Biomedical subjects

Sebastian Kozerke

Publications and source records attributed to Sebastian Kozerke.

At least 19 recordsLinked to original sources

PubMed2006

Assessing arterial blood flow and vessel area variations using real-time zonal phase-contrast MRI.

PURPOSE: To measure peripheral artery function using a real-time phase-contrast (PC)-MRI sequence with tailored image-processing algorithms for flow computation. MATERIALS AND METHODS: An approach to real-time flow measurements was developed based on two-dimensional spatially selective excitation pulses and consecutive tailored processing of the data to derive blood flow and vessel area variations. The data acquisition strategy allows for flow measurements at high spatial and temporal resolutions of 1 mm(2) and 50 msec, respectively. In postprocessing the vessel area is automatically extracted using correlation measures in conjunction with morphological image operators. By means of in vitro and in vivo validations, it is shown that the current methods provide accurate and reproducible measurements of flow and vessel area variations. RESULTS: In vitro the comparison between the lumen area measured with the presented method and the values obtained by caliper gauge measurement showed a difference of 3.4% +/- 3.4% (mean +/- 2 SD). Similarly, the comparison between the stroke volumes determined with the presented method and by stopwatch and bucket measurements yielded a difference of 6.1% +/- 2.1%. In vivo the results from the real-time measurements for lumen area and stroke volume were compared with those from a gated PC-MRI technique with differences of 4.8% +/- 14% and 3.0% +/- 24.7%, respectively. CONCLUSION: The presented method constitutes a reliable tool set for quantifying the variations of blood flow and lumen area in the superficial femoral artery during reactive hyperemia and for studying their correlation with cardiovascular risk factors.

Algorithms↗

PubMed2006

k-t BLAST reconstruction from non-Cartesian k-t space sampling.

Current implementations of k-t Broad-use Linear Acqusition Speed-up Technique (BLAST) require the sampling in k-t space to conform to a lattice. To permit the use of k-t BLAST with non-Cartesian sampling, an iterative reconstruction approach is proposed in this work. This method, which is based on the conjugate gradient (CG) method and gridding reconstruction principles, can efficiently handle data that are sampled along non-Cartesian trajectories in k-t space. The approach is demonstrated on prospectively gated radial and retrospectively gated Cartesian imaging. Compared to a sliding window (SW) reconstruction, the resulting image series exhibit lower artifact levels and improved temporal fidelity. The proposed approach thus allows investigators to combine the specific advantages of non-Cartesian imaging or retrospective gating with the acceleration provided by k-t BLAST.

Artifacts↗

PubMed2006

Coil setup optimization for 2D-SENSE whole-heart coronary imaging.

Accelerated "whole-heart" coronary imaging with sensitivity encoding applied in both phase encoding directions (2D-SENSE) was investigated. In order to maximize the signal-to-noise ratio (SNR), coil configuration optimization was performed. To this end, 10 practical coil configurations each consisting of six standard coil elements were investigated and the local SNR was assessed by means of phantom experiments. Based on the experimental data, a symmetric configuration was found to yield the highest SNR. In a volunteer study, 2D-SENSE coronary images were obtained with total reduction factors of 3 and 4. Excellent depiction of the right and left coronary systems was possible in all cases.

Adolescent↗

PubMed2006

2D-spatially-selective real-time magnetic resonance imaging for the assessment of microvascular function and its relation to the cardiovascular risk profile.

