Yue-Houng Hu

Image Artifact in Digital Breast Tomosynthesis and Its Dependence on System and Reconstruction Parameters

Digital breast tomosynthesis (DBT) is a three-dimensional (3D) imaging modality that produces cross-sectional image slices parallel to the detector plane using a limited number of views (<50) and a limited angular range (< 50 degrees). Due to the incomplete sampling of DBT, image artifacts are unavoidable. In this paper we investigated the dependence of out-of-plane image artifact on image acquisition and reconstruction parameters. The image artifact of a microcalcification (MC) was correlated with the 3D point spread function (PSF) of the DBT system, which was calculated using a cascaded linear system model. Our results show that the linear system model can be used to quantitatively predict the image artifact in DBT, and can be used as a unified tool for maximizing lesion detectability and minimizing artifact.

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.

Breast structural noise in digital breast tomosynthesis and its dependence on reconstruction methods

Digital breast tomosynthesis (DBT) is being investigated to overcome the obscuring effect of overlapping breast tissue in projection mammography To quantify the effectiveness of DBT in reducing overlapping breast structures, it is important to investigate how breast structural noise propagates during the reconstruction process Others have found that breast structure may be characterized as power law noise of the form κ/ fβ We investigate how the power law exponent, β, varies as a function of reconstruction methods Clinical DBT data sets were used to analyze breast structural noise in both projection and reconstructed domains using different filter schemes of a filtered back projection (FBP) reconstruction algorithm The dependence on filter settings was compared with cascaded linear system theory The goal this work is to combine frequency domain analysis of breast structural noise with previous work on quantum noise in DBT and develop a generalized framework to optimize DBT for breast lesion detection.

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.

Experimental quantification of lesion detectability in contrast enhanced dual energy digital breast tomosynthesis

Digital breast tomosynthesis (DBT) is a three-dimensional (3D) x-ray imaging modality that has recently been employed to increase lesion conspicuity through the removal of overlying tissue. Recently, a great deal of work has been devoted to the development of contrast enhanced (CE) DBT. Radio-opaque contrast agents (e.g. iodine) are injected into patients with suspicious breast lesions, with the goal of differentiating malignant tumors from benign by imaging the contrast uptake signature associated with angiogenesis. Either temporal subtraction (TS) or dual energy (DE) subtraction may be performed to further remove structural noise from the images. The current work quantifies the change in power-law noise after either DE subtraction or TS using structured breast tissue equivalent phantoms. Additionally, iodine contrast filled phantoms were used to determine the effect of x-ray energy and image subtraction technique on the signaldifference- to-noise ratio (SDNR). Finally, we investigate the improvement in imaging performance of an amorphous selenium (a-Se) direct conversion flat panel detector with increased a-Se thickness.

Impact of subtraction and reconstruction strategies on dual-energy contrast enhanced breast tomosynthesis with interleaved acquisition

Contrast enhanced digital breast tomosynthesis can yield superior visualization of tumors relative to conventional tomosynthesis and can provide the contrast uptake kinetics available in breast MR while maintaining a higher image spatial resolution. Conventional dual-energy (DE) acquisition protocols for contrast enhancement at a given time point often involve two separate continuous motion sweeps of the X-ray tube (one per energy) followed by weighted subtraction of the HE (high energy)and LE (low energy) projection data. This subtracted data is then reconstructed. Relative to two-sweep acquisition, interleaved acquisition suffers from a lesser degree of patient motion artifacts and entails less time spent under uncomfortable breast compression. These advantages for DE interleaved acquisition are reduced by subtraction artifacts due to the fact that each HE, LE acquisition pair is offset in angle for the usual case of continuous tube motion. These subtraction artifacts propagate into the reconstruction and are present even in the absence of patient motion. To reduce these artifacts, we advocate a strategy in which the HE and LE projection data are separately reconstructed then undergo weighted subtraction in the reconstruction domain. We compare the SDNR of masses in a phantom for the subtract-then-reconstruct vs. reconstruct-then-subtract strategies and evaluate each strategy for two algorithms, FBP and SART. We also compare the interleave SDNR results with those obtained with the conventional dual-energy double-sweep method. For interleave scans and for either algorithm the reconstruct-thensubtract strategy yields higher SDNR than the subtract-then-reconstruct strategy. For any of the three acquisition modes, SART reconstruction yields better SDNR than FBP reconstruction. Finally the interleave reconstruct-then-subtract method using SART yields higher SDNR than any of the double-sweep conventional acquisitions.

