened.
• Zone III: the restricted area around the control room, locked and entered only by trained MR personnel or by screened people under their supervision.
• Zone IV: the scanner room itself, entered only under direct supervision of trained MR personnel.

Everyone who enters Zone III or IV must be screened for implants and metal, every time, including staff. Patients and visitors complete a written screening form, change into hospital clothing and have it reviewed by an MR technologist. Staff who work in the area receive MR safety training and are designated MR personnel; those who only occasionally need to enter, such as porters, cleaners, security and maintenance staff, need training too, or must be escorted.

Equipment entering Zone IV must be labelled MR safe, or MR conditional for the conditions of that scanner. Nothing unlabelled goes in. Hospitals keep MR-conditional stretchers, wheelchairs, IV poles, oxygen cylinders and fire extinguishers dedicated to the MRI suite, and many use ferromagnetic detectors at the Zone IV door as an extra check. In an emergency, such as a patient in cardiac arrest, the patient is moved out of Zone IV for resuscitation, rather than bringing a crash cart into the room. Firefighters are told in advance that the magnet is always on, even during a fire or power failure.

A near miss at Northbridge

One evening, an environmental services worker covering for a colleague started to push a floor polisher toward the open door of the MRI scanner room after the last patient had left. A technologist still finishing paperwork in the control room saw her through the window, shouted, and stopped her at the door. The polisher was steel, and it would almost certainly have been pulled into the magnet.

The JHSC review found that the regular evening cleaner had been trained and knew the rules, but the replacement had never been told; the door to Zone IV had been propped open for ventilation; and there was no sign at the Zone III entrance saying the magnet is always on. The fixes were a locked Zone III door with badge access only for trained staff, a sign on every door, an MR-conditional cleaning kit kept in the suite, a rule that nobody cleans Zone IV without a technologist present, and MRI awareness training for every environmental services worker, not just those regularly assigned.