WO2021059290A1

WO2021059290A1 - Mri safe cardiac pacemaker

Info

Publication number: WO2021059290A1
Application number: PCT/IN2020/050466
Authority: WO
Inventors: Jonath SUJAN
Original assignee: Sujan Jonath
Priority date: 2019-09-23
Filing date: 2020-05-25
Publication date: 2021-04-01

Abstract

MRI Safe Cardiac Pacemaker is a small battery-operated device that is placed underneath the surface of the skin in the chest or abdomen regions. Cardiac Pacemaker has a sensing unit which senses the abnormal beating of the heart. Once abnormal rhythms are sensed the discharge unit of the pacemaker generates a voltage pulse that is delivered to the heart through implantable electrical leads connected to the heart muscles. MRI Safe Cardiac Pacemaker has external housing made of bio-materials like Carbon Fibre and Tantalum which blocks the magnetic field thereby eliminating false triggers of pacemaker due to malfunctioning of reed switch due to external magnetic field. An intelligent electronic circuit (Fig 4) is also included inside the housing of the pacemaker to protect the patient from false triggers during MR Imaging. The Reed Switch is replaced by RF switch to operate the pacemaker in synchronous and asynchronous mode as the external housing for the pacemaker will shield the Magnetic fields. A specific band of Radio Frequencies apart from the environmental Radio Frequencies and Radio Frequencies used in MRI should be allocated for controlling the RF Switch used to operate the Pacemaker in Synchronous and Asynchronous Mode. The RF from MRI will travel to the pacemaker along the leads causing RF Interferences to the Pacemaker Circuitry. This is eliminated by using a Low Pass Filter inside the pacemaker housing, in the terminal connecting the pacemaker and the lead.

Description

MRI SAFE CARDIAC PACEMAKER

FIELD OF THE INVENTION

The present invention relates to Medical Implants, in specific to Cardiac implants like pacemaker which can be made MRI safe and compatible by using a pacemaker housing made of diamagnetic and paramagnetic materials and making using of electronics to check the proper functioning of sensing and pacing unit of the pacemaker during times of MR Imaging. This invention will be a breakthrough in the field of healthcare.

BACKGROUND OF INVENTION

Cardiac Pacemakers are used to treat patients with irregular heart rhythms which leads to reduced oxygen supply to vital organs like the brain, heart and different parts of the body. The reduction in oxygen supply to the heart causes the heart to work ineffectively. As a result, the heart cannot pump efficiently causing blood to clot. The blood clot loosens and the embolus reaches the brain through the carotid artery, blocking the blood flow to the brain causing a stroke. Prolonged cardiac arrhythmia like tachycardia, bradycardia and fibrillation leads to cardiac failure. Controlling the extent of arrhythmia under the normal condition will make the heart to function efficiently. Cardiac Pacemaker plays a vital role in pacing the heart back to its normal rate. Therefore, Cardiac Pacemaker is an essential device for patients with cardiac arrhythmia as the MR Imaging is not frequent.

Many investigations have been reported on the safety of patients with cardiac pacemaker undergoing Magnetic Resonance Imaging such as

1. Tsitlik et ah, 1993 have disclosed MRI Safe Cardiac Pacing using RF Filtering Method.

2. Martin et ah, 2004 reported about Magnetic Resonance Imaging and Cardiac Pacemaker safety at 1.5T.

3. Luechinger et ah, 2004 investigated the heating effects at the myocardium - pacemaker tip interface during MRI at 1.5T.

4. Ostroff et ah, 2011 explained the usage of biostimulator and leadless pacing to safely operate under the MRI Conditions.

5. Greatbatch et ah, 2002 have done Magnetic resonance safety testing of a newly-developed fiber-optic cardiac pacing lead. 6. Koop et al.,2016 explained the usage of second implant to detect the magnetic field from MRI and automatically make the Cardiac Pacemaker enter into the MRI Safe mode.

