Nuclear magnetic resonance methods and apparatus

A method of exciting nuclear magnetic resonance at a particular location of a body comprises: exciting first nuclear spins in the body (A, Gx, -G'x, B, Gxd, C, Gy, -G'y, D) so that spins occurring in a first selected region of the body have their spin vectors lying in a first direction and any spins occurring elsewhere in the body have their spin vectors lying in a plane normal to the first direction; dephasing the spins in the body whose vectors lie in said plane (Gyd); and exciting nuclear spins in a second selected region of the body (E, Gz, -G'z) which intersects said first region at said particular location so that the vectors of the resultant spins in said particular location only are aligned in said plane. The method finds particular application in providing data for chemical analysis of material at a particular location of a body. Apparatus for carrying out the method is also described.

This invention relates to methods and apparatus for chemical analysis of a 
selected region of a body by nuclear magnetic resonance (NMR) techniques. 
NMR techniques have been used for the chemical analysis of material for 
many years, particularly by spectroscopy. More recently NMR techniques 
have been used to obtain images representing the distribution in a 
selected cross-sectional slice or volume of a body of a chosen quantity, 
e.g. the density of chosen nucleons, for example hydrogen protons, or of 
NMR spin relaxation time constants. Such distributions are similar to, 
although of different significance from, the distributions of X-ray 
attenuation provided by computerised tomography systems, and have thus 
found especial application in medical examination of patients. 
Still more recently interest has been shown in providing a facility in NMR 
imaging apparatus whereby chemical analysis of a selected region of a body 
under examination can be carried out. Current proposals to this end allow 
collection of chemical analysis data for each of a plurality of locations 
along a selected line or each of a plurality of lines in a selected plane 
in the body. However, the degree of resolution for analysis purposes 
obtainable with such proposed methods is necessarily limited due to the 
requirement for spatial as well as chemical discrimination. 
It would therefore be useful for the purpose of chemical analysis in NMR 
imaging apparatus to be able to excite material in a single location only 
in the body, e.g. a region corresponding to a single pixel of the image 
obtainable in use of the apparatus, thereby to facilitate the collection 
of sufficient data for high resolution NMR spectroscopy of the material in 
the selected location. 
A well known method of exciting material in a selected slice only of a body 
for NMR imaging comprises applying a steady magnetic field to the body 
with a gradient in a chosen direction so that the magnetic field 
throughout the selected slice has a unique uniform value, and 
simultaneously applying an RF magnetic field to the body at the nuclear 
magnetic resonance frequency of appropriate material in the selected slice 
(such frequency being dependent on the steady magnetic field value). 
At first sight it might seem that excitation of material in a single 
location might be carried out by performing successively excitation in 
three orthogonal slices which intersect at a point. Such a process, if 
effective, would amount to first exciting a whole slice, then dephasing 
excitation at all parts of the slice except the line where the first 
selected slice intersects the second selected slice and finally dephasing 
excitation at all locations along the line except that where the third 
selected slice intersects the line. Unfortunately it is found in practice 
that the integrals of the magnetic fields applied for the second and third 
slice selections are not necessarily effective to dephase excitation at 
all points in the first and second selected slices except the desired 
location. Moreover, further problems can arise due to the difficulty of 
holding the applied magnetic fields accurately at desired values 
throughout three successive slice selection sequences. 
Thus in practice it has been found impossible to use such a process to 
produce excitation only at a single location. 
It is an object of the present invention to provide an alternative NMR 
method capable of exciting material in a particular location only of a 
body to provide data for chemical analysis. 
According to the present invention a method of exciting nuclear magnetic 
resonance at a particular location in a body comprises: exciting first 
nuclear spins in the body so that spins occurring in a first selected 
region of the body have their spin vectors lying in a first direction and 
any spins occurring elsewhere in the body have their spin vectors lying in 
a plane normal to the first direction; dephasing the spins in the body 
whose vectors lie in said plane; and exciting nuclear spins in a second 
selected region of the body which intersects said first region at said 
particular location so that the vectors of the resultant spins in said 
particular location only are aligned in said plane. 
In a preferred method in accordance with the invention said first nuclear 
spins are produced by: exciting nuclear spins in the body so that the spin 
vectors lie in said first direction in a first selected slice of the body 
and elsewhere lie in said plane; dephasing the spins in the body whose 
vectors lie in said plane; and then exciting nuclear spins in the body so 
that in a second selected slice of the body which intersects the first 
slice at said first selected region the spin vectors lie in said first 
direction only where the first and second slices intersect and elsewhere 
lie in said plane. 
In such a method the second selected region is preferably a third slice, 
and the first, second and third slices are suitably mutually orthogonal. 
Each excitation of nuclear spins whose spin vectors lie in said first 
direction in a slice of the body and in said plane elsewhere in the body 
is suitably carried out by; applying a steady uniform magnetic field to 
said body in said first direction; applying in conjunction with said 
steady field a magnetic field in said first direction with a gradient in a 
direction normal to said slice to give a unique uniform field in said 
slice of said body; applying in conjunction with said gradient field a 
90.degree. RF magnetic field pulse at the Larmor frequency for the field 
in said slice to cause nuclear spins selectively therein whose spin 
vectors lie in said plane; and subsequently applying a second 90.degree. 
