You are asked to design a spectral filter that practically removes 99.0% of the low energy photons in an X-ray beam. Such photons contribute to the patient dose without contributing to the image and are defined as no more than 1% of these photons making it to the other side of the patient. Assume a patient can be modelled as a 20cm thick homogenous object with linear attenuation coefficients as shown below in Table 1. What is the thickness of the filter needed to eliminate all the energies which satisfy the above requirement? Filter linear attenuation properties are given below in Table 2. Table 1: Lincar attenuation coefficients vs. energy of patient equivalent material Energy [keV] Hjet (mm 20 0.02601 30 0.02407 40 0.02303 50 0.02151 60 0.02013 Table 2: Linear attenuation coefficients vs. energy of filter material Energy (keV Hr mm 20 30 40 50 60 70 0.1225 0.1067 0.0872 0.07541 0.06684 0.06327
You are asked to design a spectral filter that practically removes 99.0% of the low energy photons in an X-ray beam. Such photons contribute to the patient dose without contributing to the image and are defined as no more than 1% of these photons making it to the other side of the patient. Assume a patient can be modelled as a 20cm thick homogenous object with linear attenuation coefficients as shown below in Table 1. What is the thickness of the filter needed to eliminate all the energies which satisfy the above requirement? Filter linear attenuation properties are given below in Table 2. Table 1: Lincar attenuation coefficients vs. energy of patient equivalent material Energy [keV] Hjet (mm 20 0.02601 30 0.02407 40 0.02303 50 0.02151 60 0.02013 Table 2: Linear attenuation coefficients vs. energy of filter material Energy (keV Hr mm 20 30 40 50 60 70 0.1225 0.1067 0.0872 0.07541 0.06684 0.06327
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The filter asked to be designed should be able to remove 99% of the photons incident on it, and only let 1% of the photos pass through.
This can be done as the X-ray beam passing through the filter gets attenuated as it goes through.
The attenuated intensity is given in terms of the original intensity as
Since 99% of the incident radiation is to be blocked, and only 1% is to be passed,
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