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Papers by Stefanie Remmele

Medical Imaging 2011: Image Processing, 2011

Segmentation of brain tumours in Magnetic Resonance (MR) images and classification of the tumour ... more Segmentation of brain tumours in Magnetic Resonance (MR) images and classification of the tumour tissue into vital, necrotic, and perifocal edematous areas is required in a variety of clinical applications. Manual delineation of the tumour tissue boundaries is a tedious and error-prone task, and the results are not reproducible. Furthermore, tissue classification mostly requires information of several MR protocols and

NMR in Biomedicine, 2010

The consistent determination of changes in the transverse relaxation rate R 2 Ã (DR 2 Ã ) is esse... more The consistent determination of changes in the transverse relaxation rate R 2 Ã (DR 2 Ã ) is essential for the mapping of the effect of hyperoxic and hypercapnic respiratory challenges, which enables the noninvasive assessment of blood oxygenation changes and vasoreactivity by MRI. The purpose of this study was to compare the performance of two different methods of DR 2 Ã quantification from dynamic multigradient-echo data: (A) subtraction of R 2 Ã values calculated from monoexponential decay functions; and (B) computation of DR 2 Ã echo-wise from signal intensity ratios. A group of healthy volunteers (n ¼ 12) was investigated at 3.0 T, and the brain tissue response to carbogen and CO 2 -air inhalation was registered using a dynamic multigradient-echo sequence with high temporal and spatial resolution. Results of the DR 2 Ã quantification obtained by the two methods were compared with respect to the quality of the voxel-wise DR 2 Ã response, the number of responding voxels and the behaviour of the 'global' response of all voxels with significant R 2 Ã changes. For the two DR 2 Ã quantification methods, we found no differences in the temporal variation of the voxel-wise DR 2 Ã responses or in the detection sensitivity. The maximum change in the 'global' response was slightly smaller when DR 2 Ã was derived from signal intensity ratios. In conclusion, this first methodological comparison shows that both DR 2 Ã quantifications, from monoexponential approximation as well as from signal intensity ratios, are applicable for the monitoring of R 2 Ã changes during respiratory challenges.

Magnetic Resonance in Medicine, 2013

This work presents a novel method for the rapid and simultaneous measurement of R1 and R2* relaxa... more This work presents a novel method for the rapid and simultaneous measurement of R1 and R2* relaxation rates. It is based on a dynamic short repetition time steady-state spoiled multigradient-echo sequence and baseline R1 and B1 measurements. The accuracy of the approach was evaluated in simulations and a phantom experiment. The sensitivity and specificity of the method were demonstrated in one volunteer and in four patients with intracranial tumors during carbogen inhalation. We utilized (ΔR2*, ΔR1) scatter plots to analyze the multiparametric response amplitude of each voxel within an area of interest. In normal tissue R2* decreased and R1 increased moderately in response to the elevated blood and tissue oxygenation. A strong negative ΔR2* and ΔR1 response was observed in veins and some tumor areas. Moderate positive ΔR2* and ΔR1 response amplitudes were found in fluid-rich tissue as in cerebrospinal fluid, peritumoral edema, and necrotic areas. The multiparametric approach was shown to increase the specificity and sensitivity of oxygen-enhanced MRI compared to measuring ΔR2* or ΔR1 alone. It is thus expected to provide an optimal tool for the identification of tissue areas with low oxygenation, e.g., in tumors with compromised oxygen supply.

Journal of Magnetic Resonance Imaging, 2010

To compare two DeltaR2* quantification methods for analyzing the response of intracranial tumors ... more To compare two DeltaR2* quantification methods for analyzing the response of intracranial tumors to different breathing gases. The determination of changes in the magnetic resonance imaging (MRI) relaxation rate R2* (DeltaR2*), induced by hyperoxic and hypercapnic respiratory challenges, enables the noninvasive assessment of blood oxygenation changes and vasoreactivity. Sixteen patients with various intracranial tumors were examined at 3.0 T. The response to respiratory challenges was registered using a dynamic multigradient-echo sequence with high temporal and spatial resolution. At each dynamic step, DeltaR2* was derived in two different ways: 1) by subtraction of R2* values obtained from monoexponential decay functions, 2) by computing DeltaR2* echo-wise from signal intensity ratios. The sensitivity for detection of responding voxels and the behavior of the "global" response were investigated. Significantly more responding voxels (about 4%) were found for method (1). The "global" response was independent from the chosen quantification method but showed slightly larger changes (about 6%) when DeltaR2* was derived from method (1). Similar results were observed for the two methods, with a slightly higher detection sensitivity of responding voxels when DeltaR2* was obtained from monoexponential approximation.

