Classroom Demonstrations of some Physical Principles important in Medicine

Introduction

            The best demonstration of each of the seven technologies focused on in this book is, of course, the real thing.  Most colleges and universities are located close enough to a hospital whose facilities can be tapped for fieldtrips to operating rooms and medical imaging departments.  Many hospitals also have facilities for radioisotope imaging and SPECT, as well as radiation therapy centers.  Only major research centers will have some of the specialized tools, such as PET scanners or ultrafast spiral CT scanners.  With adequate advanced notice, many physicians will arrange visits of small numbers of students so long as these visits accommodate their restricted schedules.  Students should be clearly briefed on the ground rules in advance, such as the need for silence during some procedures, sensitivity to the privacy of any patients involved, and taking sterile environments seriously.

            However, in many situations it will be impossible to arrange such fieldtrips.  Also, lecture demonstrations of simple physical phenomena enhance students' understanding of the fundamental principles behind each technology.  The following listing concentrates principally on inexpensive classroom demonstrations which utilize equipment readily accessible to most college instructors.  A short bibliography of helpful books and a listing of major scientific instructional equipment suppliers is given at the end of the chapter.

            An evolving resource for supplementing classroom lectures with images and information is the World Wide Web, accessible by many programs such as Mosaic and Netscape.  Several Web sites presently exist showing medical images generated by the technologies discussed herein, and others are under development.  Instructors and students should definitely try browsing and search for keywords such as medical imaging, ultrasound, magnetic resonance, etc.

 

Ward Scientific has many anatomical models, including one of the eye, various internal organs, heart, the brain, joints, etc. I recommend a basic stock of these for accompanying the discussions of the relevant anatomy and physiology. We own: a knee joint model (for teaching about arthroscopic surgery of the knee, in conjunction with the hour-long video The Operation: Knee Surgery", available from Insight Media) (joint models are available from http://www.einsteins-emporium.com/science/human-anatomy/sh248.htm and .; a model eye (from Pasco Scientific to see the optics, or from an anatomical supplier like Ward to show the anatomy); a model of the torso and abdominal organs; and a model skull and brain. We also own a working model ear (with mechanical vibrating ear drum, moving oscicles and vibrating rubber hair cells!) from Ward Scientific.

 

Budget Demos--those that require only very inexpensive or very common lab equipment--are indicated by bold-face.

Chapter 2:  Fiber optics and medical fiberscopes

            Many of the standard demonstrations used to teach optics in freshman physics courses are appropriate for illustrating the optics used in constructing laparoscopes and in medical laser applications.  Actual endoscopes or laparoscopes are fairly expensive (upwards of a thousand dollars), but you may be able to gather a collection of associated disposable attachments and equipment from your local hospital.  (Since these become nonsterile if opened without being used, you can ask for samples at no cost to them.)

Demonstrations:

Light rays  (Requires intense lamp source with slits)  Students often regard the use of rays in geometrical optics as an artificial construction applicable only to the laboratory.  A quick demonstration that any light source, not just lasers, can be described using sample rays of light is helpful in clarifying the general nature of this idea.  Using slits to separate out individual "rays" of light from an ordinary white light source and tracing their paths should suffice.  Also, students generally like to remind themselves that one cannot see down an ordinary curved tube, and that light is ordinarily not transmitted from one end of such a tube to the other.  (While this seems quite obvious, students may not clearly distinguish at first that special optics are necessary to achieve the wave guiding properties of fiber optics.)

Reflection, refraction, Snell's Law and total internal reflection:  For demonstrating the refraction and reflection of light rays at the interface between two media and total internal reflection, very simple apparatuses suffice.  A laser pointer is a good light source, but very bright white light sources can work if properly collimated and used with a slit. A ray of light from either a laser or collimated white light source can be directed onto a tank of water containing a small amount of powdered milk.  Scattering from the milk makes the ray's path visible.  (For best results, the tank should have flat sides to reduce distortions.  A small aquarium can be used in lieu of special containers.)  Total internal reflection is achieved for rays traveling from the water and hitting the water-air interface from below. Equally effective are smoked optics (available from scientific supply houses) which make visible the path of light rays.  For quantitative measurements of angles for verifying Snell's Law, one can use the simple apparatuses above along with graduated circles or protractors.  A more flexible (and expensive) arrangement involves either a Hartl disk or other Blackboard Optics set with accompanying optics set or a reflection/refraction tank (both available from CENCO).)  The former requires a high intensity lamp source, available through the same vendor.  Both of these apparatuses have large, easily visible graduated circles, and the reflected and refracted rays are visible even at large distances.

