Surgery – Truss – Pad
Patent
1991-10-24
1994-02-22
Smith, Ruth S.
Surgery
Truss
Pad
1286534, 324309, 324316, A61B 5055
Patent
active
052878546
DESCRIPTION:
BRIEF SUMMARY
This is a United States national phase application of International Application Number PCT/EP90/00604, filed Apr. 12, 1990.
BACKGROUND OF THE INVENTION
The present invention relates to improvements in and relating to magnetic resonance imaging (MRI) and in particular to apparatus for and methods of electron spin resonance enhanced magnetic resonance imaging (ESREMRI).
ESREMRI, described by us in EP-A-296833 (Leunbach), is a method of magnetic resonance imaging in which amplification of the nuclear magnetic resonance signal, the free induction decay (FID) signal, is achieved by stimulating an electron spin resonance transition of a paramagnetic species present in the subject being imaged. Stimulation of the ESR transition leads to a polarization of the nuclear spin system responsible for the FID signals from which the magnetic resonance (MR) image of the subject is generated. This so called dynamic nuclear polarization is in effect an overpopulation, relative to equilibrium values, of the excited nuclear spin state and can be so large that the FID signal may be amplified by a factor of well over 100.
Using this technique, MR images may be generated by conventional imaging procedures, such as for example two and three dimensional Fourier transform, with enhanced signal to noise (SN) ratios (due to the amplification of the FID signal) and/or with shorter image acquisition times (since the nuclear spin system does not have to be allowed to relax towards equilibrium over a period comparable with T.sub.1, the spin-lattice relaxation time, for example about 1 second, between each excitation /FID signal detection cycle) and/or at lower strength primary magnetic fields than are conventionally utilized in MRI, e.g. 0.002 to 0.1 T or lower.
ESREMRI involves exposing the subject being imaged to pulses of electromagnetic radiation of frequencies selected such that ESR and NMR transitions are stimulated.
These frequencies are of course dependent on the strength of the primary magnetic field of the imaging apparatus; however, since, at the field strengths conventionally used in MRI, the ESR and NMR stimulating radiations are generally microwave (MW) and radiofrequency (RF) radiations, for the sake of convenience the ESR and NMR stimulating radiations will be referred to hereinafter as being MW and RF radiations respectively.
To maximize the FID signal enhancement in ESREMRI, the ESR transition(s) of the paramagnetic species, which may be naturally present in the subject being imaged but more generally will be administered to the subject as a contrast agent, should be stimulated at or near saturation level for a period leading up to the initial RF pulse of the RF pulse/FID signal detection cycle of the MR image acquisition procedure.
Exposure of live subjects to electromagnetic radiation of RF or MW frequencies (including the radiations of lower frequencies than are conventionally considered to be MW or RF but which are encompassed herein by those terms by virtue of the definition given above) may cause undesirable heating of the subject's tissues to occur and clearly it is essential for a diagnostic technique such as MRI (and ESREMRI) that the temperature increases in the tissue be kept down to an acceptable level.
To avoid excessive RF heating, there are recommendations for conventional MRI that the maximum radiation exposure, the specific absorption rate (SAR), should be about 1-8 W/kg bodyweight during the imaging procedure. If MRI is conducted in accordance with these recommendations, any tissue temperature rises should be acceptably low, e.g. less than about 1.degree. C., even for extended imaging periods.
Irradiation at power levels well above those recommended values however can be tolerated as long as the pulse duration of such radiation is short. Indeed, where pulsed RF or MW radiation is applied its heating effect may be lower than that of continuous wave radiation even where the SAR averaged over the whole exposure period may be much higher. Thus, some MR imagers operate using pulsed RF radiation where the
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Nycomed Innovation AB
Smith Ruth S.
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