Journal of Magnetic Resonance Imaging, 2010

To compare two magnetic resonance (MR) contrast mechanisms, R*(2) BOLD and balanced SSFP, for the... more To compare two magnetic resonance (MR) contrast mechanisms, R*(2) BOLD and balanced SSFP, for the dynamic monitoring of the cerebral response to (C)O(2) respiratory challenges. Carbogen and CO(2)-enriched air were delivered to 9 healthy volunteers and 1 glioblastoma patient. The cerebral response was recorded by two-dimensional (2D) dynamic multi-gradient-echo and passband-balanced steady-state free precession (bSSFP) sequences, and local changes of R*(2) and signal intensity were investigated. Detection sensitivity was analyzed by statistical tests. An exponential signal model was fitted to the global response function delivered by each sequence, enabling quantitative comparison of the amplitude and temporal behavior. The bSSFP signal changes during carbogen and CO(2)/air inhalation were lower compared with R*(2) BOLD (ca. 5% as opposed to 8-13%). The blood-oxygen-level-dependent (BOLD) response amplitude enabled differentiation between carbogen and CO(2)/air by a factor of 1.4-1.6, in contrast to bSSFP, where differentiation was not possible. Furthermore, motion robustness and detection sensitivity were higher for R*(2) BOLD. Both contrast mechanisms are well suited to dynamic (C)O(2)-enhanced MR imaging, although the R*(2) BOLD mechanism was demonstrated to be superior in several respects for the chosen application. This study suggests that the R*(2) BOLD and bSSFP-response characteristics are related to different physiologic mechanisms.

IEEE Transactions on Medical Imaging, 2000

In dynamic magnetic resonance imaging (MRI) studies, the motion kinetics or the contrast variabil... more In dynamic magnetic resonance imaging (MRI) studies, the motion kinetics or the contrast variability are often hard to predict, hampering an appropriate choice of the image update rate or the temporal resolution. A constant azimuthal profile spacing (111.246 degrees), based on the Golden Ratio, is investigated as optimal for image reconstruction from an arbitrary number of profiles in radial MRI. The profile order is evaluated and compared with a uniform profile distribution in terms of signal-to-noise ratio (SNR) and artifact level. The favorable characteristics of such a profile order are exemplified in two applications on healthy volunteers. First, an advanced sliding window reconstruction scheme is applied to dynamic cardiac imaging, with a reconstruction window that can be flexibly adjusted according to the extent of cardiac motion that is acceptable. Second, a contrast-enhancing k-space filter is presented that permits reconstructing an arbitrary number of images at arbitrary time points from one raw data set. The filter was utilized to depict the T1-relaxation in the brain after a single inversion prepulse. While a uniform profile distribution with a constant angle increment is optimal for a fixed and predetermined number of profiles, a profile distribution based on the Golden Ratio proved to be an appropriate solution for an arbitrary number of profiles.

IEEE Transactions on Medical Imaging, 2000

The steady-state free precessing (SSFP) sequences, widely used in MRI today, acquire data only du... more The steady-state free precessing (SSFP) sequences, widely used in MRI today, acquire data only during a short fraction of the repetition time (TR). Thus, they exhibit a poor scan efficiency. In this paper, a novel approach to extending the acquisition window for a given TR without considerably modifying the basic sequence is explored for radial SSFP sequences. The additional data are primarily employed to increase the signal-to-noise ratio, rather than to improve the temporal resolution of the imaging. The approach is analyzed regarding its effect on the image SNR (signal to noise ratio) and the reconstruction algorithm. Results are presented for phantom experiments and cardiac functions studies. The gain in SNR is most notable in rapid imaging, since SNR enhancement for a constant repetition time may be used to compensate for the increase in noise resulting from angular undersampling.

