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Fibre optic sensor for non-invasive monitoringof blood pressure during MRI scanning

Teemu S. Myllyla *; 1, Ahmed Abou Elseoud 2, Hannu S. S. Sorvoja 1, Risto A. Myllyla 1,Juha M. Harja 1, Juha Nikkinen 2, Osmo Tervonen 2, and Vesa Kiviniemi 2

1 University of Oulu, Department of Electrical and Information Engineering, Optoelectronics and Measurement Techniques Laboratory,P.O. Box, 4500 University of Oulu Oulu 90014, Finland

2 Oulu University Hospital, Department of Diagnostic Radiology, Oulu, Finland

Received 23 December 2009, revised 24 March 2010, accepted 24 March 2010Published online 16 April 2010

Key words: Non-invasive blood pressure measurement, fibre optical accelerometer, magnetic resonance imaging, pulsewave velocity, MR compatibility

Æ Supporting information for this article is available free of charge under http://dx.doi.org/10.1002/jbio.200900105

1. Introduction

Being non-invasive and requiring no radiation ex-posure, functional magnetic resonance imaging(fMRI) has become the most widely used tool fordetecting functional brain activity. However, fMRIsignals that reflect neuronal brain activity are rela-tively weak, producing only a variation of 2–5% inthe amplitude of the measured NMR signal inten-sity. Detecting such small variations requires the ab-

sence of external interfering signals. As the de-tected signals are weak relative to the noise presentin scanner images, careful attention must be paidon the comfortability, motionlessness and flexibilityof the used physiological co-registration instru-ments. Since the measurement of the so-calledblood oxygen level dependent (BOLD) signals re-quires repetitive MRI scans covering the wholebrain, measurement times may vary from a fewminutes to hours.

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This report focuses on designing and implementing anon-invasive blood pressure (NIBP) measuring devicecapable of being used during magnetic resonance ima-ging (MRI). Based on measuring pulse wave velocity inarterial blood, the device uses the obtained result to es-timate diastolic blood pressure. Pulse transit times aremeasured by two fibre optical accelerometers placedover the chest and carotid artery. The fabricated acceler-ometer contains two static fibres and a cantilever beam,whose free end is angled at 90 degrees to act as a re-flecting surface. Optical fibres are used for both illu-minating the surface and receiving the reflected light.When acceleration is applied to the sensor, it causes adeflection in the beam, thereby changing the amount of

reflected light. The sensor’s output voltage is propor-tional to the intensity of the reflected light. Tests con-ducted on the electronics and sensors inside an MRIroom during scanning proved that the device is MR-compatible. No artifacts or distortions were detected.

Optical fibres for both illuminating the cantilever surfaceand receiving the reflected light

* Corresponding author: e-mail: teemu.myllyla@ee.oulu.fi, Phone: +358 8 553 2762, Fax: +358 8 553 2774

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Most functional imaging signals are related toBOLD signals, which have had a huge impact onour knowledge of brain functions. Neural activity inthe cortex produces alterations in the brain’s bloodoxygenation level such that capillaries over acti-vated areas become hyperoxygenated relative tothe baseline. At present, the most rapidly increasingarea within BOLD research is the detection ofspontaneous fluctuations in BOLD signals. Thesefluctuations are also related to spontaneous vaso-motor waves, because the precapillary vasomotortone governs the perfusion of cerebral tissue. Un-derstanding the effects of vasomotor tone fluctua-tions on BOLD signal fluctuations is therefore ofvital importance. Nonetheless, non-invasive toolshave not been applied to assessing vasomotorwaves in humans during fMRI scanning prior to thisstudy.

The scanner rooms of magnetic resonance ima-ging (MRI) suites present special challenges tophysiological measurements, including blood pres-sure measurements of the imaged subject. SinceMRI scanning is based on detecting weak NMRresonance signals emitted from the imaged tissue,the room must be a tightly shielded Faraday cagepreventing electromagnetic interference, such asRF signals, from outside the room. Moreover, theelectronic parts of the device have to be well-shielded and placed at a certain distance from theMRI scanner to prevent imaging artifacts such asdistortions and phase/frequency direction artifacts.All materials in the scanning volume must be MRcompatible to ensure undisturbed MR imaging.Being immune to electromagnetic fields, optical fi-bres are ideally suited for use in this environment.Thus, the application of fibre optics offers a practi-cal approach to non-invasive blood pressure meas-urements.

