FR2557298A1 - INTEGRATED RADIOFREQUENCY COIL FOR NUCLEAR MAGNETIC RESONANCE TECHNIQUES - Google Patents

Abstract

THE INVENTION CONCERNS NUCLEAR MAGNETIC RESONANCE TECHNIQUES. WE MANUFACTURE, BY ENGRAVING TECHNIQUES, ON THE SAME PRINTED CIRCUIT BOARD 13, AN RF COIL 1, 3 AND A CAPACITIVE ELEMENT 9 NECESSARY TO RESONATE THE COIL. THIS SIMPLIFIES THE CONSTRUCTION OF COIL STRUCTURES WITH COMPLEX CONDUCTIVE PATTERNS. BY USING A FLEXIBLE PRINTED CIRCUIT BOARD WHOSE SUBSTRATE HAS HIGH DIELECTRIC CONSTANT, LOW LOSSES, AND GOOD VOLTAGE RESISTANCE, CAPACITORS CAN BE MADE ESPECIALLY USEFUL IN NMR APPLICATIONS. APPLICATION TO MEDICAL IMAGING.

Description

The present invention relates to an apparatus for

  nuclear magnetic resonance (NMR). More particularly, the invention relates to integrated radio frequency (RF) coils useful in such apparatus for transmitting and / or receiving RF signals. The phenomenon of NMR has been used in the past by chemists to study, in vitro, the structure

  molecular organic molecules. In a typical way

  The NMR spectrometers used for this purpose were designed to receive relatively small samples of the test substance. However, NMR has been developed more recently to provide a technique

  imagery used, for example, to obtain

  of anatomical features of living human beings.

  Such images representing parameters associated with nuclear spins (typically protons

  of hydrogen associated with the water in the tissues) can

  have an interest in a medical diagnosis, in the determination of the state of tissue health in the region examined. NMR techniques have also been extended to in vivo spectroscopy of elements such as, for example, phosphorus and carbon, which offers researchers the means for the first time to study chemical processes.

  in a living organism. The use of NMR for

  to create images and to carry out spectroscopic studies

  of the human body necessitated the use of specially designed system components, such as magnet coils,

gradient and RF.

  As a background of the invention, we can note

  that the phenomenon of nuclear magnetic resonance

  in atomic nuclei with an odd number of protons and / or neutrons. Because of the spin of protons and neutrons, each of these nuclei has a magnetic moment, so that when placing in a homogeneous static magnetic field, Bo, a sample composed of

  such nuclei, a large number of nuclear magnetic moments

  res align with the field to produce a resultant macroscopic magnetization M in the field direction. Under the influence of the magnetic field Bo, the magnetic moments perform a movement of precession around the field axis, at a frequency which depends on the intensity of the applied magnetic field and the characteristics of the nuclei. The

  angular frequency of precession, u, also called frequency

  Larmor's equation, is given by the Larmor equation (* = <B, where Y is the gyromagnetic ratio (which is constant for each isotope exhibiting the NMR phenomenon), and in which B is the magnetic field (B plus other fields) acting on the nuclear spins, we see that the resonant frequency depends on the intensity

  of the magnetic field in which the sample is placed.

  The orientation of the magnetization can be disrupted

  M, which is normally directed in the direction of the

  magnetic field Bo, by the application of magnetic fields

  oscillating at the Larmor frequency or at a nearby frequency. Typically, such magnetic fields, designated B1, are applied in a direction orthogonal to the magnetization direction M by means of radio frequency pulses in a coil connected to a radiofrequency transmitter apparatus. The magnetization M rotates around the direction of the field B1. In NMR, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate the

  magnetization M so as to bring it into a perpendicular plane

  to the direction of the Bo field. This plane is commonly called the transverse plane. At the moment when the RF excitation ceases, the nuclear moments that have turned to come in the transverse plane begin to realign with the B 0 field by various physical processes. During this re-lineage process, the nuclear moments emit radio frequency signals, which are called NMR signals, which are characteristic of the magnetic field and the particular chemical environment in which the nuclei are located. The same RF coil or a second RF coil can be used to receive the signals from the cores. In NMR imaging applications, one observes the signals of

  NMR in the presence of magnetic field gradients

  read to encode spatial information in the NMR signal. This information is subsequently used to reconstruct images of the object under study, in a manner

well known to those skilled in the art.

  In conducting whole-body NMR studies, it has been found that it is advantageous to

  to increase the intensity of the homogeneous magnetic field Bo.

