Breast Phantom Images Research Articles

Method of measuring contrast‐to‐noise ratio (CNR) in nonuniform image area in digital radiography

SummaryOne of the physical indices for evaluating the image quality of digital radiography is the contrast‐to‐noise ratio (CNR). The CNR was reported to simulate human perception. Thus, the CNR is a variable index for evaluating detectability in digital radiography. However, it is difficult to evaluate the CNR in a nonuniform image area. In this study, we have devised a new method for measuring the CNR in a nonuniform image area on digital radiography using the top‐hat transform. Furthermore, we investigated the radiation dose dependence of CNR in the digital image of an anthropomorphic breast phantom containing a simulated calcification. We have shown that the CNR in a nonuniform image area can be analyzed with high accuracy by the devised method for measuring the CNR. As for the radiation dose dependence of the CNR, the CNR increased with the mAs values and fell depending on the increase in the density of mammary glands in the anthropomorphic breast phantom. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(7): 32–41, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11416

SU‐F‐500‐09: Molecular Breast Imaging Using Emission Tomosynthesis

Purpose: Tour objective is to design a novel SPECT system for molecular breast imaging (MBI) and evaluate its performance. The limited angle SPECT system, or emission tomosynthesis, is designed to achieve 3D images of the breast with high spatial resolution/sensitivity. The system uses a simplified detector motion and is conducive to on‐board biopsy and multi‐modal imaging with mammography. Methods: The novel feature of the proposed gamma camera is a variable‐angle, slant‐hole (VASH) collimator, which is well suited for limited angle SPECT of a mildly compressed breast. The collimator holes change slant angle while the camera surface remains flush against the compression paddle. This allows the camera to vary the angular view (±30°, ±45°) for tomographic imaging while keeping the camera close to the object for high spatial resolution and/or sensitivity. Theoretical analysis and Monte Carlo simulations were performed assuming a point source and isolated breast phantom. Spatial resolution, sensitivity, contrast and SNR were measured. Results were compared to single‐view, planar images and conventional SPECT. For both conventional SPECT and VASH, data were reconstructed using iterative algorithms. Finally, a proof‐of‐concept VASH collimator was constructed for experimental evaluation. Results: Measured spatial resolution/sensitivity with VASH showed good agreement with theory including depth‐of‐interaction (DOI) effects. The DOI effect diminished the depth resolution by approximately 2 mm. Increasing the slant angle range from ±30° to ±45° resulted in an approximately 1 mm improvement in the depth resolution. In the breast phantom images, VASH showed improved contrast and SNR over conventional SPECT and improved contrast over planar scintimmammography. Reconstructed images from the proof‐of‐concept VASH collimator demonstrated reasonable depth resolution capabilities using limited angle projection data. Conclusion: We conclude that this limited angle SPECT approach using a VASH collimator can achieve superior 3D image quality for MBI in a design that has attractive characteristics for clinical imaging.

TH-A-103-08: System Optimization of Spectral Breast CT Based On An Energy-Resolved Photon-Counting Si Strip Detector: A Simulation Study

Purpose: To determine the optimal design parameters for a high resolution spectral breast CT system based on an energy-resolved photon counting Si strip detector for breast imaging. Methods: A simulation package was developed for a spectral breast CT system. The focal spot size (100 to 300 μm), magnification (1.1 to 1.4), motion blurring and detector pixel size (intrinsic pixel pitch and 4×4 pixel binning) were considered to optimize the system design. Simulations were conducted using single slice fan beam geometry at 60 kVp with 2 mm Al filter. The system modulation transfer functions (MTFs) were calculated from reconstructed image of a 10 μm diameter tungsten wire. The performance of the system with optimal design parameters was evaluated with a simulated breast phantom. A 14 cm diameter cylindrical phantom made of breast tissue with 20% glandularity was used to simulate an average-sized breast. The soft tissue lesions and microcalcifications were simulated with spheres of glandular tissues (1,2, and 4 mm in diameter) and hydroxyapatite (100 to 250 μm in diameter), respectively. Results: The spatial frequencies at 10 % of the maximum MTF was calculated to be 1.6 lp/mm and 6.4 lp/mm with and without a 4 x 4 pixel binning, respectively. The optimal system magnification was estimated to be 1.2. There was minimal improvement in system MTF when the x-ray tube focal spot size was changed from 300 to 100 μm. In the simulated breast phantom image, 100 μm microcalcifications can be clearly identified and the soft tissue contrast-to-noise ratio was improved with separate image processing, where 4×4 pixel binning was used at a dose level of 6 mGy. Conclusion: The simulation results show that the proposed system can potentially detect micro-calcifications and soft tissue lesions with a dose level equivalent to that of a standard two-view mammography.