The aim of this project was to produce a methodfor the non-invasive measurement of vasomotor tonefluctuations during fMRI scanning without disturb-ing the patient or the scanning procedure.

2. Magnetic resonance compatibility

Instruments used near or inside the scanning volumemust not be affected by the scanner’s magnetic fieldand all selected materials need to be MR compati-ble. Hence, metals like aluminium, copper, brass andtitanium are allowed, while any ferromagnetic mate-rials, such as steel, are forbidden, and this must beconsidered when designing and selecting sensors,electronics, connectors and enclosures. Many con-nectors, for example, are available in coated brass.Of course, the pins of integrated circuits include fer-romagnetic material, which cannot be avoided [1, 2].

Fortunately, by using fibre optics, electronics, whichare often magnetic incompatible, may be placedfurther from the scanner or even outside the MRIroom.

MR compatibility also includes electrical compat-ibility. Thus, to maintain the accuracy of imaging, in-strumentation used in physiological measurementsmust not produce electromagnetic interference. Onthe other hand, the radio frequency pulses of thescanner used to excite tissue spins for NMR/reso-nance are 1� 106 stronger than the measured signals.Producing these excitation RF pulses requires strongelectrical pulses that may disturb the equipment in-side the MRI room as well. Electromagnetic interfer-ence near the scanner appears typically in a widefrequency range [3], from �0 Hz to 1.5 GHz. Thus,electromagnetic shielding of electronic devices is acrucial factor.

When the developed non-invasive blood pressure(NIBP) device was to be placed in the MRI room,electromagnetic shielding of its electronic compo-nents had to be considered carefully to prevent thedevice from affecting MR imaging and vice versa.Therefore, all electronics were placed in an alumi-nium box, serving as a Faraday cage with the MRcompatible optical fibres extending outside the boxas shown in Figure 1. In addition, to collect the meas-ured data from the NIBP device, a USB interfacewas required. The device’s supply voltage was regu-lated from the USB bus power. As a result, the USBconnector was placed on the opposite side of thebox. Finally, a well-shielded USB cable was used toconnect the NIBP device to a laptop computerplaced outside the MRI room.

Figure 1 Shielded NIBP device: an aluminium box, servingas a Faraday cage with the MR compatible optical fibresextending outside the box. A USB connector was placedon the opposite side of the box for voltage supply and datatransfer.

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3. Measurement method and device

3.1 Optical blood pressure measurementmethods

At each heartbeat, blood is forced through the per-ipheral vessels, constantly changing their diameter.This corresponds to tiny movements of the skin,especially near the arteries [4]. Fibre optics can beused to sense both pressure and skin vibrations. Of-ten, optical blood pressure measurement methods in-volve fibre optics and a light source/detector paireither in contact with or in close proximity to theskin surface.

Most non-invasive blood pressure measuringmethods are based on using a cuff which, after infla-tion, will detect pressure variations during cuff defla-tion. Blood pressure can afterwards be determinedon the basis of the cuff oscillation amplitude. Thisoscillometric method can easily be made MR compa-tible by using fibre optical pressure sensors insidethe cuff. However, the method gives only one diasto-lic and systolic blood pressure value for each meas-urement, and the measurement time is about oneminute. In addition, frequent cuff inflation may pro-duce pain and contribute to the patients’ stress levelwhich, in turn, affects their blood pressure.

Another widely used optical blood pressure meth-od, invented in 1973 by Penaz, is the volume clampmethod, which employs a LED and photodiode pairattached on the finger tip. In this method, opticalcomponents are placed inside the cuff, which is trans-parent on the side against the skin. This photo-plethysmograph method is based on the fact that cuffpressure changes in step with arterial pressure, as theartery tries to maintain a constant blood flow in thefinger. Commercially known as FinapresTM, the meth-od requires a pressure pump near the cuff, but alsothe LED and photodiode need electronics, whichmay interfere with MR images. The problem may besolved by using fibre bundles and by moving the elec-tronics further from the imaging area. Nonetheless,the cuff will produce pain in constant use [5].

