Methods of Blood Flow Measurement
2.6 Research Aims and Methodology
The aim of this research is to design and develop a non-invasive, EM induction technique that can be used for measuring the total blood flow rate in blood vessels within a cross-sectional area of an upper or lower limb. The limb is to be inserted into a circular multi-electrode array across which a uniform magnetic field is applied. Flow induced potentials measured by the electrodes are then related to the total volumetric flow rate due to the blood flow in a single blood vessel or in multiple blood vessels. The
technique is intended to overcome the previously encountered problems noted in the literature for EM flow meters such as sensitivity to asymmetric velocity profiles at the measurement site and the need for frequent calibration. The development of a robust mathematical model for this method will also allow the device, built using this method, to be calibrated offline (‘dry’ calibration).
This device should also be low cost due to its simple design and thus, it could be available in health centres and clinics to be used by General Practitioners, nurses, physiotherapists and podiatrists in various applications including the diagnosis of PAD and DVT. The methodologies that will be used to achieve the objectives are listed below.
1. To develop a mathematical model using the “virtual current” theory for a cross-sectional area with multiple flow channels (multiple blood vessels) bounded by a multi-electrode system and across which a uniform magnetic field is applied. This model will allow the measurement of the total instantaneous volumetric blood flow rate in all major blood vessels at the position of the electrode array. This total flow rate measurement will be independent of the size and location of the flow channels within the cross-sectional area bounded by the electrode array.
2. To develop a finite element (FE) model to simulate a cross-sectional area with multiple flow channels. The FE model will consist of an electromagnet to generate a uniform magnetic field across a geometric model which will consist of one or multiple tubes, simulating blood vessels. From the FE model, flow induced potential differences will be obtained. These potential differences will be compared with the those obtained from the mathematical model developed in 1. The aim of this work is to validate the developed mathematical model.
3. To design a practical experiment in order to validate the mathematical model developed in 1. This work comprises the design of a physical pipe model (mechanical system) that has multiple flow channel and an electromagnet – with its power supply – to generate a uniform magnetic field. Signal conditioning and processing systems will be designed to allow measurement of low level and noisy flow induced potential differences.
4. To perform flow tests on the practical SVS model and obtain flow induced potential differences related to water flow imposed in the channels within the SVS. The flow rate will be varied in the channels and their location will be changed with respect to the magnetic field direction. The flow induced potential differences obtained from the practical experiment will be quantitatively compared with the flow potential differences obtained from the mathematical model developed in 1 above.
2.7 Summary
MRI and X-ray devices were discussed in terms of their operation, clinical applications and drawbacks. MRI and X-ray-based devices are advanced screening instruments that are usually used for advanced diagnostic tests and not for routine check-ups. They are not available in many health centres and hospitals as they require a large space for installation, they are expensive to buy and operate, and must be operated by experienced radiologists for accurate diagnosis. The injected dye in MRI and X-ray devices may cause bad reactions in patients. Additionally, MRI may cause discomfort for patients as it is performed in an enclosed space. Impedance plethysmography has been used in the past for cardiac output and peripheral blood flow measurements. However, currently it has more limited application due to its poor accuracy compared with other medical devices such as duplex ultrasound and nowadays it is often only used for DVT diagnosis. Clinical studies described in Section 2.3.3 showed that IPG can have an accuracy of 58% which is significantly poor and may lead to false diagnosis.
Currently for the diagnosis of PAD and DVT diseases, the most common device used is the duplex ultrasound. Ultrasound waves are any vibrations generated with a frequency above the human hearing range, i.e. 20 kHz. The Doppler method is used for image reconstruction of targets and also measurement of blood velocity if required. Despite its acceptance by health practitioners for use in clinical applications, the duplex ultrasound has its drawbacks and limitations. From a duplex ultrasound scan, the peak velocity measurement is the most relied upon for estimating the severity of stenosis. This measurement can be affected by errors that are caused by the technologist operating the Doppler device, the device itself or the patient. Errors arising from the technologist can be due to the angle of insonation, improper placement and size of sample volume, pulse repetition frequency, gain setting and interobserver variability. In a stenosed artery, it
was found that an error of up to 28% can be caused by the technologist. Errors from the Doppler device itself, including the sample volume shape, the system noise and the assumption that the sound velocity is constant in all tissues, can cause an error of up to 8%. Hence, all the error sources noted above can limit the accuracy of duplex ultrasound and may lead to false diagnosis. Moreover, dialysis patients are more likely to suffer from plaque calcification. Such a condition prevents ultrasound beams from penetrating the wall of the blood vessel to allow the velocity measurement to be taken.
