In order to prevent the shadows cast onto the film by the grid from interfering with visualization of diagnostic detail, certain principles must be followed:
the lead should be as thin as possible to be consistent with adequate absorption of scattered radiation. The thinner the lead, the narrower the shadow it will produce on the film and the less visible it will be to the eye.
Also, the thinner it is the less absorption of primary radiation will be in the grid. BUT it must be noted that adequate absorption of scattered radiation is the function of the grid and lead must be thick enough to provide this function.
Another factor is the relative fineness of the grid. This quality is represented by the number of lines per inch.
In general, the greater the number of lines per inch, the less visible will the individual lines be, but this is subject to certain practical considerations which modify it in actual use.
In the absence of the grid there is the possibility of scattered radiation generated at any point in the pt. reaching the film
Presence of grid results in a reduction in scatter at the film.
A reasonable grid is able to remove as much as 80-90% of scattered radiation.
This removal of scatter increases the contrast.
Majority of primary radiation will pass thro the interspace & reach the film.
Some scattered rays produced in the pt are potentially able to reach the film.
It also removes some of the useful primary radiation
BEAM CUT OFF.
The primary radiation will be traveling radially from the xray tube focal spot to the film & will encounter the strips obliquely.
This primary absorption creates an overall pattern of thin parallel white lines (shadow of lead strips.) known as the grid lines.
The closer together the grid lines are on the film, less objectionable they become
Absence of grid ,scattered radiatn also would have contributed for film density , so tube exposure must be further increased on this account.
The kv has to be increased ,which decreases the contrast & increases the pt dose.
Filter in x-ray physics is a sheet of metal through which the rays go before striking the object to be examined or treated.
Filters are sheets of metal placed in the path of the x-ray beam near the x-ray tube housing to absorb low energy radiation before it reaches the patient.
The use of absorbers or filters for the preferential attenuation of radiation of certain wavelengths from a useful primary beam of x radiation.
Minimizing patients exposure while maintaining a diagnostically acceptable radiograph which is a major goal in diagnostic radiology.
is used in order to modify the spectral or spatial distribution of X-rays, or both
In March 1896, William Francis Maggie, Professor of Physics at Princeton, described the effectiveness of the aluminum filter.
He noted that an obstacle to the x-rays actually improved the shadows that they cast.
The interposition of a thin plate of aluminum between the vacuum tube and the fluorescent paper of the skin scope did not merely leave the fluorescence undimmed, but actually intensified it considerably.
George E. (1905) was the first to use filter as a protective device.
He set out to find a filter that would strain out the soft skin burning rays while letting the others through in much the same fashion as window glass strains out the UV rays of sun that cause sunburn.
He used the principle of selective absorption. Each, material absorbs rays of a particular quality from the heterogeneous x-ray beams.
He thought that skin has got a peculiar absorbing power. To filter these harmful rays he should select a substance that resembles the skin as close as possible – Leather. He selected thickest leather i.e. Sole leather, which is 4 times as thick as human skin.
The filter was a simple disc of sole leather 5 inches in diameter soaked in water, in order to resemble skin more closely.
Although the filter did provide considerable protection to the skin a substantial part of this protection. was overcome if a radiologists, relying on the filter, increased the dose too enthusiastically.
The sudden change in the attenuation of the radiation occurs at photon energies equal to the binding energies of the different electron shell –
Low energy photons are less attenuated and are more penetrating than higher energy photons
Any substance is relatively transparent to its own characteristic radiation , the energies of which are always at least a little less than the corresponding binding energies