Biomedical subjects

H Bruder

Publications and source records attributed to H Bruder.

12 recordsLinked to original sources

PubMed2005

[On the way to isotopic spatial resolution: technical principles and applications of 16-slice CT].

The broad introduction of multi-slice CT by all major vendors in 1998 was a milestone with regard to extended volume coverage, improved axial resolution and better utilization of the tube output. New clinical applications such as CT-examinations of the heart and the coronary arteries became possible. Despite all promising advances, some limitations remain for 4-slice CT systems. They come close to isotropic resolution, but do not fully reach it in routine clinical applications. Cardiac CT-examinations require careful patient selection. The new generation of multi-slice CT-systems offer simultaneous acquisition of up to 16 sub-millimeter slices and improved temporal resolution for cardiac examinations by means of reduced gantry rotation time (0.4 s). In this overview article we present the basic technical principles and potential applications of 16-slice technology for the example of a 16-slice CT-system (SOMATOM Sensation 16, Siemens AG, Forchheim). We discuss detector design and dose efficiency as well as spiral scan- and reconstruction techniques. At comparable slice thickness, 16-slice CT-systems have a better dose efficiency than 4-slice CT-systems. The cone-beam geometry of the measurement rays requires new reconstruction approaches, an example is the adaptive multiple plane reconstruction, AMPR. First clinical experience indicates that sub-millimeter slice width in combination with reduced gantry rotation-time improves the clinical stability of cardiac examinations and expands the spectrum of patients accessible to cardiac CT. 16-slice CT-systems have the potential to cover even large scan ranges with sub-millimeter slices at considerably reduced examination times, thus approaching the goal of routine isotropic imaging.

Cardiovascular Diseases↗

PubMed2005

Image reconstruction and image quality evaluation for a 64-slice CT scanner with z-flying focal spot.

We present a theoretical overview and a performance evaluation of a novel z-sampling technique for multidetector row CT (MDCT), relying on a periodic motion of the focal spot in the longitudinal direction (z-flying focal spot) to double the number of simultaneously acquired slices. The z-flying focal spot technique has been implemented in a recently introduced MDCT scanner. Using 32 x 0.6 mm collimation, this scanner acquires 64 overlapping 0.6 mm slices per rotation in its spiral (helical) mode of operation, with the goal of improved longitudinal resolution and reduction of spiral artifacts. The longitudinal sampling distance at isocenter is 0.3 mm. We discuss in detail the impact of the z-flying focal spot technique on image reconstruction. We present measurements of spiral slice sensitivity profiles (SSPs) and of longitudinal resolution, both in the isocenter and off-center. We evaluate the pitch dependence of the image noise measured in a centered 20 cm water phantom. To investigate spiral image quality we present images of an anthropomorphic thorax phantom and patient scans. The full width at half maximum (FWHM) of the spiral SSPs shows only minor variations as a function of the pitch, measured values differ by less than 0.15 mm from the nominal values 0.6, 0.75, 1, 1.5, and 2 mm. The measured FWHM of the smallest slice ranges between 0.66 and 0.68 mm at isocenter, except for pitch 0.55 (0.72 mm). In a centered z-resolution phantom, bar patterns up to 15 lp/cm can be visualized independent of the pitch, corresponding to 0.33 mm longitudinal resolution. 100 mm off-center, bar patterns up to 14 lp/cm are visible, corresponding to an object size of 0.36 mm that can be resolved in the z direction. Image noise for constant effective mAs is almost independent of the pitch. Measured values show a variation of less than 7% as a function of the pitch, which demonstrates correct utilization of the applied radiation dose at any pitch. The product of image noise and square root of the slice width (FWHM of the respective SSP) is the same constant for all slices except 0.6 mm. For the thinnest slice, relative image noise is increased by 17%. Spiral windmill-type artifacts are effectively suppressed with the z-flying focal spot technique, which has the potential to maintain a low artifact level up to pitch 1.5, in this way increasing the maximum volume coverage speed that can be clinically used.

Equipment Design↗

PubMed2004

Performance evaluation of a 64-slice CT system with z-flying focal spot.

