Radioisotopes in medicine



Radioisotopes in medicine

Natural radioactivity was more amazing to physicist than x-ray since to produce x-ray it was necessary to put in electrical energy , while natural radioactivity provided a steady source of high – energy particles for an indefinite period of time with no input energy .

Radioactivity:

A certain natural elements ,heavy have unstable nuclei that disintegrate to emit various rays . Alpha (α ) ,Beta (ß) and Gamma (γ) rays.

|Alpha (α ) |Beta (ß) |Gamma (γ) |

|1-Positive charge |Negative charge |Without charge |

|2-Affected by magnetic +electric field |Affected by magnetic +electric field |Doesn’t affected |

|3-Stop in a few centimeter of air (low |It is stopped in a few meters of air and a few |High energy photon (high penetrating |

|penetrating power) |millimeters of a tissue(the penetrating power |power). |

| |is more than α and less than γ ) | |

|4- Is Helium atom (2He4) |High speed electrons |It is photon |

|5-Has a fixed energy for a given source |Has a spread of energy up to max |Has a fixed energy for a given source |

Isotopes:-

Nuclei of a given element with different numbers of neutrons. There are two types:

1- Stable isotopes if they are not radioactive. Ex:(12C, 13C)

2- Radioisotopes if they are radioactive. Ex:(11C, 14C, 15C)

Radio-nuclides:-

Is used when several radioactive elements are involved.(Radioisotopes are used when referring to single element).

Neutrino:-

A mass less, charge less, particle. Takes up the difference in energy between the actual beta energy and the maximum beta energy.

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 γ-ray is very penetrating a gamma emitting radioactive element inside the body can be detected outside the body.

13153I 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 γ-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 radio-nuclides 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 .

Metastable:-

Some man –made radio-nuclides emit type of radiation not emitted by natural radioactive substance. 99m Tc, the m stands for metastable , which mean (half stable) .Decay by emitting gamma rays only , and the daughter nucleus differs from its parent only in having less energy. For example:

99m Tc decay 99m Tc γ -rays (140 kev)

This is useful for N.M application ,because :

1- It is penetrating enough to get out of the body .

2- It is easy to shield with a few millimeters of lead.

• when a metastable radionuclide is used internally, the absence of bet rays greatly reduces the radiation dose to the patient.

[pic] Extremely minute quantities of radioactive substances are used in (N.M) and these small quantities (typically less than 1 mg) . Don’t affect the normal physiological functioning of the body.

• Nuclear reactors produce radioactivity by adding neutrons to stable nuclei. The resulting nuclei thus have too many neutrons and usually decay by emitting beta particles , which make the nuclei more positive.

Half life (T1/2 ):

The time needed for half of the radioactive nuclei to decay.

Radioactivity Half-life

100

50 ------

Time Log Scale

2 4 6 8

100 Radioactivity

Half-life

50 ----------------

T1/2

Time

2 4 6 8

A=AOe-λt ………………..(1)

A is the activity after time (t)

AO is the initial activity

λ is the decay constant (sec -1 ,hour -1 , year -1)

t is the time (sec, hours, year)

A= λ N…………………(2)

N=no. of radioactive atoms.

T1/2 = 0.693 / λ……………(3)

A = λ N =(0.693 /T1/2 )x(mass/ atomic weight)x Avogadro , s number

1 year =3.15 X10 7 sec.

T1/2 =should be in sec.

The average or mean time T=1/ λ

1/ λ from the equation (3)=1.44 T1/2

So T=1.44 T1/2

Ex: A- If you have 1 gm of pure potassium 40 (40K) that is experimentally determined to emit about 105 beta rays per second what is the decay constant?

Solution :-

A = λ N = λ x(mass/ atomic weight)x Avogadro , s number

105= λ x 1/40 x 6.02 x 1023

So λ=6.7 x 10-18 sec-1

B-Estimate the half life of 40K

T1/2 = 0.693 / λ sec=1017

T1/2 = 107/3.15 x107 =3 x109 years

Units of radioactivity:

The unit of radioactivity is ci(Curic)

1 ci=3.7 x1010 dis/sec or Bq(Becquerel)

(micro)μ ci=10-6 ci

(nano)η ci=10-9 ci

(pico)ρ ci=10-12 ci

Question:-

Q 1) at the end of 20 days one-eight of a sample of radioactive material remains. What is the half life?

