Erkki Vihriälä

Human Heart Pulse Wave Responses Measured Simultaneously at Several Sensor Placements by Two MR-Compatible Fibre Optic Methods

This paper presents experimental measurements conducted using two noninvasive fibre optic methods for detecting heart pulse waves in the human body. Both methods can be used in conjunction with magnetic resonance imaging (MRI). For comparison, the paper also performs an MRI-compatible electrocardiogram (ECG) measurement. By the simultaneous use of different measurement methods, the propagation of pressure waves generated by each heart pulse can be sensed extensively in different areas of the human body and at different depths, for example, on the chest and forehead and at the fingertip. An accurate determination of a pulse wave allows calculating the pulse transit time (PTT) of a particular heart pulse in different parts of the human body. This result can then be used to estimate the pulse wave velocity of blood flow in different places. Both measurement methods are realized using magnetic resonance-compatible fibres, which makes the methods applicable to the MRI environment. One of the developed sensors is an extraordinary accelerometer sensor, while the other one is a more common sensor based on photoplethysmography. All measurements, involving several test patients, were performed both inside and outside an MRI room. Measurements inside the MRI room were conducted using a 3-Tesla strength closed MRI scanner in the Department of Diagnostic Radiology at the Oulu University Hospital.

MEMS Technology Sensors as a More Advantageous Technique for Measuring Foot Plantar Pressure and Balance in Humans

Locomotor activities are part and parcel of daily human life. During walking or running, feet are subjected to high plantar pressure, leading sometimes to limb problems, pain, or foot ulceration. A current objective in foot plantar pressure measurements is developing sensors that are small in size, lightweight, and energy efficient, while enabling high mobility, particularly for wearable applications. Moreover, improvements in spatial resolution, accuracy, and sensitivity are of interest. Sensors with improved sensing techniques can be applied to a variety of research problems: diagnosing limb problems, footwear design, or injury prevention. This paper reviews commercially available sensors used in foot plantar pressure measurements and proposes the utilization of pressure sensors based on the MEMS (microelectromechanical systems) technique. Pressure sensors based on this technique have the capacity to measure pressure with high accuracy and linearity up to high pressure levels. Moreover, being small in size, they are highly suitable for this type of measurement. We present two MEMS sensor models and study their suitability for the intended purpose by performing several experiments. Preliminary results indicate that the sensors are indeed suitable for measuring foot plantar pressure. Importantly, by measuring pressure continuously, they can also be utilized for body balance measurements.

Case study of ECG signal used as a reference signal in optical pulse transit time measurement of blood flow - The effect of different electrode placements on pulse transit time

The electrocardiography (ECG) signal is often used as a reference signal when calculating pulse transit times (PTT) measured by photoplethysmographic (PPG) sensors. In addition, ECG measurements are widely used in clinical health monitoring. In clinical measurements, small changes in the time delays of R waves in relation to blood flow pulsations between each ECG measurement are not relevant. In most cases, they would not even be observed, due to the rather low sampling rates used in clinical ECG devices. However, in PTT measurements, where time delays are measured with an accuracy of milliseconds, the placement of ECG electrodes can have a distinct effect on the results. This paper presents case studies of ECG signals measured simultaneously and independently by two ECG devices. We explore what effect different placements of ECG electrodes have on the R wave of the QRS complex and how it should be taken into account when used as a reference signal in pulse transit time measurements of blood flow. Additionally, we study what kind of ECG electrode placements are most suitable for PTT measurements.

Cardiovascular effects of mannitol infusion: a comparison study performed on mouse and human

Monitoring blood-brain barrier (BBB) opening is of great interest in terms of brain drug delivery in the treatment of brain lymphoma and maybe in the future in other diseases like dementia. A method involving BBB disruption (BBBD) by mannitol infusion has been developed in University of Portland, USA, and then exploited in Oulu University Hospital in treatment of primary CSN lymphoma. Proper opening of the BBB is crucial for the treatment, yet there are no methods available for its real-time clinical monitoring. Recently, we presented a combined method using direct-current electroencephalography (DC-EEG) and near-infrared spectroscopy (NIRS) for monitoring BBBD in human. Carotid artery mannitol infusion generated a strongly lateralized DC-EEG response and in NIRS a prolonged increase in the oxy/deoxyhemoglobin ratio. This study explores further BBBD, by focusing on monitoring its cardiovascular effects, when measured in human and mouse. For this, we used photoplethysmography (PPG) and opto-electro-mechanical sensors to gather the signals in human and mouse. Mannitol infusion in human causes strong fluctuations in blood pressure, heart rate and PPG signals, and here we discuss how the acquired signals in mouse model compares to human data. In addition, we present our scale-free monitoring concept that enables monitoring physiological signals similarly when performing experiments in mouse and human neuroimaging setups. By combining microscopic and macroscopic imaging in mouse setup enables us to study correlations between mechanistic cellular data and clinical functional data. Further, this allows us to validate and optimize macroscopic sensing and imaging techniques aimed to be used in human imaging.

