Radiofrequency transducer and method of using same

A radiofrequency transducer, for nuclear magnetic resonance spectroscopy, comprising a radiofrequency transmission line formed into a loop, having terminals for the connection of a radiofrequency source or receiver thereto in such a manner that the transmission properties of the loop counteract the impedance due to its loop geometry. The transducer may comprise first and second elongate conductors (1,2) spaced by a dielectric material (3), for example polytetrafluoroethylene, and shaped to form the said loop, terminals (4,5) for the two conductors being provided at opposite ends of the loop.

The present invention relates to a radiofrequency transducer and is 
concerned in particular, but not exclusively, with the use of such a 
transducer in conjunction with a known technique of nuclear magnetic 
resonance. 
Nuclear magnetic resonance (NMR) is a well known laboratory technique, 
which can be used, inter alia for the determination and characterization 
of chemical species. In the classic NMR experiment, a substance is 
subjected to a static magnetic field ("B") and an oscillatory 
electromagnetic field of angular frequency .omega.. A condition of 
resonance occurs when .omega.=.gamma.B, where .gamma. is the gyromagnetic 
ratio which is characteristic of a particular nucleus present in the 
substance. Although the oscillatory field is normally and electromagnetic 
one for practical reasons, it should be pointed out that it is only the 
magnetic component which interacts with the test substance to produce the 
NMR effect. The resonance condition can be detected by absorption of, or 
absorption and re-emmision of, the radiofrequency field applied, and is 
indicative of the presence of a particular element comprising that 
nucleus. In general, the way of coupling the radiofrequency source to the 
particular substance has been to provide a simple loop or coil transducer, 
designed to generate a radiofrequency magnetic field connected to the RF 
source by a coaxial cable or other type of RF transmission line. 
It is necessary to tune the coil so as to be resonant at the required 
frequency for the particular nucleus under investigation at the particular 
magnetic field employed, and to match the impedance of the coil to that of 
the transmission line (eg. coaxial cable) employed. Examples of typical 
circuits for tuning and matching are shown in FIGS. 1a and 1b, which will 
be described in more detail hereinafter. These are the well known "half-T" 
and PI networks respectively. 
At the high frequencies commonly used in NMR techniques, very low values of 
variable capacitors are required in the arrangement shown in FIGS. 1a and 
1b for capacitors C1 and C3, to successfully tune and match the simple 
coil. In particular, when large coil sizes are employed, for example loops 
greater than 5 cm or so, and at high field strengths, for example of 1.5 
Tesla and above, the stray capacitances of the coil become significant 
compared with the values required to tune and match the coil. The self 
capacitance of the coil is even more significant if the sample being 
analyzed is conductive, or if it has a high dielectric constant. Under 
these conditions, the tune and match methods become inefficient and 
unworkable. This is particularly true for so-called "surface coils" for 
example of the kind suggested by J. J. H. Ackermann et al (Nature 283,167 
(1980)), which are often used in medical and other in vivo applications in 
which there is physical difficulty in locating the sample at the centre of 
a loop transducer. 
One method of overcoming this problem has been to introduce into the region 
of the coil one or more series capacitors to lower the effective 
inductance of the coil. An example of such a solution is that proposed by 
D. W. Alderman and D. M. Grant (J. Magn. Res. 36, 447, (1979)). As well as 
reducing the effective inductance of the coil, the introduction of series 
capacitance in this way means that the high RF voltages employed are 
divided across more components. A schematic diagram of such an arrangement 
as shown in FIG. 2, which will be described in more detail hereinafter. 
A difficulty with such proposals has been that they are relatively fragile 
and prone to break when applied to large rf coils. 
It is an object of the present invention to provide a radiofrequency 
transducer, particularly but not exclusively for use in nuclear magnetic 
resonance techniques, which overcomes or reduces the problems set out 
above where a relative large coil size is utilised at high frequencies and 
the stray capacitances of the coil become significant compared with the 
capacitance values required to tune and match the coil to a source or 
receiver. 
