Personal Protection: Basic Concepts :Karina Caboverde, Jorge Artamendi, Danny Ferro Personal Protection: Basic Concepts


Brief Introduction :Brief Introduction Positron Emission Tomography (PET) is a method for determining biochemical and physiological processes in vivo (inside the body) Possible by using externally-detectable radiolabeled molecules PET employs mainly short-lived positron emitting radiopharmaceuticals. The radionuclides employed most widely are: 11C (t½ = 20 min) 13N (t½ = 10 min) 15O (t½ = 2 min) 18F (t½ = 110 min)


Brief Introduction :Brief Introduction The annihilation process forms the basis of PET imaging


Brief Introduction :Brief Introduction Carbon, oxygen, nitrogen and hydrogen are the elements of life and the building stones of nearly every molecule of biological importance. In PET, a fluorine isotope is often a replacement for a hydrogen atom in a molecule This occurs since hydrogen has no radioactive isotope decaying with emission of radiation In other words, hydrogen can’t be detected outside the human body.


Basic Concepts for PET :Basic Concepts for PET During a PET scan, the patient receives radiation exposure from two sources: the administered radiopharmaceutical the attenuation correction segment of the protocol. Attenuation- decreases the intensity of electromagnetic radiation due to absorption or scattering of photons. The latter can be from either CT, or an external radionuclide source.


Objectives :Objectives Discuss ALARA and percentage for occupational dose Understanding how time, distance, and shielding can reduce radiation exposure for PET Demonstrate the mathematics of time, distance and shielding (inverse square law) Discuss absorbers for high Z materials Demonstrate how the thickness of a pig or lead container is essential for exposure protection for PET


ALARA :ALARA A- as L- low A- as R- reasonably A- achievable If the limit are exceeded, corrective action must be taken or higher limits must be justified for particular situations. Even though PET radiopharmaceuticals do not fall under the NRC, all authorized users must participate in the program as requested by the 10CFR. NRC has two goals for a radiation worker to achieve: 10% of the occupational dose per quarter (Action level I) 30% of the occupational dose per quarter (Action level II)


ALARA :ALARA To limit personnel exposure to ionizing radiation, control the variables/ basic radiation principles: 1.Time 2.Distance 3. Shielding


Time :Time Source (absorbed dose) is Proportional to the time its exposed Task should be carefully planned to minimize exposure time Training of a particular task without the radionuclide source helps increase your efficiency identify problem areas possibly reduce the time needed to complete a task.


Distance :Distance Inverse Square Law: Dose rate α 1/(distance)² Dose Rate Distance Since the exposures drops significantly, and most diagnostic procedures use small doses, with this combination the risk to hospital staff is very small.


Inverse Square Law :Inverse Square Law Radiation exposure follows the inverse square law Makes it dependant on both time and distance from the source. If you double your distance from the source your exposure rate drops by a factor of four If you triple your distance, your exposure drops by a factor of nine.


Inverse Square Law :Inverse Square Law Example: Calculate the intensity of a radioactive source at a different distance than the distance it was originally measured.   Q: If the intensity of a Iridium 192 source was found to be 620 Roentgen/hour 25 meters, what is the exposure at a distance of 1 meter.


Inverse Square Law :Inverse Square Law Answer: Reworking the equation to solve for I2 : I2= 62 mR/hr (100 ft)² 1 (ft)² I2= 620,000 mR/ hr or I2= 620 R/hr


Patient with 18F-FDG (Exposure) :Patient with 18F-FDG (Exposure) 0 0.5 1m 2m Distance 0.5 0.1 0.06 0.03 mSv/ Hr 10 mCi 18F-FDG


Properties of commonly used positron emitting radio-isotopes for PET :Properties of commonly used positron emitting radio-isotopes for PET


PET Shielding :PET Shielding Radiations passing through absorbers lose energy by interaction with absorber material. This loss of energy is very effective on high Z materials: Lead Tungsten Lead is mostly shielding material of choice since it’s the least expensive. Example of shields made of lead: Brick Syringe shield L -blocks Syringe container


Shielding :Shielding In PET facilities, 511 keV photons from positron emitters are highly penetrating, therefore, larger amounts of shielding material are needed in all aspects of radiation protection For example, pigs or lead containers used to transport cyclotron-produced materials should be sufficiently thick to reduce external levels to an acceptable value. Not actual “pigs” Actual “Pigs”


Half Value Layer :Half Value Layer The half-value-layer (HVL) in lead for the 511 keV annihilation is about 4.1mm. The use of a pig with a wall thickness of about an inch (25.4 mm) will result in external exposure levels that are about 1.4% of those that would have resulted without the presence of the shield for most of the positron emitters. As a useful note: it is helpful to use a Lucite sleeve over a syringe, a so-called "beta shield", to absorb energetic positrons.


Shielding in PET :Shielding in PET Once again! Protection against high energy photons requires lead shield of significant thickness (cm)


Summary :Summary Briefly introduced PET and most employed radionuclides Reintroduced ALARA and its principles Explained the Inverse Square Law and its importance FDG shielding and example of absorbers Properties for commonly used positron emitting radioisotopes for PET


Question 1 :Question 1 What kind of imaging modality is Positron Emission Tomography (PET)?


Answer 1 :Answer 1 Positron Emission Tomography (PET) is a method for determining biochemical and physiological processes in vivo (inside the body).


Question 2 :Question 2 What is the basis of PET imaging?


Answer 2 :Answer 2 The annihilation process


Question 3 :Question 3 What is the result of annihilation?


Answer 3 :Answer 3 Two anti-parallel 511 keV photons are produced


Question 4 :Question 4 Which of the four basic elements of “life” is replaced by a fluorine isotope? Explain why


Answer 4 :Answer 4 Hydrogen, This occurs since hydrogen has no radioactive isotope decaying with emission of radiation


Question 5 :Question 5 During a PET procedure, what are the two sources that may expose radiation to the patient?


Answer 5 :Answer 5 The administered radiopharmaceutical The attenuation correction segment of the protocol.


Question 6 :Question 6 What are the basic radiation principles to limit personal exposure to ionizing radiation?


Answer 6 :Answer 6 ALARA- Time, Distance, and Shielding


Question 7 :Question 7 True or False There is a high risk of receiving radiation exposure in a hospital?


Answer 7 :Answer 7 False! Since the exposures drops significantly, and most diagnostic procedures use small doses, with this combination the risk to hospital staff is very small.


Question 8 :Question 8 Math Equation: Question: If the intensity of a source was found to be 100 mR/hour at 12 inches, what is the exposure at a distance of 1 meter?


Answer 8 :Answer 8 Solve for I2 : I2= 100 mR/hr (12 )² (36)² I2= 11.11 mR/hr


Question 9 :Question 9 Name a few examples of shields made of lead


Answer 9 :Answer 9 Brick Syringe shield L -blocks Syringe container


Question 10 :Question 10 For the 511 keV annihilation, what is the half value layer of lead?


Answer 10 :Answer 10 4.1 mm


Resources :Resources Christian Waterstram-Rich. (2007).Nuclear Medicine and Pet/CT. St. Louis, Mo. Dale L. Bailey. (2008).Positron Emission Tomography-Basic Science. Published 2005, Springer. Peter E. Valk, Dale L. Bailey, David W. Townsend, Michael N. Maisey . (2008). Positron Emission Tomography-Basic Science and Clinical Practice.Published 2003, Springer