2. Materials and Methods
Molecular Breast Imaging
MBI is the non-invasive and quantitative imaging of cellular and molecular processes of normal and malignant breast tissue – with the use of an injectable contrast agent - to aid in detection,
diagnosis,
and treatment assessment.
CZT-based MBI Scanner
Semiconductor CZT
The 140 keV gamma-ray emissions from 99mTc are detected and localized by semiconductor CZT modules (Figure 1). The modules are arranged into two large (15.2 cm x 20.3 cm) plates each with a pixel array of 96x128 ( at a 1.59 mm pitch). CZT has high energy resolution (<4.5% ~ 140 keV) for scatter rejection and dual-isotope applications. Our previous work [Pa 97] concluded that pixel pitch for parallel hole collimation is optimized at 1.5 mm.
Registered-hole,
tungsten collimator
Figure 2 shows a photograph of the first near-field,
registered tungsten collimator,
now in use at the Mayo Clinic. Each of the 12,288 holes is registered (or aligned) with a CZT pixel. Sensitivity is improved by covering the pixel edges and shortening hole depth for near-field (<3.0 cm) optimized imaging.
Gamma Medica’s LumaGEM™
Figure 5 is a photograph of the dual-head LumaGEM™ (Gamma Medica,
Northridge,
CA) MBI scanner. It is capable of obtaining cranio-caudal (CC) and medio-lateral oblique (MLO) views of a seated female patient by moving the pair of detectors to the correct vertical height (using vertical tower motion) and the correct orientation (CC or MLO) using the circular track shown in Figure 3. Four views of 10 minutes are acquired; compression is light and comfortable (15 lbs compared with 45 lbs for MMG).
BEIR VII Risk Calculations
Excess Absolute Risk (EAR),
Lifetime Attributable Risk (LAR)
The BEIR VII report [NRC 06,
Chapter 12] prefers the EAR model for estimation of induced breast cancer risk [Law 01]. This model was used to compare cancer incidence and mortality for positron emission tomography (PEM) as well as the 99mTc-MIBI single-photon nuclear technique. Lifetime attributable risks (LAR) are estimated by summing EAR estimates for each year of life after exposure [OC 10a].
The LAR estimates were adjusted for the fraction of a population of 100,000 females expected to survive to various ages between 0-80. EAR was calculated from Equation 12-2 of the BEIR report; LAR was constructed by summing EAR at each age,
with weighting for survival. PEM and MBI dose estimates were taken from published sources [Ha 02]. An average level of background radiation in the U.S.
is estimated at 3.1 mSv [NCRP 09]. Estimates of lifetime attributable risk of cancer incidence and cancer mortality were calculated for PEM,
MBI,
Digital Mammography,
Screen-Film Mammography,
and Background (without medical tests).