White Paper. Medical Imaging

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Shonda Rice
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1 White Paper Medical Imaging

2 Introduction Today, everywhere in the world, we increasingly need to rely on our healthcare systems: our life expectancy is increasing, as are the possibilities of self-treatment, therefore medical facilities need to be at the cutting edge. Patients today no longer expect hospitals to simply carry out high-level treatments ; they also want to feel good while being treated. The quality of treatment is no longer the patients only consideration, with safety being a main concern. All hospital facilities should provide safe solutions. Medical Imaging plays an important role within these facilities: Magnetic Resonance Systems, X-Ray Imaging Systems, Computed Tomography These machines are now well-known to us as they are used a great deal in the pre-diagnosis process. They must possess characteristics that fulfil the hospitals needs such as adaptability in terms of space and technical environment, flexibility in terms of loading and technological evolution, and electricity network availability to ensure the machines will work when needed. As well as considering medical treatment, it is important to acknowledge that hospitals are managed as private companies and need to be profitable. To ensure they meet their economic targets the entire network must be able to guarantee a high level of availability to avoid losing money. We must also remember that these machines are crucial to emergency diagnoses in the case of accidents, to ensure the correct treatment of patients. For example, Medical Imaging is essential to the use of Angiographic machines, which require placing a catheter inside the patient s body. Therefore, to provide good quality treatments, the electricity network must also be able to meet its service continuity targets. As the utility network is not sufficient, Medical Imaging manufacturers often rely on a system where a UPS has been added. wp 051 a This is where the difficulties arise: which UPS should be used? Our experience in this specific field has taught us how important it is to properly choose your UPS, as Medical Imaging machines are very particular from an electrical point of view. The purpose of this document is to explain these concepts and guide the user in his choice of UPS. We will focus only on on-line UPSs (VFI according to IEC standards or double conversion in common language). This is because we are dealing with machines with a high sensibility to spurious signals. A UPS functioning in line-interactive mode (or, even worse, in off-line mode), where the load is not fully protected and where some distortions can go through the UPS, amounts to not using the machine at full potential. Finally, the term UPS refers here to the relative standard, IEC series, while IEC is used to refer to medical electrical equipment (the UPS is part of the electro-medical system). Glossary XR MRI MI PWM THD MEE X-Ray Imaging Systems Magnetic Resonance Systems Medical Imaging Pulse Width Modulation Total Harmonic Distortion Medical Electrical Equipment 2 White paper : Medical Imaging - SOCOMEC

3 Contents Introduction... 2 Glossary... 2 The electrical behaviour of MI... 4 High di / dt... 4 Asymmetrical current... 5 High harmonics... 6 Cooling system supply... 6 Consequences on the electrical installation... 7 High di / dt... 7 Asymmetrical current... 7 High harmonics... 8 Cooling system supply... 8 How to address these issues... 8 High di / dt... 8 Asymmetrical current... 9 High harmonics... 9 Cooling system supply... 9 On-site activities Conclusion... 10

The electrical behaviour of MI MI using MRI, X-Ray or Computed Tomography usually requires a machine which creates a lot of distortions when it is operating, and with an apparent nominal power of 20

4 The electrical behaviour of MI MI using MRI, X-Ray or Computed Tomography usually requires a machine which creates a lot of distortions when it is operating, and with an apparent nominal power of 20 to 100 kva. As the machine starts to operate, the nominal current is in line with the nominal power indicated above, with no pollution of the electrical signal due to the MI. This ideal state does not last long as the machine will move on to a specific electrical behaviour when it starts doing the analysis. We will study this by focusing on the following 4 main aspects: very high di / dt (very high current peaks in a very short time) with a pulse phenomenon, asymmetrical currents (with a crest factor much higher than 2), high harmonics, how to handle the cooling system (chiller) supply. High di / dt When carrying out an analysis, the MI machine indicates very high di / dt behaviour with peaks that can be 8 to 12 times the nominal current during 2 to 10 ms: wp 019 a The direct start of an electrical motor usually produces currents that are 5 to 7 times higher than the nominal current. Here, the impact is bigger. The power cables and all other electrical installations must therefore be sized so as to bear these very high current peaks: High di / dt wp 020 a wp 019 a With only one peak, as is the case for the direct start of an electrical motor, the situation is not as serious. Here, however, we have pulse trains: a high peak during a few ms, followed by nothing, then a high peak again and so on. 4 White paper : Medical Imaging - SOCOMEC

