LIFETIME MANAGEMENT SYSTEMS FOR

MEDICAL DEVICES

Specific Methods for Life Extension of Equipment and Systems in Medical Devices

Oliver Heuermann and Wolfgang Fengler

Computer Architecture Group, Technische Universität Ilmenau, Ilmenau, Germany

Keywords: Reliability, Lifetime extension, High power tubes, Klystron, Magnetron, Thyratron, Accelerator, x-Ray

tubes.

Abstract: This position paper describes research activities in the scope of targeted lifetime extension of components

which are used in medical devices and high energy physics. The considered medical areas are in the

diagnostics and the therapy field. The focus of the analysis of medical machines and systems with high-

power tubes is on the x-ray-radiation or rf-power performance. On this occasion, the operational behaviour

of such tubes is of special interest. In this paper a methodology will be presented to examine the specific

influence of service life-determining parameters. For the implementation of the methodology a discrete

event simulation is constructed using the realtime design tool MLDesigner from MLDesign Technologies,

Inc. With the help of different example tubes in the form of workable specifications the mean physical

behaviour is copied. The base specification is extended by additional functions for special types of tubes.

Therefore, studies can be carried out with regard to the tube service life in different components. The

simulation shows that the targeted specific influence on the service life-determining parameters can prolong

useful service life of a high power tube.

1 INTRODUCTION

As part of research work at the Computer

Architecture Group of the Technical University

Ilmenau (Technische-Universität-Ilmenau) the

default behavior of high-power tubes used in

medical equipment is investigated. The focus of this

research work aims to develop new business and

application models for service life extension of

equipment in medical technology. To develop

appropriate additional sensors and condition

monitoring concepts, it is especially necessary to

provide a detailed look at the life-defining

parameters. With the help of modeling a realtime

discrete event simulation, the theoretical

assumptions of the research work, meaning that by

means of a targeted control of service life-

determining the parameters, the whole useful service

life of high power tubes can be extended essentially,

will be investigated. The expected outcome of this

investigation is the consolidation of the theoretical

assumptions by means of an appropriate physical

experiment with the implementation of all required

information about the tube specific life-defining

parameters.

Functions in a medical system (eg radiotherapy

equipment, particle therapy, computed tomography,

mammography and angiography equipment) use for

diagnosis or treatment high-power tubes such as

klystrons, magnetrons, thyratrons, x-ray tubes, and

linear accelerators. The flow of diagnostic and

therapeutic applications is to be modeled and

investigated by means of a simulation. An

investigation of the relationship between the

loadprofile of a system and the service life of a tube

used in that system is possible.

Partial of models for hardware and software of

the control system as well as of the electronic and

electromechanical components are necessary.

Exemplary models of high tubes are established and

inserted into the simulation system. Partial models

are to be interchangeable for use in simulations for

different application fields.

It is necessary to establish a basic tube model for the

simulation tool MLDesigner (MLDesign

Technologies, Inc. 2007), as well as the

implementation of predefined algorithms and

301

Heuermann O. and Fengler W..

LIFETIME MANAGEMENT SYSTEMS FOR MEDICAL DEVICES - Speciﬁc Methods for Life Extension of Equipment and Systems in Medical Devices.

DOI: 10.5220/0003112803010306

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2011), pages 301-306

ISBN: 978-989-8425-37-9

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

Figure 1: MLDesigner simulation structure overview.

methods for evaluation of the tube-data (Heuermann,

2006).

The structure of the model is done in several

phases. The first priority is the development of a

basic tube, complying with a typical x-ray tube, just

the way it is used in most cases in practice. Based on

these results of the modeled tube, an optimized

design is created, in which the predetermined factors

affecting service life-determining parameters are

changed selectively. By a direct comparison of the

two models, with and without optimization, a very

accurate statement on the expected life of such a

tube is possible (Wippler, 2007, Krestel, 1988).

