GB1603560A - Imaging systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO IMAGING SYSTEMS (71) We, EMI LIMITED, a British company of Blyth Road, Hayes, Middlesex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to systems for providing images of the distribution of a quantity, in a chosen region of a body, by gyromagnetic resonance, for example nuclear magnetic resonance (NMR) techniques.

Such techniques may be used for examining bodies of different kinds. However a particularly beneficial application is the examination of patients for medical purposes.

Nuclear magnetic resonance is known for the analysis of materials, particularly by spectroscopy. Recently it has been suggested that the techniques be applied to medical examination to provide distributions of water content or relaxation time constants in sectional slices or volumes of patients. Such distributions are similar to, although of different significance from, the distributions of x-ray attenuation provided by computerised tomography (CT) systems.

Practical NMR systems operate by applying suitable combinations of magnetic fields to the body being examined, via coil systems, and detecting induced currents in one or more detector coil systems.

According to one aspect of the invention there is provided a method of examining, by nuclear magnetic resonance, a body placed in an examining region, the method including (a) the selective excitation of a slice of said body by: applying a steady magnetic field along an axis in the body; applying a gradient field, which in conjunction with said steady field gives a predetermined total field in said slice; and in conjunction with the gradient field, applying a periodic magnetic field at the Larmor frequency for the field in said slice to cause resonance of a predetermined nuclear species, and (b) removing the gradient and periodic fields and sensing the resonance signal resulting from the slice within a predetermined frequency band, wherein the magnitude of the gradient field is chosen in relation to the steady field such that where positions outside said slice, at which different values of said steady and gradient fields combine to give a resultant field substantially equal to said predetermined value, are within said region the change of field therein when the gradient field is removed is sufficient to shift the resonant frequency of said predetermined nuclear species, at said positions outside said slice, out of said predetermined frequency band.

According to another aspect of the invention there is provided an apparatus for examining a cross-sectional slice of a body by nuclear magnetic resonance the apparatus including: an examining region in which the body is situated; means for applying to the body a steady magnetic field along an axis therein; means for applying to the body a periodic magnetic field; means for applying in conjunction with the periodic field, a further field having a gradient in the body, which in conjunction with said steady field gives a resultant field in the slice, for which th Larmor frequency is the frequency of the periodic field, to cause resonance of a predetermined nuclear species therein and thereafter for removing said gradient field; and means for sensing the resonance signal, over a predetermined frequency band, resulting from said slice during a field measurement period after removal of the gradient field wherein the means for applying the steady and gradient fields are arranged to provide relative magnitudes of said fields which ensure that when positions outside said slice, at which different values of said steady and gradient fields combine to give a resultant field substantially equal to that in said slice, are within said region the resonance frequencies, of said predetermined nuclear species at said positions outside said slice, are shifted out of the frequency band of the sensing means when the gradient field is removed.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 shows the form of the HR field, Figure 2 shows the relationship of the HR field to the other fields, Figure 3a shows the field pulse sequence of the invention Figure 3b shows the effects of the pulses of Figure 3a on the proton spin vectors, Figure 4a and 4b show in two views the HO, Hx and H field coils, of a practical NMR machine Figure 5 shows the H1 field coils of the machine, Figure 6 shows the He field coils of that machine, and Figure 7 illustrates how the aliasing effect is caused.

For the examination of a sample of biological tissue NMR primarily relates to protons, (hydrogen nuclei) of water molecules in the tissue. In principle however, other nuclei could be analysed, for example, those of deuterium, tritium, fluorine or phosphorus.

Protons each have a nuclear magnetic moment and angular momentum (spin) ab out the magnetic axis. If then a steady magnetic field Ho is applied to the sample the protons align themselves with the magnetic field, many being parallel thereto and some being anti-parallel so that the resultant spin vector is parallel to the field axis. Application of an additional field H1 which is an R.F. field of frequency related to H,. in a plane normal to HO, causes resonance at that frequency so that energy is absorbed in the sample. The resultant spin vectors of protons in the sample then rotate from the magnetic field axis (z-axis) towards a plane orthogonal thereto (x,y). The R.F.

field is generally applied as a pulse and if I Hl dt for that pulse is sufficient to rotate the resultant spin vectors through 90" into the x, y plane the pulse is termed a 90" pulse.