Making frequent measurements calls for a cuff-less, continuous type of blood pressure method, giv-ing diastolic and systolic values for each heartbeat.A recently developed and widely used method isbased on measuring the blood pressure pulse delaybetween two measurement points on the arterialtree. This may be done by using photoplethysmo-graph sensors (a LED and a photodiode) or otheroptical sensors, such as a laser diode with a monitor-ing diode, which may detect skin vibrations near/onthe artery, when the laser is tuned to the self-mixingmode. Because this method has been successfullyused in fibre optic measurements, it may be MRcompatible [6–9].

3.2 Pulse wave velocity

The designed device is based on detecting the pulsewave velocity (PWV) of blood pressure. PWV is de-termined by two sensors, placed over the chest andcarotid artery, measuring the transmit time t betweenthe starting time of a pressure pulse (opening of theaortic valve) and the ensuing diameter change of thecarotid. PWV can be calculated, by using Formula(1), once the distance s between the sensors has beendetermined.

PWV ¼ s

tð1Þ

Since PWV depends on the pressure in the aorta(the higher the pressure, the greater the velocity),diastolic pressure in the aorta can be determined byPWV. Moens (1878) and Korteweg (1878) derived amathematical expression for the velocity of the pulsefront travelling along an artery, treating it as a func-tion of the elasticity coefficient, the thickness of thearterial wall and the end-diastolic diameter of thevessel lumen. This is shown in Formula (2), where his the thickness of the vessel wall, d the diameter ofthe vessel, r the density of blood and E stands forYoung’s modulus describing the elasticity of the ar-terial wall [10, 11].

PWV ¼ffiffiffiffiffiffiffiffiffiffih � Ed � r

sð2Þ

Young’s modulus of the arterial wall E is not con-stant, but depends on the blood pressure P insidethe vessel. The relationship between the elasticitycoefficient of the vessel and the pressure inside thevessel is shown in Formula (3), where E is Young’smodulus (elasticity), E0 is the zero pressure modu-lus, z is a constant that depends on the vessel(1,20� 104 N/m2–1,35� 104 N/m2) and e is the mathe-matical constant 2.71828.

E ¼ E0 � ez �P ð3Þ

By combining Formulas (2) and (3), blood pressurecan be expressed as Formula (4)

P ¼ 1z� ln d � r � ðPWVÞ2

h � E0

!ð4Þ

and Formula (4) can be expressed as

P ¼ 1z� ln ððPWVÞ2Þ þ 1

z� ln d � r

h � E0

� �

¼ k1 � ln ððPWVÞ2Þ þ k2 ð5Þ

If two blood pressures, P1 and P2, and their relatedpulse wave velocities PWV1 and PWV2 are meas-

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ured, then the factors k1 and k2 can be determinedusing Formulas (6) and (7)

k1 ¼ln ððPWV1Þ2Þ � ln ððPWV2Þ2Þ

P1 � P2ð6Þ

k2 ¼ P� k1 ln ððPWVÞ2Þ ð7ÞTo know these relationships between blood pressureand pulse wave velocity, the device must be cali-brated separately for each patient. Thus, the accu-racy of NIBP measurements depends significantly oncalibration.

This calibration was conducted in two steps just be-fore scanning started with the subject first in a sittingand then in a lying position. A clinical MRI compati-ble anaesthesia monitor (Schiller Maglife C 400 GBaar, Switzerland) was used to measure blood pres-sure from each subject’s right arm with an automatedcuff system. These base measurements were then usedto calibrate the blood pressure values of the NIBP de-vice. The used calibration procedure was based onmethods presented in the referenced papers [12, 13].

