David A. Scaduto

Optimization of contrast-enhanced breast imaging: Analysis using a cascaded linear system model.

Purpose: Contrast‐enhanced (CE) breast imaging involves the injection contrast agents (i.e., iodine) to increase conspicuity of malignant lesions. CE imaging may be used in conjunction with digital mammography (DM) or digital breast tomosynthesis (DBT) and has shown promise in improving diagnostic specificity. Both CE‐DM and CE‐DBT techniques require optimization as clinical diagnostic tools. Physical factors including x‐ray spectra, subtraction technique, and the signal from iodine contrast, must be considered to provide the greatest object detectability and image quality. We developed a cascaded linear system model (CLSM) for the optimization of CE‐DM and CE‐DBT employing dual energy (DE) subtraction or temporal (TE) subtraction. Methods: We have previously developed a CLSM for DBT implemented with an a‐Se flat panel imager (FPI) and filtered backprojection (FBP) reconstruction algorithm. The model is used to track image quality metrics — modulation transfer function (MTF) and noise power spectrum (NPS) — at each stage of the imaging chain. In this study, the CLSM is extended for CE breast imaging. The effect of x‐ray spectrum (varied by changing tube potential and the filter) and DE and TE subtraction techniques on breast structural noise was measured was studied and included as a deterministic source of noise in the CLSM. From the two‐dimensional (2D) and three‐dimensional (3D) MTF and NPS, the ideal observer signal‐to‐noise ratio (SNR), also known as the detectability index (d′), may be calculated. Using d′ as a FOM, we discuss the optimization of CE imaging for the task of iodinated contrast object detection within structured backgrounds. Results: Increasing x‐ray energy was determined to decrease the magnitude of structural noise and not its correlation. By performing DE subtraction, the magnitude of the structural noise was further reduced at the expense of increased stochastic (quantum and electronic) noise. TE subtraction exhibited essentially no residual structural noise at the expense of increased quantum noise, even over that of the DE case. For DE subtraction, optimization of dose weighting to the HE view (fh) results in the minimization of quantum noise. Both subtraction weighting factor (wSub) and the iodine contrast signal were dependent on the LE and HE x‐ray spectra. To best detect a 5 mm Gaussian lesion with 5 mg/ml of iodine within a 4 cm thick breast, it was found that the high energy (HE) view should be acquired with a tube potential of 47 kVp (W/Ti spectrum) and the low energy (LE) view with a potential of 23 kVp (W/Rh spectrum). Due to the complete removal of structural noise, TE subtraction produced much higher d′ than DE subtraction both as a function of mean glandular dose and iodine concentration. Conclusions: We have shown the effect of increasing x‐ray energy as well as projection domain subtraction on breast structural noise. Further, we have exhibited the utility of the CLSM for DE and TE subtraction CE imaging in the optimization of imaging parameters such as x‐ray energy, fh, and wSub as well as guiding the understanding of their effects on image contrast and noise.

A scatter correction method for contrast-enhanced dual-energy digital breast tomosynthesis

Contrast-enhanced dual energy digital breast tomosynthesis (CE-DE-DBT) is designed to image iodinated masses while suppressing breast anatomical background. Scatter is a problem, especially for high energy acquisition, in that it causes severe cupping artifact and iodine quantitation errors. We propose a patient specific scatter correction (SC) algorithm for CE-DE-DBT. The empirical algorithm works by interpolating scatter data outside the breast shadow into an estimate within the breast shadow. The interpolated estimate is further improved by operations that use an easily obtainable (from phantoms) table of scatter-to-primary-ratios (SPR)--a single SPR value for each breast thickness and acquisition angle. We validated our SC algorithm for two breast emulating phantoms by comparing SPR from our SC algorithm to that measured using a beam-passing pinhole array plate. The error in our SC computed SPR, averaged over acquisition angle and image location, was about 5%, with slightly worse errors for thicker phantoms. The SC projection data, reconstructed using OS-SART, showed a large degree of decupping. We also observed that SC removed the dependence of iodine quantitation on phantom thickness. We applied the SC algorithm to a CE-DE-mammographic patient image with a biopsy confirmed tumor at the breast periphery. In the image without SC, the contrast enhanced tumor was masked by the cupping artifact. With our SC, the tumor was easily visible. An interpolation-based SC was proposed by (Siewerdsen et al 2006 Med. Phys. 33 187-97) for cone-beam CT (CBCT), but our algorithm and application differ in several respects. Other relevant SC techniques include Monte-Carlo and convolution-based methods for CBCT, storage of a precomputed library of scatter maps for DBT, and patient acquisition with a beam-passing pinhole array for breast CT. Our SC algorithm can be accomplished in clinically acceptable times, requires no additional imaging hardware or extra patient dose and is easily transportable between sites.

