Papers by Ferdinand Schweser
PLos One, 2013
Objectives: The application of susceptibility weighted imaging (SWI) in brain tumor imaging is ma... more Objectives: The application of susceptibility weighted imaging (SWI) in brain tumor imaging is mainly used to assess tumorrelated ''susceptibility based signals'' (SBS). The origin of SBS in glioblastoma is still unknown, potentially representing calcifications or blood depositions. Reliable differentiation between both entities may be important to evaluate treatment response and to identify glioblastoma with oligodendroglial components that are supposed to present calcifications. Since calcifications and blood deposits are difficult to differentiate using conventional MRI, we investigated whether a new postprocessing approach, quantitative susceptibility mapping (QSM), is able to distinguish between both entities reliably.
Quantitative magnetic susceptibility mapping (QSM) has recently been introduced to provide a nove... more Quantitative magnetic susceptibility mapping (QSM) has recently been introduced to provide a novel quantitative and local MRI contrast. However, the anatomical contrast represented by in vivo susceptibility maps has not yet been compared systematically and comprehensively with gradient (recalled) echo (GRE) magnitude, frequency, and R2⁎ images.
NeuroImage, Jan 1, 2012
Quantitative susceptibility mapping (QSM) is a novel magnetic resonance-based technique that dete... more Quantitative susceptibility mapping (QSM) is a novel magnetic resonance-based technique that determines tissue magnetic susceptibility from measurements of the magnetic field perturbation. Due to the ill-posed nature of this problem, regularization strategies are generally required to reduce streaking artifacts on the computed maps. The present study introduces a new algorithm for calculating the susceptibility distribution utilizing a priori information on its regional homogeneity derived from gradient echo phase images and analyzes the impact of erroneous a priori information on susceptibility map fidelity. The algorithm, Homogeneity Enabled Incremental Dipole Inversion (HEIDI), was investigated with a special focus on the reconstruction of subtle susceptibility variations in a numerical model and in volunteer data and was compared with two recently published approaches, Thresholded K-space Division (TKD) and Morphology Enabled Dipole Inversion (MEDI). HEIDI resulted in susceptibility maps without streaking artifacts and excellent depiction of subtle susceptibility variations in most regions. By investigating HEIDI susceptibility maps acquired with the volunteers' heads in different orientations, it was demonstrated that the apparent magnetic susceptibility distribution of human brain tissue considerably depends on the direction of the main magnetic field.Copyright © 2012 Elsevier Inc. All rights reserved.
Magnetic Resonance in Medicine, Jan 1, 2012
Magnetic susceptibility is an intrinsic tissue property that recently became measureable in vivo ... more Magnetic susceptibility is an intrinsic tissue property that recently became measureable in vivo by a magnetic-resonance based technique called quantitative susceptibility mapping (QSM). Although QSM may be performed without additional acquisition time, for example, in the course of the well-established susceptibility weighted imaging, the applicability of QSM is currently hampered by the numerical complexity and computational cost associated with the reconstruction procedure. This work introduces a novel QSM framework called superfast dipole inversion which allows rapid online reconstruction of susceptibility maps from wrapped raw gradient-echo phase data. The algorithm relies on the extension and combination of several recent algorithms involving the precalculation of convolution kernels and the correction of inversion artifacts. Reconstruction of three-dimensional high resolution susceptibility maps of the human brain was achieved with superfast dipole inversion in less than 20 s on a conventional workstation computer. Thus, superfast dipole inversion opens the door to an implementation of QSM on MR scanner hardware as well as to the routine reconstruction of large cohorts of datasets. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
J Cerebr Blood Flow Metabol, Jan 1, 2011
Cerebral microbleeds (CMBs) are findings in patients with neurological disorders such as cerebral... more Cerebral microbleeds (CMBs) are findings in patients with neurological disorders such as cerebral amyloid angiopathy and Alzheimer's disease, and are indicative of an underlying vascular pathology. A diagnosis of CMBs requires an imaging method that is capable of detecting ironcontaining lesions with high sensitivity and spatial accuracy in the presence of potentially confounding tissue abnormalities. In this study, we investigated the feasibility of quantitative magnetic susceptibility mapping (QSM), a novel technique based on gradient-recalled echo (GRE) phase data, for the detection of CMBs in the arcAb mouse, a mouse model of cerebral amyloidosis. Quantitative susceptibility maps were generated from phase data acquired with a high-resolution T 2 *-weighted GRE sequence at 9.4 T. We examined the influence of different regularization parameters on susceptibility computation; a proper adjustment of the regularization parameter minimizes streaking artifacts and preserves fine structures. In the present study, it is shown that QSM provides increased detection sensitivity of CMBs and improved contrast when compared with GRE magnitude imaging. Furthermore, QSM corrects for the blooming effect observed in magnitude and phase images and depicts both the localization and spatial extent of CMBs with high accuracy. Therefore, QSM may become an important tool for diagnosing CMBs in neurological diseases.
