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M Kachelriess

Publications and source records attributed to M Kachelriess.

At least 19 recordsLinked to original sources
PubMed2005

Measuring the 13 neutrino mixing angle and the CP phase with neutrino telescopes.

The observed excess of high-energy cosmic rays from the Galactic plane in the energy range around 10(18) eV may be explained by neutron primaries generated in the photodissociation of heavy nuclei. In this scenario, lower-energy neutrons decay before reaching the Earth and produce a detectable flux in a 1 km(3) neutrino telescope. The initial flavor composition of the neutrino flux, phi(nu(e)):phi(nu(mu)):phi(nu(tau))=1:0:0, permits a combined nu(mu)/nu(tau) appearance and nu(e) disappearance experiment. The observable flux ratio phi(nu(mu))/phi(nu(e)+nu(tau) at Earth depends on the 13 mixing angle theta(13) and the leptonic CP phase delta(CP), thus opening a new way to measure these two quantities.

Journal Article↗
PubMed2005

Reconstruction from truncated projections in CT using adaptive detruncation.

If the object exceeds the field of measurement (FOM) of a given CT scanner, severe artifacts may result. In this work, we propose an adaptive detruncation (ADT) method to reconstruct images from medical CT projections which are truncated in the transaxial direction. The truncated projections are extrapolated by estimating the convex hull of the patient. The ADT method allows us not only to achieve artifact-free images in the FOM but also to extend the images beyond the FOM, and can therefore be very attractive, for example, in PET/CT scanners for attenuation correction.

Algorithms↗
PubMed2005

Image quality of volume rendering of the bronchial tree: Kymogram-gated versus retrospectively ECG-gated and non-gated multi-row detector CT.

RATIONALE AND OBJECTIVE: To compare the image quality of three different heart-cycle-synchronized computed tomography (CT) reconstruction algorithms for volume-rendered (VR) 3D visualization of the bronchial tree. MATERIALS AND METHODS: Kymogram-gated, retrospectively ECG-gated, and non-ECG-gated reconstructions of the bronchial tree were performed from 4-detector-row CT data in 10 subjects. The raw data were reconstructed in 10 phases of the cardiac cycle using ECG-gated and kymogram-gated technique, respectively. For both reconstructions, the optimal artifact-free diastolic phase was determined. VR reconstructions of the bronchial tree were generated from these two data sets and from the non-gated data. Stairstep artifacts of the main bronchi, artifacts in the lung parenchyma, and the extent of bronchial tree visualization were rated by two blinded, independent readers. RESULTS: Kymogram-gated reconstruction showed stairstep artifacts in the main bronchi to the same extent as non-gated reconstruction, but less compared to ECG-gated reconstruction (P < .001). Artifacts in the lung parenchyma were similar with kymogram-gated and non-gated reconstruction, but less compared to ECG-gated reconstruction (P < 0.01). Kymogram-gated reconstruction showed no differences in visualization of segmental and subsegmental bronchi compared to ECG-gated reconstruction, but was inferior to the non-gated reconstruction (P < 0.05). CONCLUSION: Kymogram-gated reconstruction of CT data results in fewer artifacts when compared to the ECG-gated algorithm. Best visualization of the bronchial tree at a low artifact level is obtained with non-gated CT data. It remains therefore the method of choice for VR 3D data post-processing of the bronchial tree.

Adult↗
PubMed2003

[Improvement of the image quality of MSCT of the pelvis with a raw data-based, multidimensional filter].

PURPOSE: To evaluate the potential of raw data-based multidimensional adaptive filtering (MAF) by determining its effects on image noise and image quality in multi-slice spiral CT (MSCT) of the pelvis. MATERIALS AND METHODS: Fifty patients with rectal and bladder cancer were examined with MSCT using a high-resolution protocol. Reconstructions were performed with dedicated reconstruction software and a standard algorithm, both without and with MAF, with four different modification fractions selected from 5 % to 20 %. The noise was measured at six anatomic sites of the pelvis. Image quality and image noise were rated on a 5-point-scale. RESULTS: Modification fractions of 15 % (15 % MAF) and 20 % (20 % MAF) significantly reduced the noise level at all measurement points in comparison with lower modification fractions (p < 0.05). Overall quality of the reconstructed images was rated better with 15 % MAF and 20 % MAF than with other modification fractions (p < 0.05). No further improvement of the image quality was observed by changing from 15 % MAF to 20 % MAF (p > 0.05). 15 % MAF achieved a mean noise reduction of 33 %. All examinations showed an improved visualization of the rectal wall and perirectal lymph nodes. CONCLUSIONS: MAF improves the image quality by reducing the noise level and by removing noise structures. This technique offers new perspectives to radiation dose reduction in CT.