7. US5705014A (Carbon fiber magnetic resonance compatible instruments) - John Frederick Schenck and Kenneth William Rohling showed that instruments with carbon fibre can be used in Magnetic Resonance (MR) Imaging.

8. T Dill et ah, 2008 reported that Stents composed of Tantalum and other materials do not possess a risk when Magnetic Resonance (MR) Imaging of 3T or below is performed.

9. J Endod et ah, 2017 concluded that Epoxy resin-based sealers were more biocompatible compared with the other sealers.

10. US5217010A (ECG amplifier and cardiac pacemaker for use during MRI) - Joshua E. Tsitlik, Howard Levin, Henry Halperin and Myron Weisfeldt used a unique RF Filtering method to attenuate RF signals produced during MRI.

11. US20030036776A1 (MRI-compatible implantable device) - Thomas Foster and Patrick Connelly proposed optical methods to protect the patient from currents induced by the pulsed Radio Frequency fields.

To solve the complication of using Cardiac Pacemaker during MRI Procedure, MRI housing can be constructed with paramagnetic material like Tantalum and diamagnetic Carbon Fibre which are biocompatible. Electronics can also be used to check the proper functioning of the sensing and pacing unit ensuring the safety of the patient.

REFERENCES

1. J.G. Webster, Medical Instrumentation: Application and Design, Wiley Publications, 3rd Edition, 2008, ISBN: 9788126511068.

2. Leslie Cromwell, Fred J Weibell, Erich a Pfeiffer, Biomedical Instrumentation & Measurements, Wiley Publications, 2nd Edition, 2010, ISBN:9780130771315.

3. Richard Aston, Principles of Biomedical Instrumentation and Measurement, Prentice Hall of India, 4th Edition, 2005, ISBN: 9780675209434.

4. Joseph J. Carr, John M. Brown, Introduction to Biomedical Equipment Technology, Pearson Education, 4th Edition, ISBN: 9788177588835. 5. R. S Khandpur, Handbook of Biomedical Instrumentation, Tata McGraw-Hill, 2nd Edition, 2008, ISBN:9780070473553.

6. L.A. Geddes, L.E.Baker, Principles of Applied Biomedical Instrumentation, Wiley Publications, 2nd Edition, ISBN: 9788126518074.

7. Amin Al-Ahmad, How-to Manual for Pacemaker and ICD Devices: Procedures and Programming, Wiley-Blackwell Publications, 1st Edition, 2018, ISBN: 9781118820599.

8. Robert L. Boylestad and Louis Nashelsky, Electronic Devices And Circuits Theory, Pearson Publication, 10th Edition, 2009, ISBN: 9788131727003.

9. Muhammad Ali Mazidi and Janice Gillispie Mazidi, The 8051 Microcontroller and Embedded Systems Using Assembly and C, Pearson Publications, 2 edition, 2008, ISBN: 9788131710265.

PRIOR ART OF THE INVENTION

Present work is a novel attempt to develop Cardiac implants which are MRI Safe and

Compatible. The citations of work in the related field are

1. US8433421B2 (Ergin Atalar, Justin Allen, Paul Bottomley, William Eldelstein and Parag V. Karmarkar) MRI Safe High Impedance Lead Systems.

2. US9002471B2 (Robert A. Stevenson & Gabriel A. Kustra) Independently actuatable switch for selection of an MRI compatible band-stop filter placed in series with a particular therapy electrode of an active implantable medical device.

3. US7363090B2 (Henry R. Halperin, Robert A. Stevenson) Bandstop filter employing a capacitor and an inductor tank circuit to enhance MRI compatibility of active implantable medical devices.

4. Forleo et ah, 2010 Safety and efficacy of a new magnetic resonance imaging-compatible pacing system.

5. Cronin et ah, 2012 Magnetic Resonance Imaging in patients with cardiac implantable electronic devices

6. EP2198914A1 (Robert A. Stevenson) Switch for turning off therapy delivery of an active implantable medical device during MRI scans. DISADVANTAGES OF PRESENT STATE OF ART

The pain-points in the present state of art are as follows:

1. Lead tip of the pacemaker act as an antenna for radio frequency used in Magnetic Resonance Imaging causing induced heating.