  This is desirable in the case of image formation using protons, to improve the signal-to-noise ratio of NMR signals. However, this is an obligation in the case of spectroscopy, because some of the chemical species studied (eg phosphorus and carbon) are relatively scarce in the body, so a high magnetic field is needed to detect usable signals. As it is obvious

  according to Larmor's equation, the increase of the magnetic field

  tick B is accompanied by a corresponding increase in

  Wu and, consequently, the resonance frequency of the bobbins

  from the transmitter and the receiver. This complicates the concept

  RF coils that are large enough to accommodate the human body. A source of difficulty is that the RF field generated by the coil must

  be homogeneous over the entire body region to be studied.

  In order to produce a homogeneous RF field over all the part of the body to be studied and to achieve the desired nuclear spin excitation, the RF transmitter coil (which frequently also acts as a receiver coil)

  must withstand high voltages at frequencies of interest

  cient. For example, the coil may be subjected to voltages exceeding 5000 volts at specified frequencies

  by the Larmor equation. Conventional capacitors use

  Emissions with vacuum tubes are usually ferromagnetic and are therefore unsuitable for use in an NMR apparatus. Other conventional capacitors have a quality factor Q which is too low and therefore degrade unacceptably the performance of the coils. Capacitors must also be flexible to conform to the shape of the RF coils used for NMR head and body examination, which typically have a cylindrical configuration. According to the invention, it has been found that it is advantageous to engrave

  on a printed circuit board, in the form of a set of

  wavelength, the conductive pattern of the RF coil and the

  capacitive elements necessary to resonate the bobbin

  born. This ensures not only the obtaining of the capacitors having the desirable properties indicated previously,

  but also the simplification of the manufacture of

  with complex conductive patterns, with an improvement

  concomitant performance of the reproducibility of the coils when it is necessary to have many coils

having the same configuration.

  The object of the invention is therefore to provide a bobbin

  integrated RF, including a flexible RF capacitor, no

  magnetic and supporting a high voltage, for use

in an NMR apparatus.

  Another object of the invention is to provide a

  integrated RF coil-capacitor package that can be manufactured

  easy to use and suitable for use in a

NMR.

  A further object of the invention is to provide

  an integrated RF coil structure for NMR, including

  capacitor with low losses and dielectric

high cudgel.

  The invention provides an NMR apparatus which

  comes for the excitation and detection of NMR signals in a sample analyzed by NMR. The apparatus comprises a radio frequency coil having at least one coil coil and at least one capacitive element arranged to resonate the coil at the Larmor frequency of the sample, and the coil

  coil and the capacitive element consist of a conductive pattern

  manufactured on a single printed circuit board.

  The rest of the description refers to the drawings

  attached which represent respectively: Figure 1A: a two-turn NMR RF coil

  connected in parallel, manufactured in accordance with the

  tion; Figure 1B: the coil shown in Figure 1A, to which the configuration of a cylinder has been given; Figure 2: a section along the line 2-2 of the coil shown in Figure 1A;

  Figure 3: a representation similar to the figure

  re 2 but which shows another embodiment;

  Figures 4A and 4B: Schematic representations

  of a coil that can be manufactured in accordance with the

  vention; and FIGS. 5A and 5B: conductive patterns used in the manufacture, according to the invention, of

  the coil shown in FIG.

  Figure i shows schematically a coil

  NMR RF with two coils connected in parallel, of

  standard, but which is manufactured in accordance with the

  cention. The coil consists of single turns 1

2557298-

  and 3 connected in parallel and etched at points 5 and 7 between which is connected a tuning capacitor 9. In coils of conventional structure, the coil turns 1 and 3 are typically formed by copper tube and are mounted on a non-conductive cylindrical coil mandrel, consisting of a low loss dielectric material. The conventionally constructed coil uses

  a component consisting of a discrete capacitor intercon-

  electrically connected to the copper tube. Although such coils operate satisfactorily, they suffer from the disadvantages described above. In addition, such conventional construction techniques do not lend themselves to use with more complex coil structures, such as will be described in connection with FIGS. 4A.

and 4B.

  According to the invention, the coil is manufactured

  and the capacitor in the form of a single integrated set.

  Returning to FIG. 1A, it is noted that the coil is made by etching (using conventional techniques) a conductive pattern 11, comprising a capacitor 9, on a single flexible integrated circuit board 13. The The finished coil has the shape of a cylinder, as shown in FIG. 1B, fixing the edge EF at the edge GH. In general, each of the turns of the coil is sized to extend about 120 of the circumference

  of the cylinder. The region of the coil in which are formed

  Connections 5 and 7 are sized to extend about 60 of the circumference. To ensure maximum uniformity of the RF field, the sides of the coils parallel to the cylinder axis must be equal to two diameters of the cylinder (D). However, a coil having a 2D side length is not practical because RF energy is placed in regions of the patient that

  do not offer interest. Therefore, we reduce in practice

  that the length of the coil side to about a diameter.