BACKGROUND: While local endothelial dysfunction of conduit arteries is well recognized as an early step in atherogenesis, contradictory observations are reported with regard to alterations in the microcirculation and their association with cardiovascular risk factors (RFs). A real-time MR approach was developed to investigate the relationship between the RFs profile and microcirculatory alterations assessed as impairment of reactive hyperemic flow in the leg circulation. METHODS: The MR technique was applied to patients (n = 17, Pats1) with 1.8 +/- 0.8 RFs but without peripheral arterial occlusive disease (PAD), to age-matched healthy controls (n = 13, Con1), to young controls (n = 12, 23 +/- 4 y), and to patients with RFs and PAD (n = 8, Pats2). RESULTS: Superficial femoral artery (SFA) peak hyperemic flow in Pats1 was reduced vs Con1 (24.6 +/- 4.2 vs 30.4 +/- 7.3 mL min-1 100 mL-1 calf tissue, p < 0.02), and minimal vascular resistance increased incrementally with the number of RFs and with Framingham and Procam risk scores. Flow-mediated vasodilation (FMD) of the SFA was blunted in both Pats1 and Con1 (-0.5 +/- 3.4% and +0.6 +/- 3.2%, respectively, both ns vs 0). In young controls, peak hyperemic flow (30.1 +/- 3.3 mL min-1. 100 mL-1) and endothelium-independent vasodilation (9.2 +/- 10.0%) were preserved, while FMD was minimal (2.0 +/- 5.9%,p < 0.02 vs endothelium-independent vasodilation). In Pats2, peak hyperemic flow was severely reduced (12.2 +/- 3.6 mL min-1 100 mL-1, p < 0.0003 vs Con1 and Pats1), and both FMD and endothelium-independent vasodilation were absent. CONCLUSIONS: Reactive hyperemic flow in the SFA, reflecting microcirculatory function of the lower limb, gradually decreases with increasing cardiovascular risk suggesting a role for microvascular dysfunction in atherogenesis. The presented MR approach might become a valuable tool to study (micro)-vascular pathophysiology.

Adult↗

PubMed2005

Linear response equilibrium.

A new periodic pulse sequence employing weak excitation is presented. This type of sequence drives the system into a steady-state with periodic time evolution from which the data can be reconstructed to a spectrum. It is demonstrated that the frequency response of such a sequence can be analyzed using perturbation methods and linear system analysis. A mathematical framework is proposed allowing the frequency response to be tailored by weighting a periodic flip function. The weak excitation level used implies very low specific absorption rates while generating a highly frequency selective signal in the order of 1/T2 with signal strengths comparable to those obtainable with conventional large flip angle balanced steady-state free precession techniques. The concept is illustrated with phantom experiments and in vivo feasibility of water fat separation is shown on human knee images.

Adipose Tissue↗

PubMed2005

Improved artery delineation in dual-stack coronary magnetic resonance angiography using parallel imaging at 3 T.

PURPOSE: To improve vessel sharpness and T2 preparation (T2Prep) in dual-stack three-dimensional coronary magnetic resonance angiography (MRA) by shortening the time delays between the magnetization preparation pulses and the imaging stacks using sensitivity encoding (SENSE) at 3 T. MATERIALS AND METHODS: By combining dual-stack three-dimensional coronary MRA with the parallel imaging technique SENSE at 3 T, the acquisition duration of each three-dimensional imaging stack was shortened by a factor of 2. The proposed technique was implemented and tested in experiments with a moving phantom and in measurements on six healthy volunteers. RESULTS: The time delay between the navigator, T2Prep, and second imaging stack was reduced by 37%, relative to conventional dual-stack angiography without parallel imaging. This enabled the achievement of comparable high-vessel sharpness values for the left and the right coronary arteries relative to values known from conventional single-stack three-dimensional coronary MRA at 3 T. CONCLUSION: Parallel imaging allows for improved vessel visualization in dual-stack coronary MRA, given shorter temporal delays between navigator, T2Prep, and the actual image acquisitions, and thus considerably facilitates simultaneous acquisition of high-resolution angiograms of the left and right coronary systems.

Adult↗

PubMed2005

Optimizing spatiotemporal sampling for k-t BLAST and k-t SENSE: application to high-resolution real-time cardiac steady-state free precession.

In k-t BLAST and k-t SENSE, data acquisition is accelerated by sparsely sampling k-space over time. This undersampling in k-t space causes the object signals to be convolved with a point spread function in x-f space (x = spatial position, f = temporal frequency). The resulting aliasing is resolved by exploiting spatiotemporal correlations within the data. In general, reconstruction accuracy can be improved by controlling the k-t sampling pattern to minimize signal overlap in x-f space. In this work, we describe an approach to obtain generally favorable patterns for typical image series without specific knowledge of the image series itself. These optimized sampling patterns were applied to free-breathing, untriggered (i.e., real-time) cardiac imaging with steady-state free precession (SSFP). Eddy-current artifacts, which are otherwise increased drastically in SSFP by the undersampling, were minimized using alternating k-space sweeps. With the synergistic combination of the k-t approach with optimized sampling and SSFP with alternating k-space sweeps, it was possible to achieve a high signal-to-noise ratio, high contrast, and high spatiotemporal resolutions, while achieving substantial immunity against eddy currents. Cardiac images are shown, demonstrating excellent image quality and an in-plane resolution of approximately 2.0 mm at >25 frames/s, using one or more receiver coils.