Photon counting spectral breast CT: Effect of adaptive filtration on CT numbers, noise, and contrast to noise ratio

Photon counting spectral (PCS) computed tomography (CT) shows promise for breast imaging. An issue with current photon-counting detectors is low count rate capabilities, artifacts resulting from nonuniform count rate across the field of view, and suboptimal spectral information. These issues are addressed in part by using tissue-equivalent adaptive filtration of the x-ray beam. The purpose of the study was to investigate the effect of adaptive filtration on different aspects of PCS breast CT. The theoretical formulation for the filter shape was derived for different filter materials and evaluated by simulation and an experimental prototype of the filter was fabricated from a tissue-like material (acrylic). The PCS CT images of a glandular breast phantom with adipose and iodine contrast elements were simulated at 40, 60, 90, and 120 kVp tube voltages, with and without adaptive filter. The CT numbers, CT noise, and contrast-to-noise ratio (CNR) were compared for spectral CT images acquired with and without adaptive filters. Similar comparison was made for material-decomposed PCS CT images. The adaptive filter improved the uniformity of CT numbers, CT noise, and CNR in both ordinary and material decomposed PCS CT images. At the same tube output the average CT noise with adaptive filter, although uniform, was higher than the average noise without adaptive filter due to x-ray absorption by the filter. Increasing tube output, so that average skin exposure with the adaptive filter was same as without filter, made the noise with adaptive filter comparable to or lower than that without adaptive filter. Similar effects were observed when energy weighting was applied, and when material decompositions were performed using energy selective CT data. An adaptive filter decreases count rate requirements to the photon counting detectors which enables PCS breast CT based on commercially available detector technologies. Adaptive filter also improves image quality in PCS breast CT by decreasing beam hardening artifacts and by eliminating spatial nonuniformities of CT numbers, noise, and CNR.

Needle segmentation using 3D Hough transform in 3D TRUS guided prostate transperineal therapy

Prostate adenocarcinoma is the most common noncutaneous malignancy in American men with over 200,000 new cases diagnosed each year. Prostate interventional therapy, such as cryotherapy and brachytherapy, is an effective treatment for prostate cancer. Its success relies on the correct needle implant position. This paper proposes a robust and efficient needle segmentation method, which acts as an aid to localize the needle in three-dimensional (3D) transrectal ultrasound (TRUS) guided prostate therapy. The procedure of locating the needle in a 3D TRUS image is a three-step process. First, the original 3D ultrasound image containing a needle is cropped; the cropped image is then converted to a binary format based on its histogram. Second, a 3D Hough transform based needle segmentation method is applied to the 3D binary image in order to locate the needle axis. The position of the needle endpoint is finally determined by an optimal threshold based analysis of the intensity probability distribution. The overall efficiency is improved through implementing a coarse-fine searching strategy. The proposed method was validated in tissue-mimicking agar phantoms, chicken breast phantoms, and 3D TRUS patient images from prostate brachytherapy and cryotherapy procedures by comparison to the manual segmentation. The robustness of the proposed approach was tested by means of varying parameters such as needle insertion angle, needle insertion length, binarization threshold level, and cropping size. The validation results indicate that the proposed Hough transform based method is accurate and robust, with an achieved endpoint localization accuracy of 0.5 mm for agar phantom images, 0.7 mm for chicken breast phantom images, and 1 mm for in vivo patient cryotherapy and brachytherapy images. The mean execution time of needle segmentation algorithm was 2 s for a 3D TRUS image with size of 264 × 376 × 630 voxels. The proposed needle segmentation algorithm is accurate, robust, and suitable for 3D TRUS guided prostate transperineal therapy.