NeuroImage, Jan 1, 2012
Quantitative susceptibility mapping (QSM) is a novel technique which allows determining the bulk ... more Quantitative susceptibility mapping (QSM) is a novel technique which allows determining the bulk magnetic susceptibility distribution of tissue in vivo from gradient echo magnetic resonance phase images. It is commonly assumed that paramagnetic iron is the predominant source of susceptibility variations in gray matter as many studies have reported a reasonable correlation of magnetic susceptibility with brain iron concentrations in vivo. Instead of performing direct comparisons, however, all these studies used the putative iron concentrations reported in the hallmark study by Hallgren and Sourander (1958) for their analysis. Consequently, the extent to which QSM can serve to reliably assess brain iron levels is not yet fully clear. To provide such information we investigated the relation between bulk tissue magnetic susceptibility and brain iron concentration in unfixed (in situ) post mortem brains of 13 subjects using MRI and inductively coupled plasma mass spectrometry. A strong linear correlation between chemically determined iron concentration and bulk magnetic susceptibility was found in gray matter structures (r=0.84, p<0.001), whereas the correlation coefficient was much lower in white matter (r=0.27, p<0.001). The slope of the overall linear correlation was consistent with theoretical considerations of the magnetism of ferritin supporting that most of the iron in the brain is bound to ferritin proteins. In conclusion, iron is the dominant source of magnetic susceptibility in deep gray matter and can be assessed with QSM. In white matter regions the estimation of iron concentrations by QSM is less accurate and more complex because the counteracting contribution from diamagnetic myelinated neuronal fibers confounds the interpretation.Copyright © 2012 Elsevier Inc. All rights reserved.
Quantitative imaging of intrinsic magnetic tissue properties using MRI signal phase: An approach to in vivo brain iron metabolism?
Neuroimage, Jan 1, 2010
Quantitative susceptibility mapping (QSM) based on gradient echo (GRE) magnetic resonance phase d... more Quantitative susceptibility mapping (QSM) based on gradient echo (GRE) magnetic resonance phase data is a novel technique for non-invasive assessment of magnetic tissue susceptibility differences. The method is expected to be an important means to determine iron distributions in vivo and may, thus, be instrumental for elucidating the physiological role of iron and disease-related iron concentration changes associated with various neurological and psychiatric disorders. This study introduces a framework for QSM and demonstrates calculation of reproducible and orientation-independent susceptibility maps from GRE data acquired at 3T. The potential of these susceptibility maps to perform anatomical imaging is investigated, as well as the ability to measure the venous blood oxygen saturation level in large vessels, and to assess the local tissue iron concentration. In order to take into account diamagnetic susceptibility contributions induced by myelin, a correction scheme for susceptibility based iron estimation is demonstrated. The findings suggest that susceptibility contrast, and therewith also phase contrast, are not only linked to the storage iron concentration but are also significantly influenced by other sources such as myelin. After myelin correction the linear dependence between magnetic susceptibilities and previously published iron concentrations from post mortem studies was significantly improved. Finally, a comparison between susceptibility maps and processed phase images indicated that caution should be exercised when drawing conclusions about iron concentrations when directly assessing processed phase information.►A framework for quantitative susceptibility mapping was introduced and evaluated. ►Susceptibility maps demonstrate rotation-invariant, quantitative anatomical contrast. ►Tissue susceptibility is not only influenced by tissue storage iron. ►Diamagnetic susceptibility contributions from myelin can be corrected. ►Direct phase measurements are unreliable for estimation of tissue iron concentration.