Hartl disk with slits and lens or Blackboard Optics set:  Using the Hartl disk described above, several parallel rays of light can be directed onto various types of lenses.  The focusing of parallel light to a point by a converging lens is of special importance for the discussion of laser surgery. This apparatus can be used to verify Snell's Law, to examine total internal reflection, etc.

Construction of a simple telescope.  (Requires lenses, optical mounts)  Laparoscopes and arthroscopes utilize fiber optic waveguides for illumination, but for viewing purposes many simply use straight optical pathways, consisting essentially of telescopes with working distances of several centimeters.  Thus, a simple demonstration of the construction of a refractive telescope can give students an idea of the optics involved in constructing such a device.

Fiber optics demonstrations Several excellent and simple fiber optics demonstrations can be assembled easily to fit even modest budgets. Some of these demonstrations involve the use of a laser.  Laser pointers or nexpensive helium-neon lasers still are the best choice.  Many of the devices listed below are available from Edmund Scientific, Educational Innovations or Pasco Scientific.

Optical signal path demonstrator  Two smoked plastic rods, one straight and one curved, used with a laser.  These allow students to see the path followed by laser beams introduced into the rods, and trace the reflection of the laser around corners in the curved rod.

Optical fibers are available from many scientific supply houses, as individual strands with or without cladding.  Students seem to find it helpful to have a bundle with the fibers loose at one end and bound at the other, since this helps them visualize the construction of a waveguide.

Flashlight with optical fibers  This simple toy provides an inexpensive way to illustrate the wave guiding of light.

Optical fiber image conduit  These consist of rigid bundles of optical fibers roughly six inches in length, with varying outer diameters and fiber diameters/numbers of fibers.  They act as inflexible image conduits, transmitting an image of an object present at one end to the other end by total internal reflection. (Edmund Optics)

Fiber optics bundles  These are sold for use with lamp sources as fiber optic illuminators (such as those used in medical fiberscopes).  These bundles come in various lengths and sizes, and are useful for illustrating the ability of fiber optics to act as waveguides for light.  These bundles are incoherent, i.e., the orientation of the optical fibers is not preserved from end to end.  When they are contrasted with image conduits (previous demonstration), their inability to reconstruct images presented at the other end is quite striking.  The difference between the two helps students understand the principle behind image transmission.

Flexible fiberscope  A roughly three foot long fiber optic inspection scope with eyepiece and lens for imaging objects at variable distances from the viewing end.  (Edmund Optics)

CCD (charge coupled device) chip.  While these devices are present in most modern video cameras, you can easily see them only on cameras in which the lenses are removable.  If you are able to remove the lenses from your video camera, the CCD appears as a small (slightly less than 1 cm square) integrated circuit chip.

Purchasing an actual endoscope, arthoscope or laparoscope is actually more affordable than you might think. We got ours from the many online vendors who offer used equipment. You do not need an FDA license if you are using it only for educational purposes. This offers students a remarkable opportunity to see the physics at work in a real instrument. I recommend getting one equipped to you can use it for viewing with an eyepiece, rather than a video endoscope (arthroscope, etc.) Our flexible sigmoidoscope cost about $1500 in 2006, but we were able to purchase an arthroscope for around $200.

Chapter 3:  Medical lasers

Demonstrations

Spectrum of continuous light source  This requires a good quality prism, a screen onto which the spectrum is projected, and a slide projector (or other white light source).  The slide projector's lamp can be used without additional collimation if placed several feet away from the prism. Several suppliers also sell diffraction gratings mounted in eyeglasses, and these can be used very inexpensively with room lighting to show the visible spectrum.