  Referring now to FIG. 2, a section of the NMR coil of the invention is seen according to FIG.

  line 2-2 of FIG. 1A, through the capacitive element 9.

  In FIG. 2, the similar elements bear the same numerical references as in FIG. 1A. The capacitor 9 is constituted by a conductive region 15 etched on one side of the printed circuit board 13 and by a conductive region 17 disposed facing the region 15 on the other side of the printed circuit board. The region 17 is connected to the conductor 7 by means of a shunt 19 which

  crosses the circuit board. Regions 15 and 17 are separated

  This is achieved by the printed circuit substrate 20 which, in the preferred embodiment, is a low loss dielectric material capable of holding a high voltage. It has been found that double-sided printed circuit boards of the

  trade using as a substrate a synthetic resin

  that consisting of Teflon, are particularly suitable.

  It will be appreciated that PCB circuit boards employing low loss other substrate materials can be used to fabricate coils according to the invention.

than Teflon resin.

  FIG. 3 shows a section of another embodiment of the capacitive element 9 manufactured in accordance with FIG.

  to the invention. This embodiment is practically identical.

  to that of Figure 2, with the notable exception

  that one of the conductive regions (eg

  17) is divided into conductive zones 21-23 by means of

  at intervals 24 and 25. The fact of shunting one of

  24, 25, or both, by a shunt conductor (no

  represented) will increase the area of region 17 by overlapping

  with the region 15. In this way, it is possible to

  capacity of the capacitor 9 to thereby vary the frequency

  resonance of the coil. Note that we can increase

  to reduce or reduce as necessary the number of intervals

who is here two.

  The manner in which relatively simple coil patterns and capacitive elements can be constructed is described above in connection with FIGS. 1A-1B, 2 and 3. In accordance with the invention, more complex coils have been successfully constructed, such as that shown diagrammatically in FIG. 4B. The coil configuration shown in Figure 4B is described and claimed in

  U.S. Patent Application No. 548 745 which is incor-

  hereby reference in the context of the context of the invention.

  tion. Referring now to FIG. 4A, it will be noted that the coil in the saddle with a single turn comprises two

  parallel conductive segments 30 and 32, with a condensing

  34 connected in series with each of them. The extremities

  conductors 30 and 32 are connected to diametrical dots

  strongly opposed on a pair of conductive loops

  parallel 36 and 38 which are spaced along a longitudinal axis

  The coil could be driven by a source such as an RF amplifier generally designated by the reference 42, which is connected between terminals 44 and 46, in parallel with the capacitor in the segment 30. Arrows 48 indicate the current paths that occur to produce a radio frequency field B1

  perpendicular to the plane defined by the wire segments con-

  30 and 32, which will be referred to for convenience as a

  vertical plane. It should be noted that one can determine the direction

  B1 field by the classic rule of the right hand.

  The rule indicates that if the fingers of the right hand are placed around the segment carrying the current so that

  the thumb is directed in the direction of movement of the neck.

  rant, the fingers will be pointed in the direction of the field

magnetic (that is B1).

  In the preferred embodiment, the complete NMR coil structure comprises a set of vertical wire segments 50 uniformly spaced and connected around the upper and lower conductive circular loops 36 and 38, as shown in FIG. 4B. It will be noted that he

  it is not necessary for the loops to be precisely

  eyelids, and they can also be ellipsoidal or

  to have another geometric form generally comprising

  the opening to accommodate the object to be examined. Each vertical conductive segment comprises at least one capacitive element 52. The arrows 48 in FIG. 4B indicate the multiple current paths, each of which is equivalent to that of FIG. 2A. The homogeneity of the B1 field increases

  when increasing the number of vertically

  cal. This is due to the fact that, when increasing the number

  segments, the resulting field is produced by many

  contributions, which reduces the effect of a single driver

  any type. The number of conductors can not be increased without limit because open spaces between adjacent vertical conductors are required to define a path through which the magnetic flux, due to current flow, can escape and thereby produce a homogeneous B1 field. Coils having 4, 8, 11 and 32 vertical conductors have been constructed. It should be noted that the vertical conductive segments need not be

  evenly spaced. In fact, we have made a way of realizing

  the RF coil in which is formed a window for facilitating observation of the patient. What is needed to produce a homogeneous B1 field is a set of vertical conductors distributed around the periphery of the conductive loops, in such a way that the current in the

  Vertical conductors approach a sinusoidal distribution.