Humans↗
PubMed2002

Electroweak jet cascading in the decay of superheavy particles.

We study decays of superheavy particles X into leptons. We show that they initiate cascades similar to QCD parton jets, if m(X) greater or similar 10(6) GeV. Electroweak cascading is studied and the energy spectra of the produced leptons are calculated in the framework of a broken SU(2) model of weak interactions. As application, important for the Z-burst model for ultrahigh energy cosmic rays, we consider decays of superheavy particles coupled on tree level only to neutrinos and derive a stringent limit for these decays from the observed diffuse extragalactic gamma-ray flux.

Journal Article↗
PubMed2001

Generalized multi-dimensional adaptive filtering for conventional and spiral single-slice, multi-slice, and cone-beam CT.

In modern computed tomography (CT) there is a strong desire to reduce patient dose and/or to improve image quality by increasing spatial resolution and decreasing image noise. These are conflicting demands since increasing resolution at a constant noise level or decreasing noise at a constant resolution level implies a higher demand on x-ray power and an increase of patient dose. X-ray tube power is limited due to technical reasons. We therefore developed a generalized multi-dimensional adaptive filtering approach that applies nonlinear filters in up to three dimensions in the raw data domain. This new method differs from approaches in the literature since our nonlinear filters are applied not only in the detector row direction but also in the view and in the z-direction. This true three-dimensional filtering improves the quantum statistics of a measured projection value proportional to the third power of the filter size. Resolution tradeoffs are shared among these three dimensions and thus are considerably smaller as compared to one-dimensional smoothing approaches. Patient data of spiral and sequential single- and multi-slice CT scans as well as simulated spiral cone-beam data were processed to evaluate these new approaches. Image quality was assessed by evaluation of difference images, by measuring the image noise and the noise reduction, and by calculating the image resolution using point spread functions. The use of generalized adaptive filters helps to reduce image noise or, alternatively, patient dose. Image noise structures, typically along the direction of the highest attenuation, are effectively reduced. Noise reduction values of typically 30%-60% can be achieved in noncylindrical body regions like the shoulder. The loss in image resolution remains below 5% for all cases. In addition, the new method has a great potential to reduce metal artifacts, e.g., in the hip region.

Algorithms↗
PubMed2001

Advanced single-slice rebinning for tilted spiral cone-beam CT.

Future medical CT scanners and today's micro CT scanners demand cone-beam reconstruction algorithms that are capable of reconstructing data acquired from a tilted spiral trajectory where the vector of rotation is not necessarily parallel to the vector of table increment. For the medical CT scanner this case of nonparallel object motion is met for nonzero gantry tilt: the table moves into a direction that is not perpendicular to the plane of rotation. Since this is not a special application of medical CT but rather a daily routine in head exams, there is a strong need for corresponding reconstruction algorithms. In contrast to medical CT, where the special case of nonperpendicular motion is used on purpose, micro CT scanners cannot avoid aberrations of the rotational axis and the table increment vector due to alignment problems. Especially for those micro CT scanners that have the lifting stage mounted on the rotation table (in contrast to setups where the lifting stage holds the rotation table), this kind of misalignment is equivalent to a gantry tilt. We therefore generalize the advanced single-slice rebinning algorithm (ASSR), which is considered a very promising approach for medical cone-beam reconstruction due to its high image quality and its high reconstruction speed [Med. Phys. 27, 754-772 (2000)], to the case of tilted gantries. We evaluate this extended ASSR approach (which we will denote as ASSR+, for convenience) in comparison to the original ASSR algorithm using simulated phantom data for reconstruction. For the case of nonparallel object motion ASSR+ shows significant improvements over ASSR, however, its computational complexity is slightly increased due to the broken symmetry of the spiral trajectory.

Algorithms↗
PubMed2000

Noninvasive coronary angiography by retrospectively ECG-gated multislice spiral CT.