2. Mechanical dislocation of the pacemaker due to the applied magnetic field.

3. Reed Switch malfunctioning - Reed switch is an electric switch that is controlled by the applied magnetic field. Reed Switches are used to make the pacemaker work in asynchronous and synchronous modes.

4. False Discharge of Pacemaker due to the strong magnetic field from the Magnetic Resonance Imaging.

5. Faster Battery depletion due to the strong magnetic field from Magnetic Resonance Imaging thereby reducing battery life.

6. RF Signals from Magnetic Resonance Imaging will travel from the leads to the internal circuits of the pacemaker.

OBJECTS OF THE INVENTION

1. The ultimate object of the present invention is to develop pacemaker housing to make the pacemaker compatible and safe with Magnetic Resonance Imaging thereby preventing mechanical dislocation of the Pacemaker, faster battery depletion and thereby increasing longevity.

2. Another object of the invention is to prevent false triggers in the pacemaker due to the unwanted triggering of Reed Switch due to the external strong magnetic fields from strong magnets used in MRI.

3. Another object of the invention is to protect the pacemaker circuitry from external magnetic fields despite magnetic shielding provided by the external pacemaker housing.

4. Yet another object of the invention is to facilitate simultaneous handling of Pacemaker action and MRI operation. 5. Another object is to ensure the proper working of the sensing unit and pacing unit of the Pacemaker during the time of MR Imaging.

6. Yet another aim is to protect the internal circuitry of the Cardiac Pacemaker from Radio Frequency generated by the MRI.

BRIEF DESCRIPTION OF THE FIGURES

Fig 1: is the cross-sectional view of the external housing of the cardiac pacemaker made of Carbon fibre and Tantalum showing the two different layers of the pacemaker housing. Outer layer - Tantalum, Inner layer - Carbon Fibre

Fig 2: is the top view of the external housing of the cardiac pacemaker made of Carbon fibre and Tantalum showing the small exposure of carbon fibre through the cavity in the first layer of housing (Tantalum).

Fig 3: is the cross-sectional side view of the external housing of the pacemaker made of Carbon fibre and Tantalum showing the two layers of the pacemaker housing and the small projection of Carbon fibre.

Fig 4: is the block diagram of the circuit used in the invention to check the sensing and pacing element of the pacemaker during MR Imaging.

Fig 5: is the block diagram of the circuit used in the invention to control different modes of the Pacemaker.

Fig 6: is the block diagram of the circuit used in the invention to attenuate the RF frequencies protecting the pacemaker circuitry from RF Interferences.

DETAILED DESCRIPTION OF FIGURES

Description of Fig 1 is given below la. The inner layer of the pacemaker housing made of Carbon fibre. lb. The outer layer of the pacemaker housing made of Tantalum.

Description of Fig 2 is given below

2a. The exposed carbon fibre tip from the housing acting as an Antenna for RF signals.

2b. The outer layer of the pacemaker housing made of Tantalum. Description of Fig 3 is given below

3a. The exposed carbon fibre tip from the housing acting as an Antenna for RF signals.

3b. The inner layer of the pacemaker housing made of Carbon fibre.

3c. The outer layer of the pacemaker housing made of Tantalum.

Description of Fig 4 is given below

4. Battery to power up the pacemaker. The battery used is Lithium Iodine Polyvinylpyride with an output voltage of 2.8V.

5. 2x Voltage Multiplier is used to double the battery voltage from 2.8V to 5.6V.

6. 5V Regulator is used to produce regulated 5V DC.

7. Hall Switch is the magnetic switch which produces output voltage according to the magnetic field strength experienced.

8. Vref(Magnetic) - Voltage corresponding to the magnetic field strength beyond which the pacemaker’s components malfunction due to electromagnetic interference.

