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

Abstract

Purpose: Molecular breast imaging (MBI) has shown promise as an adjunct screening technique to mammography for women with dense breasts. The demonstration of reliable lesion detection with MBI performed at low administered doses of Tc-99 m sestamibi, comparable in effective radiation dose to screening mammography, is essential to adoption of MBI for screening. The concept of performing low-dose MBI with dual-head cadmium zinc telluride (CZT) gamma cameras has been investigated in phantoms in Part I. In this work, the objectives were to evaluate the impact of the count sensitivity improvement methods on image quality in patient MBI exams and to determine if adequate lesion detection could be achieved at reduced doses.

Methods: Following the implementation of two count sensitivity improvement methods, registered collimation optimized for near-field imaging and energy acceptance window optimized for CZT, MBI exams were performed in the course of clinical care. Clinical image count density (counts/cm(2)) was compared between standard MBI [740 MBq (20 mCi) Tc-99 m sestamibi, standard collimation, standard energy window] and low-dose MBI [296 MBq (8 mCi) Tc-99 m sestamibi, optimized collimation, wide energy window] in a cohort of 50 patients who had both types of MBI exams performed. Lesion detection at low doses was evaluated in a separate cohort of 32 patients, in which low-dose MBI was performed following 296 MBq injection and acquired in dynamic mode, allowing the generation of images acquired for 2.5, 5, 7.5, and 10 min/breast view with proportionately reduced count densities. Diagnostic accuracy at each count density level was compared and kappa statistic was used to assess intrareader agreement between 10 min acquisitions and those at shorter acquisition durations.

Results: In patient studies, low-dose MBI performed with 296 MBq Tc-99 m sestamibi and new optimal collimation/wide energy window resulted in an average relative gain in count density of 4.2 ± 1.3 compared to standard MBI performed with 740 MBq. Interpretation of low-dose 296 MBq images with count densities corresponding to acquisitions of 2.5, 5, 7.5, and 10 min/view and median lesion size of 1.4 cm resulted in similar diagnostic accuracy across count densities and substantial to near-perfect intrareader agreement between full 10 min-views and lower count density views.

Conclusions: Review of patient studies showed that registered optimized collimation and wide energy window resulted in a substantial gain in count sensitivity as previously indicated by phantom results. This proof of concept work indicates that MBI performed at administered doses of 296 MBq Tc-99 m sestamibi with the applied count sensitivity improvements permits the detection of small breast lesions in patients. Findings suggest that further reductions in acquisition duration or administered dose may be achievable.

Figure 1

Count density measured in patients who had both standard MBI (performed with a 740 MBq injection, standard collimation, and standard energy window) and low-dose MBI (performed with either 148 or 296 MBq injection, optimized collimation, and wide energy window). Count density (counts/cm²) is expressed per 10 min acquisition per MBq.

Figure 2

Example views from four patients undergoing both standard MBI and low-dose screening MBI with approximately 2–3 yr between exams. On the top row, standard 10 min/view MBI was performed with a 740 MBq injection, standard collimator, and standard energy window. On the bottom row, low-dose 10 min/view MBI was performed using either 148 or 296 MBq injection, optimized collimation, and wide energy window. All images were interpreted as negative.

Figure 3

A patient with multifocal invasive lobular cancer and nipple adenoma. MBI was performed following 296 MBq injection Tc-99 m sestamibi and with the wide energy window using the standard collimation [shown in (a)] and the optimized collimation [shown in (b)]. Count density was improved by a factor of 2.4 with the optimized collimation.

Figure 4

MBI study acquired for 10 min following 740 MBq injection Tc-99 m sestamibi, using standard collimation, and (a) wide energy window (110–154 keV), (b) standard energy window (126–154 keV). The difference of images in panels (a) and (b) is shown in panel (c). Each image is displayed on the range from its individual minimum to maximum count. Count density in this patient was improved by a factor of 2.6 with the wider energy window, A 1.3 cm × 1.0 cm × 0.9 cm invasive ductal carcinoma was detected, and the wide energy window resulted in improved detection of an extension of ductal carcinoma in situ (see arrow).

Figure 5

In this patient with a 0.8 cm tubular carcinoma, low-dose MBI was performed with 296 MBq injection Tc-99 m sestamibi and acquired in eight dynamic frames. Images representing acquisition durations of 2.5, 5, 7.5, and 10 min/breast view were simulated by summing counts from either two, four, six, or eight frames, respectively. Images were acquired with optimized collimation and wide energy window. The average final assessment and individual assessments from four readers are provided, where an assessment of 3 or higher is considered positive.

Figure 6

The count density (counts/cm² in a 1.25 min frame) measured in 19 MBI clinical studies as a function of time postinjection of 296 MBq Tc-99 m sestamibi. Studies were performed with optimized collimation and wide energy window. The patient with the median count density is highlighted in bold.