Continuous blood pressure recordings simultaneously with functional brain imaging: studies of the glymphatic system

The lymph system is responsible for cleaning the tissues of metabolic waste products, soluble proteins and other harmful fluids etc. Lymph flow in the body is driven by body movements and muscle contractions. Moreover, it is indirectly dependent on the cardiovascular system, where the heart beat and blood pressure maintain force of pressure in lymphatic channels. Over the last few years, studies revealed that the brain contains the so-called glymphatic system, which is the counterpart of the systemic lymphatic system in the brain. Similarly, the flow in the glymphatic system is assumed to be mostly driven by physiological pulsations such as cardiovascular pulses. Thus, continuous measurement of blood pressure and heart function simultaneously with functional brain imaging is of great interest, particularly in studies of the glymphatic system. We present our MRI compatible optics based sensing system for continuous blood pressure measurement and show our current results on the effects of blood pressure variations on cerebral brain dynamics, with a focus on the glymphatic system. Blood pressure was measured simultaneously with near-infrared spectroscopy (NIRS) combined with an ultrafast functional brain imaging (fMRI) sequence magnetic resonance encephalography (MREG, 3D brain 10 Hz sampling rate).

Prototype of an opto-capacitive probe for non-invasive sensing cerebrospinal fluid circulation

In brain studies, the function of the cerebrospinal fluid (CSF) awakes growing interest, particularly related to studies of the glymphatic system in the brain, which is connected with the complex system of lymphatic vessels responsible for cleaning the tissues. The CSF is a clear, colourless liquid including water (H2O) approximately with a concentration of 99 %. In addition, it contains electrolytes, amino acids, glucose, and other small molecules found in plasma. The CSF acts as a cushion behind the skull, providing basic mechanical as well as immunological protection to the brain. Disturbances of the CSF circulation have been linked to several brain related medical disorders, such as dementia. Our goal is to develop an in vivo method for the non-invasive measurement of cerebral blood flow and CSF circulation by exploiting optical and capacitive sensing techniques simultaneously. We introduce a prototype of a wearable probe that is aimed to be used for long-term brain monitoring purposes, especially focusing on studies of the glymphatic system. In this method, changes in cerebral blood flow, particularly oxy- and deoxyhaemoglobin, are measured simultaneously and analysed with the response gathered by the capacitive sensor in order to distinct the dynamics of the CSF circulation behind the skull. Presented prototype probe is tested by measuring liquid flows inside phantoms mimicking the CSF circulation.

RELATIONSHIP BETWEEN WEIGHT CHANGE AND CHANGES IN 3D ACCELERATION SIGNALS GENERATED BY WALKING

This study set out to investigate if a relationship exists between weight change and changes in 3D acceleration signals associated with walking. In addition to giving biomechanical information, this relationship could be applied in conjunction with new weight management solutions to address the excess weight problem currently plaguing the world. The study was conducted with 15 subjects. For a period of two months, they were weighed every morning and carried a 3D accelerometer during the working day. Daily accelerometric signals were recorded and signals recognized as walking were analyzed. To obtain information in a more controlled situation and higher weight change, a separate follow-up study was carried out involving one test subject performing controlled walking exercises. Our results show that a relationship does exist between weight change and 3D acceleration signals. The obtained correlation coefficient between weight change and the acceleration-related parameter was 0.21 for the combined result of all test subjects (n = 147, p = 0.01). Higher correlations were recorded for individual subjects (r = 0.97, p < 0.001). Also the follow-up with controlled walking exercises showed a high correlation (r = 0.89, p < 0.001). On the other hand, statistically significant results were not obtained for all subjects, and identical signal parameters did not always produce similar results.