Working Open Lasers are available from several suppiers. These allow students to see inside a working helium-neon laser, which is safely enclosed in a transparent plastic box.

Diffusion Mist is available from several suppliers, and can be used to illustrate scattering, the paths of light rays in air, etc.

Measurement of absorption spectra Most chemistry departments have spectrophotometers with which you can measure absorption spectra of a few relevant molecules.  For example, you can either use diluted solutions of blood, or purchase hemoglobin from a chemical supply house such as Sigma-Aldritch.

Absorption of light by colored filters  To show how the absorption of different colored lasers depends upon the absorption of tissues, colored filters available from many scientific and optical supply companies can be used along with colored helium neon lasers.  One effective combination is the red and green helium neon lasers available from Edmund Scientific.  They can be used with red, green, and blue filters.  (Be sure to check the absorption and transparency properties of your filters in advance.)  To demonstrate at a very basic level the relation between the apparent color of objects and their absorption properties (e.g., why green laser light is absorbed by red pigments in blood) use the red and green laser light passed through the red filter--the red light is transmitted and the green is absorbed.  The reverse effect can be observed with the green filter, and the blue filter absorbs both red and green laser light.

Absorption of light by pigments in the body: The exponential attenuation of light by pigments like blood, and its dependence on concentration, is nicely demonstrated using a standard 1cm to an edge spectrometer cuvette. (Any flat sided glass or plastic container would work, but the disposable cuvettes are inexpensive and should be available from any chemistry department.) I use red food dye from a grocery store as the pigment. One drop of this from the original container in a full cuvette of water gives very nice, vislble attenuation over the 1cm width of the cuvette when a green laser pointer beam is directed into the cuvette. The appearance of the exponential attenuation is easily visible from the side. You can vary the dye concentration to show concentration dependence of penetration depth.

Photocoagulation and photovaporization with a laser  If you have access to a fairly powerful laser (our Nd:YAG, with 1 watt CW at 1064 nm does well, as does a pulsed Nd:YAG working in the green), you can show the effect of focusing the beam to achieve high power densities.  Be sure to have the class wear laser goggles and BE VERY CAREFUL with the beam!  An unfocused beam can often nicely photocoagulate a thin tissue sample.  (We use a Minit steak with good results).  The burn looks like a small cooked region.  Experiment with exposure times and intensities in advance.  With a lens, one can achieve higher power densities, enabling one to photovaporize tissue with a high power pulsed laser.  (You will probably have to hold the steak up to the light to see the clean hole produced by the focused beam.)

Hoberman sphere can be used to illustrate the fall-off of light intensity from a lamp sources varies as an inverse square law. Students can visualize how the area of the growing or expanding sphere depends on its radius.

Surgery with the sun:  A low tech version which the students can perform for themselves uses the favorite childhood trick of focusing the sun using a magnifying glass.  It is of course essential to teach the importance of never looking directly at the Sun with or without an external lens! With a large enough lens, one can achieve very high power densities and easily burn paper, etc.  You can also purchase heat lamps and use a large Fresnel lens to focus the IR onto a small spot. Students can the put their hands at the focus to experience the heating that results.  This lets them experience the effects of power density directly by feeling the difference in heating as they bring their hand closer to the focus.  Another version of this experiment involves using plastic sheets incorporating liquid crystals which change color on heating.  These sheets absorb sunlight effectively, and by choosing the right temperature range one can see a colored spot at the region of highest power density.  Using a key relating color to temperature, students can see the difference between the temperature rises caused by different diameter beams.  (Of course this exercise requires a good sunny day, preferably not in the winter!)

Lamp source with focusing lens:  The above demos should be followed up by contrasting this with the case of an intense lamp source.  Allow the students to convince themselves of the small power densities achievable with a variety of converging lenses.  They can of course safely shine even a small diameter beam onto their skin, use the liquid crystal sheets to compare the temperature rises, or use the tissue sample from the laser demonstration to see that no photocoagulation occurs.

A Model Eye with Optics is available from Pasco Scientific.