What is the mean life?

Q2) find the activity of 1 gm of a sample of 90SR who’s half life is 28 years ?

Q3)a solution containing a radioactive isotope with T1/2 =12.26 days surrounded a Geiger counter with record 480 c/min .what counting rate will be obtained 49.04 days later?

Q4) The half life of radioactive 60CO is 5 years, calculate how long it will tares for the activity of a specimen of this radioactive isotopes to decrease to 20 percent of its initial value?

Collimator:-

It is used on scintillation detector to limit its counting volum.There are two types of collimator:-

1-Open or flat collimator ,it is used to detect γ-ray from large volume (thyroid,kidney).

2-Focused collimator , it is used to detect small volume.

Use of scintillation:-

• Detector in N.M:-

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 μ ci ) in liquid or capsule patient by mouth after 24 hours I131 (in the thyroid ) counted for 1 min.

131I (8 μ 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%

[pic]Hyperthyroid above 40%

• Hypothyroid less than 10%

• Note : The new thyroid test do not require the administration of 131I to the patient.

|Old test |New test |

|131I is given to the patient (vivo test) |No 131I is used (vitro test) |

| | |

| | |

|Not safe (patient is exposed to |It is safer (patient is not exposed to radiation) |

|radiation) | |

Comparison between new thyroid test and old thyroid test.

Kidney function:-

Kidney function is also often studied with scintillation detectors . About 7 MBq (200 μ 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 Renogram.

The component involved in making a Renogram

NaI(Ti)

[pic]

3-The dilution techniques:-

The technique is used to determine the blood volume.

About 200KBq (5μ 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 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:

Vblood x Count rate blood = Vinjected albumin x Count rateinjected albumin

X x 102 = 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

[pic]

Detector Film Lamp

Patient

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 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 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 half-life 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 х 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).

Physics of radiation therapy:-

There is evidence that an error of (5-10)% in radiation dose to tumor can have a significant effect on the results of the therapy .Too little radiation does not kill the entire tumor;while too much can produce serious complications in normal tissue.

The basic principle of radiation therapy is to maximize damage to the tumor while minimizing damage to normal tissue.This is generally accomplished by directing a beam of radiation at the tumor from several directions, so that the maximum dose occurs at the tumor.

Some normal tissues are more sensitive radiation than others. Ionizing radiation ,such as x-rays and γ-rays, tearing electrons off atoms to produce (+ve)and (-ve)ions.

It also breaks up molecules; the new chemicals formed are no use to the body and can be considered a form of poison.

The units are used to measure the amount of radiation to the patient:-

1-Erythematic dose :- the quantity of x-rays that caused redding of the skin.

2-Exposure (Roentgens (R) ),see ch16.4

1R=2.58х10-4 c/kg of air.

3-Absorbed dose (rad).The (rad) is defined as 100 ergs/g. that is a radiation beam that gives 100ergs of energy to 1g of tissue an absorbed dose of 1(rad) or gray =100(rad).

• The (rad) can be used for any type of radiation in any material;the roentgen ( R ) is defined only for x-rays and γ-rays in air.

[pic] RBE(Relative Biological Effect)

The ratio of the number of gray of 250 KVp x-rays needed to produce a given biological effect to the number of grays of the test radiation needed to produce the same effect.

LD50:-

The quantity of radiation that will kill half of the organisms in a population (cells,mice,people,……etc) is called the lethal dose for 50% or LD50.

30 in 30 days

LD50 kill 50%

Surviving %

100

[pic]

Dose(Gy)

1 2 3 4 5 6 7

LD50

Oxygen effect:- see ch.7(HOT)

Brachy therapy :-

A short distance therapy .Radium source is put into or on the surface of tumors.

• Advantage:- of brachy therapy is that gives a very large dose to the tumors with minimum radiation to the surrounding tissue.

• Disadvantage:-is the non uniformity of the dose since the radiation is much more intense near the source .Concerns radiation safety .The therapist is close to the source and to the patient(another source).

EX:- In 24 hours 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.[pic][pic][pic][pic][pic][pic][pic][pic][pic][pic][pic][pic]

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H.V+ Amp

D.S.P.H.A.

Chart Record

ren

kidney

Electronic Circuits

Signal processor ppprocessorprocessor

p

m

t

s

50

5

1

0.2

X-rays or

γ -ray

α -particles

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