How the Doppler Effect Works

7/26/2019 How the Doppler Effect Works

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How the Doppler Effect Works

A Doppler weather radar tower

If you like riddles, you'll like this one: How can a musician playing a single note on a hornchange that note without changing the way he or she plays that note? At first, you might thinkthis is a trick question. learly, the musician must do something to change the pitch, right?!rong. If the musician plays the same note while mo"ing toward or away from a stationarylistener, the note heard #y the listener will indeed change $$ e"en if the musician does nothingdifferent.

Dutch scientist hristoph Hendrik Diederik %uys %allot conducted this "ery e&periment in()*. He assem#led a group of horn players and placed them in an open cart attached to alocomoti"e. +hen he had the engineer start up the locomoti"e so it could carry the cart,complete with the horn players, #ack and forth along the track. As they were #eing pulled, themusicians played a single note on their horns. %allot stationed himself #eside the track andlistened carefully, #oth as the train approached and receded. And the notes he heard weredifferent than the notes #eing played #y the musicians.

Although unusual, %allot's e&periment demonstrated clearly one of the most important wa"ephenomena known to scientists. +he phenomenon is called the Doppler effectafter Austrianmathematician hristian ohann Doppler, who first predicted this odd #eha"ior of sound in()-. +oday, scientists know that the Doppler effect applies to all types of wa"es, includingwater, sound and light. +hey also ha"e a good idea why the Doppler effect occurs. Andthey'"e incorporated its principles into a "ariety of useful tools and gadgets.

In this article, we'll e&amine e"erything Doppler: the man, the science and the technologies.%ut first we ha"e to lay some groundwork. %ecause the Doppler effect is a phenomenonassociated with wa"es, let's start #y co"ering some #asics a#out the two #asic types of wa"es$$ light and sound.
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Wave Basics

!hen most people think of wa"es, they think of waterwa"es. %ut lightand sound also tra"elas wa"es. A light wa"e, like a water wa"e, is an e&ample of a transverse wave, which causesa distur#ance in a medium perpendicular to the direction of the ad"ancing wa"e. In thediagram #elow, you can also see how trans"erse wa"es form crests and troughs.

+he distance #etween any two crests or any two troughs/ is the wavelength, while the heightof a crest or the depth of a trough/ is the amplitude. Frequencyrefers to the num#er ofcrests or troughs that pass a fi&ed point per second. +he frequency of a light wa"e determinesits color, with higher frequencies producing colors on the #lue and "iolet end of the spectrumand lower frequencies producing colors on the red end of the spectrum.

0ound wa"es are not trans"erse wa"es. +hey are longitudinal waves, created #y some typeof mechanical "i#ration that produces a series of compressions and rarefactions in a medium.+ake a woodwind instrument, such as a clarinet. !hen you #low into a clarinet, a thin reed

#egins to "i#rate. +he "i#rating reed first pushes against air molecules the medium/, thenpulls away. +his results in an area where all of the air molecules are pressed together and,

right #eside it, an area where air molecules are spread far apart. As these compressions andrarefactions propagate from one point to another, they form a longitudinal wa"e, with thedistur#ance in the medium mo"ing in the same direction as the wa"e itself.
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If you study the diagram of the wa"e a#o"e, you'll see that longitudinal wa"es ha"e the same#asic characteristics as trans"erse wa"es. +hey ha"e wa"elength the distance #etween twocompressions/, amplitude the amount the medium is compressed/ and frequency the num#erof compressions that pass a fi&ed point per second/. +he amplitude of a sound wa"edetermines its intensity, or loudness. +he frequency of a sound wa"e determines its pitch,with higher frequencies producing higher notes. 1or e&ample, the open si&th string of a guitar

"i#rates at a frequency of (-.)23 hert4 cycles per second/ and produces a lower pitch. +heopen first string "i#rates at a frequency of 5-6.75 hert4 and produces a higher pitch.

As we'll see in the ne&t section, the Doppler effect is directly related to the frequency of awa"e, whether it's made of water, light or sound.

Wave Frequency
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8et's #egin our dissection of the Doppler effect #y considering a source that creates wa"es inwaterat a certain frequency. +his source produces a series of wa"e fronts, with each mo"ingoutward in a sphere centered on the source. +he distance #etween wa"e crests $$ thewa"elength $$ will remain the same all the way around the sphere. An o#ser"er in front of thewa"e source will see the wa"es equally spaced as they approach. 0o will an o#ser"er located

#ehind the wa"e source.

9ow let's consider a situation where the source is not stationary, #ut is mo"ing to the right asit produces wa"es. %ecause the source is mo"ing, it #egins to catch up to the wa"e crests onone side while it mo"es away from the crests on the opposite side. An o#ser"er located infront of the source will see the crests all #unched up. An o#ser"er located #ehind the sourcewill see the wa"es all stretched out. emem#er, the frequency equals the num#er of wa"esthat pass a specific point per second, so the o#ser"er in front actually sees a higher frequencythan the o#ser"er in #ack of the source.

+he scenario a#o"e descri#es wa"es formed in water, #ut it also applies to sound wa"es and

lightwa"es. 0ound wa"es are heard, not seen, so the o#ser"er will hear the #unched$upwa"es as a higher$pitched sound, the stretched$out wa"es as a lower$pitched sound. 1ore&ample, consider acartra"eling down a highway #etween two o#ser"ers, as shown #elow.+he roar of the engineand friction #etween the tiresand the road surface create a noise $$"room $$ that can #e heard #y #oth o#ser"ers and #y the dri"er.

