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2012 August 7

Medical Instrumentation, first 5 chapters

For the past few days, I’ve been slogging through one of the potential text books for the circuits course: Medical Instrumentation: Application and Design, 4th Edition. John G. Webster. Publisher: Wiley, 2009. # ISBN-10: 0471676004; # ISBN-13: 978-0471676003, which is used for a bioengineering course at George Mason University, as I commented on in Looking at bioengineering measurements courses.

I wanted to make notes on each section, so I copied the table of contents from Amazon using OCR software.  Unfortunately, their table of contents seems to have been for a different edition, and I ended up having to edit the page numbers and add section titles by hand anyway.  So far I’ve only read the first 5 chapters, and while the book has some material that would be useful to students in our course, it looks like it would not be a good textbook for the course, as it assumes that the students have already learned what our course is trying to teach.  Actually the book is very inconsistent in its assumptions about the readers, probably because the multiple authors for the different chapters did not do a good job of communicating with each other.  There are huge differences in style and level of detail between the chapters also.  I’ve always disliked staple-gun text books, where different authors write independent chapters, and this one seems to have all the standard faults of such books.
Here are my notes on the first 5 chapters:

Walter H. Olson

Starts with a rather boring anecdote before the first section.

1.1         Terminology of Medicine and Medical Devices   4
Boring list of references.

1.2         Generalized Medical Instrumentation System   5
Vocabulary, not much content.

1.3         Alternative Operational Modes   7
More vocabulary.

1.4         Medical Measurement Constraints   9
Useful table of physiological properties, ranges, and what sensor is used to measure them

1.5         Classifications of Biomedical Instruments   12
Low content (4 classification systems for biomedical instruments), but short.

1.6         Interfering and Modifying Inputs   12
Useful, but brief, description of interfering signals.

1.7         Compensation Techniques   13
Rather generic description of compensation techniques that just waves hands at the problem, while assuming that students already know what a transfer function is and how to solve an equation for negative feedback.  Who is the audience for this? Anyone who can follow the text doesn’t need the vague intro material.

1.8         Biostatistics    16
A list of definitions of common statistics (mean, standard deviation, Pearson correlation).  Not much motivation and not enough to actually teach from.

1.9         Generalized Static Characteristics   19
Definitions of accuracy, precision, resolution, … This looks like a useful collection of definitions for students to learn.

1.10       Generalized Dynamic Characteristics   25
Assumes students are comfortable enough with Laplace transforms to think in terms of transfer functions.  This is unlikely to be true of anyone taking their first electronics course.

1.11       Design Criteria   35
Mildly interesting background material.

1.12       Commercial Medical Instrumentation Development Process   37
Irrelevant to our course.

1.13       Regulation of Medical Devices   39
Irrelevant to our course.

Problems   42
References   43

Overall, there seems to be only a little in Chapter 1 worth having our students read: perhaps 1.4, 1.5, 1.6, and 1.9. If I thought that they could follow it, 1.10 might be useful also.  The style here is not encouraging—the chapter is presented in about as boring a manner as could be devised.  The only other chapter by this author is Chapter 14, Electrical Safety, so I tried not to be too discouraged by the poor writing.  Perhaps we could simply skip this chapter.

BASIC SENSORS AND PRINCIPLES                               45
Robert A. Peura and John C. Webster

2.1         Displacement Measurements   45
Just an intro to the idea of measuring displacement.

2.2         Resistive Sensors   46
2 paragraphs on potentiometers  (but only wire wound, which are a bit old fashioned).

6.5 pages on strain gauges, with a rather confusing explanation of how they work.  Shows bridge circuits, expecting students to already understand them, though they aren’t introduced until the next section.

2.3         Bridge Circuits   53
Bridge circuits are now introduced (after having already been used without explanation) and half a page of voltage results from an unbalanced bridge given without explanation.

