A Thinker`s Guide for Electrical Leakage Testing of Ultrasound Probes
Contents
1. A Thinker s Guide for Electrical Leakage Testing of Ultrasound Probes James Gessert B Sc BSEE G Wayne Moore B Sc MA FASE Acertara Acoustic Laboratories Correspondence G Wayne Moore 1860 Lefthand Circle Suite H Longmont CO 80501 USA E mail gwmoore acertaralabs com www acertaralabs com PREFACE This text is designed to provide the reader with sufficient knowledge of the principles of electrical leakage testing as it relates to diagnostic ultrasound probes to serve as a basis for understanding the why and how such testing is required and performed in the clinical setting A secondary objective is to provide sufficient references from various stake holders in this testing e g regulatory standards clinical manufacturing etc to enable you to read and understand contextually the various requirements methodologies and vernacular associated with the text in these references This White Paper is structured to help the reader learn Basic principles are mentioned early in the text and then are repeated and expanded upon and integrated into later sections Our research for this paper revealed a good deal of confusion and open for interpretation requirements within the various stake holder documents We strongly urge the reader to look carefully at ultrasound user manuals for specific instructions related to probe electrical leakage testing and ensure those instructions are compatible with standards and re2. testing of ultrasound probes and to provide the reader with additional informative data that will provide further context around the why s and how s of this testing a Thinker s Guide Note The measurement of insulation resistance shall be considered in addition to the leakage current measurement if there is any doubt about the insulation of the equipment IEC 62353 Page 23 5 3 3 1 This test is an in addition to electrical leakage not a substitute for electrical leakage testing 29 Page REFERENCES 1 2 3 4 5 6 T 8 9 IEC 60601 1 IEC 62353 2014 IEC 60601 2 37 IEC 60529 Intersocietal Accreditation Commission IAC Guideline Document 7 15 14 Section 2 2 3B Fluke ULT800 User s Manual BC Group ULT 2000 User s Manual GE Medical Systems TEE Probes User s Manual Philips Medical Systems Transducer and System Care and Cleaning Dec 2013 10 Siemens Acuson SC2000 User s Manual 11 Sonosite TEE User s Manual 12 Toshiba PET512 MC TEE Safety Kit User s Manual 13 American Society of Echocardiography 14 US FDA Guidance Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems and Transducers 9 9 08 Center for Devices and Radiological Health 30 Page ABOUT ACERTARA ACOUSTIC LABORATORIES AND THE AUTHORS ABOUT ACERTARA Founded by ultrasound industry pioneers G Wayne Moore and Jim Gessert Acertara was created to serve the
3. and to set the array center operating frequency The distribution of PZ and polymer provides a specific pattern of connectivity and coupling Dicing Slots Diced Filled with PZ Material Polymer amp Lapped Ready for To PZ Material Polymer Spine is Spine Holds Lapped to The Polymer Fill oe y Together Connectivity refers to the continuity of each material along each of the three axes Specific dicing and filling can produce an array that looks like a PZ polymer sandwich common to many current linear convex and phased arrays or an array of PZ elements where each is surrounded by polymer found in 1 5D and 2D Matrix arrays Coupling refers to the ability to transfer ultrasound out of an array and into the tissues and the reverse when echoes return Acoustic coupling improves when quarter wave layers of material with intermediate acoustic impedance values are bonded to the array face The lens attached to the array is the path to the patient If the lens is compromised an electrical pathway can be established from the probe into the patient A sample of this is shown on Page 28 Photos 1 amp 2 9 Page Overall Probe Design Considerations Signal to noise performance is essential from the design of the probe as the probe must be relatively noise immune This is required because modern composite arrays are very broadband relative to frequency and highly sensitive Additionally modern ultrasound systems have front end electronic de
4. context for understanding why the various probe testing devices including electrical leakage testers are designed the way they are and provide the kind of information they do Currently available real time diagnostic ultrasound systems and the transducers associated with them can be divided into three main categories 1 linear sequenced arrays e g linear and curved linear 2 phased arrays e g 1D 1 5D and 2D matrix and 3 mechanical sector normally a single element probe that is mechanically steered The most commonly used probes today are from categories 1 and 2 To establish common terms for the various parts of a probe please see the diagrams on Page 8 A modern electronic array ultrasound probe normally consists of the following major components 1 An acoustic stack comprised of a Lens b Matching layers c Array d Backing material 2 An ergonomically designed housing matching the form fit and intended function of the probe 3 Cable termination within the probe housing see Photo on Page 10 4 Acable with multiple co axial wires connecting the probe housing to the probe connector 5 A probe connector normally having one or more of the following inside the connector a Multiplexer circuit b Tuning circuit c Cable Termination board In addition to the above a number of modern ultrasound probes have dedicated electronics integrated into the probe housing as part of the acoustic stack for example the X7 2t TEE probe b
5. expected service life is X years if the specified maintenance and inspection procedures are performed Single Fault Condition condition of ME EQUIPMENT in which a single means for reducing a risk is defective or a single abnormal condition is present Standard A standard is a document that provides requirements specifications guidelines or characteristics that can be used consistently to ensure that materials products processes and services are fit for their purpose Supplemental Insulation Independent insulation applied in addition to basic insulation in order to provide protection against electrical shock in the event of a failure of basic insulation TEE Transesophageal echocardiography an ultrasound examination where the probe is introduced into the patient s esophagus Ventricular Fibrillation a heart rhythm problem that occurs when the heart beats with rapid erratic electrical impulses This causes pumping chambers in the heart the ventricles to quiver uselessly instead of pumping blood During ventricular fibrillation blood pressure plummets cutting off blood supply to vital organs and can be a fatal occurrence 4 Page VARIOUS PROBE WARNING LABELS FROM ULTRASOUND OEM USER MANUALS gt gt gt P PPE Pe All intraoperative studies must be performed with a Type CF classified transducer All TEE transducers are designed to be defibrillator safe in accordance with IEC 60601 1 Nevertheless always remove the transduce