9. DC Comparator compares the output DC voltage of the Hall Switch and V_ref(Magnetic) and produces logic output_.

10. The Lead System controls the input sensing of the heart’s natural depolarizing potential and output pacing of the heart when needed.

11. Switch Control controls the switches SI, S2, S3, S4, S5, S6, S7, S8.

11a. Switch SI, controls the contact between the Lead system and the Sensing element of the pacemaker.

1 lb. Switch S2, controls the voltage supply for the generation of Vref(Source).

11c. Switch S3, controls the contact between Sensing and Pacing element of the pacemaker. lid. Switch S4, controls the testing voltage to the pacing unit of the pacemaker. lie. Switch S5, controls the inclusion of Rtestin the circuit for the current generator and pacing unit to generate current according to the test resistance. 1 If. Switch S6, controls the signal from Impedance Calculator to the current generator used for testing the Pacing unit of the pacemaker.

1 lg. Switch S7, controls the contact between the Pacing element and the current comparator.

1 lh. Switch S8, is used to enable and disable the output pacing of the lead system. +V is the 5V power supply to Vref(Source) generator and artificial stimulus to Pacing Element. V ref(Source) - Voltage which is equivalent to the natural depolarization potential of the heart. It is used as an input to the sensing unit of the pacemaker to check its proper functioning. 5V DC Converter will convert the V_ref potential to 5V DC. The Sensing element is an important part of the pacemaker which senses the depolarizing potential of the heart. Logic Comparator will compare the logic states in the input and produce the desired output. Voref - Voltage at which the current generator produces the desired current to pace the heart. The current is equivalent to the output of the Pacing unit of the Pacemaker. Rtest - This impedance is equivalent to the lead impedance of the pacemaker lead. Impedance Calculator calculates the lead impedance and test impedance. This helps the pacing unit to generate current accordingly. The Current Generator will produce Current according to the Test Resistance used. Pacing element is the most important part of the pacemaker which produces an artificial current pulse when the natural depolarizing potential of the heart is not sensed. The Current Comparator will compare the current outputs of the pacing element and the current generator during the pacing element checking. Pacing Element Checker is made of electronic latches and logic comparators. It is used to check the proper functioning of the Pacing Element of the pacemaker. 24. Sensing Element checker is made of electronic latches and logic comparators. It is used to check the proper functioning of the Sensing Element of the pacemaker.

Description Fig 5 is given below

25. The transmitter will modulate the signal for synchronous or asynchronous mode selection to the desired RF signal and transmit.

26. The receiver will receive the modulated signal from the transmitter. Reception and transmission take place in wireless medium.

27. Bandpass filter will attenuate the signal with frequencies other than the pass-band frequencies. The bandpass filter will attenuate the RF generated by MRI.

28. The decoder will demodulate and decode the received signal to binary logic to guide the switch control. Switch control will close and open S3 to make the pacemaker work in synchronous and asynchronous mode.

28a. Logic 1 - Synchronous mode

28b. Logic 0 - Asynchronous mode

Description of Fig 6 is given below

29. Leads are hooked to heart muscles in the atrium and ventricle. They are used to sense the electrical activity of the heart and pace the heart when needed. Leads used are usually made of titanium or titanium alloys insulated with polymers like Polyurethane.

30. Lowpass filter will attenuate the signal with frequencies lesser than the cutoff frequency. The cutoff frequency will be equal to the frequency of natural periodic depolarizing potential generated by the heart.

DETAILED DESCRIPTION OF THE INVENTION

1. Construction of an MRI (Magnetic Resonance Imaging) safe and compatible Cardiac pacemaker housing using biocompatible materials like Tantalum and Carbon fibre.