Medical Imaging 2011: Image Processing, 2011

Segmentation of brain tumours in Magnetic Resonance (MR) images and classification of the tumour ... more Segmentation of brain tumours in Magnetic Resonance (MR) images and classification of the tumour tissue into vital, necrotic, and perifocal edematous areas is required in a variety of clinical applications. Manual delineation of the tumour tissue boundaries is a tedious and error-prone task, and the results are not reproducible. Furthermore, tissue classification mostly requires information of several MR protocols and

NMR in Biomedicine, 2010

The consistent determination of changes in the transverse relaxation rate R 2 Ã (DR 2 Ã ) is esse... more The consistent determination of changes in the transverse relaxation rate R 2 Ã (DR 2 Ã ) is essential for the mapping of the effect of hyperoxic and hypercapnic respiratory challenges, which enables the noninvasive assessment of blood oxygenation changes and vasoreactivity by MRI. The purpose of this study was to compare the performance of two different methods of DR 2 Ã quantification from dynamic multigradient-echo data: (A) subtraction of R 2 Ã values calculated from monoexponential decay functions; and (B) computation of DR 2 Ã echo-wise from signal intensity ratios. A group of healthy volunteers (n ¼ 12) was investigated at 3.0 T, and the brain tissue response to carbogen and CO 2 -air inhalation was registered using a dynamic multigradient-echo sequence with high temporal and spatial resolution. Results of the DR 2 Ã quantification obtained by the two methods were compared with respect to the quality of the voxel-wise DR 2 Ã response, the number of responding voxels and the behaviour of the 'global' response of all voxels with significant R 2 Ã changes. For the two DR 2 Ã quantification methods, we found no differences in the temporal variation of the voxel-wise DR 2 Ã responses or in the detection sensitivity. The maximum change in the 'global' response was slightly smaller when DR 2 Ã was derived from signal intensity ratios. In conclusion, this first methodological comparison shows that both DR 2 Ã quantifications, from monoexponential approximation as well as from signal intensity ratios, are applicable for the monitoring of R 2 Ã changes during respiratory challenges.

Magnetic Resonance in Medicine, 2013

This work presents a novel method for the rapid and simultaneous measurement of R1 and R2* relaxa... more This work presents a novel method for the rapid and simultaneous measurement of R1 and R2* relaxation rates. It is based on a dynamic short repetition time steady-state spoiled multigradient-echo sequence and baseline R1 and B1 measurements. The accuracy of the approach was evaluated in simulations and a phantom experiment. The sensitivity and specificity of the method were demonstrated in one volunteer and in four patients with intracranial tumors during carbogen inhalation. We utilized (ΔR2*, ΔR1) scatter plots to analyze the multiparametric response amplitude of each voxel within an area of interest. In normal tissue R2* decreased and R1 increased moderately in response to the elevated blood and tissue oxygenation. A strong negative ΔR2* and ΔR1 response was observed in veins and some tumor areas. Moderate positive ΔR2* and ΔR1 response amplitudes were found in fluid-rich tissue as in cerebrospinal fluid, peritumoral edema, and necrotic areas. The multiparametric approach was shown to increase the specificity and sensitivity of oxygen-enhanced MRI compared to measuring ΔR2* or ΔR1 alone. It is thus expected to provide an optimal tool for the identification of tissue areas with low oxygenation, e.g., in tumors with compromised oxygen supply.

Journal of Magnetic Resonance Imaging, 2010

To compare two DeltaR2* quantification methods for analyzing the response of intracranial tumors ... more To compare two DeltaR2* quantification methods for analyzing the response of intracranial tumors to different breathing gases. The determination of changes in the magnetic resonance imaging (MRI) relaxation rate R2* (DeltaR2*), induced by hyperoxic and hypercapnic respiratory challenges, enables the noninvasive assessment of blood oxygenation changes and vasoreactivity. Sixteen patients with various intracranial tumors were examined at 3.0 T. The response to respiratory challenges was registered using a dynamic multigradient-echo sequence with high temporal and spatial resolution. At each dynamic step, DeltaR2* was derived in two different ways: 1) by subtraction of R2* values obtained from monoexponential decay functions, 2) by computing DeltaR2* echo-wise from signal intensity ratios. The sensitivity for detection of responding voxels and the behavior of the "global" response were investigated. Significantly more responding voxels (about 4%) were found for method (1). The "global" response was independent from the chosen quantification method but showed slightly larger changes (about 6%) when DeltaR2* was derived from method (1). Similar results were observed for the two methods, with a slightly higher detection sensitivity of responding voxels when DeltaR2* was obtained from monoexponential approximation.

