Weixing Cai

Cone beam breast CT with multiplanar and three dimensional visualization in differentiating breast masses compared with mammography.

OBJECTIVE This pilot study was to evaluate cone beam breast computed tomography (CBBCT) with multiplanar and three dimensional (3D) visualization in differentiating breast masses in comparison with two-view mammograms. METHODS Sixty-five consecutive female patients (67 breasts) were scanned by CBBCT after conventional two-view mammography (Hologic, Motarget, compression factor 0.8). For CBBCT imaging, three hundred (1024 × 768 × 16b) two-dimensional (2D) projection images were acquired by rotating the x-ray tube and a flat panel detector (FPD) 360 degree around one breast. Three-dimensional CBBCT images were reconstructed from the 2D projections. Visage CS 3.0 and Amira 5.2.2 were used to visualize reconstructed CBBCT images. RESULTS Eighty-five breast masses in this study were evaluated and categorized under the breast imaging reporting and data system (BI-RADS) according to plain CBBCT images and two-view mammograms, respectively, prior to biopsy. BI-RADS category of each breast was compared with biopsy histopathology. The results showed that CBBCT with multiplanar and 3D visualization would be helpful to identify the margin and characteristics of breast masses. The category variance ratios for CBBCT under the BI-RADS were 23.5% for malignant tumors (MTs) and 27.3% for benign lesions in comparison with pathology, which were evidently closer to the histopathology results than those of two-view mammograms, p value <0.01. With the receiver operating characteristic (ROC) curve analysis, the area under the curve (AUC) of CBBCT was 0.911, larger than that (AUC 0.827) of two-view mammograms, p value <0.01. CONCLUSION CBBCT will be a distinctive noninvasive technology in differentiating and categorizing breast masses under BI-RADS. CBBCT may be considerably more effective to identify breast masses, especially some small, uncertain or multifocal masses than conventional two-view mammography.

Dose efficiency consideration for volume-of-interest breast imaging using x-ray differential phase-contrast CT

The newly developed differential phase-contrast (DPC) imaging technique has attracted increasing interest among researchers. In a DPC system, the self-imaging effect and the phase-stepping method are implemented through three gratings to manifest phase contrast, and differentiated phase images can be obtained. An important advantage of this technique is that hospital-grade x-ray tubes can be used, allowing much higher x-ray output power and faster image processing than with micro-focus in-line phase-contrast imaging. A DPC-CT system can acquire images from different view angles along a circular orbit, and tomographic images can be reconstructed. However, the principle of DPC imaging requires multiple exposures to compute any differentiated phase image at each view angle, which raises concerns about radiation exposure via x-ray dose. Computer simulations are carried out to study the dose efficiency for DPC-CT for volume-of-interest breast imaging. A conceptual CBCT/DPC-CT hybrid imaging system and a numerical breast phantom are designed for this study. A FBP-type reconstruction algorithm is optimized for the VOI reconstruction. Factors including the x-ray flux and detector pixel size are considered and their effects on reconstruction image quality in terms of noise level and contrast-to-noise ratio are investigated. The results indicate that with a pixel size of 20 microns and a dose level of 5.7mGy, which is equivalent to the patient dose of a two-view mammography screening or a dedicated CBCT breast imaging scan, much better tissue contrast and spatial resolution can be achieved using the DPC-CT technique. It is very promising for possible application at pathology-level in vivo study for human breasts.

Scatter correction using beam stop array algorithm for cone-beam CT breast imaging

In flat-panel detector-based cone-beam CT breast imaging (CBCTBI), scatter is an important factor that degrades image quality. It has been shown that despite the use of a large air gap, scatter still causes problems when imaging a large breast phantom with our CBCTBI prototype. The SPR at the center region near the chest wall of a C-cup phantom is about 0.5, and this value goes up to 0.9 for the D-cup phantom. As a result, the linear attenuation coefficient (LAC) distortion and reduced contrast were obvious in the reconstruction slices. To conquer the scattering problem, the beam-stop array (BSA) algorithm was presented in previous papers by our group. Since the breast is nearly axially symmetric, only one or two more projections for scatter images are required for the BSA algorithm. Therefore, the angular interpolation part in the algorithm could be simplified. The accuracy of the BSA algorithm is evaluated with a water phantom and the error of LAC reconstruction is proven to be less than 2%. The results of a C-cup phantom study shows that the LAC distortion in CBCT breast imaging could be corrected, and the CNR of target tumors is increased by a small amount. The number of sampled scattering patterns for CBCTBI is also discussed. The results showed that the BSA algorithm worked well for our CBCTBI prototype system. It could remove the scatter efficiently and improve the image quality.

