Biomedical subjects

Matthias Weigel

Publications and source records attributed to Matthias Weigel.

5 recordsLinked to original sources

PubMed2006

Contrast behavior and relaxation effects of conventional and hyperecho-turbo spin echo sequences at 1.5 and 3 T.

To overcome specific absorption rate (SAR) limitations of spin-echo-based MR imaging techniques, especially at (ultra) high fields, rapid acquisition relaxation enhancement/TSE (turbo spin echo)/fast spin echo sequences in combination with constant or variable low flip angles such as hyperechoes and TRAPS (hyperTSE) have been introduced. Due to the multiple spin echo and stimulated echo pathways involved in the signal formation, the contrast behavior of such sequences depends on both T2 and T1 relaxation times. In this work, constant and various variable flip angle sequences were analyzed in a volunteer study. It is demonstrated that a single effective echo time parameter TE(eff) can be calculated that accurately describes the overall T2 weighted image contrast. TE(eff) can be determined by means of the extended phase graph concept and is practically independent of field strength. Using the described formalism, the contrast of any TSE sequence can be predicted. HyperTSE sequences are demonstrated to show a robust and well-defined T2 contrast allowing clinical routine MRI to be performed with SAR reductions of typically at least 70%.

Adolescent↗

PubMed2006

Fast and quantitative high-resolution magnetic resonance imaging of the optic nerve at 3.0 tesla.

A novel and fast magnetic resonance imaging approach for imaging the optic nerve and the surrounding cerebrospinal fluid sheath is presented. The method provides high contrast between the nerve and cerebrospinal fluid and allows for accurate quantification of the optic nerve and its cerebrospinal fluid sheath diameter within 1.5 seconds scan time. Results of a volunteer study illustrate that measurements can reliably be performed even in the distal part of the intraorbital optic nerve track. Accuracy of quantification of the new technique is demonstrated by the assessment of changes in the optic nerve and CSF sheath diameter between straight gaze and 30 degrees abduction.

Adult↗

PubMed2004

Calculation of flip angles for echo trains with predefined amplitudes with the extended phase graph (EPG)-algorithm: principles and applications to hyperecho and TRAPS sequences.

The article presents an algorithm for calculation of flip angles in multiecho experiments to generate echoes with predefined amplitudes based on the extended phase graph algorithm. The algorithm can be used to optimize the echo envelope and thus the point spread function (PSF) in hyperecho and TRAPS (transition into the pseudosteady state) experiments while minimizing the total RF power. Implementations at 3 T using echo trains with Gaussian and Lorentzian PSF demonstrate a reduction in RF power by a factor of 3-5 while maintaining high image quality.

Algorithms↗

PubMed2004

Myocardial perfusion measurements by spin-labeling under different vasodynamic states.

In this study absolute myocardial perfusion was determined using a spin-labeling magnetic resonance imaging (MRI) technique at 2 Tesla. The technique was applied to 16 healthy volunteers at resting conditions, adenosine-induced stress, and oxygen breathing. Overall myocardial quantitative perfusion was determined as 2.3 +/- 0.8 mL/g/min (rest), 4.2 +/- 1.0 mL/g/min (adenosine), and 1.6 +/- 0.6 mL/g/min (oxygen), respectively. T1 of left ventricular blood pool decreased from 1709 +/- 101 ms (rest) to 1423 +/- 61 ms (oxygen), whereas T1 of right ventricular blood did not change significantly (1586 +/- 126 ms and 1558 +/- 150 ms). In conclusion, the presented technique for quantification of myocardial perfusion is an alternative to contrast agent-based methods. The spin labeling method is noninvasive and easily repeatable and it could therefore become an important tool to study changes in myocardial perfusion under different vasodynamic states.

Adenosine↗

PubMed2003

Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS).

A variation of the rapid acquisition with relaxation enhancement (RARE) sequence (also called turbo spin-echo (TSE) or fast spin-echo (FSE)) is presented. This technique uses variable flip angles along the echo train such that magnetization is initially prepared into the static pseudo steady state (PSS) for a low refocusing flip angle (alpha < 180 degrees ). It is shown that after such a preparation, magnetization will always stay very close to the static PSS even after significant variation of the subsequent refocusing flip angles. This allows the design of TSE sequences in which high refocusing flip angles yielding 100% of the attainable signal are applied only for the important echoes encoding for the center of k-space. It is demonstrated that a reduction of the RF power (RFP) by a factor of 2.5-6 can be achieved without any loss in signal intensity. The contribution of stimulated-echo pathways leads to a reduction of the effective TE by a factor f(t), which for typical implementations is on the order of 0.5-0.8. This allows the use of longer echo readout times, and thus longer echo trains, for acquiring images with a given T(2) contrast.

Computer Simulation↗