2. The housing of the Cardiac pacemaker is made of two layers of materials. The first layer is made of Tantalum which is biocompatible. The second layer is Carbon fibre which is biocompatible and shows great strength and flexibility. The magnetic susceptibility of Tantalum used in the first layer of the housing is 0.8490+/- 0.0006. Low magnetic susceptibility means they are poorly magnetized by the applied magnetic field. Tantalum is nonmagnetic material with magnetic permeability of 1.0001 indicating it to be a paramagnetic material. The low magnetic susceptibility and permeability of the material make it compatible with Bio-implant applications and they are biocompatible. Paramagnetic materials are weakly attracted by the strong magnetic field and they induce magnetic fields in the direction of the applied magnetic field. They allow the magnetic field of lines to pass through them. Tantalum is MRI Conditional. The second layer is made of Carbon fibre. Carbon fibre has a negative magnetic susceptibility of -1.6. Negative susceptibility signifies that the material repels the magnetic field. Carbon fibre is diamagnetic. They induce magnetic fields in the direction opposite to the applied magnetic field. They do not allow the magnetic field lines to pass through them, so the field avoids the material and passes along the surface. They are biocompatible. They are used in Bio-implants because of its high durability and corrosive resistance. Carbon fibre has less magnetic torsional force, therefore Carbon fibre is not affected by magnetic fields. The first layer of housing (Tantalum) will allow the magnetic field to pass through it and it is not magnetically attracted. The next layer of the housing (Carbon Fibre) will not allow the magnetic field to pass through them protecting the electronic circuits of the pacemaker from magnetic interferences. The two layers of the housing are adhered together with the help of epoxy resin which is biocompatible. Hall effect sensor is also placed inside the housing of the pacemaker to ensure the safety of the patients from false triggers. The Hall effect sensor acts as a Magnetic switch. The Hall effect sensor will produce an output voltage which is proportional to the magnetic field strength. Though the external housing of the pacemaker will effectively shield the magnetic field, hall effect sensor is used to simultaneously achieve the function of pacemaker and MRI. If the magnetic field strength inside the pacemaker after shielding exceeds the desirable level, the reed switch opens and the pacemaker works in asynchronous mode. To prevent the false triggers of the pacemaker due to the action of the magnetic field from the MRI, the Reed Switch is replaced by a digital switch which is controlled by the Switch Control unit. Furthermore, an intelligence circuit is included inside the housing to check the proper functioning of the sensing and pacing unit of the pacemaker during the period of MR Imaging. The output of the hall switch is compared with Vref(Magnetic), the potential beyond which the external magnetic fields cause electromagnetic interference with the electronic circuit of the pacemaker. The output of the DC Comparator will be High if the output of the hall switch is lesser than Vref(Magnetic). The output of the DC Comparator will be Low if the output of the hall switch is greater than Vref(Magnetic). If the patient is not undergoing an MRI scan, the Hall switch will not produce an output voltage. The DC Comparator will produce a High Output and the output of AND gate goes High. The switch control is disabled by the inverter. The input to the OR gate stands High which in turn enables the pacing unit and the switches SI, S3, S8 are maintained closed. Therefore, it functions as a normal pacemaker. An AND operation is carried out between the output of the DC Comparator and Battery output. If the Battery drains the pacemaker circuit fails to work. If the battery functions properly and if the output of the DC Comparator stands Low the output of the AND Gate stands Low, else it stands High. If the output of the AND gate is Low, the intelligence circuit detects that the magnetic field strength exceeded the permissible level beyond which the components of the pacemaker faces electromagnetic interference and begins to malfunction. This enables the Switch control unit, Sensing element checker and Pacing element checker. Initially, the switches SI, S2, S3, S4, S5, S6, S7, S8 are opened. When S2 is open Vref(source) is not generated and the output of DC Converter is Low. The sensing element will assume that the heart didn’t depolarize and produce a High logic to the pacing unit to pace the heart. As switch S3 is open the signal doesn’t reach the pacing unit and is sampled by the Voltage regulator. The output of the regulator is inverted. The logic comparator will compare the inverted output of the regulator and the output of the 5 V DC Converter. As both inputs to the logic converter are Low the output of the Logic converter is Low. The Low logic is latched into the Sensing Element Checker. The switches SI, S3, S4, S5, S6, S7, S8 