Artifacts↗

PubMed2005

Accelerating cine phase-contrast flow measurements using k-t BLAST and k-t SENSE.

Conventional phase-contrast velocity mapping in the ascending aorta was combined with k-t BLAST and k-t SENSE. Up to 5.3-fold net acceleration was achieved, enabling single breath-hold acquisitions. A standard phase-contrast (PC) sequence with interleaved acquisition of the velocity-encoded segments was modified to collect data in 2 stages, a high-resolution under sampled and a low-resolution fully sampled training stage. In addition, a modification of the k-t reconstruction strategy was tested. This strategy, denoted as "plug-in," incorporates data acquired in the training stage into the final reconstruction for improved data consistency, similar to conventional keyhole. "k-t SENSE plug-in" was found to provide best image quality and most accurate flow quantification. For this strategy, at least 10 training profiles are required to yield accurate stroke volumes (relative deviation <5%) and good image quality. In vivo 2D cine velocity mapping was performed in 6 healthy volunteers with 30-32 cardiac phases (spatial resolution 1.3 x 1.3 x 8-10 mm(3), temporal resolution of 18-38 ms), yielding relative stroke volumes of 106 +/- 18% (mean +/- 2*SD) and 112 +/- 15% for 3.8 x and 5.3 x net accelerations, respectively. In summary, k-t BLAST and k-t SENSE are promising approaches that permit significant scan-time reduction in PC velocity mapping, thus making high-resolution breath-held flow quantification possible.

Algorithms↗

PubMed2004

Accelerated dynamic Fourier velocity encoding by exploiting velocity-spatio-temporal correlations.

OBJECTIVE: To describe how the information content in a Fourier velocity encoding (FVE) scan can be transformed into a very sparse representation and to develop a method that exploits the compactness of the data to significantly accelerate the acquisition. MATERIALS AND METHODS: For validation, fully sampled FVE datasets were acquired in phantom and in vivo experiments. Fivefold and eightfold acceleration was simulated by using only one fifth or one eighth of the data for reconstruction in the proposed method based on the k-t BLAST framework. Reconstructed images were compared quantitatively to those from the fully sampled data. RESULTS: Velocity spectra in the accelerated datasets were comparable to the spectra from fully sampled datasets. The detected peak velocities remained accurate even at eightfold acceleration, and the overall shape of the spectra was well preserved. Slight temporal smoothing was seen in the accelerated datasets. CONCLUSION: A novel technique for accelerating time-resolved FVE scan is presented. It is possible to accelerate FVE to acquisition speeds comparable to a standard time-resolved phase-contrast scan.

Algorithms↗

PubMed2004

Comparative study of FAST gradient echo MRI sequences: phantom study.

PURPOSE: To investigate a balanced steady state free precession sequence (b-SSFP) under a large range of conditions and to compare its performance with other types of gradient echo sequences for dynamic imaging. MATERIALS AND METHODS: Balanced turbo field echo (b-TFE; Philips Medical Systems, Best, The Netherlands) was investigated in vitro at a range of T2/T1 along with T1-contrast enhanced turbo field echo (T1-TFE) and turbo field echo (TFE) so that a comparison could be made. Performance was quantified in terms of the initial slope of the signal-to-noise ratio (SNR) vs. 1/T1 curve (sensitivity) and the range of 1/T1 before signal saturation (contrast dynamic range [CDR]). RESULTS: The b-TFE sequence was found to best perform, in terms of an optimal CDR, with a 90 degrees flip angle (FA), saturation preparation, and short inversion time. Using these parameters, the sensitivity was also higher than that of the TFE sequence and T1-TFE sequence under their respective optimal conditions. For detection of small changes in contrast agent concentration (0.0-0.1 mM), b-TFE was also found to be the sequence of choice, with optimized parameters as follows, 90 degrees FA, shortest TR/TE, and no magnetization preparation. The smallest matrices gave the highest signal sensitivity for all three sequences. CONCLUSION: The CDR of b-TFE was much narrower than that of T1-TFE but could be widened under optimized conditions. The sensitivity of the b-TFE technique was the highest of the three sequences under all conditions tested.

Contrast Media↗

PubMed2004

Cardiac SSFP imaging at 3 Tesla.