Image-based 3D modeling and validation of radiofrequency interstitial tumor ablation using a tissue-mimicking breast phantom

Minimally invasive treatment of solid cancers, especially in the breast and liver, remains clinically challenging, despite a variety of treatment modalities, including radiofrequency ablation (RFA), microwave ablation or high-intensity focused ultrasound. Each treatment modality has advantages and disadvantages, but all are limited by placement of a probe or US beam in the target tissue for tumor ablation and monitoring. The placement is difficult when the tumor is surrounded by large blood vessels or organs. Patient-specific image-based 3D modeling for thermal ablation simulation was developed to optimize treatment protocols that improve treatment efficacy. A tissue-mimicking breast gel phantom was used to develop an image-based 3D computer-aided design (CAD) model for the evaluation of a planned RF ablation. First, the tissue-mimicking gel was cast in a breast mold to create a 3D breast phantom, which contained a simulated solid tumor. Second, the phantom was imaged in a medical MRI scanner using a standard breast imaging MR sequence. Third, the MR images were converted into a 3D CAD model using commercial software (ScanIP, Simpleware), which was input into another commercial package (COMSOL Multiphysics) for RFA simulation and treatment planning using a finite element method (FEM). For validation of the model, the breast phantom was experimentally ablated using a commercial (RITA) RFA electrode and a bipolar needle with an electrosurgical generator (DRE ASG-300). The RFA results obtained by pre-treatment simulation were compared with actual experimental ablation. A 3D CAD model, created from MR images of the complex breast phantom, was successfully integrated with an RFA electrode to perform FEM ablation simulation. The ablation volumes achieved both in the FEM simulation and the experimental test were equivalent, indicating that patient-specific models can be implemented for pre-treatment planning of solid tumor ablation. A tissue-mimicking breast gel phantom and its MR images were used to perform FEM 3D modeling and validation by experimental thermal ablation of the tumor. Similar patient-specific models can be created from preoperative images and used to perform finite element analysis to plan radiofrequency ablation. Clinically, the method can be implemented for pre-treatment planning to predict the effect of an individual's tissue environment on the ablation process, and this may improve the therapeutic efficacy.

TH‐A‐217BCD‐11: A Practical Approach to Evaluate the Performance of Automatic Exposure Controls in Digital Mammography

Purpose: To establish a practical methodology to quantify the performance of different automatic exposure controls (AEC) options in digital mammography in terms of dose efficiency. Methods: A Hologic Selenia digital mammography system was used in the study. A figure of merit (FOM), defined as the squared signal difference to noise ratio normalized by the mean glandular dose (MGD), was utilized to delineate the AEC performance. The measurements and analyses were conducted under different AEC options on a realistically shaped tissue equivalent phantom (CIRS 18‐222). The phantom consists of simulated masses, fibers, and microcalcifications. Slabs of BR12 were added to reach different thicknesses. Based on the results, a computer simulation was performed for lesions on the images of an anthropomorphic breast phantom (Gammex, Rachel) acquired under the same AEC options. The pathological findings from the resulted images were compared. Results: For the mammography system used in the experiment, it was found that as compared with the default AEC, the lower kVp and Mo‐Mo target‐filter AEC table results in a FOM improvement on low contrast by 11% to 20%, except for 7 cm thickness where the FOM is reduced by 17%. The FOM improves negligibly on high contrast details for the thicknesses of 4.5 – 6 cm and decreases by 18% for the thickness of 7 cm. The simulated lesions on the Rachel phantom showed enhanced contrast with the lower kVp and Mo/Mo option, but the enhancement gradually decreases for the thicker breast. Conclusions: A methodology was established to quantify the performance of different AEC options using the figure‐of‐merit and the commercially available phantoms. For the tested mammography system, the lower kVp and Mo/Mo option is preferred if the breast thickness ranges 4.5 – 6 cm, whereas the default AEC is recommended if the breast thickness is 7cm and above.

UWB microwave imaging of heterogeneous breast phantoms

UWB microwave imaging has potential for early-stage screening diagnostics of breast cancer. We present an experimental measurement setup for breast phantom trials based on M-sequence radar technology and active small dipole antennas as pre-stage for real in vivo measurements in a prone examination position. We describe current development steps as well as first experiences concerning the imaging of heterogeneous breast phantoms.

Quantitative Microwave Imaging of Realistic Numerical Breast Phantoms Using an Enclosed Array of Multiband, Miniaturized Patch Antennas.

We present a 3-D microwave breast imaging study in which we reconstruct the dielectric profiles of MRI-derived numerical breast phantoms from simulated array measurements using an enclosed array of multiband, miniaturized patch antennas. The array is designed to overcome challenges relating to the ill-posed nature of the inverse scattering system. We use a multifrequency formulation of the distorted Born iterative method to image four normal-tissue breast phantoms, each corresponding to a different density class. The reconstructed fibroglandular distributions are very faithful to the true distributions in location and basic shape. These results establish the feasibility of using an enclosed array of miniaturized, multiband patch antennas for quantitative microwave breast imaging.

