Projective Fourier angiography

Tissue perfusion in humans studied by Fourier velocity distribution, line scan, and echo-planar imaging

In tissue perfusion studies, FT velocity distribution imaging (VDI) intrinsically distinguishes signals from moving blood and volume-averaged tissue. Results in human thyroid gland, in vivo, using VDI line scan technique demonstrated separation of moving blood signal from glandular tissue, while VDI inner-volume echo-planar imaging of brain showed only CSF velocity above the image noise level. New alternating polarity gradient sequences which permit separation of diffusion and slow velocity are discussed. A novel method of 3D FT imaging (two spatial and one velocity dimension) combining inner-volume imaging and echo-planar imaging with velocity resolution of 0.15 mm/s per pixel is demonstrated. A novel graphical method of calculation and display of diffusion dependence in pulsed gradient sequences is presented.

MR Fourier transform arteriography using spectral decomposition

Reliable separation of arteries from other stationary tissues is accomplished through spectral decomposition by exploiting the pulsatile nature of the blood flow in the arteries. Fourier transformation of a series of projection images in the temporal direction along the cardiac cycle results in spectral images where the arteries are a part of the harmonic component images while stationary tissues and veins are represented as a de component image. From the magnitude of the spectral images an arteriogram can be obtained by summation of the harmonic component images excluding the de component image. This principle is applied to Fourier imaging in a cine mode data acquisition as well as line scan imaging. Since there is no need for the encoding of the flow-sensitive gradient, this technique is free from eddy current artifacts which have been one of the major obstacles to projection angiography using the flow-encoding gradient.

Projection images of the position-velocity joint spin density distribution

A new concept of phase encoding called position-velocity combined oblique Fourier phase encoding is introduced. It encodes both spatial and velocity information in a single oblique direction in the position-velocity space. Using this method, two-dimensional projection images of the three-dimensional position-velocity joint spin density distribution along different directions can be obtained. These projection images can provide detailed information on the flow system under study. The imaging of the two-dimensional projection is less time consuming compared to three-dimensional Fourier flow imaging and can be easily implemented on a conventional magnetic resonance imaging scanner.

A method for MR quantification of flow velocities in blood and CSF using interleaved gradient-echo pulse sequences

The aim of this study was to establish a rapid method for in vivo quantification of a large range of flow velocities using phase information. A basic gradient-echo sequence was constructed, in which flow was encoded along the slice selection direction by variation of the amplitude of a bipolar gradient without changes in sequence timings. The influence of field inhomogeneities and eddy currents was studied in a 1.5 T interleaved sequences for calibration and in vivo flow determination were constructed, and flow information was obtained by pairwise subtraction of velocity-encoded from velocity non-encoded phase images. Calibration was performed in a nongated mode using flow phantoms, and the results were compared with theoretically calculated encoding efficiencies. In vivo flow was studied in healthy volunteers in three different areas using cardiac gating; central blood flow in the great thoracic vessels, peripheral blood flow in the popliteal vessels, and flow of cerebrospinal fluid (CSF) in the cerebral aqueduct. The results show good agreement with results obtained with other techniques. The proposed method for flow determination was shown to be rapid and flexible, and we thus conclude that it seems well suited for routine clinical MR examinations.