Optimization of clinical protocols for contrast enhanced breast imaging

Contrast enhanced (CE) breast imaging has been proposed as a method to increase the sensitivity and specificity of breast cancer detection. Because malignant lesions often exhibit angiogenesis, the uptake of radio-opaque contrast agents (e.g. iodine) results in increased attenuation compared to the background tissue. Both planar CE digital mammography (CE-DM) and digital breast tomosynthesis (CE-DBT) have been proposed, using temporal or dual energy (DE) subtraction to remove tissue backgrounds. In the current study, we apply a cascaded linear systems model approach to analyze CE techniques with DE subtraction for designing a diagnostic imaging study, including the effects of contrast dynamics. We apply the model for both CE-DM and CE-DBT to calculate the ideal observer signal-to-noise ratio (SNR) for the detection of I contrast objects of different sizes and concentrations. The calculation of this figure-of-merit (FOM) was be used to optimize CE clinical imaging protocols.

Application of the ordered-subsets transmission reconstruction algorithm to contrast-enhanced dual-energy digital breast tomosynthesis

Contrast-enhanced dual-energy digital breast tomosynthesis (CE-DE-DBT) has a potential clinical impact for diagnostic breast imaging. Typically, CE-DE-DBT involves iodine contrast injection, image acquisitions at high energy (HE) and low energy (LE), weighted subtraction of the acquired data and finally reconstruction of the subtracted data. The resulting reconstruction displays iodinated structures against a noisy background that is the residual of imperfect subtraction of the anatomical breast structure. We hypothesize that CE-DE-DBT can be improved by using a reconstruction that yields better signal contrast and stronger background suppression as compared to that obtained using conventional reconstructions. We replace the commonly used FBP and SART reconstructions by a penalized likelihood reconstruction, OSTR (ordered-subsets transmission reconstruction). We used an ordinary quadratic smoothing regularizer as well as an edge-preserving Lange prior. OSTR is applied separately to the HE and LE data with weighted subtraction performed in the reconstructed domain. We obtained projection data using a Siemens CEDET DBT scanner and a customized CIRS020 phantom that reflected adipose and glandular spatial variability and included an array of iodine inserts of varying contrast and size. The data was scatter corrected and reconstructed using the three methods. We measured image quality by SDNR (signal-difference-to-noise-ratio) at each of the 16 iodine inserts. In all 16 cases, SDNR was best for OSTR and worst for FBP. Our iodine inserts had high contrast, but in a clinical setting, it may be important to visualize weaker iodine signals. OSTR, a form of penalized likelihood reconstruction, may find use in improving CE-DE-DBT.

The effect of amorphous selenium thickness on imaging performance of contrast enhanced digital breast tomosynthesis

Digital breast tomosynthesis (DBT) and contrast enhancement (CE) for both DBT (CEDBT) and planar mammography (CEDM) are being investigated to increase conspicuity of malignant lesions. To image above the k-edge of iodine (33 keV), CEDBT requires x-ray energies higher than those of typical mammograms (˜28 kVp). Increasing the thickness of the detectors amorphous selenium (a-Se) layer improves x-ray absorption and detective quantum efficiency (DQE), particularly at higher energies. For DBT, where systems are often designed with partially isocentric geometries, thicker a-Se layers may result in degradation of the modulation transfer function (MTF) for oblique views. We employed a cascaded linear system model to analyze the effect of oblique entry on MTF. Also, the model was experimentally validated using 200 and 300 μm a-Se flat panel imagers. Finally, we use an ideal-observer SNR model for projection and DBT imaging to optimize a-Se layer thickness for detectability of iodinated objects.

Dependence of Contrast-Enhanced Lesion Detection in Contrast-Enhanced Digital Breast Tomosynthesis on Imaging Chain Design

Contrast-enhanced digital breast tomosynthesis CEDBT may improve contrast-enhanced lesion conspicuity and relative contrast quantification by improving three-dimensional visualization of lesion morphology, and reducing the integration of attenuation information along the axial direction. Improved visualization of patterns of contrast-enhancement and improved iodine quantification may help differentiate between malignant and benign enhancing lesions. The dependence of dual-energy contrast-enhanced lesion detectability on imaging chain design is investigated. Lesion detectability and relative iodine quantification is comparable for subtraction in either reconstruction or projection domains for both phantom and patient images. SART generally produces greater SDNR than FBP, and scatter correcting projections further improves SDNR.

Impact of angular range of digital breast tomosynthesis on mass detection in dense breasts.

The detection of cancerous mass lesions using digital breast tomosynthesis (DBT) has been shown to be limited in patients with dense breasts. Detection may potentially be improved by increasing the DBT angular range (AR), which reduces breast structural noise and increases object contrast in the reconstructed slice. We investigate the impact of DBT AR on the detection of masses in a simulation study using a cascaded linear system model (CLSM) for DBT. We compare the mass conspicuity between wide- and narrow-AR DBT system in a clinical pilot study. The simulation results show reduced in-plane breast structural noise and increased in-plane detectability of masses with increasing AR. The clinical results show that masses are more conspicuous in wide-AR DBT than narrow-AR DBT. Our study indicates that the detection of mass lesions in dense breasts can be improved by increasing DBT AR.

Lesion assessment and radiation dose in contrast-enhanced digital breast tomosynthesis.

