Grid Errors, Anode Heel Effect, Spatial Resolution

Off-level error

• Decreased receptor exposure across the entire radiograph
(grid cut-off)
• Possible during portable exams, not when using stationary
equipment

Off-center error​

• Grid not placed directly under CR​

• Only affects focused grids​

• Decreased receptor exposure across the entire radiograph (grid cut-off)​

• Possible during portable exams, not when using stationary equipment

Off-focus error ​

• SID does not match the design of the focused grid​

• Only affects focused grids​

• Results in grid cut-off at outer edges of image

Angulation error​

• Beam is angled against the direction of grid lines​

• Affects focused and non-focused grids ​

• Stationary systems only allow longitudinal angulation of CR, this matches the direction of the grid lines​

• Results in grid cut-off over entire radiograph 

Upside-down error​  

• Focused grid is placed upside down​

• Only affects focused grids​

• Results in grid cut-off on outer edges of radiograph  

• Anode is angled 6-20 degrees​

• Increases surface area, increases heat capacity​

• Decreases effective focal spot size, increasing spatial resolution​

• Drawback: Causes a variation of beam intensity across exposure field 

Anode Heel Effect​

X-ray photons get created throughout the anode, not just on the surface, as a result some of the x-ray photons get absorbed by the anodes heel. ​

When can we use the anode heel effect to our advantage?

Abdominal xrays- anode over upper abdomen, cathode over the lower more dense part of the abdomen. ​

Decreased anode angle =

increased heel effect​

Shortened SID =

increased heel effect 

increasing field size =

increase heel effect​

OID can be included in patient factors too along with pathologies. ​Smaller DEL’s increase

spatial resolution. ​Small pitch increases spatial resolution. ​High fill factor increases spatial resolution. ​

Spatial resolution is described (measured) in

LP/mm

Sharpness recorded in the image is

Spatial Resolution