On removal of the H1 field the equilib rium alignments reestablish themselves with a time constant T1, the spin-lattice relaxa tion time. In addition a proportion of the absorbed energy is re-emitted as a signal "centred at the resonant frequency" which can be detected by suitable coils. The emitted energy is a measure of the water content of the sample.

As so far described, the resonance signal detected relates to the entire sample. If individual resonance signals can be determined for elemental samples in a slice or volume of a patient then a distribution of proton densities, in effect of water content, can be determined for that slice or volume.

Additionally or alternatively it is possible to determine a distribution of T1 or the spinspin relaxation time T2.

In general the principles of analysing proton density by NMR in a slice of a body have been extensively discussed and a great many different but related methods have been proposed. Many of the known techniques have been reviewed by P. Mansfield in Contemp. Phys. 17 (b) 1976, 553-576.

Furthermore other related techniques, including those used in spectroscopy, are relevant. Consequently the techniques will only be discussed in detail herein to the extent necessary to understand the improved arrangement of this invention.

Considering now the nature of a suitable examining procedure, the first step is to ensure that resonance occurs at the chosen frequency only in the selected slice. Since the resonance frequency (the Larmor frequency) is related to the value of HO, slice selection is achieved by imposing a gradient on HO so that steady field is of different magnitude in different slices of the patient.

The steady and uniform Ho field is applied as before, usually longitudinal to the patient. An additional magnetic field HZ is also applied, being a gradient HZ = 6H/6z. If then the pulsed H1 field is applied at the appropriate frequency, resonance occurs where the resonance frequency as set by Ho and the local value of Hz, is equal to the frequency of Hl. This should occur only in a single cross-sectonal slice. If the H1 pulse is a 90" pulse, it brings the spin vectors into the x, y plane only for the resonant slice. Since the value of the field is only significant during the Hl pulse it is only necessary that He be applied when H1 is applied and in practice Hz is also pulsed. The H1 and HZ fields are therefore then removed. It is still, however, possible to change the resonant frequencies of the spin vectors which are now in the x, y, plane. This is achieved by applying a further field HR (actually BH)/6R which is parallel to HO. The intensity of HR, however, varies from a maximum at one extreme of the slice, through zero in the centre to a maximum in the reverse direction on the opposite surface. The HR field is illustrated in Figure 1, the arrows indicating only magnitudes at points on a rectangle 1.

There will of course be a smooth variation through and between the magnitudes illustrated. The fields are also illustrated diagrammatically in the side elevation of Figure 2 in relation to a patient 2. Correspondngly the resonant frequencies will vary smoothly across the slice from one side to the other.

As mentioned before, the signal which now occurs is at the resonant frequency.

Consequently the signals received from the slice will also have frequencies which vary across the slice in the same manner. The amplitude at each frequency then represents inter-alia the proton density in a corresponding strip parallel to the zero plane of HR.

One procedure for slice selection together with frequency dispersion between strips at a selected orientation within a slice has been described by Lauterbur et al (18th Ampere Congress, Nottingham, 1974, ed. P.S.Allen et al. Vol.1. p.27-29). The amplitude for each strip can be obtained by varying the detection frequency through the range which occurs across the slice. Preferably however the total signal at all frequencies is measured. The disperson during the HR field is then Fourier analysed by well known techniques to give a frequency spectrum.

The frequency appropriate to each strip will be known from the field values used and the amplitude for each frequency is given by the spectrum.

As discussed for the field HR illustrated in Figure 1, the individual signals derived from the frequency spectrum, for increments of frequency, correspond to incremental strips parallel to the zero plane of HR. These signals are similar in nature to the edge values derived and analysed for x-ray beams in computerised tomography.

The X-ray edge values are obtained for sets at a plurality of different orientations in an examined slice and then are processed by a suitable method, such as that described in British Patent No.1283915 and the further development thereof described in British Patent No.1471531.

It will be apparent that by changing the orientation, in the x, y plane, and about the z -axis of the zero plane of HR, further sets of signals can be obtained representing proton densities along lines of further sets of parallel lines at corresponding further directions in the examined slice. The procedure is therefore repeated until sufficient sets of "edge values" have been derived to process by methods like those used for sets of X-ray beams. In practice the HR field is provided by combination of two fields Hx and Hy, which are both parallel to HO but have gradients in orthogonal directions. The direction of the gradient of the resultant HR is therefore set by the relative magnitudes of Hx and Hv. At many points in the following description reference will be made to the creation of HR field pulses and it should be remembered even where Hx and Hv fields are not individually discussed that reference is to the resultant of Hx and Hv field pules.

