1. Lecture 6: Attenuation and Transmission of Photons
Unit I: Physics of Nuclear Medicine

2. Lecture 6 Objectives (Adapted from your Textbook)
Write the general form of the attenuation equation for gamma photons.
Calculate the reduction of gamma radiation using the general attenuation equation.
State the relationship between the linear attenuation coefficient and the half-value layer.
Recognize the equation and use of the mass attenuation coefficient.

3. Attenuation and Transmission of Photons
This is how all the gamma radiation eventually succumbs to matter
It is absorbed or attenuated. This is how it relates to instrumentation
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 52.

4. Attenuation and Transmission of Photons
Combined effects of attenuation is expressed by the linear attenuation coefficient (μ), which is in the units 1/distance(cm-1).
The attenuation of incident radiation (I) can be expressed as follows:
Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 52.
X is the distance through which the incident radiation travels through the attenuating material.
Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6th Ed. (St. Louis: Mosby 2004), p 57.

5. Attenuation and Transmission of Photons
Half-Value Layer (HVL)
Similar concept to T1/2
Layer of attenuating material that will absorb ½ the incident radiation
Specific for type of material and energy of incident radiation
Is related to μ according to the following:
Where have we seen this before???
Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6th Ed. (St. Louis: Mosby 2004), p 57.

6. Attenuation and Transmission of Photons
Substituting the previous for μ, our attenuation equation now looks like…
I is the incident radiation intensity after it has passed through a thickness of an absorbing material
I0 is the original incident radiation intensity before it passes through an absorbing material
x is the thickness of the absorbing material
HVL is the half-value layer, which is obtained from a source document (or your instructor).
Note: for this equation to work, x and HVL have to be the same units of distance

7. Attenuation and Transmission of Photons
An example: A survey meter reveals an exposure level of 5mR/hr at 1 meter from a capsule of 131I. The half-value layer of lead for 131I’s 364 keV photons is 0.3 cm. If you place the capsule in a lead container that is 0.9cm thick, what should be the expected reading of the shielded capsule at 1 meter.
What do we know?
Original incident radiation field (I0) is 5mR/hr
131I
(364 keV principle gamma photon E)
Lead is the shielding
HVL is 0.3 cm for 364 keV photons
Thickness (x)of the lead is 0.9cm
What are we trying to find out? (I)

8. Attenuation and Transmission of Photons
Mass Attenuation Coefficient
Based on material density
Is related to the linear attenuation coefficient
They use this to calculate linear attenuation coefficient since attenuation can vary depending on mass density.
Physicists can break this down so that they can measure attenuation according to Compton scatter, photoelectric effect, and pair production
Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6th Ed. (St. Louis: Mosby 2004), p 57.

9. Basic Electrical Concepts
Next time:
Basic Electrical Concepts