Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1995-12-29
1997-04-22
O'Shea, Sandra L.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324314, G01R 3320
Patent
active
056232063
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/GB97/01640, filed Jul. 29, 1994.
This invention relates to Nuclear Magnetic Resonance (NMR) Imaging and more particularly to an apparatus for and method of obtaining an improved NMR image of a solid object using a gradient echo technique.
The gradient echo technique, and techniques derived from discussed by S. P. Cottrell, M. R. Halse and J. H. Strange, Meas. Sci. Technol., 1 (1990) 624. and Y. Mat Daud and M. R. Halse, Physica B, 176 (1992) 167; have been developed for the magnetic resonance imaging of solids and other short T.sub.2 systems. The basic apparatus and method employs a rapidly oscillating magnetic field gradient of large amplitude and is described and claimed in published UK Patent Application GB-A-8915090-8. In the arrangement a 90.degree. radio frequency (rf) pulse is applied to a sample at a gradient zero crossing and a gradient echo, frequency encoded with spatial information, is acquired one gradient period later. Profile degradation due to finite pulses can be recovered in large part either by shifting the center of the pulse relative to the gradient zero crossing or alternatively by leaving the pulse centered on the zero crossing and applying a spatially dependent phase rotation to the resulting profile. In either case a modification to the linearisation procedure is required. The echo is linearised and Fourier transformed to yield a one dimensional profile in the gradient direction.
Although the arrangement described in the aforementioned Patent Application provided good resolution when used with liquids when used with solids, resolution was not perfect. It can be shown that the maximum number of pixels across a profile in normal gradient echo imaging is restricted to: ##EQU1## where L is the sample length, wr the resolution, t the gradient period and t.sub.90 the radio frequency pulse length. For a gradient period of t=80 ms and a pulse length of t.sub.p =10 ms, N.sub.max is less than 42. This does not afford sensible spatial resolution.
An advantage of the aforementioned gradient echo technique is that the required bandwidth of the pulse is substantially reduced compared to previous techniques which, for example, require the gradient field to be on during the application of the rf pulse. However, in practice it is not possible to have infinitely short pulses, which would in any case, have an infinite bandwidth. Consequently the gradient is necessarily switching through zero during the period of application of the rf pulse and significantly degrades it. An understanding of the effect of the gradient during the period of application of the pulse leads to an understanding of a number of artifacts and errors previously encountered in gradient echo imaging experiments. In particular it is invariably found that the `best` echo is double, rather than single, peaked, and phase rotated at its center. The resulting profiles suffer from a loss of resolution and intensity in the "wings". One way of overcoming these problems, is described in a paper by P. J. McDonald, K. L. Perry, S. P. Roberts entitled "A repetitive pulse variant of gradient echo imaging" now published: reference P. J. McDonald, K. L. Perry., S. P. Roberts Meas. Science Technology 4 (1993) 896-898. According to the paper, by using repetitive low flip angle pulses in a FLASH variant of gradient echo imaging, resolution is improved. However, a problem suffered by this type of gradient echo imaging has been that the length of pulses has been shortened and this has reduced the signal to noise of the echo following each pulse.
The present invention arose from a consideration of the pulse bandwidth and double peaked echo problems and overcomes the aforementioned problems and retains the advantages of using true 90.degree. pulses.
International Patent Application Publication Number WO-AI-9006523 describes a method for selective excitation of NMR signals in which slice selection is achieved by use of a pulse and a gradient. The method is applicable to soft tissues.
Accord
REFERENCES:
patent: 4509015 (1985-04-01), Ordidge et al.
patent: 4833411 (1989-05-01), McDonald et al.
patent: 5252923 (1993-10-01), Cottrell et al.
patent: 5254949 (1993-10-01), McDonald et al.
patent: 5369362 (1995-12-01), Counsell
Y. Mat Daud et al, "Two-Dimensional Fourier transform NMR imaging of solids using large oscillatin field gradients", Physic B 176 (1992) 167-172.
Miller et al: "1H-Refocused Gradient Imaging of Solids", Journal of Magentic Resonance, vol. 82, No. 3, May 1, 1989, pp. 529-538, see p. 529--p. 532; figures 1,2.
Cory, et al: "NMR Images of Rotating Solids", Journal of Magnetic Resonance, vol. 76, No. 3, Feb. 15, 1988, pp. 543-547, see p. 543--p. 544; figure 1.
Roos, et al: "Spatial Localization in Stochastic NMR Imaging with Oscillating Gradients", Journal of Magnetic Resonance, vol. 87, No. 3, May 1, 1990, pp. 554-566, see the whole document.
Li et al: "41P NMR Imaging of solid Bone with Solid Echos Combined with Refocused Gradients", Physics in Medicine and Biology, vol. 35, No. 8, Aug. 1, 1990, pp. 1153-1158, see the whole document.
Cottrell, et al: "NMR Imaging of Solids Using Large Oscillating Field Gradients", Measurment Science & Technology, vol. 1, No. 7, Jul. 1, 1990, pp. 624-629, cited in the application, see the whole document.
Benson Timothy B.
McDonald Peter J.
British Technology Group Limited
Mah Raymond Y.
O'Shea Sandra L.
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