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SLICE MOVEMENT in NMR IMAGING

IP.com Disclosure Number: IPCOM000039612D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 20K

Publishing Venue

IBM

Related People

Harrison, CG: AUTHOR

Abstract

In NMR (nuclear magnetic resonance) imaging selective excitation of a slice of spins in any extended object is accomplished by applying an RF (radio frequency) pulse with a narrow band width in the presence of a slice-selecting gradient. The excited slice will be located at the position r, where r satisfies the relation: (Image Omitted) (1) where f is the carrier frequency of the rf pulse, fo is the normal spin resonance frequency and is the magnetogyric ratio: (2) where Bo is the (homogeneous) static magnetic field.

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SLICE MOVEMENT in NMR IMAGING

In NMR (nuclear magnetic resonance) imaging selective excitation of a slice of spins in any extended object is accomplished by applying an RF (radio frequency) pulse with a narrow band width in the presence of a slice-selecting gradient. The excited slice will be located at the position r, where r satisfies the relation:

(Image Omitted)

(1) where f is the carrier frequency of the rf pulse, fo is the normal spin resonance frequency and is the magnetogyric ratio: (2) where Bo is the (homogeneous) static magnetic field. In the trivial experiment: f=fo and the slice is excited at the zero crossing of the gradient field In the more general case, f / fo and the slice is displaced from the zero crossing by: (3) This is called

"slice movement" and can be accomplished generally by programming a frequency synthesizer from a control computer. During the signal acquisition phase of an NMR imaging experiment, the slice selecting gradient will be off, and so the nominal spin frequency will revert to fo. (The resonance will, of course, be broadened by the read-out gradient but this does not shift the center frequency of the excited spins.) In NMR in general and NMR imaging in particular, it is important to maintain a fixed phase relationship between the excitation RF signal and the demodulating frequency applied during read-out. Unless the phase relationship is constant, the phase of the acquired signal will vary in an unknown manner and require separate phase correction for each experiment; this would be a major disadvantage. For NMR imaging with slice movement, there are thus two options for the demodulating frequency: 1. Demodulation takes place at fo, but the frequency synthesizer is specially adapted to move from f to fo

in a way that ensures a known phase relationship. 2. Demodulation takes place at f. In this case the read-out signal is displaced in frequency by the

difference (f-fo). This has two practical

consequences: (i) the band width of the receiver must

be increased by 2 (f-fo), which admits more noise, and

(ii) the image becomes displaced in the image space

because the coordinate origin has moved. Most commercial imaging systems use option 1, a complex, expensive phase-coherent synthesizer. Apparatus employing option 2, while avoiding the problem of image shift, is schematically shown in Fig. 1. The modulation/demodulation frequency f is generated by frequency synthesizer 10 and fed to modulator 11 and demodulator 15. Modulator 11 and amplifier 12 are part of the RF transmitter, and signals therefrom excite an object located near probe 13. Resonant signals from the object are picked up by the probe and fed through

1

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pre-amplifier 14 to demodulator 15. An exemplary NMR experiment is illustrated in Fig. 2. SLICE MOVEMENT IN NMR IMAGING - Continued With reference to Fig. 3, consider a two-dimensional sl...