Medical physics, Jan 1, 2010
Purpose: Identification of calcifications and hemorrhages is essential for the etiological diagno... more Purpose: Identification of calcifications and hemorrhages is essential for the etiological diagnosis of cerebral lesions. The purpose of this work was to develop a robust method for characterization of para-and diamagnetic intracerebral lesions based on clinical gradient-echo magnetic resonance phase data acquired at 1.5 Tesla. Methods: The magnetic susceptibility distribution of biological tissue produces a distinct magnetic field pattern, which is directly reflected in gradient-echo magnetic resonance phase images. Compared to brain parenchyma, iron-laden tissues are more paramagnetic, whereas mineralized tissues usually possess more diamagnetic susceptibilities. Magnetic resonance phase data were inverted to the underlying susceptibility distribution utilizing additional geometrical information about the lesions, which was obtained from the gradient-echo magnitude signal void corresponding to the lesions. Clinical magnetic resonance exams of three patients with multiple brain lesions ͑total n=70͒ were processed and evaluated. For one patient, the results were validated by an additionally available computed tomography scan. Numerical simulations were conducted to evaluate the robustness of the method. Results: The obtained susceptibility maps showed impressive delineation of lesions, vessels, and potentially iron-laden tissue. Compensation of the nonlocal field perturbations was clearly discernable on the susceptibility maps. In all cases, discrimination of para-from diamagnetic lesions was achieved and the results were confirmed by the additional computed tomography. The numerical simulations demonstrated that robust determination of the total magnetic moment of lesions is possible. Thus, the proposed method is able to yield quantitative values for the minimum magnetic susceptibility of lesions. Conclusions: A method has been developed for noninvasive, semiautomatic characterization of brain lesions based on magnetic resonance imaging data. Initial clinical results demonstrated that the proposed technique can be applied to diagnosis of lesions with calcifications or hemorrhages. If confirmed by larger studies, it bears the potential to obviate the need for confirmation with computed tomography.
PIERS Online, Jan 1, 2008
Parallel transmission bears the potential of compensating B1 field inhomogeneities induced by wav... more Parallel transmission bears the potential of compensating B1 field inhomogeneities induced by wave propagation effects in (ultra) high field whole body MR imaging. However, with increasing field strength, the RF power deposition and the associated local specific absorption rate (SAR) represent an important attention point with respect to patient safety. This paper presents simulations of a 3 T whole body eight-channel transmit/receive body coil loaded with a human bio-mesh model. Phantom SAR simulations were carried out and validated by temperature measurements. A good correlation between SAR simulations and measured temperature was obtained, so that the FDTD method can be considered to be a valuable tool in determining (local) SAR for patient safety in multi-channel transmission MRI systems.
Conference Proceedings by Ferdinand Schweser
Proceedings of the …, Jan 1, 2009
The local specific absorption rate (SAR) becomes a limiting factor for many MR imaging applicatio... more The local specific absorption rate (SAR) becomes a limiting factor for many MR imaging applications with increasing field strength. However, tailoring of the underlying electric fields (E-fields) in the RF pulse design process is possible when using parallel Tx systems. Different approaches exist to reduce SAR in parallel transmission. Minimal SAR RF pulses can be selected from the large solution space due to the extra degree of freedom in the RF pulse design . This paper presents a novel approach for local SAR reduction by exploitation of the temporal degree of freedom of multi-shot imaging sequences. It is based on successive application of RF pulses with the same target excitation pattern, but different spatial distributions of SAR, levelling out by time averaging. The concept was validated by simulations. Both computation of RF pulses and spatial SAR distributions were significantly accelerated implementing it on a graphicsprocessing unit (GPU). The local SAR calculation was carried out in real-time for a whole body bio-mesh according to .
Background field correction with SHARP and PDF: A comparison using different convolution kernels
First Intl Workshop on MRI …, Jan 1, 2011
... Pages: 7. Find this paper at ... Gradient-echo (GRE) magnetic resonance phase data are propor... more ... Pages: 7. Find this paper at ... Gradient-echo (GRE) magnetic resonance phase data are proportional to the magnetic field, providing useful information for several ... Quan Chen, Mingli Chen, Weiguo Lu in Medical Physics (2011). 4 readers Save reference to library · Related research ...
