Energy-resolved Computed Tomography Research Articles

Urinary stone component identification by energy-resolved X-ray computed tomography

ABSTRACT Urinary stones have four main types of components: calcium, uric acid, ammonium magnesium phosphate, and cystine, and the type of treatment must be selected according to these types. Although X-ray computed tomography (CT) is currently the most promising method for differentiating the component types of urinary stones in vivo, both conventional and dual-energy CTs are subject to limits in this component differentiation. In this paper, we show the usefulness of an energy-resolved CT system for determining urinary stone components. Standard laboratory chemicals were used as substitutes for true urinary stones, and the effective atomic numbers of the chemicals were estimated to see if they could be used to identify the different types of urinary stones.

A scattered X-ray correction method for transmission measurements and its verification using energy-resolved computed tomography

ABSTRACT Scattering distorts images obtained from X-ray transmission, especially those measured using computed tomography (CT). We previously proposed a scatter-correction method involving a simple measurement on a cylindrical phantom with the same dimensions as a subject under study and a deterministic calculation. This method works well, but in practice, it requires a separate phantom size-matched to every subject. In this paper, we demonstrate an interpolated scatter correction method that uses phantoms smaller and larger than the subject. We also applied this correction method to a subject with similar shape to a human body. The corrected results were verified using energy-resolved CT, and the obtained effective atomic numbers and electron densities were compared with those in the NIST database.

Correction of X-ray scattering in energy-resolved computed tomography imaging of 20-cm-diameter phantom

ABSTRACT In computed tomography (CT) of human-sized subjects, scattered X-rays distort CT images, which medical doctors then use to diagnose diseases. X-ray scattering causes further difficulties in energy-resolved CT (ER-CT) because the scattered X-rays affect the electric currents used to calculate X-ray energy spectrum. Conventional strategies for correcting X-ray scattering have undesirable requirements such as increased dose exposure, lengthy calculations, and large amounts of input data. In this paper, we propose a scattered X-ray correction method that combines experiment and calculation. Electric current induced by both primary and scattered X-rays was measured for a ‘scatter-correction phantom’ made of acrylic, which had the same dimensions with the one having six other resins. The contribution of the primary X-rays to the electric current was estimated by a simple calculation. The scattered X-ray contribution in the electric current was given as the difference between the experimental and the calculated values. Effective atomic numbers and electron number densities of seven resins obtained by ER-CT with and without scatter correction are compared. With the correction of X-ray scattering, excellent proportionalities between estimated and theoretical values are obtained for the effective atomic number and electron number density.

Photon-counting CT review.

Photon-counting detectors are a promising new technology for computed tomography (CT) systems. They provide energy-resolved CT data at very high spatial resolution without electronic noise and with improved tissue contrasts. This review article gives an overview of the principles of photon-counting detector CT, of potential clinical benefits and limitations, and of the experience gained so far in pre-clinical installations.

A Monte Carlo study to investigate the feasibility of an on-board SPECT/spectral-CT/CBCT imager for medical linear accelerator.

