Medical Physics Ib2 08

7/21/2019 Medical Physics Ib2 08

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Medical Physics

The Ear and Hearing

1. Distinguish among the:

a) outer ear

b) middle ear

c) inner ear

2. Describe the function of the:

a) ear canal

b) eardrum (tympanic membrane)

c) Eustachian tube

ossicles (consisting of . . .

d) cochlea

e) auditory nerves


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. !o" are sound pressure variations in air changed to larger pressure

variations in the cochlear fluid#

a) $ever action of ossicles %

b) Different areas&

'. hy is the middle ear necessary# hy not have the sound "aves vibrate the oval "indo" directly#

. hat is the range of audible fre*uencies e+perienced by a person "ith normal hearing#

,esult:

-. hat is a sound "ave#

ressure: /ormula: 0nits: ir pressure:

. hat fre*uency is a normal ear most sensitive to#


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Intensity and Loudness

o"er: /ormula: 0nits:

Intensity: /ormula: 0nits:

3otal area of sound energy:

s distance from source doubles . . .

rea of eardrum (or oval "indo") "here sound is received:

2. hat is the difference bet"een intensity and loudness#

$oudness:

453E:

-. hy discuss intensity rather than loudness#

1. 3he po"er output of a spea6er is 177 .

a) Determine the intensity of the sound heard by a person -.7 m a"ay from the spea6er.

b) Determine the energy delivered to the person8s eardrum each second. (3he total area of theeardrum is appro+imately 7 mm2.)


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Intensity $evel (I$):

Intensity Level

0nits:

-. 9alculate the intensity level of a 177 spea6er at a

distance of .7 meters.

. sound meter placed 7. meter from a circular sa" measures 2 dB.

a) hat is the intensity of the sound that corresponds to this intensity level#

b) !o" much sound po"er does the sa" produce#


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;. hat are the effects of short&term and long&term e+posure to noise#

a)

b)

c)

'. Discomfort is e+perienced by a person "ith normal hearing "hen the intensity is appro+imately 1.7


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I$ versus fre*uency diagram for a person "ith normal hearing

>cale on fre*uency a+is:

9urve:a)

b)

c)

1. a) 9an a 177 !? note from a piano be heard by a person sitting in the audience at such a distance that

the intensity level of the sound is 27 dB# E+plain.

b) hat is the minimum intensity and intensity level that the listener could hear for this note#

c) hat is the lo"est fre*uency note that the listener could hear at 27dB#

2. >6etch on the graph the results of a hearing test for an elderly person.


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'

Medical Imaging

X-Rays

@arious imaging techni*ues are used for diagnostic purposes:

1) A&rays

2) 9omputed 3omography (93)

-) 0ltrasound

) 4uclear agnetic ,esonance (4,)

) $asers

roduction of A&rays:

a)

b)

c)

d)

Imaging techni*ue:

a)

b)

Increasing sharpness: Increasing contrast:


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Attenuation:

a)

b)

Attenuation coefficient:

Half-value Thicness: >ymbol: 0nits:

3he graph at right sho"s data for the intensity of the A&rays transmitted through a muscle versus the thic6ness

the muscle. hat is the relationship bet"een the t"o#

Intensity

E!uation:

,elationship bet"een !alf&value thic6ness and

attenuation coefficient

>ymbol: 0nits:


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2. 5utline the basis for the formation of A&ray image of a limb.

a)

b)

c)

1. 3he graph at right sho"s the transmitted intensity of a parallel

beam of A&rays versus the thic6ness of a certain type of tissue.

Determine the:

a) half&value thic6ness of this tissue

b) attenuation coefficient of this tissue

-. naly?e the intensity graph at right by straightening it.


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. parallel beam of A&rays passes through muscle and reduces to one&eighth of its initial intensity.Determine the fraction of the intensity of this beam "hen it is transmitted through the same thic6ness of

fatty tissue. 3he half&value thic6ness of muscle is .7 mm and the half&value thic6ness of fatty tissue is

.7 mm.

. 3he half&value thic6ness for soft tissue (muscle) is about 27 cm and for bone is 17 times less than this.

Determine the attenuation coefficient for bone and for soft tissue.

