HIGH VOLTAGE TECHNIQUES IN BIOMEDICAL ENGINEERING

1.1    INTRODUCTION

High voltage can be defined in many forms for example; it can be defined in terms of the voltage magnitude example;500KV,1000KV etc., its application, example; high voltage transmission, its effect on electrical circuits, the hazards it can cause to the people in contact with it and the environment etc.

Looking at the context of the discussion, we can consider high voltage in two areas; the danger of causing electric shock by contact or proximity and its possibility of causing dielectric breakdown or insulation breakdown.

Biomedical Engineering, BME has and will continue to play a crucial role in understanding the fundamental principles of human life sciences, especially those related to health care and clinical medicine. The focus is on the application of the principles of electrical engineering particularly high voltage engineering to both biology and medicine in clinical and research settings. Biomedical engineering can be defined as the application of engineering principles and techniques to the medical field. It combines the design and problem solving skills of engineering with the medical and biological science to help improve patient health care and the quality of life of healthy individuals.

Biomedical engineering is widely considered an interdisciplinary field, resulting in a broad spectrum of disciplines that draw influence from various fields and sources. Due to the extreme diversity, it is not atypical for a biomedical engineer to focus on a particular aspect. This assignment tends to point out the application of the high voltage in biomedical engineering.

Accidental contact of a person with high voltage will usually result in severe injury or death. This can occur as a person's body provides a path for current flow causing tissue damage and heart failure. Other injuries can include burns from the arc generated by the accidental contact. These can be especially dangerous if the victim’s airways are affected. Injuries may also be suffered as a result of the physical forces exerted as people may fall from height or be thrown considerable distance.

SOME OF THE HIGH VOLTAGE TECHNIQUES IN BIOMEDICAL ENGINEERING INCLUDE;

·         Fluoroscopy

·         Electron Microscope

·         Ultrasound

·         Artificial Pacemaker

1.11 Fluoroscopy:
Imaging technologies are often essential to medical diagnosis, and are typically the most complex equipment found in a hospital. Medical imaging refers to the techniques and processes used to create images of the human body (or parts thereof) for clinical purposes (medical procedures seeking to reveal, diagnose or examine disease) or medical science (including the study of normal anatomy and function)
Fluoroscopy is an imaging technique commonly used by physicians to obtain real-time images of the internal structures of a patient through the use of a fluoroscope. In its simplest form, a fluoroscope consists of an x-ray source and fluorescent screen between which a patient is placed. The fluorescent screen receives its power from the normal 240V socket outlet. However, modern fluoroscopes couple the screen to an x-ray image intensifier and CCD video camera allowing the images to be played and recorded on a monitor. The use of x-rays, a form of ionizing radiation, requires that the potential risks from a procedure be carefully balanced with the benefits of the procedure to the patient. The x-ray source should not be more than 420 KV, x-ray photons are produced by an electron beam striking a target. While physicians always try to use low dose rates during fluoroscopy procedures, the length of a typical procedure often results in a relatively high absorbed dose to the patient. Recent advances include the digitization of the images captured and flat-panel detector systems which reduce the radiation.

Fig. 1 A Modern Fluoroscope

Risks

Because fluoroscopy involves the use of x-rays, a form of ionizing radiation, all fluoroscopic procedures pose a potential health risk to the patient. Radiation doses to the patient depend greatly on the size of the patient as well as length of the procedure, with typical skin dose rates quoted as 20-50 mGy/min. Exposure times vary depending on the procedure being performed, but procedure times up to 75 minutes have been documented. Because of the long length of some procedures, in addition to standard cancer-inducing stochastic radiation effects, deterministic radiation effects have also been observed ranging from mild erythema, equivalent of a sun burn, to more serious burns.  Where adequate electrical protection is not ensured, the event of short circuit or over voltage from the fluorescent or x-ray source can increase the intensity of the radiation and can even increase the risk of shock.
A study has been performed by the Food and Drug Administration (FDA) entitled Radiation-induced Skin Injuries from Fluoroscopy with an additional publication to minimize further fluoroscopy-induced injuries, Public Health Advisory on Avoidance of Serious X-Ray-Induced skin Injuries to Patients During Fluoroscopically-Guided Procedures.
While deterministic radiation effects are a possibility, radiation burns are not typical of standard fluoroscopic procedures. Most procedures sufficiently long in length to produce radiation burns are part of necessary life-saving operations.

1.12 ELECTRON microscope:

An electron microscope is a type of microscope that uses electrons to illuminate a specimen and create an enlarged image. Electron microscopes have much greater resolving power than light microscopes and can obtain much higher magnifications. Some electron microscopes can magnify specimens up to 2 million times, while the best light microscopes are limited to magnifications of 2000 times. Both electron and light microscopes create images with electromagnetic radiation, with their resolving power and magnification limited by the wavelength of the electromagnetic radiation being used to obtain the image. The greater resolution and magnification of the electron microscope is due to the wavelength of an electron being much smaller than that of a light photon.
The electron microscope uses electrostatic and electromagnetic lenses in forming the image by controlling the electron beam to focus it at a specific plane relative to the specimen in a manner similar to how a light microscope uses glass lenses to focus light on or through a specimen to form an image.