+o the dri"er, this noise will not change. %ut the o#ser"er located in front of the car will heara higher$pitched noise. !hy? %ecause the sound wa"es compress as the "ehicle approachesthe o#ser"er located in front. +his increases the frequency of the wa"e, and the pitch of the"room rises. +he o#ser"er located #ehind the car will hear a lower$pitched noise #ecause thesound wa"es stretch out as the car recedes. +his decreases the frequency of the wa"e, and the

pitch of the "room falls.

8ight wa"es are percei"ed as color, so the o#ser"er will sense the #unched$up wa"es as a#luer color, the stretched$out wa"es as a redder color. 1or e&ample, consider an astronomero#ser"ing a gala&ythrough a telescope. If the gala&y is rushing toward ;arth, the light wa"esit produces will #unch up as it approaches the astronomer's telescope. +his increases thefrequency of the wa"e, which shifts the colors of its spectral output toward the #lue. If thegala&y is rushing away from ;arth, the light wa"es it produces will spread apart as it recedesfrom the astronomer's telescope. +his decreases the frequency of the wa"e, which shifts thecolors of its spectral output toward the red.

As you can imagine, astronomers routinely take ad"antage of the Doppler effect to measurethe speed at which planets, stars and gala&ies are mo"ing. %ut its usefulness isn't limited toouter space. Doppler's disco"ery is integral to se"eral applications right here on ;arth.

The Origin of the Universe: A Shift in Thinking

In 6-6, ;dwin Hu##le noticed that light coming from almost e"ery gala&y he studied wasshifted, according to the Doppler effect, to the red end of the spectrum. He argued that onlygala&ies mo"ing away from our gala&y could produce these


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Busting the Boom

+he Doppler effect is used in many technologies that #enefit people. %ut it can ha"e anegati"e impact, as well. 1or e&ample, sonic #ooms, which are caused #y supersonicaircraft,can cause o#=ectiona#le sounds and "i#rations on the ground, which is why supersonic

airplanes are not allowed to fly o"erpopulatedareas. 0onic #ooms are directly related to theDoppler effect. +hey occur when airplanes, flying at the speed of sound or higher, actually flyfaster than the sound wa"es they are producing. All of the wa"es #unch up #ehind the craft, inan e&tremely small space. !hen the #unched$up wa"es reach an o#ser"er, they are ne such in"ention is a spike e&tending from the nose of the airplane. +his spikeessentially lengthens the plane and distri#utes the wa"es o"er a greater distance. +his reducesthe #oom e&perienced #y an o#ser"er on the ground.

Practical Applications of the Doppler Effect

In the 72 years or so since Doppler first descri#ed the wa"e phenomenon that would cementhis place in history, se"eral practical applications of the Doppler effect ha"e emerged to ser"esociety. In all of these applications, the same #asic thing is happening: A stationarytransmitter shoots wa"es at a mo"ing o#=ect. +he wa"es hit the o#=ect and #ounce #ack. +hetransmitter now a recei"er/ detects the frequency of the returned wa"es. %ased on theamount of the Doppler shift, the speed of the o#=ect can #e determined. 8et's look at a fewspecific e&amples.

olice adar

+he handheld radarguns used #y police to check for speeding "ehicles rely on the Dopplereffect. Here's how they work:

1. A police ofcer takes a position on the side o the road.2. The ofcer aims his radar gun at an approaching vehicle. The gun sends

out a burst o radiowaves at a particular requency.

3. The radio waves strike the vehicle and bounce back toward the radar gun.

. The radar gun measures the requency o the returning waves. !ecausethe caris moving toward the gun" the requency o the returning waveswill be higher than the requency o the waves initially transmitted by thegun. The aster the car#s speed" the higher the requency o the returningwave.

$. The di%erence between the emitted requency and the re&ected requencyis used to determine the speed o the vehicle. A computer inside the gunperorms the calculation instantly and displays a speed to the ofcer.

Doppler adar
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@eteorologists use a similar principle to read weather e"ents. In this case, the stationarytransmitter is located in a weather station and the mo"ing o#=ect #eing studied is a stormsystem. +his is what happens:

1. 'adio waves are emitted rom a weather station at a speci(c requency.

2. The waves are large enough to interact with clouds and other atmosphericob)ects. The waves strike ob)ects and bounce back toward the station.

3. * the clouds or precipitation are moving away rom the station" therequency o the waves re&ected back decreases. * the clouds orprecipitation are moving toward the station" the requency o the wavesre&ected back increases.

. +omputersin the radar electronically convert ,oppler shit data about there&ected radio waves into pictures showing wind speeds and direction.

Doppler images are not the same as reflecti"ity images. eflecti"ity images also rely on

radar, #ut they are not #ased on changes in wa"e frequency. Instead, a weather station sendsout a #eam of energy, then measures how much of that #eam is reflected #ack. +his data isused to form the precipitation intensity images we see all the time on weather maps, where

#lue is lightprecipitation and red is hea"y precipitation.

Doppler ;chocardiogram

A traditional echocardiogram uses sound wa"es to produce images of theheart. In thisprocedure, a radiologist uses a transducer to transmit and recei"e ultrasoundwa"es, which arereflected when they reach the edge of two structures with different densities. +he image

produced #y an echocardiogram shows the edges of heart structures, #ut it cannot measurethe speed of#loodflowing through the heart. Doppler techniques must #e incorporated to

pro"ide this additional information. In a Doppler echocardiogram, sound wa"es of a certainfrequency are transmitted into the heart. +he sound wa"es #ounce off #lood cellsmo"ingthrough the heart and #lood "essels. +he mo"ement of these cells, either toward or away fromthe transmitted wa"es, results in a frequency shift that can #e measured. +his helpscardiologists determine the speed and direction of #lood flow in the heart.

hristian Doppler
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ame !ecognition

