Extending the Life of X-ray Tubes in Medical Imaging Systems
For over 100 years, the X-ray tubes used in medical diagnostic systems have proven to be a dependable and cost effective way of producing X-radiation for a variety of healthcare applications. When handled, operated, and maintained properly, they can provide many years of functional service. However, without proper care, tube life can be shortened significantly due to a variety of factors. While some of these are beyond the control of technicians and engineers, the functional life expectancy of a tube, and hence its cost per hour of operation, is ultimately determined by those who operate and maintain the systems.
The Role of the Manufacturer in Minimizing Premature X-ray Tube Failure
Even the best manufacturers cannot produce consistently flawless, identical tubes. No matter how hard they may try to prevent it, small differences and defects will creep into the product. Normally, it’s the manufacturer who needs to make sure these factors do not negatively impact the operation of the tube in a clinical setting. Aside from selecting a reputable brand and distributer, there is little a technician or engineer can do to prevent tube failure prior to taking receipt of it.
From the moment an imaging department takes receipt of a new or used X-ray tube, however, the responsibility for its life expectancy rests firmly on the shoulders of the engineers who service and maintain the equipment and the technicians who perform the scans.
Four Common Reasons Medical X-ray Tubes Burn Out
Provided the tube has been quality tested, packed, shipped, and handled properly and there are no manufacturing defects, failure is typically attributable to four different factors: normal aging, application mismatch, improper power supply setup or operation, and issues with the tube enclosure.
1. Normal Aging – X-ray tubes eventually “burn out” due to prolonged use. This happens because the hot tungsten filament inside the tube steadily loses mass as material evaporates from its surface. Hot spots evaporate tungsten faster and the wire thins more rapidly at these locations, ultimately burning through the filament.
While normal aging can’t be prevented, it can be slowed down significantly by keeping the filament temperature as low as possible. Higher power, especially for extended periods, causes higher temperatures which, in turn, accelerates fatigue. Simply stated: Operating an X-ray tube at the lowest useable power extends life.
Aging can be further accelerated by other factors that increase the filament’s temperature or create localized hot spots and arcing. These include a loss of vacuum within the tube due to slow leaks, failure to perform prescribed warmup after inactivity, crazing or etching of the glass vessel and other insulating components, microcracks on the target disc, binding or jamming due to worn bearings, and damage caused by a failure to follow recommended protocols during installation and operation.
2. Application Mismatch – Ideally, the X-ray tube being used for a specific application is perfectly suited to the equipment and conditions of use. Unfortunately, this isn’t always feasible due to lack of tube availability. For example, a tube designed for high voltage may function at lower voltages and can therefore be used as a substitute if needed, but the filament will need to run at a higher current to overcome the limited ray emission. This results in increased temperatures, accelerated aging, and premature failure.
One of the best ways to minimize application mismatch is to establish a relationship with a reputable supplier who carries a wide variety of X-ray tubes and has the expertise necessary to ensure the selected tube fits both the equipment and the intended usage.
It’s also important to contact your supplier in advance regarding the typical availability of any tube(s) you may need. Some tubes, like the THA OPTITOP, are readily available due to their popularity. Others, like the THA Dura 422-MV or the Straton MX P46, may have longer lead times as a result of recent supply chain issues.
3. Improper Power Supply Setup/Operation – The power supply provides the electric current needed to operate the tube, including the filament and rotor, and also contains the logic and interlocks used by the system.
Many of the parameters of the power supply can be adjusted manually, such as frequency, rotational speed and braking, stator boost, filament boost, exposure high voltage pulse, and idle current. Each of these settings has an impact on both system performance and the operating conditions of the X-ray tube. Adjusting them improperly to improve scanner performance could potentially lead to increased temperatures, excessive vibration, and other suboptimal conditions, all of which may result in premature tube failure.
Of all these settings, one of the most critical is the filament current limit. This set point limits the maximum output energy of the power supply to protect the filament from excess current. It should be set at or below the X-ray tube manufacturer’s specification.
4. Tube Enclosure Issues – The X-ray tube is enclosed in a sealed housing to prevent X-rays from emanating in all directions, provide high voltage insulation, and allow for cooling of the tube and system. Many aspects of this housing have a direct bearing on the life expectancy of the tube it contains, but none more so than overheating of the housing. This condition can be due to oil leaks, dusty heat exchangers, improper housing attitude, and high ambient temperatures. Proper routine inspections and preventive maintenance can go a long way in minimizing and preventing these problems.
The Best Way to Reduce Accelerated Aging and Damage in Medical X-ray Tubes
Above all else, the key to X-ray tube longevity is to operate within the published ratings of the tube and always follow recommended protocols. Familiarity and planning are paramount, as are caution and care. It is very easy to mix up charts, misread specifications, make incorrect assumptions, or not pay attention to details. Always double check before making functional, mechanical, or operational changes.
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