6. from l Normal Single Fault Condition MA Condition mA ype B Type BF ts iy ofl i or Type CF 14 Page PROBE TYPES Surface Probes Endocavitary Probes TEE Probes Intraoperative Probes C5 1 etc E8C etc 6Tc etc VF13 5SP etc Although IEC 60601 1 doesn t stipulate which classification is to be used for various ultrasound probe model types IEC 60601 2 37 does IEC 60601 2 37 also lists other probe related limits such as probe aperture surface temperature and acoustic power reporting Surface probes normally carry a BF classification while intraoperative probes that come into direct contact with the heart must have a CF classification TEE probes may carry either a BF or BF Defibrillation proof classification The defibrillation proof applied part e g TEE probe marking indicates that an applied part can safely remain attached to a patient who is being defibrillated without any adverse effect on the subsequent use of the TEE probe by way of example BF Isolated from ground Maximum patient leakage current under e Normal condition lt 100uA e Single fault condition lt 500uA Although the ultrasound probes carry BF and CF marks the ultrasound system itself is considered and labeled as a B mark device This mark is the human form only icon as shown in the Table on the previous page NOTE BENE It is noted here that TEE probes can have either a BF designation or BF Defibrillation design
7. hand held battery operated device the potential for electrical shock from a damaged probe is called out Electrical Hazard A damaged probe may increase the risk of electric shock if conductive solutions come in contact with internal live pads Inspect the probe often for cracks or openings in the housing and holes in and around the acoustic lens or other damage that could allow moisture to enter Become familiar with the probe s care precautions outlined in Vscan maintenance on page 6 7 GE V Scan Op Man Even direct wired probes being used on a battery operated hand held system must be tested for electrical leakage Type BF Applied part TYPE BF APPLIED PART providing a specified degree of protection against electric shock with particular regard to allowable LEAKAGE CURRENT Patient leakage current lt 100 microA lt 500 microA 21 Page WHY MEASURE ELECTRICAL LEAKAGE CURRENT cont Ultrasound Laboratory Accreditation Requirements The purpose of IAC accreditation is to ensure high quality patient care and to promote health care by providing a mechanism to encourage and recognize the provision of quality imaging diagnostic evaluations by a process of accreditation Through the accreditation process facilities assess every aspect of daily operation and its impact on the quality of health care provided to patients While completing the accreditation application facilities often identify and correct potential probl
8. issues can also be caused by physical trauma and tears to the probe Ps Cable Tear cable specifically the co axial wires within the cable or to the ground shielding itself see Photo to right 10 Page VISUAL AND TACTILE INSPECTION OF PROBES IN SERVICE Because probes are fragile and used in sometimes difficult environments all ultrasound probes should be thoroughly inspected prior to each use to look for any cracks tears holes or any other structural damage In each OEM probe user manual we reviewed this visual inspection was stressed in its importance Carefully inspect the connector cable and the transducer housing Special attention should be paid when inspecting TEE and Intraoperative probes For detailed information on probe care please request our White paper entitled Common Ultrasound Probe Failures As a general guideline from all manufacturers the following is a minimum probe inspection recommendation An inspection should be both visual and tactual Acertara also recommends using a 10x Illuminated Loupe to closely inspect the insertion tube sleeve for any small holes see Photo below It should be noted that even when using a 10x loupe there may be fluid pathways allowing ingress too small to detect by visual acuity as in the example below again underscoring the importance of electrical leakage testing NOTE If an obvious hole is detected in the protective covering do not perform a leakage test as fluid ingress may occur
9. longer functioning as a Class BF device and would fail a leakage current test The patient would then be at ground potential and leakage currents from other devices within the patient s environment could travel through the patient compromising the safety of both the patient and the operator One leakage testing device User s Manual also states that n addition to verifying that the ultrasound transducers are safe for patient use the test makes it possible to reduce expensive repairs Identifying transducers that exceed safe leakage currents early may allow for repairs to be made before a transducer becomes non repairable Although a TEE probe does not actually come into direct contact with the heart it is only separated from the heart by a very thin membrane As it takes very little current to excite the muscle of the heart and potentially cause ventricular fibrillation an obvious life threatening condition it is vital that TEE probes are tested for electrical leakage testing prior to every use TEE probes are not normally covered during use with a protective sheath as other probes are Non TEE probes used in surgical applications are normally covered in a latex free intraoperative cover similar to the one shown below Endocavitary probes like the one shown below are also almost always covered during use but that does not mean they should not be tested for electrical leakage as a tear or puncture in the sheath would provide a conductivity
10. Anterior Posterior American Society of Echocardiography Body Body Floating Bending Neck Sheath Celsius European Conformity Conformit Europ ene Cardiac Floating Canadian Standards Association Degree s Direct Current Device Under Test European Community Electromagnetic Compatibility Fahrenheit United States Food and Drug Administration Full Scale Hertz cycle per second Intersocietal Accreditation Commission International Electrotechnical Commission International Organization for Standards Organisation internationale de normalisation Insertion tube e g TEE probe kilogram s microamp s milliampere s Medical Electrical Medical Imaging Technology Alliance millimeter s National Electrical Manufacturers Association Ohm Root Mean Square second s Transesophageal Echocardiography Underwriters Laboratory Ultrasound Leakage Tester Volt s Volts Alternating Current Volts Direct Current GENERAL DISCUSSION ABOUT ULTRASOUND PROBE COMPONENTS AND CONSTRUCTION This section contains a very high level discussion on ultrasound probes For the reader who would like to delve in deeper in understanding this technology additional detailed information can be obtained by contacting us at Acertara and requesting our published paper entitled The Silent Revolution Catching Up With the Contemporary Composite Transducer Understanding the basic construction of an array can yield insight into and provide
11. IEC 60601 1 that limit the effects of electrical current on a patient user or other individuals in accordance with this standard Electrical Leakage Current The total current flowing from MAINS PARTS to earth via a the PROTECTIVE EARTH CONDUCTOR and ACCESSIBLE CONDUCTIVE PARTS of the enclosure and APPLIED PARTS differential and alternative method or b the ACCESSIBLE CONDUCTIVE PARTS of the enclosure and APPLIED PARTS direct method Enclosure Leakage Current Total current flowing from the enclosure and all accessible parts excluding applied parts through an external conductive connection other than the protective ground conductor to ground or another part of the enclosure Ground Leakage Current Current flowing from all mains parts through or across the insulation into the protective ground conductor of the grounded power cord IEC The International Electrotechnical Commission is a non profit non governmental international standards organization that prepares and publishes International Standards for all electrical electronic and related technologies collectively known as electrotechnology The various published Standards use the format of IEC XXXXX X where X a number for example IEC 60601 1 Leakage Test Tests designed to simulate a human body coming in contact with different parts of the equipment The measured leakage current values are compared with acceptable limits Maintenance Involves the combination of all technica
