Proof of concept for low-dose molecular breast imaging with a dual-head CZT gamma camera. Part I. Evaluation in phantoms

Abstract

Purpose: Molecular breast imaging (MBI) is a nuclear medicine technology that uses dual-head cadmium zinc telluride (CZT) gamma cameras to image functional uptake of a radiotracer, Tc-99m sestamibi, in the breast. An important factor in adoption of MBI in the screening setting is reduction of the necessary administered dose of Tc-99m sestamibi from the typically used dose of 740 MBq to approximately 148 MBq, such that MBI's whole-body effective dose is comparable to that of screening mammography. Methods that increase MBI count sensitivity may allow a proportional reduction in the necessary administered dose. Our objective was to evaluate the impact of two count sensitivity improvement methods on image quality by evaluating count sensitivity, spatial resolution, and lesion contrast in phantom simulations.

Methods: Two dual-head CZT-based MBI systems were studied: LumaGem and Discovery NM 750b. Two count sensitivity improvement methods were implemented: registered collimators optimized for dedicated breast imaging and widened energy acceptance window optimized for use with CZT. System sensitivity, spatial resolution, and tumor contrast-to-noise ratio (CNR) were measured comparing standard collimation and energy window setting [126-154 keV (+10%, -10%)] with optimal collimation and a wide energy window [110-154 keV (+10%, -21%)].

Results: Compared to the standard collimator designs and energy windows for these two systems, use of registered optimized collimation and wide energy window increased system sensitivity by a factor of 2.8-3.6. Spatial resolution decreased slightly for both systems with new collimation. At 3 cm from the collimator face, LumaGem's spatial resolution was 4.8 and 5.6 mm with standard and optimized collimation; Discovery NM 750b's spatial resolution was 4.4 and 4.6 mm with standard and optimized collimation, respectively. For both systems, at tumor depths of 1 and 3 cm, use of optimized collimation and wide energy window significantly improved CNR compared to standard settings for tumors 8.0 and 9.2 mm in diameter. At the closer depth of 1 cm, optimized collimation and wide energy window also significantly improved CNR for 5.9 mm tumors on Discovery NM 750b.

Conclusions: Registered optimized collimation and wide energy window yield a substantial gain in count sensitivity and measurable gain in CNR, with some loss in spatial resolution compared to the standard collimator designs and energy windows used on these two systems. At low-count densities calculated to represent doses of 148 MBq, this tradeoff results in adequate count density and lesion contrast for detection of lesions ≥8 mm in the middle of a typical breast (3 cm deep) and lesions ≥6 mm close to the collimator (1 cm deep).

Figure 1

Components of the simulated energy spectrum for CZT (with 3.8% energy resolution) for an MBI acquisition using standard energy window and wide energy window.

Figure 2

System spatial resolution of the LumaGem detector [shown in (a)] and the Discovery NM 750b detector [shown in (b)] with their respective standard and optimal collimators. A 95% confidence interval of uncertainty of the linear fit is indicated. Vertical lines indicated b_min, the distance below which spatial resolution plateaus to equal collimator hole size.

Figure 3

Schematic of tumor placement within the breast phantom. Tumor diameter given in millimeter.

Figure 4

Images of breast phantom acquired on the LumaGem and Discovery NM 750b CZT detectors using their respective standard collimators and a standard 126–154 keV energy window (top row) and respective optimized collimators and wide (110–154 keV energy window (bottom row). Activity concentration ratio in tumors versus background was ∼20:1. Images from the LumaGem are shown in (a) with tumors were placed at a depth of 1 cm and in (b) depth was 3 cm from the collimator face. Images from Discovery NM 750b are shown in (c) with tumors at 1 cm depth and in (d) with tumors at 3 cm depth. The highest simulated count density with standard settings of 800 counts/cm² was chosen to match that observed in patient studies performed with injection of 740 MBq Tc-99m sestamibi. A proportionally decreased dose was calculated to correspond with phantom images of decreased count densities.

Figure 5

Lesion CNR as a function of lesion size measured on breast phantom images with respective count densities representing a 148 MBq dose of Tc-99m sestamibi. Images were acquired on LumaGem at lesion depths of 1 and 3 cm from the collimator face [shown in (a) and (b)] and Discovery NM 750b also at lesion depths of 1 and 3 cm [shown in (c) and (d)] with each combination of standard and optimized collimators and standard and wide energy windows. Total simulated breast thickness was 6 cm. Error bars represent the standard error of the CNR measurements. Lesions were considered detectable at a CNR greater than 1, as indicated by the dotted line. Significantly improved CNR relative to that obtained with standard collimator and standard energy window is noted with “*.”

Figure 6

Breast phantom images acquired on the LumaGem system performed using (a) wide energy window (110–154 keV), (b) standard energy window (126–154 keV), and (c) their difference.