Magnetic resonance imaging

In a magnetic resonance apparatus for providing an image of a sample a birdcage resonator 12 is provided with its axis parallel to the magnetic field of the apparatus. A radio-frequency source 36 is connected to the resonator 12 so as to generate a magnetic field by a first resonance mode of the resonator and a detector 44, 46 is connected to the resonator 12 so as to detect any signal received by the resonator in its second, orthogonal mode.

This invention relates to an improved method and apparatus for imaging 
samples, using either Nuclear Magnetic Resonance (NMR) techniques or 
Electron Paramagnetic Resonance techniques. 
The principle of NMR was first developed in 1946, and the technique of 
field gradients originated in 1973. In early arrangements, resonance in a 
swept field was detected either by a single coil resonator (bridge method) 
or a double coil resonator, i.e. with two coils at right angles (induction 
method). Both methods are disclosed in "High-resolution Nuclear Magnetic 
Resonance" by J. A. People et al., McGraw Hill, on pages 52 and 54. 
NMR is now widely used for imaging liquids and liquid-like material such as 
biological tissues, in which NMR spin-spin relaxation times T.sub.2 are in 
the millisecond range. Such long relaxation times allow the use of pulsed 
Fourier transform NMR with pulsed magnetic field gradients, and good 
quality images with high spatial resolution can be obtained in a short 
time. In solids, the NMR lines are much broader and T.sub.2 is very much 
shorter. Nevertheless several techniques have been developed to provide 
images of solids using NMR. The techniques include multi-pulse line 
narrowing and Stray Field Imaging, STRAFI, but such techniques are 
severely restricted by limitations of available radio frequency (RF) power 
to produce sufficiently short pulses; the result is that the size of 
sample which can be imaged is small, approximately 10 millimetres, as the 
power increases as at least the cube of sample dimensions. In addition, 
STRAFI requires the sample to be translated by small distances between NMR 
excitations, which is difficult with large objects. 
Magic angle spinning has also been used but requires the sample to be spun 
rapidly within the magnet, which clearly imposes restrictions on sample 
size and geometry. Another technique is the oscillating gradient method, 
but this requires very strong gradients to be generated, which again 
limits sample size as the DC gradient power requirement increases as the 
fifth power of the coil dimensions. 
In all of the above techniques for imaging solids, there is the further 
restriction that they can be used only if the spin-spin relaxation time 
T.sub.2 is greater than about 10 microseconds. 
In a different approach, swept-field continuous wave (CW) NMR has been used 
to perform NMR spectroscopy of solids since 1949, see The Journal of 
Chemical Physics, Vol. 17, No. 10, October 1949, H. S. Gutowsky et al. 
Swept field pulsed NMR has also been used for spectroscopy, with the 
magnetic field swept in step-wise manner, and low intensity RF pulses 
applied to detect NMR resonance. In yet another spectroscopic technique, a 
series of pulses of increasing or decreasing frequency has been applied at 
constant magnetic field; this technique of variable pulse frequency swept 
field NMR has been used for wide line NMR spectroscopy. 
In a different technology, electron paramagnetic resonance (EPR), a sample 
is continuously irradiated with electromagnetic radiation, and an applied 
magnetic field is swept past the likely resonances. The technique of 
continuous wave (CW) EPR has been known since 1979 and has been developed 
more recently to provide images of free radicals in biological systems, 
and for spatial localisation of paramagnetic centres in solids, as 
disclosed by M. J. R. Hoch and A. R. Day in Solid State Communications 
1979, Vol. 30, pages 211-213. 
It is an object of the invention to provide apparatus for imaging solids 
using NMR techniques but without the previously-applicable limitations of 
sample size and of spin-spin relaxation time. 
It is a further object of the invention to provide apparatus for imaging in 
vivo samples using EPR technique which is less sensitive to sample 
movement than the conventional reflection bridge methods. 
According to the invention, apparatus for providing a magnetic resonance 
image of a sample comprises: 
means for providing a magnetic field having: 
first, homogeneous, component, 
a second, gradient, component which varies in strength across the sample, 
and 
a third, swept, component which varies in strength with time, 
the second and third components being parallel to the first component and 
being of substantially lower magnitude; 
a birdcage resonator arranged within the magnetic field with its axis 
parallel to the field; 
means to connect a radio-frequency source to the resonator so as to 
generate a magnetic field in a first resonance mode of the resonator, and 
means to connect detection means to the resonator so as to detect any 
signal received by the resonator in its second, orthogonal resonance mode. 
Optionally the connections between the resonator and the source and the 
resonator and the detection means are inductive connections, but 
alternatively they may be capacitive connections. 
Optionally the means to provide the third component of the magnetic field 
comprises means to supply a ramp signal to the ZO coils of a conventional 
superconducting NMR magnet. 
The apparatus may be arranged to operate by the technique of nuclear 
magnetic resonance, or by the technique of electron paramagnetic resonance 
.

