Physics of diagnostic methodsThis chapter discusses physics-based diagnostictechnology—especially non-invasive methods.We begin with the classic method of auscultation,that is, listening to the characteristic sounds madeby internal organs.1. AuscultationDoubtless the doctor-priests of the Greek healingshrines of Æsklepios, as well as the Egyptian priestsof Thoth knew thousands of years ago that certainbody sounds are normal whereas others signal thepresence of disease. However, the science of aus-cultation did not come into its own until theinvention of the stethoscope by René Laënnec1 atthe beginning of the 19th Century.The first stethoscope was a hollow wooden tubewith a bell-shaped end that was pressed against thechest or abdomen of the patient. It may have beeninvented to spare the modesty of female patientsas well as improving the hygeine of the procedure.However, Laënnec realized that it provided themore important benefits of reducing backgroundnoise and amplifying the sound from the patient,thereby greatly increasing the chance of a correctdiagnosis. In particular the stethoscope made possible thediagnosis of such heart malfunctions as arrythmias,inflammation of the pericardium and damagedvalves. The latter manifest themselves through“murmurs”, resulting from the backflow of bloodfrom the aorta to the left ventricle (or from theventricle back into the atrium) through an incom-pletely closed valve.The sounds produced in the air tubes and alveolarsacs of the lungs during breathing are modified bythe presence of fluid, phlegm or inflammation, andcan be detected by auscultation. The technique of percussion was devised in 1761by a Viennese physician, Leopold Auenbrugger.The son of a publican, Auenbrugger applied themethod he had learned as a boy, of gauging thecontents of a cask by tapping it and listening to theresultant echos, to the detection of fluid and otherabnormalities in the chest cavity. This method,widely ignored during his lifetime, was greatly im-proved by the invention of the stethoscope.The standard binaural stethoscope was inventedby G.P. Cammann, a New York physician, in theearly 20th Century. It has survived with only minorimprovements into the 21st Century. Today, how-ever, electronically amplified models of the binau-ral stethoscope are widely available. Physics of the Human BodyChapter 13: Physics of diagnostic methods 1191. born Feb. 17, 1781, Quimper, France; died Aug. 13, 1826, Kerlouanec, France.2. Blood pressureNumerous pathological conditions are signalled bya change in blood pressure. Since there is a mod-erate range of blood pressures within a populationof healthy individuals, and since a given healthyindividual’s pressures can vary widely based ontime of day, activity level, current diet and psycho-logical state2, in order that blood pressure meas-urements have diagnostic value the physician mustknow the patient’s normal levels under similarcircumstances. This is one of the main reasons thathealthy individuals should subject themselves toannual or biennial physical examinations.Blood pressure is traditionally measured in milli-meters of mercury3. The most precise blood pres-sure readings are obtained by inserting cathetersinto arteries and veins. However, the sphygmoma-nometer4, shown to the right is accurate enough formany purposes and completely non-invasive. Itwas invented in 1896 by Italian physician ScipioneRiva-Rocci, who combined an inflatable cuff witha mercury manometer. One records both the peakpressure produced by the heartbeat (systolic pres-sure) as well as the resting pressure (diastolic pres-sure). The measurement involves compressing amajor artery (most often the brachial artery) untilno pulse is heard by a stethoscope placed down-stream (that is, in the portion of the artery distalto the point of compression). The external pres-sure required to do this is the systolic pressure. Thecuff pressure is gradually reduced while listening tothe pulse. Since the artery is only partially com-pressed, one hears a pulse that gets louder, thenceases. The pressure at the second cessation ofsound is the diastolic pressure.Elevated blood pressure—especially diastolic pres-sure—can signal the presence of various pathologi-cal conditions, generally related to narrowing ofthe arteries, or reduction of elasticity of the arterialwalls. For example, some renal malfunctions causeelevated blood pressure. Excessive systolic pressurecan cause hemorrhages in the brain, leading to oneform of stroke.120 Chapter 13: Physics of diagnostic methods2. For example, many patients exhibit “white coat syndrome” wherein having one’s pressures measured bya nurse or assistant, in the venue of a doctor’s office or hospital emergency room, can raise the systolicblood pressure by 10 mm or more. 3. The original pressure gauges employed columns of mercury, akin to barometers.4. …from the Greek word sphygmo- (“of the pulse”) and the French word manometre (“pressure meter”).3. Ultrasound imagingThe use of short wavelength sound waves to imageinternal structures non-invasively has become oneof the staple tools of modern medicine. This is anexample of “spin-off”—the application of a tech-nology developed for specialized purposes to abroader market. Ultrasound imaging is an out-growth of sonar5, developed originally to locatesubmarines in naval warfare; then applied to locat-ing fish schools and depth sounding; and finally, tomedical diagnosis.The typical speed of sound in the body is about thatin water, 1500 m/sec. In order to distinguish twofeatures that are spatially separated by a distanceD, any visualization method using waves mustemploy waves of wavelength λ ≤ D. Thus, to“see” structures about 2 mm in size, the sound musthave a frequencyf ≈ usD = 7.5×105 Hz .Below we exhibit an example ultrasound image6 ofa human carotid artery at about 2 mm resolution,exhibiting about 80% blockage. In order to examine a structure at a given depthwith the body, the ultrasounic transducer emitsbrief pulses of sound—rather the way a bat echolo-cates its prey—and times the reflected pulses. Byincreasing the time interval before the receiver can“hear” the echo, the operator increases the roundtrip distance the sound has traveled, thereby in-creasing the depth. In this way, three-dimensionalvisualizations can be created.A major virtue of ultrasound imaging is that soundreflected from a moving structure will have itsfrequency shifted