3.3 Measurement device

To sense skin movements, the developed device usesfibre optic accelerometer sensors [14, 15]. The sensorhas two static optical fibres –– a light source and a lightsensor. These fibres are attached to a cantilever andlight is aimed at the free end of the cantilever, which isangled at 90 degrees to act as a reflecting surface, Fig-ure 2. When the sensor is attached on the skin, the ac-celeration, as a consequence of heartbeat/skin move-ment, bends the cantilever causing deflection of thebeam, whereupon the amount of reflected lightchanges. The sensor output voltage is proportional tothe intensity of the reflected light. As no mirrors or

prisms are required, the structure of the sensor is sim-ple and affordable. Although the response is not line-ar, it is more important for this application to measurethe instant moment of the heartbeat.

The sensor platform, shown in Figure 3, is madeof PolyOxyMethylene plastic and the cantilever is

Figure 2 (online color at: www.biophotonics-journal.org)Optical fibres and the cantilever.

Figure 3 (online color at: www.biophotonics-journal.org)Fabricated accelerometer. Two fibres, coming from the left,are attached to a cantilever, which is placed in the middleof the platform box measuring 41 mm � 28 mm � 15 mm.The size of the cantilever is 18 mm � 5 mm � 0.05 mm. Theright side end of the cantilever is angled downwards at 90degrees and the size of the reflecting area is 5 mm � 4 mm.

Figure 4 Impulse response and the resonance frequency ofthe fabricated accelerometer. A resonance frequency of58 Hz and Q-factor of 129 are sufficiently high for themeasurement application.

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made of brass foil, thickness 0.05 mm. Plastic opticalfibres between the sensor and electronics are madeof poly methyl methacrylate (PMMA). All these ma-terials are MR compatible.

The sensor measures acceleration as a conse-quence of heartbeat/skin movement, and these accel-erations have a bandwidth between 10 Hz and 30 Hz.Since the resonance frequency of the accelerometermust be well above this bandwidth, the resonance fre-quency of the fabricated accelerometer was examined.Figure 4 shows the measured impulse response andthe corresponding resonance frequency of the acceler-ometer.

Sensor electronics for the NIBP device includethree identical transfer/receiver modules. Figure 5shows a flow chart of one module. Using standardmethods, received signals are first amplified and fil-tered, before the received analog data is collectedon a National Instruments USB-6009 data acquisi-tion card. Finally, the data are processed and ana-lysed using a LabView program.

3.4 Accuracy of the NIBP device

The PWV blood pressure measurement method isbased on measurements of short time differences.Thus, it is important that the response curve of eachheartbeat from both accelerometers is similar and

that the delays are identical. The accuracy of the fab-ricated accelerometer was compared to a commer-cial accelerometer, VTI SCA620-CF8H1A, whichhas an operating range of �1.7 g and a sensitivity of1.2 V/g at the operating voltage of 5 V. In the com-parison test, both accelerometers were attached onthe patients’ chest close to one another. As shownon the upper graph in Figure 6, the measured volt-age/time response curves for heartbeats were quitesimilar. Additionally, a comparison test was con-ducted between two fabricated accelerometers. Alsotheir response curves were almost identical, asshown on the lower graph in Figure 6.

4. Measurements in the MRI room

4.1 Testing place

Testing the MR compatibility of the fabricated NIBPdevice was performed in an MRI scanning operatingroom housing a 1.5 Tesla-strength closed MRI scan-ner (manufactured by GE) in the Dept. of Diagnos-tic Radiology at the Oulu University Hospital.

Figure 5 (online color at: www.biophotonics-journal.org)Block diagram of a transfer/receiver module above andPC board below.

Figure 6 (online color at: www.biophotonics-journal.org)Comparison of voltage/time response curves of heartbeats.

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4.2 Measurement environment

In these measurements, the accelerometer sensorwas placed inside the scanning volume of the MRIscanner, Figure 7. The fibres coming from the sen-sors were connected to the NIBP device, placed atsome distance from the scanner.

A laptop computer was placed outside the MRIroom to collect the measured data from the NIBPdevice via a USB cable, which was brought througha waveguide on the RF shielded wall, Figure 8.

5. Results

5.1 Effects on MR imaging

Four independent scanning measurements were con-ducted. In the first measurement, a reference imagewas taken of a round CuSO4 calibration phantomwith an eight-channel parallel imaging coil, which is

normally placed around the patient’s head. This cali-bration phantom is represented by the evenly andsymmetrically white area in Figure 9, on the left,without the sensor inside the coil. In the secondmeasurement, the fibre optic accelerometer sensorwas placed inside the parallel imaging coil, as shownin Figure 7. Because the sensor contains some metal,it disturbed the MR image, as indicated by Figure 9,on the right. However, when conducting real mea-surements on living patients, the sensor would beplaced inside the gradient coil.

