Biomedical subjects

J P Groen

Publications and source records attributed to J P Groen.

5 recordsLinked to original sources

PubMed1992

MR angiography with pulsatile flow.

To achieve acceptable scan times, current multiple thin slice and 3D MR angiography (MRA) methods usually are based on continuous data acquisition, without ECG-synchronization. The purpose of this work is to study consequences of pulsatile blood flow for the 2D inflow method. Arterial blood flow and blood signal intensity versus cardiac phase were studied by a 2D phase based method with retrospective cardiac synchronization. Such studies were performed in different parts of the body and with different excitation flip angles. As expected, a clear relation between intensity enhancement and time dependent flow can be demonstrated. The raw data of these multiphase studies was used to simulate alternative inflow MRA data acquisition strategies to improve image quality, without the excessive increase in scan time implied by standard cardiac triggering. The alternatives investigated were data collection during part of the cardiac cycle and cardiac-ordered phase encoding. Simulation results indicate that the best results are obtained by a combination of both strategies. This method was implemented on Philips Gyroscan systems to compare it with standard nontriggered 2D inflow in practical MRA studies. For highly pulsatile flow, much better MR angiograms were obtained in this way.

Blood Vessels↗

PubMed1989

Experimental approaches to image localized human 31P NMR spectroscopy.

Experimental procedures for obtaining localized 31P NMR spectra of humans by means of the ISIS sequence are discussed in detail. The technique is optimized for use with volume coils and with surface coils in order to measure localized 31P NMR spectra of different tissues and organs. Selective frequency-modulated (FM) inversion and excitation pulses are applied for optimal inversion or excitation despite B1 inhomogeneity. Pulse imperfection may lead to spurious signal contributions from outside the selected volume; this contamination is reduced by using long pulse intervals, by properly ordering the ISIS acquisitions, and by using FM excitation pulses. Simultaneous measurement of multiple volumes was implemented by including an additional selective inversion pulse, and an extension of the ISIS addition/subtraction scheme. Localized T1 measurements with surface coils are implemented by using a B1-insensitive inversion pulse in the inversion recovery sequence. The quantitative reproducibility of localized 31P NMR spectra was verified. Absolute metabolite concentration can be determined after a suitable calibration of the 31P NMR spectrum. Localized shimming is required to obtain localized 31P NMR spectra of excellent spectral resolution. This is done by monitoring the 1H NMR signal from water by a single-shot localization technique. The techniques discussed can be applied to obtain spectra of brain, liver, heart, and other organs. 31P NMR spectra of intracranial tumors demonstrate its applicability in the examination of patients.

Brain↗

PubMed1985

Very fast MR imaging by field echoes and small angle excitation.

An MR imaging technique has been developed producing head and body images of diagnostic quality in only a few seconds acquisition time. The Fourier type imaging technique uses excitation with relatively small excitation angels, echoes produced by gradient inversion, and extremely fast profile repetition. A typical result at 0.5 T is an artifact-free head image of 128 x 128 resolution, 10 mm slice thickness in an acquisition time of 2 seconds.

Humans↗

PubMedDate not supplied

Fast Field Echo imaging: an overview and contrast calculations.

Current fast imaging techniques are based on gradient echo sequences with reduced flip angle excitation pulses and very short repetition times TR. Practical T2 values may be of the order of TR or longer. In this situation, a different image contrast can be obtained, depending on details of the sequence. Four essentially different versions of the basic Fast Field Echo (FFE) sequence can be distinguished and are described systematically in this article. For these sequences, image contrast formulas are presented. Practical imaging should tolerate small field inhomogeneities. This requirement can be satisfied by only three of the four versions. Numerical simulations are used to study the influence of a modified phase alternation scheme on image contrasts of two of the remaining sequences. The results of the calculations are verified by phantom studies on a 1.5-T whole-body imager. Implications for contrast in clinical images are discussed in relation to head images obtained on the same machine.

Humans↗

PubMedDate not supplied

Magnetic resonance fast Fourier imaging.

A new method for fast magnetic resonance imaging is presented. It provides a more rapid data acquisition than two-dimensional Fourier imaging (2DFI) by a factor which may be chosen depending on the required signal-to-noise ratio of the image. In addition to the readout gradient of 2DFI, the present method employs an oscillating modulation gradient. In this way, a curved alternating trajectory in k space is sampled after each spin excitation. For a p-times accelerated data acquisition, the trajectory consists of p periods, where p is of the order of 2 to 8 for low-frequency gradient modulation but can be chosen higher if certain hardware requirements are met. Adequate sampling density in k space is obtained by scanning shifted trajectories after subsequent spin excitations. The method can be combined with volume imaging (3DFI) and multiple slice 2DFI. It was implemented on a standard Philips Gyroscan system without any hardware modifications. Results obtained for an acceleration factor p = 4 are shown.

Brain↗