"Our spectral mammography technique is accurate, reproducible, fully automatic, and most importantly can be readily incorporated into current clinical screening programs," Molloi, vice-chairman of research within the Department of Radiological Sciences at UCI, told medicalphysicsweb. "The breast density measurement can be done within a few seconds and the radiation dose absorbed by patients during the screening exams will be up to 50% less than the current level."

Identifying breast tissue types

Measuring the volumetric breast density requires assessment of two independent variables: thickness and density. The human breast is predominately composed of two different tissue types: adipose and glandular. In order to calculate the volumetric breast density, it is necessary to measure the thickness of glandular and adipose tissues separately, which is not possible using only a standard mammogram. This has led to multiple research groups proposing different ways to solve the problem.

The spectral mammography approach developed by Molloi and Ding measures the thickness and density via dual energy decomposition. Their approach uses a silicon-based photon-counting detector, which differentiates incoming X-ray photons according to their energy, and a scanning multi-slit geometry. A "splitting energy" is set such that the gathered X-ray photons are sorted into either a low-energy or a high-energy bin. It is then possible to derive the glandular and adipose tissue thicknesses, as well as the density using these acquired images.

"The low-energy and high-energy images are acquired simultaneously in a single exposure without additional dose to the patient," explained Molloi. "The combination of an energy-sensitive photon-counting detector and the multi-slit geometry makes this system nearly ideal for dual energy material decomposition. It is also worth noting that the sum of the two energy images can be used as the standard mammogram for lesion detection."

Tests on phantoms

Once their system had undergone rigorous calibration, Molloi and Ding devised four phantom experiments, each designed to test one representative variable. The first test investigated the accuracy of volumetric breast density quantification for glandular and adipose equivalent phantoms of different thicknesses and known density. Next, the thickness was fixed at 4 cm, and the density was varied. Then, with fixed thickness and known densities, the area was varied from 62.5 cm up to 250 cm2. Finally, phantoms of different shapes were constructed.

"Our four phantom studies indicate that spectral mammography can be used to measure volumetric breast density with an RMS error of less than 2%," stated Molloi. "The development of an accurate volumetric breast density measurement technique can potentially have a high impact in the assessment of breast cancer risk."

The researchers say that their proposed technique could easily be translated to a clinical setting, where neither the patients nor the radiologists would notice any changes in the screening exams. "We are now planning to have a pilot patient study to evaluate the performance of the spectral mammography system in the clinical setting," commented Molloi.

Related articles in PMB
Quantification of breast density with spectral mammography based on a scanned multi-slit photon-counting detector: a feasibility study
Huanjun Ding and Sabee Molloi Phys. Med. Biol. 57 4719
MicroCT with energy-resolved photon-counting detectors
X Wang et al Phys. Med. Biol. 56 2791
Photon counting spectral CT versus conventional CT: comparative evaluation for breast imaging application
Polad M Shikhaliev and Shannon G Fritz Phys. Med. Biol. 56 1905