Surgery – Truss – Pad
Patent
1990-10-29
1994-06-21
Cohen, Lee S.
Surgery
Truss
Pad
A61B 505
Patent
active
053220650
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to paramagnetic materials for use in magnetic resonance imaging (MRI), and in particular to media containing deuterated stable free radicals and the use of deuterated free radicals for the manufacture of such contrast media.
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 for example the X-radiation of conventional radiography.
This technique however suffers from several serious draw-backs, including in particular the expense of manufacture and operation of the MRI apparatus, the relatively long scanning time required to produce an image of acceptable spatial resolution, and the problem of achieving contrast in the magnetic resonance (MR) images between tissue types having the same or closely similar imaging parameters, for example in order to cause a tissue abnormality to show up clearly in the images.
The expense of manufacture and operation of an 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 time (the image acquisition time). In general, magnets capable of generating field strengths of 0.1 to 2 T have been used and image acquisition times are commonly of the order of 10-30 minutes. For relatively low field strengths of up to 0.15 T, resistive magnets (generally adjacent coaxial metal coils) may be 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 improvements in S/N ratio, especially if such apparatus would enable lower field magnets to be used without undue loss in spatial resolution.
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 all 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. .omega..sub.o =E). This frequency is termed the Larmor frequency and is proportional to the strength of the applied field. 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 produced by radiation at the Larmor frequency, generally radiofrequency (RF) radiation, transitions between ground and excited spin states occur. If the exposure is for a relatively short duration then the r
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Cohen Lee S.
Gilbert Samuel
Nycomed Innovation AB
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