Compensation of the non-uniformity of back-lighting sources when digitising X-ray films for video-densitometric measures

Abstract

Background: computerised image analysis and the building of electronic archives require the digitisation of x-ray films performed in the past, when direct digitisation was not yet available. Different techniques can be used, based on laser scanning or camera acquisition; both can be heavily impaired by the non-uniformity of the backlighting source. This work wants to develop a methodology, able to reduce this noise source.

Methods: The introduced algorithm is a step beyond the common compensation of backlighting non-uniformity through subtraction; in fact, each grey level is corrected taking into account not only its position on the back-lighting system and its raw value, but also the interaction between these two parameters.

Results: Given a certain acquisition system, the introduced algorithm has allowed to reduce the maximum grey level standard deviation, read on uniform grey level films, from 9.15 to 2.00 (on a scale from 0 to 255) against traditional subtraction techniques that achieved a minimum standard deviation equal to 5.39.

Conclusions: The employed algorithm has proved to be effective in order to obtain same measured grey levels, almost independent from the position of the radiograph on the back-lighting system, and therefore makes x ray image digitisation more accurate. Only 26.4% had FADS prior to 20 weeks. Sonogram performed anytime during the pregnancy could detect 55.9% (62 of 121)) of congenital malformations suggesting that it was missed in 44.1% cases.  Those who submitted for FADS between 14 – 24 weeks of pregnancy, the malformations were detected in 53.2% of 66 pregnancies. The majority (45.2%) of anomalies diagnosed by ultrasonography were neural tube defects.