wp 021 a This continuous cycling is damaging as it repeatedly stresses the UPS. Robustness is therefore a key characteristic required to offset this phenomenon.

vicinity. Asymmetrical current Alternating current usually indicates symmetrical behaviour, but not in this case. Here, the current does not only have peaks, it is also asymmetrical.

5 wp 021 a This continuous cycling is damaging as it repeatedly stresses the UPS. Robustness is therefore a key characteristic required to offset this phenomenon. Furthermore, it is important to be aware that these current peaks can cause voltage drops to values out of the working range of the MI machine, as well as for machines in the vicinity. Asymmetrical current Alternating current usually indicates symmetrical behaviour, but not in this case. Here, the current does not only have peaks, it is also asymmetrical. As is shown above, there is no symmetrical sine-wave with a 2 crest factor, and furthermore the current is not the same in each of the three phases: wp 023 a wp 022 a 5 White paper : Medical Imaging - SOCOMEC

wp 024 a By studying the signal, we can observe that the MI machine is an atypical load. Cooling system supply For some operations, the cooling system is a critical element of the MI system.

6 High harmonics To further complicate the situation, MI machines are considered to be nonlinear loads. The current signal below clearly shows that the current THD increases when the machine is taking the photograph. wp 024 a By studying the signal, we can observe that the MI machine is an atypical load. Cooling system supply For some operations, the cooling system is a critical element of the MI system. The cooling liquid and other components of the system need to be kept at a specific controlled temperature to ensure a perfect image quality and to avoid causing damage to them. These components are expensive and not easy to repair or replace, therefore the users often choose to supply the cooling system through a UPS. This means that the extra load must also be taken into consideration: wp 026 a wp 025 a 6 White paper : Medical Imaging - SOCOMEC

Consequences on the electrical installation High di / dt Electrical equipment does not respond well to current peaks.

for this type of application. The high risk of damage to and dysfunction of the electrical equipment leads to its oversizing and generates a loss of efficiency.

In this case, the UPS automatically transfers to the bypass circuit: the load drops onto the mains input feed and the expensive machine is no longer protected by a double conversion (see the

7 Consequences on the electrical installation High di / dt Electrical equipment does not respond well to current peaks. It leads to stress on the transformers, the cables, the panels It also leads to an increase in the risk of nuisance (or spurious) tripping and therefore the loss of continuity, an important element for this type of application. The high risk of damage to and dysfunction of the electrical equipment leads to its oversizing and generates a loss of efficiency. A high di / dt can be interpreted as a short circuit event by an upstream UPS. In this case, the UPS automatically transfers to the bypass circuit: the load drops onto the mains input feed and the expensive machine is no longer protected by a double conversion (see the explanation in the introduction). Mains Mains Q2 Q2 Mains UPS Aux Mains Mains UPS Aux Mains Q4 Q4 wp 027 a Furthermore, if the current is too high (even for a short period of time), the inverter (UPS output) may not be able to handle all the current required by the load and will transfer onto the bypass. As mentioned above, if the UPS is on bypass the load no longer has a double conversion protection and some distortions from the grid may reach the MI machine. Conversely, the machine itself can cause distortions to other loads around it because the filter set up by the double protection is no longer in effect. Asymmetrical current Electrical equipment is designed to operate using symmetrical signals (voltage and current) within a tolerance range. This range is not infinite ; if the asymmetry is too important some devices may not work. This can be problematic if the UPS being used has been built to function with a crest factor of between 3 and 4 as opposed to the typical crest factor of 2. Moreover, if the machine is installed within a high impedance network, high asymmetrical currents can create asymmetrical voltage. The distortions can then travel from the machine upstream. High impedance network e (t) Line wp 028 a Load 7 White paper : Medical Imaging - SOCOMEC