BACKGROUND: We investigated the applicability and image quality of contrast-enhanced coronary artery visualization by multislice spiral CT using retrospective ECG gating. METHODS AND RESULTS: Twenty-five patients in sinus rhythm (significant coronary artery stenoses ruled out by invasive angiography) were studied with a multislice spiral CT (Siemens SOMATOM Volume Zoom). In inspiration (mean breath-hold, 37 seconds), a volume data set of the heart was acquired (intravenous contrast agent; 4 x 1-mm slice thickness; 500-ms rotation; table feed, 1.5 mm/360 degrees ). Simultaneous recording of the ECG permitted retrospective reconstruction of contiguous cross sections in intervals of 1 mm at any desired interval of the cardiac cycle. The mean duration of the image reconstruction window was 185 ms. Next to 3-dimensional reconstructions of the heart and coronary arteries, multiplanar reconstructions were rendered to determine the visualized length of the coronary arteries, the contrast-to-noise ratio, and the correlation of coronary artery diameters to quantitative coronary angiography. CONCLUSIONS: The coronary arteries could be visualized over long segments (left main, 9+/-4 mm; left anterior descending, 112+/-34 mm; left circumflex, 80+/-29 mm; right coronary artery, 116+/-33 mm). On average, 78+/-16% of these distances were visualized free of motion artifacts. The mean contrast-to-noise ratio was 9.3+/-3.3. Coronary artery diameters in multislice spiral CT showed close correlation to quantitative coronary angiography (CT, 3.3+/-1.0 mm; angiography, 3. 2+/-0.9 mm; mean difference, 0.38 mm; r=0.86). Contrast-enhanced multislice spiral CT permits visualization of the coronary artery lumen. Further studies are necessary to determine whether image quality is sufficient to reliably detect coronary artery stenoses.

Adult↗
PubMed2000

[Subsecond multislice spiral CT as an alternative to electron beam computerized tomography].

X-ray computed tomography (CT) has exhibited rapid technological advances in recent years which has made it a potential alternative to electron beam computed tomography (EBCT). In addition to the development of spiral CT, rotation times in the subsecond range and the development of multislice detectors were decisive steps. These technical developments will be sketched briefly; the necessary adaptation of image reconstruction and evaluation will be explained. Particular emphasis will be placed on the necessary steps of quality assurance and calibration in quantitative procedures, as for example coronary calcium measurements. The exposure for typical CT examinations of the heart are in the order of half to five times the natural exposure per year. With synchronous recording of the ECG, the complete heart can be imaged continuously with thin slices and high spatial resolution in less than 30 s. In spite of the very short validation phase up to now, we consider multislice spiral CT an alternative to EBCT.

Calibration↗
PubMed2000

Technical advances in multi-slice spiral CT.

X-ray computerised tomography (CT) scanning with continuous patient transport has been established under the name Spiral CT since several years as the standard clinical examination procedure. This technique has been improved continuously with respect to scan speed, temporal response and z-axis resolution by the use of latest technical developments: Rotation times up to 0.5 s and multi-row detector array systems. Today detector systems with M + 4 simultaneously measured slices are available. We report about recent progress of spiral CT reconstruction algorithms that are based on multi-slice data. It is demonstrated that the new technology not only provides significant reduction in overall scan times and thereby of the CT scanner?s X-ray tube load; beyond that, the new technology allows CT imaging of the beating heart with high level image quality in standard clinical routine.

Algorithms↗
PubMed2000

Single-slice rebinning reconstruction in spiral cone-beam computed tomography.

At the advent of multislice computed tomography ICT) a variety of approximate cone-beam algorithms have been proposed suited for reconstruction of small cone-angle CT data in a spiral mode of operation. The goal of this study is to identify a practical and efficient approximate cone-beam method, extend its potential for medical use, and demonstrate its performance at medium cone-angles required for area detector CT. We will investigate two different approximate single-slice rebinning algorithms for cone-beam CT: the multirow Fourier reconstruction (MFR) and an extension of the advanced single-slice rebinning method (ASSR), which combines the idea of ASSR with a z-filtering approach. Thus, both algorithms, MFR and ASSR, are formulated in the framework of z-filtering using optimized spiral interpolation algorithms. In each view, X-ray samples to be used for reconstruction are identified, which describe an approximation to a virtual reconstruction plane. The performance of approximate reconstruction should improve as the virtual reconstruction plane better fits the spiral focus path. The image quality of the respective reconstruction will be assessed with respect to image artifacts, spatial resolution, contrast resolution, and image noise. It turns out that the ASSR method using tilted reconstruction planes is a practical and efficient algorithm, providing image quality comparable to that of a single-row scanning system even with a 46-row detector at a table feed of 64 mm. Both algorithms tolerate any table feed below the maximum value associated to the detector height. Due to the z-filter approach, all detector data sampled can be used for image reconstruction.

Algorithms↗
PubMed2000

ECG-correlated imaging of the heart with subsecond multislice spiral CT.

