Optimization of Digital Mammography Resolution Using Magnification Technique in Computed Radiography

Hur, Gham; Hwang, Yoon Joon; Cha, Soon Joo; Kim, Su Young; Kim, Yong Hoon

Abstract

PURPOSE
To determine whether magnified digital mammography using a computed radiography system can produce better spatial resolution by reducing the focus-object distance, and to define the optimal magnification factor when a large x-ray tube focal spot is used for digital mammography using a CR system.
MATERIALS AND METHODS
Digital images obtained using computed radiography of a breast phantom were obtained using various magnification factors. Up to twelve acrylic blocks each measuring one centimeter in height were used to increase the distance between the breast phantom and the base plate (screen holder), in order to create the magnification images. The large (0.3 mm) focal spot of the x-ray tube was used for the entire series of images. Three radiologists participated in the evaluation of the images, in order to determine which had the best resolution. The resolving ability of the line pair structures and image clarity of the detectable artificial microcalcifications (specs) were the two factors used to determine the resolution of the images. The images were not compressed aFnd the viewing conditions, including the magnification factors, brightness and contrast, were fixed. The images were displayed on four high resolution PACS dedicated monitors (5 mega pixel LCD, BARCO Belgium).
RESULTS
A focus-object distance of 590 mm and a source-to-image receptor distance of 650 mm (set by the manufacturer) resulted in the best resolution, when combined with a magnification factor of 1.1. All three radiologists agreed on this result. Two of the radiologists believed that at least two more line pairs were better separated on the magnified image having the best resolution than on the unmagnified image, while one radiologist believed that three more line pairs were better separated on this magnified image. Using images with still larger magnification factors did not improve the resolution due to edge blurring. It was easier to determine the resolving power by means of the line-pair structures than by assessing the clarity of the artificial microcalcifications (specs). A 10% decrease in focal spot-object distance resulted in a 21% increase in radiation to the breasts.
CONCLUSION
Magnified digital breast images taken with a computed radiography system using a large focal spot produced better quality images, because of their utilizing more pixels per volume of the breast phantom with a minimal increase in radiation dosage.