8 High harmonics On high impedance networks, high current harmonics create voltage harmonics which lead to issues of power quality. Some devices cannot handle the flow of current with a high harmonic content. Most new generation devices have been designed to be cost effective and are therefore less robust against network pollution. This can lead to the common problem of harmonics on electrical installations (transformers, cables, thermal effect ). Cooling system supply As mentioned above, if the user chooses to have the cooling system supplied through a UPS, the additional power supply required must be taken into account. However, this is not sufficient because, as a mechanical machine, the cooling system will most likely create a high starting current. This will add further stress on both the mains and the UPS supply. How to address these issues High di / dt In order to avoid the transfer onto bypass, the UPS must be designed to sustain the high transient currents with the use of a stronger inverter and rectifier. If not, the entire load will be fed directly by the upstream network (via the UPS bypass). This means that the internal components must be oversized compared to standard applications. The inverter must be able to supply high current peaks with no damage to its internal components, which are usually IGBTs. The rectifier must be built with an adapted curve of consumption (the use of a normal rectifier during a peak would lead to an overcurrent in the upstream line). In addition, the UPS firmware must be adapted to uphold the periods of high current without transferring onto bypass. For UPS designed for an MI application, Socomec uses a 3-level technology to allow for a more powerful inverter: E + (LEVEL 1) + Vdc 2 E zzz zzz E - (LEVEL 3) 2 0 (LEVEL 2) N N _ Switch + Switch V out - Switch N - Vdc wp 029 a This is the case for the Green Power 2.0 range. The battery system must also be designed and sized accordingly, as the batteries take over to provide the necessary power and energy to complete the imaging cycle in the case of a line interruption. 8 White paper : Medical Imaging - SOCOMEC

9 Asymmetrical current The components of the inverter must be designed so as to continue supplying a load even if the crest factor exceeds 4. The UPS manufacturer must provide a system capable of enduring the higher temperatures caused by the asymmetrical behaviour. Due to the lack of a perfect sine wave, the PWM characteristics must be adjusted to cope with what the MI will absorb. In addition to the 3-level technology, Socomec uses a specific PWM control system that constantly adapts to the current required and ensures a low output impedance to address these issues: MI manufacturers usually require a maximum impedance of 100 mω for the supply network. Socomec s UPS use 10 to 20 mω (using specific cables based on the load value). The impedance is therefore 5 to 10 times lower than the required maximum, which allows for flexibility for any future modification of the installation (for example if the MI machine is moved to another room). Because we are dealing with a double conversion, the asymmetrical behaviour remains downstream of the inverter. Thus, the UPS cleans the distortions and the network never experiences them. wp 031 a wp 030 a High harmonics To avoid accelerated ageing, a derating factor should be adopted if a normal UPS is used with a high THDI load. The inverter s components must therefore be more robust to feed the non-linear load. The IGBTs are chosen carefully because they will be faced with a lot of harmonic current, which can damage the electric supply. The double conversion UPS also acts as a filter for the network from a harmonic point of view. Cooling system supply Damage to the cooling system leads to important economic consequences because the liquid used to cool the MI system is expensive, and there can be a loss of turnover while the system is non-operational. It is therefore highly advised to use a UPS to supply the cooling system, to avoid any network disruption in the case of a black-out. The UPS must supply additional current and power to handle the current peaks of the cooling system (although they are not as high as those of the MI machine). 9 White paper : Medical Imaging - SOCOMEC