In 66-, Austriamarked the *2th anni"ersary of the disco"ery of the Doppler effect #yreleasing a stamp featuring the thin face of hristian ohann Doppler. Although Doppler

ne"er could ha"e imagined such a tri#ute, he did grasp the significance of his work from the"ery #eginning. In the ()- paper that first descri#ed the phenomenon, Doppler offered this

prediction:


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"Bedside" or continuous wave Doppler#+his type uses the change in pitch of thesound wa"es to pro"ide information a#out #lood flow through a #lood "essel. +hedoctor listens to the sounds produced #y the transducer to e"aluate the #lood flowthrough an area that may #e #locked or narrowed. +his type of ultrasound can #e doneat the #edside in the hospital with a porta#le machine to pro"ide a fast estimate of the

e&tent of #lood "essel damage or disease. Duple$ Doppler#Duple& Doppler ultrasound uses standard ultrasound methods to

produce a picture of a #lood "essel and the surrounding organs. Also, a computercon"erts the Doppler sounds into a graph that gi"es information a#out the speed anddirection of #lood flow through the #lood "essel #eing e"aluated.

%olor Doppler#olor Doppler uses standard ultrasound methods to produce a pictureof a #lood "essel. Also, a computer con"erts the Doppler sounds into colors that areo"erlaid on the image of the #lood "essel and that represent the speed and direction of

#lood flow through the "essel. ower Doppler is a special type of color Doppler.ower Doppler can get some images that are hard or impossi#le to get using standard

color Doppler. ower Doppler is most commonly used to e"aluate #lood flow through"essels within solid organs.

Why It Is Done

Doppler ultrasoundis done to:

-ind blood clots and blocked or narrowed blood vessels in almost any parto the body" especially in the neck" arms" and legs.

valuate leg pain that may be caused by intermittent claudication" acondition caused by atherosclerosiso the lower e/tremities.

valuate blood &ow ater a stroke or other condition that might be causedby a problem with blood &ow. valuation o a stroke can be done through atechnique called transcranial ,oppler 0T+, ultrasound.

valuate abnormal veins causing varicose veinsor other problems.

ap veins that may be used or blood vessel grats. *t also can check thecondition o grats used to bypass blockage in an arm or leg.

-ind out the amount o blood &ow to a transplanted kidneyor liver.

onitor the &ow o blood ollowing blood vessel surgery.

-ind out the presence" amount" and location o arterial plaque. laque inthe carotid arteries can reduce blood &ow to the brainand may increasethe risk o stroke.

4uide treatment such as laser or radiorequency ablation o abnormalveins.

+heck the health o a etus. !lood &ow in the umbilical cord"through theplacenta" or in the heartand brain o the etus may be checked. This testcan show i the etus is getting enough o/ygen and nutrients. ,oppler

ultrasound may be used to guide decisions during pregnancy when5
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o The etus is smaller than normal or his or her gestational age 0etalgrowth restriction. !lood &ow through the large blood vessel in theumbilical cord 0the umbilical artery can be looked at.

o 'h sensiti6ationhas occurred. !lood &ow through a blood vessel inthe brain 0the middle cerebral artery" or +A can be used tomonitor etal health.

o The mother has other problems" such as preeclampsiaor sickle celldisease.

A transcranial Doppler ultrasound +D/ may #e used in children with sickle celldisease toe"aluate their risk of stroke. In adults, +D can #e used to e"aluate #lood flow in the #rain.

How It Is Done continued...

Eel is applied to the skin to promote the passage of the sound wa"es. +he transducer is placedin the gel and mo"ed along the skin. Fou need to lie "ery still during the procedure. Fou mayhear sounds that represent the flow of #lood through the #lood "essels.

+he test usually takes 52 to 72 minutes.

Arteries in the rms nd !egs

+his test is often performed on #oth arms or #oth legs. ;"en if the suspected #lood flowpro#lem is in only one lim#, #oth may #e tested for comparison. If your arms are #eingtested, they will #e tested first while you are lying down and then again while you are sitting.

Depending on which #lood "essels are #eing tested, a#lood pressurecuff may #e wrappedaround one or #oth lim#s so that the #lood pressure can #e taken at se"eral different places.!hen testing the legs, a #lood pressure cuff may #e wrapped first around the calfand thenaround the thigh. +he test may #e done at se"eral locations on your leg. !hen testing thearms, the pressure cuff may #e wrapped first around the forearm and then around the upperarm.

+esting may #e done #efore and after e&ercise, if you are healthy enough.

"eins in the rms nd !egs

1or this test, you will #e asked to lie down and #reathe normally. Fou must lie "ery still. Anychanges in #lood flow that occur as a response to your #reathing patterns are noted.

+he test may #e repeated while the e&aminer presses on the "eins close to the surface of yourskin to help detect a clot in the "ein called a compression maneu"er/. +he e&aminer may dothis with your legs or arms in different positions to ensure that the #lood supply is not

#locked in these positions. +he e&aminer may also squee4e your calf or forearm to help #loodmo"e more quickly through the "eins called an augmentation maneu"er/. +his is done toe"aluate #lood flow toward your heart.
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!hile your legs are #eing tested, you may also #e asked to try to #reathe out strongly withyour nose pinched and your mouth closed called aalsal"a maneu"er/. +his maneu"erusually causes a sudden change in #lood flow through the "eins.

Arteries in the neck

Fou will #e asked to lie down with a pillow underneath your head for support. +he test isperformed on #oth sides of your neck, and then the results are compared to standard "alues todetermine the amount of #lockage or narrowing of the arteries.

Trnscrni! u!trsound

1or a transcranial ultrasound, the transducer is passed lightly o"er the skin at the #ase or sideof your skull.