12. acoustic testing and quality control product development needs of ultrasound engineering and service professionals worldwide Acertara is an independent ISO IEC 17025 2005 accredited acoustic measurement testing and calibration laboratory providing advanced ultrasound testing products inventors of the probe tester known as FirstCall such as Aureon and Active Z Acertara also provides IS09001 2008 1S013485 2003 certified probe repairs developed and refined by over 15 years of experience in the legacy of Sonora Medical Systems founded by G Wayne Moore The Acertara team has authored and co authored more than 40 United States patents and is highly published in both clinical and engineering journals As an active member of the Medical Imaging Technology Alliance MITA NEMA IEEE and AdvaMed Acertara and its employees are intimately involved in the development of various domestic and international diagnostic ultrasound regulatory and compliance standards G Wayne Moore B Sc MBA FASE President and CEO A 30 year veteran of the diagnostic ultrasound market Wayne has held senior level positions with several major medical equipment manufacturers including Honeywell Medical Systems and Siemens Medical Solutions Wayne has been directly involved in the development and commercialization of more than 15 technologically intensive ultrasound systems He is widely published in diagnostic ultrasound literature a sought after speaker at medical imag
13. andards In order for any test to be widely accepted standards regarding that test as well as guidance on how to perform the test must be provided to ensure the test is done properly and the subsequent results informative and repeatable This type of structure will ensure that each ultrasound laboratory or HTM professional conducting the testing will arrive at the same result as every other laboratory Electrical leakage testing is intended to test exactly what the name implies electrical leakage This then is a measure of current expressed in Amperes or in the case of ultrasound probes micro amperes What it is not is a simple pass fail test using a red and green LED as this does not inform the user of any trending from test to test nor does it tell the user how close to being over the limit it actually is It is also not an Ohmic measurement designed to detect a fluid ingress into a probe e g insertion tube insulation integrity please see note below that IEC 62353 does not specify a limit value for this test which although a useful measurement does not address the IEC standards please note this is not a dielectric test of the insulating material of the insertion tube What electrical leakage testing is a test to ensure that patient leakage current is less than or equal to what the IEC standards prescribe expressed in micro amps The intent and goal of this paper was to shine a light on the current state of the test for electrical leakage
14. ase for any particular system or probe the HTM is responsible for we encourage you to contact the respective OEM for clarification Difference between Manufacturing Testing and In service Testing IEC 62353 2014 in service testing requirements apply to a tests prior to placing the medical device into service b recurrent testing and c tests to be performed after a repair is performed on the device IEC 60601 1 manufacturing testing requirements applies to a design of the device b type testing c testing of each manufactured device 19 Page WHY MEASURE ELECTRICAL LEAKAGE CURRENT cont Patient and User Safety Leakage current is measured to ensure that direct contact with the medical equipment is highly unlikely to result in electrical shock The tests are designed to simulate a human body coming in contact with different applied parts of the equipment The measured leakage current values are compared with acceptable limits These limits are based on the type of applied part being tested the point of contact with the applied part i e earth enclosure patient and the operation of the product under both normal and single fault conditions As a function of repeated use a TEE probe can develop punctures or cracks somewhere along the insertion tube These conditions could likely expose the patient s esophagus to chassis leakage current and cause the patient to be at the chassis ground potential If this occurs then the probe is no
15. ate Leakage Fail lt SuA With regard to performing electrical leakage testing on normal surface probe transducers e g trans abdominal curved array probes or vascular linear array or trans thoracic phased array probes not all manufacturers provide guidance on how or if this testing should be done Our recommendation is that all probes should always be tested if they are dropped or banged or if there is a suspected compromise to the cable or transducer housing or the acoustic lens prior to the next use A Note on How the Grounding Works in all Ultrasound Probes Generally all ultrasound probes are completely surrounded by an outer shield that is connected to earth ground with care taken that the shield carries no operational current 26 Page MANUFACTURER S RECOMMENDATIONS FOR TEE TESTING FREQUENCY General Electric t is strongly recommended that before the probe is used again conduct a current leakage test to ensure the electrical safety of the probe and the patient Philips f the outer layer of the shaft is punctured or cracked a patient s esophagus could be exposed to chassis leakage current Carefully feel the tip and shaft and inspect the entire transducer If you suspect an electrical problem follow the electrical safety check procedure described Siemens Perform a leakage current test on the transducer at the end of each transducer high level disinfection procedure Retest prior to use if
16. ation and caution must be exercised by the clinical user to insure the use of the proper TEE designation for the clinical application intended TEE Probe with Defibrillation Proof Label TEE Probe without Defibrillator Proof Label 15 Page PROBE TYPES cont Not all probes labeled with the BF Defibrillation mark are intended to be used near the heart as the photo of the Philips C10 3v transvaginal probe shown to the right attests However not all transvaginal or endocavitary are marked in that way as shown by the General Electric model IC5 9 D shown below The Philips Laparoscopic probe Lap L9 5 shown below is CF marked with the icon of the heart inside the square Some non intraoperative probes such as the Philips X5 1 shown below also carry the CF label Philips Model L9 5 Philips Model X5 1 NOTE If the specified IPX classification is applicable for only part of the TRANSDUCER ASSEMBLY the marking of the IPX code on the TRANSDUCER ASSEMBLY is not required 16 Page WHY MEASURE ELECTRICAL LEAKAGE CURRENT ON ULTRASOUND PROBES SPECIFICALLY 1 Patient and User Safety 2 To Comply with Standards 3 Ultrasound Laboratory Accreditation 4 Use Changes the Protective Layers of a Probe 5 Catch Failures Early in the Process Save Money on Repairs What is electrical leakage current and what specifically is the potential hazard The Hazard It is generally understood that standing in a puddle of water and stickin
