Lecture 6 Attenuation and Transmission of Photons Unit

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, 5 th Ed. (St. Louis: Mosby 2004) p 52.

Attenuation and Transmission of Photons • Attenuation 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, 5 th 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, 6 th Ed. (St. Louis: Mosby 2004), p 57.

Attenuation and Transmission of Photons • Half-Value Layer (HVL) • • Similar concept to T 1/2 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: Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6 th Ed. (St. Louis: Mosby 2004), p 57. Where have we seen this before? ? ?

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 I 0 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 5 m. R/hr at 1 meter from a capsule of 131 I. The half-value layer of lead for 131 I’s 364 ke. V photons is 0. 3 cm. If you place the capsule in a lead container that is 0. 9 cm thick, what should be the expected reading of the shielded capsule at 1 meter. • What do we know? • Original incident radiation field (I 0) is 5 m. R/hr • 131 I • (364 ke. V principle gamma photon E) • Lead is the shielding • HVL is 0. 3 cm for 364 ke. V photons • Thickness (x)of the lead is 0. 9 cm • 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, 6 th Ed. (St. Louis: Mosby 2004), p 57.

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