The meanwhile established generation of 16-slice CT systems enables routine sub-millimeter imaging at short breath-hold times. Clinical progress in the development of multidetector row CT (MDCT) technology beyond 16 slices can more likely be expected from further improvement in spatial and temporal resolution rather than from a mere increase in the speed of volume coverage. We present an evaluation of a recently introduced 64-slice CT system (SOMATOM Sensation 64, Siemens AG, Forchheim, Germany), which uses a periodic motion of the focal spot in longitudinal direction (z-flying focal spot) to double the number of simultaneously acquired slices. This technique acquires 64 overlapping 0.6 mm slices per rotation. The sampling scheme corresponds to that of a 64 x 0.3 mm detector, with the goal of improved longitudinal resolution and reduced spiral artifacts. After an introduction to the detector design, we discuss the basics of z-flying focal spot technology (z-Sharp). We present phantom and specimen scans for performance evaluation. The measured full width at half maximum (FWHM) of the thinnest spiral slice is 0.65 mm. All spiral slice widths are almost independent of the pitch, with deviations of less than 0.1 mm from the nominal value. Using a high-resolution bar pattern phantom (CATPHAN, Phantom Laboratories, Salem, NY), the longitudinal resolution can be demonstrated to be up to 15 lp/cm at the isocenter independent of the pitch, corresponding to a bar diameter of 0.33 mm. Longitudinal resolution is only slightly degraded for off-center locations. At a distance of 100 mm from the isocenter, 14 lp/cm can be resolved in the z-direction, corresponding to a bar diameter of 0.36 mm. Spiral "windmill" artifacts presenting as hyper- and hypodense structures around osseous edges are effectively reduced by the z-flying focal spot technique. Cardiac scanning benefits from the short gantry rotation time of 0.33 s, providing up to 83 ms temporal resolution with 2-segment ECG-gated reconstruction.

Algorithms↗

PubMed2003

Image reconstruction and image quality evaluation for a 16-slice CT scanner.

We present a theoretical overview and a performance evaluation of a novel approximate reconstruction algorithm for cone-beam spiral CT, the adaptive multiple plane reconstruction (AMPR), which has been introduced by Schaller, Flohr et al. [Proc. SPIE Int. Symp. Med. Imag. 4322, 113-127 (2001)] AMPR has been implemented in a recently introduced 16-slice CT scanner. We present a detailed algorithmic description of AMPR which allows for a free selection of the spiral pitch. We show that dose utilization is better than 90% independent of the pitch. We give an overview on the z-reformation functions chosen to allow for a variable selection of the spiral slice width at arbitrary pitch values. To investigate AMPR image quality we present images of anthropomorphic phantoms and initial patient results. We present measurements of spiral slice sensitivity profiles (SSPs) and measurements of the maximum achievable transverse resolution, both in the isocenter and off-center. We discuss the pitch dependence of image noise measured in a centered 20 cm water phantom. Using the AMPR approach, cone-beam artifacts are considerably reduced for the 16-slice scanner investigated. Image quality in MPRs is independent of the pitch and equivalent to a single-slice CT system at pitch p approximately 1.5. The full width at half-maximum (FWHM) of the spiral SSPs shows only minor variations as a function of the pitch, nominal, and measured values differ by less than 0.2 mm. With 16 x 0.75 mm collimation, the measured FWHM of the smallest reconstructed slice is about 0.9 mm. Using this slice width and overlapping image reconstruction, cylindrical holes with 0.6 mm diameter can be resolved in a z-resolution phantom. Image noise for constant effective mAs is nearly independent of the pitch. Measured and theoretically expected dose utilization are in good agreement. Meanwhile, clinical practice has demonstrated the excellent image quality and the increased diagnostic capability that is obtained with the new generation of multislice CT systems.

Algorithms↗

PubMed2003

Image reconstruction and performance evaluation for ECG-gated spiral scanning with a 16-slice CT system.