17. con or the heart icon in a box is explained on Page 14 NOTE f the specified IPX classification is applicable for only part of the TRANSDUCER ASSEMBLY the marking of the IPX code on the TRANSDUCER ASSEMBLY is not required Degree of protection against water Second Characteristic Numeral IPx 1 The enclosure is exposed to dripping water with a flow rate of 1 mm min for ten minutes Acceptance for Class IPx 1 requires no water to have penetrated into the item in such a quantity or in such a position as to prevent satisfactory operation of the item or to present a safety risk Protection against ingress of solid foreign objects IPx 7 First Characteristic As seen on Page 13 an example of this indicator is seen on the Toshiba probe The number 7 in Numera n Norrequireg 0 Non protected the label IPx 7 means that ingress of water in a 1 gt 50mm diameter quantities causing harmful effects shall be 5 Scene diaiieter prevented when the enclosure is immersed in 1 3 sjem diaineter meter of water for 30 minutes This applies to the 4 gt 1 0 mm diameter cable and probe housing only not to the probe 5 Dust protected connector The x means that there is no 6 Dust tight requirement for protection of ingress of solid foreign objects IPx 8 Second Protection against ingress of This label indicates the probe s degree of Characteristic water with harmful effects protection against ingress of fluid The test is Numeral X Not
18. de trending information relative to the electrical leakage levels over a recurrent testing period recommended in IEC 62353 The low end of the current reading is to ensure there is an adequate circuit formed and that current is indeed flowing While there is no standard on the low end of the leakage current range we recommend at least 5uA 23 Page COMPONENTS USED FOR ELECTRICAL LEAKAGE TESTING Solution Strength Although there is a standard for solution strength relative to probe electrical leakage testing the amount may vary from OEM to OEM For example the SonoSite User Manual recommends 50g NaCl liter of H20 the Philips Manual recommends 9g NaCl liter of H20 and the Toshiba Manual requires a physiological saline solution of 0 9 which is the amount called out in the IEC 60601 1 standards document Given that manufacturers must comply with the IEC standard it seems reasonable to use the solution strength called out in the standard for consistency of testing across all probes being tested If a particular OEM recommends something other than the standard solution strength we would recommend you contact the OEM and find out why they vary from the standard Once a particular solution strength is decided upon it should be used for all recurrent testing and documented Some manufacturers of electrical leakage test devices also supply a dual element probe see below that is connected to the test device that allows the operator to test the co
19. e tested Passed or Failed must be documented in the routine TEE probe cleaning maintenance log along with action taken if Failed Concerning frequency of testing the IEC 60601 1 standard states that all transducers that are designated Type BF must be checked once a year to ensure that they still comply with the requirements of this standard Ultrasound imaging is the most frequently applied imaging method in medical diagnostics These probes are often harshly treated either through use or through disinfecting cleaning and storage Therefore daily not yearly inspection is highly recommended and in some cases required 22 Page MEASURING DEVICES BACKGROUND AND TEST SET UP The purpose of any measurement is to provide information about a quantity of interest With regard to electrical leakage testing of ultrasound probes that quantity of interest is the amount of current conducted through the probe expressed in pA No measurement is exact When a quantity is measured the outcome depends on the measuring system the measurement procedure the skill of the operator the environment and other effects Even if the quantity were to be measured several times in the same way and in the same circumstances a different measured value would in general be obtained each time assuming the measuring system has sufficient resolution to distinguish between the values The accuracy of a measurement system is the degree of closeness of measur
20. edical Electrical Equipment Recurrent Test and Test after Repair Says a tests prior to placing the medical device into service b recurrent testing and c tests to be performed after a repair is performed on the device 27 Page SAMPLES OF PROBE ELECTRICAL LEAKAGE TESTING FAILURES CAUSES Photo 2 Hole in Lens Photo 3 Hole in Insertion Tube Sleeve Photo 4 Fluid Ingress and the Damage Done Photo 5 Insertion Tube Sleeve Damage Photo 6 Using a bite guard on a consistent basis eliminates many of the TEE issues 28 Page CONCLUSION Because the transducer is the most sensitive and most often damaged link in the ultrasound image quality chain and because it is always in contact with the patient while being used great care must be taken ona disciplined and consistent basis to ensure its proper performance from both an imaging perspective as well as an electrical safety perspective The push by the Intersocietal Accreditation Commission to mandate electrical leakage testing for Transesophageal echocardiography probes prior to every use underscores the growing awareness of the risks to patients associated with using these devices As we researched for this White Paper we discovered a lack of consistency in various ultrasound User Manuals and test device User manuals relative to electrical leakage testing processes tools and even common language describing how to perform a test and how to interpret results with respect to the IEC St
21. ements of a quantity to that quantity s actual true value The precision of a measurement system related to reproducibility and repeatability is the degree to which repeated measurements under unchanged conditions show the same results The Probability Density shown on the Y axis in the diagram below is a statistical measure that defines a probability distribution for a random variable and is often denoted as f x When the PDF function is graphically portrayed the area under the graph will indicate the interval under which the variable will fall Actual Value Probability Density Accuracy Precision Value Measuring electrical leakage testing will require using various components including a current tester These testers have minimum and maximum values that they can read These minimum and maximum values are also specified with a range of error usually specified against full scale expressed as x It is important to know the ranges the meter you are using specifies and the percentage of error they may have as it can have an impact on a Pass Fail determination It is also important to know exactly what the Pass Fail criteria for each manufacturer are as they will vary we have seen this multiple times especially with the low limit We DO NOT recommend using meters with simple Pass Fail light without an actual leakage and or conductivity current value being presented and retained for testing records as these hard numbers can provi
