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X-Ray Interaction (Part II) in This Lecture Attenuation in Radiography

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X-Ray Interaction (Part II) in This Lecture Attenuation in Radiography In this lecture X-ray Interaction (Part II) Differential Attenuation Effect on Radiographic Image Contrast Examinations Exponential Attenuation Differential Attenuation Attenuation in Radiography • At particular photon energies, some or all of X-ray Tube the attenuation processes may be competing • Of the five interaction to remove photons form the X-ray beam processes two are • Consider linear attenuation coefficients for important for diagnostic radiography radiological purposes radiography Uniform intensity − • The transmitted x-rays = ( μ ) x • Ix Ioe are more important than Compton Photoelectric or those which interact Scatter absorption −( τ +σ +π ) x Ix = Ioe Remnant Beam Dependence on Atomic Number, Z Dependence on Atomic Number, Z 10 •Example: Extremity examination Photoelectric Contribution Logarithmic scale! •More photons are absorbed for bone & soft tissue Soft tissue photoelectrically in bone than in 1 Bone soft tissue •Photoelectric absorption is 3 proportional to Z 0.1 •Probability of PE interaction in bone is 7 times for bone than Relative Probabilityinteractionof PE 0.01 soft tissue 20 40 60 80 100 120 X-ray energy (keV) 1 Dependence on Atomic Number, Z Dependence on Atomic Number, Z 10 Material Atomic Combined Contribution of Number (Z) Logarithmic scale! Photoelectric & Compton Logarithmic scale! Effective atomic numbers Human Tissue of materials important to Fat 6.3 Soft tissue scattering for bone & soft Muscle/soft tissue 7.4 1 Bone diagnostic radiography tissue Lung 7.4 Bone 13.8 Contrast Agents Air 7.6 0.1 Barium 53 Iodine 56 Other Materials Relative Probabilityinteractionof PE Concrete 17 Molybdenum 42 0.01 20 40 60 80 100 120 Tungsten 74 X-ray energy (keV) Lead 82 Dependence on Atomic Number, Z Dependence on Mass Density Material Atomic • Intuitively, we could image bone Material Mass Example: Number (Z) Density even if differential absorption (Kgm-3) How much more likely Human Tissue How much more likely Human Tissue Fat 6.3 were not related to Z is an x-ray photon to Lung 320 Muscle/soft tissue 7.4 Fat 910 interact with bone Lung 7.4 – Bone has a higher Mass density Muscle/soft tissue 1000 Bone 13.8 than soft tissue than fat? Bone 1850 Contrast Agents Contrast Agents Air 7.6 Mass density: mass per unit • Air 1.3 Barium 53 3 volume (Kg/m ) Barium 3500 Iodine 56 Iodine 4930 Other Materials All interactions are proportional Other Materials Concrete 17 • All interactions are proportional Calcium 1550 Molybdenum 42 to mass density Concrete 2350 Tungsten 74 Molybdenum 10200 Lead 82 Tungsten 19350 Lead 11350 Characteristics of Differential Dependence on Mass Density Characteristics of Differential Dependence on Mass Density Absorption Example: Material Mass Density As x-ray energy •Fewer Compton interactions What is the relative probability -3 (Kgm ) increases •Much fewer photoelectric interactions that 60 keV X-ray photons will Human Tissue •More transmission through tissue undergo Compton scattering in bone Lung 320 compared to soft tissue? Fat 910 Muscle/soft tissue 1000 Muscle/soft tissue 1000 As tissue atomic •No change in Compton interactions Bone 1850 Bone 1850 number Contrast Agents •Many more photoelectric interactions Contrast Agents increases Air 1.3 •Less transmission through tissue Barium 3500 Iodine 4930 Other Materials As tissue mass •Proportional increase in Compton interactions Calcium 1550 density increases •Proportional increase in photoelectric interactions Concrete 2350 •Proportional decrease in transmission through tissue Molybdenum 10200 Tungsten 19350 Lead 11350 2 Example Contrast Examinations • Barium & Iodine • Assume that all interactions during • Both have high Z & density than soft tissue mammography are photoelectric. What • Example: is the differential absorption of x-rays in microcalcifications relative to fatty – What is the probability that an x-ray will tissue? interact with iodine rather than soft tissue? Exponential Attenuation Exponential Attenuation For exponential law to be 100 applied −( μ ) x −( μ ) x Ix = Ioe Ix = Ioe 80 1. Radiation beam must be or or parallel −( τ +σ +π ) x 60 −( τ +σ +π ) x Ix = Ioe Ix = Ioe 2. Radiation must be Monoenergetic 40 3. Attenuator must be 20 Transmitted Intensity (%) Intensity Transmitted homogeneous 0 Attenuator Thickness Half-Value-Thickness Half-Value-Thickness 100 100 −( μ ) x = −( μ ) x Ix = Ioe I x Ioe 80 80 or Io −( μ ) HVT = Ioe 60 −( τ +σ +π ) x 60 2 Ix = Ioe 40 40 1 −( μ ) HVT Definition: Thickness of = e a substance that will 2 20 transmit one-half of the 20 Transmitted Intensity (%) Transmitted Intensity (%) 0.693 intensity of radiation μ = 0 incident upon it 0 HVT Attenuator Thickness Attenuator Thickness HVT HVT HVT HVT HVT HVT HVT HVT 3 Summary X-ray Interaction 2 - Problem Sheet 1. Define Differential Absorption? 2. In a contrast chest x-ray, what is the relative probability that an x-ray photon will interact with lung tissue rather than air? • Differential Attenuation 3. What is the relationship between the atomic number of tissue and differential • absorption? • Effect on Radiographic Image 4. In a contrast examination using iodine, what is the relative probability that the x-ray beam will interact with iodine rather than soft tissue? • Contrast Examinations 5. What is the relationship between the atomic number of a tissue and the differential • Exponential Attenuation absorption? • Half-Value-Thickness 4.
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