The full examination for one direction of the HR gradient is achieved by applying, via appropriate coils, the sequence of field pulses shown in Figure 3a. Figure 3b indicates the effect which each pulse has on the spin vector. It will be realised that H1 field is a periodic field effectively rotating about the z-axis. Correspondingly the spin vectors thereafter precess about the z-axis. For clarity of explanation the spin vectors are shown in Figure 3b on a coordinate system which rotates with H1.

Referring to Figures 3a and 3b together, the pulse cycle comprises six phases, AB to FG, and a recovery period shown by the broken line. The Ho field is continuously present throughout the cycle.

Prior to the first pulse, or after the recovery period if an earlier cycle has been implemented, the mean spin moments are substantially aligned wth the z-axis (A).

The gradient field Hz pulse and H1 pulses (AB), simultaneously applied, respectively select the slice and bring the resultant spin moments into the x, y plane (still, of course, precessing about the z-axis). Although the resonant frequency is the same throughout the slice selected, there is a phase dispersion introduced because the excitation occurred in a field gradient. Thus the spin moments are shown at B, though dispersed between limits L much greater than can be satisfactorily illustrated. The limits shown at L are merely indicative of the value of the dispersion. It has been found that this phase dispersion can be reversed by the application of a negative field gradient pulse, that is a pulse of the correct relative magnitude as Hz but 180C displaced (the relation being about 55%). The use of such a 1800 displaced pulse for rephasing has been proposed see for example Hoult. J. Mag. Res.

26, 1977, 165. The preferred magnitude disclosed herein has not, however been previously disclosed. This pulse BC is therefore applied to bring the spin moments in the x, y plane into place as at C. The H1 field need not be continued into the negative gradient pulse (Hz') but it can be continued during that pulse if required to ensure that the spin moments go into the x, y plane.

At that time a signal could be sensed to give proton density for the whole slice.

However in this sequence, the signal is sensed in the presence of an HR pulse CD which gives frequency dispersion in a selected direction (r) in the slice as previously described. The change to the new frequencies is almost instantaneous with the applicaton of the HR pulse and is maintained proportionately throughout the pulse. As discussed the signal is sensed and frequency analysed to give the proton densities for a plurality of adjacent parallel strips of the slice. After the HR pulse the spin moments, which are still largely in the x, y plane despite some relaxation, have a considerable phase dispersion as shown at D (which is illustrative - as the actual dispersion is n;t (n being 100 or more). At that stage, if a further cycle as described so far were to be required, it would be necessary to wait for spin-lattice relaxation to realign the spin moments with the z-axis. This could take as much as 5 seconds which, since several thousand cycles are required, is much too long.

It is proposed to return the spin moments substantially back to the starting position (A) by repeating the pulse sequence up to D in the reverse order and reverse sense. Since the -HR pulse is substantially the same as the HR pulse except for its sense, further signals may be detected during it. These will be for the same r direction as for the forward pulse and help to improve the signal to noise ratio.

It will be realised that this procedure bears some relation to the so-called "DEFT" (Driven Equilibrium Fourier Transform) technique as disclosed for example by Becker et al. J.Amer.Chem.Soc. 91 (27) DEC 31.1969. 7784-5. DEFT is disclosed however only for spectroscopy systems and no disclosure is given of its relevance to body imaging. Consequently the disclosure only relates to reversal of the R.F. pulse sequence in the absence of gradient fields.

After the reverse pulse sequence the spin moments still show some deviation from the z axis due to phase dispersion caused by spin-spin coupling. This can not be reversed by this pulse sequence nor, it is believed, by any other. The period GA therefore allows some relaxation to thermal equilibrium (time constant T1) which eliminates the effect of the phase dispersion and also reduces the effects of any mismatching between the forward and reverse pulses.