2.4         Inductive Sensors   53
Self inductance, mutual inductance, and linear variable differential transformers are mentioned, with considerable space given to LVDTs. While I would like to include an inductance sensor in the course, nothing in the section struck me as a valuable way to introduce the concept. Note that they don’t include dynamic microphones (coil moving in magnetic field) in this category, since those are voltage-output sensors—they don’t include such sensors in their classification scheme.

2.5         Capacitive Sensors   56
Describes capacitance sensing for capacitor microphones and explains why they are not suitable for a lot of physiological measurements (where frequencies lower than 20Hz are needed, but the explanation is a bit too abstract. Does not cover electret microphones and their included FET output transistor, so seems a bit old-fashioned.

2.6         Piezoelectric Sensors   58
A reasonable description of piezoelectric sensors, but assuming a much higher level of EE circuits background than we can assume for our students.  (The examples would be good for the very end of our class, not the beginning.)

2.7        Temperature Measurements  62
A basic intro for why temperature measurement is useful medically. Not very interesting nor informative.

2.8         Thermocouples   63

Misleading description of thermocouples that first talks about a single junction, then assumes that students know that there are two junctions.  Unless you already knew how thermocouples worked, the passage would be nearly incomprehensible.

An inverting amplifier circuit is given, but not explained. It has two crossing wires with neither a connecting dot nor a bridge—an ambiguous documentation style that should never be used in a textbook.  (The wires need to be connected for the circuit to make sense, but the standard for crossing wires without a dot is that they don’t connect.)

The LT1025 “electronic cold junction” is used but not explained.  The LT1025 is not, in fact, a cold junction, but a compensator for correcting for the cold junction being at room temperature rather than 0 C.  I can’t tell if the author knew that well and just explained it badly (given how badly thermocouples were explained) or hadn’t even read the data sheet.

2.9         Thermistors   66

The thermistor plots in Figure 2.15 a are rather hard to read. The explanation of self-heating is pretty good. Only the simplest version of the thermistor resistance equation is given. Differential temperature measurement is described but not well explained.

The notes about use of thermistors seem to be a random collection of factoids, with no coherence.

2.10       Radiation Thermometry   69
The explanation of blackbody radiation is good, but their claim that chopped-beam radiation thermometers are standard seems a bit odd—I find very few chopped-beam IR thermometers on the market.  There are some, and they are more tolerant of ambient temperature changes, but the book falsely implies that they are the dominant method.

2.11       Fiber-Optic Temperature Sensors   74
OK explanation of what these do, though they aren’t very relevant to a circuits course.

2.12       Optical Measurements   74
Another 2-paragraph intro.

2.13       Radiation Sources   75
Figure 2.21 e, referred to in the caption and the text, is not included in the printing.

Describes incandescent and arc lamps fairly well. LED description seems a bit out of date (no mention of blue or white LEDs).

The section on lasers also seems rather out of date—they seem to be talking only about high-power lasers and make the ludicrous claim that semiconductor lasers only operate in the infrared and need to be pulsed. I guess they’ve never seen a red, green, or violet laser pointer.

2.14       Geometrical and Fiber Optics   79
Not relevant for a circuits course.

2.15       Optical Filters   82
Not very relevant for a circuits course.

2.16       Radiation Sensors   83
Pretty good explanation of phototubes, photoresistors, photodiodes, and phototransistors.  Not so clear on photovoltaic devices.

2.17       Optical Combinations   86
A one-paragraph section that seems rather artificially tacked on.

Problems   87
References   88

Overall, Chapter 2 had little of use for our applied circuits course. The classification of sensors could have been useful, if the descriptions had been better and the classification scheme more complete. I’m disappointed that a book that has a 2010 copyright still has parts that read like they were written in 1977 (the copyright for the first edition).  This looks like publisher churn rather than a proper updating of the book.

John C. Webster

3.1         Ideal Op Amps   91
Basic rules for ideal op amps are fairly standard, but they don’t seem to be aware of rail-to-rail op amps, which are fairly common now (particularly for low-voltage, single-power-supply op amps).  Assuming that all op amps operate on ±13v power supplies and are limited to ±10v outputs seems quaint.

