To investigate how the radiation dose level affects the detection of microcalcifications (MCs) in cone beam breast CT (CBCT), simulated MCs were embedded in simulated breast tissue and imaged with an experimental CBCT system. The system employs a 30 x 40 cm2 a-Si/CsI based flat panel detector with a pixel size of 194 microns. Three 5 x 5 clusters of simulated calcifications (212-224, 250-280, and 300-355 μm) were embedded in a stack of 11 cm diameter lunch meat and positioned at the center of each slice of lunch meat. 300 projection images over 360 degrees were acquired in the non-binning mode at various dose levels (4.2, 6, 12, 18, and 24 mGy) three times, and were reconstructed with the Feldkamp algorithm. After that, 767 x 767 x 9 volume data were extracted from the fifteen reconstructed images for each size group, resulting in 45 CBCT MC phantom images. An observer experiment was performed by counting the number of visible MCs for each MC phantom image. The phantom images were displayed on a review workstation with a 1600 x 1200 CRT monitor and reviewed by six readers independently. The order of the images was randomized for each reader. The ratios of the visible MCs were averaged over all readers and plotted as a function of the dose level. The CNR was calculated for each MC size and each doe level as well. The results showed that the performance of the reconstructed images acquired with 4.2 mGy was similar to the images acquired with 6 mGy, and the images acquired with 18 mGy performed similarly to those acquired with 24 mGy.
We developed and investigated a scanning sampled measurement (SSM) technique for scatter measurement and
correction in cone beam breast CT imaging. A cylindrical polypropylene phantom (water equivalent) was mounted on a
rotating table in a stationary gantry experimental cone beam breast CT imaging system. A 2-D array of lead beads, with
the beads set apart about ~1 cm from each other and slightly tilted vertically, was placed between the object and x-ray
source. A series of projection images were acquired as the phantom is rotated 1 degree per projection view and the lead
beads array shifted vertically from one projection view to the next. A series of lead bars were also placed at the phantom
edge to produce better scatter estimation across the phantom edges. Image signals in the lead beads/bars shadow were
used to obtain sampled scatter measurements which were then interpolated to form an estimated scatter distribution
across the projection images. The image data behind the lead bead/bar shadows were restored by interpolating image
data from two adjacent projection views to form beam-block free projection images. The estimated scatter distribution
was then subtracted from the corresponding restored projection image to obtain the scatter removed projection images. Our preliminary experiment has demonstrated that it is feasible to implement SSM technique for scatter estimation and
correction for cone beam breast CT imaging. Scatter correction was successfully performed on all projection images
using scatter distribution interpolated from SSM and restored projection image data. The resultant scatter corrected
projection image data resulted in elevated CT number and largely reduced the cupping effects.
In this work, we investigated the effects of scattered radiation and beam quality on the low contrast performance
relevant to cone beam breast CT imaging. For experiments, we used our benchtop conebeam CT system and constructed
a phantom consisting of simulated fat and soft tissues. We varied the field of view (FOV) along the z direction to observe
its effect on scattered radiation. The beam quality was altered by varying the tube voltage from 50 to 100 kV. We
computed the contrast-to-noise ratio (CNR) from reconstructed images and normalized it to the square root of dose
measured at the center of the phantom. The results were used as the figure of merit (FOM). The effect of the beam
quality on the scatter to primary ratio (SPR) had minimal impact and the SPR was primarily dominated by the FOV. In
the central section of the phantom, increasing the FOV from 4 to 16 cm resulted in drop of CNR in the order of 15-20%
at any given kVp setting. For a given FOV, the beam quality had insignificant effect on the FOM in the central section of
the phantom. In the peripheral section, a 10 % drop in FOM was observed when the kVp setting was increased from 50
to 100. At lower kVp values, the primary x-ray transmission through the thicker parts of the phantom was severely
reduced. Under such circumstances, ring artifacts were observed due to imperfect flat field correction at very low signal
intensities. Higher kVp settings and higher SPRs helped to increase the signal intensity in highly attenuating regions and
suppressed the ring artifacts.