Although the relaxation period GA is still necessary, the use of the reversed pulse sequence D to G has much reduced that period and allows faster repetition of the total sequence for other r-directions. The length of the signal measurement period CE is determined by the phase dispersion caused by HO field inhomogeneity and also by the dispersion caused by spin-spin coupling. If the effect of HO field inhomogeneity is considered excessively to shorten the period CE then the H1 part of pulse FG may be a 1800 r.f. pulse rather than a 90" pulse.

Turning the spin moments through 1800 produces a so-called "spin-echo" of known form and the HR pulses similar to CD and can can be repeated to give a further signal measurement period. The spin-echo procedure is known to reverse the dispersion due to field inhomogeneity and can be repeated here several times until sufficient signal has been obtained or until spin-spin dispersion, which cannot be reversed, becomes excessive. As in the sequence of Figure 3A, a spin-echo sequence should end with pulses EF, FG and recovery period GA.

In the case where a 90" H1 pulse is used, the ratio of period GA to period AG should preferably be approximately the ratio of T1 to T2 for maximum sensitivity. An H1 pulse less than 90" can also be used when the ratio of the periods GA:AG should be appropriately reduced. Typically the total period AGA is 40m sec where AG is approximately 5.5m sec, AB is 300+ sec and CD is 2m sec.

The H1 pulse is typically of 0.6 Oe and has a frequency of 4.26 MHz for an HO of 1000 Oc.

All other pulses are at envelope frequencies HZ being typically + 30 Oe to - 30 Oe. HR being 15 jOe to - 15 Oe.

In the preferred embodiment HZ' is less than Hz; typically f HZ' dt = 0.55 f He dot to 0.57 f HZ dt.

Figures 4a and 4b show in end and side elevation respectively a practical coil arrangement to provide the Hq field and HR field pulses. To show approximate dimensions the patient 2 is shown in cross-section in the end elevation of Figure 4a. A suitable couch or other supporting means for the patient 2 may readily be provided and has not been shown in the figure.

As mentioned hereinbefore the HR field pulses are the resultant of HX and Hy components. The HX components are provided by four coils 3 and the Hy components by four coils 4. The steady Ho field is provided by four coils 5 connected in series, although a smaller number could be used.

Further details of the coil windings will not be given since suitable coils can readily be devised, by those with the appropriate skills, to provide the fields required.

Figures 4a and 4b do not show the H1 and Hz coils so as to reduce the complexity of the drawing.

The H1 coils are shown in Figure 5 in perspective. They are two saddle shaped coils 6 which are driven in parallel to provide the rotating H1 field and which are also used to detect the signals which are of approximately the same frequency.

Figure 6 shows, also in perspective the two circular coils 7 which provide the Hz field component for the gradient superimposed on H,.

It will be understood that. although the Ho field is made substantially uniform over a central volume to be scanned, for practical systems the nature of the coil systems will mean that the field falls rapidly outside that volume.

This is shown in Figure 7 where, superimposed on an outline of the patient 2, there is shown in solid line the distribution of Ho provided by practical field coils for examination of a slice 8. It will be seen that the field at this region is the uniform field at 9.

As discussed hereinbefore, for slice selection, an Hz gradient field is applied and the broken line in Figure 7 shows the total field including Hz. Hz is chosen in the preferred embodiment to be zero at 9 but a slice could be selected for a finite, known, Hz value.

It will be seen from Figure 7 that, because of the nature of the practical field there will be a second slice 10 for which, with Hz applied, the field value 11 is the same as that at 9.

As the slice 8 has been selected the shaped R.F. pulse H1 is applied and the protons in slice 8, and slice 10 because it is subject to the same field, are caused to resonate. Thus in addition to the selected slice, a second spurious slice has been excited in a form of aliasing.

Now when the gradient Hz is removed the resonance in slice 8 stays at the same frequency as the field does not change. The field in slice 10 drops to the value at 12 but the slice continues to resonate at the corresponding new frequency. There will be frequency shifts in both slices due to the HR field and resonance signals will be detected for both slices to give a confused and unsatisfactory output.

It is proposed to avoid aliasing due to the excitation of a second slice such as 10, by taking an appropriate value of the Hz gradient in relation to the variation of H,.

The solution is to ensure that the value of Hz is such that the frequency shift from 11 to 12 will take resonance of slice 10 out of the frequency range of the detection system.

