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Optimal breast imaging radiation discussed at SPIE conference

During the annual meeting of SPIE, the professional scientific society dedicated to optical science, researcher Michael O’Connor of the Mayo Clinic gave a presentation titled “Development of radiation dose reduction techniques for cadmium zinc telluride detectors in molecular breast imaging.”

O’Connor’s August 5 presentation during a medical applications session of the August 1-5 SPIE conference at the San Diego Convention Center also utilized the work of Mayo Clinic researchers Carrie Hruska, Amanda Weinmann, Armando Manduca, and Deborah Rhodes.

“This is a new technology,” O’Connor said. “We knew it was going to get a lot better.”

In 1997 the U.S. Food and Drug Administration approved the use of Technetium-99m sestamibi for breast imaging. The standard dosage of Tc-99m was initially 30 millicuries, which provides a sensitivity rate of between 35 percent and 64 percent for masses under one centimeter. The breast cancer survival rate for tumors under one centimeter is 98 percent while the survival rate for three-centimeter tumors is 70 percent.

Conventional imagery instrumentation which has placed cameras ten centimeters away from the breast can be an impediment to better detection.

“You can’t get breast tissue close to the camera,” O’Connor said. “You can never get good resolution with conventional gamma cameras breast imaging.”

The use of the cadmium zinc telluride semiconductor allowed for closer imaging.

“You can image all the way up to the edge of the breast,” O’Connor said. “We kind of hung our hat on CZT.”

CZT imaging can also be performed at room temperature. Testing at the Mayo Clinic over the past six years has also shown that Tc-99m tracer material accumulates in cancer cells and is not affected by breast density.

The drawback of CZT imaging, including the use of the TC-99m radioactive isotope, is the radiation.

“You’re getting a lot more than you’re getting in a mammogram,” O’Connor said. “This becomes a problem.”

The Mayo Clinic initially used a standard dose of 25 millicuries, but over a 40-year period the accumulated radiation would lead to a mortality rate approximately 10 times higher than that of digital mammography. The Mayo Clinic began a program to reduce dosage to between two and four millicuries while maintaining the sensitivity necessary for early detection.

Four dose reduction strategies were developed and evaluated: collimator optimization, energy window optimization, noise reduction algorithms and composite imaging for opposite detectors.

The conventional use of a hexagonal hole collimator over a square hole detector seemed like a prime target for optimization. The design optimization provided a gain in sensitivity by a factor of 2.2. When patients were given doses of eight millicuries,

sensitivity factors between 2.0 and 7.0 over hexagonal collimators were observed.

Although a wide energy window of between 110 and 154 keV (thousands of electron volts) produced better sensitivity, the Mayo Clinic researchers set a goal of attaining 90 keV which would still allow for capture of backscatter. A non-local means

denoising function allowed for noise reduction.

The standard dosage for patients was reduced to from 25 millicuries to eight millicuries with the various improvements. Small tumors were still detectable at four

millicuries. The two-millicurie dosage shows promise but is not yet to the researchers’ satisfaction.

“You can still see the lesion, but there is some graininess,” O’Connor said.

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