The third and fourth scanning measurementswere conducted on a patient who had a fibre opticsensor placed near the carotid artery. The imagingsequence was called T2*-weighted gradient-recalledecho planar imaging sequence and it was used to ex-amine blood oxygen level changes in the brain dur-

Figure 7 (online color at: www.biophotonics-journal.org)Sensors placed inside the MRI scanner. In this picture,scanner calibration is in progress.

Figure 8 (online color at: www.biophotonics-journal.org)Waveguide in the RF shielded wall.

Figure 9 Reference undisturbed image on the left. Dis-turbed image, on the right, with the sensor placed withinthe head coil.

Figure 10 Images (A) and (B) represent BOLD imaging,while image (C) represents 3D anatomy imaging. As seen,the MR images are undisturbed when the sensor is placedappropriately outside the parallel imaging coils.

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ing activation. The fourth measurement was a so-called 3D_FSPGR (3-dimensional fast spoiled gradi-ent echo) imaging sequence, used to image the threedimensional anatomy of the brain. Placed near thecarotid artery, the fibre optic sensor did not disturbMR imaging, see Figure 10. This is mainly because,in contrast to the second measurement, the sensorwas placed outside the coil.

5.2 Effects on NIBP measurement

Figure 11 presents an example of an acceleration sig-nal recorded during MRI scanning by a sensorplaced near a patient’s carotid artery. In this meas-urement, the acceleration signal coming from thesensor is slightly disturbed, as indicated by the extralow acceleration peaks between higher peaks. How-ever, the opening of the aortic valve is clearly visibleas a high peak.

When analyzing the acceleration signals, it wasnoticed that the disturbance occurred during 3DFSPGR imaging sequences, but not during T2* EPIsequence scanning. This indicates that the distur-bance is caused by vibrations of the MRI scanner’sgradient coils. Furthermore, it was clear that the sen-sor electronics were not disturbed during the scan-ning, as verified by acceleration signals from the sen-sor placed on the chest. In actuality, when thissensor was not placed on the chest during MRI meas-urements and only measured light in the MRI room,the signal stayed constant during the entire measure-ment. Therefore, we may conclude that the MRIscanner did not affect the electronics.

5.3 NIBP measuring test

Finally, the MRI compatibility of the NIBP device asa measuring system was tested. The purpose of thesemeasurements was to examine the functionality ofthe device in routine MRI measurements of patients.Fibre optic sensors were attached on the patients’chest and carotid, before conducting the four differ-ent MR measurements described above.

Figure 12 shows one heartbeat, scaled from themeasured acceleration signals of two sensors. Redindicates a sensor signal from the chest, blue fromthe carotid. The circles, marked on the curves, showthe moment of opening of the aortic valve. Noticethe difference in time, reflecting the pulse propaga-tion difference that can be used as a basis for calcu-lating blood pressure according to the formulas andcalibration method presented in Subsection 3.2.

Figure 13 presents an example of two non-inva-sive blood pressure recordings during functionalMRI scanning. The red dots indicate diastolic bloodpressure values of a patient in a resting state, whilethe black boxes show diastolic blood pressure valuesof a patient who has just hyperventilated.

Figure 11 (online color at: www.biophotonics-journal.org)Acceleration signal output of four consecutive cardiac cy-cles of a sensor placed above the carotid artery on a hu-man subject during 3D FSPGR scanning.

Figure 12 (online color at:www.biophotonics-journal.org)One heartbeat, measured by thesensors. Red indicates recordingfrom the chest, blue from thecarotid. Two circles, marked onthe curves, show the moment ofopening of the aortic valve.There is no significant interfer-ence caused by the scanner.