are maintained open and switch S2 is closed. When S2 is closed Vref(source) is generated and the output of DC Converter is High. The sensing element will sense that the heart depolarized and produce a Low logic to the pacing unit not to pace the heart. As switch S3 is open the signal doesn’t reach the pacing unit and is sampled by the Voltage regulator. The output of the regulator is inverted. The logic comparator will compare the inverted output of the regulator and the output of the 5 V DC Converter. As both inputs to the logic converter from DC Convertor and Sensing element is High the output of the Logic converter is Low. The Low logic is latched into the Sensing Element Checker. If Logic 0’s are latched in the Sensing element checker, the Sensing element works properly, else the intelligence circuit understands that the sensing unit is malfunctioning. The pacing element is checked if the sensing element works properly or sensing element is checked again. The switches SI, S3, S4, S8 are maintained open and switch S2 is opened switches S5, S6, S7 are closed. The switch control makes one input of the OR Gate High. This enables the pacing element. When S5 is closed the test resistance Rtest is included in the circuit. The impedance calculator measures the test resistance. When S6 is closed, the Current generator will produce an output current according to Rtest and V₀ref. When S4 is open, Low signal is sent to the pacing element and it assumes that the sensing element has sensed a depolarizing potential from the heart and doesn’t produce output current for pacing the heart. The inputs to the current comparator are zero current from the pacing unit and the reference current generated by the current generator. Therefore the output of the current comparator is High which is latched to the pacing element checker. When S4 is closed, a High signal is sent to the pacing element and it assumes that the sensing element has sensed that the heart is not depolarized and produces output current for pacing the heart. The inputs to the current comparator are pacing current pulse from the pacing unit and the reference current generated by the current generator. Therefore the output of the current comparator is Low which is latched to the pacing element checker. If Logic 1 and Logic 0 are latched in the Pacing element checker, the pacing element works properly, else the intelligence circuit understands that the pacing unit is malfunctioning. Finally, if the sensing element and pacing element works good, switches SI, S3, S8 are closed and switches S2, S4, S5, S6, S7 are opened. So it functions as a normal pacemaker analyzing the electrical activity of the heart and pacing when needed. The checking of the pacing unit and sensing unit is carried out by the intelligence circuit only when the magnetic field strength exceeds the desired level. The above discussed electronic circuitry for checking of Sensing and Pacing element of the pacemaker will be enclosed inside the second layer of Carbon Fibre to protect it from Electromagnetic Interferences. As the reed switch is replaced by a digital switch, to make the pacemaker work in synchronous or asynchronous mode the pacemaker is controlled externally by using Radio Frequencies. The first layer of the housing, Tantalum will have a small cavity or aperture exposing the second layer of carbon fibre. Carbon fibres have high conductive property so it will not allow the RF signals to pass through it. But it will act as an antenna for RF signals. The inner side of the second layer of housing, Carbon fibre will have a small projection. The RF signal is picked up by the Carbon fibre and conducted to the projection in the inner side. The projection of the carbon fibre is connected to the Bandpass filter which is tuned to a narrow band of frequencies other than the frequencies used by MRI and environmental Radio frequencies thereby allowing only a very specific band of frequencies. To make the pacemaker work in synchronous mode, the input signal is modulated to any Radio Frequencies in the allowed band of frequencies and transmitted. The RF signal for which the bandpass filter is tuned will be received and other frequencies will be attenuated. The allowed RF signal is demodulated to logic 1. The demodulated signal is sent to the switch control and switch S3 is closed making the pacemaker work in synchronous mode. To make the pacemaker work in asynchronous mode, the input signal is modulated to any Radio Frequencies in the allowed band of frequencies and transmitted. The RF signal for which the bandpass filter is tuned will be received and other frequencies will be attenuated. The allowed RF signal is demodulated to Logic 0. The demodulated signal is sent to the switch control and switch S3 is opened making the pacemaker work in asynchronous mode. 27. Thus the function of Reed Switch is replaced by Digital Switch. As frequencies are chosen not in the range of Radio Frequencies used in MRI or Environmental Radio Frequencies, there is no possibility for triggering the pacemaker to work in synchronous or asynchronous mode by accident.