. 3he half&value thic6ness of a -7 6e@ A&ray photon in aluminum is 2. mm. 3he initial intensity of

the A&ray beam is .7 + 1726


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"utline the #asis of $T scanning

% A&ray image of target ta6en at different angles (many

different directions)

& computer produces detailed image of slice (these images are

combined using computers to form a t"o&dimensional image ofsection)

% images of many sections


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hy "ould patients accept an increased health ris6 from e+posure to A&ray radiation#

(iological Effects of Ioni)ing Radiation(short&term and long&term)

a) ,adiation causes direct damage to D4 by removing electrons

b) Damages cells indirectly via ioni?ation of "ater (removing electrons)c) >hort&terms effects include death of cellhort&term effects include production of to+ins


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ltrasound

"%erating %rinci%les: !igh fre*uency sound "aves aretransmitted from a probe into the patient8s body and are reflected

at each boundary bet"een different types of tissue and bone. 3he

same probe both transmits and receives the ultrasound "aves.

By measuring the time bet"een transmission and reception= thedistance to each boundary can be calculated using the speed of

sound and thus the location and surface of each organ can be

mapped.

Ty%ical o%erating fre!uencies of ultrasound .aves:

Ty%e of energy used:

dvantage:

Production and detection of ultrasound .aves

ie?oelectric crystal:

roduction:

Detection:

/actors affecting choice of diagnostic fre!uency:

a) ,esolution:

/avors:

b) ttenuation:

/avors:

9ompromise:


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,eflection at a boundary:

Acoustic Im%edance: >ymbol: 0nits:/ormula:

2. 9alculate the acoustic impedance of room temperature air.

. hat is the purpose of putting gel on probe and the patient8s s6in#

-. Based on the table above= from "hat boundary "ill most of the energy of the ultrasound "aves bereflected#

1. 3he time delay for an ultrasound pulse going through body fat to reach and be reflected from the liver

is 7.1-- ms. 3he speed of the ultrasound through fat at the chosen fre*uency is 17 m


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Ty%es of ultrasound scans

1) A-scan:

resentation:

2) (-scan:

resentation:

3he diagram above sho"s an ultrasound transducer (probe) in contact "ith the s6in in an effort todetermine the depth and "idth of the organ sho"n. n &scan of the results is sho"n also. 3he

average speed of the ultrasound used in tissue and muscle is 2.7 + 17-m


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0MR

4,:

,I:

"%erating Princi%les:

a) $arge constant uniform magnetic field causes hydrogen atoms to line up (align their spin a+es)% act li6e tiny magnets

b) >mall non&uniform magnetic field is superimposed on top of larger field % locali?ed magneticfield % "ea6 oscillating field in the form of pulses of radio "aves

c) If fre*uency of radio "aves matches that of the hydrogen atoms (resonance) then the smallerfield ma6es some hydrogen atoms realign

d) hen small non&uniform field is removed= atoms rela+ bac6 to original alignment

e) s they rela+ they emit radio&"aves

f) 3ime it ta6es to rela+ is measured

g) /re*uency of emitted radio "aves and rela+ation times are processed to produce the 4,

image

0ses:

1)

2)

-)

3ype of energy used:

1)

2)

dvantages:

1) ny situation "here detailed tomography


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$om%arisons of ,iagnostic Imaging Techni!ues

. part from health ha?ards= "hy are different means of diagnosis needed#

a) Different types of tissues and bone have different absorptionome are better at distinguishing boundaries of organs

c) >ome provide t"o dimensional slice imaging % some provide complete three dimensional images

d) >ome are better at monitoring static or dynamic conditionse) >ome are better to investigate at large or small scales

f) >ome can be used to study concentrations of specific types of tissue or pharmaceuticals

g) >ome are better at monitoring specific organ functions

1. hat are the advantages and disadvantages of using

ultrasound instead of A&rays#

dvantage: not as harmful since no ioni?ing radiation

nd so can be used for pregnant "omen

Disadvantages:>mall depth of penetration$imit to si?e of obCects that can be imaged

Blurring of images due to reflection at boundaries

2. hy are A&rays preferred over ultrasound for bone fractures#

4early all ultrasound is reflected by bone (at bone


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Lasers

1) Pulse "*imetry

,ed and infrared laser light is shone through a thin part of a patient8s anatomy= for e+ample anearlobe or fingertip. 3he amount of absorbance of each "avelength "ill depend on many factors but a

changing relative absorbance bet"een t"o "avelengths can be used to determine the ratio of blood

cells "ith o+ygen ("hich are bright red) and those "ithout o+ygen ("hich are dar6 red). 3his allo"sthe overall o+ygen content of the blood to be determined "ithout having to ta6e a sample of blood.

