The authors introduce an algorithm to estimate the spatial dose distributions in computed tomography (CT)
images. The algorithm calculates dose distributions due to the primary and scattered photons separately. The
algorithm only requires the CT data set that includes the patient CT images and the scanner acquisition parameters.
Otherwise the scanner acquisition parameters are extracted from the CT images. Using the developed
algorithm, the dose distributions for head and chest phantoms are computed and the results show the excellent
agreements with the dose distributions obtained using a commercial Monte Carlo code. The developed algorithm
can be applied to a patient-specific CT dose estimation based on the CT data.
Single-shot dual-energy sandwich detector can produce sharp images because of subtraction of images from two sub-detector layers, which have different thick x-ray converters, of the sandwich detector. Inspired by this observation, the authors have developed a microtomography system with the sandwich detector in pursuit of high-resolution bone-enhanced small-animal imaging. The preliminary results show that the bone-enhanced images reconstructed with the subtracted projection data are better in visibility of bone details than the conventionally reconstructed images. In addition, the bone-enhanced images obtained from the sandwich detector are relatively immune to the artifacts caused by photon starvation. The microtomography with the single-shot dual-energy sandwich detector will be useful for the high-resolution bone imaging.
We present a theoretical framework describing projections obtained from computed tomography systems considering physics of each component consisting of the systems. The projection model mainly consists of the attenuation of x-ray photons through objects including x-ray scatter and the detection of attenuated/scattered x-ray photons at pixel detector arrays. X-ray photons are attenuated by the Beers-Lambert law and scattered by using the Klein-Nishina formula. The cascaded signal-transfer model for the detector includes x-ray photon detection and light photon conversion/spreading in scintillators, light photon detection in photodiodes, and the addition of electronic noise quanta. On the other hand, image noise is considered by re-distributing the pixel signals in pixel-by-pixel ways at each image formation stage using the proper distribution functions. Instead of iterating the ray tracing over each energy bin in the x-ray spectrum, we first perform the ray tracing for an object only considering the thickness of each component. Then, we assign energy-dependent linear attenuation coefficients to each component in the projected images. This approach reduces the computation time by a factor of the number of energy bins in the x-ray spectrum divided by the number of components in the object compared with the conventional ray-tracing method. All the methods developed in this study are validated in comparisons with the measurements or the Monte Carlo simulations.
We have developed a novel sandwich-style single-shot (single-kV) detector by stacking two indirect-conversion flat-panel detectors for preclinical mouse imaging. In the sandwich detector structure, extra noise due to the direct x-ray absorption in photodiode arrays is inevitable. We develop a simple cascaded linear-systems model to describe signal and noise propagation in the flat-panel sandwich detector considering direct x-ray interactions. The noise-power spectrum (NPS) and detective quantum efficiency (DQE) obtained from the front and rear detectors are analyzed by using the cascaded-systems model. The NPS induced by the absorption of direct x-ray photons that are unattenuated within the photodiode layers is white in the spatial-frequency domain like the additive readout noise characteristic; hence that is harmful to the DQE at higher spatial frequencies at which the number of secondary quanta lessens. The model developed in this study will be useful for determining the optimal imaging techniques with sandwich detectors and their optimal design.
We revisit the doubly-layered sandwich detector configuration for single-shot dual-energy x-ray imaging. In order to understand its proper operation, we investigated the contrast-to-noise performance in terms of the x-ray beam setup using the Monte Carlo methods. Using a pair of active photodiode arrays coupled to phosphor screens, we have built a sandwich detector. For better spectral separation between the projection images obtained from the front and rear detectors during a single x-ray exposure, we inserted a copper sheet between two detectors. We have successfully obtained soft tissue- and bone-enhanced images for a postmortem mouse with the developed sandwich detector using weighted logarithmic subtraction, and the image quality was comparable to those achieved by the conventional kVp-switching technique. Although some problems to be mitigated for the optimal and practical use, for example, the scatter effect and image registration, are still left, the performance of the sandwich detector for single-shot dual-energy x-ray imaging is promising. We expect that the active sandwich detector will provide motion-artifact-free dual-energy images with a reasonable image quality.
Detectors for computed tomography (CT) typically consist of scintillator and photodiode arrays which are coupled using optical glue. Therefore, the leakage of optical photons generated in a scintillator block to neighboring pixel photodiodes through the optical glue layer is inevitable. Passivation layers to protect the silicon photodiode as well as the silicon layer itself, which is inactive to the optical photons, are another causes for the leakage. This optical crosstalk reduces image sharpness, and eventually will blur CT images. We have quantitatively investigated the optical crosstalk in CT detectors using the Monte Carlo technique. We performed the optical Monte Carlo simulations for various thicknesses of optical components in a 129 × 129 CT detector array. We obtained the coordinates of optical photons hitting the user-defined detection plane. From the coordinate information, we calculated the collection efficiency at the detection plane and the collection efficiency at the single pixel located just below the scintillator in which the optical photons were generated. Difference between the two quantities provided the optical crosstalk. In addition, using the coordinate information, we calculated point-spread functions as well as modulation-transfer functions from which we estimated the effective aperture due to the optical photon spreading. The optical crosstalk was most severely affected by the thickness of photodiode passivation layer. The effective aperture due to the optical crosstalk was about 110% of the detector pixel aperture for a 0.1 mm-thick passivation layer, and this signal blur was appeared as a relative error of about 3-4% in mismatches between CT images with and without the optical crosstalk. The detailed simulation results are shown and will be very useful for the design of CT detectors.