3.2         Inverting Amplifiers   93
OK presentation of standard inverting amplifier and summing inverting amplifier, with application of summing amplifier to cancelling a DC voltage bias on one input.

3.3         Noninverting Amplifiers   96
Standard non-inverting op amp circuit.

3.4         Differential Amplifiers   97
Gives common-mode rejection ratio as in the range 100 to 10000, without using the usual dB scale.  The cheap chip I’ve been using (INA126P) has a minimum common-mode rejection of 83 dB (which is 14000) and typical of 94dB (which is 50000), and for $10 the INA118P has a typical CRR of  120dB (1000000) at a gain of 100, so their numbers seem rather dated.

Gives both 1-op-amp and 3-op-amp differential amplifiers, but no 2-op-amp circuits (the INA126P is a 2-op-amp differential amplifier).

3.5         Comparators   100
Reasonable explanation of hysteresis.  I would have preferred some explanation of the dynamic behavior also, as op amps make rather slow Schmitt triggers, which can make a huge difference in some applications.

3.6         Rectifiers   102
2-op-amp circuit with 4 diodes, a resistor, and a potentiometer to make a “perfect” full-wave rectifier circuit.  Also a one-op-amp circuit (which really needs both an input and an output amplifier, since it has a low input impedance and requires a constant load). The application given (using with an integrator to quantify the amplitude of electromyographic signals) seem ok, but I suspect that it would be easier and cheaper to do that in a microprocessor nowadays.

3.7         Logarithmic Amplifiers    103
OK circuits, but probably not all that useful.  It would have been better if an example of “device with logarithmic or exponential input-output relation” had been given, as I can’t think of such a sensor off the top of my head.

3.8         Integrators    104
Standard integrator circuit.  Example of use as a charge amplifier for piezoelectric transducer.  Good explanation of the need to restore initial conditions because of drift.

3.9         Differentiators    107
Standard circuit.  Mentions problems with oscillation and noise amplification, but doesn’t say what to do about them. Gives application to detection of R wave in an EKG, though everyone would do that in software nowadays.

3.10       Active Filters    108
Standard one-op-amp low-pass, high-pass, and band-pass filters. Students would need to understand transfer functions to understand how these work.

3.11       Frequency Response    110
Starting to look at non-ideal op amps. Good explanation of loop gain, gain-bandwidth product, and slew rate.

3.12       Offset Voltage    112
Offset voltage is explained, but the nulling pot is rather vaguely described—not all amplifier chips have the “terminals indicated on the specification sheet” for doing external nulling.  (The ones I’ve been using, for example, do not.)

3.13       Bias Current    114
Their use of old op amps makes for some rather large bias currents (200 pA, rather than ±1pA).  The INA126P instrumentation amp I’ve been using, however, has an enormous bias current of -10 nA. Their explanation of compensation for bias current and calculation of noise due to variations in bias current is more confusing than enlightening.

3.14       Input and Output Resistance    115
Reasonable discussion of input and output resistance, with appropriate caveats about how other limitations (input bias current and output current limits) make the calculated input and output resistances rather useless.

3.15       Phase-Sensitive Demodulators   117
The transformer-based ring demodulator seems like rather old tech. Even the solid-state part they mention (1495, a 4-quadrant multiplier) is obsolete and no longer available.  More recent parts like the AD633 4-quadrant multiplier or HFA3101 RF mixer would make more sense to describe. It looks like this section has not been updated for 15 years.

3.16        Timers                  120
Describes using a 555 timer to make an oscillator.  This is not really any better than using an op-amp relaxation oscillator and hardly seems worth the trouble of adding to the book between editions.  They don’t explain the 555 in any detail, so I fail to see the benefit added by this section.

3.17       Microcomputers in Medical Instrumentation   122
Reads like an ad for LabVIEW.  Has no content other than to say that everything is done in microprocessors these days (contradicting a big chunk of hte chapter).

