Electronic detectors spring from an x-ray heritage

Digital mammography and general radiography developed from the same x-ray root, but mammography has veered off, driven by a need to meet clinical demands for low radiation exposure and high image quality.

Because mammography is done in a screening environment in which most patients are imaged even when no obvious signs of pathology are present, the technique must minimize patient radiation dose. Mammography also must provide an extremely high degree of image resolution, because pathologies may be minute, subtle, and difficult to differentiate from normal tissue.

Nevertheless, radiography and mammography can be traced to the standard x-ray imaging technique of Roentgen's time, when a screen or phosphor was used to absorb x-rays, release energy, and convert that energy into a light flash, or scintillation, that could capture an image and represent it on film. The first forms of digital detector technology invented for general radiography and mammography evolved from that simple technique. Indirect conversion detectors still use a material like a phosphor that absorbs x-rays and gives off light. The difference is that the x-ray film has been replaced by a digital light-sensitive detector or a cassette.

Other types of digital mammography detectors convert x-rays that have passed through the breast directly into electronic signals. But all seek to foster the detection of breast cancers by improving penetration in dense regions of tissue, characterization of suspicious areas, and depiction of the periphery.

CHALLENGING SCREEN-FILM

Screen-film mammography systems have been the standard for breast cancer screening because of their high spatial resolution. With the ability to resolve as many as 20 line pairs per millimeter, screen-film mammography can identify minuscule soft-tissue aberrations such as microcalcifications and spiculations. High contrast allows screen-film mammography to distinguish shadowy differences in density.

The modality has been credited with reducing mortality from breast cancer by 20% to 35%. Screen-film mammography, however, does not visualize 10% to 20% of malignancies detected on self- or physical examination, and up to 80% of the questionable findings it discovers turn out to be benign at biopsy.

Among the drawbacks to screen-film mammography are a limited 40:1 dynamic range and noise from the granularity of film, which can influence detective quantum efficiency (DQE) and compromise the detection of microcalcifications.

Digital mammography, which has been in development since the 1980s, has a wide 1000:1 dynamic range, which enhances the visualization of all areas of the breast. It also allows the adjustment of gray-scale intensity to heighten contrast; enables postprocessing to expand options for cancer detection and correct under- or overexposures; and offers the opportunity for lowering radiation dose. But it is not a technological panacea.

"Digital mammography is not going to magically find a bunch more cancer than you could find on screen-film, because breast cancer is going to be hidden behind breast tissue regardless of what 2D technique you use, whether it be screen-film or digital," said Dr. Bruce Schroeder, a radiologist from Eastern Radiologists in Greenville, North Carolina.

Nevertheless, Schroeder, who has several years of experience reading digital mammograms and still reads screen-films from outlying breast imaging centers, prefers the detail he can see on digital images.

"Certain parts of the breast, such as a centimeter or two under the skin, you can never see in screen-film," he said. "In digital, it's right there for you."

The ability to provide such images without ramping up dose comes from the digital detector technology.

FIRST DIGITAL DETECTORS

Screen-film systems have intensifying phosphor screens that collect x-rays and release light scintillation in the form of light photons that spread out and illuminate the film. The systems have been designed to include thick screens to absorb x-ray scatter and make images more efficient. If screens are too thick, however, they can increase the scattering of light and blur images. Screen-film systems strive for a happy medium.

Early digital mammography systems were designed in a similar fashion. Employing what are known as indirect conversion digital detectors, the systems have a scintillator made of cesium iodide that captures x-ray energy and converts it to light. Light is detected by means of a series of photodiodes or charge-coupled devices and is transformed into electronic signals via a series of thin-film transistors.

Because of their thin and tubular structure, cesium iodide crystals can decrease light scatter by directing light to the surface of the optical detector. Indirect conversion digital technology still has at least some degree of light scatter, however, which reduces image sharpness.

The first digital mammography system to become commercially available in 1999, the Senographe 2000D by GE Healthcare, applied such an indirect capture approach. The Senographe 2000D has a cesium iodide scintillator that has been bonded to a flat-panel imaging plate made of amorphous silicon. Underneath is a thin-film transistor array with a light-sensitive diode at every pixel.

Since the Senographe 2000D was introduced, GE has been updating its digital detector topology to improve DQE and to follow newer imaging pathways-such as digital tomosynthesis-by altering the thickness and the structure of the cesium iodide, as well as the characteristics of the thin-film transistors and photo diodes, said Mike Barber, chief technology officer for GE Healthcare.

THE DIRECT APPROACH

Another approach is direct conversion. Rather than using a phosphor coating, direct digital detectors use a photoconductor made of amorphous selenium or silicon that converts x-rays directly to electronic signals.

"Direct conversion is quite different philosophically from the first digital detectors because it uses a metal that absorbs an x-ray and converts it directly into an electrical charge without the intermediate step of light generation. This has certain advantages for mammography," said Dr. Andrew Smith, principal scientist for Hologic in Bedford, Massachusetts.

The phosphor layer that is characteristic of indirect conversion systems must be thick enough to absorb enough x-rays at a reasonable patient dose, according to Smith. The greater the phosphor layer thickness, however, the lower the resolution will be.

"With direct conversion detectors, you can make the radiation-absorbing layer very thick without affecting resolution because the electrical charges that are generated don't diffuse through selenium in the same way that the light pulse diffuses through cesium iodide," he said.

An amorphous selenium detector can absorb more than 95% of the x-rays generated in the mammographic system, even at a thickness of 250 microns, according to Smith.

In addition to the Selenia mammography system by Hologic, amorphous selenium direct conversion detectors are also used in the digital mammography Novation system by Siemens, the Giotto by IMS, and the Nuance by Planmed. A silicon-based direct conversion detector is used in the Sectra MicroDose model.

SLIT SCANNING

Sectra MicroDose Mammography is the only commercially available photon-counting system, which extracts the maximum amount of information from each x-ray, said Prof. Mats Danielsson, a professor of physics at the Royal Institute of Technology in Stockholm and cofounder of Sectra Mamea.

"The conversion step, which normally adds noise to the image and is associated with detection of x-rays, is eliminated by incrementing a counter for each x-ray hit," Danielsson said. "This enables high image quality at lower dose."

Sectra is the only company to use crystalline silicon material, which is considered the gold standard in the semiconductor industry, for its digital detector.

"In a comparison with other detector materials made of compounds such as cesium iodide or selenium, crystalline silicon has production volumes and industry infrastructure in place that are many orders of magnitude larger and more mature," he said.

Sectra currently has clinical installations of the MicroDose system in 14 countries, mostly in Europe but also including New Zealand and Australia. In several markets, Sectra has more than 50% market share for digital mammography. In the U.S., the system is pending Food and Drug Administration approval.

WHAT'S AHEAD

Digital mammography detectors are already highly efficient and produce stunning images. So reductions in dose or enhancements in resolution most likely will not be driven by a new type of detector, but by advanced techniques that remove overlapping tissue, such as 3D tomosynthesis or dual-energy subtraction, which captures two images at different energy levels, separates areas containing contrast material, and provides a view of the vasculature and mammary ducts and other structures.

"X-rays, just as visible light, come in different colors," Danielsson said. "It is only because current technology is color-blind that we always see x-rays in black and white."

Just like the color of a flower can help it stand out in the midst of other vegetation, the color of an object in a mammogram may also aid detection, he said. With a photon-counting system, this information comes almost for free, once the instrumentation is in place. And it requires no extra work for the radiographer, since the energy discrimination is taking place in the detector electronics.

"The extra information from dual-energy imaging, therefore, may be used to enhance structures of clinical interest, such as calcifications," he said.