The new spiral multislice computed tomography (CT) scanners and the significant increase in rotation speed offer great potential for cardiac imaging with X-ray CT. We have therefore developed the dedicated cardiac reconstruction algorithms 180 degrees multislice cardio interpolation (MCI) and 180 degrees multislice cardio delta (MCD) and here offer further details and validation. The algorithm 180 degreesMCI is an electrocardiogram (ECG)-correlated filtering (or weighting) algorithm in both the cardiac phase and in the z-position. Effective scan times (absolute temporal resolution) of as low as t(eff) = 56 ms are possible, assuming M 4 simultaneously measured slices at a rotation time of t(rot) = 0.5 s and S < or = d < or = 3S for the table feed d per rotation, where S denotes the collimated slice thickness. The relative temporal resolution w (fraction of the heart cycle depicted in the image), which is the more important parameter in cardiac imaging, will then be as low as w = 12.5% of the heart cycle. The second approach, 180 degreesMCD, is an ECG-correlated partial scan reconstruction of 180 degrees + delta data with delta << phi (fan-angle). Its absolute temporal resolution lies in the order of 250 ms (for the central ray, i.e., for the center of rotation), and the relative temporal resolution w increases with increasing heart rate, e.g., from typically w = 25% at fH = 60 min(-1) to w = 50% at fH = 120 min(-1), assuming again t(rot) = 0.5 s. For validation purposes, we have done simulations of a virtual cardiac motion phantom, measurements of a dedicated cardiac calibration and motion phantom, and we have reconstructed patient data with simultaneously acquired ECG. Both algorithms significantly improve the image quality compared with the standard reconstruction algorithms 180 degrees multislice linear interpolation (MLI) and 180 degrees multislice filtered interpolation (MFI). However, 180 degreesMCI is clearly superior to 180 degreesMCD for all heart rates. This is best illustrated by multiplanar reformations (MPR) or other three-dimensional (3-D) displays of the volume. 180 degreesMCI, due to its higher temporal resolution, is best for spatial and temporal four-dimensional (4-D) tracking of the anatomy. A tunable scanner rotation time to avoid resonance behavior of the heart rate and the scanner's rotation and shorter rotation times would be of further benefit.

Algorithms↗
PubMed2000

Direct comparison of a xenon and a solid-state CT detector system: measurements under working conditions.

Measurements of various image quality parameters were carried out with two different detector systems in an otherwise unchanged medical computed tomography (CT) scanner. As all other components of the scanner and the image reconstruction system remained identical, we were able to quantify the difference in performance between a Xenon gas ionization detector and a new solid-state scintillation detector in an isolated fashion. We determined noise, spatial resolution, and artifact behavior and assessed the potential for dose reduction. No significant impact of the detector change on absolute CT values of a calibration phantom was observed. Spatial resolution was improved by more than 10% for the solid-state system. As the system's modulation transfer functions were measured with a wire phantom and otherwise unchanged scanner geometry and image reconstruction algorithm, the increase of resolution is explained by the improved temporal response of the solid-state detector. At the same time, noise was reduced by 12% for a 20-cm diameter water phantom. The noise reduction corresponds to a possible reduction of patient dose by 23% for constant image quality, which is in good agreement with our prediction by estimations of both systems total detective quantum efficiency. Also, a significant improvement of scatter rejection was found for the solid-state system.

Ceramics↗
PubMed2000

ECG-correlated image reconstruction from subsecond multi-slice spiral CT scans of the heart.