Effects of exposure equalization on image signal‐to‐noise ratios in digital mammography: A simulation study with an anthropomorphic breast phantom

The scan equalization digital mammography (SEDM) technique combines slot scanning and exposure equalization to improve low-contrast performance of digital mammography in dense tissue areas. In this study, full-field digital mammography (FFDM) images of an anthropomorphic breast phantom acquired with an anti-scatter grid at various exposure levels were superimposed to simulate SEDM images and investigate the improvement of low-contrast performance as quantified by primary signal-to-noise ratios (PSNRs). We imaged an anthropomorphic breast phantom (Gammex 169 "Rachel," Gammex RMI, Middleton, WI) at various exposure levels using a FFDM system (Senographe 2000D, GE Medical Systems, Milwaukee, WI). The exposure equalization factors were computed based on a standard FFDM image acquired in the automatic exposure control (AEC) mode. The equalized image was simulated and constructed by superimposing a selected set of FFDM images acquired at 2, 1, 1/2, 1/4, 1/8, 1/16, and 1/32 times of exposure levels to the standard AEC timed technique (125 mAs) using the equalization factors computed for each region. Finally, the equalized image was renormalized regionally with the exposure equalization factors to result in an appearance similar to that with standard digital mammography. Two sets of FFDM images were acquired to allow for two identically, but independently, formed equalized images to be subtracted from each other to estimate the noise levels. Similarly, two identically but independently acquired standard FFDM images were subtracted to estimate the noise levels. Corrections were applied to remove the excess system noise accumulated during image superimposition in forming the equalized image. PSNRs over the compressed area of breast phantom were computed and used to quantitatively study the effects of exposure equalization on low-contrast performance in digital mammography. We found that the highest achievable PSNR improvement factor was 1.89 for the anthropomorphic breast phantom used in this study. The overall PSNRs were measured to be 79.6 for the FFDM imaging and 107.6 for the simulated SEDM imaging on average in the compressed area of breast phantom, resulting in an average improvement of PSNR by ∼35% with exposure equalization. We also found that the PSNRs appeared to be largely uniform with exposure equalization, and the standard deviations of PSNRs were estimated to be 10.3 and 7.9 for the FFDM imaging and the simulated SEDM imaging, respectively. The average glandular dose for SEDM was estimated to be 212.5 mrad, ∼34% lower than that of standard AEC-timed FFDM (323.8 mrad) as a result of exposure equalization for the entire breast phantom. Exposure equalization was found to substantially improve image PSNRs in dense tissue regions and result in more uniform image PSNRs. This improvement may lead to better low-contrast performance in detecting and visualizing soft tissue masses and micro-calcifications in dense tissue areas for breast imaging tasks.

Dosimetric evaluation of breast radiotherapy in a dynamic phantom

This phantom study quantifies changes in delivered dose due to respiratory motion for four breast radiotherapy planning techniques: three intensity-modulated techniques (forward-planned, surface-compensated and hybrid intensity-modulated radiation therapy (IMRT)); using a combination of open fields and inverse planned IMRT) and a 2D conventional technique. The plans were created on CT images of a wax breast phantom with a cork lung insert, and dose distributions were measured using films inserted through slits in the axial and sagittal planes. Films were irradiated according to each plan under a static (modeling breathhold) and three dynamic conditions—isocenter set at mid-respiratory cycle with motion amplitudes of 1 and 2 cm and at end-cycle with 2 cm motion amplitude (modeling end-exhale). Differences between static and moving deliveries were most pronounced for the more complex planning techniques with hot spots of up to 107% appearing in the anterior portion of all three IMRT plans at the largest motion at the end-exhale set-up. The delivered dose to the moving phantom was within 5% of that to the static phantom for all cases, while measurement accuracy was ±3%. The homogeneity index was significantly decreased only for the 2 cm motion end-exhale set-up; however, this same motion increased the equivalent uniform dose because of improved posterior breast coverage. Overall, the study demonstrates that the effect of respiratory motion is negligible for all planning techniques except in occasional instances of large motion.