The on-board flat-panel cone-beam computed tomography (CBCT) lacks molecular/functional information for current online image-guided radiation therapy (IGRT). It might not be adequate for adaptive radiation therapy (ART), particularly for biologically guided tumor delineation and targeting which might be shifted and/or distorted during the course of RT. A linear accelerator (Linac) gantry-mounted on-board imager (OBI) was proposed using a single photon counting detector (PCD) panel to achieve single photon emission computed tomography (SPECT), energy-resolved spectral CT, and conventional CBCT triple on-board imaging, which might facilitate online ART with an addition of volumetric molecular/functional imaging information. The system was designed and evaluated in the GATE Monte Carlo platform. The OBI system including a kV-beam source and a pixelated cadmium zinc telluride (CZT) detector panel mounted on a medical Linac orthogonally to the MV beam direction was designed to obtain online CBCT, spectral CT, and SPECT tri-modal imaging of patients in the treatment room. The spatial resolutions of the OBI system were determined by imaging simulated phantoms. The CBCT imaging was evaluated by a simulated contrast phantom. A PMMA phantom containing gadolinium was imaged to demonstrate quantitative imaging of spectral-CT/CBCT of the system. The capability of tri-modal imaging of the OBI was demonstrated using three different spectral CT imaging methods to differentiate gadolinium, gold, calcium within simulated PMMA and the SPECT to image radioactive 99m Tc distribution. The dual-isotope SPECT imaging of the system was also evaluated by imaging a phantom containing 99m Tc and 123 I. The radiotherapy-related parameters of iodine contrast fraction and virtual non-contrast (VNC) tissue electron density in the Kidney1 inserts of a simulated phantom were decomposed using the Bayesian eigentissue decomposition method for contrast-enhanced CBCT/spectral-CT of the OBI in a single scan. The spatial resolutions of CBCT and SPECT of the OBI were determined to be 15.1lp/cm at 10% MTF and 4.8-12mm for radii of rotation of 10-40cm, respectively. In CBCT image of the contrast phantom, most of the soft-tissue inserts were visible with sufficient spatial structure details. As compared to the CBCT image of gadolinium, the spectral CT image provided higher image contrasts. Calcium, gadolinium, and gold were separated well by using the spectral CT material imaging methods. The reconstructed distribution of 99m Tc agreed with the spatial position within the phantom. The two isotopes were separated from each other in dual-isotope SPECT imaging of the OBI. The iodine fractions and the VNC electron densities were estimated in the iodine-enhanced Kidney1 tissue inserts with reasonable RMS errors. The main procedures of the tri-modal imaging guided online ART workflow were presented with new functional features included. Using a single photon counting CZT detector panel, an on-board SPECT, spectral CT, and CBCT tri-modal imaging could be realized in Linacs. With the added online molecular/functional imaging obtained from the new OBI for the online ART proposed, the accuracy of radiation treatment delivery could be further improved.

Basic principles and clinical potential of photon-counting detector CT

Photon-counting detectors are a new technology for future computed tomography (CT) systems, with the potential to overcome major limitations of conventional CT detectors. They provide energy-resolved CT data at very high spatial resolution without electronic noise. This review article gives an overview of the basic principles of photon-counting detector CT, of potential benefits and limitations, and of the clinical experience gained so far in pre-clinical installations.

Full-Spectrum-Knowledge-Aware Tensor Model for Energy-Resolved CT Iterative Reconstruction.

Energy-resolved computed tomography (ErCT) with a photon counting detector concurrently produces multiple CT images corresponding to different photon energy ranges. It has the potential to generate energy-dependent images with improved contrast-to-noise ratio and sufficient material-specific information. Since the number of detected photons in one energy bin in ErCT is smaller than that in conventional energy-integrating CT (EiCT), ErCT images are inherently more noisy than EiCT images, which leads to increased noise and bias in the subsequent material estimation. In this work, we first deeply analyze the intrinsic tensor properties of two-dimensional (2D) ErCT images acquired in different energy bins and then present a F ull- S pectrum-knowledge-aware Tensor analysis and processing (FSTensor) method for ErCT reconstruction to suppress noise-induced artifacts to obtain high-quality ErCT images and high-accuracy material images. The presented method is based on three considerations: (1) 2D ErCT images obtained in different energy bins can be treated as a 3-order tensor with three modes, i.e., width, height and energy bin, and a rich global correlation exists among the three modes, which can be characterized by tensor decomposition. (2) There is a locally piecewise smooth property in the 3-order ErCT images, and it can be captured by a tensor total variation regularization. (3) The images from the full spectrum are much better than the ErCT images with respect to noise variance and structural details and serve as external information to improve the reconstruction performance. We then develop an alternating direction method of multipliers algorithm to numerically solve the presented FSTensor method. We further utilize a genetic algorithm to tackle the parameter selection in ErCT reconstruction, instead of manually determining parameters. Simulation, preclinical and synthesized clinical ErCT results demonstrate that the presented FSTensor method leads to significant improvements over the filtered back-projection, robust principal component analysis, tensor-based dictionary learning and low-rank tensor decomposition with spatial-temporal total variation methods.

