In-vitro assessment of the behavior of magnetic resonance angiography in the presence of constrictions.

RATIONALE AND OBJECTIVES: Time-of-flight magnetic resonance angiography (TOF/MRA) is increasingly used to assess the nature and severity of stenotic blood vessels. Flow artifacts associated with high flows and/or narrow constrictions may confuse the interpretation of these studies. Accurate TOF/MRA evaluations demand an understanding of the nature of these flow effects. METHODS: A two-dimensional TOF pulse sequence was used to acquire images of five smoothly constricted phantoms at various flows. Analysis included assessment of phantom appearance and quantification of apparent constriction diameter and signal variations. RESULTS: Most notable flow phenomena were a cone-shaped region of high signal, a region of signal void, and signal preservation along the wall. When visible, constriction diameter was accurately measured. CONCLUSIONS: The behavior observed in TOF/MRA images can be understood by considering the contributing mechanisms of phase dispersion, turbulence, poststenotic flow eddies, flow reversal, and flow separation.

Interleaved magnetic resonance and ultrasound by electronic synchronization.

Increasing attention has been directed toward using magnetic resonance imaging (MRI) to assess blood flow velocity. Complete acceptance of this application requires validation of MRI-derived flow measurements against an accepted flow measurement technique such as Doppler ultrasound in an in vivo situation. To provide an accurate correlation in the presence of rapid changes in blood flow, the MR acquisition should be made nearly simultaneously with the ultrasonic measurements. Unfortunately, standard ultrasound equipment generates radio frequency signal which interferes with MRI. Near-simultaneous acquisition of MR data and ultrasonic blood flow data should be possible if the two measurements are properly synchronized. In the technique presented, ultrasound is made to peacefully coexist with MRI by gating the ultrasound so that it is disabled during the time of MR data acquisition. Phantom and animal experiments confirm the use of this procedure. Although we did not specifically test new fast-scan MR techniques, our technique is completely general and should work equally well with spin-echo as well as newer fast scanning MRI techniques.

Image processing on Macintosh II: a practical boundary finding algorithm for biomedical measurement.

With increasing research interest in displaying and analyzing biomedical images, a practical personal computer based off-line image processing software would be useful. This paper describes the implementation of an image processing work station on a Macintosh II which features a novel edge detection capability useful for biomedical measurement. The boundary finding algorithm is coded in Turbo Pascal, and operates at a speed comfortable for interactive operation. Depending on the complexity of the problem, it usually takes less than a minute for the measurement of an image. The edge detection algorithm has an built-in edge detector with decision-making capability, and can be efficiently controlled by a mouse. In this way, the local accuracy of an automatic edge detection operator, and the global accuracy of the human eye (through manual control of a mouse) are combined.

Single-acquisition chemical-shift imaging of a binary system with use of stimulated echoes.

A method for separating binary chemical-shift components with a single image data acquisition by means of stimulated echoes is demonstrated. With a strategy analogous to the modified Dixon method, three stimulated echoes were acquired to form three complex images. In each of the images, the complex pixel intensities were imparted, by design of the pulse sequence, with a phase factor carrying chemical-shift or field inhomogeneity information. With these three images, true fat/water separation can be obtained in biologic tissues. Studies at high field strength (4.7 T) on a toluene phantom, a pseudo-binary chemical-shift system, were used to evaluate the applicability of the method. Its clinical feasibility was demonstrated on a healthy human subject in a 0.6-T whole-body imaging system.