TU‐E‐220‐02: Combined Pulse Echo, X‐Ray Tomosynthetic, Photoacoustic and Speed of Sound Imaging in the Mammographic Geometry

Purpose: Ultrasound (US) is sensitive to many cancers that are not well detected by mammography in dense breasts, and characterizes those cysts requiring no further diagnosis. But US screening sees many apparent abnormalities, increasing callbacks, and misses many cancers detected by their microcalcifications. If sufficient US image quality and coverage can be achieved in this geometry, a combined US/Breast Tomosynthesis (US/BT) system should allow rapid identification of structures of interest in corresponding modalities, reducing misidentification errors in diagnosis and allowing reduction of diagnostic follow up of probably benign masses in screening. In screening, adding high quality ultrasound registered to BT should increase detection of cancers in dense breasts as with manual scanning.Photoacoustic tomography (PAT) is sensitive to vascular anomalies including negligible flow. The penetration appears to be adequate for PAT imaging from both sides of the compressed breast. The ability of such optical absorption imaging at different wavelengths (spectroscopic PAT, S‐ PAT) to distinguish oxy‐ and deoxyhemoglobin appears to add to discrimination of benign from malignant masses and might find use in selecting and tracking therapies. Coregistered BT, US and optical imaging might well provide similar screening and diagnostic effectiveness in a single, 15 minute examination, as the combination of current mammography and contrast MRI examinations.Methods: The combined US/BT system has been described earlier. A pilot study was performed of BT and AUS in 51 cases going to biopsy, with the mass visible in BT and in the volume covered by AUS.US images from a new Breast Light and Ultrasound Combined Imaging (BLUCI) system are registered to the US images from the US/BT system for initial studies. BLUCI allows limited angle, transmission speed of sound (SOS) imaging with the same type of 570 element, broadband array as is used for PAT. Our PAT is performed with an 8 cm diameter PFDF polymer transducer array. This added imaging mode might provide similar information as elasticity imaging, but with fewer artifacts.Results: In the preliminary study, addition of automated US did not change the average accuracy, Az, over BT in this small number of cases where only masses visible in BT were analyzed. Each reader, on a case by case question, responded very positively to ‐ If screening were performed by BT (and US) in this fashion, would tomosynthesis alone allow you to avoid categorizing this case as BIRADS 0 and instead enable you to categorize it to avoid additional imaging?The challenge of pulse echo imaging through two layers of lobes (on either side of the midline) with single or two view automated scanning appears to be met in the, currently, separate ultrasound/S‐PAT scanner on a mammographic frame using compound scanning from both sides of the breast (Carson, et al. at this meeting). Initial success in achieving rapid, full acoustic coupling in humans, and in performing PAT and SOS imaging of breast phantoms and specimens will be presented. PAT revealed simulated vessels to 5 cm depth. SOS shows a cylindrical object with the expected resolution of limited angle tomography.Conclusion: Essentially full, high resolution pulse echo coverage of the breast and large volume PAT (∼ an 8 cm diameter cylinder) appear possible in reasonable exam times. The promise of these particular 3 modalities in the mammographic geometry and the level of our current systems justifies initiation of an enhanced screening study of US/BT, with SOS and/or PAT added as method development and exam times allow.

A comparison between the electronic magnification (EM) and true magnification (TM) of breast phantom images using a CDMAM phantom

To provide a comparison between the image quality of electronically magnified (EM) and geometric, or true, magnification (TM) mammographic images. One Computed Radiography (CR), one Digital Radiography (DR) and two screen-film (S-F) imaging systems were investigated. A Contrast-Detail Mammography (CDMAM) phantom was used as a test object. Three contact images and three sets of TM images with a magnification factor of 1.8 were taken on all systems. Software was used to zoom the contact images by a factor of 1.8 to produce EM images. Two observers evaluated all of the images. An Image Quality Figure and contrast detail curve were used to analyze the observer data and Mann-Whitney U-tests were performed to determine the statistical significance of the results. No significant differences were found between soft copy and hard copy for any imaging modality. No significant difference in contrast detail detectability (CDD) was seen between EM images from the two digital systems and TM images on S-F systems. The results for the DR EM images and S-F TM images also showed no differences. The CDD of DR TM images was significantly better than both EM and S-F TM images. Digitally zoomed images offer the same level of CDD as S-F TM images, and so may be viably used in their place. DR systems offer greater CDD than conventional S-F images, when comparing the TM images. This implies that doses can be greatly reduced for TM views using DR systems, while maintaining acceptable image quality.