Simplified method of scatter correction using a beam-stop-array algorithm for cone-beam computed tomography breast imaging

In flat-panel detector-based cone-beam computed-tomography breast imaging (CBCTBI) systems, scatter is an important factor that degrades image quality. It has been shown that despite the use of a large air gap, scatter still causes problems when imaging breast phantoms with our CBCTBI prototype. As a result, linear attenuation coefficient (LAC) distortion is obvious in the reconstruction; it appears as cupping artifacts and contrast loss. A simplified beam-stop array (BSA) algorithm is presented in this paper to solve this problem practically and efficiently. When the breast is positioned along the rotational axis, the scatter profiles from different views along a circular orbit are similar, and thus it is possible to use only one x-ray shot with the BSA in place for scatter pattern estimation, and this BSA image is used to generate a scatter pattern for all projections. The result of this scatter correction algorithm is compared with the reconstruction image over a small field of view, where scatter is assumed to be negligible, and the relative reconstruction error (RRE) is evaluated. The reconstruction is not sensitive to the estimation of scatter profile curvature, but is sensitive to the estimation of scatter intensity. It is shown as well that for any angular position where the BSA image is taken, the RRE is very small. The results show that the BSA algorithm works well for our CBCTBI prototype system with almost unchanged x-ray exposure.

A novel cone beam breast CT scanner: preliminary system evaluation

The clinical goal of breast imaging is to detect tumor masses when they are as small as possible, preferably less than 10 mm in diameter. Conventional screen-film mammography is the most effective tool for the early detection of breast cancer currently available. However, conventional mammography has relatively low sensitivity for the detection of small breast cancers (under several millimeters). Specificity and the positive predictive value of mammography remain limited owing to an overlap in the appearance of benign and malignant lesions, and surrounding structure. We propose to address the limitations accompanying conventional mammography by incorporating a cone beam CT reconstruction technique with a recently developed flat panel detector (FPD). We have performed a computer simulation study and preliminary phantom studies to prove the feasibility of developing an FPD-based cone beam CT breast imaging technique for a small size normal breast phantom. In this study, we report the design and construction of a novel FPD-based cone beam breast CT scanner prototype. In addition, we present the results of phantom studies performed on our current FPD-based cone beam CT scanner prototype, which uses the same flat panel detector proposed for the cone beam breast CT scanner prototype, to predict the image performance of the novel cone beam breast CT scanner, while we are completing the construction of the system.

Scatter correction for clinical cone beam CT breast imaging based on breast phantom studies

In flat-panel detector-based cone beam CT breast imaging (CBCTBI) systems, scattering is an important factor that degrades image quality. It is not practical to measure the scattering profiles of a breast for all view angles in a patient study, but it is possible to develop a method to estimate the scattering profiles based on information acquired from breast phantom studies. A new scattering correction method is proposed for clinical CBCTBI in this study. The scattering profiles of three anthropomorphic uncompressed breast phantoms of different sizes were thoroughly investigated, and the results indicated that though phantom size differed, the scattering profiles were mainly determined by local breast diameters, which are the approximate diameters of coronal slices that are perpendicular to the nipple-to-chestwall direction. Thus for scattering correction purposes it is possible to establish a relationship between location breast diameters and local scattering profiles, namely the fitted smooth curves of scatter-to-primary ratios (SPR) and normalized scattered radiations (NSR). In clinical CBCTBI studies, after the local breast diameters are sampled and measured on projection images, the scattering image for every projection image can be generated based on the established relationship, and the projection images can be corrected using either the SPR based method or the NSR based method. Phantom studies and clinical studies showed that both the SPR and NSR methods are able to correct cupping artifacts and reduce reconstruction error. The SPR method does not increase tissue contrast or noise while the NSR method increases both.

