Dieter Matthaei

Rapid three-dimensional MR imaging using the FLASH technique.

Fast low-angle shot (FLASH) imaging is a new technique for rapid magnetic resonance (MR) imaging that reduces acquisition times to seconds while retaining spatial resolution. This article deals with a three-dimensional (3D) variant of the FLASH method that allows the recording of a 3D-data set of 128 X 128 X 128 pixels within an acquisition time of only 4 min. The method is demonstrated using a 2.35 T 40 cm bore MR system. Experiments are carried out on rabbit head and human extremities. Depending on the field of view, the isotropic resolution is 1 mm or even less leading to cross-sectional images with a 1 mm slice thickness. In principle, FLASH imaging techniques are applicable to any MR system without the need of major hardware modifications. However, high-speed computers, large storage capacity, and rapid image display routines greatly facilitate an advantageous use of the 3D-FLASH variant.

Inversion recovery snapshot FLASH MR imaging.

Snapshot fast low angle shot (FLASH) magnetic resonance (MR) imaging techniques have been developed to enable real time imaging of MR parameters. The method is based on a 64 x 128 FLASH tomogram acquired within less than 200 ms. This work describes snapshot FLASH MR using a single 180 degrees pulse prior to the acquisition of a series of FLASH images. The experiment creates continuous dynamic inversion recovery (IR) T1 contrast in successive images. The total acquisition time of 16 images displaying the IR behavior is less than 4 s. Representative snapshot FLASH IR MR images of the abdomen of healthy rats and of an implanted hepatic tumor are illustrated.

Rapid NMR imaging using stimulated echoes

Abstract Rapid NMR imaging with measuring times of the order of 200 ms (32 × 64 pixels) to 1 s (128 × 128 pixels) is demonstrated for the first time using a superconducting magnet supplied with a commercial gradient system. The technique is a simple extension of STEAM ( st imulated e cho a cquisition m ode) imaging recently presented in this journal. In the basic 90°- t 1 -90°- t 2 -90°- t 3 STEAM experiment, a single stimulated echo (STE) is recorded at t 3 = t 1 . For rapid imaging the final part of the sequence, i.e., the 90°- t 3 (STE) period, is replaced by a series of acquisition periods with small angle pulses. Each of these pulses “reads” only a small part of the longitudinal magnetization prepared by the two leading 90° pulses. Application of a different phase-encoding gradient for each STE signal yields, after Fourier transformation, a series of projections sufficient for reconstruction of an image using a standard 2D FT algorithm. Since the entire acquisition time of the rapid STEAM imaging sequence is limited only by the T 1 relaxation process, and since further the rf power deposition is considerably lower than for a multiple spin-echo sequence, the method is particularly suitable for high-field NMR imaging. 1 H images of phantoms and human extremities have been obtained at 100 MHz using a 2.3 T magnet with a bore of 40 cm.

Fast inversion recovery T1 contrast and chemical shift contrast in high-resolution Snapshot FLASH MR images

Fast MR imaging attracts the interest of both clinicians and physicists because new diagnostic information arises with reduced artifacts due to short investigational times. With the acceleration of the Snapshot FLASH MR sequence, the measurement of high-resolution images with 256 x 256 matrix is reported, together with contrasting prepulses that are applied to attain contrast in combination with higher in-plane resolution. Measuring times are in the range of a second. For whole-body imaging, a TR = 5.2 msec and a TE = 2.6 msec could be attained measuring omit 256 x 256 matrix images. Artifact-free images demonstrating T1 contrast and contrast from chemical shift are performed on moving organs (heart, intestine) in different experiments. These applications can easily be performed in a couple of minutes for clinical use. Especially in the lung, short TE and high resolution result in a new imaging quality of pulmonary and mediastinal vessels.

Three-dimensional FLASH MR imaging of thorax and abdomen without triggering or gating

FLASH (fast low-angle shot) imaging is a new technique for rapid two- and three-dimensional (2D and 3D) MRI with high signal-to-noise, high spatial resolution and low sensitivity to motions. In particular, transverse magnetizations are strongly dephased by rapid motions with time constants of several milliseconds. They are not refocused by the gradient echo signal used for FLASH imaging and therefore do not contribute to the image. Artifacts due to motions with longer time constants are averaged out by multiple data acquisitions either because of a statistical nature of the dynamic process itself or, in the case of periodic motions, because of the incoherent data sampling with respect to the time period. In general, the large number of signal acquisitions necessary for a true 3D data set turns out to be sufficient for this purpose. Since 1283 3D FLASH MR images require measuring times of only 4 min, even two or four accumulations of entire 3D data sets remain within acceptable clinical measuring times. The method for the first time depicts high-quality three-dimensional views of the thoracic and abdominal anatomy in enddiastole and endexspiratory rest without the need of gating or triggering. 3D FLASH images will be of great medical utility, especially in cases where topographic information is needed as, for example, for radiation therapy and angiography planning or preoperative diagnosis.

ECG-triggered arterial FLASH-MR flow measurement using an external standard

In ECG-triggered FLASH-MR images, the inflow of unsaturated spins into the imaging plane results in the reproducible delineation of time variant flow in the arterial system. With the additional acquisition of an external reference image upstream the arterial vessel under investigation, the quantification of flow is possible with the FLASH-MR sequence in one measurement. The method allows the rapid measurement of arterial flow at least in great vessels.