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6. Conclusions

The designed NIBP device utilizes the reflection oflight to detect the pulse wave velocity of blood pres-sure (PWV). A sensor with two static optical fibres ––a light source and a light sensor –– were attached onpatients’ chest and carotid. The transition time ofpressure pulses was determined by measuring theperiod between the starting time of each pressurepulse (opening of the aortic valve) and the ensuingdiameter change of the carotid.

On the basis of our on-site measurements, wemay conclude that the presented method, based onnon-invasive fibre optic sensing, lends itself to deter-mining continuous blood pressure during MRI scan-ning in a magnetic resonance imaging operatingroom. Although it was noticed that acceleration sig-nals measured by the sensors were disturbed duringsome imaging sequences, due to vibrations of theMRI scanner, the opening of the aortic valve wasstill clearly visible. Moreover, the electromagnetic in-terference produced by the MRI scanner did not af-fect the sensor electronics, and the NIBP device didnot affect MR imaging.

Figure 13 (online color at: www.biophotonics-journal.org)Diastolic blood pressure recorded using the NIBP deviceduring functional MRI scanning: The red dots indicate di-astolic blood pressure values during a normal resting-statescan, while the black boxes show blood pressure values im-mediately after a 2-minute hyperventilation task, showinginitial elevation in blood pressure followed by s gradualreturn to the baseline. Calibration was conducted as de-scribed in the Methods Section.

Teemu Myllyla studied atthe University of Oulu,Technical University of Ber-lin and Humboldt Universityof Berlin. He received theM.Sc. degree (2005) in ap-plied electronics from theUniversity of Oulu, wherehe is working in the Opto-electronics and MeasurementTechniques Laboratory. Hisresearch interests includeEMC, wireless sensors and

measuring techniques, especially in the hospital envir-onment.

Ahmed Abou Elseoud re-ceived his M.D. (2006)from the Suez Canal Uni-versity, Egypt. He is cur-rently working toward aPh.D. degree in multimodalneuroimaging at the fMRIresearch unit, Oulu Univer-sity Hospital, Finland. Hisresearch focuses on devel-oping methods to alter thespontaneous activity of thebrain and on using multi-

modal neuroimaging techniques to obtain functionalbiomarkers which will help the diagnosis and treat-ment of various brain disorders.

Hannu Sorvoja received hisMaster’s (1993), Licentiate(1998) and Doctoral (2006)degrees in electrical engi-neering from the Universityof Oulu, Finland. Havingbeen employed as a researchscientist, senior researcher,assistant, senior assistantand professor at the Univer-sity of Oulu, he is presentlyworking as a laboratory

manager in the Department of Electrical and Informa-tion Engineering at the University of Oulu. His re-search interests include different sensors and biomedi-cal measurements.

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Risto Myllyla received hisD.Sc. (Tech) degree fromthe University of Oulu, Fin-land, in 1976. He has beenan associate professor in theDepartment of ElectricalEngineering, University ofOulu, since 1974 and a fullprofessor since 1995. From1988 to 1995, he was a re-search professor in the Tech-nical Research Centre ofFinland. Among his research

interests are industrial and biomedical instrumentationdevelopment, particularly in optical measurements. Re-cently, he has taken an interest in printed intelligence.

Juha Harja received theM.Sc. degree in electricalengineering (2006) and theLic.Sc degree in appliedelectronics (2008) from theUniversity of Oulu, Fin-land. From 2006 to 2009,he worked as a researcherat the Optoelectronics andMeasurement TechniquesLaboratory, University ofOulu. His research focuseson magnetic resonance ima-

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Osmo Tervonen is Professorof Diagnostic radiology atthe University of Oulu andChairman of the Institute ofDiagnostics at the Oulu Uni-versity Hospital, Finland. Healso received his doctoratedegree in medicine in 1989from the University of Oulu.His scientific work has cen-tered on functional MRmethods in medicine.

Vesa Kiviniemi received hisM.D. (1996) and Ph.D.(2005) degrees from theUniversity of Oulu, Finland.He has pioneered in devel-oping methods for detectingspontaneous brain activity incollaboration with BharatBiswal in MCW, Wisconsin,USA. Based at the Univer-sity of Oulu with manycollaborating research units

around the world, he is working on developing diagnos-tic and therapeutic methods related to spontaneousbrain activity fluctuations.

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