Comparison of breast specific gamma imaging and molecular breast tomosynthesis in breast cancer detection: Evaluation in phantoms

Abstract

Purpose: Breast specific gamma imaging or molecular breast imaging (BSGI) obtains 2D images of (99m)Tc sestamibi distribution in the breast. Molecular breast tomosynthesis (MBT) maps the tracer distribution in 3D by acquiring multiple projections over a limited angular range. Here, the authors compare the performance of the two technologies in terms of spatial resolution, lesion contrast, and contrast-to-noise ratio (CNR) in phantom studies under conditions of clinically relevant sestamibi dose and imaging time.

Methods: The systems tested were a Dilon 6800 and a MBT prototype developed at the University of Virginia. Both systems comprise a pixelated sodium iodide scintillator, an array of position sensitive photomultipliers, and a parallel hole collimator. The active areas and energy resolution of the systems are similar. System sensitivity, spatial resolution, lesion contrast, and CNR were measured using a Petri dish, a point source phantom, and a breast phantom containing simulated lesions at two depths, respectively. A single BSGI projection was acquired. Five MBT projections were acquired over ±20°. For both modalities, the total scan count density was comparable to that observed for each in typical 10 min human scans following injection of 22 mCi (814 MBq) of (99m)Tc-sestamibi. To assess the impact of reducing the tracer dose, the pixel counts of projection images were later binomially subsampled by a factor of 2 to give images corresponding to an injected activity of approximately 11 mCi (407 MBq). Both unprocessed (pixelated) BSGI projections and interpolated (smoothed) BSGI images displayed by default on the Dilon 6800 workstation were analyzed. Volumetric images were reconstructed from the MBT projections using a maximum likelihood expectation maximization algorithm and extracted slices were analyzed.

Results: Over a depth range of 1.5-7.5 cm, BSGI spatial resolution was 5.6-11.5 mm in unprocessed projections and 5.7-12.0 mm in interpolated images. Over the same range, the in-slice MBT spatial resolution was 6.7-9.4 mm. Lesion contrast was significantly improved with MBT relative to BSGI for five out of eight lesions imaged at either the 22 mCi or the 11 mCi dose level (p < 0.05). At both dose levels, significant improvements in CNR with MBT were also found for five out of eight lesions (9.8, 7.8, 6.2 mm lesions at water depth of 1.7 cm and 9.8, 7.8 mm lesions at water depth of 4.5 cm, p < 0.05). The 6.2 and 4.9 mm lesions located at 4.5 cm below the water surface were not visible in either modality at either activity level.

Conclusions: Under conditions of equal dose, imaging time and similar detectors, compared to BSGI, MBT provided higher lesion contrast, higher CNR, and spatial resolution that was less depth dependent.

FIG. 1.

Dilon 6800 BSGI system (left) and MBT scanner (right). MBT is a subsystem of the DMT scanner. During a DMT scan, the patient is seated. X-ray DBT is performed with the gamma camera positioned anteriorly, out of the field of view. Following DBT, the camera is positioned at the chest wall and the MBT scan is performed. For clarity, the photo shows the gamma camera at the chest wall and positioned far above the paddle; however during MBT, the camera is lowered to just above the paddle as shown in Fig. 2.

FIG. 2.

Schematics of BSGI and MBT acquisition. In BSGI, the breast is compressed between the camera and a high attenuation plate. In MBT, the breast is compressed between a breast support and a low-attenuation compression paddle. In this study, lesion (or source) depth was defined relative to the collimator input surface in BSGI and relative to the lower surface of the compression paddle in MBT.

FIG. 3.

Schematic of point source phantom (a). Schematic of lesion size and location within the box phantom (b). The water surface in the box phantom was 5 mm below either the collimator in BSGI or the lower surface of the compression paddle in MBT. The water height was 7.2 cm. The dashed rectangular region represents background ROI drawn on either BSGI projections or MBT slices.

FIG. 4.

BSGI images and MBT slices of the point source phantom. The zero-degree projection view of the MBT scan is also presented.

FIG. 5.

Spatial resolution in BSGI images and MBT reconstructed slices. The spatial resolution of the MBT camera is plotted and labeled as “MBT camera.” For the MBT camera, the depths were measured from the collimator input surface.

FIG. 6.

Count density in human MBT projection images (zero-degree view) vs compressed breast thickness, under conditions of 22 mCi injected ^99mTc-sestamibi and 2 min acquisition time per projection view.

FIG. 7.

BSGI projection images and MBT reconstructed slices of the box phantom. Lesion-to-background concentration ratio was ∼20:1. MBT slices through the lesion centers at depths of 2.2 cm (1.7 cm below the water surface) and 5.0 cm (4.5 cm below the water surface) are shown in the third and forth columns, respectively. Images in the first row were produced from the original projections in which the count density is about equal to that in a 22 mCi, 10 min scan of a 7.2 cm thick breast. Those in the bottom row were produced from subsampled projections in which the count density corresponds to an 11 mCi, 10 min scan.

FIG. 8.

Lesion contrast in BSGI images and MBT reconstructed slices of the box phantom with count densities corresponding to 22 and 11 mCi of injected ^99mTc-sestamibi and 10 min total scan time. Lesions whose diameters are labeled with a single quotation mark were located at 2.2 cm depth (1.7 cm below the water surface) and those labeled with a double quotation mark were located at 5.0 cm depth (4.5 cm below the water surface). Error bars represent 95% confidence intervals of the measurements over eight nominally identical trials. Statistically significant (p < 0.05) improved contrast relative to that of BSGI is noted with an asterisk.

FIG. 9.

Lesion CNR in BSGI images and MBT reconstructed slices of the box phantom with count densities corresponding to 22 and 11 mCi of injected ^99mTc-sestamibi and 10 min total scan time. Lesions whose diameters are labeled with a single quotation mark were located at 2.2 cm depth (1.7 cm below the water surface) and those labeled with a double quotation mark were located at 5.0 cm depth (4.5 cm below the water surface). Error bars represent 95% confidence intervals of the measurements over eight nominally identical trials. Statistically significant (p < 0.05) improved CNR relative to that of BSGI is noted with an asterisk.