10 On-site activities Choosing a UPS using the criteria above is only 50 % of the task. On-site activities are crucial to making sure the chosen UPS is appropriate and will remain fit to function. The first of these activities is a measurement campaign to have a good understanding of the distortion levels and the entities of specific operations. This is to avoid any unpleasant surprises when the UPS is first turned on. Once the UPS is put in position and connected (both power and signal cables), an expert technician adjusts the settings of the machine. These machines are very specific, it is therefore necessary to examine all the data to adapt the settings such as voltage limits or the handling of overloads. The expert technician can then advise on which options are needed for the application, to avoid any costly but unnecessary extras. Finally, the UPS must be properly maintained to ensure that it will remain functional at the level it is required to after several months or years. Maintenance is necessary to monitor the lifespan of the UPS, and it must be carried out by an expert who knows precisely how to handle the equipment. A maintenance contract with the UPS manufacturer is therefore highly recommended to make sure any problems are fixed as soon as they arise and to ensure the longevity of the UPS. Conclusion Medical Imaging machines are too important and their installation is too specific to be treated as standard, particularly if a UPS is added to the general system. Their electrical behaviour requires a dedicated range of UPS. Socomec s on-site experience as a manufacturer, with specialised knowledge regarding standards and production quality, allows it to understand these characteristics well. It has developed a specific UPS range along with accompanying maintenance programmes and training for the technicians who use this equipment daily. 10 White paper : Medical Imaging - SOCOMEC

11 Socomec worldwide IN EURoPE IN ASIA PACIfIC IN MIddlE EAST BELGIUM Tel Fax info.be@socomec.com FRANCE Tel Fax dcm.ups.fr@socomec.com GERMANy Tel Fax info.ups.de@socomec.com Power Control & Safety / Tel Fax info.scp.de@socomec.com ITALy Tel Fax info.ups.it@socomec.com Power Control & Safety / Tel Fax info.scp.it@socomec.com NETHERLANDS Tel Fax info.nl@socomec.com POLAND Tel Fax info.ups.pl@socomec.com Power Control & Safety / Tel Fax info.scp.pl@socomec.com head office SOCOMEC GROUP SAS SOCOMEC capital R.C.S. Strasbourg B B.P , rue de Westhouse F Benfeld Cedex - FRANCE Tel Fax info.scp.isd@socomec.com PORTUGAL Tel Fax info.ups.pt@socomec.com ROMANIA Tel Fax info.ro@socomec.com SERBIA Tel Fax info.rs@socomec.com SLOvENIA Tel Fax info.si@socomec.com SPAIN Tel Fax info.es@socomec.com SwITZERLAND Tel Fax info@socomec.ch TURKEy Tel Fax info.tr@socomec.com UNITED KINGDOM Tel Fax info.uk@socomec.com Power Control & Safety / Tel Fax info.uk@socomec.com AUSTRALIA / Power Control & Safety Tel Fax info.ups.au@socomec.com CHINA Tel Fax info.cn@socomec.com INDIA Tel Fax & 51 info.in@socomec.com SINGAPORE Tel Fax info.sg@socomec.com THAILAND Tel Fax info.ups.th@socomec.com YoUR distributor / PARTNER UNITED ARAB EMIRATES Tel Fax info.ae@socomec.com IN AMERICA USA, CANADA & MEXICO Power Control & Safety / Tel Fax info.us@socomec.com other CoUNTRIES NORTH AFRICA Algeria / Morocco / Tunisia info.naf@socomec.com AFRICA Other countries info.africa@socomec.com SOUTH EUROPE Cyprus / Greece / Israel / Malta info.se@socomec.com SOUTH AMERICA Tel info.es@socomec.com MORE DETAILS Non contractual document. 2016, Socomec SAS. All rights reserved. - document printed on paper from sustainably managed forests. DOT / 17 - Photo: Martin Bernhart - Réalisation: Socomec

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