The purpose of this study was to evaluate the use of digital tomosynthesis (DT) for pediatric facial bone imaging. We
compared the eye lens dose and diagnostic image quality of DT facial bone exams relative to digital radiography (DR)
and computed tomography (CT), and investigated whether we could modify our current DT imaging protocol to reduce
patient dose while maintaining sufficient diagnostic image quality. We measured the dose to the eye lens for all three
modalities using high-sensitivity thermoluminescent dosimeters (TLDs) and an anthropomorphic skull phantom. To
assess the diagnostic image quality of DT compared to the corresponding DR and CT images, we performed an observer
study where the visibility of anatomical structures in the DT phantom images were rated on a four-point scale. We then
acquired DT images at lower doses and had radiologists indicate whether the visibility of each structure was adequate for
diagnostic purposes. For typical facial bone exams, we measured eye lens doses of 0.1-0.4 mGy for DR, 0.3-3.7 mGy for
DT, and 26 mGy for CT. In general, facial bone structures were visualized better with DT then DR, and the majority of
structures were visualized well enough to avoid the need for CT. DT imaging provides high quality diagnostic images of
the facial bones while delivering significantly lower doses to the lens of the eye compared to CT. In addition, we found
that by adjusting the imaging parameters, the DT effective dose can be reduced by up to 50% while maintaining
sufficient image quality.
The purpose of this study was to evaluate the diagnostic quality of digital tomosynthesis (DT) images for pediatric
imaging of the spine. We performed a phantom image rating study to assess the visibility of anatomical spinal structures
in DT images relative to digital radiography (DR) and computed tomography (CT). We collected DT and DR images of
the cervical, thoracic and lumbar spine using anthropomorphic phantoms. Four pediatric radiologists and two residents
rated the visibility of structures on the DT image sets compared to DR using a four point scale (0 = not visible; 1 =
visible; 2 = superior to DR; 3 = excellent, CT unnecessary). In general, the structures in the spine received ratings
between 1 and 3 (cervical), or 2 and 3 (thoracic, lumbar), with a few mixed scores for structures that are usually difficult
to see on diagnostic images, such as vertebrae near the cervical-thoracic joint and the apophyseal joints of the lumbar
spine. The DT image sets allow most critical structures to be visualized as well or better than DR. When DR imaging is
inconclusive, DT is a valuable tool to consider before sending a pediatric patient for a higher-dose CT exam.
Easy particle propagation (Epp) is a Monte Carlo simulation EGSnrc user code that we have developed for dose
calculation in a voxelized volume, and to generate images of an arbitrary geometry irradiated by a particle source.
The dose calculation aspect is a reimplementation of the function of DOSXYZnrc with new features added and
some restrictions removed. Epp is designed for x-ray application, but can be readily extended to trace other
kinds of particles.
Epp is based on the EGSnrc C++ class library (egspp) which makes modeling particle sources and simulation
geometries simpler than in DOSXYZnrc and other BEAM user codes based on EGSnrc code system. With Epp
geometries can be modeled analytically or voxelized geometries, such as those in DOSXYZnrc, can be used.
Compared to DOSXYZnrc (slightly modified from the official version for saving phase space information of
photons leaving the geometry), Epp is at least two times faster. Photon propagation to the image plane is
integrated into Epp (other particles possible with minor extension to the current code) with an ideal detector
defined. When only the resultant images are needed, there is no need to save the particle data. This results in
significant savings of data storage space, network load, and time for file I/O.
Epp was validated against DOSXYZnrc for imaging and dose calculation by comparing simulation results
with the same input. Epp can be used as a Monte Carlo simulation tool for faster imaging and radiation dose
We investigated the potential for digital tomosynthesis (DT) to reduce pediatric x-ray dose while maintaining
image quality. We utilized the DT feature (VolumeRad<sup>TM</sup>) on the GE Definium<sup>TM</sup> 8000 flat panel system installed in the
Winnipeg Children's Hospital. Facial bones, cervical spine, thoracic spine, and knee of children aged 5, 10, and 15 years
were represented by acrylic phantoms for DT dose measurements. Effective dose was estimated for DT and for
corresponding digital radiography (DR) and computed tomography (CT) patient image sets. Anthropomorphic phantoms
of selected body parts were imaged by DR, DT, and CT. Pediatric radiologists rated visualization of selected anatomic
features in these images. Dose and image quality comparisons between DR, DT, and CT determined the usefulness of
tomosynthesis for pediatric imaging.
CT effective dose was highest; total DR effective dose was not always lowest - depending how many
projections were in the DR image set. For the cervical spine, DT dose was close to and occasionally lower than DR
dose. Expert radiologists rated visibility of the central facial complex in a skull phantom as better than DR and
comparable to CT. Digital tomosynthesis has a significantly lower dose than CT. This study has demonstrated DT
shows promise to replace CT for some facial bones and spinal diagnoses. Other clinical applications will be evaluated in
We are investigating methods for computational scatter estimation for scatter correction in cone-beam computed
tomography. We have developed an analytical method for estimating single scatter. The paper discusses our analytical
method and its validation using Monte Carlo simulations. The paper extends previous results to include both Compton
and Rayleigh single scatter interactions. The paper also discusses the potential for hybrid scatter estimation, in which
empirical measurements of the total scatter signal in the collimator shadow may be used to augment computational single
scatter estimates and thus account for multiple scatter.
