Effects of kV, filtration, dose, and object size on soft tissue and iodine contrast in dedicated breast CT

Abstract

Purpose: Clinical use of dedicated breast computed tomography (bCT) requires relatively short scan times necessitating systems with high frame rates. This in turn impacts the x-ray tube operating range. We characterize the effects of tube voltage, beam filtration, dose, and object size on contrast and noise properties related to soft tissue and iodine contrast agents as a way to optimize imaging protocols for soft tissue and iodine contrast at high frame rates.

Methods: This study design uses the signal-difference-to-noise ratio (SDNR), noise-equivalent quanta (NEQ), and detectability (d´) as measures of imaging performance for a prototype breast CT scanner that utilizes a pulsed x-ray tube (with a 4 ms pulse width) at 43.5 fps acquisition rate. We assess a range of kV, filtration, breast phantom size, and mean glandular dose (MGD). Performance measures are estimated from images of adipose-equivalent breast phantoms machined to have a representative size and shape of small, medium, and large breasts. Water (glandular tissue equivalent) and iodine contrast (5 mg/ml) were used to fill two cylindrical wells in the phantoms.

Results: Air kerma levels required for obtaining an MGD of 6 mGy ranged from 7.1 to 9.1 mGy and are reported across all kV, filtration, and breast phantom sizes. However, at 50 kV, the thick filters (0.3 mm of Cu or Gd) exceeded the maximum available mA of the x-ray generator, and hence, these conditions were excluded from subsequent analysis. There was a strong positive association between measurements of SDNR and d' (R² > 0.97) within the range of parameters investigated in this work. A significant decrease in soft tissue SDNR was observed for increasing phantom size and increasing kV with a maximum SDNR at 50 kV with 0.2 mm Cu or 0.2 mm Gd filtration. For iodine contrast SDNR, a significant decrease was observed with increasing phantom size, but a decrease in SDNR for increasing kV was only observed for 70 kV (50 and 60 kV were not significantly different). Thicker Gd filtration (0.3 mm Gd) resulted in a significant increase in iodine SDNR and decrease in soft tissue SDNR but requires significantly more tube current to deliver the same MGD.

Conclusions: The choice of 60 kV with 0.2 mm Gd filtration provides a good trade-off for maximizing both soft tissue and iodine contrast. This scanning technique takes advantage of the ~50 keV Gd k-edge to produce contrast and can be achieved within operating range of the x-ray generator used in this work. Imaging at 60 kV allows for a greater range in dose delivered to the large breast sizes when uniform image quality is desired across all breast sizes. While imaging performance metrics (i.e., detectability index and SDNR) were shown to be strongly correlated, the methodologies presented in this work for the estimation of NEQ (and subsequently d') provides a meaningful description of the spatial resolution and noise characteristics of this prototype bCT system across a range of beam quality, dose, and object sizes.

Keywords: MTF; NPS; breast CT; iodine contrast; spectral optimization; x-ray imaging.

Fig. 1.

(a) Photographs of the V3 phantom contrast wells (red- and black-taped “x” marks) and the phantom suspended in the breast computed tomography scanner field of view. (b) Example coronal and sagittal slices through the V3 phantom reconstructions depicting the location of the iodine, glandular, and adipose regions. The circular ROIs in the coronal image depict the regions used for the signal-difference-to-noise ratio measurements and the contrast term in the task function for the detectability index. The dotted square ROIs indicate the region where the modulation transfer function calculation was performed.

Fig. 2.

Tube current (mA) required for each kV/filter combination in order to deliver the target mean glandular dose of 6 mGy. Results shown are the average +/− one standard deviation across all three phantom sizes. The solid horizontal lines indicate the max mA allowed for the Doheny scanner which corresponds to 220, 180, and 150 mA at 50, 60, and 70 kV, respectively. Specifically, for the thicker filtration choices at 50 kV (0.3 mm Cu & 0.3 mm Gd), the required mA is beyond the power limitations of the scanner investigated.

Fig. 3.

Modeled x-ray spectra generated for the 50 and 70 kV spectra with 0.2 mm Cu and 0.2 mm Gd filtration. The photon fluence is scaled to the air kerma (see Table I) that delivers a 6 mGy mean glandular dose for the V3 (median)-sized breast phantom.

Fig. 4.

Signal-difference-to-noise ratio (SDNR) results measured for iodine contrast (left column) and soft tissue (right column) for phantom sizes V1, V3, and V5 using a constant mean glandular dose of 6 mGy. Several kV/filter combinations were beyond the tube power limitations of the scanner under investigation and are indicated by “N/A.” The SDNR value is the average and standard deviation (error bars) across seven consecutive slices centered about the central plane in the breast computed tomography scanner.

Fig. 5.

Radially averaged modulation transfer function for the median-sized (V3) phantom. The average value across all 10 kV/filter combinations and both imaging tasks is shown along with +/− one standard deviation from the mean.

Fig. 6.

(a–c) Orthogonal central slices through the three-dimensional (3D) NPS measured in the reconstructed images of the V3 phantom scanned with 50 kV and 0.2 mm Gd filtration. Radially averaged central slices through the 3D NPS (f_z = 0) are also shown for a subset of 4 kV/filter combinations and the V3 phantom size.

Fig. 7.

Comparisons of the two-dimensional noise equivalent quanta (NEQ) for the median-sized (V3) phantom at 6 mGy mean glandular dose for a subset of 4 kV/filter combinations. The results for 60 kV, 0.3 mm Gd, and 0.3 mm Cu filtration are not shown for ease in visual comparisons.

Fig. 8.

Correlation comparisons between the two-dimensional slice detectability index (d′) measured in the frequency domain and the signal-difference-to-noise ratio measured in the image domain. Results are shown for the V3 phantom and all 10 kV/filter combinations for the iodine contrast and soft tissue tasks. Results for the thin filters (0.2 mm Gd/Cu) are shown using solid markers and thick filter results (0.3 mm Gd/Cu) are shown using outlined markers.

Fig. 9.

Signal-difference-to-noise ratio (SDNR) results for the V3 (median)-sized phantom from the posterior (z = 0 cm) to anterior (z = 50 mm) region of the contrast wells in the breast-shaped phantom. Results are shown for iodine contrast and soft tissue contrast tasks for tube potentials of 50 and 70 kV with either 0.2 mm Cu or 0.2 mm Gd added filtration. The y-axis is the SDNR as a function of z normalized to the SDNR at z = 0 (i.e., posterior boundary of scanner field of view).

Fig. 10.

Signal-difference-to-noise ratio (SDNR) results at various mean glandular dose (MGD) levels for 60 kV with 0.2 mm Gd filtration. Results are shown for the V1, V3, and V5 phantoms along with a linear fit. The dashed line indicates the SDNR for the V3 phantom at 6 mGy and can be used to interpolate the dose that is necessary to provide identical image quality across all phantom sizes.