2 BACKGROUND

The life of vacuum tubes, used to produce radiation

(reception, screening, treatment and therapy) in the

medical technology, is determined to a large extent

by the emission of the cathode. In all type of tubes,

directly as well as indirectly heated cathodes, and

“cold” emitting cathodes, during the period of

usability a reduction of the electron emitting

material can be noticed (eg filament evaporation rate

and barium evaporation rate). Some of the service

life-determining parameters for the vacuum tubes

used in medical technology are as follows:

x-ray/carbon nano tubes:

• anode roughening, anode heat capacity,

filament evaporation rate, scan-seconds

load (load profile), temperature, timing,

arcing

They have a finite, but not in all applications reliably

predictable service life and must be replaced by the

facility to ensure availability.

RF-components:

• cathode roughening, barium evaporation

rate, beam-seconds load (load profile),

temperature, gascomposition/vacuum

quality, ion back-bombardement, timing,

arcing

They have a finite, but unpredictable service life and

must be replaced at short notice by the facility to

ensure availability (Heuermann, 2010).

In the field of x-ray tubes, there are procedures

for lifetime prediction known, e.g. used in high

resolution CT-systems. The analysis of input

vectors, taking into account disturbance vectors,

generates output vectors. These output vectors do

reliably produce predictable lifetime calculations

with high confidence. In prior art solutions mostly

the condition monitoring is restricted to the view

from the “outside” on the physical behaviour of the

tube (Heuermann, 2006, 2007).

Figure 2: TubeGuard@CT structure overview.

In the field of high power tubes, usually the

resistance of the heater coil is measured. With the

knowledge of the used materials and the dimensions,

a thermal model can be created and the cathode

surface temperature of the direct heated tungsten

filament can be calculated. The emitter deterioration

is based on sensor data of tube current and filament

current (Siemens Guardian Programm, 2007).

Figure 3: Measurement of emitter deterioration.

This procedure is the calculation of the cathode

surface, from which the evaporation rate is

dependent, but not sufficiently accurate. Many

disturbance vectors, such as tube-stray distribution,

time dependant varying parameters of the tube itself,

and different ambient temperatures of the object to

be considered, alter the thermal balance of the

system, which is used for the calculation. As a

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result, a heating scheme materializes that does not

match the actual existing surface temperature. For

example figure 4 shows a thermal investigation done

on an e-gun. The simulation was performed by the

manufacturer of the e-gun with a COSMOS/M

model. Cathode is 40°C, other points 20°C higher in

the specific tube model. This results in a shorter

predicted service life. On the other hand, if the

model does not reflect the real thermal balance the

cathode temperature could be much higher. As an

example for the given dimensions of the used e-gun

back-heating as a cause of ion-back-bombardement

(beam but no RF: 50°C, beam and RF: 110°C) adds

60°C to the cathode surface.

Figure 4: Mismatch between simulated and measured

temperatures in an e-gun assembly.

As an example of the importance of accurate

surface temperature estimation the effects in a

klystron will be explained as follows:

For a nominal surface temperature given with

890° C, production of only 50° C more temperature

on the surface results in twice as high barium

evaporation. The same is true for all types of high-

power tubes (klystron, magnetron, thyratron,

accelerator), which use barium enriched materials as

an electron emitter in the gun because of the low

work function. This released barium is deposited on

the cold spots in the tube and provides over time a

reduction of dielectric strength in the tube. The

result is a high voltage low impedance breakthrough

(so called arcing) (Heuermann, 2007, 2010).

Researchers working on that topic also published

solutions like continuously measuring the µP (micro

perveance) and keep the cathode current to 98% of

the nominal value (Wright, Oiessen, 2000). Another

solution is to implement thermo-couples in the

cathode surface structure (Noguchi, 1996).

These solutions represent the state of the art in

the field of condition monitoring for electron tubes.

The usual practice today is that tubes, depending

Figure 5: Example of evaporation rate vs. temperature.

on the type (x-ray, klystron, magnetron, linac,

thyratron), are assigned to according maintenance

contracts, which stipulate an exchange at a certain

time.

It is the top priority of the equipment

manufacturers, to avoid tube-failures of this manner

from the very beginning. However, there is no

possibility to ensure a complete avoidance of

incidents. This is why, in so called unavoidable

circumstances, one would like to have at least a big

enough lead time, to ensure the exchange can be

made before there is a downtime of a system.

