Björn Heismann

Density and atomic number measurements with spectral x-ray attenuation method

X-ray attenuation measurements are widely used in medical and industrial applications. The usual results are one- to three-dimensional representations of the attenuation coefficient μ(r). In this paper, we present the ρZ projection algorithm for obtaining the density ρ(r) and atomic number Z(r) with an energy-resolving x-ray method. As input data the algorithm uses at least two measurements μ1,μ2,… with different spectral weightings of the source spectrum S(E) and/or detector sensitivity D(E). Analytically, ρ is a function of μ1−cμ2, c=const, and Z is a function of μ1/μ2. The full numerical treatment yields ρ(μ1,μ2) and Z(μ1,μ2) with S(E) and D(E) as commutative parametric functions. We tested the method with dual-energy computed tomography measurements of an organic sample and a set of chemical solutions with predefined ρ and Z. The resulting images show ρ and Z as complementary information: The density ρ reflects the morphology of the objects, whereas the atomic number Z=number of electrons/atom describ...

Quantitative image-based spectral reconstruction for computed tomography

Computed tomography (CT) devices are routinely employed to obtain three-dimensional images of the human body. The reconstructed CT numbers represent weighted x-ray attenuation coefficients. Their spectral weighting is influenced by the selected x-ray source spectrum, the detector characteristics, and the attenuating object itself. The quantitative ground truth of the scanned object is given by the spectral attenuation coefficient. It is not directly measurable in standard CT. For spectral CT measurements, algorithms like the basis material decomposition yield parametrized representations of the spectral mass attenuation coefficient. In practical applications, image-based formulations are commonly used. They are affected by both the CT system characteristics and the object self-attenuation effects. In this article the authors introduce an image-based spectral CT method. It expresses measured CT data as a spectral integration of the spectral attenuation coefficient multiplied by a local weighting function (LWF). The LWF represents the local energy weighting in the image domain, taking into account the system and reconstruction properties and the object self-attenuation. A generalized image-based formulation of spectral CT algorithms is obtained, with no need for additional corrections of, e.g., beam hardening. The iterative procedure called local spectral reconstruction yields both the mass attenuation coefficients of the object and a representation of the LWF. The quantitative accuracy and precision of the method are investigated in several applications: First, beam hardening corrections to various target energy weightings and attenuation correction maps for SPECT/CT and PET/CT are calculated. Second, an iodine density evaluation is performed. Finally, a direct identification of spectral attenuation functions using the LWF result is demonstrated. In all applications, the ground truth of the objects is reproduced with a quantitative accuracy in the subpercent to 2% range. An exponential convergence behavior of the iterative procedure is observed, with one to two iteration steps as a good compromise between quantitative accuracy and precision. The authors conclude that the method can be used to perform image-based spectral CT reconstructions with quantitative accuracy. Existing algorithms benefit from the intrinsic treatment of beam hardening and system properties. Novel algorithms are enabled to directly compare material model functions to spectral measurement data.

Sequence-based acoustic noise reduction of clinical MRI scans.

Clinical MRI patients typically experience elevated acoustic noise levels of 80–110 dB(A). In this study, standard clinical turbo spin echo (TSE) and gradient echo (GRE) sequences were optimized for reduced acoustic noise at preserved diagnostic image quality.

Single and dual energy attenuation correction in PET/CT in the presence of iodine based contrast agents

In present positron emission tomography (PET)/computed tomography (CT) scanners, PET attenuation correction is performed by relying on the information given by a single CT scan. The scaling of the linear attenuation coefficients from CT x-ray energy to PET 511 keV gamma energy is prone to errors especially in the presence of CT contrast agents. Attenuation correction based upon two CT scans at different energies but performed at the same time and patient position should reduce such errors and therefore improve the accuracy of the reconstructed PET images at the cost of introduced additional noise. Such CT scans could be provided by future PET/CT scanners that have either dual source CT or energy sensitive CT. Three different dual energy scaling methods for attenuation correction are introduced and assessed by measurements with a modified NEMA 1994 phantom with different CT contrast agent concentrations. The scaling is achieved by differentiating between (1) Compton and photoelectric effect, (2) atomic number and density, or (3) water-bone and water-iodine scaling schemes. The scaling method (3) is called hybrid dual energy computed tomography attenuation correction (hybrid DECTAC). All three dual energy scaling methods lead to a reduction of contrast agent artifacts with respect to single energy scaling. The hybrid DECTAC method resulted in PET images with the weakest artifacts. Both, the hybrid DECTAC and Compton/photoelectric effect scaling resulted also in images with the lowest PET background variability. Atomic number/density scaling and Compton/photoelectric effect scaling had problems to correctly scale water, hybrid DECTAC scaling and single energy scaling to correctly scale Teflon. Atomic number/density scaling and hybrid DECTAC could be generalized to reduce these problems.