Problems    123
References   125

Chapter 3 has more-or-less standard op-amp circuits, with a few rather old-fashioned choices thrown in.  It could be mildly useful, but I’m pretty sure that there are dozens of books that present the same material better.  All circuits stuff that comes before op amps is completely lacking from this book, which lowers its value for our course enormously.  At this point, it is clear that this book will not be useful for at least the first half of our course.

THE ORIGIN OF BIOPOTENTIALS                              126
John w. Clark. Jr.

4.1         Electrical Activity of Excitable Cells   126
A good, basic introduction to where voltages in the body come from. I think that the bioengineers would benefit from reading this, and it would tie into their chemistry and biology courses well.

4.2         Volume Conductor Fields    135
This section is not as clear, but could still be useful in a run-up to the EKG lab.

4.3         Functional Organization of the Peripheral Nervous System   138
Background material that isn’t really relevant to the course.

4.4         The Electroneurogram (ENG)    140
Useful to medical people, but a bit irrelevant for this course. There is no way that we’re going to hook up 100v stimulators to students!  The chances of them doing real damage to themselves by using pulses longer than 300 microseconds is too high. (I’ve also had treatment with electrical stimulation when Ihad ulnar tunnel inflammation—it can get quite painful if the level is set too high.) I also think that having them try to pull out 10 μvolt signals from the noise is too challenging for a first circuits course.

So while this section has some cool background material, there isn’t much in it directly applicable to the course.

4.5         The Electromyogram (EMG)    144
Explains why EMGs are often done with needle electrodes rather than surface electrodes, but otherwise not of much use.  We’re not going to be puncturing the skin in our course.

4.6         The Electrocardiogram (ECG)    139
Somewhat detailed explanation of how the electrical system of the heart works.  It is clear that modeling the heart is one of the author’s loves, and this material might be useful as background before the EKG lab.

4.7         The Electroretinogram (ERG)    158
No way that we are putting electrodes on or in the eye.

The EOG (electro-oculogram), which attempts to measure eye movements from electrodes on the bridge of the nose and the temple is interesting,as it requires DC amplification of small signals.  I found a DIY EOG project—they use a $10 instrumentation amp (the INA118P).  I’m not sure whether the better specs of that part are needed for this application, or whether the $2.50 INA126P that I’ve been using would suffice (they have the same pinout). I’ve not priced small disposable electrodes of EOGs, but the ECG ones we have are too big for the bridge of the nose.

4.8         The Electroencephalogram (EEG)    163

I’ve looked at the open-source EEG stuff before, but I was having enough trouble with the EKG circuits, that I decided not to tackle EEG.  Now that I have EKG circuits that work reliably, I may revisit the EEG.  The difficulty of pulling out 50–100 μvolt signals from the noise due to facial muscles and so forth may make this lab impractical.  Unlike the EKG, where a functioning circuit provides a characteristic output that is easily recognized, it is difficult to tell whether an EEG is working correctly, especially as the subject generally needs to be lying down, relaxing, with their eyes closed, so students would have a hard time watching the oscilloscope while taking their own EEG.
Input electrodes can be a problem also—I don’t think that there’s a cheap disposable electrode that works well (unlike the EKG).

This section has way too much info on brain structure for my taste.  There are good subsections on the brain waves and different “montages”: differential, referential, or “Laplacian” reference voltages for the signals.
4.9         The Magnetoencephalogram (MEG)    181
Superconducting quantum interference devices (SQUIDs) are well beyond the scope of this circuits class.

Problems    182
References    186

Overall Chapter 4 had some useful material for our students—particularly on the sources of biopotentials and to a lesser extent on how the EKG signal is related to the internals of the heart.  It might be worth having students read bits of this chapter before the EKG lab (put the book on reserve in the library?).