2.1 Simulation

The hospital-specific diagnostic and treatment

requirements are implemented into the simulation

environment. The daily routine of a clinic is

considered in the simulation as well as a statistically

spread patient number, the load profile given by

logging files recorded over months will give all

necessary operating points.

Particularly interesting is the implementation of

the "optimization". The calculation of residual life is

based on the fact that all calculated life-critical

values are afflicted with an error reflected from

practise of about + -15%. This is due to

manufacturing tolerances of the tube and its

environmental factors. The thermal balance

calculated with the knowledge of the geometries and

materials does not show the correct value for the

surface temperature of the anode plate or the cathode

surface. The "optimization" deals with the

simulation exactly as before, but with a smaller

error: + -2%. This error is the assumed total residual

error of the measurement chain (pyrometer,

operational amplifiers, AD-converter) to measure

the surface temperature.

A solid basis for creating a discrete event

simulation, which is extended and detailed, and with

the load profiles of hospitals, enables reliable

investigations. The work has shown that

LIFETIME MANAGEMENT SYSTEMS FOR MEDICAL DEVICES - Specific Methods for Life Extension of Equipment

and Systems in Medical Devices

303

MLDesigner (MLDesign Technologies, Inc. 2007) is

the right tool for the reconstruction of the technically

physical processes within a medical facility.

During observation it soon becomes clear that

this is a classic optimization problem. It is a

balancing act between maximized service life (carry

out the exchange as late as possible), and realizing

the avoidance of potential downtime. A statement of

this quality on the life of a high-performance tube

can not be given to this day in a satisfactory manner.

The existing studies and investigations are only

estimates and approaches. The complex

relationships and calculations within such a tube are

seen analyzed and evaluated from the outside of the

tube (Wippler, 2009, Heuermann, 2006).

The underlying research work pursues a

fundamentally new approach. This means a direct

view on the processes within the tube, instead of just

estimating. This allows examining the condition of

the tube much more in detail, with the result that the

statements on the processes are significantly more

related to reality.

A simple example is the surface temperature of

the cathode. So far, the temperature is calculated

according to complex procedures. Despite all

precision and complexity of observation, the result is

still estimation. The idea of the research works

however is just to measure the surface temperature

of the cathode. Thus, it is possible to respond to

changes almost immediately. The model to be

developed will shed light on whether it is precisely

this optimization, that will be prove decisive for the

substantial extension of the economic life of a high

power tube.

The realization of this comparison is carried out

by two simulation models. The basic model

corresponds to the current usage of high power

tubes, ie without any optimizations. Based on that

first developed basic model, a model extension is

designed. This serves a direct comparison between

the basic and extended model. These extensions

include the optimizations as discussed. Thereafter,

the data of the two simulation runs can be compared.

With the optimization, a service life extension

should be observed under normal circumstances.

The simulation run for a klystron (Figure 6) and

a x-ray tube (Figure 7) shows patient count per hour,

statistically spread over one day, machine load

profile, actual condition of the gun and the anode.

The optimization option was off. Within the

optimization option two specific calculations will be

used. Once the exact cathode surface temperature

and second the gas pressure inside of the tube. Both

parameters will give the control system the most

significant service life-determining parameters. The

rate of change of µP and ion-back-bombardement

will indicate how fast the cathode is loosing

emission (Heuermann, 2007, 2009, 2010).

Figure 6: Simulation run example for a klystron.

Figure 7: Simulation run example for a x-ray tube.

2.2 Temperature Measurement

The in section 2 mentioned problem with electron

emitting cathode material and the evaporation of

cathode material in

vacuum tubes is solved by using

an integrated optical measuring equipment for

continuous cathode or anode surface temperature

measurement (in x-ray tubes the anode temperature

is of special interest too) for calculating the

evaporation rate in vacuum tubes of medical

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technology. By a direct measurement of the actual

surface temperature an effectively working control

of the heating system can be realized. Therefor a

photo semiconductor is integrated in the tube.