Fabrication and characterization of NbN, AlN and NbN/AlN/NbN on MgO substrates

At ambient substrate temperatures, NbN and AlN thin films as well as NbN/AlN/NbN sandwiches are prepared on single crystal MgO substrates, using direct current (dc) or radio frequency (RF) magnetron sputtering techniques. Excellent single crystal orientations of these structures are revealed by x-ray diffraction and transmission electron microscopy (TEM), while x-ray photoelectron spectroscope (XPS) shows that the stoichiometric composition of the NbN films is 1: 0.9 (Nb:N). Furthermore, AFM (atomic force microscope) scans indicate a root mean square (rms) roughness of 0.755 nm for AlN/NbN, and 0.83 nm for AlN, both over an area of 5 µm × 5 µm.

Fabrication of AlN thin films on different substrates at ambient temperature

Aluminium nitride (AlN) is very useful as a barrier in superconductor?insulator?superconductor (SIS) device or as an insulating layer in many other applications. At ambient temperature, we deposit AlN thin films onto different substrates (such as MgO, LaAlO3 and Si) by using radio-frequency magnetron sputtering and pure Al target. X-ray diffraction (XRD) and ?-scan patterns show that the films grown on MgO substrates are excellent epitaxial films with (101) orientation of a hexagonal lattice. A possible structure of the interface between the film and the substrate is suggested and discussed.

Preliminary study of a phase-contrast cone-beam computed tomography system: the edge-enhancement effect in the tomographic reconstruction of in-line holographic images

A phase-contrast cone-beam computed tomography (PC CBCT) system is proposed for small-animal imaging that incorporates the in-line holography technique into CBCT system. Theoretical analysis shows that the in-line holographic image can be approximately transformed into line integrals of an object function composed of an attenuation term and a phase term. The Fresnel diffraction theory is applied to generate in-line holographic images along a circular orbit, and the Feldkamp-Davis-Kress algorithm is applied to reconstruct the object function. The proposed system was investigated using a numerical phantom, and the reconstruction was evaluated using the edge-enhancement factor and the relative reconstruction error. The reconstruction results show that all the structures in the numerical phantom are bounded with enhanced edges with negligible artifacts. These enhanced edges make the reconstruction visually sharper and clearer. The results show that while the relative reconstruction errors are very close to that of the conventional CBCT reconstruction, having a small cone angle, weak attenuation, small focal spot size, and high-resolution detector are preferred for a greater edge-enhancement effect.

Performance Investigation of a Hospital-grade X-ray Tube-based Differential Phase-contrast Cone Beam CT System

Differential phase contrast technique could be the next breakthrough in the field of CT imaging. While traditional absorption-based X-ray CT imaging is inefficient at differentiating soft tissues, phase-contrast technique offers great advantage as being able to produce higher contrast images utilizing the phase information of objects. Our long term goal is to develop a gantry-based hospital-grade X-ray tube differential phase contrast cone-beam CT (DPC-CBCT) technology which is able to achieve higher contrast noise ratio (CNR) in soft tissue imaging without increasing the dose level. Based on the micro-focus system built last year, a bench-top hospital-grade X-ray tube DPC-CBCT system is designed and constructed. The DPC-CBCT system consists of an X-ray source, i.e. a hospital-grade X-ray tube and a source grating, a high-resolution detector, a rotating phantom holder, a phase grating and an analyzer grating. Threedimensional (3-D) phase-coefficients are reconstructed, providing us with images enjoying higher CNR than, yet equivalent dose level to, a conventional CBCT scan. Three important aspects of the system are investigated: a) The The systems performance in term of CNR of the reconstruction image with regard to dose levels, b) the impacts of different phase stepping schemes, i.e. 5 steps to 8 steps, in term of CNR on the reconstruction images, and c) the influence of magnification or position of the phantom on image quality, chiefly CNR. The investigations are accomplished via phantom study.