Types of Electron Microscope

Transmission Electron Microscope (TEM):

The original form of electron microscopy, Transmission electron microscopy (TEM) involves a high voltage electron beam emitted by a cathode, usually a tungsten filament and focused by electrostatic and electromagnetic lenses. The electron beam that has been transmitted through a specimen that is in part transparent to electrons carries information about the inner structure of the specimen in the electron beam that reaches the imaging system of the microscope. The spatial variation in this information (the "image") is then magnified by a series of electromagnetic lenses until it is recorded by hitting a fluorescent screen, photographic plate, or light sensitive sensor such as a CCD (charge-coupled device) camera. The image detected by the CCD may be displayed in real time on a monitor or computer.

Scanning Electron Microscope (SEM)

Unlike the TEM, where electrons of the high voltage beam form the image of the specimen, the Scanning Electron Microscope (SEM) produces images by detecting low energy secondary electrons which are emitted from the surface of the specimen due to excitation by the primary electron beam. In the SEM, the electron beam is rastered across the sample, with detectors building up an image by mapping the detected signals with beam position.
Generally, the TEM resolution is about an order of magnitude greater than the SEM resolution, however, because the SEM image relies on surface processes rather than transmission it is able to image bulk samples and has a much greater depth of view, and so can produce images that are a good representation of the 3D structure of the sample.

Reflection Electron Microscope (REM)

In addition there is a Reflection Electron Microscope (REM). Like TEM, this technique involves electron beams incident on a surface, but instead of using the transmission (TEM) or secondary electrons (SEM), the reflected beam is detected. This technique is typically coupled with Reflection High Energy Electron Diffraction and Reflection high-energy loss spectrum (RHELS). Another variation is Spin-Polarized Low-Energy Electron Microscopy (SPLEEM), which is used for looking at the microstructure of magnetic domains.

 

1.13 Ultrasound

This is another imaging technique. It is the cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound. Ultrasound is manually produced in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium. The most well known application of this technique is its use in sonography to produce pictures of fetuses in the human womb. There are a vast number of other applications as well.

Fig. 2 Ultrasound Machine

 

 

 

 

1.14 Artificial pacemaker

An artificial pacemaker is a medical device which uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart. The primary purpose of a pacemaker is to maintain an adequate heart rate, either because the heart's native pacemaker is not fast enough, or there is a block in the heart's electrical conduction system. Modern pacemakers are externally programmable and allow the cardiologist to select the optimum pacing modes for individual patients. Some combine a pacemaker and implantable defibrillator in a single implantable device. Others have multiple electrodes stimulating differing positions within the heart to improve synchronization of the lower chambers of the heart.

Fig. 3  A pacemaker, scale in centimeters

The Basic Function of a Pace Maker

Modern pacemakers usually have multiple functions. The most basic form monitors the heart's native electrical rhythm. When the pacemaker doesn't sense a heartbeat within a normal beat-to-beat time period, it will stimulate the ventricle of the heart with a short low voltage pulse. This sensing and stimulating activity continues on a beat by beat basis.
The more complex forms include the ability to sense and/or stimulate both the atrial and ventricular chambers.

The act of putting a pacemaker in use can be termed pacing. Two methods of pacing include; temporary or transvenous pacing and permanent pacing.

Transvenous pacing (temporary):

In this method, a pacemaker wire is placed into a vein, under sterile conditions, and then passed into either the right atrium or right ventricle. The pacing wire is then connected to an external pacemaker outside the body. Transvenous pacing is often used as a bridge to permanent pacemaker placement. It can be kept in place until a permanent pacemaker is implanted or until there is no longer a need for a pacemaker and then it is removed.

Permanent pacing

Permanent pacing with an implantable pacemaker involves transvenous placement of one or more pacing electrodes within a chamber, or chambers, of the heart. The procedure is performed by incision of a suitable vein into which the electrode lead is inserted and passed along the vein, through the valve of the heart, until positioned in the chamber. The procedure is facilitated by fluoroscopy which enables the physician or cardiologist to view the passage of the electrode lead.

POWER SOURCE FOR THE PACE MAKER:
After satisfactory lodgment of the electrode is confirmed the opposite end of the electrode lead is connected to the pacemaker generator. The pacemaker generator is an hermetically sealed device containing a power source, usually a lithium battery, a sensing amplifier which processes the electrical manifestation of naturally occurring heart beats as sensed by the heart electrodes, the computer logic for the pacemaker and the output circuitry which delivers the pacing impulse to the electrodes.
Most commonly, the generator is placed below the subcutaneous fat of the chest wall, above the muscles and bones of the chest. However, the placement may vary on a case by case basis.
The outer casing of pacemakers is so designed that it will rarely be rejected by the body's immune system. It is usually made of titanium, which is inert in the body.

The body of the device is about 4 centimeters long, the electrode measures between 50 and 60 centimeters.