Effective atomic number estimation by energy-resolved X-ray computed tomography with a current-mode detector system

We have developed a current-mode detector system, called a “transXend” detector, consisting of several segmented detectors aligned along the direction of X-ray incidence, for energy-resolved computed tomography (CT). The transXend detector measures X-rays as electric currents and after analysis gives the energy spectrum of the incident X-ray. In previous work, we measured the linear attenuation coefficients of resins as a function of X-ray energy with the energy-resolved CT using the transXend detector. This paper shows a method to estimate the effective atomic number and electron density of nine resins from the linear attenuation coefficients using a photon–atom cross-section lookup table. The estimated and agreed well with calibration lines with accuracies of about 2%.

Filter-based energy-resolved X-ray computed tomography with a clinical imager

ABSTRACT Filter-based energy-resolved X-ray computed tomography (CT) is an approach for implementing energy-resolved CT imaging using a flat-panel-detector-based cone-beam system. In this study, we performed experiments with a 20-cm-diameter phantom on a clinical X-ray imager. The material density results showed good agreement with the ideal values. We also propose an improved method for obtaining the detector response function and the X-ray spectrum, which requires fewer measurements and will be practical for future clinical use. Issues such as scatter and image noise remain to be addressed.

Effective Atomic Number Measurement with Energy-Resolved Computed Tomography Using Two-Dimensional “transXend” Detector

Introduction: We have previously developed an effective atomic number (Zeff) measurement method using linear attenuation coefficients (LACs) obtained by energy-resolved computed tomography (CT) with one-dimensional (1D) detector. The energy-resolved CT was performed with a “transXend” detector, which measured X-rays as electric current and then gave X-ray energy distribution with unfolding analysis using pre-estimated response function (RF). The purpose of this study is to measure Zeff by the energy-resolved CT using a flat panel detector (FPD). Methods: To demonstrate a 2D transXend detector, we developed the stripe absorbers for the FPD. Eleven human tissue-equivalent material rods which were grouped into four material categories were measured by X-rays with 120 kVp tube voltage, 2.3 mA tube current, and 1.0 s exposure time. Zeff is measured by the ratio of LACs with two different pseudo-monochromatic X-ray energies. RFs of each rod material were estimated by numerical calculation. First, we employed the RF estimated for the same rod material (self-RF scenario). Second, we employed the RF estimated for the different rod materials in the same material category (cross-RF scenario). The purpose of the cross-RF scenario was to find representative rod materials in each material category. Results: Upon the self-RF scenario, measured Zeffs were systematically underestimated. Median relative error to theoretical Zeff was -6.92% (range: -7.89% - -4.60%). After normalizing measured Zeffs to the theoretical one for Breast, median relative error improved to -0.75% (range: -1.79% - +1.73%). Upon the cross-RF scenario, the representative rod materials were found in two material categories. Conclusion: Zeff measurements were performed by energy-resolved CT using 2D transXend detector with numerically-estimated RF data. Normalized Zeffs for all rod materials in the self-RF scenario were in good agreement with the theoretical ones.

A simulation study on the influence of scattered X-rays in energy-resolved computed tomography

ABSTRACTIn computed tomography (CT), scattered X-rays are known to cause image quality degradation including low contrast and poor uniformity. Energy-resolved CT (ERCT) is a specialized imaging method that utilizes the measured energy distribution and intensity of detected X-rays when creating images. The influence of scatter in ERCT is expected to differ from conventional CT, which only relies on energy (charge) integration during detection. Monte Carlo simulations were performed with different X-ray beam qualities and contrast agents to determine the influence of scatter on ERCT images. The simulation geometry included a 16 cm diameter cylindrical polymethylmethacrylate phantom containing a 1 cm diameter region filled with iodine or gold contrast agent solution at the center. A detector array collected the photon energy, position, and number of scattering. The image contrast and scatter artifact in the simulated ERCT images were compared as a function of photon energy range. Results show that the scatter artifact in ERCT is correlated to energy and is affected by the beam hardness. Care should be taken while obtaining ERCT images in the low-energy region, specifically near the K-edge of iodine.

Two-dimensional “transXend” detector with band structure absorbers for third-generation energy-resolved computed tomography with improved spatial resolution

ABSTRACTEnergy-resolved computed tomography (CT) is a method that can generate multiple CT images reconstructed by the X-rays in specific energy ranges. We have previously discussed a lattice absorber type of X-ray detector for energy-resolved CT. We have now developed a band absorber type detector with the aim of improving spatial resolution. We compare spatial resolution and contrast-to-noise ratios for the two types of detectors.