The modulation transfer function (MTF) is a typical parameter to measure the spatial resolution, which is an essential
factor for evaluating the performance of computed tomography (CT) systems. It is known that the CT system does not
follow the shift-invariant manner because of the cone-beam geometry and the transformation from the cylindrical
coordinates to the axial coordinates when the image reconstruction is employed. Several studies reported that if the
position of impulse receded from the center of a region of interest (ROI), the MTF degraded continuously. In this study,
the trend of shift-variant characteristics of CT systems was measured and analyzed using a novel multi-cylindrical
phantom. This study used to determine a point spread function (PSF) and MTF of a CT system using a simple cylindrical
phantom. First of all, the optimal diameter of cylinder phantoms was experimentally determined as 70 mm to obtain
reliable PSFs. Two kinds of field of views (FOVs), 40 cm and 60 cm, were used to vary reconstructed pixel sizes. The
shift-variant MTF curves were acquired at five off-center positions per FOV. For the effective analysis of MTF shiftvariance,
the integrated MTF values were calculated and used. In the result, the MTF slightly decreased as diameter
increased from CT center in the central region within the distance of 10 cm. Moreover, a considerable MTF decrease
suddenly occurred around the distance of 15 cm in the actual FOVs. The decreasing trend of the off-center spatial
resolution of CT cannot be neglected in recent radiologic and radio-therapeutic fields requiring high degree of image
precision, especially in sub-mm images. It is recommended that the ROI is laid on the CT center as close as possible. A
novel cylindrical phantom was finally suggested to effectively measure PSFs with optimal diameters for clinical FOVs in
this study. This phantom is cheap and convenient to use because it was only made of acryl with simple geometry. It is
expected that the spatial resolution of CT can be easily monitored using our methodology in clinical CT sites.
Recent developments in large-area flat-panel detectors have made tomosynthesis technology revisited in multiplanar xray
imaging. However, the typical shift-and-add (SAA) or backprojection reconstruction method is notably claimed by a
lack of sharpness in the reconstructed images because of blur artifact which is the superposition of objects which are out
of planes. In this study, we have devised an intuitive simple method to reduce the blur artifact based on an iterative
approach. This method repeats a forward and backward projection procedure to determine the blur artifact affecting on
the plane-of-interest (POI), and then subtracts it from the POI. The proposed method does not include any Fourierdomain
operations hence excluding the Fourier-domain-originated artifacts. We describe the concept of the self-layer
subtractive tomosynthesis and demonstrate its performance with numerical simulation and experiments. Comparative
analysis with the conventional methods, such as the SAA and filtered backprojection methods, is addressed.
We exploit the development of a clinical computed microtomography (micro-CT) system for dental imaging. While the
conventional dental CT simply serves implant treatment, the clinical dental micro-CT may provide clinicians with a
histologic evaluation. To investigate the feasibility of the realization of a dental micro-CT, we have constructed an
experimental test system which mainly consists of a microfocus x-ray source, a rotational subject holder, and a flat-panel
detector. The flat-panel detector is based on a matrix-addressed photodiode array coupled to a CsI:Tl scintillator. The
detective quantum efficiency (DQE) of the detector was measured as a function of magnification based on the measured
modulation-transfer function (MTF) and noise-power spectrum (NPS). The best MTF and DQE performances were
achieved at the magnification factor of 3. Similar tendency of the spatial resolving power in tomography was also
observed with a wire phantom having a 25 μm diameter. From the investigation of tomographs reconstructed from a
humanoid skull phantom, the application of magnification in the system largely reduced both signal-to-noise ratio (SNR)
and contrast-to-noise ratio (CNR) for a fixed dose at the entrance surface of the detector, 1.2 mGy, while this setup
increased the dose at the object plane from 4.7 mGy to 19.1 mGy for the magnification factor from 2 to 4, respectively.
Although the quantum mottles at the high magnification factor tackled the practical use in the clinic, the information
contained in the magnified CT images was quite promising.
For image-guided proton therapy, we investigated the feasibility of CBCT (cone-beam computed tomography) and
CBDT (cone-beam digital tomosynthesis) technologies in the gantry treatment room. A fully equipped x-ray projection
system, which was originally operated for patient alignment, in parallel to proton-beam direction was utilized for
acquiring CBCT/CBDT. The performance of the imaging detector was analyzed in terms of MTF (modulation-transfer
function), NPS (noise-power spectrum) and DQE (detective quantum efficiency). Tomographic imaging performances,
such as spatial resolving power, linearity of CT numbers, SNR (signal-to-noise ratio), and CNR (contrast-to-noise ratio),
were analyzed by using the AAPM (American Association of Physicists in Medicine) CT QC phantom. Geometric
alignment of CBCT/CBDT system was analyzed by using a calibration phantom, which consists of steal ball bearings.
The determined calibration parameters were applied to the image reconstruction procedures. The overall CBCT
performances of the system were demonstrated with reconstructed humanoid phantom images. In addition, we
implemented the CBDT with a selected number of projection views acquired for CBCT in limited angle ranges. From the
reconstructed phantom images, the CBCT system in the gantry treatment room will be very useful as a primary patient
alignment system for image-guided proton therapy. The CBDT may provide fast patient positioning with less motion
artifact and patient doses.