How to determine Failure Tolerance Time of Laser Equipment?

[Roland chung]

Hello,

Anyone in the forum has experience with laser equipment?

I am wondering how to determine the failure tolerance time of laser equipment which required by the clause 51.2 in 60601-2-22.

The laser equipment has a laser power monitoring system (by sensing current). 60601-2-22 requires the monitoring shall be carried out at intervals shorter than the failure tolerance time. Our monitoring system is real time. Is it necessary to evaluate the failure tolerance time?

Thanks and regards,
Roland

 

[plantengineer]

Why type of laser equipment? Is it a laser interferometer? If so, their should probably be a failure tolerance associated with it since measurements with those types of lasers are continuous, and based on pulses. Might need to verify the measurement rate is what the manufacturer says it is more than the failure tolerance time.

I believe that laser autocollimators have a calibration responses measured in arc-seconds. It measures rotation about the axis, so in this case, IU would assume their would not be a failure time tolerance associated with this type.

 

[Peter Selvey]

This is an old requirement which has some historical basis. Older lasers were not very stable, so the output could fluctuate significantly even in normal condition. The feedback system, whether used for closed loop or open loop control should be fast enough to respond in time to such fluctuations.

In a modern lasers the cavity designs are more stable. Feedback loops can be tuned such that there is no significant overshoot/settling time to a step change. In such a case, the response time of the feedback loop is not critical.

In the event of a genuine fault, the standard specifies a 100ms response time for the protection system.

 

[Roland chung]

Hello both, thank you for your input.

Why type of laser equipment?

This is Nd:YAG laser equipment.

This is an old requirement which has some historical basis.

The requirement remain unchanged in 60601-2-22:2007. So I think it is not outmoded.

Any thoughts on the failure tolerance time?

 

[plantengineer]

I would recommend contacting the laser manufacturer. They should know what the capabilities and requirements of their laser should be. Too difficult to set the parameter yourself.

 

[Peter Selvey]

Yes, I saw it is word for word copied in the 2007 edition.

But I still think the requirement is clear in a historical background.

I have worked with many types of lasers since around 1995 in Australia and Japan (both countries seem to be hot spots for medical lasers). Some were very unstable, others perfectly stable. Some used a lot of tricks to get a stable output.

The impression I had was that with the invention of new types of lasers (e.g. diode lasers), improvement in cavity design for existing types (e.g. CO2 lasers), new ways to get to useful wavelengths (e.g. frequency doubling) and improvements in feedback loops (e.g. speed of sensors, op-amps, digital processing etc), stability in normal condition is becoming less of an issue.

But there are lots of laser types and some types may still be unstable in normal condition, thus the requirement should remain.

If you want to measure the "monitoring time" I recommend investigate laser output response to step change (i.e. step response). Even "real time" control is not perfect, and takes time to settle and there may be overshoot.

The settling time and overshoot energy (if applicable) should be negligible with respect to the clinical application ("failure tolerance time"). Some applications like ophthalmic work have very short tolerance time, others for general surgical procedures may have long tolerance time.

 

[Marcelo]

The requirement remain unchanged in 60601-2-22:2007. So I think it is not outmoded.

It's not unusual to standards to keep requirements with can be easily fulfilled just per se... for example, the third edition was to have removed the requirements for flamable anaesthetic gases because no one in the world uses them anymore. However, some people said that they could use it someday, so it remained, as an Annex, in the standard.

Another example were the old requirements for CRTs...the usual problems with CRTs were solved a lot of time ago, but the standard still have requirements.

Those in a way might be there just to keep things in check.

Just a generic comment, anyway...not totally sure this is the case for the laser requirements.

 

[Roland chung]

Actually, I am not concerned the "monitoring time" since this time is up to the speed of feedback loop. It is not difficult to obtain.

The question of failure tolerance time is still open. I have worked with infusion pump. It is clear to determine the failure tolerance time (FTT) for air infusion. That is the volume of the ADMINISTRATION SET between the air detector and the venous cannula connected to it divided by the maximum flow rate of the pump.

For the laser equipment, the FTT is indeed short (mS). But how to calculate it?

 

[Peter Selvey]

It's different than an infusion pump: in that case it is assumed any air is bad (death), so fault tolerance time is based on time it takes the air bubble to reach the patient. This is true of any situation where it takes time for something "bad" to travel to the patient.

For a laser, the "bad" thing (e.g. overpower) will for all intentional purposes reach the patient instantaneously.

But, it takes time for the tissue to respond, increase in temperature, and be damaged. And, even if damage occurs, the severity may not be the same (in many locations the body can deal with tissue damage). So the fault tolerance time is really the time in which significant or serious unintended damage to the tissue occurs.

This depends very much on the clinical application, type of laser etc.

 

[Roland chung]

Do you think it is necessary for us to simulate the clinical application on mimic tissue and record the tissue damage time (i.e. fault tolerance time)?