Chapter 4: Ultrasound Imaging

Standard Wave Demonstrations: Many standard freshman physics demonstrations for sound can be used at the beginning, including using a Slinky(TM) to demonstrate longitudinal waves.

Stethoscope: We also use a real stethoscope to remind students that sound can indeed be transmitted readily through the body.

Elasticity of Gases: We use a large hypodermic syringe sealed at one end to show students that the gas captured in the syringe has an elastic modulus, something many find surprising at first. When they push the plunger in, the trapped gas resists further compression. They can feel the spring-like force generated on compression.

Ball and spring Matter Model (Pasco Scientific): Literally a set of balls held together by springs in 1, 2 or 3 dimensions, this Matter Model allows students to visualize the deformations within solids as sound waves travel through them.

Ultrasound imaging demo with a sonar ranger (these are available from either Vernier Software or Pasco Scientific): A computerized sonar ranger (operating frequency 50 kHz) can be used to illustrate many topics in ultrasound imaging with expensive equipment. We set up a ranger on a table, so it is fixed, while the objects to be imaged move. Students provide the targets that reflect the ultrasound pulses. We explain how the echo ranging technique works, then show how it can be used to determine distances along the direction of travel of the sound wave. Next, students walk across the ultrasound beam, which typically fans out to subtend a cone 15 to 20 degrees wide. This shows how an ultrasound beam can be scanned from side to side to determine the dimensions of an object in a direction perpendicular to the direction of travel of the sound. Finally, you can show spatial resolution effects. The wavelength of the sound pulses are several mm typically. We get very good echoes from objects larger than this, but no echoes from a very small diameter object (like a very fine thread hung before the ranger). This last part is tricky, I warn you. We also show the effects of lateral spatial resolution (resolution perpendicular to the direction of travel of the sound) by using two meter sticks that move across the beam. If the two sticks are far apart compared to the 15 to 20 degree width of the beam, then they give separate echoes. If they are closer than this, they give a single echo.

An Actual Ultrasound Imaging Experiment is available complete with samples (phantoms) and gel from American 3B Scientifc.

Function generator with speaker and oscilloscope display  (A computer datalogger can be used to display the output signal also.)  Use to show the frequency range of audible sound, and to identify loudness with amplitude, and pitch with frequency.  You can only go a short way into the ultrasound regime, but it should be enough to convince the students of the existence of this limit.

Wave generator:  Available from Pasco, this consists of numerous parallel thin rods mounted like ribs on a stiff wire in such a way that torsional displacements of the wire result in easily visible displacements of the rods.  The wave generator allows one to nicely illustrate several wave properties.  Sections with different rod lengths, and hence different speeds, are available and can be linked together.  This allows one to demonstrate the reflection and transmission of waves at an interface between different speeds of sound.  Impedance matching can be demonstrated using a special matching section.  A simple damping device is provided to demonstrate absorption.

Ceramic piezoelectric disk:  Available from Radio Shack and many electronics suppliers.  The electronic buzzers commonly available have these as active elements, and by breaking open their packaging you can expose the disk.  Allows students to see materials similar to the active element in transducers.

Doppler effect demonstration:  Swinging a loudspeaker or electronic buzzer (securely tethered to a string) in circles at moderate speeds will create a very distinct change in pitch. Pasco Scientific sells a Doppler Cannon device that works well, but a low-tech homemade model is easy to make with a battery-powered speaker from Radio Shack or elsewhere.

Radar Gun that uses the Doppler Effect to measure speed: These are now available for about $100 (in 2006) from various vendors, including Edmund Scientific, Radio Shack and Educational innovations.

Wave tank:  Available from Sargent Welsh, among other suppliers.  Best used in a configuration which allows the projection of the waves with an overhead projector.  Use to show the effect of reflecting waves from objects greater than or less than one wavelength in size.  Use with an extended, linear source to generate very low frequency, long wavelength ripples.  Also an effective way of illustrating the effects of interference.

Ultrasound Doppler heart monitors are now commercially available for very little money (ours was about $100) for use in home fetal monitoring.  While we do not endorse the latter use, we do use them as a lecture demonstration, with the instructor showing how they can be used to monitor the heart rate by reflecting ultrasound off the heart itself or a carotid artery.

