Device and method for adjusting a radiofrequency antenna of a nuclear magnetic resonance apparatus

A device and method are provided for adjusting a radiofrequency antenna of a nuclear magnetic resonance apparatus. In the invention, using capacities preset in the factory, the antenna detuning of an NMR apparatus is limited to a range such that the standing wave rate of a high frequency line which conveys the radiofrequency signal is limited. This line is cut and, at a short distance, a tuning adjustment circuit is inserted. This circuit may be automated: the electric adjustment motors, thus spaced away, do not disturb the magnetic fields of the apparatus. By measuring the real part of the admittance at the input of the adjustment circuit and the phase shift between voltage and current this circuit can be adjusted.

FIELD OF THE INVENTION 
The present invention relates to a device and method for adjusting a 
radiofrequency antenna of a nuclear magnetic resonance apparatus. It finds 
its application more particularly in the medical field where nuclear 
magnetic resonance (NMR) examinations provide a precious aid in diagnosis. 
It may nevertheless find its application in other fields. The aim of the 
present invention is to contribute to the creation of images using NMR 
apparatus which are most exact and more accurate. 
BACKGROUND OF THE INVENTION 
An NMR apparatus traditionally includes means for subjecting a body to a 
constant and intense magnetic field. Thus conditioned, some regions of the 
body may be energized by radiofrequency excitation and, when this 
excitation ceases, induce a resonance signal which is measured and which 
includes information useful for creating images of parts of the body. The 
excitation and measuring means include a radiofrequency antenna; this 
antenna is disposed about the body to be examined. The load seen by this 
antenna at the time of emission, or which comes to the same thing, the 
internal impedance of the generator which it forms at the time of the 
measurement, depends essentially on the body subjected to the examination. 
From one body to another it changes. To provide the best excitation and 
the best measurement, it is necessary to tune the antenna. Furthermore, 
the NMR antenna must be matched to the high frequency line which connects 
it, through a duplexer, either to the emitter or to the receiver. An 
adjustable device must then be associated with the antenna for matching 
its impedance. In practice, since high frequency lines are coaxial cables 
characterized by their characteristic impedance, for example of 50 ohms, 
the antenna must further be adjusted so that it presents such a real 
impedance at the operating frequency. 
DESCRIPTION OF THE PRIOR ART 
A circuit generally used for matching the antennae uses two adjustable 
capacitors. A first capacitor is connected in parallel with the antenna, 
it is followed by another connected in series. These two capacitors are 
conventionally installed in the direct proximity of the antenna itself. 
They are located as it were in the tunnel of the machine. They are 
accessible only with difficulty and adjustment thereof is ordinarily 
carried out manually by means of two linkage rods. In fact, in the present 
state of the technique, it is impossible to place electric control motors 
in the tunnel of the machine. They would disturb the magnetic fields too 
much. Under these conditions, the adjustment must still remain manual 
which is troublesome. The solution which might consist in extending the 
rods so that the control motors are at a distance from the tunnel of the 
machine does not seem feasible because of the complexity of the linkage 
which this choice would involve. 
SUMMARY OF THE INVENTION 
The object of the present invention is to overcome the above-mentioned 
drawbacks in that matching of the antenna is always provided, and that it 
is provided automatically. In the invention, the tuning capacities known 
for limiting the impedance mismatching of the antenna to the line are 
preset in the factory. Then the high frequency line (in practice a coaxial 
cable) is cut into two parts: a first relatively short part disposed close 
to the antenna and a longer part which continues it and which is connected 
to the emission-reception means. Between these two parts an antenna tuning 
circuit is installed. The purpose of the pre-adjustment is to limit the 
standing wave rate (SWR) in the small part of the line, for example less 
than or equal to 3.5. 
At the end of the small part of the line it is then possible to dispose a 
tuning circuit whose settings may be made automatically (with electric 
motors because, from this point of view, the spacing apart is sufficient). 
The invention provides an adjustment device for a nuclear magnetic 
resonance apparatus comprising an antenna, for emitting and/or receiving 
from a body under examination in the apparatus radiofrequency resonance 
signals, connected by a high frequency line to emission/reception means, 
which device comprises, inserted between the antenna and the 
emission.reception means, a portion of the high frequency line followed in 
cascade by an antenna tuning circuit. 
