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.
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.
Currently, the patient dose of proton therapy and treatment planning are based on the x-ray photon CT (computed
tomography) data. However, material interactions between protons and photons are different. Proton tomograms are
useful to make with accuracy the dose calculations for planning and positioning of patients. For applying CT techniques,
many projection images during at least half rotation are need. It makes extremely high patient dose. We apply the proton
tomosynthesis, which is the limited angle tomography using
proton-beams. The proton tomosynthesis may provide more
accuracy of dose calculation and verifications with less patient dose. We describe the concept of the proton
tomosynthesis and demonstrate its performance with GEANT4 simulation and experiments. We confirmed the feasibility
of proton digital tomosynthesis through comparative analysis with the x-ray photon methods. This study is useful for
proton therapy planning and verification.
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.
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.