28. The leads used in the pacemaker will act as an antenna picking up Radio Frequencies. This problem can be rectified by using an LC Low pass filter inside the pacemaker housing at the terminal end of the lead connected to the pacemaker circuit. The cutoff frequency of the filter should be the frequency of the depolarizing potential of the heart. Therefore the depolarizing signal from the heart will be allowed and RF used in MRI is attenuated protecting the pacemaker circuit from RF Interferences.

Example 1

If the heart doesn’t depolarize when MR scanning is carried out and the Magnetic Field strength is enough to cause electromagnetic interferences.

The block diagram for the checking of the Sensing and Pacing unit is shown in (Fig 2). The paramagnetic Tantalum will allow the magnetic field to pass through it. The diamagnetic Carbon Fibre will effectively block the magnetic field. Carbon fibre has zero magnetic torsional force so it is not affected by strong magnetic fields. But due to the nonideality of the paramagnetic and diamagnetic materials, there is a possibility for a small amount of Magnetic field to pass through the pacemaker housing. The Hall switch will measure the magnetic field and produce an output voltage. If the voltage exceeds the desired level DC comparator will turn the logic level to Low turning on the Switch Control. Switch Control opens all the switches. As S2 is open the reference voltage equivalent to the depolarizing potential of the heart is not given to the sensing unit. The sensing element will give a High signal to the pacing unit to pace the heart, which is sampled by the Logic comparator. The logic comparator will compare the voltage sampled and the output of the 5 V DC convertor, producing a Low logic which is latched to the Sensing Element Checker. S2 is closed and the reference voltage is given to the sensing unit and the sensing unit sends a Low signal to the Pacing unit instructing not to pace the heart, which is sampled by the Logic comparator. The logic comparator will compare the voltage sampled and the output of the 5 V DC convertor producing a Low logic which is latched to the Sensing Element Checker. Logic 0’s are latched to the Sensing Element Checker indicating the proper functioning of the Sensing unit. If the sensing unit functions properly, S2 is opened, S5 is closed and test resistance is measured by the impedance calculator. S6 and S7 are closed. The current generator will produce current according to the test resistance and Voref. Switch S4 is open and the pacing unit won’t pace producing zero depolarizing current. The current comparator will compare the current from the pacing element and current generator producing a logic High output which is latched to the pacing element checker. Switch S4 is closed and the pacing unit will pace, producing depolarizing current. The current comparator will compare the current from the pacing element and current generator producing a logic Low output which is latched to the pacing element checker. Logic 0 and logic 1 are latched to the Pacing Element Checker indicating the proper functioning of the pacemaker. If the Sensing Element and the Pacing Element functions properly, Switch control closes switches SI, S3, S8 and opens the other switch making it work like a normal pacemaker measuring the electrical activity of the heart and pacing when needed. If the sensing and pacing element check fails the checking process will be carried out again.

Example 2

If the heart doesn’t depolarize when MR scanning is carried out and the Magnetic Field strength is not enough to cause electromagnetic interferences.

The block diagram for the checking of the Sensing and Pacing unit is shown in (Fig 2). The paramagnetic Tantalum will allow the magnetic field to pass through it. The diamagnetic Carbon Fibre will effectively block the magnetic field. Carbon fibre has zero magnetic torsional force so it is not affected by strong magnetic fields. But due to the non-ideality of the paramagnetic and diamagnetic materials, there is a possibility for a small amount of Magnetic field to pass through the pacemaker housing. The Hall switch will measure the magnetic field and produce an output voltage. As the magnetic strength is not strong enough to cause electromagnetic interference, the output voltage of the Hall Switch doesn’t exceed the desired level, DC comparator will turn the logic level to High turning off the Switch Control, enabling the pacing unit as one of the inputs to the OR gate is high from the AND Gate. The switches SI, S3, S8 are closed. Thus the setup works as a normal pacemaker measuring the electrical activity of the heart and pacing when needed.