During #regnncy

+he transducer is mo"ed #ack and forth on your #elly until the doctor finds the #lood "esselthat needs to #e studied. After the doctor has found the #lood "essel, it may take some time tocheck the #lood flow.

How It $ee!s

+here is normally no discomfort in"ol"ed with ha"ing a Doppler ultrasound test. +he gel mayfeel cold when it is put on your skin unless it is first warmed to #ody temperature. If your

#lood pressure is taken during the test, you will feel pressure when the #lood pressure cuff isinflated.

%isks

+here are no known risks associated with a Doppler ultrasound test. +his test will not harm afetus.

%esu!ts

A Doppler ultrasoundtest uses reflected sound wa"es to see how #lood flows through a #lood"essel.

Do##!er u!trsound

&orm!: There are no (ndings o signi(cant narrowing or other abnormality in

any o the arteries e/amined.

There is no evidence o a clot in any o the veins e/amined. The si6e

and position o veins are normal.

7ormal blood &ow is ound in the blood vessels that supply o/ygen and
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nutrients to a etus.

A'norm

!:

-or continuous wave ,oppler or duple/ ,oppler" di%erences in blood

&ow between the right and let sides o the body may be heard. At the

e/act location where an artery is blocked or narrowed" the sound may

be high8pitched or turbulent. !lockage 0such as rom a blood clot" an

aneurysm" or narrowing o a blood vessel may be detected. The speed

o blood &ow may be compared to standard values to (nd out the

amount o blockage or narrowing o the blood vessel.

A duple/ ,oppler ultrasound graph may show irregular &ow that

means a blocked or narrowed blood vessel.

A color ,oppler image may show a blocked or narrowed blood vessel

or an aneurysm.

*n the veins" a blood clot may be indicated i blood &ow does notchange in response to breathing or does not increase in response to

either a compression maneuver or 9alsalva maneuver. *ncomplete

blockage o a vein by a blood clot may be seen on color ,oppler or

during a compression maneuver.

Abnormal veins" such as varicose veins" are seen.

!lood &ow through the blood vessels that supply o/ygen and nutrients

to a etus is abnormally increased or decreased.

Wht A(ects the Test

easons you may not #e a#le to ha"e the test or why the results may not #e helpful include:

!ones above the area being studied or gas in the intestines. 7ot being able to remain still during the test.

/treme obesity.

*rregular heart rhythms 0arrhythmias or heart disease" which may cause

changes in blood &ow patterns even though the blood vessels are notabnormal.

:aving a cold arm or leg. !lood &ow through that limb may be slowed.

:aving an open wound in the area that needs to be viewed
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How &ltrasound Works

+here are many reasons to get an ultrasound. erhaps you're pregnant, and your o#stetricianwants you to ha"e an ultrasound to check on the de"eloping #a#y or determine the due date.

@ay#e you're ha"ing pro#lems with#loodcirculation in a lim# or your heart, and your doctorhas requested a Doppler ultrasound to look at the #lood flow. Gltrasound has #een a popularmedical imaging technique for many years.

&ltrasoundor ultrasonographyis a medical imaging technique that uses high frequencysound wa"es and their echoes. +he technique is similar to the echolocation used #y #ats,whales and dolphins, as well as 0>9A used #y su#marines.

In this article, we'll look at how ultrasound works, what type of ultrasound techniques area"aila#le and what each technique can #e used for.
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&ltrasound image of a growing fetus 'appro$imately () weeks old* inside a mother+s

uterus# ,his is a side view of the -a-y. showing 'right to left* the head. neck. torso and

legs#

What is &ltrasound/

In ultrasound, the following e"ents happen:

. +he ultrasound machine transmits high$frequency to * megahert4/ sound pulsesinto your #ody using a pro#e.-. +he sound wa"es tra"el into your #ody and hit a #oundary #etween tissues e.g.

#etween fluid and soft tissue, soft tissue and #one/.

5. 0ome of the sound wa"es get reflected#ack to the pro#e, while some tra"el on furtheruntil they reach another #oundary and get reflected.

). +he reflected wa"es are picked up #y the pro#e and relayed to the machine.

*. +he machine calculates the distance from the pro#e to the tissue or organ #oundaries/using the speed of sound in tissue *,22* fts or,*)2 ms/ and the time of the each

echo's return usually on the order of millionths of a second/.

7. +he machine displays the distances and intensities of the echoes on the screen,forming a two dimensional image like the one shown #elow.

In a typical ultrasound, millions of pulses and echoes are sent and recei"ed each second. +hepro#e can #e mo"ed along the surface of the #ody and angled to o#tain "arious "iews.
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An ultrasound machine

,he &ltrasound 0achine

A #asic ultrasound machine has the following parts:

,ransducer pro-e$ pro#e that sends and recei"es the sound wa"es %entral processing unit '%P&*$ computer that does all of the calculations and

contains the electrical power supplies for itself and the transducer pro#e

,ransducer pulse controls$ changes the amplitude, frequency and duration of thepulses emitted from the transducer pro#e

Display$ displays the image from the ultrasound data processed #y the G

1ey-oard2cursor$ inputs data and takes measurements from the display

Disk storage devicehard, floppy, D/ $ stores the acquired images

Printer$ prints the image from the displayed data

+he transducer pro#e is the main part of the ultrasound machine. +he transducer pro#e makesthe sound wa"es and recei"es the echoes. It is, so to speak, the mouth and ears of theultrasound machine. +he transducer pro#e generates and recei"es sound wa"es using a

principle called the pie3oelectricpressure electricity/ effect, which was disco"ered #yierre and acques urie in ((2. In the pro#e, there are one or more quart4crystals calledpie3oelectric crystals. !hen an electric current is applied to these crystals, they changeshape rapidly. +he rapid shape changes, or "i#rations, of the crystals produce sound wa"esthat tra"el outward. on"ersely, when sound or pressure wa"es hit the crystals, they emit

electrical currents. +herefore, the same crystals can #e used to send and recei"e sound wa"es.
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+he pro#e also has a sound a#sor#ing su#stance to eliminate #ack reflections from the pro#eitself, and an acoustic lens to help focus the emitted sound wa"es.

