Task-based strategy for optimized contrast enhanced breast imaging: analysis of six imaging techniques for mammography and tomosynthesis

Abstract

Purpose: The use of contrast agents in breast imaging has the capability of enhancing nodule detectability and providing physiological information. Accordingly, there has been a growing trend toward using iodine as a contrast medium in digital mammography (DM) and digital breast tomosynthesis (DBT). Widespread use raises concerns about the best way to use iodine in DM and DBT, and thus a comparison is necessary to evaluate typical iodine-enhanced imaging methods. This study used a task-based observer model to determine the optimal imaging approach by analyzing six imaging paradigms in terms of their ability to resolve iodine at a given dose: unsubtracted mammography and tomosynthesis, temporal subtraction mammography and tomosynthesis, and dual energy subtraction mammography and tomosynthesis.

Methods: Imaging performance was characterized using a detectability index d', derived from the system task transfer function (TTF), an imaging task, iodine signal difference, and the noise power spectrum (NPS). The task modeled a 10 mm diameter lesion containing iodine concentrations between 2.1 mg/cc and 8.6 mg/cc. TTF was obtained using an edge phantom, and the NPS was measured over several exposure levels, energies, and target-filter combinations. Using a structured CIRS phantom, d' was generated as a function of dose and iodine concentration.

Results: For all iodine concentrations and dose, temporal subtraction techniques for mammography and tomosynthesis yielded the highest d', while dual energy techniques for both modalities demonstrated the next best performance. Unsubtracted imaging resulted in the lowest d' values for both modalities, with unsubtracted mammography performing the worst out of all six paradigms.

Conclusions: At any dose, temporal subtraction imaging provides the greatest detectability, with temporally subtracted DBT performing the highest. The authors attribute the successful performance to excellent cancellation of inplane structures and improved signal difference in the lesion.

Figure 1

TTF calculation. An iodine-doped wafer was placed in an oil bath phantom and imaged with conventional mammography and tomosynthesis. The edge of the wafer was segmented and used to determine the TTF.

Figure 2

(a) and (b) Lesion profiles with DM (solid) and DBT (dotted) acquisition. Note the slightly larger radius for DBT after 3D reconstruction, and the corresponding increase in task profile.

Figure 3

Regions of interest from breast phantom images illustrating intensity of signal difference in various imaging schemes. Shown are (a) unsubtracted mammography, (b) unsubtracted tomosynthesis, (c) DE tomosynthesis, and (d) TS tomosynthesis.

Figure 4

Schematic of how d^′ was optimized using every LE/HE image combination and weighting factor.

Figure 5

(a) Low energy mammogram of CIRS 020 breast phantom with (b) log–log NPS plot. Power fitting the NPS on a log–log plot yielded a slope of −2.9.

Figure 6

Line trace of 2D NPS from averaging seven rows about the u-axis. (a) In unsubtracted DM, large-scale structures remain similarly visible as patient dose is increased, but quantum noise is reduced. (b) In TS, anatomical structure is substantially eliminated with dose.

Figure 7

NPS of TS mammography (right axis) compared to task function (left axis). This direct comparison useful in conveying which frequencies contribute the most to d^′.

Figure 8

Line trace of NPS, seven rows about u-axis. (a) Unsubtracted DBT shows little reduction in low frequency anatomical noise with dose, but diminished quantum noise. (b) TS substantially reduces anatomical noise, and improvement is seen with increased dose.

Figure 9

Noise power spectra of DE images. The plots illustrate the effect of weight on total noise reduction in (a) mammography and (b) tomosynthesis. Note that at optimal weight, 0.15 in (a) and 0.30 in (b), there is excellent suppression of anatomical noise at low frequencies but slightly increased quantum noise at higher frequencies.

Figure 10

Relative d^′ of DE-DBT across weight and dose allocation. Here, DE-DM was computed at 1.5 mGy as weighting and dose allocation are varied. d^′ peaks at w = 0.15.

Figure 11

Relative d^′ values with respect to total MGD and weight, obtained using different high kVp filters: (a) and (c) copper and (b) and (d) titanium. For mammography, the titanium filter yielded greater d^′, while copper produced better results in tomosynthesis.

Figure 12

Absolute d^′ as a function of iodine concentration. (a) at 1.25 mGy, DE-DM yields higher detectability than other modes, with a slight crossover above 6.4 mg/cc. TS schemes are absent as the combined dose exceeded 1.25 mGy. (b) As expected, TS offers the highest d^′, followed by DE and unsubtracted imaging. There is no crossover between DE-DM and DE-DBT at this dose.

Figure 13

(a)–(d) Relative d^′ for each mode, after normalization to TS DBT at 1.25 mGy. d^′ was calculated over a range of doses from 1.75 to 3 mGy. Overall, subtraction techniques substantially improved lesion detectability.