Dual-Source Multi-Energy CT with Triple or Quadruple X-ray Beams.

Energy-resolved photon-counting CT (PCCT) is promising for material decomposition with multi-contrast agents. However, corrections for non-idealities of PCCT detectors are required, which are still active research areas. In addition, PCCT is associated with very high cost due to lack of mass production. In this work, we proposed an alternative approach to performing multi-energy CT, which was achieved by acquiring triple or quadruple x-ray beam measurements on a dual-source CT scanner. This strategy was based on a "Twin Beam" design on a single-source scanner for dual-energy CT. Examples of beam filters and spectra for triple and quadruple x-ray beam were provided. Computer simulation studies were performed to evaluate the accuracy of material decomposition for multi-contrast mixtures using a tri-beam configuration. The proposed strategy can be readily implemented on a dual-source scanner, which may allow material decomposition of multi-contrast agents to be performed on clinical CT scanners with energy-integrating detector.

LabPET II, an APD-based Detector Module with PET and Counting CT Imaging Capabilities

Computed tomography (CT) is currently the standard modality to provide anatomical reference for positron emission tomography (PET) in molecular imaging applications. Since both PET and CT rely on detecting radiation to generate images, using the same detection system for data acquisition is a compelling idea even though merging PET and CT hardware imposes stringent requirements on detectors. These requirements include large signal dynamic range with high signal-to-noise ratio for good energy resolution in PET and energy-resolved photon-counting CT, high pixelization for suitable spatial resolution in CT, and high count rate capability for reasonable CT acquisition time. To meet these criteria, the avalanche photodiode (APD)-based LabPET II module is proposed as the building block for a truly combined PET/CT scanner. The module is made of two monolithic $ 4\times 8$ APD pixel arrays mounted side-by-side on a custom ceramic holder. Individual APD pixels have an active area of $ 1.1\times 1.1~\hbox{mm}^{2}$ at a 1.2 mm pitch. The APD arrays are coupled to a 12-mm high, $8 \times 8$ LYSO scintillator array made of $1.12 \times 1.12~\hbox{mm}^{2}$ pixels also at a pitch of 1.2 mm to ensure direct one-to-one coupling to individual APD pixels. The scintillator array was designed with unbound specular reflective material between pixels to maximize the difference between refractive indices and enhance total internal reflection at the crystal side surfaces for better light collection, and the APD quantum efficiency was improved to $ \sim60\%$ at 420 nm to optimize intrinsic detector performance. Mean energy resolution was $20 \pm 1\%$ at 511 keV and $ 41\pm 4\%$ at 60 keV. The measured intrinsic spatial and time resolution for PET were respectively $0.81 \pm 0.04~\hbox{mm}~{\rm FWHM}/1.57 \pm 0.04~\hbox{mm}$ FWTM and $ 3.6\pm 0.3~\hbox{ns}$ FWHM with an energy threshold of 400 keV. Initial phantom images obtained using a CT test bench demonstrated excellent contrast linearity as a function of material density. With a magnification factor of 2, a CT spatial resolution of 0.66 mm FWHM/1.2 mm FWTM, corresponding to 1.18 lp/mm at ${\rm MTF}_{10\%}/0.67~\hbox{lp/mm}$ at ${\rm MTF}_{50\%}$ , was measured, allowing 0.75 mm air holes in an Ultra-Micro Hot Spot resolution phantom to be clearly distinguished.

Two-dimensional “transXend” detector for third-generation energy-resolved computed tomography

The “transXend” detector measures X-rays as electric currents and provides the energy distribution of the measured X-rays after analysis. Capabilities of material distinction, effective atomic number measurement, and low-dose exposure computed tomography (CT) with high K-edge contrast agent from the use of the transXend detector in energy-resolved CT have been demonstrated via the first-generation CT measurements. For application of the principle of the transXend detector to the third-generation CT for human subjects in future work, a method for fabrication of a two-dimensional transXend detector is proposed and demonstrated using a commercial two-dimensional detector and two kinds of strip absorbers. The energy-resolved CT is performed by placing a proposed absorber system in front of a two-dimensional detector, which is used for conventional current measurement CT.