According to the present invention in one aspect there is provided a 
radiofrequency transducer comprising a radiofrequency transmission line 
formed into at least one loop, means being provided for the connection of 
a radiofrequency source or receiver thereto in such a manner that the 
transmission properties of the loop counteract the impedance due to its 
loop geometry. 
It has now been found, at least in preferred embodiments of the invention 
that the problem of tuning and matching relative large RF coils can be 
overcome by constructing an RF transducer loop as in effect an RF 
transmission line, in which additional self capacitance is provided, 
distributed relatively uniformly around the loop. This may be achieved by 
providing a loop made from first and second elongate electrical conductors 
spaced by dielectric material. The radiofrequency signal source can be 
applied to the conductors by means of connectors at opposite ends of the 
conductors, whereby the effect of series capacitance is provided, which 
has the effect of lowering the inductance of the loop. This capacitance is 
distributed relatively evenly around the loop. 
In accordance with one main aspect of the present invention there is 
provided a radio frequency transducer comprising a radio frequency 
transmission line formed in a configuration including at least one loop, 
said transmission line comprising at least first and second elongate 
conductors spaced by dielectric material, and connection means for 
connecting the transmission line to a radio frequency source or receiver, 
the connection means comprising first connection means for the said first 
conductor and second connection means for the said second conductor, the 
connection means for the first conductor being provided at one end of the 
loop and the connection means for the second conductor being provided at 
the other end of the loop. 
In some particularly preferred forms of the present invention it is 
arranged that in the loop (or in each loop of a plurality of loops) of the 
transmission line at least one of the conductors has a connection means 
coupled to it at one end of the loop and has an open circuit at the other 
end of the loop. In some preferred forms it is arranged that in the loop 
each of the conductors has a connection means coupled to it at one end of 
the loop and has an open circuit at the other end of the loop. 
However in alternative arrangements, it may be arranged that in the loop at 
least one of the conductors has a connection means coupled to it at one 
end of the loop and has a selectively variable impedance component at the 
other end of the loop, for example a selectively variable capacitor. Such 
a variable capacitor allows an extension of the normal working range of 
the transducer over which a suitable tune and match capacitive network may 
couple the loop to an NMR spectrometer, as will be described in more 
detail hereinafter. 
In many applications it will be preferred that the transducer comprises a 
single loop, but in many cases two or more loops may be provided. For 
example in one configuration the transmission line may include two or more 
loops arranged substantially parallel to each other and spaced apart in a 
direction perpendicular to the planes of the loops. 
In one preferred arrangement the transmission line is formed in a 
configuration including two connected loops, the said connection means 
being provided at a junction between the two loops, the arrangement being 
such that the first connection means for the first conductor is provided 
at one end of one loop and the second connection means for the second 
conductor is provided at the other end of the said one loop, the 
arrangement also being such that the first connection means for the first 
conductor is provided at one end of the other loop, and the second 
connection means for the second conductor is provided at the other end of 
the said other loop. 
It is particularly preferred that the transmission line may be formed with 
two loops arranged in a Helmholtz coil configuration with the loops 
approximately circular and spaced apart along a common axis, the axial 
spacing of the two loops being substantially equal to the mean radius of 
the loops. Such an arrangement gives a particularly uniform magnetic 
field. 
It is to be appreciated that more than two loops may be provided in a 
parallel configuration, such as has been set out above, for example a 
third loop may be provided mid way between the said two loops, and may 
also be connected electrically in parallel with the first two loops, to 
the first and second connection means in corresponding manner. In other 
arrangements additional loops may be added in various configurations 
depending upon the function required, for example additional turns may be 
added to the said two loops, provided this meets the functional 
requirements. 
In other arrangements the transmission line may be formed in a 
configuration including two or more consecutive loops, the connection 
means for the first conductor being provided at one end of the series of 
loops, and the connection means for the second conductor being provided at 
the other end of the series of loops. For example the series of loops may 
comprise a helix with the connector means connected at opposed ends of the 
helix. 
It will normally be preferred that the loop is a substantially complete 
loop with the said ends of the loop in close proximity to each other and 
with the said connection means for the conductors of the loops in close 
proximity to each other. 
