lated at a frequency which, in the presence of the said
IMAGING SYSTEMS magnetic fields and pulsed magnetic field, is the resonance frequency for a position in the plane lying outside
The present invention relates to systems for provid- said slice,
ing images of distributions of a quantity, in a chosen 5 In order that the invention may be clearly understood
region of a body, by gyromagnetic resonance, particu- and readily carried into effect it will now be described
larly nuclear magnetic (resonance (NMR) techniques. with reference to the accompanying drawings, of
Such techniques may be used for examining bodies of which,
different kinds. However a particularly beneficial appli- piG. 1 shows in schematic form an NMR apparatus
cation is the examination of patients for medical pur- 10 with which the invention may be used,
poses. FIG. 2 shows the distribution of field measurement
It has been proposed that nuclear magnetic resonance probes for the apparatus of FIG. 1,
be used to provide distributions of water protons or FIG. 3 shows how the graajent field is disposed in an
other molecules or relaxation time constants in sectional examined slice in a prior arrangement,
slices or volumes of patients. It is particularly beneficial 15 FIGS ^ and 4b show how the gradient is disposed in
to perforin the examination to provide data which can different directions in the prior arrangement,
be analysed by techniques similar to thoselcnown for FIGS 5a and sb show how the radient field is dis.
distributions of x-ray attenuation provided by comput- osed ^ different directions in one Embodiment of this
ensed tomography (CT) systems, invention
An apparatus for and method of achieving the exami- 20 FIQ ... how the dient ^ drive lifiers
nation, and^improvements thereto, have been described of plG 1 afe arra d for one embodiment of this
and claimed m our co-pending patent applications Ser. *• .■
Nos. 039,650, 040,289, 039,649 and 048,777 and U.S. , ^ r.A , , , v
Pat No 4 254 778 FIG. 7 shows how the field probes may be used to
Apparatus of the form described in the said applica- 25 0O^fS} the gradient field,
tions has produced satisfactory pictures. These pictures . FIG" 8 shows in diagrammatic form the matrix inver
have been found to include artefacts resulting from «on arrangement for coil current error determinations,
noise introduced by the system. In particular some f9^owtst,how exPected gradient fieId values are
artefacts have been found to appear consistently at or calcdated for this invention
close to the centre of the picture. 30 ^ 10 « ^ed to show the significance of another
It is an object of this invention to provide an arrange- embodiment of this invention and FIG. 11 is a block
ment by which such noise originating artefacts can be dlagram of a clrcult for operating the embodiment dis
reduced. cussed with reference to FIG. 10.
It should also benoted that the prior system provides Nuclear magnetic resonance techniques are generally
signals with double sidebands, after demodulation with 35 used t0 analyse distributions of protons of water mole
a reference frequency. These sidebands must be sepa- cu»es ln a body. They can, however, be used to analyse
rated. This may be achieved in the Fourier transform other molecules.
following phase correction. This invention allows in Molecules subject to a magnetic field have a resonant
one embodiment an arrangement which can avoid that frequency related to the value of the field. Then by
problem. > , 40 application of ah RF magnetic field at the resbnant
According to the invention there is provided a nu- frequency they can be excited and the excitation al
clear magnetic resonance apparatus, for examining a lowed to decay. The decay causes an induced signal at
substantially planar slice of a body, the apparatus in- the resonant frequency in suitable coils around the
eluding means for applying magnetic fields to cause body.
resonance preferentially in the plane of said slice, means 45 In the arrangement described in the aforesaid applica
for applying a pulsed magnetic field having a gradient tions the magnetic field is adjusted to have different
across the plane of the slice to produce phase dispersion values in different parts of the body. Only those parts
in said resonance, means for sensing resonance signals having a resonant frequency equal to the frequency of
induced during said pulsed field for a plurality of differ- the RF field are excited.
ent directions thereof, means for demodulating the reso- 50 The basic magnetic field is in the z direction, usually
nance signals and means for further processing the de- coaxial with a patients body and can be called Hzo if all
modulated signals to provide a representation of said fields in that direction are referred to as Hz. A further
slice, wherein the resonance signals are demodulated at Hz field Gz is applied to have a gradient in the z direc
a frequency which, in the presence of the said magnetic tion so that Gz=(aHz/3z). This provides a unique total
fields and pulsed magnetic field, is the resonance fre- 55 field value in a chosen cross-sectional slice of the pa
quency for a position in the plane lying outside said tient. A rotating RF field Hi, of frequency chosen to
slice. cause resonance in the selected slice, is then applied.
A method of examining a substantially planar slice of Thus ideally only the molecules in the slice resonate, a body by nuclear magnetic resonance, the method The resonance signal from the slice can then be deincluding: applying magnetic fields to cause resonance 60 tected. However as it is detected there is applied a furpreferentially in the plane of the slice; applying a pulsed ther field, G^=(3Hz/3r) which is in the z-direction but magnetic field having a gradient across the plane of the has a gradient in a direction r perpendicular to z. This slice to produce phase dispersion in said resonance; causes dispersion of the resonant frequencies in the sensing the resonance signals induced during said pulsed r-direction and consequent dispersion of the resonance field for a plurality of different, directions thereof; de- 65 signal detected. Frequency analysis of this signal, prefmodulating the resonance signals and further processing erably. by Fourier Transformation, yields a plurality of the demodulated signals to provide a representation of resonance signals each for a different one of a plurality said slice; wherein the resonance signals are demodu- of strips in the chosen slice perpendicular to r.