Effective atomic number measurement with energy-resolved X-ray computed tomography

Abstarct The effective atomic number, Z eff , is measured by the ratio of linear attenuation coefficients at two different X-ray energies obtained by monochromatic X-ray computed tomography (CT). However, monochromatic X-rays are available in synchrotron facilities only. By defining narrow energy ranges in energy-resolved CT using a current mode “transXend” detector, Z eff is measured for sugar and K2HPO4 solutions, as well as for water and acrylic. The experimental Z eff values obtained for water and acrylic were in excellent agreement with the theoretical values. The experimental Z eff values obtained for the sugar and K2HPO4 solutions had larger errors.

Energy-resolved CT imaging with a photon-counting silicon-strip detector

Photon-counting detectors are promising candidates for use in the next generation of x-ray computed tomography (CT) scanners. Among the foreseen benefits are higher spatial resolution, better trade-off between noise and dose and energy discriminating capabilities.Silicon is an attractive detector material because of its low cost, mature manufacturing process and high hole mobility. However, it is sometimes overlooked for CT applications because of its low absorption efficiency and high fraction of Compton scatter. The purpose of this work is to demonstrate that silicon is a feasible material for CT detectors by showing energy-resolved CT images acquired with an 80 kVp x-ray tube spectrum using a photon-counting silicon-strip detector with eight energy thresholds developed in our group.We use a single detector module, consisting of a linear array of 50 0.5 × 0.4 mm detector elements, to image a phantom in a table-top lab setup. The phantom consists of a plastic cylinder with circular inserts containing water, fat and aqueous solutions of calcium, iodine and gadolinium, in different concentrations.By using basis material decomposition we obtain water, calcium, iodine and gadolinium basis images and demonstrate that these basis images can be used to separate the different materials in the inserts. We also show results showing that the detector has potential for quantitative measurements of substance concentrations.

Measurement of effective atomic numbers using energy-resolved computed tomography

For ion radiation therapy, the measurement of effective atomic numbers, Zeff, is necessary to know the material distribution in a human body; the range of ions entering the human body is influenced by the material distribution along their paths. Zeff, however, cannot be measured at hospitals because monochromatic X-rays with different energies are necessary and are used only at synchrotron facilities. To make Zeff measurements at hand, we propose energy-resolved computed tomography (CT) using a “transXend detector”. By assigning two narrow energy ranges in the unfolding process of the data obtained by the transXend detector, Zeff for acrylic and aluminum can be estimated by energy-resolved CT. The estimated Zeff are compared with those obtained by dual-energy and monochromatic X-ray CT.

Using energy-resolved X-ray computed tomography with a current mode detector to distinguish materials

In conventional X-ray computed tomography (CT), X-rays are measured as electric current. Materials inside a subject are described by the linear attenuation coefficients averaged by the energy spectrum of the X-rays. A CT image cannot distinguish materials such as iodine and calcium, because the linear attenuation coefficient is not inherent to a material, but the product of X-ray mass attenuation coefficient and the density of the material. Materials such as iodine and calcium can be distinguished using an energy-resolved CT technique, with a current-mode detector system, using segment detectors aligned in the direction of X-ray incidence: the energy-resolved CT images are reconstructed by the X-rays with the energy of interest, by unfolding electric currents measured by the segment detectors.

Low-dose exposure energy-resolved X-ray computed tomography using a contrast agent with a high-energy K-edge

X-ray computed tomography (CT) with iodine contrast agent is widely employed to locate cancers. However, this method has shortcomings such as high-radiation dose exposure, iodine side effects, and a beam hardening effect. We have been working on the energy-resolved CT measurement method using a novel X-ray detection system, the “transXend” detector, which measures X-rays as electric currents and gives the energy distribution of incident X-rays after analysis. In the present study, we propose a method for low-dose exposure CT that involves the combination of the energy-resolved CT method, which is free from the beam hardening effect, and a harmless contrast agent with high-energy K-edge absorption, such as gold nanoparticles expected as a future contrast agent. Comparisons of radiation dose exposures as functions of aluminum filter thickness at the exit aperture of an X-ray tube and the K-edge energies of contrast agents are described.