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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