The invention further provides a method of adjusting a nuclear magnetic 
resonance apparatus having an antenna for emitting and receiving a 
radiofrequency resonance signal, connected by a high frequency line to 
emission/reception means, wherein: 
between the antenna and the emission/reception means is inserted a portion 
of the high frequency line, connected in cascade to an antenna tuning 
circuit and to a sensor, this sensor being itself connected to a circuit 
for measuring the detuning, 
and the apparatus is adjusted by adjusting the tuning circuit as a function 
of the results delivered by the circuit measuring the detuning.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 shows an NMR apparatus equipped with the adjustment device of the 
invention. This NMR apparatus includes means 1 for subjecting a body 
examined 2 to an intense and constant magnetic field B.sub.O. Subjected to 
this influence, body 2 further receives through an antenna 3 a 
radiofrequency excitation emitted by generating means 4. The apparatus 
further includes so-called gradient coils 5 for particularizing certain 
regions of body 2 of which an image is desired. The resonance signal 
emitted by body 2 is picked up by the antenna 3 and is directed, in 
particular by means of a duplexer 6, towards reception and processing 
means 7. The generating means 4 and the reception means 7 are drive by a 
sequencer 8. In the direct proximity of the antenna are placed 
conventional capacities 9 and 10 provided for tuning the antenna. In the 
state of the technique, the adjustment of these capacities is effected by 
means of rods respectively 11 and 12 projecting from the rear face 13 of 
the apparatus. 
In the invention, the connection which connected antenna 3 (with its 
intrinsic tuning circuit 9,10) to the duplexer 6 is cut. This high 
frequency connection often comprises a coaxial cable whose braid is 
connected to ground. A first relatively short section 14 is thus followed 
by a second section 15. The length of the first section 14 is sufficiently 
great so that, at its end 16 the furthest away from the access tunnel 17 
of the machine, an automatic device 18 may be disposed for motorizing an 
inserted antenna tuning circuit 19. Since section 14 is not matched, 
additional losses result due to the mismatching, in addition to the losses 
in the dielectric and in the conductor. These losses are limited by 
limiting the length of the section to the minimum required and by limiting 
the SWR in this section. Limitation of the SWR is obtained by presetting 
the capacities 9 and 10. In practice, if the impedance of the antenna is 
correctly set, the SWR in this section does not exceed 3 in cases as 
different as the presentation of an adult or a child for examination. The 
reception and processing means 7 are matched to the characteristic 
impedance of the high frequency line section 15. In one example where the 
field B.sub.0 is equal to about 0.5 tesla, the resonance frequency of the 
NMR signal is equal to 21.3 MHz. With a coaxial cable of the RX 4/RG 213 U 
type, a section 14 of 3 m in length, and if the load presented by the body 
2 to the antenna 3 is matched from the input to the output of section 14, 
the resonance signal is attenuated by 0.10 dB. On the other hand, if this 
load is mismatched, so that the standing wave rate in section 14 remains 
substantially less than 3.5, the attenuation is equal to 0.16 dB. In other 
words, the additional loss of 0.06 dB is negligible with regard to the 
advantage obtained by section 14: that of allowing the presence of drive 
means 18. 
Preferably, the tuning circuit 19 is formed of two adjustable capacitors 
C.sub.1 and C.sub.2 and an inductance L mounted in the form of .pi.. 
Capacitor C.sub.1 is the capacitor the closest to section 14. For the 
convenience of the explanations which follow the downstream output of the 
tuning circuit 19 is called A. The values of the capacities C.sub.1 and 
C.sub.2, as well as their adjustment range and the value of the self 
inductance L are calculated as a function of the load impedance range 
which it is desired to match. For example, the self impedance L has a 
value of 0.2 microHenry and capacities C.sub.1 and C.sub.2 are capacities 
adjustable between 220 pF and 720 pF. Circuit 19 could however have 
another form. For example, it could include more than three reactive 
elements. Or else, it could include a T assembly of two adjustable 
inductances connected in series, on each side of a fixed parallel 
capacity. The .pi. mounting presented is preferred because it is the 
simplest and because adjustable capacities are industrially more 
accessible than adjustable inductances. 
The procedure for adjusting capacities C.sub.1 and C.sub.2 will now be 
described. For that, the reasoning will be based on the admittances at 
point A. In a first stage, it can be seen that capacity C.sub.2 only adds 
a purely imaginary admittance to the admittance seen from point A. 
Consequently a variation of C.sub.2 only modifies the imaginary part of 
this admittance whereas the real part of this admittance only depends on 
the value of C.sub.1. This judicious remark leads to choosing a 
particularly simple adjustment procedure. For matching the antenna, the 
real part and the imaginary part of the admittance seen from point A must 
then be measured. The adjustment procedure consists then in adjusting 
C.sub.1 so that the real part of the admittance is equal to the 
characteristic admittance of section 15, and in adjusting C.sub.2 so as to 
cancel out the imaginary part of this admittance. The procedure is 
preferably carried out in this order since tuning of the antenna may thus 
be obtained with two single adjustments. 
