BIO-MEDICAL INSTRUMENTATION
BIO-TELEMETRY
Bio-telemetry is an electrical technique for conveying biological information from a living organism and its environment to a location where this information can be recorded. It refers to communication between a living system and an observer.
ELEMENTS OF BIO-TELEMETRY SYSTEM
Transducer: Converts biological variable into electrical signal. Signal conditioner: Amplifies and modifies the signal for effective transmission. Transmission link: Connects the signal input blocks to readout device by wire.
DESIGN OF BIO-TELEMETRY SYSTEM
Telemetering system should transmit bio-electric signals with maximum fidelity and simplicity. No reaction or interference with the living system. Size and weight of the system should be small. For long term or implant units the weight and size limit is of the order of 1%.For shorter duration about 5%. More stability and reliability. Power consumption should be small for extension of source life time. For wire transmission , shielding of cables is done to reduce noise level. Differential amplifiers are used to reject common mode interference.
RADIO-TELEMETRY SYSTEMS They are of two types: Single channel Multi-channel
SINGLE CHANNEL TELEMETRY SYSTEM
A miniature battery operated radio transmitter is connected to the electrodes of the patients. The transmitter broadcasts the bipotential over a limited range to the receiver. The receiver detects the radio signals and recovers it for further processing. Negligible connections and stray capacitances between the electrode circuit and the system. Risk to the patient is the battery operated transmitter but the power is kept low.
Radio frequency-few hundred KHZ-300MHZ. Beyond this frequency range, attenuation becomes excessive. Hence the transmitter and the man made noise is made small. Amplitude modulation is not adopted coz when signal amplitude is varied it introduces error. Frequency or pulse modulation techniques are adopted.
TRANSMISSION OF BIOELECTRIC VARIABLES
Active measurements: Bioelectric variables like ECG, EMG and EEG are measured directly without using excitation voltage. Passive measurements: Physiological variables like blood pressure, temperature, blood flow are measured indirectly using transducers and excitation voltage
TELEMETRY CIRCUITS
Tunnel diode FM transmitter Hartley type FM transmitter Pulsed Hartley oscillator
TUNNEL DIODE FM TRANSMITTER
Used for transmission of EMG, ECG, EEG, respiration rate..
Tunnel diodes are active devices (TD, BD) and circuit has higher sensitivity and fidelity. Total weight with battery -1.44 gm. Size-0.8 x 0.22 cm2 Radio frequency used: 100-250 MHZ Frequency response: 0.01HZ-20KHZ Input impedance: 300 kilo ohms-Mega ohms. Temperature stability of carrier frequency: 0.05%/degree c. Varactor diodes- voltage sensitive semiconductor capacitors are used for frequency modulation. Signal transmission is through inductor.
Advantages
Signal transmission from the surface of the object to the receiver is in a normal hospital environment. No shielded room is necessary. Interference is greatly reduced.
HARTLEY TYPE FM TRANSMITTER
Used for the transmission of ECG, EEG and EMG.
The capacitor C1 and inductor L form the tank components of Hartley oscillator. C2-coupling capacitor T1-driver amplifier transistor T2-oscillator transistor Capacitance between emitter and base is voltage sensitive to frequency modulate the carrier. Amplitude of the signal-10 microvolt-several mV Transmission range for power consumption level in mW-few metres to 30 metres. Bandwidth-100 to 1000 HZ.
PULSED HARTLEY OSCILLATOR
To measure temperature, a thermistor is placed instead of R1. To measure pressure, pressure changes are given to core M. To measure pH or voltage , electrodes are connected across XX’. The transmitter is modulated by varying the pulses of RF oscillations Circuit simple with low power consumption-5 to 10 microwatt
RADIO TELEMETRY WITH A SUBCARRIER
When the relative position of transmitter to the body or other conduction object changes, the carrier frequency and amplitude change due to loading change of carrier frequency resonant circuit. If the signal has a frequency different from the loading effect, they can be separated by filters. Otherwise real signal will be distorted by loading effect. To avoid loading effect, sub carrier system is needed.
The signal is modulated on a sub carrier to convert signal frequency to the neighbourhood of sub carrier frequency. RF carrier is modulated by this sub carrier. The receiver detects RF and recovers the subcarrier.
IMPLANT STRESS TRANSMITTER
The signal from the strain gauge changes the resistance of the two arms of the bridge to amplify modulate the 20KHZ sub carrier generated by tunnel diode. This is fed to a FM circuit to frequency modulate the 100MHZ RF carrier. The tunnel diode serves as RF carrier oscillator and modulator. The transistor amplifies the carrier and radiates it through coil L2.
The 20KHZ sub carrier produced by the FM receiver is filtered. It is then amplified and detected to get stress information. The stress signal with about 1/30 HZ can be recorded for several weeks with this circuit.
MULTIPLE CHANNEL TELEMETRY SYSTEMS It is of two types: Frequency division multiplex Time division multiplex
FREQUENCY DIVISION MULTIPLEX SYSTEM
Each signal is frequency modulated on a sub carrier frequency. These are then combined to modulate the main RF carrier. At the receiver side, the modulated sub carriers are separated by band pass filters. Individual signals are recovered by 2nd set of discriminators. LPF’s are used to extract the signal without noise.
TIME DIVISION MULTIPLEX SYSTEM
TDM system is used because bio-medical signals have low frequency bandwidth requirements. The transmission channel is connected to each signal input for sampling. When all the channels in a cycle have been scanned the next cycle will start.
At the receiver end, the process is reversed. Sequentially arranged signal pulses are distributed to individual channels by a synchronized switching circuit. If the no of scanning cycles/second is large the signal in each channel is recovered without distortion.
CONDITIONS
The scanning frequency fn should be greater than twice the maximum signal frequency. fn>2fs max If Tn=1/fn=scanning period tn is the sampling time of each channel Maximum no of channels n=Tn/tn n should be small to avoid interference between channels
PROBLEMS IN IMPLANT TELEMETRY
Long term telemetry-electronic circuit packed in a capsule-implanted subcutaneously deep into the body closer to signal source-avoids mechanical difficulties. Size and weight limitations are serious-reliability requirement more critical.
Body reaction: size, weight, surface condition and shape affect it. E.G: medical grade silastic, teflon, glass are used as enclosures.
Protection of electronic circuits: coating materials are silicon rubber, epoxy, plastics, glass and metal. For implants of a few months we can have three layers of structure with silastic outer coating to reduce tissue reaction and inner layers of epoxy or paraffin for protection from leakage. Glass and metal seal to avoid air pockets.
Power supplies: two special types: Environmental power supply: Radio induction to transmit mW of power to implant telemetry unit for months. Microwatt power supply circuits using piezoelectric crystals placed on blood vessel or aorta.
USES OF BIO-TELEMETRY
To record bio signals over long periods while patient is engaged in normal activities. Easy diagnosis of the nature of disease. No mechanical or physical disturbance to the patient. For future reference or to study treatment effect. For recording on animals for research. For monitoring persons who are in action.