  The resulting NMR coil can be considered as a

  resonant cavity formed by a cylinder with open ends

  with an oscillating magnetic field transverse to the axis of the cylinder, when the coil is excited

  by a source of voltage or sinusoidal current.

  We will now describe in relation to the figures

  res 5A and 5B the way in which we have dimensioned phy-

  Electrically and electrically for NMR head studies, the preferred embodiment of a 32 segment coil has been constructed in accordance with the invention. The same construction method is used in the construction of whole body bobbins, which are typically sized to have a larger size.

  diameter. The coil for the head can operate at a frequency of

  21.31 MHz, which is determined by the intensity of the main magnetic field Bo and the NMR isotope being studied. The coil is usually made by engraving

  using conventional techniques) four circuit boards

  double-sided printed, in Teflon resin coated with

  fever. The pads are mounted on a cylindrical support mandrel having an outer diameter of about 26.7 cm.

  of diameter. A different conductive pattern is engraved on each

  that face circuit boards. Each circuit board

  cooked measures approximately 20.3 cm by 30.5 cm. Note however that it is possible to engrave the complete coil (including

  capacitors) on a single printed circuit board.

flexible me.

  We will now consider Figure 5A, on which we see the conductive pattern used to engrave the face of the circuit board (which will be called hereinafter the internal etched surface) which is mounted on the mandrel

  cylindrical and which is the closest to the sample

  lysed by NMR which is positioned in the coil. Tapes

  71 and 73, each having a width of 2.2 cm,

  quarter of the length of the conductive loop elements

  tors. There are eight rectilinear conductive elements, designated

  generally denoted by reference 75, each measuring approximately 25.4 cm in length and 12.7 mm in width, which

  extend between the loop elements 71 and 73. The elements

  Straight sections are separated by inactive areas 12.7 mm wide in which the copper must be removed by etching. The rectilinear elements are offset by about 9.5 mm from the ends of the loop elements 71 and 73. A space 77 is formed between the adjacent rectilinear elements 75 to alternately leave a third (carrying

  reference 81) of the element connected to the elements of

  71 and 73. A second space 79 is formed in each element.

  rectilinear way to separate the remaining two-thirds (

  both the reference 83) of the rectilinear element relative to the corresponding loop element. A pattern is formed in this way in which each rectilinear element is constituted by a third of the element which is connected and by two thirds of the element which are not connected. In adjacent rectilinear conductors, the unconnected element 83 has the same extent as the connected third party and extends beyond

  space 77 to have the same extent as one-third of the

  not connected in the adjacent straight conductor.

  The other reason is shown in FIG. 5B, and

  it is on what will be called hereinafter the

  outer wing. This pattern is symmetrical with the pattern shown in Figure 5A and is sized to have the same dimensions. The pattern of FIG. 5B differs from that of FIG. 5A insofar as each of the parts of FIG.

  rectilinear conductor 81 and 83 characteristically comprises

  four copper zones 85 (although it can be used

  number or a smaller number), trained

  by engraving narrow spaces 87.

  The inner and outer etched surfaces are overlapped so that points S, T, U, V (Fig. 5A) are above respective points 0, P, Q, R (Fig. 5B). In this way, the spaces 77 that

  found on each engraved surface (interior and exterior)

  re) are shunted by continuous parts of the two-thirds unconnected, 83, rectilinear elements 75 on each

  area. Spaces 79 are shunted by continuous portions

  naked 81 of the rectilinear element. The combination of the copper foil segments and the printed circuit dielectric form three capacitors connected in series along the length of each rectilinear conductor. The number of capacitors can be changed by increasing or decreasing the number of spaces. The resultant capacitance of each rectilinear conductor is typically adjusted so that all these capacitances are approximately equal. We

  make the adjustment by electrically connecting to the

  81 or 83 (FIG. 5B) one or more of the copper zones 85, shunting the gaps 87 to change the area of coverage of the inner and outer surfaces. In the preferred embodiment, the interior patterns are engraved

  outside on opposite sides of a circuit board.

baked double-sided print.

