noise in biomedical instrumentation

NOISE IN BIOMEDICAL

INSTRUMENTATION

MADIHA WAJEEH

PGICE/126318

INTRODUCTION In biomedical instrumentation, noise is

considered as a disturbance that affects a signal and may distort the information carried by that signal.

Some sources of noise can be operator error, machine malfunction ,or other influences.

Noise is characterised as external or internal. Examples of external sources of noise are 60

Hz electric line frequency or magnet field interference.

An internal source of noise may be patient motion,such as eye blinking or muscle movement while the signal is being recorded.

EXAMPLES OF NOISE IN MEDICAL INSTRUMENTATION Artifact is a term used to describe a

wave on an ECG or an EEG that arises from sources other than the heart and brain.

Patient movement or respiration at frequencies between 0.15 Hz to 0.30 Hz can affect how the signal is recorded. Power line interference can also occur with a narrow band noise centered at approx 60 Hz or 50 Hz radiated with a bandwidth less than 1 Hz.

Arterial blood pressure waveforms are also affected by noise.

MUSCLE ARTIFACT

WANDERING ARTIFCTS ON ECG

Sometimes this 60 Hz noise is at least 50 percent peak to peak of the amplitude of the bio-signal. Additionally,this noise may result inperiodic interference.

Electrode Contact Noise: If the electrode is not properly in

contact with the skin,there are rapid and random baseline transitions of the biosignal.The transitions decay to the baseline.

Respiration Noise:

Respiration can also damage the real signal by moving the baseline with the waveform of the respiration.The amplitude and frequency of respiration can vary.This is called breathing artifact.

Motion artifact: A shift in the baseline of the bio-signal

can be caused by patient vibration or movement. This baseline shift is similar to a DC offset of the signal and the change can be abrupt.

These transient baseline changes are due to electrode motion resulting in changes in electrode-skin impedance.The electrode-skin impedance is similar to a source impedance of the bio-signal source and if there are abrupt changes to its integrity,the resulting input voltage to the amplifier will also change.

Motion artifacts are the noise caused by movement of the electrode.This movement results in skin deformation ,which changes the impedance and capacitance of the skin around the electrode site.For example,a patient walking on a treadmill during an ECG procedure may generate motion artifacts in his or her ECG traces.These motion artifacts may lead to misdiagnosis or inappropriate decisions.

Muscle contraction noise: Muscle contractions can generate EMG

noise in ECG signals.This is because this type of EMG signal has smaller amplitudes but has larger bandwidth than the ECG signal.There are more than one type of EMG signals with varying amplitudes and frequency ranges.

Electronic devices noise:If the input amplifier gain is such that the bio-signal sends the amplifier into saturation or cut-off mode,then the output amplified signal is not a true representation of the bio-signal.All electronic devices are temperature-sensitive and the circuit should never be allowed to operate in the saturation or cut-off mode.

Electrode-Surgical Noise: An electro-surgical machine operatyes at

frequencies over 100kHz,and high-frequency noise from the machine can radiate or leak into the ECG signals.As a result,the ECG signalwill be noisy and this noise will appear in the trace.

Image Artifact Noise: Images can have distortions due to patient

movement,metallic artifacts,or operator errors.In MRI imaging,three types of noise can appear in actual images:

1)Patient motion and resulting blood flow; 2)electromagnetic spikes due to MRI

electromagnets and abrupt or intense changes in the object discontinuities (such as brain/skull interface):

3)chemical shift and sensitivity to any metal around.

An image artifact is a structure not normally present in the image as a result of changes in patient position,electromagnetic spikes or even malfunction in the software.The image distortions may appear and the quality of the image can be affected due to these artifacts.

These are possible in CT,MRI,or any other types of image modalities.Several techniques using computer software or hardware have been developed to identify and reduce these artifacts

Operator Errors:Operator errors can result in bad signals.For example,the operator may unintentionally fail to follow te correct procedure in collecting patient data.

Machine Malfunctions: If the noise cannot be recognised as one

of the preceding types,it could be the result of a machine malfunction.

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