New NIST-Invented Device to Help Radiologists Improve Image Quality in Mammography

A new device invented at the National Institute of Standards and Technology will help radiologists improve image quality in mammography, one of medicine's most important breast cancer screening tools.

The device, an X-ray crystal diffraction spectrometer, measures the distribution of X-ray energies that a patient would receive from a mammography unit more accurately than existing field calibration methods, NIST scientists say.

"Accurate measurement of kilovoltage is a key step toward improving the image quality for the millions of mammograms performed annually in the United States," said NIST physicist Bert Coursey.

The American Cancer Society estimates that 180,000 women were diagnosed with breast cancer and that the disease claimed 46,000 lives in 1992.

"The clinical community needs to be able to put tighter limits on the voltage applied to X-ray sources," said Dr. Richard Deslattes, inventor of the diffraction spectrometer device. Deslattes and colleagues will describe the new device in the January issue of Medical Physics.

The quality of a mammogram, an X-ray image of breast tissue, is determined, in part, by the electrical voltage that generates X-rays in a mammography unit. Lower voltages produce lower energy X-rays, and higher voltages produce higher energy X-rays. The exact voltage required for optimum image quality varies from woman to woman.

A radiologic technologist sets the voltage on the unit based on the thickness and tissue density of the breast. The existing non-invasive voltage measurement systems that are practical for mammography are accurate to within one or two kilovolts. Image quality, on the other hand, is influenced by sub-kilovoltage changes.

More accurate voltage measurement is available by use of calibrated potential dividers, but this kind of "invasive" measurement is complex, labor intensive and disturbing to the clinical environment.

In response to this measurement need, NIST scientists have developed a new approach based on two very old ideas. They first noted that the highest energy X-rays emitted by a radiological source correspond exactly in energy to the voltage applied to the X-ray tube. They then took advantage of a spectrometer design originally described by Sir Ernest Rutherford and E.N. da C. Andrade in 1914 to produce a convenient instrument requiring neither precise alignment nor external calibration to determine the high energy limit of the X-ray spectrum.

The NIST X-ray crystal diffraction spectrometer will be used as a calibration device. When placed in the X-ray beam, the device tells whether the actual voltage agrees with the indicated voltage. The NIST device, a metal box about 46 centimeters (18 inches) in length, measures the electrical voltage over the range used in mammography more accurately than any existing methods.

A patent for this approach to high voltage measurement has now been issued, and a license for commercial manufacture is pending.

More recent developments have extended the applicable range to 150 kV and demonstrated a new spectrometer design in which use of a slightly curved crystal permits high-resolution spectra to be obtained independent of the size and placement of the X-ray source.

"Tube voltage is an important parameter relating to mammography image quality and is one of the most difficult to measure accurately in the field. This new crystal spectrometer from NIST promises much more accurate measurements of tube kilovoltages made on the 12,000 mammography units in the United States," said R. Edward Hendrick, associate professor at the University of Colorado Health Sciences Center and chairman of the American College of Radiology Committee on Mammography Quality Assurance.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growh by working with industry to develop and apply technology, measurements and standards.