Balanced steady-state free precession (SSFP) techniques provide excellent contrast between myocardium and blood at a high signal-to-noise ratio (SNR). Hence, SSFP imaging has become the method of choice for assessing cardiac function at 1.5T. The expected improvement in SNR at higher field strength prompted us to implement SSFP at 3.0T. In this work, an optimized sequence protocol for cardiac SSFP imaging at 3.0T is derived, taking into account several partly adverse effects at higher field, such as increased field inhomogeneities, longer T(1), and power deposition limitations. SSFP contrast is established by optimizing the maximum amplitude of the radiofrequency (RF) field strength for shortest TR, as well as by localized linear or second-order shimming and local optimization of the resonance frequency. Given the increased SNR, sensitivity encoding (SENSE) can be employed to shorten breath-hold times. Short-axis, long-axis, and four-chamber cine views obtained in healthy adult subjects are presented, and three different types of artifacts are discussed along with potential methods for reducing them.

Adult↗

PubMed2004

Sensitivity-encoded coronary MRA at 3T.

Long scan times are still a main limitation in free-breathing navigator-gated 3D coronary MR angiography (MRA). Unlike other MRI applications, high-resolution coronary MRA has not been amenable to acceleration by parallel imaging techniques due to signal-to-noise ratio (SNR) concerns. In the present work, mitigating SNR limitations by the transition to higher static magnetic field strength is proposed, thus enabling scan time reduction by the parallel sensitivity encoding (SENSE) technique. The study reports the implementation and evaluation of free-breathing navigator-gated 3D coronary MRA with SENSE at 3T. Results from 11 healthy subjects indicate that the approach permits significant scan time reduction in MRA of the left and right coronary systems. Quantitative image analysis and visual grading suggest that two-fold scan acceleration can be accomplished at nearly preserved image quality. The additional experiments appear to demonstrate that parallel MRA equally permits enhancing volume coverage and spatial resolution while maintaining scan time.

Adult↗

PubMed2004

Modified Look-Locker inversion recovery (MOLLI) for high-resolution T1 mapping of the heart.

A novel pulse sequence scheme is presented that allows the measurement and mapping of myocardial T1 in vivo on a 1.5 Tesla MR system within a single breath-hold. Two major modifications of conventional Look-Locker (LL) imaging are introduced: 1) selective data acquisition, and 2) merging of data from multiple LL experiments into one data set. Each modified LL inversion recovery (MOLLI) study consisted of three successive LL inversion recovery (IR) experiments with different inversion times. We acquired images in late diastole using a single-shot steady-state free-precession (SSFP) technique, combined with sensitivity encoding to achieve a data acquisition window of < 200 ms duration. We calculated T1 using signal intensities from regions of interest and pixel by pixel. T1 accuracy at different heart rates derived from simulated ECG signals was tested in phantoms. T1 estimates showed small systematic error for T1 values from 191 to 1196 ms. In vivo T1 mapping was performed in two healthy volunteers and in one patient with acute myocardial infarction before and after administration of Gd-DTPA. T1 values for myocardium and noncardiac structures were in good agreement with values available from the literature. The region of infarction was clearly visualized. MOLLI provides high-resolution T1 maps of human myocardium in native and post-contrast situations within a single breath-hold.

Contrast Media↗

PubMed2004

Navigator gating and volume tracking for double-triggered cardiac proton spectroscopy at 3 Tesla.

Respiratory motion compensation based on navigator echoes for double-triggered cardiac proton spectroscopy at 3.0 T is presented. The navigators measure the displacement of the liver-lung interface during free breathing. This information allows for double triggering on a defined window within the respiratory cycle and on a defined trigger delay after the R-wave based on the ECG. Furthermore, it allows the excitation volume to be shifted by the determined respiratory displacement within the defined window in real-time (volume tracking). Static and motion phantom experiments were performed in this study, and it was demonstrated that volume tracking permits the suppression of signal from tissue next to the localized volume. However, triggering on a defined respiratory position is still necessary to achieve high spectral quality, because shimming and water suppression calibration are only optimal for a small window of the respiratory cycle. Single-volume spectra obtained in the myocardial septum of healthy subjects are presented.

Electrocardiography↗

PubMed2004

Accelerating cardiac cine 3D imaging using k-t BLAST.