You also may be able to obtain videotapes of ultrasound imaging from a local hospital or from major medical manufacturers of diagnostic ultrasound equipment.  We own: 21st Century Medicine: Volume One: Operating in the Future, from Ambrose Video and Discovery Health Channel, and The Vision of Modern Medicine.

Chapter 5, 6, and 7:  X-ray Imaging, Nuclear Medicine, Radiation Therapy: Radioactivity and ionizing radiation

These chapters share similar demonstrations and equipment.

Sample X-ray Images are available, with explanations, from Ward Scientific

Geiger counters:  Versions for under $300 are available from Vernier Software and Edmund Scientific.  These are also calibrated in Sieverts/hour.  Vernier also supplies software and computer interfaces.  With these you can perform statistical analyses of counting experiments and plot counts as a function of time.  Useful for demonstrating the basic ideas behind counting statistics and for measuring background radiation levels.

Half-life demonstration:  Short half-life isotopes can be generated using kits available from Pasco.  A cesium-137 cow immobilized on a gel chromatography column decays with half-life of 30 years, yielding as a decay product barium-137m with half-life of only 2.55 minutes.  By flushing the column with a buffer solution (supplied), the short half-life daughter barium can be separated out on an ongoing basis.  Be sure to use with adequate shielding and protective gear (gloves, labcoats, and provisions for spills.)  Because of the very short half-life, no special provisions for disposing of the wastes are necessary if they are allowed to decay to background levels before disposal.  This use of "cows" to generate short half-life radioisotopes is the same idea used to generate technetium-99m (half-life 6 hours) supplies for medical imaging using a radioactive generator (cow) of Mo-99 (half-life 67 hours). The main experiment involves measuring the count rate from a given sample using  the Geiger counter and software described above.  This allows one to measure and plot the decay of source activity with time, extracting the half-life either by measuring the time for the count rate to diminish by a factor of two or by using an integral nonlinear least squares fitting routine built into the Vernier software.

You can emphasize the probabilistic roots of radioactive decay with a simple "enactment" of exponential decay.  Have everyone in the class stand and have each student roll a die on command.  (Special many-sided die from game store, or a calculator with a random number generator can also be used, with obvious modifications to the instructions.)  Each student obtaining a one sits down, while those with other numbers remain standing.  On each cycle, count the number of students who sat down--"decayed"--then repeat. Observe the number of cycles required to halve the number of students standing;  obviously this will work best with large classes.

Priscilla Laws (Dickinson College) and Ronald Thornton (Tufts University) suggest using the popping of popcorn in a microwave as a simple contrasting case.  The "decay" can be followed using a microphone and plotting the sound intensity as a function of time.)

Radiation shielding:  The Geiger counter and software described above can be used to measure the count rate from a long half-life source with a fairly large source activity.  A rigid, stable source-detector geometry should be used, with adequate shielding between the source and students.  Aluminum foils folded into varying thicknesses can then be used to determine the effect of absorber thickness on detected count rate at a fixed source-detector distance.  Students also can measure both the dependence of counts on distance from the source to see the inverse-square-law fall-off.  The Vernier software has provisions for plotting an additional variable (such as source-detector distance or absorber thickness) vs. measured detector counts, so the functional dependencies can also be determined for both cases.

Simple demonstrations of x-ray imaging using projections can be found on our Labs section of this website

Projection vs. cross-section:  It is helpful for students to distinguish clearly behind the projection given by a standard x-ray and cross-sections measured in tomography.  Simple ways of doing so involve making projections of partially transparent objects using an overhead (or the photographic paper sold for making sunprints).  Cross-sections can be generated from anatomical models if you have access to them, or simply by cutting cross-sections from a piece of fruit.  (This simple distinction seems to be surprisingly difficult for students to grasp properly, and is well worth demonstrating.) They can directly observe issues involved in the optics of x-ray imaging (particularly limits on resolution and magnification) using a "point" visible light source to cast shadows of hands or other objects on a screen. The distances between the object and screen, and the source and object can be varied to illustrate various topics.