Method for Measuring Contrast-to-Noise Ratio (CNR) in Non-uniform Image Area on Digital Radiography

One of physical indices for evaluating the image quality of digital radiography is contrast-to-noise ratio (CNR). The CNR was reported to simulate the human perception. Thus, the CNR is a variable index for evaluating detectability for digital radiography. However, it is difficult to evaluate the CNR in non-uniform image area. In this study, we have devised a new method for measuring CNR in non-uniform image area on digital radiography using top-hat transform. Furthermore, we investigated the radiation dose dependence of CNR in the digital image of an anthropomorphic breast phantom containing a simulated calcification. We have shown that the CNR in non-uniform image area can be analyzed with high accuracy by our devised method for measuring CNR. In the radiation dose dependence of CNR, CNR increased with mAs values and reduced depending on the increase in the concentration of glandular in the anthropomorphic breast phantom.

Ｃｏｍｐｕｔｅｄ Ｒａｄｉｏｇｒａｐｈｙ（ＣＲ）マンモグラフィにおける乳腺含有率の推定

Measurement of a percent glandular tissue composition (%GTC) is important in terms of the estimation of individual patient exposure dose and the prediction of malignancy, and thus a number of reports for estimating %GTC by use of a mammogram have been published. In this study, we propose a method for estimating individual %GTC by use of computed radiography (CR) mammograms. By employing breast-equivalent phantoms that are able to create breast phantom images with various combinations of fat and glandular tissue, as well as the thickness of whole breast, we determined a reference table for converting an each pixel value on CR mammography to the glandular tissue ratio. Therefore, the %GTC for individual breast was estimated by averaging glandular tissue ratio for a whole region. The clinical image data set that consisted of 49 CR mammograms were used for estimating %GTC. A paired comparison method for determining subjective ranking of the degree of breast density was employed in order to demonstrate the validity of our method. The results indicate that the average estimated %GTC was 35.0% (ranged from 12.0% to 67.0%) and they had a increased correlation with the ranking of those obtained by observer test. Therefore, it was suggested that our proposed method would be utilized for estimating the %GTC in objective manner.

Reduction of motion artefacts in on-board cone beam CT by warping of projection images

We describe the development and testing of a motion correction method for flat panel imager-based cone beam CT (CBCT) based on warping of projection images. Markers within or on the surface of the patient were tracked and their mean three-dimensional (3D) position calculated. The two-dimensional (2D) cone beam projection images were then warped before reconstruction to place each marker at the projection from its mean 3D position. The motion correction method was tested using simulated cone beam projection images of a deforming virtual phantom, real CBCT images of a moving breast phantom and clinical CBCT images of a patient with breast cancer and another with pancreatic cancer undergoing radiotherapy. In phantom studies, the method was shown to greatly reduce motion artefacts in the locality of the radiotherapy target and allowed the true surface shape to be accurately recovered. The breast phantom motion-compensated surface was within 1 mm of the true surface shape for 90% of surface points and greater than 2 mm from the true surface at only 2% of points. Clinical CBCT images showed improved image quality in the locality of the radiotherapy target after motion correction. The proposed method is effective in reducing motion artefacts in CBCT images.

Three‐dimensional microwave imaging of realistic numerical breast phantoms via a multiple‐frequency inverse scattering technique

Breast density measurement has the potential to play an important role in individualized breast cancer risk assessment and prevention decisions. Routine evaluation of breast density will require the availability of a low-cost, nonionizing, three-dimensional (3-D) tomographic imaging modality that exploits a strong properties contrast between dense fibroglandular tissue and less dense adipose tissue. The purpose of this computational study is to investigate the performance of 3-D tomography using low-power microwaves to reconstruct the spatial distribution of breast tissue dielectric properties and to evaluate the modality for application to breast density characterization. State-of-the-art 3-D numerical breast phantoms that are realistic in both structural and dielectric properties are employed. The test phantoms include one sample from each of four classes of mammographic breast density. Since the properties of these phantoms are known exactly, these testbeds serve as a rigorous benchmark for the imaging results. The distorted Born iterative imaging method is applied to simulated array measurements of the numerical phantoms. The forward solver in the imaging algorithm employs the finite-difference time-domain method of solving the time-domain Maxwell's equations, and the dielectric profiles are estimated using an integral equation form of the Helmholtz wave equation. A multiple-frequency, bound-constrained, vector field inverse scattering solution is implemented that enables practical inversion of the large-scale 3-D problem. Knowledge of the frequency-dependent characteristic of breast tissues at microwave frequencies is exploited to obtain a parametric reconstruction of the dispersive dielectric profile of the interior of the breast. Imaging is performed on a high-resolution voxel basis and the solution is bounded by a known range of dielectric properties of the constituent breast tissues. The imaging method is validated using a breast phantom with a single, high-contrast interior scattering target in an otherwise homogeneous interior. The method is then used to image a set of realistic numerical breast phantoms of varied fibroglandular tissue density. Imaging results are presented for each numerical phantom and show robustness of the method relative to tissue density. In each case, the distribution of fibroglandular tissues is well represented in the resulting images. The resolution of the images at the frequencies employed is wider than the feature dimensions of the normal tissue structures, resulting in a smearing of their reconstruction. The results of this study support the utility of 3-D microwave tomography for imaging the distribution of normal tissues in the breast, specifically, dense fibroglandular tissue versus less dense adipose tissue, and suggest that further investigation of its use for volumetric evaluation of breast density is warranted.