+ransducer pro#es come in many shapes and si4es, as shown in the photo a#o"e. +he shape ofthe pro#e determines its field of "iew, and the frequency of emitted sound wa"es determines

how deep the sound wa"es penetrate and the resolution of the image. +ransducer pro#es maycontain one or more crystal elements in multiple$element pro#es, each crystal has its owncircuit. @ultiple$element pro#es ha"e the ad"antage that the ultrasounc #eam can #e ncethe raw data are processed, the G forms the image on the monitor. +he G can also storethe processed data andor image on disk.
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+he transducer pulse controls allow the operator, called the ultrasonographer, to set andchange the frequency and duration of the ultrasound pulses, as well as the scan mode of themachine. +he commands from the operator are translated into changing electric currents thatare applied to the pie4oelectric crystals in the transducer pro#e.

5$D ultrasound images

Different ,ypes of &ltrasound

+he ultrasound that we ha"e descri#ed so far presents a two$dimensional image, or


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0a6or &ses of &ltrasound

Gltrasound has #een used in a "ariety of clinical settings, including o#stetrics andgynecology, cardiology and cancer detection. +he main ad"antage of ultrasound is that

certain structures can #e o#ser"ed without using radiation. Gltrasound can also #e done muchfaster than J$rays or other radiographic techniques. Here is a short list of some uses forultrasound:

O'stetrics nd )yneco!ogy

measuring the si6e o the etus to determine the due date determining the position o the etus to see i it is in the normal head down

position or breech

checking the position o the placenta to see i it is improperly developingover the opening to the uterus 0cervi/

seeing the number o etuses in the uterus

checking the se/ o the baby 0i the genital area can be clearly seen

checking the etus#s growth rate by making many measurements overtime

detecting ectopic pregnancy" the lie8threatening situation in which thebaby is implanted in the mother#s -allopian tubes instead o in the uterus

determining whether there is an appropriate amount o amniotic &uid

cushioning the baby monitoring the baby during speciali6ed procedures 8 ultrasound has been

helpul in seeing and avoiding the baby during amniocentesis 0sampling othe amniotic &uid with a needle or genetic testing. ;ears ago" doctors useto perorm this procedure blindly< however" with accompanying use oultrasound" the risks o this procedure have dropped dramatically.

seeing tumors o the ovary and breast

*rdio!ogy

seeing the inside o the heart to identiy abnormal structures or unctions measuring blood &ow through the heart and ma)or blood vessels

Uro!ogy

measuring blood &ow through the kidney seeing kidney stones

detecting prostate cancer early

In addition to these areas, there is a growing use for ultrasound as a rapid imaging tool fordiagnosis in emergency rooms.
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+here ha"e #een many concerns a#out the safety of ultrasound. %ecause ultrasound is energy,the question #ecomes

the e&am.

5. +he ultrasonographer applies a mineral oil$#ased =elly to your skin $$ this =ellyeliminates air #etween the pro#e and your skin to help pass the sound wa"es into your

#ody.

). +he ultrasonographer co"ers the pro#e with a plastic co"er.

*. Heshe passes the pro#e o"er your skin to o#tain the required images. Dependingupon the type of e&am, the pro#e may #e inserted into you.

7. Fou may #e asked to change positions to get #etter looks at the area of interest.

3. After the images ha"e #een acquired and measurements taken, the data is stored ondisk. Fou may get a hard copy of the images.

(. Fou are gi"en a towelette to clean up.

6. Fou get dressed.

(7# ,he Future of &ltrasound11.As with other computer technology, ultrasound machines will most likely get faster

and ha"e more memory for storing data. +ransducer pro#es may get smaller, and moreinserta#le pro#es will #e de"eloped to get #etter images of internal organs. @ostlikely, 5$D ultrasound will #e more highly de"eloped and #ecome more popular. +heentire ultrasound machine will pro#a#ly get smaller, perhaps e"en hand$held for usein the field e.g. paramedics, #attlefield triage/. >ne e&citing new area of research is

the de"elopment of ultrasound imaging com#ined with heads$up"irtual reality$type
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displaysthat will allow a doctor to


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+hat may #e little comfort to you when you're getting ready for an @I e&am. Fou're strippedof your =ewelry and credit cards and asked detailed questions a#out all the metallicinstruments you might ha"e inside of you. Fou're put on a tiny sla# and pushed into a holethat hardly seems large enough for a person. Fou're su#=ected to loud noises, and you ha"e tolie perfectly still, or they're going to do this to you all o"er again. And with each minute, you

can't help #ut wonder what's happening to your #ody while it's in this machine. ould itreally #e that this ordeal is truly #etter than another imaging technique, such as an J$ray or aA+ scan? !hat has aymond Damadian wrought?

0!8 0agnets9 the 0a6or Players

+he components of an @I system

@I scanners "ary in si4e and shape, and some newer models ha"e a greater degree ofopenness around the sides. 0till, the #asic design is the same, and the patient is pushed into atu#e that's only a#out -) inches 72 centimeters/ in diameter source: HornakB. %ut what's inthere?

+he #iggest and most important component of an @I system is the magnet. +here is ahori4ontal tu#e $$ the same one the patient enters $$ running through the magnet from front to

#ack. +his tu#e is known as the -ore. %ut this isn't =ust any magnet $$ we're dealing with anincredi#ly strong system here, one capa#le of producing a large, sta#le magnetic field.