We present an image reconstruction approach and a performance evaluation for ECG-gate cardiac spiral scanning with recently introduced 16-slice CT equipment. We present an extension of the Adaptive Cardio Volume (ACV) reconstruction approach for ECG-gated multislice spiral scanning. We discuss the image z reformation introduced to control the spiral slice width of the final images and give an overview of the reformation functions chosen. We investigate image quality and discuss the maximum number of slices that can be reconstructed without severe cone-beam artifacts. Slice sensitivity profiles (SSPs) and transverse resolution are evaluated as a function of the patient's heart rate. We demonstrate the influence of slice width on the visualization of stents and plaques and show the impact of reduced gantry rotation time (0.42 s) on temporal resolution. Deviating from general purpose spiral scanning cone-beam reconstruction is not required for ECG-gated cardiac CT with up to 16 slices. Using the ACV approach with image reformation, SSPs are well defined and independent of the patient's heart rate. With 0.75 mm collimated slice width, the measured full width at half-maximum (FWHM) of the smallest reconstructed slice is about 0.83 mm. Using this slice width and overlapping image reconstruction, cylindrical holes 0.6-0.7 mm in diameter can be resolved in a z-resolution phantom. Adequate visualization of the coronary arteries requires reconstruction slice widths not larger than 1.5 mm. Visualization of stents and severe calcifications is significantly improved with sub-mm slice width. Experimental evidence for the theoretically predicted temporal resolution and for the variation of temporal resolution depending on the position in the field of measurement (FOM) is presented. With 0.42 s gantry rotation temporal resolution reaches its optimum of 105 ms in the center of the FOM at 81 bpm. First scans on human subjects demonstrate the potential to expand the range of heart rates accessible to routine clinical examinations. A 16-slice platform can cover the heart with sub-mm slices within short breath-hold times, allowing for improved cardiac imaging due to isotropic sub-mm spatial resolution.

Algorithms↗

PubMed2002

New technical developments in multislice CT--Part 1: Approaching isotropic resolution with sub-millimeter 16-slice scanning.

The introduction of multislice CT was a breakthrough with regard to increased scan speed, improved axial resolution and better utilization of the tube output. The new generation of multislice CT scanners offering simultaneous acquisition of up to 16 sub-millimeter slices represents an important leap on the way towards true isotropic scanning. We present an evaluation of a state-of-the-art 16-slice CT system (SOMATOM Sensation 16, Siemens AG, Forchheim, Germany). After an introduction to the detector design we discuss dose utilization and finally elaborate on multislice spiral scanning with 16 slices. Due to the increased number of slices dose utilization is improved compared to current 4-slice CT scanners, and sub-millimeter collimation needs no longer be restricted to special applications. For CT systems with 8 or more slices, the cone-beam geometry causes severe artifacts if not corrected for by a so-called cone-correction, which thus becomes mandatory in this case. With the Adaptive Multiple Plane Reconstruction AMPR, cone beam artifacts are effectively suppressed, while the benefits of Adaptive Axial Interpolation are maintained: free selection of the spiral pitch according to the clinical needs of an examination, slice width independent of the pitch, full dose utilization at all pitch values. Clinical practice will have to demonstrate the application spectrum that is opened with the new generation of multislice CT systems.

Artifacts↗

PubMed2002

New technical developments in multislice CT, part 2: sub-millimeter 16-slice scanning and increased gantry rotation speed for cardiac imaging.

Despite all promising advances, some challenges remain for ECG-gated multislice CT examinations of the heart and the coronary arteries with current 4-slice detectors: adequate visualization of stents and severely calcified coronary arteries, examination of patients with higher heart rates and patients, who cannot adequately hold their breath for at least 30 sec. The new generation of multislice CT systems offering simultaneous acquisition of up to 16 sub-millimeter slices and gantry rotation times shorter than 0.5 sec has the potential to overcome these limitations. We describe the technical principles of cardiac scanning with a state-of-the-art 16-slice CT equipment (SOMATOM Sensation 16, Siemens AG, Forchheim, Germany). We discuss an extension of the Adaptive Cardio Volume (ACV) reconstruction approach for ECG-gated multislice spiral CT. We show the impact of reduced gantry rotation time (0.42 sec) on temporal resolution, and we demonstrate the influence of slice width on the visualization of stents and plaques. Deviating from general purpose applications a cone-correction is not required for cardiac scanning with 16-slice CT systems. In addition to the absolute improvement, the temporal resolution shows a different dependence on the patient's heart rate for 0.42 sec rotation time, reaching its optimum (105 msec) at 81 BPM. This has the potential to expand the range of heart rates accessible to routine clinical examinations. Owing to sub-millimeter slice width and optimized in-plane resolution characteristics, visualization of stents and severe calcifications in coronary arteries is significantly improved. Clinical experience will be needed to fully evaluate the potential of 16-slice technology for cardiac imaging.