In FIG. 1 a sample 10, comprising two blocks 10A, 10B, is shown inside a 
high-pass birdcage resonator 12. The resonator comprises a first 
cylindrical polymethyl methacrylate (PMMA) former 14 carrying two 
conducting end rings 16 joined by eight conducting legs 18. Between the 
junctions of the end rings and each adjacent pair of legs is a variable 
capacitor 20, shown schematically. 
Arranged coaxially around the resonator 12 is a second cylindrical PMMA 
former 22 on the outer surface of which is a radio frequency shield 24 in 
the form of a 25 .mu.m layer of copper foil. The former 22 carries 
magnetic field modulation coils 26 in the form of a split solenoid; turns 
of e.g. 1 mm copper wire 28 with PVC insulation are wound on the former; 
although only two sets of four turns are shown, in practice a greater 
number, e.g. ten turns, would be used. 
In FIG. 2 the sample 10, birdcage resonator 12 and the former 22 for the 
field modulation coils are shown positioned within the various coils 30 
within the bore 32 of a superconducting magnet 34. The coils 30, shown 
shaded, include the room temperature shim, gradient and ZO coils of a 
conventional NMR magnet. The magnet provides an axial magnetic field, 
shown as B.sub.H in FIG. 1. 
FIG. 2 also illustrates, in highly schematic form, the inductive coupling 
loops L.sub.a, L.sub.b, situated between the resonator 12 and the 
radio-frequency shield 24 carried by the former 22. Each loop is connected 
to a coaxial cable (not shown) for connection to the circuit in FIG. 3. 
Neither the loops nor the cable are shown in FIG. 1 for reasons of 
clarity, but in FIG. 2 the direction of the field B.sub.1.sup.a by the 
loop L.sub.a is indicated. 
In an alternative arrangement (not illustrated), the resonator may be 
capacitively coupled to the rest of the apparatus. 
FIG. 3 shows the electrical equipment used to provide a one-dimensional 
image of the sample 10. 
An RF source 36 is connected through a power amplifier 38 to the inductive 
coupling loop L.sub.a of the birdcage resonator 12, and the inductive 
coupling loop L.sub.b of the resonator is connected through a DC biassed 
diode detector 44 to a lock-in amplifier 46. The amplifier 46 is connected 
to the field modulation coils 26 and to a computer 56. 
A DC power supply unit 48 is connected to the field gradient coils 50 of 
the magnet 34, and a field ramp controller or sweep generator 52 supplies 
the ZO field offset coils 54 of the magnet 34; the magnet is not shown in 
this Figure. 
The computer 56, through a bus 58, controls the amplifier 46, the RF source 
36, and the sweep generator 52. 
In operation, the RF source 36 generates a constant amplitude sinewave at 
about 300 MHz; the signal is amplified by amplifier 38 and passes to the 
resonator 12, which in its first resonance mode generates an RF magnetic 
field B.sub.1.sup.a at the sample 10. 
When the sample 10 resonates, the resonator 12 in its second resonance 
mode, orthogonal to the first, receives a signal. This signal passes via 
the loop L.sub.b to the DC biassed diode detector 44; the output of the 
detector is a DC level depending on the amplitude of the resonance signal. 
The field ramp controller 52 supplies a sawtooth waveform to the ZO coils 
54; the field gradient applied by DC unit 48 to the field gradient coils 
50 can be set at a selected fixed level. 
In combination, the fields applied to the sample 10 are therefore: 
a. the axial field B.sub.H provided by the magnet 22, 
b. a gradient magnetic field parallel to B.sub.H and provided by the 
gradient coils 50, 
c. a swept field, parallel to B.sub.H, provided by the ZO coils 54 of the 
magnet; and 
d. a constant field B.sub.1.sup.a, perpendicular to field B.sub.H, provided 
by one resonance mode of the birdcage resonator 12. 
To record a spectrum the swept field is swept through an NM resonance of 
the sample 10, either from below or from above resonance. At magnetic 
resonance, i.e. when 
EQU .omega.=.gamma.B.sub.o 
when .omega. is the RF angular frequency 
.gamma. is the gyromagnetic ratio of the nucleus to be detected in the 
sample 10, and 
B.sub.o is the (total local) magnetic field strength, 
the nuclear spins are disturbed from the Boltzmann population, and the 
nuclear magnetisation in the sample 10 starts to precess about the 
direction of B.sub.H at frequency .omega., resulting in an RF signal being 
received by the resonator in its second mode of resonance. 
To increase the sensitivity, a field modulation technique is used, the 
lock-in amplifier 46 generates a "reference" sinewave at a frequency 
f.sub.m which is much less than the RF frequency of generator 36; the 
reference sinewave is applied to the field modulation coils 26. When 
resonance is approached, the RF signal emitted by the resonator 12 through 
the coupling loop L.sub.b contains a component at frequency f.sub.m which 
remains on the output of the diode detector 44 and is fed to the input of 
lock-in amplifier 46; amplifier 46 contains a narrow-bandwidth filter, 
automatically centred on f.sub.m, so that it is capable of recovering the 
modulated signal from large out-of-band noise signals. 