22. ems revise protocols and validate Quality Improvement QI Programs Because accreditation is renewed every three years a long term commitment to quality and self assessment is developed and maintained Facilities may use IAC accreditation as the foundation to create and achieve realistic quality care goals Health care organizations are held to very high levels of accountability by peers and by the general public In numerous states reimbursement directives that require accreditation of the facility have been instituted by Medicare carriers as well as private third party insurers Similar draft payment policies are pending throughout the United States Facilities attaining accreditation before it is required for reimbursement demonstrate a willingness to surpass current expectations The general public and members of the imaging community will recognize an unmatched commitment to providing quality health care by facilities that achieve IAC accreditation The IAC Intersocietal Accreditation Commission www intersocietal org Guideline Document published 7 15 13 section 2 2 3B says The manufacturer s guidelines must be followed for the appropriate care and cleaning of the TEE transducer and adhere to the appropriate infectious disease standards to prevent the transmission of disease Effective December 31 2015 the structural and electrical integrity of the transducer must be checked between each use using an ultrasound transducer leakag
23. g a screw driver into a wall outlet is very dangerous Directly contacting a 120 or 240VAC source will result in electric shock that could result in death It is less well known that capacitance coupled or leakage current can still present a significant hazard even when direct physical contact to a conductor is not made Leakage current is current that is the result of capacitance between AC power sources people and equipment Any two conductors with insulation between them constitute a capacitor which can conduct AC current A simple example is an extension cord Wrapping your hand around the hot lead of an extension cord forms a capacitor The wire in the cord and your hand are the two conductors and the insulation on the wire is the dielectric Since any two conductors and an insulator form a capacitor the possibilities for creating many capacitive connections with AC mains power sources is very large To be a hazard it is necessary that a complete circuit is formed Because AC mains power sources are referenced to ground a human that is in contact with the ground will conduct current when they form a capacitor with an AC voltage source If you stand in a puddle of water and pick up an extension cord that is connected to AC power you WILL be conducting a current The current is proportional to the size of the capacitor formed by the wire and your hand and the AC voltage on the conductor The actual hazard to the person receiving the shock is a func
24. gulatory requirements Any variances from these requirements or ambiguous language found in User Manuals should be brought to the attention of the OEM and clarification sought In this paper we have highlighted or noted in red certain key concepts warning of things to look out for we label NOTE BENE or items to pay special attention to in any additional future reading you may do The information presented in this White Paper while greatly simplified is accurate to the maximum possible degree Finally a brief philosophical thought Medical imaging devices are some of the most brilliantly conceived systems ever developed They provide insights into disease processes providing the ability to diagnose much earlier making treatments significantly more effective But these systems must also be properly maintained and this relates to the safety of the device both for the patient as well as the user Acertara Acoustic Laboratories has dedicated itself to creating technologies that allow an HTM Healthcare Technology Manager and other technical personnel to validate the safe performance of these products We sincerely hope that you will share this commitment of always putting the patient first 1 Page INDEX Definitions Warning Labels Abbreviations Used Throughout this Document General Discussion about Ultrasound Probes Visual and Tactical Inspection Probe Labeling Why Measure Electrical Leakage in Probes Ultrasound Laboratory Accredita
25. ing conferences has served as an expert witness in multiple ultrasound litigations and holds more than 16 United States ultrasound related patents Wayne obtained his MBA from the University of Denver Daniels College of Business He was elected as a Fellow of the American Society of Echocardiography FASE in 2009 Jim Gessert B Sc BSEE Executive Vice President of Engineering and CTO Jim s engineering and design career spans 35 years and has included direct responsibility for bringing several technologically intensive diagnostic ultrasound systems to market Jim holds more than 20 important ultrasound patents and was awarded the H W Sweatt Award for engineering excellence from Honeywell Medical Systems In addition to developing traditional ultrasound imaging and Doppler systems Jim was also a pioneer developer in the field of intravascular ultrasound IVUS and is well versed in high intensity focused ultrasound HIFU Mr Gessert obtained a Bachelor of Science Degree in Electrical Engineering from the University of Michigan and a Bachelor of Science Degree in Physics from Central Michigan University ACCREDITED E pl FN Wh F E avas TESTING 31 Page Acertara Acoustic Laboratories 1860 Lefthand Circle Suite H Longmont CO 80501 303 834 8413 www acertaralabs com sales acertaralabs com
26. l and administrative means including supervisory ones to keep Medical Electrical ME Equipment or an ME System in a normal working condition or restored to normal working condition Patient A living being person or animal undergoing medical or dental investigation or treatment Patient Auxiliary Current Current flowing in the patient in normal use between parts of the applied part and not intended to produce a physiological effect for example bias current of an amplifier current used in impedance plethysmography 3 Page Patient Leakage Current Current flowing from the applied part via the patient to earth or flowing from the patient via an F Type applied part to earth originating from the unintended appearance of a voltage from an external source on the patient Recurrent Test Testing conducted at a defined time interval for the assessment of safety Reinforced Insulation A single system of insulation that provides two levels of protection against electrical shock Repair Restoring to a safe functional and normal condition Servicing The combination of all means for maintaining the ME Equipment or ME System within the specified requirements of the Manufacturer Service Life The expected service life of a probe is defined by each manufacturer and is arrived at by considering a number that includes anticipated use as well as several manufacturing variables Most manuals spell out the Caveat to the expected service life such as The
27. nductivity of the fluid the probe will be immersed in This is generally referred to as a conductivity test The actual current value may be displayed or a simple pass fail LED will inform the user if there is an appropriate level of conductivity This applies both to a saline solution or if the electrical leakage test is being conducted in a cleaning disinfecting solution As a reminder conductivity is the ability of a solution to pass an electric current The conductivity is roughly proportional to the number of ions present in the conducting solution In the Figure shown below two electrodes are placed in the solution described above The separation between the two electrodes is given by the length L this length may vary between manufacturers of these electrodes depending on design approach When an AC voltage AC is connected across the two electrodes placed in the physiological solution the ions will move between the electrodes and an electric current i will start to flow Conductivity Testing 24 Page ELECTRICAL LEAKAGE TEST PROCESS General Discussion The manufacturer s recommendations for set up to perform leakage testing on probes also vary By way of example Philips recommends performing the test while the probe is attached to the connector on the system mainframe which is how the IEC 60601 1 standard reads while other manufacturers recommend testing the probe independent of the ultrasound system Not all manufacture