Journal of Magnetic Resonance Imaging, 2010

To compare two magnetic resonance (MR) contrast mechanisms, R*(2) BOLD and balanced SSFP, for the... more To compare two magnetic resonance (MR) contrast mechanisms, R*(2) BOLD and balanced SSFP, for the dynamic monitoring of the cerebral response to (C)O(2) respiratory challenges. Carbogen and CO(2)-enriched air were delivered to 9 healthy volunteers and 1 glioblastoma patient. The cerebral response was recorded by two-dimensional (2D) dynamic multi-gradient-echo and passband-balanced steady-state free precession (bSSFP) sequences, and local changes of R*(2) and signal intensity were investigated. Detection sensitivity was analyzed by statistical tests. An exponential signal model was fitted to the global response function delivered by each sequence, enabling quantitative comparison of the amplitude and temporal behavior. The bSSFP signal changes during carbogen and CO(2)/air inhalation were lower compared with R*(2) BOLD (ca. 5% as opposed to 8-13%). The blood-oxygen-level-dependent (BOLD) response amplitude enabled differentiation between carbogen and CO(2)/air by a factor of 1.4-1.6, in contrast to bSSFP, where differentiation was not possible. Furthermore, motion robustness and detection sensitivity were higher for R*(2) BOLD. Both contrast mechanisms are well suited to dynamic (C)O(2)-enhanced MR imaging, although the R*(2) BOLD mechanism was demonstrated to be superior in several respects for the chosen application. This study suggests that the R*(2) BOLD and bSSFP-response characteristics are related to different physiologic mechanisms.

IEEE Transactions on Medical Imaging, 2000

In dynamic magnetic resonance imaging (MRI) studies, the motion kinetics or the contrast variabil... more In dynamic magnetic resonance imaging (MRI) studies, the motion kinetics or the contrast variability are often hard to predict, hampering an appropriate choice of the image update rate or the temporal resolution. A constant azimuthal profile spacing (111.246 degrees), based on the Golden Ratio, is investigated as optimal for image reconstruction from an arbitrary number of profiles in radial MRI. The profile order is evaluated and compared with a uniform profile distribution in terms of signal-to-noise ratio (SNR) and artifact level. The favorable characteristics of such a profile order are exemplified in two applications on healthy volunteers. First, an advanced sliding window reconstruction scheme is applied to dynamic cardiac imaging, with a reconstruction window that can be flexibly adjusted according to the extent of cardiac motion that is acceptable. Second, a contrast-enhancing k-space filter is presented that permits reconstructing an arbitrary number of images at arbitrary time points from one raw data set. The filter was utilized to depict the T1-relaxation in the brain after a single inversion prepulse. While a uniform profile distribution with a constant angle increment is optimal for a fixed and predetermined number of profiles, a profile distribution based on the Golden Ratio proved to be an appropriate solution for an arbitrary number of profiles.

IEEE Transactions on Medical Imaging, 2000

The steady-state free precessing (SSFP) sequences, widely used in MRI today, acquire data only du... more The steady-state free precessing (SSFP) sequences, widely used in MRI today, acquire data only during a short fraction of the repetition time (TR). Thus, they exhibit a poor scan efficiency. In this paper, a novel approach to extending the acquisition window for a given TR without considerably modifying the basic sequence is explored for radial SSFP sequences. The additional data are primarily employed to increase the signal-to-noise ratio, rather than to improve the temporal resolution of the imaging. The approach is analyzed regarding its effect on the image SNR (signal to noise ratio) and the reconstruction algorithm. Results are presented for phantom experiments and cardiac functions studies. The gain in SNR is most notable in rapid imaging, since SNR enhancement for a constant repetition time may be used to compensate for the increase in noise resulting from angular undersampling.