Nuclear medicine

Lec.3
Applications of radioactivity in medicine
-Nuclear medicine (N.M)
-Applications of radioactive material in medicine
Nuclear Medicine:-
The clinical uses of radioactivity for the diagnosis of
disease .
The most useful radio-nuclides for nuclear
medicine are those that emit gamma rays.
Since ?-ray is very penetrating a gamma emitting
radioactive element inside the body can be
detected outside the body.
131
53I indicate an atom of iodine with a nucleus
containing 131 protons and neutron and a
subscript indicates the atomic number (53).
The most common emission from radioactive
elements are beta particles and ?-ray. Because beta
particles are not very penetrating they are easily
absorbed in the body and generally of little use for
diagnosis. However some beta-emitting radionuclides
such as 3H and 14C play an important role
in medical research.
32P is used for diagnosis of tumors in the eye because
some of its beta particles have enough energy to
emerge from the eye.
1- Evaluate thyroid function (24 hours uptake test)
The thyroid uses iodine the production of
hormones that control the metabolic rate of the
body, a person with an under active thyroid
(hypothyroid)will take up less iodine than a person
with normal thyroid function (euthyroid), and a
person with over active thyroid (hyper thyroid ) will
take up more iodine.
131I(8 ? ci ) in liquid or capsule patient
by mouth after 24 hours I131 (in the thyroid )
counted for 1 min.
131I (8 ? ci ) standard place in a
neck phantom then after 24 hours I131 counted for
1 min.
After correction are made for background counts ,
the ratio of the thyroid counts to the standard
counts X 100 gives the percent 24 hours up take.
Normal 10 -40% average 20%
Hyperthyroid above 40%
Hypothyroid less than 10%
Note : The new thyroid test do not require the
administration of 131Ito the patient.
Old test
New test
131I is given to the
patient (vivo test)
No131I is used (vitro test)
Not safe (patient is
exposed to
radiation)
It is safer (patient is
not exposed to
radiation)
In 24 h EX:- ours uptake test the standard counts rate
is 2000 c/min and the thyroid counts rate is 20 c/sec
.What is the type of thyroid?
Solution :-
20 x 60=1200 c/min
(thyroid counts/standard)x 100
=(1200/2000)x100=60%
.?. thyroid is hyperthyroid.
Ex :- In 24 hour uptake test ,the standard counts rate
is 2000 c/min and the background counts rate is 200
c/min and the thyroid counts rate is 20 c/sec .What
is the type of thyroid ?
Solution :-
20 x 60 =1200 c/min
(thyroid counts – background)/(standard counts –
background)x100
(1200-200)/(2000-200)x100=55%
So thyroid is hyperthyroid.
Kidney function:-
Kidney function is also often studied with scintillation
detectors . About 7 MBq (200 ? ci) of 131I .Labeled
hippuric acid is injected into the blood stream , and
as it’s removed from the blood by the kidneys the
radio activity of each kidney is monitored .The signals
from each detector are fed to an electronic instrument
called a rate meter to record the change in the
radioactivity with time .The rate meter averages the
signals over a short period of time and indicates the
count rate in counts per minute or counts per second
.It is often connected to a strip chart record to make a
permanent record of the count rate versus time, or a
Reno gram.
The component involved in making a Renogram
NaI(Ti)
3-The dilution techniques:-
The technique is used to determine the blood volume.
About 200KBq (5? ci) of 131I labeled albumin is
injected into an arm vein , and after about 15 min . A
blood sample is drawn from the other arm and
counted(if the patient’s blood contains radioactivity
from a previous study , a pre-injection sample of the
blood must also be drawn and counted).The net count
rate and volume of the sample is compared to the
kidney
H.V+AMP
AAAAaamp
li Amp
Dis or P.H.A
Chart Record Pen
count rate and volume of the injected material to
determine the blood volume.
Ex:-What is the blood volume of patient if 5 ml of 131I
–labeled albumine with a net count rate of 105 c/sec
was injected into the blood and the net count rate of 5
ml blood sample drawn 15 min later was 102 c/sec ?
Solution:
Vbloodx Count rateblood= Vinjected albuminx Count rateinjected albumin
X x102 = 5 x 105
.·. X =5000 ml
Nuclear medicine imaging device :-
Imaging : producing picture of the distribution of the
radioactivity .The two principal devices used to
produce nuclear medicine images are :
1- The rectilinear scanner .
2- The gamma camera.
1-Rectilinear scanner :-
The NaI (Ti) detector of a rectilinear scanner
moves in a raster pattern over the area of interest ,
making a permanent record of the count rate , or a
map of the radiation distribution in the body .The
image is made by moving a small light source over
a photographic film as shown in figure below.The
intensity of the light increases with an increase in
activity , producing corresponding dark areas on
the film.
Raster Pattern Rectilinear Scanner
Detector Film
Patient lamp
2-The gamma camera:-
it has a large NaI(Ti) scintillation crystal about 1
cm thick and 30 cm to 45 cm in diameter. The
scintillation are viewed through a light pipe by an
array of 19 or 37 PMTs .When a gamma ray
Electronic Circuits
interacts somewhere in the crystal , the light from
the scintillation produces a large signal in the
closest PMT and weaker signals in PMTs further
away . These signals are electronically processed to
determine the (x,y)coordinates of the scintillation
and causes a bright spot to appear at the
corresponding (x,y) location of the CRT.The bright
spot is then recorded on the film in the camera.
The Gamma Camera
Lead multiple
Collimator
shielding
CRT
Light pipe
NaI(Ti)
camera
Patient
Rectilinear scanner Gamma camera
1-Imaging time is 20 min Imaging time 1-2
min.(obtain dynamic
p m t s
Signal
processorppproc
essorprocessor
information)
2-Focus collimator is used A parallel hole collimator
is used
3-Less resolution High resolution (it can
distinguish two sources
about 5 mm apart when
they are held close to the
collimator)
4- It is not possible to use
short half – life radio
nuclides
It is possible to use radio
nuclides with very short
half-lives min. or less.
Radiation dose in N.M:-
In general ,the radiation dose to the body from a
nuclear medicine procedure is:
1- Non uniform since radioisotopes tend to
concentrate in certain organs, while it is
impossible to measure the radiation received by
a particular patient ,it is possible to calculate the
dose to various organs of a standard man .The
organ receiving the largest dose during a
procedure is sometimes referred to as the critical
organ.
2- The dose can vary considerably from one
individual to another.
3- The dose to a particular organ of the body
depends on the physical characteristics of the
radio-nuclides what particles it emits and their
energies and on the length of the radio-nuclides
is in the organ. Two factors determine the length
of time the radio-nuclides is in the organ,or the
effective half-life (T1/2 Bio) .The biological halflife
of an element is the time needed for one half
of the original atoms present in an organ to be
removed from the organ ,and it is independent of
whether the element is radioactive.
T1/2 eff= [(T1/2 Bio ? T1/2 Phy)/ (T1/2 Bio + T1/2 Phy) ]
Note:-
If either biological or physical half-life is much
shorter than other , the effective half-life is
essentially equal to the shorter value(see Ex 17.5
P483).
Summary
There are two types of devices used in N.M as
imaging which give us the distribution of the
radioactivity, for example rectilinear scanner and
gamma camera . and the second one give us the
quantity of radioactive material.