+he strength of a magnet in an @I system is rated using a unit of measure known as a tesla.Another unit of measure commonly used with magnets is the gauss tesla L 2,222 gauss/.

+he magnets in use today in @I systems create a magnetic field of 2.*$tesla to -.2$tesla, or
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*,222 to -2,222 gauss. !hen you reali4e that the ;arth's magnetic field measures 2.* gauss,you can see how powerful these magnets are.

@ost @I systems use a superconducting magnet, which consists of many coils orwindings of wire through which a current of electricity is passed, creating a magnetic field of

up to -.2 tesla. @aintaining such a large magnetic field requires a good deal of energy, whichis accomplished #y superconductivity , or reducing the resistance in the wires to almost 4ero.+o do this, the wires are continually #athed in liquid helium at )*-.) degrees #elow 4ero1ahrenheit -76. #elow 4ero degrees elsius/ source: oyneB. +his cold is insulated #y a"acuum. !hile superconducti"e magnets are e&pensi"e, the strong magnetic field allows forthe highest$quality imaging, and superconducti"ity keeps the system economical to operate.

,he :ther Parts of an 0!8 0achine

@I De"elopments

@I machines are e"ol"ing so that they're more patient$friendly. 1or e&ample, manyclaustropho#icpeople simply can't stand the cramped confines, and the #ore may notaccommodate o#ese people. +here are more open scanners, which allow for greater space, #utthese machines ha"e weaker magnetic fields, meaning it may #e easier to miss a#normaltissue. ery small scanners for imaging specific #ody parts are also #eing de"eloped.

>ther ad"ancements are #eing made in the field of @I. Functional 0!8f0!8/, fore&ample, creates#rain mapsof ner"e cell acti"ity second #y second and is helping researchers

#etter understand how the #rain works. 0agnetic resonance angiography0!A/ creates

images of flowing #lood, arteries and "eins in "irtually any part of the #ody.

+wo other magnets are used in @I systems to a much lesser e&tent. !esistive magnetsarestructurally like superconducting magnets, #ut they lack the liquid helium. +his differencemeans they require a huge amount of electricity, making it prohi#iti"ely e&pensi"e to operatea#o"e a 2.5 tesla le"el. Permanent magnetsha"e a constant magnetic field, #ut they're sohea"y that it would #e difficult to construct one that could sustain a large magnetic field.

+here are also three gradient magnetsinside the @I machine. +hese magnets are muchlower strength compared to the main magnetic field they may range in strength from (2gauss to -32 gauss. !hile the main magnet creates an intense, sta#le magnetic field around

the patient, the gradient magnets create a "aria#le field, which allows different parts of the#ody to #e scanned.

Another part of the @I system is a set of coils that transmit radiofrequency wa"es into thepatient's #ody. +here are different coils for different parts of the #ody: knees, shoulders,wrists, heads, necks and so on. +hese coils usually conform to the contour of the #ody part

#eing imaged, or at least reside "ery close to it during the e&am. >ther parts of the machineinclude a "ery powerful computer system and a patient ta#le, which slides the patient into the

#ore. !hether the patient goes in head or feet first is determined #y what part of the #odyneeds e&amining. >nce the #ody part to #e scanned is in the e&act center, or isocenter, of themagnetic field, the scan can #egin
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Hydrogen Atoms and 0agnetic 0oments

+he steps of an @I

!hen patients slide into an @I machine, they take with them the #illions of atoms thatmake up the human #ody.1or the purposes of an @I scan, we're only concerned with thehydrogen atom, which is a#undant since the #ody is mostly made up of water andfat.+heseatoms are randomly spinning, or precessing, on their a&is, like a child's top. All of the atomsare going in "arious directions, #ut when placed in a magnetic field, the atoms line up in thedirection of the field.

+hese hydrogen atoms ha"e a strong magnetic moment, which means that in a magneticfield, they line up in the direction of the field. 0ince the magnetic field runs straight down thecenter of the machine, the hydrogen protons line up so that they're pointing to either the

patient's feet or the head. A#out half go each way, so that the "ast ma=ority of the protonscancel each other out $$ that is, for each atom lined up toward the feet, one is lined up towardthe head. >nly a couple of protons out of e"ery million aren't canceled out. +his doesn'tsound like much, #ut the sheer num#er of hydrogen atoms in the #ody is enough to createe&tremely detailed images. It's these unmatched atoms that we're concerned with now.

What Else 8s ;oing on in an 0!8


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particular frequency, in a particular direction. +he specific frequency of resonance is calledthe =armour frequencyand is calculated #ased on the particular tissue #eing imaged and thestrength of the main magnetic field.

At appro&imately the same time, the three gradient magnets =ump into the act. +hey are

arranged in such a manner inside the main magnet that when they're turned on and off rapidlyin a specific manner, they alter the main magnetic field on a local le"el. !hat this means isthat we can pick e&actly which area we want a picture of this area is referred to as the


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+he @I system uses in6ecta-le contrast, or dyes, to alter the local magnetic field in thetissue #eing e&amined. 9ormal and a#normal tissue respond differently to this slightalteration, gi"ing us differing signals. +hese signals are transferred to the images an @Isystem can display more -*2 shades of gray to depict the "arying tissue source: oyneB. +heimages allow doctors to "isuali4e different types of tissue a#normalities #etter than they could

without the contrast. !e know that when we do


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a de"eloping fetus. +he decision of whether or not to scan apregnantpatient is made on acase$#y$case #asis with consultation #etween the @I radiologist and the patient'so#stetrician.