Subsecond spiral computed tomography (CT) offers great potential for improving heart imaging. The new multi-row detector technology adds significantly to this potential. We therefore developed and validated dedicated cardiac reconstruction algorithms for imaging the heart with subsecond multi-slice spiral CT utilizing electrocardiogram (ECG) information. The single-slice cardiac z-interpolation algorithms 180 degrees CI and 180 degrees CD [Med. Phys. 25, 2417-2431 (1998)] were generalized to allow imaging of the heart for M-slice scanners. Two classes of algorithms were investigated: 180 degrees MCD (multi-slice cardio delta), a partial scan reconstruction of 180 degrees + delta data with a < phi (fan angle) resulting in effective scan times of 250 ms (central ray) when a 0.5 s rotation mode is available, and 180 degrees MCI (multi-slice cardio interpolation), a piecewise weighted interpolation between successive spiral data segments belonging to the same heart phase, potentially providing a relative temporal resolution of 12.5% of the heart cycle when a four-slice scanner is used and the table increment is chosen to be greater than or equal to the collimated slice thickness. Data segments are selected by correlation with the simultaneously recorded ECG signal. Theoretical studies, computer simulations, as well as patient measurements were carried out for a multi-slice scanner providing M = 4 slices to evaluate these new approaches and determine the optimal scan protocol. Both algorithms, 180 degrees MCD and 180 degrees MCI, provide significant improvements in image quality, including extremely arythmic cases. Artifacts in the reconstructed images as well as in 3D displays such as multiplanar reformations were largely reduced as compared to the standard z-interpolation algorithm 180 degrees MLI (multi-slice linear interpolation). Image quality appears adequate for precise calcium scoring and CT angiography of the coronary arteries with conventional subsecond multislice spiral CT. It turned out that for heart rates fH > or = 70 min(-1) the partial scan approach 180 degrees MCD yields unsatisfactory results as compared to 180 degrees MCI. Our theoretical considerations show that a freely selectable scanner rotation time chosen as a function of the patient's heart rate, would further improve the relative temporal resolution and thus further reduce motion artifacts. In our case an additional 0.6 s mode besides the available 0.5 s mode would be very helpful. Moreover, if technically feasible, lower rotation times such as 0.3 s or even less would result in improved image quality. The use of multi-slice techniques for cardiac CT together with the new z-interpolation methods improves the quality of heart imaging significantly. The high temporal resolution of 180 degrees MCI is adequate for spatial and temporal tracking of anatomic structures of the heart (4D reconstruction).

Algorithms↗
PubMed2000

Advanced single-slice rebinning in cone-beam spiral CT.

To achieve higher volume coverage at improved z-resolution in computed tomography (CT), systems with a large number of detector rows are demanded. However, handling an increased number of detector rows, as compared to today's four-slice scanners, requires to accounting for the cone geometry of the beams. Many so-called cone-beam reconstruction algorithms have been proposed during the last decade. None met all the requirements of the medical spiral cone-beam CT in regard to the need for high image quality, low patient dose and low reconstruction times. We therefore propose an approximate cone-beam algorithm which uses virtual reconstruction planes tilted to optimally fit 180 degrees spiral segments, i.e., the advanced single-slice rebinning (ASSR) algorithm. Our algorithm is a modification of the single-slice rebinning algorithm proposed by Noo et al. [Phys. Med. Biol. 44, 561-570 (1999)] since we use tilted reconstruction slices instead of transaxial slices to approximate the spiral path. Theoretical considerations as well as the reconstruction of simulated phantom data in comparison to the gold standard 180 degrees LI (single-slice spiral CT) were carried out. Image artifacts, z-resolution as well as noise levels were evaluated for all simulated scanners. Even for a high number of detector rows the artifact level in the reconstructed images remains comparable to that of 180 degrees LI. Multiplanar reformations of the Defrise phantom show none of the typical cone-beam artifacts usually appearing when going to larger cone angles. Image noise as well as the shape of the respective slice sensitivity profiles are equivalent to the single-slice spiral reconstruction, z-resolution is slightly decreased. The ASSR has the potential to become a practical tool for medical spiral cone-beam CT. Its computational complexity lies in the order of standard single-slice CT and it allows to use available 2D backprojection hardware.

Algorithms↗
PubMed1998

Electrocardiogram-correlated image reconstruction from subsecond spiral computed tomography scans of the heart.

Subsecond computed tomography (CT) scanning offers potential for improved heart imaging. We therefore developed and validated dedicated reconstruction algorithms for imaging the heart with subsecond spiral CT utilizing electrocardiogram (ECG) information. We modified spiral CT z-interpolation algorithms on a subsecond spiral CT scanner. Two new classes of algorithms were investigated: (a) 180 degrees CI (cardio interpolation), a piecewise linear interpolation between adjacent spiral data segments belonging to the same heart phase where segments are selected by correlation with the simultaneously recorded ECG signal and (b) 180 degrees CD (cardio delta), a partial scan reconstruction of 180 degrees + delta with delta < fan angle, resulting in reduced effective scan times of less than 0.5 s. Computer simulations as well as processing of clinical data collected with 0.75 s scan time were carried out to evaluate these new approaches. Both 180 degrees CI and 180 degrees CD provided significant improvements in image quality. Motion artifacts in the reconstructed images were largely reduced as compared to standard spiral reconstructions; in particular, coronary calcifications were delineated more sharply and multiplanar reformations showed improved contiguity. However, new artifacts in the image plane are introduced, mostly due to the combination of different data segments. ECG-oriented image reconstructions improve the quality of heart imaging with spiral CT significantly. Image quality and the display of coronary calcification appear adequate to assess coronary calcium measurements with conventional subsecond spiral CT.

Algorithms↗