A generic geometric calibration method for tomographic imaging systems with flat‐panel detectors—A detailed implementation guide

To present a generic geometric calibration method for tomographic imaging systems with flat-panel detectors in a very detailed manner, in the aim to provide a useful tool to the public domain. The method is based on a projection matrix which represents a mapping from 3D object coordinate system to 2D projection image plane. The projection matrix can be determined experimentally through the imaging of a phantom of known marker geometry. Accurate implementation was accomplished through direct computation algorithms, including a novel ellipse fitting using singular value decomposition and data normalization. Benefits of the method include: (1) It is capable of being applied to systems of different scan trajectories, source-detector alignments, and detector orientations; (2) projection matrices can be utilized in image reconstructions or in the extraction of explicit geometrical parameters; and (3) the method imposes minimal limits on the design of calibration phantom. C++ programs that calculate projection matrices and extract geometric parameters from them are also provided. For validation, the calibration method was applied to the computer simulation of a cone-beam CT system, as well as to three tomosynthesis prototypes of different source-detector movement patterns: Source and detector rotating synchronizedly; source rotating and detector wobbling; and source rotating and detector staying stationary. Projection matrices were computed on a view by view basis. Geometric parameters extracted from projection matrices were consistent with actual settings. Images were reconstructed by directly using projection matrices, and were compared to virtual Shepp-Logan image for CT simulation and to central projection images of CIRS breast phantoms for tomosynthesis prototypes. They showed no obvious distortion or blurring, indicating the high quality of geometric calibration results. When the computed central ray offsets were perturbed with Gaussian noises of 1 pixel standard deviation, the reconstructed image showed apparent distortion, which further demonstrated the accuracy of the geometric calibration method. The method is suitable for tomographic imaging systems with flat-panel detectors.

Experimental validation of a radiographic simulation code using breast phantom for X-ray imaging

Computer models and simulations of X-ray imaging systems are becoming a very precious tool during the development and evaluation of new X-ray imaging techniques. To provide, however, a faithful simulation of a system, all components must be accurately modelled and tested, followed by verification through experimental measurements. This paper presents a validation study of the XRayImagingSimulator, an in-house developed X-ray imaging simulator, which is extensively used as a basic tool in carrying out complex breast imaging simulations. The approach followed compares results obtained via an experimental setup for breast phantom (CIRS 011A) imaging, using synchrotron radiation (SYRMEP beamline at ELETTRA), with those from its simulated setup under the same conditions. The study demonstrated a very good agreement between experimental and simulated images compared both in terms of subjective and objective criteria. The combination of the XRayImagingSimulator with our BreastSimulator provides a powerful tool for in silico testing of new X-ray breast imaging approaches.

Enhancement of ultrasound images by displacement, averaging, and interlacing

Ultrasound is a widely used medical imaging methodol- ogy because it is safe and relatively inexpensive. However, the qual- ity of the images is affected by the point-spread function of the sys- tem, medium effects (such as refraction, reflection, and attenuation), and coherent wave interference or speckle. The present research studies the averaging of images that have been displaced laterally and displays them using an interlaced grid. The main goals are to reduce speckle and improve contrast-to-noise ratio and resolution. The point-spread function of the ultrasound scanner was estimated using a thin nylon thread within a water bath. Then, a set of images of a breast phantom (having lateral displacements smaller than the width of the point spread function) were averaged and interlaced. The results show significant improvements in signal-to-noise ratio and contrast-to-noise ratio. © 2010 SPIE and IS&T.