That is to say the value of the field at 11 should be sufficiently large to achieve that effect.

It will be clear that the actual field gradient required depends on the Ho field variation and frequency band for a particular design of the apparatus. However those skilled in the art could readily produce the effect shown in Figure 7 for any practical design.

Considering the pulse sequence as described hereinbefore, it will be realised that there is a relatively long recovery period, GA, in which further information for the same slice cannot be obtained. It is proposed to utilise this period to recover data for other slices. Although the time for one slice is long it will be clear that this interleaving allows examination of several slices in almost the same time. This data can be used to give an effective volume scan.

Typically the time required fo any one slice examination is one eighth of the scan cycle. Thus eight slices can be examined in a single period only slightly longer than a single cycle.

The effect is that one set of data for parallel lines in the slice is obtained for each slice in sequence. Then returning to the first slice, it is repeated for lines at a different angle and so on until all data for each slice is recovered.

It should be noted that instead of the nomenclature used throughout this specificaton an alternative, and more strictly correct, form may be encountered. In this alternative form all fields longitudinal of the body are denoted Hz, then a gradient in the Z direction is denoted Gz = dHz/dz, gradient in the x direction is denoted Gx = dHz/dx, a gradient in the y direction is denoted Gy = dHz/dy and Gx and Gy together form GR = dHz/dr.

WHAT WE CLAIM IS: 1. A method of examining, by nuclear magnetic resonance, a body placed in an examining region, the method including (a) the selective excitation of a slice of said body by: applying a steady magnetic field along an axis in the body; applying a gradient field, which in conjunction with said steady field gives a predetermined total field in said slice; and in conjunction with the gradient field, applying a periodic magnetic field at the Larmor frequency for the field in said slice to cause resonance of a predetermined nuclear species, and (b) removing the gradient and periodic fields and sensing the resonance signal resulting from the slice within a predetermined frequency band, wherein the magnitude of the gradient field is chosen in relation to the steady field such that where positions outside said slice, at which different values of said steady and gradient fields combine to give a resultant field substantially equal to said predetermined value, are within said region the change of field therein when the gradient field is removed is sufficient to shift the resonant frequency of said predetermined nuclear species, at said positions outside said slice, out of said predetermined frequency band.

2. A method of examining a body substantially as herein described with reference to the accompanying drawings.

3. An apparatus for examining a crosssectional slice of a body by nuclear magnetic resonance the apparatus including: an examining region in which the body is situated; means for applying to the body a steady magnetic field along an axis therein; means for applying to the body a periodic magnetic field; means for applying in conjunction wth the periodic field, a further field having a gradient in the body, which in conjunction with said steady field gives a resultant field in the slice, for which the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. This is shown in Figure 7 where, superimposed on an outline of the patient 2, there is shown in solid line the distribution of Ho provided by practical field coils for examination of a slice 8. It will be seen that the field at this region is the uniform field at 9. As discussed hereinbefore, for slice selection, an Hz gradient field is applied and the broken line in Figure 7 shows the total field including Hz. Hz is chosen in the preferred embodiment to be zero at 9 but a slice could be selected for a finite, known, Hz value. It will be seen from Figure 7 that, because of the nature of the practical field there will be a second slice 10 for which, with Hz applied, the field value 11 is the same as that at 9. As the slice 8 has been selected the shaped R.F. pulse H1 is applied and the protons in slice 8, and slice 10 because it is subject to the same field, are caused to resonate. Thus in addition to the selected slice, a second spurious slice has been excited in a form of aliasing. Now when the gradient Hz is removed the resonance in slice 8 stays at the same frequency as the field does not change. The field in slice 10 drops to the value at 12 but the slice continues to resonate at the corresponding new frequency. There will be frequency shifts in both slices due to the HR field and resonance signals will be detected for both slices to give a confused and unsatisfactory output. It is proposed to avoid aliasing due to the excitation of a second slice such as 10, by taking an appropriate value of the Hz gradient in relation to the variation of H,. The solution is to ensure that the value of Hz is such that the frequency shift from 11 to 12 will take resonance of slice 10 out of the frequency range of the detection system. That is to say the value of the field at 11 should be sufficiently large to achieve that effect. It will be clear that the actual field gradient required depends on the Ho field variation and frequency band for a particular design of the apparatus. However those skilled in the art could readily produce the effect shown in Figure 7 for any practical design. Considering the pulse sequence as described hereinbefore, it will be realised that there is a relatively long recovery period, GA, in which further information for the same slice cannot be obtained. It is proposed to utilise this period to recover data for other slices. Although the time for one slice is long it will be clear that this interleaving allows examination of several slices in almost the same time. This data can be used to give an effective volume scan. Typically the time required fo any one slice examination is one eighth of the scan cycle. Thus eight slices can be examined in a single period only slightly longer than a single cycle. The effect is that one set of data for parallel lines in the slice is obtained for each slice in sequence. Then returning to the first slice, it is repeated for lines at a different angle and so on until all data for each slice is recovered. It should be noted that instead of the nomenclature used throughout this specificaton an alternative, and more strictly correct, form may be encountered. In this alternative form all fields longitudinal of the body are denoted Hz, then a gradient in the Z direction is denoted Gz = dHz/dz, gradient in the x direction is denoted Gx = dHz/dx, a gradient in the y direction is denoted Gy = dHz/dy and Gx and Gy together form GR = dHz/dr. WHAT WE CLAIM IS:

