Biomedical subjects

Holger Eggers

Publications and source records attributed to Holger Eggers.

7 recordsLinked to original sources

PubMed2006

Signal decay correction in 2D ultra-short echo time imaging.

OBJECTIVES: In ultra-short echo time (UTE) imaging, the transverse magnetization experiences significant decay during data acquisition, which gives rise to a loss of signal intensity and spatial resolution in conventionally reconstructed images. The present work proposes an iterative algorithm to correct these adverse effects. MATERIALS AND METHODS: The algorithm involves solving a large linear system of equations and requires a separate reference scan to map the spatially variant transverse relaxation time. It was implemented and applied to simulations and to experiments with custom-built resolution phantoms. The evaluation focused on its ability to improve the actual point-spread function (PSF) and on its influence upon the local signal-to-noise ratio (SNR). RESULTS: The algorithm is demonstrated to virtually restore the ideal PSF of the acquisition. It proves to provide images with better signal intensity and spatial resolution but reduced SNR, if the transverse relaxation time is known with sufficient accuracy. CONCLUSIONS: The present work shows the basic feasibility of correcting signal decay effects in UTE imaging.

Algorithms↗

PubMed2006

Simultaneous imaging and R2* mapping using a radial multi-gradient-echo (rMGE) sequence.

PURPOSE: To demonstrate a rapid MR technique that combines imaging and R2* mapping based on a single radial multi-gradient-echo (rMGE) data set. The technique provides a fast method for online monitoring of the administration of (super-)paramagnetic contrast agents as well as image-guided drug delivery. MATERIALS AND METHODS: Data are acquired using an rMGE sequence, resulting in interleaved undersampled radial k-spaces representing different echo times (TEs). These data sets are reconstructed separately, yielding a series of images with different TEs used for pixelwise R2* mapping. A fast numerical algorithm implemented on a real-time reconstruction platform provides online estimation of the relaxation rate R2*. Simultaneously the images are summed for the computation of a high-resolution image. RESULTS: Convenient high-resolution R2* maps of phantoms and the liver of a healthy volunteer were obtained. In addition to stable intrinsic baseline maps, the proposed technique provides particularly accurate results for the high relaxation rates observed during the presence of (super-)paramagnetic contrast agents. Assuming that the change in R2* is proportional to the concentration of the agent, the technique offers a rough estimate for dynamic dosage. CONCLUSION: The simultaneous online display of morphological and parametric information permits convenient, quantitative surveillance of contrast-agent administration.

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↗

PubMed2005

Correction for heart rate variability improves coronary magnetic resonance angiography.

PURPOSE: To address degradation of coronary MR angiography (MRA) image quality due to heart rate variability (HRV)-associated variations in coronary artery position and motion. MATERIALS AND METHODS: Free-breathing navigator-gated and -corrected coronary MRA using subject-specific trigger delays and acquisition windows was combined with a real-time HRV correction algorithm, such as commonly used in left ventricular wall motion studies. Ten healthy adults underwent free-breathing navigator-gated and -corrected coronary MRA with and without HRV correction. Signal-to-noise (SNR), contrast-to-noise (CNR), vessel length, diameter, sharpness, and subjective image quality (on a five-point scale) were compared in a blinded fashion. RESULTS: Vessel sharpness improved significantly for both the left (LCA) and right (RCA) coronary artery systems (P = 0.016 and P = 0.015, respectively) with the use of HRV correction. Subjective image quality also improved significantly when HRV correction was used (P = 0.003). There were no significant differences with regard to SNR and CNR (P > 0.1). CONCLUSIONS: Preliminary results suggest that HRV correction improves objective and subjective image quality in coronary MRA. Continued studies in patients with known or suspected coronary artery disease are warranted to investigate the clinical impact of this technique.

Adolescent↗

PubMed2005

SNR enhancement in radial SSFP imaging using partial k-space averaging.

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.

Algorithms↗

PubMed2003

Image-based tracking of optically detunable parallel resonant circuits.

In this work strategies for the robust localization of parallel resonant circuits are investigated. These strategies are based on the subtraction of two images, which ideally differ in signal intensity at the positions of the devices only. To modulate their signal amplification, and thereby generate the local variations, the parallel resonant circuits are alternately detuned and retuned during the acquisition. The integration of photodiodes into the devices permits their fast optical switching. Radial and spiral imaging sequences are modified to provide the data for the two images in addition to those for a conventional image in the same acquisition time. The strategies were evaluated by phantom experiments with stationary and moving catheter-borne devices. In particular, rapid detuning and retuning during the sampling of single profiles is shown to lead to a robust localization. Moreover, this strategy eliminates most of the drawbacks usually associated with image-based tracking, such as low temporal resolution. Image-based tracking may thus become a competitive (if not superior) alternative to projection-based tracking of parallel resonant circuits.

Catheterization↗

PubMed2002

Combined high-resolution and real-time imaging: a technical feasibility study on coronary magnetic resonance angiography.

PURPOSE: To propose a new approach to combining high-resolution and real-time imaging and to show its technical feasibility on the example of coronary magnetic resonance angiography. MATERIALS AND METHODS: The insertion of fast two-dimensional (2D) acquisitions into time intervals that have not been utilized by triggered or gated 2D or three-dimensional (3D) acquisitions so far is suggested, as well as the immediate reconstruction and display of the additional data. For a technical validation of this concept, a 2D ventricular function protocol was interleaved into a cardiac-triggered and respiratory-gated 3D coronary angiography protocol. Dedicated hardware was employed to rapidly process the data originating from the former. Since the sampling of the latter was restricted to intervals with minimal motion, remaining periods of time could be used to simultaneously image the cardiac and respiratory motion. RESULTS: The technical feasibility of the proposed approach was demonstrated by successful measurements with the combined high-resolution and real-time protocol in volunteers. All examinations provided short axis views during the acquisition and angiograms of selected parts of the coronary system after its completion. CONCLUSION: The investigated concept allows high-resolution measurements to be complemented with real-time imaging functionality without affecting the scan time or image quality. In the particular application considered, an image-based patient monitoring or motion correction is enabled, indicating potential benefits of combining two very dissimilar methods of data acquisition in one measurement.

Coronary Circulation↗