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Nuclear Medicine

Introduction

What is nuclear medicine?

• The use of radioactive tracers (radiopharmaceuticals) to obtaindiagnostic information [and for targeted radiotherapy].

• Radiation is emitted from inside the human body cf transmittedradiation in !ray imaging.

 

Tracers "!

• Trace the paths of various biochemical molecules in our body.

• #ence can obtain functional information about the bodies $or%ings(i.e. physiology).

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Radiopharmaceuticals

+ 

Biochemical

BondingPharmaceutical

Traces physiology /

localises in organs

of interest 

Radioactive

nuclide

Emits radiation for

detection or therapy 

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The &harmaceutical

 ' The ideal tracerpharmaceutical should follo$ only  the specific path$ays of interest e.g. there is

upta%e of the tracer only in the organ of interest

and no$here else in the body. *n reality this is

never actually achieved. 

 ' Typically $ant no physiological response from the

patient 

 ' The mechanism of localisation can be as simple

as the physical trapping of particles or as

sophisticated as an antigen!antibody reaction

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Radionuclides in +uclear ,edicine

The ideal radionuclide for in!vivo diagnosis "• -ptimum half life

 ' of same order as the length of the test (this minimises the radiation dose to thepatient)

• &ure gamma emitter  ' +o alpha or beta particles these do not leave the body so merely increase the

radiation dose.

• -ptimum energy for γ  emissions ' #igh enough to eit the body but lo$ enough to be easily detected. seful range

for gamma cameras is /0 ! /00 %e1 (optimum 2 3/0 %e1).

• 4uitable for incorporating into a pharmaceutical $ithout alteringits biochemical behaviour 

• Readily and cheaply available on the hospital site.

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Some Commonly Used Radionuclides in

Nuclear Medicine

Radionuclide Half-life Pure

emissions ?

Energy of main

’s (KeV)Source of

production99Tcm (Technecium) 6 h y 140 On site generator111In (Indium) 2.8 days y 173 247 !yc"otron123I (Iodine) 13 h y 160 !yc"otron131

I 8 days n 280 360 640 #eactor201T" (Tha""ium) 73.$ h y 68%80 !yc"otron

Radionuclide &roduction"•  +eutron 5apture•  +uclear 6ission•  5harged &article 7ombardment•  Radionuclide 8enerator 

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&roducing the Radiopharmaceutical

Radiopharmaceutical kits

• ,ost common radiopharmaceuticals are available as %its. These

contain all the necessary free9e!dried ingredients in an air!tight

vial usually the pharmaceutical a stannous compound and

stabili9er. -n addition of ::Tcm  0;!  the stannous reduces the::Tcm 0;!  ma%es it charged and

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Detection of the radiopharmaceutical

• Nonimaging  ' Invitro (measuring radiation levels in bodily fluids outside the body) ' e.g. 7lood sample counting for 86R analysis"

&atient

'"ectronics and

count%rate meter 

Inect radioactie

tracer 

'*tract sam+"e o,  -odi"y ,"uid

(e.g. -"ood)

easure ,"uid sam+"e

in sam+"e detector 

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• Nonimaging  ' Invivo (pta%e measurements in organs using a radiation detector probe) ' e.g. 4e#5=T study for bile salt malabsorption .

Detection of the radiopharmaceutical

'"ectronics and

count%rate meter 

/cinti""ation

 +ro-e

!o""imator 

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Detection of the radiopharmaceutical

• In !ivo imaging the gamma camera

&roperties of gamma rays•  #igh energy electromagnetic radiation•  5an be scattered and absorbed

•  5annot be focused

&atient

#adioactie

tracer 

amma

rays

amma

camera

Image

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The 8amma 5amera

&ositioncircuitry

5ollimator 

 aI

5rystal

&hoto ,ultiplier 

  Tubes

 =nalogue to

igital 5onverters

igital

circuitry

-utput position

@ energy signals

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The 5ollimator 

• The purpose of the collimator is to proAect an image of the radioactive distribution in the

patient onto the scintillation crystal.

• *t is a crude and highly inefficient device $hich is reBuired because no gamma!ray lenseists.

• *n the parallel hole collimator only incident photons that are normal to the collimatorsurface $ill pass through it.

•  =ll other photons should be absorbed by the lead septa bet$een the holes

• The collimator defines the field of vie$ and essentially determines the system spatialresolution and sensitivity.

O-(ect O-(ectImage   Image

LES

P!R!LLEL

"#LL$%!R 

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4patial resolution of an imaging device defines its ability to distinguish bet$een

t$o structures close together and is characterised by the blurred image

response to a point!source input. 6or a gamma camera the overall spatial

resolution in the image depends on the collimator (collimator resolution) and the

other gamma!camera components (intrinsic resolution).

4patial Resolution @ 4ensitivity

"o improve collimator resolution

•  *ncrease the septa depth (d)•  Reduce the si9e of the holes (s)•  Resolution C as the source is moved

  a$ay from the collimator   ! important to image $ith the camera

  as close to the patient as possible

"o improve collimator sensitivity•  ependent on the number of photons

  passing through the collimator •  *mproved $ith larger hole si9es and

  smaller length septa

resolution and sensitivity are conflicting parameters 

!o""imator 

#adioactie

 +t. source

Out+ut ,romco""imator 

/+atia" distance

/+read o, res+onse

to +t. source de,ines

co""imator reso"utiond

s

0 cm

/ cm

30 cm

3/ cm

D0 cm

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4cintillation 5rystal

• The gamma ray causes an electron release in the

crystal via the &hotoelectric Effect 5ompton 4catteringor the electronpositron pair production (Eγ  F 1.022 e5)this ecess energy gives rise to subseBuent visible lightemission $ithin the crystal (scintillation).

• +umber of light photons produced is roughly ∝  Eγ #ence this is an energy discriminating detector

(important feature as $e can use this to reAect scattered photons)

Incident

gamma

ray

 aI(T") /cinti""ation

  crysta"

ight

&hotons (41$nm)

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*mage Types*n +uclear ,edicine various forms of data acBuisition can be performed"

• Static Imaging ' The distribution of the radiopharmaceutical is fied

over the imaging period.

 ' ,ultiple images can be acBuired vie$ing from different

angles (e.g. anterior obliBue).

 ' e.g. %idneys (,4=) thyroids bone lung ::Tcm Thyroid 4can

• #hole Body imaging

 ' the camera scans over the $hole body to cover more

$idespread distributions or un%no$n locations

 ' e.g. bone scan infection imaging tumour imaging

::Tcm #& 7one 4can

• Dynamic Imaging

 ' 5onsecutive images are acBuired over a period of time

($ith the camera in a fied position) sho$ing the changing 

distribution of the radiopharmaceutical in the organ of interest.

 ' e.g. renogram 8* bleed mec%elGs diverticulum

::Tcm labelled red blood cells

8* bleed