2) Endosco%es

n endoscope provides the ability to loo6 inside the human body "ithout invasive surgery. tube is inserted into the body= usually through the mouth or anus. ithin the tube there is a collection

of optical fibers that allo"s illumination from an outside source to reach the end of the tube and

illuminate the region under investigation (stomach= lungs= colon= intestines). ,eflected light is

collected using a lens system and further optical fibers are used to allo" an image to be vie"ed.

3otal internal reflection of a laserbeam in a fiber optic cable

-) +cal%el

laser focused on a small region can increase its temperature so high that it cuts through

tissue li6e a scalpel. 3he laser heating ensures the site is 6ept free of germs and thus the ris6 ofinfection is reduced. In addition= vessels and nerves are automatically sealed off.

) $oagulator

defocused laser beam can stop bleeding by stimulating the blood to form a clot.


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hat are three factors that affect the absorbed dose#

Thera%eutic Radiation

,adiation dosimetry:

Diagnostic radiation:

3herapeutic radiation:

12 E*%osure 3 >ymbol: 0nits:/ormula:

>ignificance:

42 A#sor#ed ,ose 3

>ymbol: 0nits:/ormula:

>ignificance:

52 6uality /actor &Relative (iological Effectiveness' 3 >ymbol: 0nits:

>ignificance:

72 ,ose E!uivalent 3>ymbol: 0nits:/ormula:

>ignificance:

rad


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2. hy use the dose e*uivalent rather than the absorbed dose "hen assessing

biological effects of radiation#

-. >o "hat is this quality factor(RBE)#

It is a factor that compares the effectiveness of different types of radiation to that of A&raysbecause different types of radiation of the same intensity produce different amounts of ioni?ing

radiation and can cause different amounts of damage.

3hus 1 for A&rays.

1. hy do radiations have different effects for same absorbed dose#

. ,adiation from a gamma source creates an e+posure of .- + 17&-

9


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E+ample of therapy techni*ue using iodine&1-1 to treat thyroid cancer:

a) Iodine&1-1 decays "ith beta&minus and gamma particle

b) Iodine readily absorbed by thyroid so "ill locali?e in target area

c) Beta&particles are absorbed over small volume so dose is locali?ed ("hereas alpha&particles are

absorbed over large volume % more indiscriminate and therefore dangerous)

d) Famma "ill escape from body so can be monitored

e) Effective half&life is about si+ days so patient "ill receive effective dose for a reasonableperiod of time but not too long to cause other damage

Radiation Thera%y

im of radiotherapy:

3o target malignant cells in preference to normal healthy cells. alignant cells are slightly more

susceptible to damage from radiation than healthy cells.

3ypes of treatments:

1) radioactive source can be placed "ithin the tumor itself= either physically or chemically.

2) 5verlapping beams of radiation can be used. here they overlap the dose "ill be high (at the

tumor site) "hile else"here the does "ill be lo".

Balanced ris6:

If the dose is too high= too many healthy cells are 6illed. If the dose if too lo"= cancer is not

destroyed. 3he dose needs to be as high as possible in the region of the cancer and as lo" as

possible every"here else.

3ypes of sources:

1) radioactive element

2) !igh energy A&rays= gamma rays or protons from

particle accelerators

E+ample of therapy techni*ue using A&rays to 6ill cancer cells in a tumor:

a) A&rays are much higher intensity than those used to ta6e chest A&rays

b) 9ancer cells are targeted to receive a high dose by irradiating region from different angles "ithtumor in overlap region

c) im is to minimi?e danger to other healthy cells "hile 6illing cancerous ones since malignantcells are preferentially susceptible to +&rays.


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IB 12Half-Life

Physical Half-life &TP' &same as T184' 3a. the time ta6en for G the number of radioactive nuclei in sample to decay

b. the time ta6en for the activity of a sample to decrease to G its initial value

(iological Half-life &T(' 3

Effective Half-life &TE' 3

Half-Life

Relationshi%

1. If the physical half&life of a radioisotope is 17 days and thebiological half&life is 1 days= "hat fraction "ill remain after -7

days#

2. a) 3he isotope iodine&1-1 can be used to treat malignant gro"ths in the thyroid gland. 3he isotope has a

radioactive half&life of ; days and a biological half&life of 21 days. 9alculate its effective half&life anddetermine the time it "ould ta6e for its activity to decrease to 1

Medical Physics Ib2 08

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