S numbers with digital radiographs

RE: S numbers with digital radiographs

Evelyn Barbour-Hill — Sat, 01 Jan 2022 20:27:32 GMT

As I understand it, variation in kV or mAS has exactly the same effect in any kind of radiography; just as variation in aperture and time has the same effect on digital photography as it does on film. (*)

However, as I understand it (which may be all wrong): in translating what's hit the sensor to what appears on the screen, the digital radiography system applies various adjustments, modifications, whatever, without being asked to, to provide what it thinks you want. And some systems are better than others. And being basically stupid, as all computer systems are, it sometimes gives you something you don't want. Maybe also the sensor cells react in a different way from the chemicals in a film emulsion, to Xrays hitting them – I'm sure somebody on this forum will know.

All you can do is play about with kV and mAS until you get it right.

*(In photography I much prefer film for the end result, but I use a digital camera most of the time for convenience).

RE: S numbers with digital radiographs

Clive Ansell — Sun, 19 Dec 2021 12:55:26 GMT

S, or sensitivity, values related to the amount of exposure received by receptors within a digital system. It is an inverse numerical value, so a lower S number refers to higher exposure and vice versa. S numbers are recorded with every image processed, so can be used for monitoring exposures. For example, if S values are constantly too low, then too greater exposures are being used. More applicable in human radiography I guess where patient exposure needs to be kept to a safe minimum. I believe an optimum value is around 200.

RE: S numbers with digital radiographs

Neil Wheadon — Sun, 19 Dec 2021 12:43:20 GMT

The basic I know is that the S number has a range and yes indicates exposure.

In the case of x-ray plates if a radiograph was underexposed you wound up the KV or increased the time. This logic doesn't work with digital. In effect the S number seems to bear no relation to the KV.

The S number seems to be the key, get that right and you get a good x-ray, but what do you do to move that value about?

I realise that modern radiographs can be adjusted much like a RAW image in photography, but some processers are better at it than others. Like photography the aim is to get it right pre-production not post production

Neil

RE: S numbers with digital radiographs

Beats — Sun, 19 Dec 2021 11:14:38 GMT

it's probably the exposure index?