Contrast-Enhanced Digital Breast Tomosynthesis (CEDBT) provides a three-dimensional (3D) contrast-enhancement map with co-registered anatomical information from low-energy DBT. It combines the benefits from Contrast-Enhanced Digital Mammography (CEDM) and Digital Breast Tomosynthesis (DBT), and may improve breast cancer detection and assessment of lesion morphology. We investigate the efficacy of CEDBT in the assessment of lesion contrast enhancement and margin identification, and evaluate the dose efficiency. We generate synthetic CEDM images from CEDBT data, similar to synthesis of 2D mammograms from DBT data, which may facilitate overall lesion assessment without additional radiation dose. Preliminary results from a patient study show that CEDBT depicts lesion margins better compared to CEDM, while the contrast-enhancement level for in-plane slice is not as high as in CEDM. CEDBT delivers less radiation dose compared to CEDM + DBT. Synthetic CEDM is able to provide lesion contrast-enhancement level comparable to CEDM.

Experimental characterization of a direct conversion amorphous selenium detector with thicker conversion layer for dual‐energy contrast‐enhanced breast imaging

Purpose: Dual‐energy contrast‐enhanced imaging is being investigated as a tool to identify and localize angiogenesis in the breast, a possible indicator of malignant tumors. This imaging technique requires that x‐ray images are acquired at energies above the k‐shell binding energy of an appropriate radiocontrast agent. Iodinated contrast agents are commonly used for vascular imaging, and require x‐ray energies greater than 33 keV. Conventional direct conversion amorphous selenium (a‐Se) flat‐panel imagers for digital mammography show suboptimal absorption efficiencies at these higher energies. Methods: We use spatial‐frequency domain image quality metrics to evaluate the performance of a prototype direct conversion flat‐panel imager with a thicker a‐Se layer, specifically fabricated for dual‐energy contrast‐enhanced breast imaging. Imaging performance was evaluated in a prototype digital breast tomosynthesis (DBT) system. The spatial resolution, noise characteristics, detective quantum efficiency, and temporal performance of the detector were evaluated for dual‐energy imaging for both conventional full‐field digital mammography (FFDM) and DBT. Results: The zero‐frequency detective quantum efficiency of the prototype detector is improved by approximately 20% over the conventional detector for higher energy beams required for imaging with iodinated contrast agents. The effect of oblique entry of x‐rays on spatial resolution does increase with increasing photoconductor thickness, specifically for the most oblique views of a DBT scan. Degradation of spatial resolution due to focal spot motion was also observed. Temporal performance was found to be comparable to conventional mammographic detectors. Conclusions: Increasing the a‐Se thickness in direct conversion flat‐panel imagers results in better performance for dual‐energy contrast‐enhanced breast imaging. The reduction in spatial resolution due to oblique entry of x‐rays is appreciable in the most extreme clinically relevant cases, but may not profoundly affect reconstructed images due to the algorithms and filters employed. Degradation to projection domain spatial resolution is thus outweighed by the improvement in detective quantum efficiency for high‐energy x‐rays.

WE-DE-207B-05: Measuring Spatial Resolution in Digital Breast Tomosynthesis: Update of AAPM Task Group 245

PURPOSE Spatial resolution in digital breast tomosynthesis (DBT) is affected by inherent/binned detector resolution, oblique entry of x-rays, and focal spot size/motion; the limited angular range further limits spatial resolution in the depth-direction. While DBT is being widely adopted clinically, imaging performance metrics and quality control protocols have not been standardized. AAPM Task Group 245 on Tomosynthesis Quality Control has been formed to address this deficiency. METHODS Methods of measuring spatial resolution are evaluated using two prototype quality control phantoms for DBT. Spatial resolution in the detector plane is measured in projection and reconstruction domains using edge-spread function (ESF), point-spread function (PSF) and modulation transfer function (MTF). Spatial resolution in the depth-direction and effective slice thickness are measured in the reconstruction domain using slice sensitivity profile (SSP) and artifact spread function (ASF). An oversampled PSF in the depth-direction is measured using a 50 µm angulated tungsten wire, from which the MTF is computed. Object-dependent PSF is derived and compared with ASF. Sensitivity of these measurements to phantom positioning, imaging conditions and reconstruction algorithms is evaluated. Results are compared from systems of varying acquisition geometry (9-25 projections over 15-60°). Dependence of measurements on feature size is investigated. RESULTS Measurements of spatial resolution using PSF and LSF are shown to depend on feature size; depth-direction spatial resolution measurements are shown to similarly depend on feature size for ASF, though deconvolution with an object function removes feature size-dependence. A slanted wire may be used to measure oversampled PSFs, from which MTFs may be computed for both in-plane and depth-direction resolution. CONCLUSION Spatial resolution measured using PSF is object-independent with sufficiently small object; MTF is object-independent. Depth-direction spatial resolution may be measured directly using MTF or indirectly using ASF or SSP as surrogate measurements. While MTF is object-independent, it is invalid for nonlinear reconstructions.