In a test arrangement a 7 Tesla magnet 34 with 125 millimetre free bore 
within its shim/gradient coils 30 was used, and the field was swept by a 
value of 1.76 mili Tesla over 64 seconds in a sawtooth waveform. The 
birdcage resonator 12 was excited at 300.015 MHz, and a magnetic field 
modulation of 15 micro Tesla was applied at 881 Hertz through the field 
modulation coils 26. A field variation of 1.76 milli Tesla is equivalent 
to about 75 kHz proton frequency. The lock-in amplifier 46 had a time 
constant of 300 milliseconds. 
The sample 10 had the form illustrated in FIG. 1, it comprised two blocks 
of vulcanised rubber 10A, 10B both 1.5 cm thick and 3 cm long, spaced 3.5 
cm apart. The block 10A was of height 2.5 cm and the block 10B was of 
height 4 cm. The sample was placed in the resonator 12 of diameter 75 mm 
and length 90 mm with a 120 mm diameter copper shield 24. 
The vulcanised rubber sample 10 had a T.sub.2 of about 1 millisecond, which 
is relatively long in terms of the capabilities of the apparatus, and 
which could have been detected by known techniques, but was convenient for 
test purposes. The two rubber blocks were separated along the gradient 
direction, as shown. 
FIG. 4 shows the first derivative NMR signal S obtained from the sample 
with magnetic field gradients G.sub.Z of 0, 3.6, 5.4 and 9.0 milli Tesla 
per metre applied by coils 50. 
It can clearly be seen that with zero gradient, a single line is obtained, 
while with the three non-zero gradients, two distinct lines are visible. 
FIG. 5 shows the integrated signal I.sub.S. This was obtained by 
subtracting the zero gradient signal from each of the non-zero gradients; 
performing a linear base line correction; and performing integration. Such 
signal processing is conventional in EPR systems. The Figure indicates two 
peaks corresponding to the blocks 10A, 10B, with a spatial resolution 
increasing with the strength of the gradient. This can be calculated as 
the ratio of the zero gradient spectral line width to the field gradient 
intensity, and in the experiment the maximum spectral resolution was about 
10 millimetres. 
FIG. 6 shows the spectrum with the sample 10 removed from the birdcage 
resonator 12; both the signal and the base line-corrected integral are 
shown. The signal arises from the PMMA former 14 supporting the birdcage 
resonator 12. Although the value of the RF magnetic field B.sub.RF in the 
centre of the resonator 12 was set at a low value (about 2 micro Tesla) 
appropriate to the relatively narrow proton resonance in rubber, the 
B.sub.RF field was inevitably concentrated close to the wires 16, 18 of 
the resonator, and was sufficient to give a reasonably intense signal from 
the former 14. Glassy polymers such as PMMA exhibit extremely short 
T.sub.2 values (about 40 microseconds) and are particularly difficult to 
detect and image by conventional NMR techniques. 
The parameters of the apparatus were not optimised for the detection either 
of the rubber NM resonance or for the PMMA resonance; particularly for the 
latter, a considerably larger modulation field should ideally be used to 
achieve optimum signal to noise ratio. 
In alternative apparatus, the diode detector 38 may be replaced by a mixer 
and a reference arm. In a further alternative, use of a superheterodyne 
bridge configuration would give better sensitivity at low RF power levels. 
It will be appreciated that the results given above relate only to a 
one-dimensional projection of the test object 10. To obtain 
two-dimensional or three-dimensional images, either the sample 10 can be 
rotated, or the field gradient can be rotated; either rotation can supply 
projections at a sufficient number of orientations through the sample 10 
to permit signal analysis, for example by filtered back-projection or a 
similar technique, to reconstruct an image of the sample. If deconvolution 
of the projection spectra with the zero-gradient spectrum is required, as 
in FIG. 2, this would be carried out prior to back-projection. Such signal 
analysis techniques are conventional in NMR. 
In the apparatus described above, the continuous magnetic field gradient is 
generated by use of the gradient coils of the magnet with DC power 
supplies. In an alternative, the stray field of a superconducting magnet 
can be used, as in the STRAFI technique, when the sample 10, resonator 12, 
field modulation coils 26, and field sweep coils 54 would be placed near 
the edge of the magnet bore. 
In the apparatus described above, the magnetic field sweep is provided by 
ramping the magnetic field from the room-temperature ZO coil of the 
superconducting magnet. In an alternative, the field sweep may be 
generated by ramping the magnetic field of the superconducting magnet 
itself. 
It is an advantage of swept field continuous wave NMR imaging that it 
allows the use of RF coils or resonators with extremely high Q-factors, 
because the detection circuitry need have only a very narrow bandwidth, of 
the order of 1 Hertz. The use of high Q-factor resonators increases the 
signal to noise ratio. If such a high Q resonator is to be used, it is 
advisable to incorporate automatic frequency control circuitry to ensure 
that any frequency drift of the resonator is followed by the RF source. 
In an alternative apparatus, not illustrated, the superconducting magnet 34 
is replaced by a resistive magnet of much lower field, for example 0.01 
Tesla for operation at 300 MHz. All other parts of the apparatus are 
unchanged. The apparatus can now be operated by the technique of electron 
paramagnetic resonance to provide EPR spectroscopy and imaging.