Figure 1. (A) This prototype digital mammography system that used the Fuji 5000MA CR imager and reader (FUJIFILM Medical Systems USA, Inc., Stamford, CT) was used by the University of California, Davis during the Digital Mammographic Imaging Screening Trial (DMIST). (B) Computed radiography mammography uses a photostimulable storage phosphor (PSP) plate in a conventional cassette form-factor to record the X-ray signals transmitted through the breast. Essentially, any dedicated mammography system accredited for screen- film mammography can become Òdigital.Ó Additionally, flexibility is achieved (as it is in filmscreen mammography) through the option of 2 different detector sizes so that a particular breast size can be better matched.

Figure 2. (A) The exposed computed radiography (CR) imaging plate is placed into a special CR mammography reader, is transported to an optical reading stage, and is scanned with a stimulating laser beam to produce photostimulated luminescence (PSL) that sweeps the extracted imaging plate. The PSL signal, proportional to the incident X-ray intensity on the imaging plate, is captured by a light guide, channeled to a photomultiplier tube (PMT), converted to an electronic signal, and amplified. An analog-to-digital converter (ADC) subsequently transforms the electronic signal amplitude into an equivalent digital number that is placed in the digital image matrix corresponding to the position of the laser beam at that instant. Scanning occurs in a raster fashion by transporting the plate continuously through the optical stage. Laser scanning occurs very quickly, from left to right, and plate translation speed is coordinated to ensure the same sampling pitch (space between pixels) in the rows and columns of the digital image. Once the imaging plate is scanned, an erasure step eliminates residual signal, and the refreshed imaging plate is ready for another exposure. (B) For dedicated mammography, a specialized Òdual-sided readoutÓ and 50-µm sampling pitch is employed to optimize detection and readout efficiency of the imaging process. There are differences in the characteristics of the information acquired from the front and back light guides. Sophisticated signal processing algorithms are applied to the separate signals to optimize the characteristics of spatial resolution and contrast resolution, which are then combined at the image processor to produce the final output image.

Figure 3. (A) The modulation transfer function (MTF) illustrates how information is lost as a function of spatial frequency (inverse of object size). A perfect system would deliver 100% modulation for all spatial frequencies. The cutoff frequency (maximum spatial frequency contained in a signal averaged over an area) for a 50-µm element size is 20 line pairs per mm (lp/mm). Depending on the sampling pitch (distance between sample areas) the Nyquist frequency (maximum useful frequency) when the sampling pitch equals the aperture dimension (the situation for most digital detectors) is equal to half the cutoff frequency (known as the Nyquist sampling theorem), meaning that 10 line pairs per mm is the maximum useable frequency in the acquired image for a 50-µm spot dimension. (B) In reality, when one compares the measurements of the hypothetical perfect detector with actual computed radiography measurements, it is clear that the MTF does fall off significantly at higher spatial frequencies (smaller object size).