RF magnetic field pulse in the absence of said gradient field so that the 
spins in said slice having vectors in said plane are rotated to lie in 
said first direction and any other spins in said body have their spin 
vectors rotated to lie in said plane. 
The invention also provides apparatus for carrying out a method according 
to the invention.

The apparatus required for carrying out the method is for the most part of 
conventional form, for example, as described in UK Patent Specification 
No. 1,578,910 (corresponding to U.S. Pat. No. 4,284,948) or No. 2,056,078 
(corresponding to U.S. Pat. No. 4,355,282) to which reference should be 
made for a fuller description of the apparatus. 
The essential features of such apparatus in so far as is required for an 
understanding of the present invention are as follows: 
The apparatus includes a first coil system whereby a magnetic field can be 
applied to a body to be examined in a given direction, normally designated 
the Z-direction, with a gradient in any one or more of the three 
orthogonal directions i.e. X, Y and Z directions. 
Referring to FIG. 1, the first coil system comprises coils 1 which provide 
a steady uniform magnetic field Bo in the Z direction; coils 3 which 
provide a magnetic field gradient Gx in the X-direction, coils 5 which 
provide a magnetic field gradient Gy in the Y-direction; and coils 7 which 
provide a magnetic field gradient Gz in the Z-direction. 
In addition, the apparatus includes a second coil system 9 whereby RF 
magnetic fields can be applied to the body under examination in a plane 
normal to the direction of the magnetic field produced by the first coil 
system, and whereby RF magnetic fields resulting from nuclei in the body 
under examination which have been excited to nuclear magnetic resonance 
with a spin vector component other than in the Z-direction can be 
detected. 
In the drawing a single pair of coils 9 is shown for both applying and 
detecting RF fields, but in certain circumstances it may be preferable to 
provide separate coils for detecting the RF fields. 
The various coils 1, 3, 5, 7 and 9 are driven by Bo, Gx, Gy, Gz and RF 
drive amplifiers 11, 13, 15, 17 and 19 respectively, controlled by Bo, 
Gxy, Gz and RF control circuits 21, 23, 25 and 27 respectively. These 
circuits may take various forms which are well known to those with 
experience of NMR equipment and other apparatus using coil induced 
magnetic fields. 
The circuits 21, 23, 25 and 27 are controlled by a central processing and 
control unit 29 with which are associated inputs and other peripherals 31, 
for the provision of commands and instructions to the apparatus, and a 
display 33. 
The NMR signals detected by the coils 9 are applied via an amplifier 35 to 
a signal handling system 37. The signal handling system is arranged to 
make any appropriate calibration and correction of the signals, but 
essentially transmits the signals to the processing and control unit 29 
wherein the signals are processed for application to the display to 
produce an image representing the distribution of an NMR quantity in the 
body being examined. 
It will be appreciated that whilst shown separately to clarify the present 
description, the signal handling system 37 may conveniently form part of 
the unit 29. 
The apparatus also includes field measurement and error signal circuits 39 
which receive signals via amplifiers 41 from field probes X.sub.1, 
X.sub.2, Y.sub.1 and Y.sub.2 which are disposed at suitable positions in 
relation to a slice 43 of the body being examined, as illustrated in FIG. 
2, to monitor the applied magnetic fields. 
Referring now to FIGS. 3 and 4 of the drawings, in use of the apparatus to 
excite nuclear magnetic resonance uniquely at a selected location in the 
body to be examined, a steady magnetic field Bo is applied to the body in 
the Z-direction. A further magnetic field in the Z-direction with a 
gradient Gx in the X-direction is then applied to the body so that a 
unique magnetic field in the Z-direction is applied to a selected 
cross-sectional slice lying nominally in a Y-Z plane (see FIG. 4). During 
application of this field gradient Gx an RF magnetic field pulse at the 
Larmor frequency for the material to be analysed in the unique field 
applied to the selected slice is applied by means of the second coil 
system 9. The RF field pulse A causes the spin vectors of nuclear spins of 
the material to be analysed in the selected slice, which hitherto were 
aligned in the Z-direction, to be tipped towards the X-Y plane in known 
manner. The integral of the pulse is chosen so that the pulse is just 
sufficient to tip the spin vectors through 90.degree. into the X-Y plane, 
such a pulse being herein referred to as a 90.degree. RF magnetic field 
pulse. It will be appreciated that the spin vectors of nuclear spins in 
parts of the body outside the slice are substantially unaffected since 
there is no resonance between the applied RF field and the Larmor 
frequency of these spins. 
The gradient Gx is then removed and replaced by a reverse gradient -G'x to 
re-phase the spins in the selected slice against dephasing resulting from 
the gradient across the slice during excitation, as described, for 
example, in the above mentioned UK Patent Specification No. 1,578,910. 