  The strips 71 and 73 on each of the graded surfaces

  inner and outer layers are electrically connected together at points O and S, P and T, Q and U, and R and V, for

  forming an assembly constituting a quarter of a reel. A bobbin

  does not complete requires four of these sets placed in overlap and interconnected. One half of the coil is formed by electrically connecting two sets consisting of coil quarters. The points O and Q of a coil quarter are electrically connected to the respective points P and R of the second coil quarter. We mount the two halves of coil

  constructed in this way on a coil mandrel -

  cylindrical, without electrical connections between them. In operation, the two coil halves are coupled by their mutual inductances, when one of the halves is

  excited at the terminals of one or more of the three

  connected in series in a straight conductor, as

  for example at points 89 and 91 shown in Figure 5A.

  The impedance of the point of attack is approximately 50 ohms without any adjustment, with the head of a patient positioned in the

  coil (that is, with a loaded coil).

  In the preferred embodiment, the thickness of the dielectric (Teflon resin) of the double-sided printed circuit board is about 0.15 mm. Each of the three capacitors in each straight conductor is set to have a capacity of about 133 picofarads. It should be noted that it is not important that each capacitor has

  equal value, the only important point being that the

  The resultant cities of each rectilinear conductor are equal. The desired resonance frequency with a field

  Homogeneous RF Magnet is 21.31 MHz.

  We have constructed a simple emulation of a bobbin

  NMR according to the invention by following the patterns

  described in connection with FIGS. 5A and 5B and using

  32 vertical segments. This coil was constructed on a cylindrical mandrel having an outer diameter of 29.2 cm and a length of 41.9 cm. Band elements

  71 and 73 (FIGS. 5A and 5B) are 6.3 mm wide. The con-

  straight ducts 75 are 12.7 mm wide and are spaced

  at intervals of 15.9 mm. There are in this case in each rectilinear conductor ten spaces, similar to

  spaces 77 and 79, so that the value of each

  serial denser is less than that of the method of

  7A and 7B. The resonance frequency of the

coil is 63.86 MHz.

  We can determine the capacity, C, of a condensa-

  formed by two conductive sheets having an area A,

  separated by a Teflon resin substrate having a thickness of

  d, using the formula: C = KA (0.0886 pF / cm) d

  in which: K is the dielectric constant of the material

substrate and

pF denotes picofarads.

  For a double-sided printed circuit board using a 0.10 mm thick Teflon resin substrate to which a 28 g copper foil is attached, the dielectric constant K is about 2.4. Suitable printed circuit boards are commercially available from 3M Company, Electronic Products Division, St. Paul, Minnesota.

USA.

Claims (7)

  An NMR apparatus suitable for exciting and detecting NMR signals in a sample analyzed by NMR, said apparatus comprising a radio frequency (RF) coil having at least one coil coil (1,3) and at least one element capacitive means (9) for resonating the coil at the Larmor frequency of the sample, characterized in that the coil and the capacitive element are formed by a conductive pattern (11, 15) made on a single plate integrated circuit (13).   An NMR apparatus according to claim 1, characterized in that the printed circuit board (13) comprises a low-loss dielectric substrate (20) capable of withstanding a high voltage, with a conductive material (15, 17) disposed on main surfaces   of this substrate, and in that the capacitive element is constituted   killed by regions of overlapping conductive material   (15, 17) formed on opposite sides of the substrate.   3. NMR apparatus according to claim 2,   characterized in that at least one of the conductive regions   these overlaps (15, 17) comprise a set of electrically insulated conductive zones (21, 22, 23), and these   zones may be electrically interconnected to   re varying the area of overlap between the conductive material regions (15, 17) forming the capacitive element   (9), to vary the capacity of the latter.   An NMR apparatus according to claim 1,   characterized in that the coil comprises a conductive portion   trice formed by first and second conductive segments   the first and second segments (75) being formed on opposite sides of the printed circuit board (13), so that the space (77) in a conductive segment is bridged by a continuous region of the other conductive segment, thereby forming at least one capacitive element (52) in series with the conductive coil portion. An NMR apparatus according to claim 4, characterized in that at least one of the conductive segments (75) comprises an electrically insulated conductive area (85), in the region of the non-conductive space (77), said area can be electrically connected to the rest of the   conductive segment (75) to vary the area of   overlapping the first and second segments (75) in order to   adjust the capacitance of the capacitive element (52).   An NMR apparatus according to claim 4, characterized in that the first and second segments (75) are formed on opposite sides of a single platelet of printed circuit board.   An NMR apparatus according to claim 4,   characterized in that the conductive coil portion   takes a set of capacitive elements connected in series,   and this coil can be excited between several combinations   capacitive elements connected in series, so that several combinations of these elements provide different impedances for the coil.