By way of example, said at least one loop may be substantially circular 
and, where there are two or more loops, the loops may be coaxial. In other 
arrangements each loop may have a rectangular configuration. In some 
arrangements, each loop may lie on a cylindrical surface and may have a 
configuration in that surface such that a developed view of the surface 
shows the coil as having a rectangular configuration. 
Although in many arrangements the transmission line of the transducer will 
have only first and second spaced apart elongate conductors, in other 
arrangements three or more conductors may be provided. In particular, the 
transmission line may comprise first, second and third elongate conductors 
spaced by dielectric material, said first connection means being connected 
to said first and third elongate conductors. 
According to another aspect of the invention, there is provided a 
radiofrequency transducer, for example an r.f. radiator or probe 
comprising first and second elongate electrical conductors spaced by a 
dielectric material and formed into at least one loop, means being 
provided for the connection of a radiofrequency signal source to each of 
the said conductors, the connection means for the second conductor being 
provided at an end of the said at least one loop opposite the connection 
means for the first conductor. The two electrical conductors are 
preferably substantially uniform and the arrangement is such that the 
distribution of capacitance between the conductors around the loop is 
substantially linear. This arrangement minimizes point-to-point voltages 
around the loop, and thereby gives rise to lowered dielectric and 
conduction losses. 
The rf transducer coil according to the invention should be thought of as a 
so-called "transmission line" and its properties predicted using 
conventional transmission line theory. The transmission properties of the 
loop thus can be arranged to counteract the inductive impedance of its 
coil geometry. Preferably the transmission line is an assembly of 
conductors and insulation material which have predictable and uniform 
radiofrequency properties in the required frequency range and which can be 
used to carry rf power. The properties of such a transmission line are not 
a simple sum of the "point" static properties. 
Analysis of the dynamic behavior and transmission properties can be found 
in sources such as "Microwave Transmission Design Data", Theodore Moreno, 
Dover edition 1958; library of Congress Catalogue Card No. 58-11278. This 
reference also analyzes the effects of non-uniformity of the transmission 
line thus indicating the degree of uniformity required at any selected 
operating frequency. 
A simple introductory analysis can be found in standard text books such as 
"Electricity and Mangetism", B.I. and B Blearey, OUP 1965. 
The transducer preferably comprises a single loop or turn, although for 
certain applications multiple turns may be desirable. 
The dielectric material preferably has very low loss, and the material of 
choice is polytetrafluoroethylene (PTFE), or a glass laminate 
incorporating PTFE. 
The loop may be thought of in its simplest embodiment as at least two 
conductors spaced by a uniform amount over their length, formed into a 
loop, and connected at opposite ends to a tuning and matching circuit. In 
alternative embodiments, three or more spaced conductors may be utilized. 
When more than two conductors are used, sets of conductors may be 
connected in parallel. 
The conductors may be formed simply from an appropriate length of coaxial 
cable, or indeed from two suitable supported strands of wire provided with 
appropriate connections provided that the rf transmission line properties 
are suitable. However, in a preferred embodiment, the coil comprises a 
pair of copper strips separated by PTFE tape or like material, the 
opposite ends of the strips serving to provide the connection means. In an 
alternative preferred embodiment, the conductors may be provided in the 
form of copper or the like laminate disposed on opposite surfaces of a 
dielectric sheet. Such a radiator may be formed, for example, by etching a 
suitable double- sided copper clad laminate board. 
In addition to copper, the conductors may be made of silver, gold or any 
other metal whose magnetic susceptibility lies between approximately 
-10.sup.-4 cgs units and +10.sup.-4 cgs units. 
The transducer coil according to the invention may be tuned and matched to 
the RF source and coaxial cable by a simple "T" network, for example as 
illustrated in FIG. 3, which will be described in more detail hereinafter. 
Because the rf source is connected to opposite ends of the conductor, rf 
current flows completely round the loop in one or other of the conductors. 
For NMR purposes, this is important, because the current is the source of 
the rf magnetic field. The distributed capacitance means however that 
voltages are kept low, which minimizes the electrical component of the 
radiated field. Thus, a predictable and reasonably uniform rf magnetic 
field may be generated. 