By exploiting spatiotemporal correlations in cardiac acquisitions using k-t BLAST, gated cine 3D acquisitions of the heart were accelerated by a net factor of 4.3, making single breathhold acquisitions possible. Sparse sampling of k-t space along a sheared grid pattern was implemented into a cine 3D SSFP sequence. The acquisition of low-resolution training data, which was required to resolve aliasing in the k-t BLAST method, was either interleaved into the sampling process or obtained in a separate prescan to allow for shorter breathhold durations in patients with heart disease. Volumetric datasets covering the heart with 20 slices at a spatial resolution of 2 x 2 x 5 mm3 were recorded with 20 cardiac phases in a total breathhold duration of 25-27 sec, or 18 sec if partial Fourier sampling was additionally employed. The feasibility of the method was demonstrated on healthy volunteers and on patients. The comparison of endocardial area derived from single slices of the 3D dataset with values extracted from separate single-slice acquisitions showed no significant differences. By shortening the acquisition substantially, k-t BLAST may greatly facilitate volumetric imaging of the heart for evaluation of regional wall motion and the assessment of ventricular volume and ejection fraction.

Computer Simulation↗

PubMed2004

Catheter tracking and visualization using 19F nuclear magnetic resonance.

This work presents an investigation into catheter visualization and localization using 19F nuclear magnetic resonance (NMR) in conjunction with proton imaging. For this purpose, the imaging capabilities of a standard system were extended to allow for 19F excitation and signal detection. Two modes of operation were implemented: 1) a real-time tracking mode that provides tip tracking and automatic slice position updates interleaved with real-time, interactive proton imaging; and 2) a non-real-time catheter length visualization mode in which the entire length of a catheter can be assessed. Initial phantom experiments were conducted with the use of an angiographic balloon catheter filled with the blood substitute perfluorooctylbromide (PFOB). Using limited bandwidth excitation centered at the resonances of the CF2 groups of PFOB, we found that sufficient signal could be received to facilitate tip tracking during catheter motion and length visualization for various catheter configurations. The present approach is considered a promising alternative to existing methods, which either are associated with safety concerns (if active markers are employed) or suffer from insufficient, direction-dependent contrast (if passive visualization is used). Furthermore, our approach enables visualization of the entire length of the catheter. The proposed method provides a safe technique that, unlike electrical or optical devices, does not require modification of commercially available catheters.

Catheterization↗

PubMed2004

Calibration of echo-planar 2D-selective RF excitation pulses.

Echo-planar radiofrequency (RF) pulses (EPP) are increasingly being used for 2D-selective excitation in MRI. Pulse schemes of this kind are susceptible to eddy-current effects, timing imperfections, and anisotropy of the gradient system. As a consequence, practical EPP implementations have been restricted to robust fly-back strategies that use only every other leg of the echo-planar trajectory for RF transmission. The present work is dedicated to enabling forward-backward EPP with RF transmission during each k-space segment, hence doubling the pulses' time efficiency. This is accomplished by comprehensive pulse calibration based on preparatory measurements of the system imperfections, including potential gradient anisotropy. The effectiveness of the method is demonstrated in vitro and in vivo. By doubling the speed of k-space coverage, the proposed method enhances the potential of EPP for numerous applications. For example, motion-sensitive techniques benefit from shorter feasible echo times (TEs) and improved excitation profiles resulting from reduced in-pulse motion. In sequences with fast repetition, shorter EPP help reduce the overall scan duration. Alternatively, the higher time efficiency of forward-backward EPP can enhance their spatial selectivity.

Anisotropy↗

PubMed2004

Free-breathing radial acquisitions of the heart.

There is considerable interest in performing free-breathing acquisitions of the heart in order to obtain high-quality images without the need for multiple, long breathholds. In this article a 3D motion-correction method is described that is based on image registration of in-plane data and through-plane slice tracking. A number of fast radial undersampled images are acquired, each of which is free of motion artifacts. Initially, in-plane translational and rotational motion between each image was corrected before combining the data to give a fully sampled image. At the next stage, correction of in-plane deformation, in addition to translations and rotations, was performed in the image domain. Through-plane translational motion was compensated using a navigator echo to move the acquisition plane. Using this method, information on the motion of the heart was captured at the same time as acquiring the image data. No motion model, assumptions about the motion, or training data are required. The method is demonstrated on phantom data and cardiac images acquired on free-breathing volunteers.

Algorithms↗