Medical Instrumentation
There are many categories of medical instruments ranging from those used in surgery to the ECG for monitoring the cardiovascular system.
In this article,we will be mostly concerned with medical instruments which record physiological signals including the ECG,EMG,and EEG that measure the voltage potentials on the skin.
These measurements are done by placing electrodes on specific locations of the body.Non evasive electrodes are surface electrodes,that can be placed on the skin whereas needle electrodes are evasive in the sense that they must pierce the area to obtain measurements.
Other sensors are also available to transducer quantities such as body temperature and blood pressure.
Detected electrical signals can be processed using a computer or simply a digital signal processor (DSP) board.The processor receives the measurement signals and then performs specific tasks such as administering intravenous drips or alerting the doctor.
Other instruments such as the heart pacemaker are surgically implanted.Modern pacemakers have wireless communication capabilities,which enable them to display operating parameters or vital signs on nearby monitors.
The device parameters such as amplitude of heart stimulation may also be adjusted in a similar fashion.
A major source of noise in medical instruments is due to the polarization electrodes.When these electrodes are in direct contact with the skin.a two layer charge develops between the electrode and the gel surface.
Movement of the electrode against the electrolyte results in a change of the half-cell potential and causes distortions to the recordings made between two moving electrodes.
This is known as a motion artifact,which is the result of a super imposed bio-signal and is a serious measurement noise source.Some researches have shown that the outer skin layer,stratum corneum, possesses dielectric or piezoelectric properties that increases the probability of motion artifacts (Togawa et al.,1997). Unfortunately,removal of this skin layer is not advisable due to risk of infection.