28. ngent classification and is used for applied parts that are not functionally conductive and can be immediately released from the patient for example medical battery charges ear examination equipment Type B applied parts may be connected to earth while Type BF and CF are floating and must be separated from earth BF Is less stringent than CF and is generally for devices that have contact intentional physical contact with the patient or have medium or long term contact with the patient Examples additional to ultrasound include monitors and ventilators BF Defibrillator Proof To obtain this label additional testing is required of the manufacturer per IEC 60601 1 section 8 5 5 1 page 77 Defibrillation protection The Defibrillation Proof applied part marking indicates that the applied part can safely remain attached to a patient who is being defibrillated without any adverse effect on subsequent use of the Medical Electrical equipment NOTE BENE Although mentioned in some OEM User Manuals the term Defibrillator Safe does not appear nor is it defined in any Standard If the probe you are using is referred to as defibrillator safe in the user manual please contact your OEM and ask for a reference and definition for this term CF Is the most stringent classification being required for those clinical applications where the applied part is in direct conductive contact with the heart intentional cardiac physical contact
29. ning fluids are all corrosive and will ultimately severely damage the electrical and electronic components permanently damaging the probe Thus leakage testing ultrasound probes not only protects patients from an electrical hazard it also readily detects probe damage that may result in a biologic hazard to patients and damage that may significantly shorten the life of the probe Should I test the probe separately from the ultrasound system or connected to the ultrasound system As you will read in this paper some manufacturers suggest testing probes separate from the system while others only describe leakage testing of probes on a system There are some subtle differences in the testing done on a system versus testing on a probe alone but from a practical standpoint the data is equivalent The major disadvantage to leakage testing probes on a system is that you need a system TEE and Intraoperative probes should be leakage tested after cleaning and before the next use Having a system available for a large number of tests is a significant logistical issue Another benefit of testing probes independent of systems is that only the probe is tested so any failure is clearly a probe fault 18 Page WHY MEASURE ELECTRICAL LEAKAGE CURRENT cont Electrical Regulatory and Compliance Manufacturers of medical electrical equipment are required to test to the international electrical standard IEC 60601 1 to ensure the design of the equipment is in
30. or other clinical applications as considered necessary Examples additional to intraoperative ultrasound would include defibrillators and heart monitors Type CF classification indicates that the degree of protection from electrical shock afforded by the transducer is suitable for all patient applications including direct cardiac and intraoperative applications Although a TEE probe is inserted into the esophagus and images the heart it does not come into physical contact with the heart therefore it is classified as a BF device rather than CF Type CF as shown below has the icon of a heart in a box The label with the man figure icon inside the rectangle shown below indicates a BF level device that is NOT protected for use with defibrillation NOTE BENE It is noted here that used probes purchased from non OEM sources may not have all of the required labeling intact on the connector identifying what level of classification it was assigned For example a hospital may need to replace a TEE probe which had a BF Defibrillation label and buy a probe that has the label missing This probe should not be used in a clinical setting that requires defibrillation protection Some TEE probe user manuals use a term defibrillator safe and reference ISO 60601 1 This standard does not mention or define defibrillator safe Users of a probe so referenced should contact their OEM vendor and find out exactly what this term means and where it was derived
31. path to the patient Also since leakage current is capacitively coupled current a sheath will reduce but not eliminate leakage current from a probe with a leakage current fault Endocavitary probe cover 20 Page WHY MEASURE ELECTRICAL LEAKAGE CURRENT cont Use Changes the Efficacy of the Protective Components Wear and tear as well as other issues that occur over time and use may make the TEE probe unsafe for patient use as shown in a recent recall notice 9 3 14 for Siemens Acuson V5Ms TEE probes shown below Reports of deterioration of material covering the articulating section of the V5Ms transesophageal transducer Action Siemens sent a Customer Safety Advisory notification to all affected customers on August 13 2014 The notification identified the product the problem and the action to be taken by the customer Customers were advised that if they notice any damage or wear on their transducer or if the transducer fails to pass the leakage current test they should immediately discontinue use of the transducer and contact their local Customer Service Engineer If they have any questions customers should contact their local service support person for information regarding timelines and status Customers were instructed to share this information with all personnel within their organization who need to be aware of this issue From the GE Vscan User s Manual Even with a small
32. r from the patient before defibrillation WARNING Only use Type BF transducers with the ultrasound system to maintain a level of protection against electrical shock WARNING When using an endocavity or intraoperative transducer with a TYPE CF applied part the patient leakage currents may be additive WARNING The leakage current test for transesophageal transducers must be done prior to each use in order to reduce the likelihood of harm to the patient Refer to the manufacturer s instructions included with the tester WARNING Transducer abnormalities may cause electrical shock or injury to the patient Caution Do not bend the flexible shaft into a curve of less than 30 5cm in diameter Caveat on Warning Labels Some warning label language may be ambiguous or may not cite a reference directly or correctly An example is found in the second warning above that uses the terminology defibrillator safe a term not used or defined in the ISO Standards If you suspect this is the case you should contact the manufacturer for a more detailed explanation and the reference s they are citing 5 Page ABBREVATIONS USED THROUGHOUT THIS DOCUMENT AC AIUM ANSI A P ASE B BF BNS C CE CF CSA DC DUT EC EMC FDA FS Hz IAC IEC ISO kg uA mA ME MITA NEMA RMS sec TEE UL ULT VAC VDC 6 Page Alternating Current American Institute of Ultrasound in Medicine American National Standards Institute