The half-scan cone beam technique, requiring a scan for 180° plus detector width only, can help achieve both shorter scan time as well as higher exposure in each individual projection image. This purpose of this paper is to investigate whether half-scan cone beam CT technique can provide acceptable images for clinical application. The half-scan cone beam reconstruction algorithm uses modified Parker's weighting function and reconstructs from slightly more than half of the projection views for full-scan, giving out promising results. A rotation phantom, stationary gantry bench top system was built to conduct experiments to evaluate half-scan cone beam breast CT technique. A post-mastectomy breast specimen, a stack of lunch meat slices embedded with various sizes of calcifications and a polycarbonate phantom inserted with glandular and adipose tissue equivalents are imaged and reconstructed for comparison study. A subset of full-scan projection images of a mastectomy specimen were extracted and used as the half-scan projection data for reconstruction. The results show half-scan reconstruction algorithm for cone beam breast CT images does not significantly degrade image quality when compared with the images of same or even half the radiation dose level. Our results are encouraging, emphasizing the potential advantages in the use of half-scan technique for cone beam breast imaging.
Purpose: To compare two detector systems - one based on the charge-coupled device (CCD) and image amplifier, the
other based on a-Si/CsI flat panel, for cone beam computed-tomography (CT) imaging of small animals.
A high resolution, high framing rate detector system for the cone beam CT imaging of small animals was developed. The
system consists of a 2048×3072×12 bit CCD optically coupled to an image amplifier and an x-ray phosphor screen. The
CCD has an intrinsic pixel size of 12 μm but the effective pixel size can be adjusted through the magnification
adjustment of the optical coupling systems. The system is used in conjunction with an x-ray source and a rotating stage
for holding and rotating the scanned object in the cone beam CT imaging experiments. The advantages of the system
include but are not limited to the ability to adjust the effective pixel size and to achieve extremely high spatial resolution
and temporal resolution. However, the need to use optical coupling compromises the detective quanta efficiency (DQE)
of the system. In this paper, the imaging characteristics of the system were presented and compared with those of an a-
Si/CsI flat-panel detector system.
Digital radiographs are often processed prior to printing or display. The algorithm used is generally a combination of contrast and edge enhancement applied in a spatially varying way. However, such enhancement often resulted in objectionable noise level in heavily attenuating regions, which may compromise the low contrast performance. We are developing and investigating a Scan Equalization Digital Radiography (SEDR) technique with which the transmitted x-ray exposure falling on the detector would be equalized and the image SNRs would be made more uniform. In this study, we simulated exposure equalization by acquiring a series of digital radiographs with incrementing mAs’ (0.25, 0.5, 1, 2, 4, 8) and then adding them with binary weighting factors to achieve equalized exposures over various regions of the image. The exposure-equalized image was then processed with two algorithms: local window/level optimization or edge enhancement using unsharp masking. The processed images with and without exposure equalization were then examined and compared with each other. For quantitative comparison, identically acquired images were used to obtain two sets of equalized and processed images. These two sets of images were subtracted from each other to generate a map of normalized noise for comparison. It is found that exposure equalization resulted in more uniform SNRs in both raw and processed images. Thus the noise levels in heavily attenuated regions were kept low and had less objectionable appearance for visualization of low contrast objects.
KEYWORDS: Sensors, Fluctuations and noise, Radiography, X-rays, Radiation effects, Signal detection, Imaging systems, Chest, Signal to noise ratio, Image quality
We are constructing and investigating a Scan Equalization Digital Radiography (SEDR) system using an a-Si:H based flat-panel detector. With this system, slot-scan imaging with regionally adjustable beam width is used to achieve scatter rejection and exposure equalization. As part of the SEDR system, we have developed and implemented an electronic aft-collimation technique, referred to as the Alternate Line Erasure and Readout (ALER), by modifying the electronics of an a-Si:H/a-Se based flat-panel detector to alter the sequence of image readout. Instead of reading the image line by line, the leading edge line of the scanning fan beam is reset to erase the scatter component accumulated prior to the arrival of the fan beam while the trailing edge line is read out to acquire the exposure signals integrated following the exposure of the scanning fan beam. This resetting and readout cycle is repeated and synchronized to the motion of the scanning fan beam. To guide the selection of the slot width in implementing the SEDR system, measurements of the scatter-to-primary ratio (SPR) and relative contrast-to-noise ratio (RCNR) were made and compared for various slot widths. Our preliminary testing has demonstrated that it is feasible to implement an electronic aft-collimation technique to effectively reject scattered radiation without attenuating the primary x-rays and without using a bulky, heavy aft-collimator. The SPR and RCNR measurements indicated that the performance of the slot-scan imaging technique improves with narrower slot width and is generally better than the anti-scatter grid method.