Sensors based on complementary metal oxide semiconductors (CMOS) technology have recently been
considered for mammography applications. CMOS offers the advantages of lower cost and relative ease
of fabrications. We report on the evaluation of a CMOS imager (C9730DK, Hamamatsu Corporation)
with 14-bit digitization and 50-micron detector element (del) resolution. The imager has an active area of
5 x 5 cm and uses 160-micron layer of needle-crystal CsI (55 mg/cc) to convert x-rays to light. The
detector is suitable for spot and specimen imaging and image-guided biopsy. To evaluate resolution
performance, we measured the modulation transfer function (MTF) using the slanted edge method. We
also measured the normalized noise power spectrum (NNPS) using Fourier analysis of uniform images.
The MTF and NNPS were used to determine the detective quantum efficiency (DQE) of the detector. The
detector was characterized using a molybdenum target/molybdenum filter mammography x-ray source
operated at 28 kVp with 44mm of PMMA added to mimic clinical beam quality (HVL = 0.62 mm Al).
Our analysis showed that the imager had a linear response. The MTF was 28% at 5 lp/mm and 8% at 10
lp/mm. The product of the NNPS and exposure showed that the detector was quantum limited. The DQE
near 0 lp/mm was in the 55-60% range. The DQE and MTF performance of the CMOS detector are
comparable to published values for other digital mammography detectors.
The SenoScan full-field digital mammography scanner uses a scanning slot detector that is 10 mm wide and 220 mm long. The X-ray beam is collimated to just outside the area of the detector. One important advantage of slot scanning is its inherent scatter rejection. As previously reported, the SenoScan slot scatter rejection is better than that obtained using a 3.5:1 mammography grid, and somewhat worse than that with a 5:1 grid. Additional scatter reduction can potentially improve the contrast in images of thick breasts. We evaluate a custom-designed grid for the slot scanning system. The grid is one-dimensional, offering scatter rejection along the longitudinal axis of the detector. We evaluate the reduction in scatter fraction, grid absorption and changes in the signal-difference-to-noise ratio (SDNR). Based on phantom studies, our results show effective scatter reduction by the grid with minimal reduction of SDNR. Grid absorption and scatter elimination do not necessarily lead to an increase in patient dose, especially if there is a improvement in the number of digital values in the image that are within the useful dynamic range of the detector. A benefit of removing the scatter contribution is an improvement in system dynamic range, because electronic detector gain adjustments can compensate for the drop in the digital pixel values.
We report a novel approach for statistical image reconstruction in X-ray CT. Statistical image reconstruction depends on maximizing a
likelihood derived from a statistical model for the measurements. Traditionally, the measurements are assumed to be statistically Poisson, but more recent work has argued that CT measurements actually follow a compound Poisson distribution due to the polyenergetic nature of the X-ray source. Unlike the Poisson distribution, compound Poisson statistics have a complicated likelihood that impedes direct use of statistical reconstruction. Using a generalization of the saddle-point integration method, we derive an approximate likelihood for use with iterative algorithms. In its most realistic form, the approximate likelihood we derive accounts for polyenergetic X-rays and poisson light statistics in the detector scintillator, and can be extended to account for electronic additive noise. The approximate likelihood is closer to the exact likelihood than is the conventional Poisson likelihood, and carries the promise of more accurate reconstruction, especially in low X-ray dose situations.
Dual-energy (DE) X-ray computed tomography (CT) has shown promise for material characterization and for providing quantitatively accurate CT values in a variety of applications. However, DE-CT has not been used routinely in medicine to date, primarily due to dose considerations. Most methods for DE-CT have used the filtered backprojection method for image reconstruction, leading to suboptimal noise/dose properties. This paper describes a statistical (maximum-likelihood) method for dual-energy X-ray CT that accommodates a wide variety of potential system configurations and measurement noise models. Regularized methods (such as penalized-likelihood or Bayesian estimation) are straightforward extensions. One version of the algorithm monotonically decreases the negative log-likelihood cost function each iteration. An ordered-subsets variation of the algorithm provides a fast and practical version.
This paper describes two statistical iterative reconstruction methods for X-ray CT. The first method assumes a mono-energetic model for X-ray attenuation. We approximate the transmission Poisson likelihood by a quadratic cost function and exploit its convexity to derive a separable quadratic surrogate function that is easily minimized using parallelizable algorithms. Ordered subsets are used to accelerate convergence. We apply this mono-energetic algorithm (with edge-preserving regularization) to simulated thorax X-ray CT scans. A few iterations produce reconstructed images with lower noise than conventional FBP images at equivalent resolutions. The second method generalizes the physical model and accounts for the poly-energetic X-ray source spectrum and the measurement nonlinearities caused by energy-dependent attenuation. We assume the object consists of a given number of non-overlapping tissue types. The attenuation coefficient of each tissue is the product of its unknown density and a known energy-dependent mass attenuation coefficient. We formulate a penalized-likelihood function for this poly-energetic model and develop an iterative algorithm for estimating the unknown densities in each voxel. Applying this method to simulated X-ray CT measurements of a phantom containing both bone and soft tissue yields images with significantly reduced beam hardening artifacts.
Ultrasonic imaging has been suggested for guidance of high intensity focused ultrasound therapy. This is typically implemented using two different ultrasonic transducer systems. However the need for two transducers may pose practical difficulties such as alignment and different coordinate systems. In this paper we investigate the possibility of using the same physical transducer array for performing both therapy and imaging. A spherically shaped 1D 64-element high intensity focused ultrasound transducer capable of operating in therapeutic and imaging modes was designed and fabricated. In vitro experiments were conducted to show that this transducer is capable of creating well defined lesions 30-50 mm deep into bovine muscle samples. Furthermore, an experimental pulse-echo system was designed to collect full synthetic aperture data using this transducer. Images of multiple-wire and speckle-generating phantoms are shown to illustrate the imaging capability of this transducer. Although the image quality achieved with this array is inferior to that obtained by conventional diagnostic imaging transducers, it is sufficiently high to produce image features suitable for guidance.