The reverse gradient -G'x is then removed and a further 90.degree. RF pulse 
B is applied at the relevant Larmor frequency, this being now the same 
throughout the body due to the absence of any field gradient. This causes 
rotation by 90.degree. of all the spin vectors in the body so that the 
spin vectors of the nuclear spins in the selected slice are rotated so as 
lie in the Z-direction, and the spin vectors of any nuclear spins in any 
other part of the body are rotated so as to lie in the X-Y plane. 
A further magnetic field in the Z-direction with a gradient Gxd in the 
X-direction is then applied for a relatively long period txd. This 
dephases all the spins in the body whose spin vectors are not in the 
Z-direction, thus leaving the spins in the selected slice unaffected, but 
dephasing the spins elsewhere in the body. 
A similar sequence is then carried out in respect of a selected slice in 
the X-Z plane by the application, in turn, of a slice selection gradient 
Gy in conjunction with an RF pulse C, a re-phasing reverse gradient -G'y 
and RF pulse D, and a dephasing gradient Gyd. 
However, in this case the first 90.degree. RF pulse 5 will rotate from the 
Z-direction into the X-Y plane only the spin vectors of nuclear spins 
along the line where the selected X-Z plane slice intersects the 
previously selected Y-Z plane slice (see FIG. 2) since the spins in the 
selected X-Z plane slice not lying in the line of intersection do not lie 
in the Z-direction. Consequently the second RF pulse D will rotate into 
the Z-direction only the spin vectors along the line of intersection and 
the dephasing gradient will leave unaffected only the spins along the line 
of intersection, dephasing all other spins in the body, in particular 
those in the selected Y-Z plane slice not lying in the line of 
intersection with the selected X-Z plane slice which will have been 
rotated into the X-Y plane by the RF pulse D. 
Thus, at the end of the second sequence all spins in the body are dephased 
except those in the line of intersection of the selected Y-Z plane and X-Z 
plane slices. 
Finally a further RF pulse E is applied in conjunction with a slice 
selection field in the Z-direction with a gradient Gz in the Z-direction 
to select an X-Y plane slice which intersects the line of intersection of 
the selected Y-Z and X-Z slices at the selected location L (see FIG. 4). 
As a result, only the spin vectors of the spins in the selected location 
are rotated into the X-Y plane, the spins in other parts of the selected 
X-Y plane slice not being similarly rotated since they do not lie in the 
Z-direction. The gradient Gz also serves to dephase spins along the line 
of intersection of the Y-Z and X-Z slices other than at the selected 
location. 
After application of a further re-phasing gradient -G'z for spins in the 
selected location, the signal induced in the second coil system by these 
spins, commonly called the free induction decay (FID) signal is recorded. 
By analysis of the spectrum of the FID signal data regarding the chemical 
composition of the material in the selected location is then obtained in 
known manner. 
It will be understood that the dephasing gradients Gxd and Gyd are required 
to be applied for a sufficiently long time txd and tyd to ensure that no 
re-phasing of spins outside the selected region can occur during data 
collection. This necessarily results in the process being somewhat slow, 
but the possibility of unacceptably large spin-to-spin relaxation 
occurring before data collection is not a serious problem since such 
dephasing does not occur during times t.epsilon. and t.delta. whilst the 
spins in the selected location are in the Z-direction, but only during the 
relatively short times t.alpha., t.beta. and t.gamma. during the 
excitation steps of the process. 
Spin-to-lattice relaxation occurs during times t.epsilon. and t.delta. but 
is not normally a problem at least in medical examinations since the 
spin-to-lattice relaxation time constant T.sub.1 for human tissue is 
typically 300 milliseconds and typical values of t.epsilon. and t.delta. 
are 20 milliseconds and 6 milliseconds respectively so that the FID signal 
is typically reduced from an initial value N.sub.o to N.sub.o 
[1-2e.sup.-6/300 (1-e.sup.-20/300)] i.e. by 12.5%. 
It will be appreciated that whilst in the method described by way of 
example the three orthogonal slices are excited in the order Y-Z, X-Z, and 
X-Y, the order of excitation is immaterial. 
If desired after the re-phasing gradient -G'z and before data collection, a 
third 90.degree. RF pulse in the absence of any field gradient followed by 
a third dephasing gradient Gzd may be applied followed by excitation of a 
further slice containing the selected location to ensure dephasing of 
spins along the line of intersection of the Y-Z and X-Z slices other than 
in the selected location. However, normally gradient Gz is sufficient to 
effect this. 
It will be appreciated that whilst in the method described above by way of 
example a single location is selected, in other methods in accordance with 
the invention two or more non-contiguous locations may be selected and 
their FID signals recorded, for example, by successive selection of two or 
more spaced X-Y plane slices, the FID signal for each location being 
recorded before the selection of the slice defining the next location. 
Furthermore, in other alternative methods in accordance with the invention 
by omitting excitation in one slice and the immediately following 
dephasing step, nuclear magnetic resonance may be uniquely excited in the 
line of intersection of two slices.