In practice, the transmission line is chosen for best propagation and 
minimum loss. This is not necessary for the application to work but is 
necesasry for optimum performance. Optimum performance is obtained when 
the quality factor, Q, of the resonant tuned and matched assembly is a 
maximum. 
In practice for a particular coil application one would decide the 
transmission properties by the following considerations: 
1. How big is the rf loop to be? 
2. What are the practical tune and match capacitors? 
3. From the above the required characteristic impedance of the transmission 
line can be decided. 
4. Which type of transmission line will best satisfy step 3, and also give 
the best quality factor when assembled? 
5. For human subjects of the NMR investigation there may also be safety 
factors to be considered when the transmission line is chosen. 
In accordance with a further embodiment of the invention, there is provided 
a transducer assembly for a nuclear magnetic resonance spectrometer, 
comprising a transducer as set forth above, and means for connecting the 
asesmbly to a radiofrequency transmission line, for example a coaxial 
cable, for providing a connection to the spectrometer, and means including 
a variable capacitor for tuning the probe assembly to the radiofrequency 
signal and the said transmission line. 
An NMR spectrometer utilizing a probe according to the invention is very 
useful for in-vivo biochemical measurements, since it is in general 
important in such measurements for the probe to energize and respond to 
nuclei at some distance, for example a few cm deep, within the sample. 
Increasing depth of penetration requires increasingly large transducer 
coils. 
In accordance with a further aspect of this invention, there is provided an 
NMR spectrometer, comprising a transducer coil as set forth above, or a 
probe as defined above. 
According to yet a further aspect of the invention, there is provided a 
method of obtaining an NMR spectrum, which method involves the utilization 
of such an rf transducer coil. 
It should of course be appreciated that a wide range of variations of loop 
geometry are possible within the scope of the invention, for example, if 
it is desired to provide a non-uniform magnetic field, it may in certain 
circumstances by desirable to provide a structure in which the two 
conductors extending parallel over only a portion of the loop, the 
remainder of the loop being comprised by only one of the conductors.

FIG. 1(a) shows a circuit diagram of a conventional "T" tuning and matching 
network comprising two capacitors 21 and 22, having capacitances C1 and C2 
respectively, and connected in parallel and in series respectively across 
a coil 23, which may be a conventional surface coil for an NMR 
spectrometer. The tuning and matching network 21, 22 is connected to a 
transmission line 24, for example a coaxial cable, which leads to a 
radiofrequency signal source or receiver 25 of an NMR spectrometer 
indicated generally at 26. 
FIG. 1(b) shows a tuning and matching network of the known "PI" kind 
comprising three capacitors 27, 28 and 29 of capacitances C3, C4 and C5. 
The capacitors 27 and 28 are connected in parallel and series respectively 
across a surface coil 23, in the same manner as in FIG. 1(a), and the 
capacitor 29 is connected in parallel across the series combination of 
capacitors 27 and 28. The input and output of the tuning network 27, 28 
and 29 is again connected across a transmission line 24 which leads to a 
spectrometer 27 in the same manner as in FIG. 1(a). 
It is to be appreciated that although circuits shown in FIGS. 1(a) and 1(b) 
are known in themselves when the coils 23 are conventional surface coils 
for NMR work, the circuits of FIGS. 1(a) and 1(b) may be used in 
accordance with the invention where the coils 23 are replaced by 
transducers embodying the invention. 
FIG. 2 is a circuit diagram of a known form of NMR transducer coil, 
indicated at 30, in which a number of capacitors, shown in the Figure at 
31 and 32, are introduced in series with the coil shown diagrammatically 
at 23. The transducer coil 30 is linked to the transmission line 24 by 
three capacitors 33, 34 and 35 having capacitances C6, C7 and C8, the 
capacitors 33 and 34 being connected in series between the coil 30 and the 
transmission line 24, and the capacitor 35 being connected in parallel 
with the coil 30 at the junction of the two capacitors 33 and 34. The 
additional capacitors 31 and 32 in the coil 30 reduce the effective 
inductance of the coil and having the added advantage that the high rf 
voltages are divided across more components. However, such a coilcapacitor 
structure is mechanical unsound, and may in practice physically break. 