Howe"er, the @I suite can #e a "ery dangerous place if strict precautions are not o#ser"ed.

redit cards or anything else with magnetic encoding will #e erased. @etal o#=ects can#ecome dangerous pro=ectiles if they are taken into the scan room. 1or e&ample, paperclips,pens, keys, scissors, =ewelry, stethoscopes and any other small o#=ects can #e pulled out ofpockets and off the #ody without warning, at which point they fly toward the opening of themagnet at "ery high speeds.

%ig o#=ects pose a risk, too $$ mop #uckets, "acuum cleaners, I poles, patient stretchers,heart monitors and countless other o#=ects ha"e all #een pulled into the magnetic fields of the@I. In -22, a young #oy undergoing a scan was killed when an o&ygen tank was pulledinto the magnetic #ore source: @c9eilB. >nce, a pistol flew out of a policeman's holster, theforce causing the gun to fire. 9o one was in=ured.

+o ensure safety, patients and support staff should #e thoroughly screened for metal o#=ectsprior to entering the scan room. >ften, howe"er, patients ha"e implants inside them that makeit "ery dangerous for them to #e in the presence of a strong magnetic field. +hese include:

@etallic fragments in the eye, which are "ery dangerous as mo"ing these fragmentscould cause eye damage or #lindness

acemakers,which may malfunction during a scan or e"en near the machine

Aneurysm clips in the#rain,which could tear the "ery artery they were placed on torepair if the magnet mo"es them

Dental implants, if magnetic

@ost modern surgical implants, including staples, artificial =oints and stents are made of non$magnetic materials, and e"en if they're not, they may #e appro"ed for scanning. %ut let yourdoctor know, as some orthopedic hardware in the area of a scan can cause distortions in theimage.
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How >5rays Work

J$ray technology has allowed us to see inside the human #ody since (6*. As with many ofmankind's monumental disco"eries, >5ray technologywas in"ented completely #y accident.In (6*, a Eerman physicist named !ilhelm oentgen made the disco"ery whilee&perimenting with electron -eamsin a gas discharge tu-e. oentgen noticed that a

fluorescentscreen in his la# started to glow when the electron #eam was turned on. +hisresponse in itself wasn't so surprising $$ fluorescent material normally glows in reaction toelectromagnetic radiation $$ #ut oentgen's tu#e was surrounded #y hea"y #lack card#oard.oentgen assumed this would ha"e #locked most of the radiation.

oentgen placed "arious o#=ects #etween the tu#e and the screen, and the screen still glowed.1inally, he put his hand in front of the tu#e, and saw the silhouette of his #ones pro=ected ontothe fluorescent screen. Immediately after disco"ering J$rays themsel"es, he had disco"eredtheir most #eneficial application.

oentgen's remarka#le disco"ery precipitated one of the most important medical

ad"ancements in human history. J$ray technology lets doctors see straight through humantissue to e&amine #roken #ones, ca"ities and swallowed o#=ects with e&traordinary ease.@odified J$ray procedures can #e used to e&amine softer tissue, such as thelungs,#lood"esselsor the intestines.

What+s an >5!ay/

J$rays are #asically the same thing as "isi#le light rays. %oth are wa"elike forms ofelectromagnetic energycarried #y particles called photons seeHow 8ight !orksfordetails/. +he difference #etween J$rays and "isi#le light rays is the energy levelof theindi"idual photons. +his is also e&pressed as the wavelengthof the rays.
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>ur eyesare sensiti"e to the particular wa"elength of "isi#le light, #ut not to the shorterwa"elength of higher energy J$ray wa"es or the longer wa"elength of the lower energy radiowa"es.

isi#le light photons and J$ray photons are #oth produced #y the mo"ement of electronsin

atoms. ;lectrons occupy different energy le"els, or or#itals, around an atom's nucleus. !henan electron drops to a lower or#ital, it needs to release some energy $$ it releases the e&traenergy in the form of a photon. +he energy le"el of the photon depends on how far theelectron dropped #etween or#itals. 0ee this pagefor a detailed description of this process./

!hen a photon collides with another atom, the atom may a-sor-the photon's energy #y#oosting an electron to a higher le"el. 1or this to happen, the energy le"el of the photon hasto matchthe energy difference #etween the two electron positions. If not, the photon can'tshift electrons #etween or#itals.

+he atoms that make up your #ody tissue a#sor# "isi#le light photons "ery well. +he energyle"el of the photon fits with "arious energy differences #etween electron positions. adiowa"es don't ha"e enough energy to mo"e electrons #etween or#itals in larger atoms, so they

pass through most stuff. J$ray photons also pass through most things, #ut for the oppositereason: +hey ha"e too much energy.

>ther J$ay GsesThe most important contributions of X-ray technology have been in the world of medicine, but X-rayshave played a crucial role in a number of other areas as well. X-rays have been pivotal in researchinvolving quantum mechanics theory, crystallography and cosmology. In the industrial world, X-ray

scanners are often used to detect minute flaws in heavy metal equipment. And X-ray scanners havebecome standard equipment inairport security, of course.
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+hey can, howe"er, knock an electron away from an atom altogether. 0ome of the energyfrom the J$ray photon works to separate the electron from the atom, and the rest sends theelectron flying through space. A larger atom is more likely to a#sor# an J$ray photon in thisway, #ecause larger atoms ha"e greater energy differences #etween or#itals $$ the energyle"el more closely matches the energy of the photon. 0maller atoms, where the electron

or#itals are separated #y relati"ely low =umps in energy, are less likely to a#sor# J$rayphotons.

+he soft tissue in your #ody is composed of smaller atoms, and so does not a#sor# J$rayphotons particularly well. +he calcium atoms that make up your #ones are much larger, sothey are #etter at a-sor-ing >5ray photons.

