Methodology of magnetic resonance imaging

Ernst, R. R.

doi:10.1017/s0033583500004121

Cited by 37 publications

(17 citation statements)

References 58 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…1. It involves a standard spin echo sequence (6) in which the phase-encoding gradients are applied after the 180°refocus-ing pulse. (A gradient echo scheme would be desirable in order to reduce the time needed for the experiment and was, in fact, usable on the phantom; however, it proved infeasible in the rat because of magnetic field inhomogeneities introduced by bone-tissue-air interfaces surrounding the cochlea.)…”

Section: Methodsmentioning

confidence: 99%

Oscillatory flow in the cochlea visualized by a magnetic resonance imaging technique.

Denk¹,

Keolian²,

Ogawa³

et al. 1993

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

We report a magnetic resonance imaging technique that directly measures motion of cochlear fluids. It uses oscillating magnetic field gradients phase-locked to an external stimulus to selectively visualize and quantify oscillatory fluid motion. It is not invasive, and it does not require optical line-of-sight access to the inner ear. It permits the detection of displacements far smaller than the spatial resolution. The method is demonstrated on a phantom and on living rats. It is projected to have applications for auditory research, for the visualization of vocal tract dynamics during speech and singing, and for determination of the spatial distribution of mechanical relaxations in materials.Although motion ofthe cochlear partition can be characterized in human cadavers or in living animals (1), its noninvasive measurement is presently limited to viewing a small portion of the cochlea through the round window (2, 3). Other methods involve damage to the cochlea (4,5). The basic idea behind our method, which avoids some of these problems, is to stimulate the ear acoustically with a single frequency and to use magnetic field gradients that oscillate phase-locked to the stimulus to generate a magnetic resonance imaging (MRI) image offluid movement that is correlated with the stimulus. Any spins that move in synchrony with the gradient oscillation see, on average, a slightly different magnetic field than the static field and therefore accumulate an additional phase shift that depends on the motion's amplitude and its phase relative to the stimulus. Subtracting two data sets, one of them taken with reversed stimulus phase, eliminates all signals but those due to spins moving in synchrony with the stimulus. The sum of the data sets gives a conventional MRI image. MATERIALS AND METHODSThe pulse sequence to realize this scheme is illustrated in Fig. 1. It involves a standard spin echo sequence (6) in which the phase-encoding gradients are applied after the 180°refocus-ing pulse. (A gradient echo scheme would be desirable in order to reduce the time needed for the experiment and was, in fact, usable on the phantom; however, it proved infeasible in the rat because of magnetic field inhomogeneities introduced by bone-tissue-air interfaces surrounding the cochlea.) The oscillating gradients, which can be applied along either the x, y, or z direction to measure different components of the motion vector, must be of opposite phase before and after the 1800 pulse. By varying the phase between the oscillating gradients and the stimulation, the phase shift between stimulation and motion can be determined.The duration ofthe oscillating gradient periods was typically 16.42 ms prior to the 1800 pulse and 8.8 ms thereafter. Even though not necessary in principle, synchronization of the beginning of the pulse sequence to a zero crossing of the stimulus oscillation markedly reduced phase-encode noise. The fluid-motion images were produced by subtracting two data sets taken with otherwise identical parameters except for a revers...

show abstract

Section: Methodsmentioning

confidence: 99%

Oscillatory flow in the cochlea visualized by a magnetic resonance imaging technique.

Denk¹,

Keolian²,

Ogawa³

et al. 1993

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

show abstract

“…A detailed analysis will show that the mirror images are produced on every echo after the first and, in general, the magnetization at the mth echo is It is interesting to note, as predicted from Eq. [3], that a mirror image can even be produced from the first spin echo if QCr = 0. This is demonstrated experimentally by Fig.…”

Section: Multiple Se Imagingmentioning

confidence: 97%

Non‐fourier encoding with multiple spin echoes

Panych

Mulkern

Saiviroonporn

et al. 1997

Magnetic Resonance in Med

View full text Add to dashboard Cite

The advantages and limitations of multiple spin-echo sequences for non-Fourier encoding are investigated. Complications caused by improper encoding of alternate magnetization pathways due to imperfect refocusing pulses are analyzed. It is shown that mirror image ghosts result if the encoding RF pulse matrix is real-valued. These ghosts can be avoided as long as the rows of the RF pulse matrix are conjugate symmetric, which implies that spatial profiles are real valued. Non-Fourier encoding using bases derived from wavelet, Hadamard, and other real-valued orthogonal functions does not result in a mirror ghost artifact. A RARE sequence for non-Fourier encoding has been implemented on a clinical imaging system and successfully applied for brain imaging.

show abstract

“…Generally speaking, the time sequence of any imaging experiment consists of two phases, the preconditioning period and the imaging period (58). (This is true even when one cannot clearly divide a pulse sequence into the two phases.)…”

Section: Image Contrasts and Their Extractionmentioning

confidence: 99%