There will now be described with reference to FIGS. 3, 4 and 5 an 
embodiment of the present invention which provides a radiofrequency 
transducer for use in an NMR spectrometer. FIGS. 4 and 5 show a 
perspective view of an actual probe, and FIG. 3 shows diagrammatically a 
form of tuning and matching circuit for the probe. 
Referring to FIGS. 4 and 5, a loop for transmitting and receiving a 
radiofrequency magnetic field comprises a first conductor 1 and a second 
conductor 2 spaced by a dielectric 3. The conductors 1 and 2 are each 
formed of a rectangular copper strip approximately 4 mm.times.1 mm, and 
are substantially uniform in thickness and spacing over their lengths. The 
dielectric material 3 is formed of a PTFE material, approximately 0.5 mm 
in thickness. This may be either machined, to form a firm mechanical 
shape, or as an alternative, PTFE tape may be used, and wound around one 
or other of the conductors 1 and 2. It is important to use low loss 
dielectrics in the region close to the loop, where the electric field is 
at its highest, and also that the conductors should be of a thickness 
which is large compared with the RF skin depth of the frequency of choice. 
A 10 cm diameter coil of the kind illustrated in FIGS. 4 and 5 and having 
the material dimensions as described above may be tuned and matched using 
a simple circuit as illustrated in FIG. 3, using variable capacitors 36 
and 37, having capacitances C9 and C10 having a range of from 2 pF to 20 
pF, at a frequency of 80 MHz. By comparison, a corresponding single turn 
conventional coil has a capacitance of approximately 5 pF at the same 
frequency, and cannot therefore be tuned and matched to a 50 Ohm coaxial 
cable, when used on a sample with high loss. 
In the example shown diagrammatically in FIG. 4 and FIG. 3, the 
radiofrequency transducer comprises a radiofrequency transmission line 
which is indicated generally at 40, and comprises the first conductor 1, 
the second conductor 2, and the dielectric material 3 positioned between 
the conductors 1 and 2. The transmission line 40 is formed into a single 
loop, indicated generaly at 41. A first connection means 38, for example a 
terminal, is connected to the first conductor 1 at one end of the loop 41, 
which end is indicated generally at 42. A second connection means 39, for 
example a terminal, is connected to the second conductor 2 at the other 
end of the loop 41, which other end is indicated generally at 43. 
In the example shown in FIG. 3, the end of the first conductor 1 which is 
electrically remote from the first connection means 38 (that is to say 
which is physically at the end 43 of the loop) is open circuited, and 
similarly the end of the conductor 2 which is electrically remote from the 
second connection means 39 (that is to say at the end 42 of the loop 41) 
is also open circuited. 
In FIG. 3(a) there is shown a modification of the arrangement of FIG. 3, in 
which the only change is that the end of the second conductor 2 which is 
electrically remote from the second connection means 39 (that is to say at 
the end 42 of the loop 41) is not open circuited, but is connected to the 
first connection means 38 through a selectively variable capacitor 44, 
which for example may be variable in the range 0.2 pF to 10 pF. Such an 
arrangement permits adjustment of the transmission properties of the loop 
41 to account for a wider range of lossy samples, and extends the working 
range of the transducer. In other arrangements a fixed capacitor may 
replace the variable capacitor 44, and in yet other arrangements, both 
ends 42 and 43 of the loop 41 may be terminated by fixed or variable 
capacitors. 
FIG. 6 is a schematic exploded view of a further embodiment of a transducer 
according to the invention. The transducer comprises two glass-reinforced 
PTFE boards 10 and 11 providing dielectrics separating three elongate 
conductors 12, 13 and 14 having a loop configuration. Conductors 13 and 14 
are in fact disposed on opposite sides of board 11, but are shown exploded 
for clarity. In use, boards 10 and 11 are pressed close together so as to 
provide a uniform sandwich construction. Boards 10 and 11 may 
advantegeously be formed by a chemical etching process using a copper-clad 
laminate. 