BIOPOTENTIAL ELECTRODES                      189
Michael R. Neuman

5.1         The Electrode-Electrolyte Interface    189
Good description of half-cell potentials.

5.2         Polarization    192
More useful electrochemistry.

5.3         Polarizable and Nonpolarizable Electrodes    196
A good explanation of why platinum and Ag/AgCl electrodes are used and what the difference is between them.  Describes both electroplated and sintered Ag/AgCl electrodes (though not the “bleach” method for creating AgCl layers on silver).
I found some good instructions for both electroplating and chlorine bleach treatment from Warner Instruments:

Cleaning the Ag+ wire before chloriding

The wire should be cleaned before chloriding. An un-chlorided wire can simply be cleaned with EtOH and rinsed with H2O before proceeding. A previously chlorided wire should be wiped with dilute HCl to remove the old coating, then rinsed in EtOH and H2O.

Chloriding the Ag+ wire

There are two methods in common use to chloride a silver wire. These are the putative electrical and chemical methods. Both work very well but the electrical method yields a deeper coating.

Electrical method

Electrical chloriding of Ag+ wire is achieved by making it positive relative to a solution containing NaCl (0.9%) or KCI (1 M). One way to achieve this is to pass a current at a rate of approximately 1 mA/cm2 for about a minute, or until the wire is adequately plated. (For example, to chloride a 2 cm length of a 0.25 mm Ag+ wire (this is the diameter of the wire used in Warner electrode holders) requires 0.15 mA of current.) The color of a well plated electrode will be light gray to a purplish gray. While plating, occasionally reversing the polarity for several seconds tends to deepen the chloride coating and yield a more stable electrode.

Chemical method

An alternate to the electrical method is to immerse the wire in Clorox bleach until a light gray color is observed (typically 10–15 minutes is sufficient). At a minimum, this simpler method is commonly performed at the beginning of each day’s work.

5.4         Electrode Behavior and Circuit Models   202

A simple circuit model for an electrode is given, and the effect of different thicknesses of AgCl on an electroplated electrode is shown.  If we have students electroplate Ag/AgCl electrodes, this would be very useful information for them. They also provide an example showing how to estimate the four parameters of their electrode model from simple measurements.

This section looks useful to our students.

5.5         The Electrode-Skin Interface and Motion Artifact   205

The discussion of the electrode-skin interface is useful, and the suggestion to reduce the resistance by stripping off layers of the stratum corneum with Scotch tape seems useful. (I did some searching on line, and the standard Scotch Brand “magic tape” seems to be commonly used.)  This technique seems more suitable for our course than abrading, puncturing, or using acetone wipes.

5.6         Body-Surface Recording Electrodes   209

A good description of electrode designs, including the type we expect to use for the EKG lab.  The “neonatal” electrodes are also cheap (about 30 cents each) and may be less irritating than the ones I’ve been using, which start to itch after several hours.

5.7         Internal Electrodes   215

We won’t be using implantable electrodes, but students might be interested in knowing about them.

5.8         Electrode Arrays   220

Mildly interesting background material about electrode arrays.

5.9         Microelectrodes   222

The info about micropipette electrodes is directly relevant to the nanopipette research at UCSC.  The reference to negative-capacitance amplifiers (Section 6.6) may be useful.

5.10       Electrodes for Electric Stimulation of Tissue   231

Not relevant for this class, though the mention of carbon-filled rubber electrodes is mildly relevant, since we might have used them for the EKG (if we wanted a more long-term connection than the disposable electrodes).

5.11       Practical Hints in Using Electrodes   233

Basic, sound advice (like not using dissimilar metals and being aware that the insulation may not work as well when wet).

Problems   235
References   239

Chapter 5 looks like useful information for our students (again, mainly for the EKG lab).

Bottom line

I don’t think that we can make Medical Instrumentation our main text book.  It doesn’t have anywhere near enough of the basic circuits material, and the op-amp section is not wonderful. Big chunks of the book are showing their age, and the 4th-edition revisions aren’t nearly extensive enough to justify a recent date.  If we’re going to use an ancient book, we should use one that is available cheap.

There is some good material on biopotentials and EKGs (and I expect more EKG material in Chapter 6), but these are not enough to justify its use as a text book for this course.  They may be enough to justify putting the book on reserve in the library, though the reading time is large for reserve material.  We might have to write our own shorter handout covering just the high points of this material.


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