1. A method of examining, by nuclear magnetic resonance, a body placed in an examining region, the method including (a) the selective excitation of a slice of said body by: applying a steady magnetic field along an axis in the body; applying a gradient field, which in conjunction with said steady field gives a predetermined total field in said slice; and in conjunction with the gradient field, applying a periodic magnetic field at the Larmor frequency for the field in said slice to cause resonance of a predetermined nuclear species, and (b) removing the gradient and periodic fields and sensing the resonance signal resulting from the slice within a predetermined frequency band, wherein the magnitude of the gradient field is chosen in relation to the steady field such that where positions outside said slice, at which different values of said steady and gradient fields combine to give a resultant field substantially equal to said predetermined value, are within said region the change of field therein when the gradient field is removed is sufficient to shift the resonant frequency of said predetermined nuclear species, at said positions outside said slice, out of said predetermined frequency band.

2. A method of examining a body substantially as herein described with reference to the accompanying drawings.

3. An apparatus for examining a crosssectional slice of a body by nuclear magnetic resonance the apparatus including: an examining region in which the body is situated; means for applying to the body a steady magnetic field along an axis therein; means for applying to the body a periodic magnetic field; means for applying in conjunction wth the periodic field, a further field having a gradient in the body, which in conjunction with said steady field gives a resultant field in the slice, for which the

Larmor frequency is the frequency of the periodic field, to cause resonance of a predetermined nuclear species therein and thereafter for removing said gradient field; and means for sensing the resonance signal, over a predetermined frequency band, resulting from said slice during a field measurement period after removal of the gradient field wherein the means for applying the steady and gradient fields are arranged to provide relative magnitudes of said fields which ensure that when positions outside said slice, at which different values of said steady and gradient fields combine to give a resultant field substantially equal to that in said slice, are within said region the resonance frequencies, of said predetermined nuclear species at said positions outside said slice, are shifted out of the frequency band of the sensing means when the gradient field is removed.

4. A medical nuclear magnetic resonance apparatus for examining the body of a patient, the apparatus including a patient position in which the body is situated; means for applying to the body a steady magnetic field along an axis therein; means for applying a further magnetic field having a gradient in the body; and means for applying to the body, at least in conjunction with the gradient field, a periodic magnetic field, wherein the said means are arranged so that the steady magnetic field and gradient field produce in a cross-sectional slice of the body a resultant magnetic field for which the Larmor frequency is the frequency of the periodic field, to cause resonance of predetermined nuclear species in said slice, and so that after said resonance is caused the gradient field and periodic field are removed, the aparatus further including means for sensing the resonance signal resulting from the slice over a predetermined frequency band when the gradient field and the periodic field have been removed and wherein the means for applying the said fields are further arranged to produce in all parts of the patient position fields of individual magnitudes such that when parts outside the slice, at which different values of the steady and gradient fields combine to give a resultant field substantially equal to that in the slice, are within the patient position the resonance frequency, of the predetermined nuclear species in said parts outside the slice, is shifted out of said frequency band when the gradient field is removed.

5. An apparatus, for examining a body, substantially as herein described with reference to the accompanying drawings.