This paper investigates the feasibility of using a flat panel based cone-beam computer tomography (CT) system for 3-D breast imaging with computer simulation and imaging experiments. In our simulation study, 3-D phantoms were analytically modeled to simulate a breast loosely compressed into cylindrical shape with embedded soft tissue masses and calcifications. Attenuation coefficients were estimated to represent various types of breast tissue, soft tissue masses and calcifications to generate realistic image signal and contrast. Projection images were computed to incorporate x-ray attenuation, geometric magnification, x-ray detection, detector blurring, image pixelization and digitization. Based on the two-views mammography comparable dose level on the central axis of the phantom (also the rotation axis), x-ray kVp/filtration, transmittance through the phantom, detected quantum efficiency (DQE), exposure level, and imaging geometry, the photon fluence was estimated and used to estimate the phantom noise level on a pixel-by-pixel basis. This estimated noise level was then used with the random number generator to produce and add a fluctuation component to the noiseless transmitted image signal. The noise carrying projection images were then convolved with a Gaussian-like kernel, computed from measured 1-D line spread function (LSF) to simulated detector blurring. Additional 2-D Gaussian-like kernel is designed to suppress the noise fluctuation that inherently originates from projection images so that the reconstructed image detectability of low contrast masses phantom can be improved. Image reconstruction was performed using the Feldkamp algorithm. All simulations were performed on a 24 PC (2.4 GHz Dual-Xeon CPU) cluster with MPI parallel programming. With 600 mrads mean glandular dose (MGD) at the phantom center, soft tissue masses as small as 1 mm in diameter can be detected in a 10 cm diameter 50% glandular 50% adipose or fatter breast tissue, and 2 mm or larger masses are visible in a 100% glandular 0% adipose breast tissue. We also found that the 0.15 mm calcification can be detected for 100μm detector while only 0.2 μm or above are visible for 200 μm detector. Our simulation study has shown that the cone-beam CT breast imaging can provide reasonable good quality and
detectability at a dose level similar to that of two views\mammography. For imaging experiments, a stationary x-ray source and detector, a step motor driven rotating phantom system was constructed to demonstrate cone beam breast CT image. A breast specimen from mastectomy and animal tissue embedded with calcifications were imaged. The resulting images show that 355-425 μm calcifications were visible in images obtained at 77 kVp with a voxel size of 316 μm and a center dose of 600 mrads. 300-315 μm calcifications were visible in images obtained at 60 kVp with a voxel size of 158 μm and a center dose of 3.6 rads.
Slot scanning imaging techniques allow for effective scatter rejection without attenuating primary x-rays. The use of these techniques should generate better image quality for the same mean glandular dose (MGD) or a similar image quality for a lower MGD as compared to imaging techniques using an anti-scatter grid. In this study, we compared a slot scanning digital mammography system (SenoScan, Fisher Imaging Systems, Denver, CO) to a full-field digital mammography (FFDM) system used in conjunction with a 5:1 anti-scatter grid (SenoGraphe 2000D, General Electric Medical Systems, Milwaukee, WI). Images of a contrast-detail phantom (University Hospital Nijmegen, The Netherlands) were reviewed to measure the contrast-detail curves for both systems. These curves were measured at 100%, 71%, 49% and 33% of the reference mean glandular dose (MGD), as determined by photo-timing, for the Fisher system and 100% for the GE system. Soft-copy reading was performed on review workstations provided by the manufacturers. The correct observation ratios (CORs) were also computed and used to compare the performance of the two systems. The results showed that, based on the contrast-detail curves, the performance of the Fisher images, acquired at 100% and 71% of the reference MGD, was comparable to the GE images at 100% of the reference MGD. The CORs for Fisher images were 0.463 and 0.444 at 100% and 71% of the reference MGD, respectively, compared to 0.453 for the GE images at 100% of the reference MGD.
A photon counting energy sensitive micro-pattern gaseous detector is being developed for projection radiography. Two cascaded gas electron multipliers (GEMs) are used as gain stages following a drift region that acts as an absorption volume. Active area of the detector is 7cm x 7cm and the signal is read out by 0.2 mm pitch strips. An aluminum pressure vessel was designed as an integral part of the detector, which operates at pressures up to 10 atm. Voltage-gain curves were obtained for Ar and Kr mixtures. Energy resolution for a 5.9 keV Fe-55 was determined to be 20% FWHM. Energy spectra for a conventional x-ray generator operating at various voltages were also obtained. Detector MTF and DQE were calculated using linear systems analysis. The effect of primary electron range, a dominant factor in determining spatial resolution, and the effect of fluorescent photon reabsorption were studied. Operation of the detector for imaging at higher pressures with Kr and Xe mixtures is in progress.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.