Thus, the duplex ultrasound device is not the ideal device for PAD or DVT diagnosis in dialysis patients.
EM flow meters were successfully used invasively for blood flow measurements in different blood vessels. The EM flow metering method is attractive because it is a linear technique, insensitive to viscosity, density, temperature and pressure loss, and is unaffected by the velocity profile given that the velocity distribution is axisymmetric.
There are three types of invasive EM blood flow meters; cannula flow meters, perivascular and intravascular probes. Cannula flow meters are inserted between two ends of a blood vessel that has been cut out, and it is the least favourable method as it may cause health complications in patients. Perivascular probes are cuff- or clip-type probes that are mounted around a blood vessel and are considered the ‘gold standard’
for invasive blood flow rate measurement. Intravascular devices are catheter-type probes that are inserted via the skin, and then into the desired blood vessel.
A review of previous and current methods in the use of invasive EM blood flow meters provided an insight into the problems encountered during the design and operation of EM flow meters for blood flow rate measurement. It was found that, in all designs of blood flow meters noted in the literature, a conventional-type EM flow metering method
electrodes are sensitive to asymmetric velocity profiles. Such sensitivity can lead to an error of 100% in the measured flow induced emf. Thus, it is important to design EM flow meters that are independent of velocity profile.
It was found that the velocity profile in the circulatory system of mammals is non-axisymmetric. Moreover, cannula flow meters are short-end devices, and Bevir observed that in such structures, the velocity profile cannot be ensured to be axisymmetric. The haematocrit level in the blood was also found to affect EM flow meters in a number of ways. A higher level of haematocrit level increases the source impedance and therefore, the front-end of the measurement system must have high input impedance to avoid any signal loss. Moreover, an increase in haematocrit level increases the viscosity, and that distorts the velocity profile and, as a result, will affect the accuracy of the conventional-type EM flow meter. Other problems were noted in perivascular and catheter-tipped probes including heat dissipation and improper placement of the probe around or in the vessel. Nevertheless, perivascular and catheter-tip probes were used intensively by several authors in clinical applications, and a high accuracy of measurement was achieved.
The most notable attempt to develop a non-invasive EM flow meter for peripheral blood flow measurement was by Boccalon et al. However, there are several issues associated with the flow meter method that was designed. Firstly, the design was based on a conventional-type EM flow meter which has been found to be sensitive to asymmetric velocity profile resulting from blood velocity in vessels positioned in different locations in the measurement region and also due to the asymmetry of blood flow in the blood vessels themselves. Moreover, an analytical model was required for each region of the human body to obtain its calibration factor which seems to be cumbersome and
impractical. There was no example in the literature of a non-invasive EM flow meter that is insensitive to velocity profile.
According to the literature, there are three methods that can significantly reduce the effect of velocity profile: (1) use of relatively larger electrodes, (2) optimisation of the magnetic field distribution by enhancing coil design, and (3) use of a multi-electrode system. Based on the work by Engl, Horner developed multi-electrode EM flow meters and showed that, for a 16-electrode system, the error in the velocity measurement for distorted (asymmetric) velocity profile was less than 10%. Recent research work in the Systems Engineering group of the University of Huddersfield achieved a tomographic EM flow metering technique to determine the axial velocity within a flow cross-sectional area bounded by 16 electrodes. It is possible to adapt this research progress to develop an EM flow metering method that can be used for non-invasive blood flow rate measurements. Additionally, the mathematical modelling techniques developed by Shercliff and Bevir, i.e. the weight vector and virtual current density, are powerful techniques that can be used to model, calibrate and predict flow contribution in various regions within the flow cross-section to the output signal in EM flow meters.
Following the literature review, the research aims and methodology have been described and are given in the previous section. The research work started with developing the mathematical model for the non-invasive EM flow metering method (Chapter 3). This mathematical model was then validated using computer simulation and this is described in Chapter 4. The remaining chapters cover the design and execution of the practical experiment which aimed at validating the mathematical model in practice. The practical experiment also identified the main subsystems that are required to build a medical device based on the proposed EM induction method.