Noise transfer analysis of base material decomposition methods

A generalized method to evaluate the noise transfer properties of the base material decomposition has been developed. We apply the method to a typical dual-energy CT scan with energy weightings and doses of a 80kV / 140kV scan. For sets {P1, P2} of dual-energy projections with Pi = 10-4.5 ... 1, both the water and bone decomposition and the Compton and Photo Effect decomposition are analyzed. As a figure of merit we determine the noise amplification factors A1, A2. They are given by the ratio of the relative noise of the dual-energy projections B1, B2 to the relative noise of the combined projection data P. The B1, B2 and their variance are simulated by numerical inversion and integration. For the water and bone decomposition an average noise amplification of 3 to 5 is shown. For small contributions of one base material, the noise amplification becomes critically large. In this case the water and bone base material decomposition seems not to be usable for quantitative CT. The Compton and Photo effect decomposition are shown to be more robust in this respect. Physically, both coefficients can only reach zero simultaneously. The Compton coefficient has significantly better noise characteristics than the Photo Effect coefficient. For a partial region of the P1, P2 plane it shows better noise performance than the combined raw data P.

Impact of Photon Transport Properties on the Detection Efficiency of Scintillator Arrays

For spatially resolved X- and gamma ray detection pixelated scintillator arrays are used. In this study we simulate the quantum gain process of a scintillator array and its impact on the detective quantum efficiency of the detector system. The simulation tool comprises a full physical Monte-Carlo model of the X-ray interactions as well as the transport processes of the scintillation photons within the detector system. As an example, we analyze an Gd2O2S:Pr scintillator array with typical 1 mm pixel pitch and TiO2 based reflective material. The results indicate that for integrating systems fluorescence escape effects play a major role in the noise performance of scintillating pixel detectors. Additionally, the light generation and transport processes can have an impact on the signal-to-noise ratio.

Spectral and spatial resolution of semiconductor detectors in medical X- and gamma ray imaging

In X- and gamma ray based medical systems, detector performance is a key driver for diagnostic quality. Over the last years and decades, indirect conversion scintillator detectors have become the standard for many medical applications including X-ray Radiography, Computed Tomography and SPECT. Recently, direct conversion semiconductor detectors based on CdTe and CdZnTe (CZT) have come into focus, as they might offer improved or additional performance for specific applications.

SNR performance comparison of dual-layer detector and dual-kVp spectral CT

Tube-based dual-kVp and detector-based dual- layer sandwich detector CT systems can be used to generate spectral CT data. We investigate the influence of their energy weighting functions on the performance of reconstructed base material coefficient images and projections. A Monte-Carlo (MC) simulation of the noise propagation for typical water and bone CT projections is used. We find a large difference in the SNR of the reconstructed coefficient projections. Dual-kVp scanning methods yield a factor of 2 better SNR in the coefficient projections than dual-layer detector based CT. This can be explained by the degree of overlap of the weighting functions, especially for low-energy X-ray quanta.

Quantitative CT characterization of body fluids with spectral ρZ projection method

We have performed a clinical study on the characterization of body fluids with the spectral ρZ projection algorithm. It converts dual-energy CT scans into density ρ and atomic number Z information. We provide data on a total of 56 samples of 6 different classes. Blood, gall, pus and urine, as well as mixtures of blood + pus and blood + ethylendiamintetra acetate (EDTA, a blood anti-coagulant) have been investigated. For standard measurements with 80 kV and 140 kV tube voltages we encounter larger overlaps of the HU attenuation values. Gall fluid and pus are the only distinguishable substances. For the ρZ projected image data, we find blood, pus and the blood + pus mixtures to be clearly distinguishable in the two-dimensional ρZ plane. Gall fluid shows a small overlap to pus. The density and the atomic number values have standard deviations in the order of Δρ = 10 mg/cm3 and ΔZ = 0.1. The theoretical density and atomic number values for blood and water are reproduced with very small deviations in the range of Δρ = -20 mg/cm3 and ΔZ = 0.02.

Atomic number measurement precision of spectral decomposition methods for CT

We have investigated the classical attenuation decomposition and base material decomposition CT algorithms invented by Alvarez and Macovski in 1976 for their ability to provide quantitative atomic number Z of a given attenuator. For this purpose we have generated attenuation functions of the elements from Z = 1 to Z = 20 from measured attenuation data. We have fitted these functions to two sets of base functions: 1. the photoelectric absorption + Klein-Nishina scattering model and 2. the effective bone and water attenuator model. The results indicate model mismatches in the range of DeltaZ = plusmn0.5 for the base material decomposition and DeltaZ = plusmn0.7 for the attenuation decomposition