Electricity: measuring and testing – Particle precession resonance – Using an electron resonance spectrometer system
Patent
1991-01-16
1993-07-27
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using an electron resonance spectrometer system
G01R 3320
Patent
active
052313549
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to improvements in and relating to magnetic resonance imaging (MRI) apparatus and methods, and in particular to apparatus and methods for diagnostic imaging or mass screening and to contrast agents and media for use in such methods.
MRI is a diagnostic technique that has become particularly attractive to physicians as it is non-invasive and does not involve exposing the patient under study to potentially harmful radiation such as the gamma-radiation or X-radiation of conventional radiographic imaging.
Conventional MRI apparatus however is exremely costly to manufacture and operate and accordingly the occurence of MRI apparatus in hospitals, clinics and other medical or research institutions has thus far been relatively limited.
The expense of manufacture and operation of MRI apparatus is closely associated with the field strength that the primary magnet in the apparatus is required to generate in order to produce images of acceptable spatial resolution within an acceptable image acquisition time.
In general, primary magnets capable of generating field strengths of 0.1 to 2 T have been used and image acquisition times have been of the order of 10 to 30 minutes.
For relatively low field strengths of up to 0.15 T, resistive magnets (generally adjacent coaxial metal coils) have been used but the energy requirement (and as a result the heat generation) of such resistive magnets is very high. Thus a 0.1 T magnet will require about 30 kW electric power. For higher fields, superconducting magnets are conventionally used. The selection of the appropriate magnetic field strength involves balancing various factors: thus higher field results in a better signal
oise (S/N) ratio and hence better spatial resolution at a given S/N value, but also in greater manufacturing and operating expense and in poorer tissue contrast.
There is therefore a demand for MRI apparatus and techniques capable of achieving acceptable or improved in S/N ratios but using lower field magnets and without undue loss in spatial resolution.
The present invention is based on the concept of dispensing with the primary magnet and using as the constant field the earth's magnetic field. The extreme weakness of this field is a potential disadvantage which has deterred proposals for this approach until now but the ESREMRI technique which is described in detail hereinafter enables that weakness to be overcome.
The long image acquisition times generally result from the need to perform a large number (e.g. 64-1024) of pulse and detection sequences in order to generate a single image and in the need to allow the sample under study to reequilibrate between each sequence.
The degeneracy of the spin states of nuclei with non-zero spin, e.g. .sup.1 H, .sup.13 C, .sup.19 F, etc., is lost when such nuclei are placed within a magnetic field and transitions between the ground and excited spin states can be excited by the application of radiation of the frequency (.omega..sub.o) corresponding to energy difference E of the transition (i.e. .tau..omega..sub.o =E). This frequency is termed the Larmor frequency and is proportional to the strength of the field experienced by the nucleus. As there is an energy difference between the spin states, when the spin system is at equilibrium the population distribution between ground and excited spin states is a Boltzmann distribution and there is a relative overpopulation of the ground state resulting in the spin system as a whole possessing a net magnetic moment in the field direction. This is referred to as a longitudinal magnetization. At equilibrium the components of the magnetic moments of the individual non-zero spin nuclei in the plane perpendicular to the field direction are randomized and the spin system as a whole has no net magnetic moment in this plane, i.e. it has no tranverse magnetization.
If the spin system is then exposed to a relatively low intensity oscillating magnetic field perpendicular to the main field and produced by radiation at the Larmor frequency, transitions between grou
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Nycomed Innovation AB
Tokar Michael J.
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