Each conductor 12, 13 14 is provided with a leg 15, 16 and 17 respectively 
forming a terminal for connection to a radiofrequency source or receiver, 
the leg 16 of middle conductor 13 being at the opposite end of the 
conductor from that of conductors 12 and 14. 
In use conductors 12 and 14 are connected in parallel to one side (the 
screen) of a coaxial transmission line, conductor 13 being connected to 
the central conductor. 
Turning now to FIG. 7, there is shown a yet further embodiment of a 
transducer according to the invention, and again FIG. 7 is a diagrammatic 
representation showing the transducer in exploded form. 
The transducer shown in FIG. 7 comprises a transmission line indicated 
generally at 40 and comprising a first conductor 1 and a second conductor 
2, formed in a configuration including two connected loops indicated 
generally at 50 and 51, connected by linking strips of conductor 52 and 
53. The conductors 1 and 2 in loop 50 are separated by a circle of PTFE 
insulator 3 and the conductors 1 and 2 in the loop 51 are similarly 
separated by a circle of PTFE insulator material 3. In a practical 
embodiment, the conductors 1 and 2 and insulator 3 in each of the loops 50 
and 51 are pressed close together, and the linking strips 52 and 53 are 
close together, but not superimposed. the loops 50 and 51 are in practice 
arranged in a Helmholtz coil configuration with the loops 50 and 51 
circular and spaced apart along a common axis. The axial spacing of the 
two loops being substantially equal to the mean radius of the loops. 
Connection means for connecting the transmission line 40 to a matching and 
tuning network such is shown in FIG. 3, comprise first connection means 38 
for conductor 1 (for example a terminal), and second connection means 39 
for conductor 2 (for example a terminal). Thus the connection means 38,39 
is provided at a junction between the two loops 50 and 51, the junction 
being provided by the linking strips 52 and 53. 
In conformity with the nomenclature used in FIG. 3, the loop 50 is 
indicated as having one end at 42 and an opposite end at 43. Similarly the 
loop 51 is indicated as having one end at 42' and having another end at 
43'. With reference to these ends, there will now be described the manner 
in which the connection means 38 and 39 are provided at the ends of the 
loops 50 and 51, in the sense of being electrically connected to these 
ends. The arrangement is such that the first connection means 38 for the 
first conductor 1 is provided at one end 42 of the loop 50, and the second 
connection means 39 for the second conductor 2 is provided at the other 
end 43 of the loop 50. The arrangement is also such that the first 
connection means 38 for the first conductor 1 is provided at one end 42' 
of the other loop 51, and the second connection means 39 for the second 
conductor 2 is provided at the other end 43' of the said other loop 51. 
When the conductors 1, and 2 are assembled in practice, the cross-section 
of each part is as shown in FIG. 5. An alternative assembly embodying the 
principle of FIG. 7 consists of two parts each as shown in FIG. 4, linked 
in the same way as in FIG. 7. 
FIGS. 8 and 9 show a further embodiment of a transducer according to the 
invention. FIG. 8 shows diagrammatically a pair of loops indicated 
generally at 60 and 61, wrapped around a former 69. The loops 60 and 61 
are shown in developed form in FIG. 9. The loops consist again of two 
conductors 1 and 2 separated by an insulator 3 and there are provided 
first and second connection means 38 and 39 positioned on linking strips 
63 and 62 of the conductors 1 and 2. The arrangement is another example of 
two parts in parallel to give a more uniform RF magnetic field than that 
of a single part. Functionally the arrangement can be considered as a 
distorted version of the coils shown in FIG. 7, which have been wrapped 
around the outside of the former 69, which in this case is a hollow 
cylinder. In this particular example the arc lengths around the cylinder 
are 120.degree., the length along the axis having been chosen to fulfil 
other requirements for satisfactory operation. The strip widths of the 
conductors 1 and 2 insulator 3 are shown in step form for the sake of 
clarity, but in practice the widths would be as shown in FIG. 5. 
It is to be appreciated that in addition to use in the NMR spectrometer 
field, a transducer embodying the invention finds applications in other 
fields, for example rf broadcasting, and rf induction furnaces.