Temperatures from about 700° C can

measured pyrometrically with photo diode in the

visible spectral range. Pyrometers (sensor incl.

evaluation system), also called radiation

thermometers, are used for contactless temperature

measurement. Mostly the reception wavelength

range of high-temperature pyrometers is determined

by the photo detector: The lowest reception

wavelength of silicon photodiodes is, for example,

about 1.1 microns. A body with a temperature of

3000 K has its maximum radiation, but temperatures

from about 700° C can already be measured.The

surface temperatures in klystrons, magnetrons,

thyratrons and accelerators range from 890° C to

1050° C (depending on the type of cathode, ie oxide

or impregnated). The surface temperature of the

tungsten filament wire in x-ray tubes is about 2000°

To prevent deposition of evaporated electron

emitting material of the cathode on the cold surface

of the photo sensor (in klystron, magnetron,

thyratron and accelerator), a central lock (a so-called

shutter) is used.

The shutter consists of a number of

curved steel plates, which will be steered out of the

measurement beam. Such shutters are well known in

the camera technology and are available in high

volumes at low prices. The shutter protects the

sensor optics in phases, in which no measure is

taken. To measure the surface temperature, the

shutter, which is located in a vacuum, is activated

and opened electromagnetically from the outside

through a media gap (barrier of glass or ceramic

between the vacuum of the tube and the ambient

pressure). After measurement, the shutter is closed

again. The measurement itself takes place cyclically,

in time intervals still to be defined in detail. Also

usable as a shutter is a disc with an opening which

rotates when activated and releases the beam path to

the sensor.

The advantages for the use of standard integrated

optical measuring equipment for continuous cathode

or anode surface temperature measurement in

vacuum tubes are the reliable service life prediction

and the targeted life extension. With the help of the

exact measurement of surface temperature, the state

(electron emission at currently supplied heating

power) of the cathode can be detected and be used to

determine a precise heating control. The result of an

exact sequence for a heating system is the significant

extension of the life of a tube. Here the following

advantages of the invention compared to a contact

temperature measurement appear:

Very fast measurement (<1 ms to 10 ms

depending on construction). Very long, continuous

ranges possible (eg, 350 ... 3500° C), no wear (excl.

shutter mechanics), no temperature influence on the

measurement object or errors by poor thermal

contact. Possibility of measurement at high voltages,

electromagnetic fields, or corrosive materials.

The proposed novel approach allows continuing

an ongoing monitoring of the condition of the tube.

Slowly impending loss (reducing the temperature at

constant heating power supply) can be detected, an

integration of evaluation into the overall control of

the entire system allows sending of service messages

before the system fails (predictive maintenance).

With the known evaporation rate and the available

quantity of barium in the cathode from the

beginning, an arcing probability can be calculated

(Heuermann, 2009, 2010).

3 CONCLUSIONS

In the field of high-power tubes there is a large

development potential regarding service life

management and condition monitoring services to be

found.

A targeted control of the service life-determining

parameters extends the life of high-power tubes. As

proof of a life extension a simulation model is used,

which provides information about the behavior of

service life-critical parameters. Results produced by

the simulation model are transferable to reality and

can be used in a practical implementation. The

simulation shows that a targeted control of service

life-determining parameters influences the overall

lifetime of a tube. In a next step, real load profiles

recorded at customer sites will drive the tube model.

These reflect the daily routine in a hospital with the

individual patient distribution and their diagnostic

and therapy schedules and, as a result, the real tube

load. This novel approach will improve the uptime

of medical systems. First results from single x-ray-

tube systems (CT, Angiography, Fluoroscopy,

Mammography) show that in case of direct heated

cathodes the predictive maintenance works well. In

case of multiple tube systems like radiation therapy

machines, at least three high power tubes are used in

one system, the proposed specific methods for life

extension of equipment and systems in medical

devices will increase the uptime dramatically.

LIFETIME MANAGEMENT SYSTEMS FOR MEDICAL DEVICES - Specific Methods for Life Extension of Equipment

and Systems in Medical Devices

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