To achieve this adjustment, a sensor 21 connected to a circuit 22 measuring 
the detuning is disposed between the tuning circuit 19 and the input 20 of 
section 15. The sensor picks up a signal coming from the generator (on 
emission) and allows the voltage-current characteristics to be known of 
the signal which passes through the connection 14-15 at the input A of 
circuit 19. These characteristics are the same as at reception. Sensor 21 
includes a current transformer 23 for taking a signal proportional to the 
current. It further includes a divider bridge 24 for taking off a signal 
proportional to the voltage. Circuit 22 for measuring the detuning 
includes a chopping amplifier 25 for producing a so-called square signal 
synchronous with the resonance signal. The signal proportional to the 
voltage is introduced therein, amplified, then chopped so as to produce an 
AC signal with zero DC component, including successive equal pulses of 
opposite signs. 
The signal proportional to the voltage of the antenna signal is fed into a 
first mixer 26 where it is multiplied by the square signal. This 
multiplication provides full wave rectification. After low frequency 
filtering and standardizing amplification in a first shaper circuit 27, 
the signal leaving mixer 26 is transformed into a signal 28 representative 
of the modulus of the voltage of the antenna signal 
(.vertline.V.vertline.). The signal proportional to the current of the 
antenna signal taken off through the coupling 23, undergoes a similar 
processing in a second mixer 29 followed by a second shaper circuit 30. 
Taking into account the phase shift .phi. between the current and the 
voltage of the antenna signal, and considering the fact that the mixers 
are driven at their second input by the same square signal, the shaper 
circuit 30 delivers a signal 31 proportional to the product of the modulus 
of the current of the antenna signal multiplied by the cosine of the phase 
shift angle between the current and the voltage of the antenna signal 
(.vertline.I.vertline. cos .phi.). The measuring means 22 further include 
computing means 32 for working out the ratio of signal 31 to signal 28. 
The ratio may be obtained for example using a microprocessor specially 
adapted for this function or by using logarithmic dividers. The ratio is 
equal to the real part Re.sub.YA of the admittance at point A. The 
computing means 32 include a display means 33 for indicating this real 
part of the admittance. The means 33 include an indication Y.sub.c 
corresponding to the characteristic admittance of section 15. 
Furthermore, the signal proportional to the current is subjected to a 
90.degree. phase shift and standardization chopping in circuit 34. Then 
the signal delivered by circuit 34 is subjected to processing similar to 
the preceding processing by introducing it into a third mixer 35 followed 
by a third shaper circuit 36. The signal 37 delivered by circuit 36 is 
also introduced into the computing means 32. These further comprise a 
second display device 38 representative of the phase shift .phi. between 
the current and the voltage of the antenna signal. In fact, the signal 37 
thus transformed is equal to the sine of this phase shift. The principle 
of adjustment of C.sub.1 and C.sub.2 is then simple: C.sub.1 is adjusted 
so that the first indicator 33 points to Y.sub.c, then C.sub.2 is adjusted 
so that the second indicator 38 points to 0. When this latter points to 0, 
.phi. is zero, the current and the voltage are in phase: the imaginary 
part I.sub.YA of the admittance at point A is zero. 
FIGS. 2a to 2c show the trends respectively of the real part Re.sub.YA of 
the admittance at point A, for three different types of body 2, when 
capacity C.sub.1 varies. FIG. 2b shows the variation of the imaginary part 
I.sub.YA of the admittance at point A for one type of body and with 
different values of C.sub.2 (three cases) when C.sub.1 varies. FIG. 2c 
shows the evolution of the phase shift .phi. at point A when C.sub.2 
varies. In the diagram of FIG. 2a, the characteristic admittance Y.sub.c 
is shown. Depending on the body examined, curves 39, 40, 41, it can be 
seen that the real part of the admittance passes through a maximum 
situated between two minima. The variation of capacity C.sub.1 has been 
limited to the values which this capacity may take in a practical example 
of use. In the preferred example, the values C.sub.1min and C.sub.1max 
corresponds respectively to 220 pF and 720 pF. It can be seen that there 
may be two possibilities in some cases for adjusting C.sub.1. In fact, the 
maximum of curves 39 to 41 is greater than Y.sub.c, and their minima are 
less than it. 