Artifacts↗

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↗

PubMed1994

The influence of inhomogeneous volume conductor models on the ECG and the MCG.

The authors investigated the influence of human body inhomogeneities such as the lungs, blood masses and the skeletal muscle layer on the electrical body surface potential and the magnetic field. The surface potentials and magnetic fields are calculated using a boundary element method. As a rule the blood masses have a large influence on both potential and magnetic field amplitude as well as on the potential and magnetic field map orientation, but the influence on the topology of the map is less in the electric case than in the magnetic case. The single-dipole reconstruction was applied to estimate the error caused by neglecting inner inhomogeneities in source localization. The neglect of lungs and blood masses results in a localization error of less than 1 cm in the electric case but more than 1 cm for deep sources at the posterior side of the heart in the magnetic case. The authors tried to assess the influence of the skeletal muscle layer by both an analytical two-layered anisotropic half-space model and the torso extension method. The skeletal muscle layer causes a smoothing effect on the electrical surface potential and to a lesser extent on the magnetic field, leading to an overestimation of the actual source depth of about 1-2 cm. In principle this can be reduced by taking data from all over the thoracic surface. The authors designed experiments for simultaneous measurement of body surface potential and extracorporeal magnetic field from the same subject. The evaluation of data from two patients showing Wolff-Parkinson-White syndrome has shown that localization results from electric potential data and magnetocardiographic data are consistent.

Anisotropy↗

PubMed1994

Biomagnetic localization of electrical current sources in the human heart with realistic volume conductors using the single-current-dipole model.

The boundary element method was applied in order to investigate the localization accuracy for focal sources measured from MCG data. Various homogeneous volume conductor models were composed: the individually shaped torso, a scaled standard torso, an unscaled standard torso, a scaled cuboid and a scaled ellipsoid. We implemented these models in single-dipole inverse solution techniques. High resolution multichannel data were analysed from two patients showing ventricular extrasystoles and two patients suffering from Wolff-Parkinson-White syndrome. Moreover, we report the localization of shallow- and deep-lying catheters (depth 9 cm and depth 17.5 cm below the measurement grid). Using an individually shaped homogeneous torso yields a localization error of less than 3 cm even for the deepest sources (mean error 2.4 cm). Probability-based dipole localization shows that the remaining error could only partly be explained by data noise statistics. Therefore it seems to be due to either inner inhomogeneities or the inadequacy of the single current dipole or a combination of the two. Thus clinically useful localization accuracy in the millimetre range requires more sophisticated volume conductor and source models. The evaluation of measurement data and simulation study shows that a scaled cuboid model can provide nearly the same localization accuracy as the individually shaped torso model. Single dipole reconstruction with this model is computationally faster than that with the individually shaped model of the human body and is fast enough for use in clinical applications.

Arrhythmias, Cardiac↗

PubMed1992

Image reconstruction for echo planar imaging with nonequidistant k-space sampling.

Echo planar imaging is characterized by scanning the 2D k-space after a single excitation. Different sampling patterns have been proposed. A technically feasible method uses a sinusoidal readout gradient resulting is measured data that does not sample k-space in an equidistant manner. In order to employ a conventional 2D-FFT image reconstruction, the data have to be converted to a cartesian grid. This can be done either by interpolation or alternatively by a generalized transformation. Filtering methods are described to minimize ghosting artifact that is typical in echo planar imaging. Results both from computer simulation and from experiments will be presented. Experimental images were obtained using a 2-T whole-body research system.

Artifacts↗

PubMed1988

A new steady-state imaging sequence for simultaneous acquisition of two MR images with clearly different contrasts.

We present a new steady-state imaging sequence, which simultaneously allows in a single acquisition the formation of two MR images with clearly different contrasts. The contrast of the first image is FISP-like, whereas the second image is strongly T2-weighted. In principle the T2 values in the image can be calculated from the combination of the first and second images. We also show calculated T2 images.

Head↗