LASERs: Visible and IR Flashcards Preview

Medical Physics: Imaging w/ Non-Ionising Radiation > LASERs: Visible and IR > Flashcards

Flashcards in LASERs: Visible and IR Deck (27)
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What are the essential properties of a LASER?

Small Divergence


What LASER is used in LASIK surgery and why?

UVC - 193nm
Penetrates to a depth of 0.1mm into the cornea, allowing the cornea to be reshaped but does not penetrate into other parts of the eye.


What LASER is used in the treatment of diabetic retinopathy surgery and why?

Green LASER (e.g. Argon 584nm)
Passes through the eye but it is absorbed by the blood sealing the leaks.


What LASER is used in LASER Hair Removal and why?

Red/IR LASER (e.g. Alexandrite 700-800nm)
Passes through dermis and tissue, absorbed by hair follicle.


What LASER is used in LASER surgery (e.g. ENT or Gynae) and why?

Visible or IR (e.g. CO2 10,600nm)
Delicate excisions with minimal collateral damage.
Excision depth of 0.1mm.


What LASER is used in Endoscopic LASER surgery and why?

Ho-YAG 2080nm
Cuts and coagulates with minimal bleeding.
Can destroy urinary stones.


What are the different classes of LASER?

Class 1 - Safe: enclosed or low power (<1mW)
Class 1M - Safe: except with viewing aids.

Class 2 - Safe for accidental viewing: 1mW max in the visible spectrum.
Class 2M - Use of collimating optics may be hazardous.

Class 3R - Intentional intra-beam viewing may be hazardous (Power <5mW)
Class 3B - Eye hazard but no skin Hazard (Power <500mW)

Class 4 - Eye, Skin, and Fire Hazard


Where is data for the calculation of MPE found?

IEC 60825-1


What is the Nominal Ocular Hazard Distance (NOHD)?

The distance within which the MPR may be exceeded, including both specular and diffuse reflection.


What is the equation for NOHD from a LASER?

NOHD = 1000*SQRT[Power/(pi*MPE)]


What is the equation for NOHD from a Fibre?

NOHD = (1/AA)*SQRT[Power/(pi*MPE)]
AA = Acceptance angle (0.1-0.4 radians)


What is the equation for the Acceptance angle of a fibre?

AA= = arcsin(NA)
NA = numerical aperture


What measures are required for safe working with LASERs?

Classification of the LASER
Risk Assessment
Designation of a controlled area
Adequate Personal Protection.
LASER local rules
LASER protection Supervisor
Contingency plans


What duty holders are required for working with class 3B and class 4 LASERs?

Responsible body (e.g. Trust) (delegates responsibility to LPA/Head of Med Phys)
Laser Protection Supervisor
Equipment Management (EBME)


What are the duties of the LASER Protection Advisor?

Advises the employer on LASER Saftey matters.
Audits LASER Safety.


What are the duties of the Clinical LASER Expert?

Assesses competency of operators.
Oversees Training of Operators.


What are the duties of the LASER Protection Supervisor?

Carries out LASER Risk assessment
Writes Local Rules
Supervises Training and Safe Working.


Give some examples of Pre-Use checks.

Condition of fibres, cables etc.
Safe initial power setting
Doors locked or controlled
Warning signs
Windows covered
Staff instructed.


Give some examples of shutdown/handover checks.

Power setting to zero
Filters to safe
Reporting of untoward incidents
Returned to usual power settings
Return Key.


What are common faults resulting in reported incidents to staff involving a Medical LASERs ?

No/Incorrect eye protection
Unitended ootsiwtch operation
Non-standard procedure
Incorrect replacement parts.


What are common faults resulting in reported incidents to patients involving a Medical LASERs ?

Incorrect eye-protection
Internal electrical fire
Electrical shock
Failure of fibre
Non-standard parts/procedure.


What two types of measurement device are used for the with a LASER power meter?

Thermopiles and Photodiodes.


How does a thermopile measure LASER power?

A Thermopile is a bank of thermocouples that use the Seebeck effect* to measure temperature that then indicates power.
The cool end must be kept at an ambient temperature.
*Two dissimilar metals joined together that creates a voltage when heated or cooled. Voltage indicates temp.


What are the characteristics of a thermopile?

10mm diameter aperture.
Low reflection (~2% as matte black)
Wavelength response of a few percent 200-20,000nm.
Can handle 10W max power or 10kW/cm^2 max power intensity.
Damage threshold 10GW/cm^2.
Response time of 1-2s.


How does a (Si) Photodiode measure LASER power?

Currents in diode proportional to LASER power.


What are the characteristics of a photodiode?

Wavelength specific
Quick response time
Limited to a few mW continuous pulse.


What is the purpose of a spectra-radiometer?

LASER power meters are designed for single wavelength sources, and so will give inconsistent results if used with LEDs of Xenon flash lamps.
Power output is therefore measured using a spectra-radiometer.