Curves 39 and 40 correspond to the cases where there are two values of 
C.sub.1 (P.sub.1 and P.sub.2) for which the real part Re.sub.YA of the 
admittance at point A is equal to Y.sub.c. For the curve 41, the range of 
variation of C.sub.1 towards the low values is insufficient for a second 
time reaching an equality between the real part of the admittance at point 
A and the characteristic admittance of section 15. By examining curves 42 
to 44 of FIG. 2b, it can be seen that for P.sub.1 the imaginary part 
I.sub.YA of the admittance at point A is positive whereas for P.sub.2 it 
is negative. For P.sub.1 a self inductance would be required in place of 
C.sub.2 so as to be able to cancel out the imaginary part. Whereas for 
P.sub.2, the imaginary part can be cancelled out with the capacity 
C.sub.2. Thus, in most cases, only the point of adjustment P.sub.2 may 
satisfy the two conditions Re.sub.YA =Y.sub.c and I.sub.YA =0. 
Optimization of the capacities C.sub.1 and C.sub.2 is obtained when they 
have the smallest adjustment range so that all the loads corresponding to 
limited SWRs, for example between 1 and 3.5, may be concerned. The 
optimization relates to the cost of the adjustable capacities. The smaller 
their range, the less expensive they are. The three curves 42 to 44 shown 
in FIG. 2b are parametered by values of C.sub.2. These curves 42 to 44 are 
included between the curves corresponding to C.sub.2min and C.sub.2max. 
Once the adjustment of C.sub.1 has been made, the value of the imaginary 
part of the admittance at point A belongs to a segment 45 of values 
depending, for a given value of C.sub.1, on different values of C.sub.2. 
I.sub.YA may then be cancelled out by adjusting C.sub.2. When C.sub.2 
varies, FIG. 2c shows us that the phase shift .phi. passes through a zero 
value and that the admittance becomes real. 
FIG. 3 shows a flow chart of automatic processing to be used by the central 
unit-sequencer 8 which receives the information from the indicators 33 and 
38. Depending on these measurements, the central unit 8 gives instructions 
to the drive means 18 for adjusting the capacities C.sub.1 and C.sub.2. 
The functioning of this flow chart should be studied with reference also 
to FIGS. 2a to 2c. In a first stage, in a test 46, the central unit 
compares the value of the real part of the admittance with the 
characteristic admittance. If this real part is greater than it (FIG. 2a), 
an instruction 47 for increasing the capacity C.sub.1 is sent to the drive 
means 18. In a test 48, the behavior of this real part of the admittance 
is studied with respect to the characteristic admittance. As soon as the 
value of the characteristic admittance is reached, the adjustment of 
C.sub.1 is finished. On the other hand, if the real part Re.sub.YA is less 
than Y.sub.c : by an instruction 49 the value of the capacity C.sub.1 is 
decreased. In a comparator 52, the evolution of Re.sub.YA is then 
measured. If this evolution is positive (.DELTA.Re.sub.YA &gt;0) and as long 
as the value Y.sub.c has not been reached (test 50), the adjustment is 
continued by a loop 51. As soon as this value is reached the adjustment is 
stopped. This adjustment may be reached if at the outset Re.sub.YA was 
between P.sub.3 (FIG. 2a) and P.sub.2. On the other hand, if Re.sub.YA was 
between P.sub.1 and P.sub.4, the decrease of C.sub.1 causes the variation 
of Re.sub.YA to be negative. The comparator 52 detects this eventuality. 
In this case, C.sub.1 is increased by an instruction 53 until a test 54 
indicates that Re.sub.YA becomes greater than Y.sub.c : the adjustment 
point P.sub.1 has then just been exceeded. In this case, it is preferably 
decided to join the adjustment point P.sub.2. In fact, for the value of 
C.sub.1 thus found the segment 60 of the values of I.sub.YA as a function 
of C.sub.2 does not necessarily include a zero value. Thus, instead of 
stopping the adjustment of C.sub.1, test 54 branches to a sequence 
including the instruction 47. 
With the adjustment of C.sub.1 thus obtained, by a test 55, the value of 
the phase shift with respect to 0 is measured. If this phase shift is less 
than 0, C.sub.2 should be increased by sending an instruction 56 to the 
drive means 18. This instruction persists as long as a test 57 does not 
show cancelling out of the phase shift. If this phase shift, at the 
outset, is greater than 0, an instruction 58 and a test 59 play the dual 
role of the instruction 56 and the test 57. As soon as the phase shift is 
cancelled out, the adjustment is terminated. On the other hand, if the 
phase shift is zero, at the outset, the value C.sub.2 is not modified.