Radiography bucky filters to remove scattering from x-ray images:  Laptop privacy filters are available for use in covering up laptop monitors to prevent others from seeing them.  These work like buckys in radiography:  they consist of a transparent plastic sheet embedded with a fine collimating grid.  Only light emitted approximately normal to the privacy filter can be transmitted, just like a bucky's operation.

Phosphorescent Vinyl Sheets (from Educational Innovations) can be used to teach about the use of similar devices in fluroscopy and other techniques.

We have built our own model Computed Tomography scanner, appropriate for use either in emission or transmission, based on the one described in this reference: "A simple medical physics experiment based on a laser pointer", Colin Delaney and Juan Rodriguez, Am. J. Phys. 70, 1068 (2002) This can be modified to provide a SPECT like instrument by using LED's mounted in place of the sample in the AJP paper. The rotational stage can also be used with laser to model multiple beam radiation therapy with small modifications. It is not necessary to have a motorized stage, since moving the apparatus by hand gets across the main points.

Chapter 8:  Magnetic Resonance Imaging

Inexpensive and easy magnetism demos:

Many experiments relevant to MRI can be performed using small bar magnets, compasses and a top.  Rather than merely demonstrating these yourself, be sure to pass the equipment around the room so each student can try it out.  Many students have never played with magnets before, so they may be surprised by the simplest phenomena, including attraction between unlike poles and repulsion between like poles.

Mapping magnetic field lines using either 1)  iron fillings in oil,  2) a compass or 3) a tiny bar magnet in a gimbal mount;  equipment for each experiment can be purchased from many scientific supply houses.  If possible use a large permanent magnet with an extremely strong field, and be sure to have the students move an actual bar magnet along the field lines, so they can directly feel with their hands the torque which results when one tries to twist a dipole off a field line, the relative ease of moving dipoles along field lines and the attractive force on a dipole due to strong magnetic field gradients.

Using a very strong permanent magnet with its poles close together as a example of the uniform fields encountered in MRI, utilizing the various ways of mapping out magnetic field lines described above.

The basic idea of magnetic resonance can be demonstrated using an analogy with the mechanical behavior of compass needles.  Compass needles mounted in air are best, although those mounted in viscous fluids can be used.  Compasses with transparent bases can be used on overhead projectors to make the demonstrations visible to the entire class.  Use a dipole magnet to displace a compass needle from its preferred orientation, then quickly remove the dipole so that the needle oscillates about North.  The time between swings gives the period of the needles "magnetic resonance", 1 Hz being a typical frequency.  This frequency depends upon magnetic field strength, as can be seen by performing the same experiment using a second dipole magnet, placed at various fixed distances, to provide the orienting field.  Then, the resonant frequency depends upon aligning field strength.  To illustrate the effects of forcing the compass needle at its resonant frequency, use a nearby dipole magnet oscillating at various frequencies (this can be accomplished most easily by rotating the magnet manually).  In addition, the time required for the oscillations to damp out is a measure of the compass's "relaxation time."

Since tops are no longer popular children's toys, you may want to demonstrate nutation, since this may be the first time many students have seen this phenomenon.

Magnetic Torque apparatus (TeachSpin) This allows students to visualize the torques on protons (or any dipole) in the various configurations used in MRI and NMR.

Pulsed NMR Experiment (TeachSpin Corp.) allows students to do actual pulsed NMR measurements. With adjustments to the sample they provide you can do spatially resolved imaging.

ESR Experiments (commercially available) to demonstrate NMR principles.

We also own a Defibrillator Training  Unit.  This looks exactly like a difibrillator from the outside, but it does not hold a charge and it costs a small fraction of the cost.

Bibliography

Ehrlich, Robert. 1990.  Turning the World Inside Out. Princeton University Press, Princeton, New Jersey.  216 pp.

The Exploratorium. 1991. The Exploratorium Science Snackbook. Exploratorium Teacher Institute, San Francisco.

Vernier Software Newsletter.  Vernier Software,  2920 S.W. 89th Street, Portland, OR 97225.  (503) 297-5317