A 3D linear system model for the optimization of dual-energy contrast-enhanced digital breast tomosynthesis

Digital breast tomosynthesis (DBT) is a three-dimensional (3D) x-ray imaging modality that has been shown to decrease the obscuring effect of breast structural noise, thereby increasing lesion conspicuity. To further improve breast cancer detection, much recent work has been devoted to the development of contrast enhanced DBT (CEDBT). Taking advantage of angiogenesis in malignant tissue, CEDBT involves the injection of radio-opaque material (i.e. iodine) and measures the relative increase in uptake of contrast in breast cancer. Either temporal or dual energy subtraction techniques may be used to implement CEDBT. Our present work is to develop a cascaded linear system model for DBT with a CEDBT option to calculate the ideal observer signal to noise ratio (SNR) of lesions in the presence of structural noise, evaluate the efficacy of CEDBT in the removal of structural noise, and examine the associated increase in x-ray quantum noise. Our model will include the effects of dual energy subtraction on signal and noise transfer, and transfer of power-law form anatomical noise through a DBT system using a modified filtered backprojection (FBP) algorithm. This model will be used for the optimization of x-ray techniques and reconstruction filters in CEDBT.

Nonuniform angular dose distribution in digital breast tomosynthesis for increased conspicuity of small high contrast objects

Digital breast tomosynthesis (DBT) has been shown to decrease breast structural noise thus improving the detection of masses. However decreased detectability of microcalcifications was observed, and several studies have been performed to investigate the benefit of taking an additional central projection view after a DBT scan. Our study investigates the effect of variable angular dose distribution within a single DBT scan. Using a prototype DBT system with uniform angular dose distribution, several DBT scans (25 projection views over 40 degree angular range) were performed using different glandular doses. A subset of projection images was selected from each scan to form composite DBT scans (25 views each) with different angular dose distribution schemes (ADS). Two examples of ADS were: 1) seven central views with four times the dose of periphery; and 2) five central projections with six times the dose of periphery. The total dose for each ADS was the same as a reference scan with uniform dose distribution (1.5 mGy). They also had the same number of views and angular range, and were reconstructed using identical reconstruction filter settings. The detectability of calcifications, the 3D MTF and NPS of the system, and the ideal observer object detectability index for all cases were compared. The results showed that higher dose for the central views improve the detectability of calcifications. However magnitude of improvement depends on the reconstruction method and the size of the object.

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

Dual-Energy Contrast Enhanced Digital Breast Tomosynthesis (DE-CE-DBT) has the potential to deliver diagnostic information for vascularized breast pathology beyond that available from screening DBT. DE-CE-DBT involves a contrast (iodine) injection followed by a low energy (LE) and a high energy (HE) acquisitions. These undergo weighted subtraction then a reconstruction that ideally shows only the iodinated signal. Scatter in the projection data leads to “cupping” artifacts that can reduce the visibility and quantitative accuracy of the iodinated signal. The use of filtered backprojection (FBP) reconstruction ameliorates these types of artifacts, but the use of FBP precludes the advantages of iterative reconstructions. This motivates an effective and clinically practical scatter correction (SC) method for the projection data. We propose a simple SC method, applied at each acquisition angle. It uses scatter-only data at the edge of the image to interpolate a scatter estimate within the breast region. The interpolation has an approximately correct spatial profile but is quantitatively inaccurate. We further correct the interpolated scatter data with the aid of easily obtainable knowledge of SPR (scatter-to-primary ratio) at a single reference point. We validated the SC method using a CIRS breast phantom with iodine inserts. We evaluated its efficacy in terms of SDNR and iodine quantitative accuracy. We also applied our SC method to a patient DE-CE-DBT study and showed that the SC allowed detection of a previously confirmed tumor at the edge of the breast. The SC method is quick to use and may be useful in a clinical setting.