Radiographic Imaging and Instrumentation I Lecture 8: Grids PDF

Radiographic Imaging and Instrumentation I Unit 1.4: Beam Controlling Devices Lecture 8: Grids 1 What are we learning today? • Contrast • The effects of scatter • Factors affecting scatter • Beam Restricting Devices • Grids Contrast Contrast • Areas of light, dark, and shades of gray on the x-...

Radiographic Imaging and Instrumentation I Unit 1.4: Beam Controlling Devices Lecture 8: Grids 1 What are we learning today? • Contrast • The effects of scatter • Factors affecting scatter • Beam Restricting Devices • Grids Contrast Contrast • Areas of light, dark, and shades of gray on the x-ray image • Variations → shades of gray • Long scale and short scale contrast Romans p.33 Contrast Resolution • Definition • Impact of scatter radiation • Noise, ↓ contrast resolution, and ↓ image contrast Scatter Radiation • Primary photon interacts with an outer shell electron and changes direction • Scattered photon • Detrimental to the image quality • Adds noise to the image Bushberg Factors Affecting Scatter Radiation • kVp • Tissue Thickness • Collimation Scatter Radiation Scatter Radiation • Minimize the amount of scatter reaching the image receptor • Collimation and by using grids Scatter – Remnant Beam Ratio or the S/R Ratio • Describes proportion of scattered photons exiting patient relative to the number of photons exiting patient on their initial trajectory • AKA → scatter:primary ratio • Do not confuse with SNR • Signal to Noise Ratio, similar, but expressed in reverse S/R Ratio • FOV • Thickness of the patient • The energies of the x-rays S/R Ratio • Abdomen x-ray • Average patient of 20 cm • S/R Ratio = 3:1 • 75 % photons striking the detector or the image receptor carry little or no information S/R Ratio • For patients larger than 20 cm, the S/R ratio can be up to 5 or 6 • Therefore, aggressive field collimation and use of grids is needed to minimize the effects of scatter reaching the detectors or image receptors Collimation Beam Restricting Devices • Aperture diaphragm • Cones or cylinders • Collimators Beam Restrictors Variable-Aperture Collimator • P.B.L = … P.B.L • Also known as A.C.S.S = … • Sensors detect cassette size and orientation • Field size is automatically limited to cassette • Can reduce collimation further, but can’t increase it • Collimation should NEVER be larger than image receptor Grids • Grid function: • to remove scattered radiation from the remnant beam that will reach the IR • Location of grid: • between object (patient) and IR Grids • Because photons travel in straight lines • Diverge from point source (focal spot) • Can determine expected path of photons at a given SID • These principles determine the grid design Grids Grids • Antiscatter grids • To improve the radiographic contrast of the image • Designed to absorb unwanted scatter radiation Grids • Stationary • Reciprocating in a bucky tray • Potter-Bucky Diaphragm Potter-Bucky Diaphragm Grids • Necessary to use a grid with thicker, larger body parts, and when using high kVp techniques. • General Rule – body part > 10cm - kVp is > 60 Grid Composition • Contains a series of radiopaque strips alternating with radiolucent strips • Radiopaque – a dense material and high atomic number = Lead (Pb) • Radiolucent – Aluminum or Fibre Grid Function Bushberg How Grids Work Summary • Inverse Square Law • Contrast • Scatter • Beam Restriction Devices • Grids Readings • Unit 1.4 Course Manual and Study Guide • Bushberg – Chapter 7, section 7.2 • Bushong – Chapter 22