33. refore we recommend establishing a depth and using it for the baseline and all future leakage tests Correct Test Set up There are two basic leakage test configurations recommended by ultrasound manufacturers 1 with the probe connected to the ultrasound system e g Philips EPIQ 7 and 2 testing the probe independently of the ultrasound system e g Sonosite An example of a test set up for recommendation number 1 is shown below for a TEE probe There is a potential problem with including the system in the electrical leakage test set up recommendation 1 Specifically there may be an issue with the transducer connector slot on the system console by way of example bent pins poor connection and ground continuity that could lead to an erroneous electrical leakage test result IEC documents give leverage to the device manufacturer in specifying how a test should be done 25 Page ELECTRICAL LEAKAGE TEST PROCESS cont The acceptable maximum leakage current levels for an intraoperative probe are lt 10uA RMS for normal condition and lt 50uA RMS for single fault condition The general guidance from manufacturers is to submerge the head of the probe approximately 5 cm into the saline solution It is good testing practice to measure low leakage as well as the maximum allowable leakage this is to ensure that the test set up is correct and a complete circuit in the test set up has been established Fail gt 100uUA Pass Steady St
34. required made by completely immersing the enclosure in 0 Non protected water Test conditions are subject to agreement 1 Vertical dripping between manufacturer and user Acceptance for 2 Dripping 15 tilted Class IPx 8 requires no water to have penetrated 3 ea into the item in such a quantity or in sucha i P ene aes 5 Jetting position as to prevent satisfactory operation of a he i f ck Thi 6 Powerful jetting the item or to present a safety risk This 7 temboraninmmersion protection applies to the endoscope portion of g Continuons mmerion the TEE probe 12 Page Various IPx Labels IPx 1 Philips X7 2t t IPx 7 Toshiba PVT 382BT IPx8 Siemens 17L5 Note 1 Although Philips labels the X7 2t probe as IPx 1 on the connector its User Manual states that the insertion tube and tip are rated at IPx 7 this allows for submersion for cleaning disinfecting and electrical leakage testing Please see Note on preceding Page IEC 62353 states Parts of the TRANSDUCER ASSEMBLIES specified by the MANUFACTURER as intended to be immersed during NORMAL USE shall meet the requirements of watertight equipment IPX7 13 Page APPLIED PARTS CLASSIFICATIONS Applied parts are classified as Type B Type BF or Type CF according to the nature of the device and the type of contact with a patient Each classification has differing requirements from the point of view of protection against electrical shock B Is the least stri
35. resulting in probe damage Before use inspect the following areas of the transducer e Transducer body including the shaft handle amp nose piece inspect for cracks abrasions or evidence of impact e Acoustic lens or cap check for cuts tears gouging abrasion swelling bubbling delamination or discoloration e Strain relief and cable assembly check for cracks cuts tears abrasion kinking or crushing e System connector check for foreign objects broken latches or bent pins and shielding e Cable assembly check for discoloration or inflexibility of the transducer cable or strain relief e Transesophageal transducers TEE inspect the insertion tube for tears rips or discoloration pay particular attention to the insertion tube sleeve shown below where bite marks may occur TEE Insertion Tube Sleeve 10x Loupe with Light Bite Marks Is there a path for fluid 11 Page PROBE LABELING DEFINITIONS AND TECHNICAL BACKGROUND There are encapsulation requirements applicable to products with high reliability requirements and where the entry of dust could cause problems IP classification testing is carried out in accordance with the global IEC 60529 2000 standard Labeling for ultrasound probes are normally shown on the probe connector as illustrated on the various examples of probe connectors on Page 13 IP stands for International Protection the other numbers and letter x are explained below The symbol of the man i
36. roximity of the heart See Page 20 What aspects of probe construction are related to leakage current To reduce noise susceptibility the connector cable probe housing and array are all electrically shielded The shield consists of a conductive layer completely encasing all probe components It is usually connected to the earth ground of the ultrasound system Since probes are purposely electrically isolated from the patient the shield is covered in an insulating layer consisting of the lens probe housing cable jacket etc Thus a capacitor that will produce leakage current is formed between the probe shield and the patient and another is formed between the probe shield and the sonographer What problems occur with probes that can result in a leakage current hazard Damage to a probe that results in fluid leakage into the probes interior is the most common cause of probes failing leakage test measurements This is because a conductive path fluid now directly connects the probe shield and possibly all other electrical probe connections to the outside world Instead of a small capacitor connecting to the shield a leak provides a much lower impedance resistance in this case connection In addition to the electrical hazard there are other bad things that result from fluid leakage The biggest is the possibility of a biohazard from patient fluids getting into places where cleaning will not remove them Additionally saline body fluids and probe clea
37. rs e g Siemens Acuson SC2000 Instructions for Use Page 6 3 and Warning Number Three detail how to perform a test rather their User Manual points to another User Manual supplied by the manufacturer of the electrical leakage testing device With transthoracic and trans abdominal probes the contact to the patient is only from the surface of the probe but with a TEE probe the entire insertion tube must be considered in measuring the leakage current Leakage current measurements must be performed after each use due to the possibility of damage to the probe and insertion tube after a procedure There may also be instances when a probe with a factory defect may present a leakage issue One such incident involved reports of deterioration of the material covering the articulation section of the transesophageal transducer the OEM indicated this would result in a failure of the electrical leakage test Immersion Depth Immersion depth recommendations also vary as a function of the manufacturer Generally surface and endocavitary probes are submerged into the saline solution to approximately 5cm Adult TEE probe insertion tubes are submerged depending on the manufacturer to a range of 40cm to 100cm Pediatric TEE insertion tubes are submerged to a range of 40cm to 70cm The amount of capacitance anticipated from submersion is related to how deeply the insertion tube is submerged i e the more insertion tube that is submerged the higher the capacitance The
