Digital Radiograph Simulator

David Berndt M.S. 4th year Medical Student; George David M.S., DABR, FAAPM, FACR, Associate Professor of Radiology Physics at Augusta University.

© 2016

Click image above to enlarge

Left Image: 80 kV - 9 mAs
Right Image: 1 mAs - 80 kV 5 mAs - 80 kV 20 mAs - 80 kV 160 mAs - 80 kV 60 kV - 1 mAs 60 kV - 5 mAs 60 kV - 25 mAs 60 kV - 52 mAs 100 kV - 1 mAs 100 kV - 2 mAs 100 kV - 3 mAs 100 kV - 32 mAs

---

Overexposure

In the days of film, over-exposures would make an image too dark. Digital systems automatically adjust the density (darkness) of an image. Generally digital over-exposures have an excellent appearance unless the detector is saturated which is unlikely in clinical usage.

An exposure index value is calculated for each digital image. These values are vendor-specific so it is important to be familiar with each vendor’s implementation. Each digital vendor provides a recommendation for the acceptable range of exposure index. Because of the excellent appearance of over-exposed digital images, the exposure index is the only way to know that more radiation may have been used than required for diagnosis.

Underexposure

In the days of film, underexposures would make an image too light. Digital systems automatically adjust the density (darkness) of an image but they cannot invent information. Thus under-exposed digital systems will appear noisy or grainy. This is analogous to the sound of a weak radio station. One can increase the volume of the radio but that will also increase the volume of the static or noise. The lower the radiation reaching the receptor the more severe image noise will appear.

An exposure index value is calculated for each digital image. These values are vendor-specific so it is important to be familiar with each vendor’s implementation. Each digital vendor provides a recommendation for the acceptable range of exposure index.

For more information on kV and mAs parameters view the presentation on 'X-Ray Tubes & Production of X-Rays' at Augusta University's Radiology Physics page.

Click image above to enlarge

Left Image: Grid Used
Right Image: No Grid Used

---

Grid Use

It is an unfortunate fact that as radiation goes through the body some of it scatters, leaving the body in a random direction. Scatter radiation degrades the image which is essentially a shadow made by a point source of radiation. While scatter can not be controlled or reduced in the body, it is possible to reduce the scatter reaching the image receptor by use of a grid.

A grid is a directional filter for radiation that passes radiation coming from the point source in the x-ray tube and blocking scatter radiation which is traveling in the wrong direction. An analogy is looking out a window through blinds. You can clearly see what is parallel to the direction of the blinds but not in other directions.

Scatter radiation seriously degrades an image and is routinely used for all imaging except for very small patients or body parts that generate little scatter. However there is a penalty for its use. In order to compensate for the radiation removed by a grid, patient exposure must be increased by approximately a factor of 5.

For more information on grids view the presentation on 'Grids' at Augusta University's Radiology Physics page.

Click image above to enlarge

Left Image: Large Focal Spot
Right Image: Small Focal Spot

---

Focal Spot Size

Our x-ray images are just shadows and like any shadow, the smaller the source of light (or in our case radiation) the sharper the shadow (or in our case image). Resolution in x-ray is dictated by the weakest element of the imaging chain and that weakest link can be a number of things ranging from the size of the focal spot to patient motion to the size of the image matrix to the resolution of the viewer’s monitor. So changing the focal spot size may or may not affect resolution of a given image depending upon what the weakest link is.

For more information on focal spot view the presentation on 'X-Ray Tubes & Production of X-Rays' at Augusta University's Radiology Physics page.

Click image above to enlarge

Left Image: Full Field Size
Right Image: 10in x 12in 8in x 10in 5in x 8in

---

Field Size

Images should be produced at the minimum size required to make the diagnosis. Not only does a smaller x-ray field size result in less irradiated but it also reduces the amount of tissue producing scatter radiation. Even modest changes in field size will substantially reduce scatter and improve the quality of an image.

For more information on field size view the presentation on 'Beam Restrictors' at Augusta University's Radiology Physics page.