SHARP and PDF may not eliminate B1-phase background components
First Intl Workshop on MRI …, Jan 1, 2011
Magnetic susceptibility in MRI is a source of both contrasts and artefacts. For instance, suscept... more Magnetic susceptibility in MRI is a source of both contrasts and artefacts. For instance, susceptibility weighted phase images provide anatomical contrast complementary to the magnitude [1, 2] by directly reflecting local magnetic field changes induced by the distribution of magnetic susceptibility of the measured object.
MATERIALS AND METHODS Data Acquisition and Pre-Processing: High-resolution volunteer data of the ... more MATERIALS AND METHODS Data Acquisition and Pre-Processing: High-resolution volunteer data of the whole brain were acquired with a 3D fully flow compensated gradient-echo sequence (TE/TR/FA/BW= 20ms/30ms/15/80 Hz/px, voxel size 0.6× 0.6× 0.6 mm³, 75% PF in phase and slice encoding direction) on a 3T MR-scanner (Tim Trio, Siemens Medical Solutions) using a 12-channel head-matrix coil.
With increasing field-strength, the local specific absorption rate (SAR) becomes a limiting facto... more With increasing field-strength, the local specific absorption rate (SAR) becomes a limiting factor for MR imaging applications. However, using parallel Tx systems, it becomes possible to tailor the E-fields in the RF pulse design. Certain SAR optimal algorithms have been proposed, eg [1], that constrain a specific local region for SAR (eg, the eyes). This is in general insufficient for whole body imaging, as the location of SAR hot spots limiting the MR sequence is not fixed in the subject. This paper presents 1.
INTRODUCTION–Gradient-echo (GRE) magnetic resonance phase data are proportional to the magnetic f... more INTRODUCTION–Gradient-echo (GRE) magnetic resonance phase data are proportional to the magnetic field, providing useful information for several applications such as quantitative susceptibility mapping [1], or anatomical contrast analyses [2]. Interpretation of phase information requires phase unwrapping and suppression of the background phase signal that results, eg, from air-and bone-tissue interfaces.
With increasing field strength, the deposition of SAR becomes a serious concern in MR imaging, in... more With increasing field strength, the deposition of SAR becomes a serious concern in MR imaging, in particular when short spatially selective RF excitation is applied. The RF power deposition scales roughly with the square of the B0 field. Variable4rate selective excitation (VERSE)[1] is an RF pulse design approach, which reduces the RF power deposition by adjusting the time4varying gradient waveform and the RF waveform simultaneously.
Due to the analogy between the design of spatially selective RF pulses and MR imaging [1], it is ... more Due to the analogy between the design of spatially selective RF pulses and MR imaging [1], it is possible to adapt the methodology of parallel imaging [2, 3] for parallel transmission [4-6]. Thus, the RF pulse calculation can be performed via direct matrix inversion [4-6], which in parallel imaging has been used by, eg,[2, 3]. On the other hand, the RF pulse calculation of parallel transmission might be performed via iterative methods [7, 8], which in parallel imaging has been used by, eg,[9].
RF safety is a prerequisite for in vivo parallel transmission MRI experiments. In this abstract, ... more RF safety is a prerequisite for in vivo parallel transmission MRI experiments. In this abstract, a comprehensive RF patient safety concept is proposed to ensure scanning within the SAR limits with multi-channel RF transmit coils. Two paths are pursued. Before the scan, SAR calculations are carried out in (almost) real-time for demand waveforms to verify the conformity with existing SAR limits [1]. During the scan, the deviation from the desired waveform is monitored to detect violations of the SAR limits or any unsafe conditions.
Introduction With increasing field strength the local specific absorption rate (SAR) becomes a li... more Introduction With increasing field strength the local specific absorption rate (SAR) becomes a limiting factor for many MR imaging applications. Minimal SAR RF pulses can be selected from the large solution space due to the extra degrees of freedom in the RF pulse design [1-4]. This paper extends the recently proposed temporal averaging approach for local SAR reduction [5] with multiple local SAR constraints.
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Papers by Ferdinand Schweser
Conference Proceedings by Ferdinand Schweser