Claims

1. The device to make Cardiac Pacemaker MRI safe comprises: a) External housing to protect the components from magnetic interference. b) Intelligence circuit to protect the patient from false triggers by checking the sensing and pacing element of the pacemaker when the magnetic field inside the housing is strong enough to disrupt the normal functioning of the pacemaker circuit. c) Circuitry to control the synchronous and asynchronous mode selection using RF signals. d) Filters to protect the pacemaker circuitry from RF Interference. e) It has a Lithium Iodine Polyvinylpyride battery source with output voltage of 2.8V.

2. The device as claimed in claim la, comprises a pacemaker housing with two layers of biocompatible materials. a) The Outer layer of paramagnetic material, Tantalum with a thickness of 3mm. b) The Inner layer of diamagnetic material, Carbon Fibre with a thickness of 1cm. c) The layers are held together with biocompatible epoxy resin.

3. The device as claimed in claim la, wherein the carbon fibre act as an antenna a) Outer layer of the housing, Tantalum will have a small circular aperture or cavity of radius 1mm, exposing the second layer of housing, Carbon fibre. b) The inner side of the housing has a carbon fibre projection of length 1mm connected to the circuitry of the pacemaker. c) The external housing of the pacemaker as herein described in the description clearly and accompanied by clear Cross-Sectional view, Top view and cross-sectional side view of the pacemaker housing in figures 1, 2, 3.

4. The device as claimed in claim lb, has an intelligence circuit made of Hall Switch, Logic

Gates, Sensing and Pacing element Checker and Comparators. a) Hall switch is chosen to preferably produce a maximum output voltage of 5 V for the magnetic field of 3T. b) Logic gates, DC Comparators, Logic Comparators are claimed to be operated at 5 V DC. c) The Sensing and Pacing element checkers have 2 latches in it to store 2 bits temporarily. d) The intelligence circuit as herein described in the description and substantiated with accompanying examples and detailed block diagram in figure 4.

5. The device as claimed in claim lb, wherein the Test Resistance Rtest used has an impedance of 250ohms and V₀ref = 5V, the current generator produces output current pulse of 20mA and the current comparator produces a logic output of 0V and 5V comparing the current inputs.

6. The device as claimed in claim lb, wherein the switch control uses a microcontroller with a clock frequency of 5Mhz.

7. The device as claimed in claim lc, wherein a) The modulator will frequency modulate the input signal to 455 kHz and is transmitted by FM Transmitter. b) Carbon Fibre exposed in the aperture of Tantalum will act as an antenna for signal reception. c) The FM receiver is tuned to receive 455kHz frequency without information loss. d) Bandpass Filter used is tuned to have a narrow passband of bandwidth 1kHz from 455kHz to 456kHz. e) The decoder will decode the input signal to a lbit logic output. Logic 1 - Synchronous mode and Logic 0 - Asynchronous mode controlling the switch control. f) The circuitry for mode selection as herein described in the description and substantiated with a detailed block diagram in figure 5.

8. The projection as claimed in claim 4b, will conduct the Radio Frequency signal of frequency 455kHz received by the Carbon Fibre Antenna of the Housing to the Band Pass Filter.

9. The device as claimed in claim Id, wherein has a Low Pass Filter with corner frequency equal to the frequency of the depolarizing potential of the heart. The Filters as claimed in claim 11 and claim 9d, are LC Filters.

10. The filter for filtering RF signals from MRI to protect the internal pacemaker circuitry as herein described in the description and substantiated with a detailed block diagram in figure 6.