,he >5!ay 0achine

+he heart of an J$ray machine is an electrode pair$$ a cathode and an anode $$ that sits

inside a glass vacuum tu-e. +he cathode is a heated filament, like you might find in anolder fluorescent lamp. +he machine passes current through the filament, heating it up. +heheat sputters electrons off of the filament surface. +he positi"ely$charged anode, a flat discmade of tungsten, draws the electrons across the tu#e.
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+he "oltage difference #etween the cathode and anode is e&tremely high, so the electrons flythrough the tu#e with a great deal of force. !hen a speeding electron collides with a tungstenatom, it knocks loose an electron in one of the atom's lower or#itals. An electron in a higheror#ital immediately falls to the lower energy le"el, releasing its e&tra energy in the form of a

photon. It's a #ig drop, so the photon has a high energy le"el $$ it is an J$ray photon.

The free electron collides with the tungsten atom, knocking anelectron out of a lower orbital. A higher orbital electron fills the

empty position, releasing its excess energy as a photon.

1ree electrons can also generate photons without hitting an atom. An atom's nucleus mayattract a speeding electron =ust enough to alter its course. 8ike a comet whipping around thesun, the electron slows down and changes direction as it speeds past the atom. +his


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In a normal J$ray picture, most soft tissue doesn't show up clearly. +o focus in on organs, orto e&amine the #lood "essels that make up the circulatory system, doctors must introducecontrast mediainto the #ody.

ontrast media are liquids that a#sor# J$rays more effecti"ely than the surrounding tissue. +o

#ring organs in the digesti"e and endocrine systems into focus, a patient will swallow acontrast media mi&ture, typically a #arium compound. If the doctors want to e&amine #lood"essels or other elements in the circulatory system, they will in=ect contrast media into the

patient's #loodstream.

ontrast media are often used in con=unction with a fluoroscope. In fluoroscopy, the J$rayspass through the #ody onto a fluorescent screen, creating a mo"ing J$ray image. Doctors mayuse fluoroscopy to trace the passage of contrast media through the #ody. Doctors can alsorecord the mo"ing J$ray images on film or "ideo.

+he high$impact collisions in"ol"ed in J$ray production generate a lot of heat. A motorrotates the anode to keep it from melting the electron #eam isn't always focused on the samearea/. A cool oil #ath surrounding the en"elope also a#sor#s heat.

+he entire mechanism is surrounded #y a thick lead shield. +his keeps the J$rays fromescaping in all directions. A small window in the shield lets some of the J$ray photons escapein a narrow #eam. +he #eam passes through a series of filters on its way to the patient.

A camera on the other side of the patient records the pattern of J$ray light that passes all theway through the patient's #ody. +he J$ray camera uses the same film technology as anordinary camera,#ut J$ray light sets off the chemical reaction instead of "isi#le light. 0ee

How hotographic 1ilm !orksto learn a#out this process./

Eenerally, doctors keep the film image as a negative. +hat is, the areas that are e&posed tomore light appear darker and the areas that are e&posed to less light appear lighter. Hardmaterial, such as #one, appears white, and softer material appears #lack or gray. Doctors can

#ring different materials into focus #y "arying the intensity of the J$ray #eam.

Are >5!ays Bad For ?ou/

J$rays are a wonderful addition to the world of medicine they let doctors peer inside a

patient without any surgery at all. It's much easier and safer to look at a #roken #one using J$rays than it is to open a patient up.

%ut J$rays can also #e harmful. In the early days of J$ray science, a lot of doctors woulde&pose patients and themsel"es to the #eams for long periods of time. ;"entually, doctors and

patients started de"eloping radiation sickness, and the medical community knew somethingwas wrong.

+he pro#lem is that J$rays are a form of ioni3ing radiation. !hen normal light hits an atom,it can't change the atom in any significant way. %ut when an J$ray hits an atom, it can knockelectrons off the atom to create an ion, an electrically$charged atom. 1ree electrons then

collide with other atoms to create more ions.
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An ion's electrical charge can lead to unnatural chemical reactions inside cells. Among otherthings, the charge can #reak D9Achains. A cell with a #roken strand of D9A will either dieor the D9A will de"elop a mutation. If a lot of cells die, the #ody can de"elop "ariousdiseases. If the D9A mutates, a cell may #ecome cancerous,and this cancer may spread. Ifthe mutation is in a sperm or an egg cell, it may lead to #irth defects. %ecause of all these

risks, doctors use J$rays sparingly today.

;"en with these risks, J$ray scanning is still a safer option than surgery. J$ray machines arean in"alua#le tool in medicine, as well as an asset in security and scientific research. +hey aretruly one of the most useful in"entions of all time.
http://science.howstuffworks.com/cell.htmhttp://science.howstuffworks.com/cell4.htmhttp://science.howstuffworks.com/cancer.htmhttp://science.howstuffworks.com/cancer.htmhttp://science.howstuffworks.com/cell.htmhttp://science.howstuffworks.com/cell4.htmhttp://science.howstuffworks.com/cancer.htm


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How %A,


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,he Basic 8dea

%omputeri3ed a$ial tomographyA+/ scan machines produceJ$rays,a powerful form ofelectromagnetic energy. J$ray photons are #asically the same thing as "isi#le light photons,

#ut they ha"e much more energy. +his higher energy le"el allows J$ray #eams to passstraight through most of the soft material in the human #ody. 0ee How J$ays !orkto findhow J$rays do this, as well as how J$ray machines produce J$ray photons/.

A con"entional J$ray image is #asically a shadow: Fou shine a


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A scanned liver slice

How the Doppler Effect Works

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