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

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.

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.

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.

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.

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

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)

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.

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