38. signs that are capable of gt 100dB dynamic range What is key in the design is that receiver sensitivity is maximized and the noise generation and susceptibility be minimized Electrical noise may be generated by the transducer itself or it can enter into the ultrasound system as interference from outside sources Immunity to interference is usually accomplished by using coax cables to carry signals between the transducer and the system additional shielding in and around the acoustic stack see Photos below and in newer arrays placing the pre amp and micro beamforming electronics near to or integrated in the acoustic stack design Array Shielding y a gt k Pener n s 7 ch Cable Termination Although the focus of this paper is on electrical leakage testing it is noted here that one of the more common complaints associated with probe repair is noise in the B mode image and especially in the various Doppler modes pulse wave PW continuous wave CW and color flow CF One of the reasons for performing electrical leakage testing is to detect fluid ingress into a probe see Page 28 this fluid ingress can also cause noise sparkling issues in all ultrasound modes of operation When fluid ingress is detected early enough a probe can normally be dried out and repaired returning the probe to normal operation both from a safety standpoint as well as from a clinical performance standpoint i e eliminating the noise Noise
39. the transducer has not been used for an extended period of time Sonosite The electrical leakage current test should be performed on the TEE transducer after taking it out of the box and prior to each exam alternatively if the bite hole inspection test is done prior to each exam then electrical leakage current test should be done yearly at a minimum Toshiba The Toshiba transesophageal phased array transducer is designed taking every consideration to insure mechanical and electrical safety however cracks or holes may result in a hazardous situation Therefore a safety kit is supplied to detect cracks and holes not detectable visually or by touch for preventive countermeasures against such hazardous situations by measuring insulation resistance electrically Use this kit for safety check before and after using the transducer MANUFACTURER S GUIDANCE FOR ELECTRCIAL LEAKAGE TESTING METHODOLOGY General Electric Perform test with the probe not attached to US system Philips Perform test with probe connected to US system Siemens Perform test according to user instructions provided by the tester manufacturer Sonosite Perform test with the probe not attached to the ultrasound system Toshiba Perform test using Toshiba proprietary meter output of test is displayed in MQ WHAT IEC 60601 1 MANDATES Perform test with probe connected to US system test results BF device lt 100UA CF device lt 10uA WHAT IEC 62353 2014 M
40. tion Measuring Devices and Background Components Used in Testing Electrical Leakage Testing Process Manufacturer s Recommendations Samples of Probe Failures to Look for Conclusion References About Acertara and the Authors 2 Page Page 11 12 17 22 23 24 25 27 28 29 30 31 DEFINITIONS Additive Current Leakage Leakage current is additive in the presence of multiple patient contact devices and must be accounted for when assessing patient electrical shock risk Applied Part IEC 60601 1 uses the term applied part to refer to the part of the medical device which comes into physical contact with the patient in order for the device to carry out its intended function Class I Equipment protection against electric shock by earthed additional protection to basic insulation through means of connecting exposed conductive parts to the protective Earth in the fixed wiring of the installation Conductivity The ability of a solution to pass an electric current Defibrillation Proof An applied part that is protected against the effects of a discharge of a cardiac defibrillator to the patient Dielectric Strength the maximum electric field that a pure material can withstand under ideal conditions without breaking down i e without experiencing failure of its insulating properties Electrical Safety The status of protective measures within an equipment system designed and produced in accordance with
41. tion of the current path and the amount of current flowing through the body Current flowing near the heart even a very small amount may result in the loss of heart rhythm resulting in death Large currents flowing through the extremities while not fatal can result in severe burns To assure electrical equipment is not hazardous electrical leakage current testing is required Testing is done during equipment design equipment manufacturing and periodically when installed in a clinical environment Safety standards such as IEC60601 1 define safe current level limits used during testing Why is it important to test medical equipment Because patients are often connected to many pieces of equipment have compromised health and are often sedated or fully unconscious there is a real possibility of having a problem and worse the patient not being able to identify that a shock is occurring There are several facilities wiring faults that increase the hazard including open earth ground connections swapped hot and return leads in the power wiring or open return connections Leakage tests are always done with these fault conditions 17 Page WHY MEASURE ELECTRICAL LEAKAGE CURRENT cont Why is it important to test ultrasound probes The main reason is that use ALWAYS involves patient contact TEE intra cavity and intraoperative probes are especially important because they have a lower resistance connection to tissue and are often used in the p
42. trinsically safe The standard specifies various type testing requirements for protection against potential electric hazards Once medical devices are placed in service the requirements of IEC 62353 2014 become the standard against which the electrical leakage should be tested With regard to diagnostic ultrasound probes the FDA points to the IEC Standards Document IEC 60601 2 37 2008 particular requirements for the safety and essential performance of ultrasonic medical diagnostic and monitoring equipment that do not include electrical leakage testing for ultrasound probes rather they point to thermal testing as well as acoustic output power requirements for patient contact transducer surface temperature limits and electrical leakage limits As mentioned on Page 5 there is always a possibility that instructions for testing provided by the ultrasound OEM may be unclear or perhaps even incorrect based on a misinterpretation of an IEC publication This is recognized by the authors of the IEC documents as a potential problem and is referenced in the following statement IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user If this appears to be the c
43. y Philips see photo below ew Array 7 Electronics x P Array a 7 Page STANDARD COMMON PROBE NOMENCLATURE 1 Transducer Lens 2 Transducer Housing z 3 Transducer Strain Relief 4 Cable 6 1 5 Connector Strain Relief 6 Connector Housing STANDARD TEE PROBE NOMENCLATURE 1 Insertion Tube 2 Cable 3 Transducer Connector 4 Transducer Control Housing 5 Distal Tip contains acoustic stack 6 Insertion tube sleeve 7 Depth markers 8 Page Acoustic Stack The heart of an ultrasound probe is the acoustic stack the main component of which is the array Composite arrays are composed of 1 a Piezo electric PZ material that provides the necessary transduction of mechanical to electrical energy and vice versa and 2 a supporting polymer that isolates the element while shaping the mechanical and electrical properties of the PZ elements It is the physical distribution of PZ and polymer that makes the array and specifies its behavior The array construction following a specific design depends on the well developed dicing and filling techniques used in today s transducer manufacturing Construction begins with a block of PZ material that is diced to form PZ elements held by a common spine see Diagram below The diced regions are then filled with polymer The final configuration is obtained by lapping the array front and back to remove polymer on the face and the PZ spine on the back
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