Biomedical Instrumentation Questions and Answers

Answer: B

A home blood glucose sensor works on the electrochemical principle.

A drop of blood on the test strip reacts with an enzyme, usually glucose oxidase or glucose dehydrogenase, which oxidises glucose. The electrons released are transferred, through a mediator, to an electrode, and the meter measures the resulting current, which is proportional to the glucose concentration (amperometric detection). This is an electrochemical biosensor.

Electrophysiological measurements record the body's own electrical activity, as in ECG and EEG. 'Physio-chemical' and 'chemical' are too general; the meter specifically measures an electrical current produced by a chemical reaction, which is what electrochemical means.

Answer: B

The primary transducer in ECG is the electrode.

In the body, current is carried by ions, while in the instrument it is carried by electrons. The biopotential electrode, usually silver-silver chloride, is the interface that turns the heart's ionic currents into an electrical signal the amplifier can measure. It is therefore the input transducer of the ECG system.

A thermistor senses temperature, a piezoelectric crystal senses pressure or vibration and makes ultrasound, and a strain gauge senses deformation, as in blood pressure transducers. None of them picks up biopotentials.

Answer: C

The input impedance should be very high (in the MΩ range).

The electrode-skin interface has a source impedance from a few kilohms up to hundreds of kilohms, and it differs between electrodes. The amplifier's input impedance forms a voltage divider with it. Only when the input impedance is far larger, megohms or more, does almost all of the biopotential appear at the input, and do mismatches between electrodes stop turning common-mode noise into a differential error.

A very low or moderate (1-10 kΩ) input impedance would load the source and lose most of the signal. Matching the tissue impedance gives maximum power transfer, but a biopotential amplifier needs to measure voltage, not draw power.

Answer: B

The interface is modelled as a resistor-capacitor network.

At the electrode-electrolyte boundary a charge double layer forms, which behaves like a capacitor, in parallel with a charge-transfer resistance. A series resistance represents the electrolyte gel and the skin. The usual model, with a further parallel RC for the stratum corneum, therefore makes the interface impedance depend on frequency.

A pure resistor misses the capacitive, frequency-dependent behaviour. The interface has no significant inductance. A half-cell potential does appear in the full model as a small DC source, but on its own a voltage source does not describe the interface's impedance.

Answer: C

The EOG records corneoretinal potentials.

The eye acts as a dipole: the cornea is positive relative to the retina by a steady potential of several millivolts. Electrodes placed beside and above or below the eyes detect how this dipole turns as the eye moves. The electro-oculogram is therefore used to record eye movements, for example in sleep studies and nystagmus testing.

Muscle potentials are recorded by the EMG, brain activity by the EEG, and cardiac rhythm by the ECG.