NMR radio frequency coil with improved axial field homogeneity

An RF coil assembly for an NMR instrument includes a cylindrical shield which encloses a set of equally spaced linear conductors that surround a central axis. A set of shielded conductors connect to the ends of the linear conductors to form loops. The effective lengths of the loops and the RF signal source which drives the loops are selected to improve the homogeneity of the RF field produced along the central axis.

BACKGROUND OF THE INVENTION 
This invention relates to nuclear magnetic resonance (NMR) apparatus. More 
specifically, this invention relates to radio frequency (RF) coils useful 
with such apparatus for transmitting and/or receiving RF signals. 
In the past, the NMR phenomenon has been utilized by structural chemists to 
study, in vitro, the molecular structure of organic molecules. Typically, 
NMR spectrometers utilized for this purpose were designed to accommodate 
relatively small samples of the substance to be studied. More recently, 
however, NMR has been developed into an imaging modality utilized to 
obtain images of anatomical features of live human subjects, for example. 
Such images depicting parameters associated with nuclear spins (typically 
hydrogen protons associated with water in tissue) may be of medical 
diagnostic value in determining the state of health of tissue in the 
region examined. NMR techniques have also been extended to in vivo 
spectroscopy of such elements as phosphorus and carbon, for example, 
providing researchers with the tools, for the first time, to study 
chemical processes in a living organism. The use of NMR to produce images 
and spectroscopic studies of the human body has necessitated the use of 
specifically designed system components, such as the magnet, gradient and 
RF coils. 
By way of background, the nuclear magnetic resonance phenomenon occurs in 
atomic nuclei having an odd number of protons or neutrons. Due to the spin 
of the protons and neutrons, each such nucleus exhibits a magnetic moment, 
such that, when a sample composed of such nuclei is placed in a static, 
homogeneous magnetic field, B.sub.o, a greater number of nuclear magnetic 
moments align with the field to produce a net macroscopic magnetization M 
in the direction of the field. Under the influence of the magnetic field 
B.sub.o, the aligned magnetic moments precess about the axis of the field 
at a frequency which is dependent on the strength of the applied magnetic 
field and on the characteristics of the nuclei The angular precession 
frequency, .omega., also referred to as the Larmor frequency, is given by 
the Larmor equation .omega.=.gamma.B in which .gamma. is the gyromagnetic 
ratio (which is constant for each NMR isotope) and wherein B is the 
magnetic field (B.sub.o plus other fields) acting upon the nuclear spins. 
It will be thus apparent that the resonant frequency is dependent on the 
strength of the magnetic field in which the sample is positioned. 
The orientation of magnetization M, normally directed along the magnetic 
field B.sub.o, may be perturbed by the application of magnetic fields 
oscillation at or near the Larmor frequency. Typically, such magnetic 
fields designated B.sub.1 are applied orthogonal to the direction of the 
B.sub.o field by means of RF pulses through a coil connected to a 
radio-frequency-transmitting apparatus. Under the influence of RF 
excitation, magnetization M rotates about the direction of the B.sub.1 
field. In NMR studies, it is typically desired to apply RF pulses of 
sufficient magnitude and duration to rotate magnetization M into a plane 
perpendicular to the direction of the B.sub.o field. This plane is 
commonly referred to as the transverse plane. Upon cessation of the RF 
excitation, the nuclear moments rotated into the transverse plane precess 
around the direction of the static field. The vector sum of the spins 
forms a precessing bulk magnetization which can be sensed by an RF coil. 
The signals sensed by the RF coil, termed NMR signals, are characteristic 
of the magnetic field and of the particular chemical environment in which 
the nuclei are situated. In NMR imaging applications, the NMR signals are 
observed in the presence of magnetic-field gradients which are utilized to 
encode spatial information into the signals. This information is later 
used to reconstruct images of the object studied in a manner well-known to 
those skilled in the art. 
In performing whole-body NMR studies, it has been found advantageous to 
increase the strength of the homogeneous magnetic field B.sub.o. This is 
desirable in the case of proton imaging to improve the signal-to-noise 
ratio of the NMR signals. In the case of spectroscopy, however, this is a 
necessity, since some of the chemical species studied (e.g., phosphorus 
and carbon) are relatively scarce in the body, so that a high magnetic 
field is necessary in order to detect usable signals. As is evident from 
the Larmor equation, the increase in magnetic field B is accompanied by a 
corresponding increase in frequency and, hence, in the required resonant 
frequency of the transmitter and receiver coils. This complicates the 
design of RF coils which are large enough to accommodate the human body. 
One source of difficulty is that the RF field generated by the coil must 
be homogeneous over the body region to be studied to produce more uniform 
measurements and images. The production of uniform RF magnetic fields over 
large volumes becomes increasingly difficult at high frequencies owing to 
unwanted effects of stray capacitances between different parts of RF coils 
and between RF coils and surrounding objects or the NMR sample, itself, 
which limit the highest frequency at which the coil can be made to 
resonate. 
Rf coils which produce substantially homogeneous fields at high frequencies 
throughout large volumes have been designed. Such coils are disclosed, for 
example, in U.S. Pat. No. 4,680,548 for use in whole body imaging of 
hydrogen nuclei at 1.5 Tesla, and U.S. Pat. No. 4,799,016 for use as a 
local coil in imaging both hydrogen and phosphorous nuclei at 1.5 Tesla. 
While such coils are capable of producing a homogeneous RF field within 
their central region of interest when no subject is present, they do not 
produce a homogeneous RF field when a typical subject is located in the 
region of interest. 
In whole body RF coils such as that described in the above-cited U.S. Pat. 
No. 4,680,548, a set of linear conductors are disposed along and around a 
central z-axis and are joined at their opposing ends by a pair of circular 
conductors. This "birdcage" structure produces an RF field which is 
transverse to the z-axis and is relatively uniform, or homogeneous, in 
amplitude throughout the cylindrical region defined by its conductors. In 
copending U.S. application Ser. No. 07/467,475 which was filed on even 
date with this application and is entitled "NMR Radio Frequency Coil With 
Dielectric Loading For Improved Field Homogeneity," a technique for 
improving the homogeneity of the RF field strength in such a coil 
structure is described. To improve the homogeneity in the radial direction 
in a region of interest at the center of the coil, this technique employs 
a dielectric material having a high relative permittivity 
(.epsilon..sub.r) to reduce the propagation constant along the direction 
of the z-axis (k.sub.z). While this technique lowers the radial 
propagation constant (k.sub.p) of the coil and smooths out variations in 
RF field strength in the radial direction, this technique exacerbates 
variations in the RF field strength along the z-axis. This is illustrated 
in FIG. 3 where FIG. 3A shows the RF field strength variations along the 
z-axis of the coil with no dielectric material used (.epsilon..sub.r =1), 
and FIG. 3B shows the Rf field strength when a dielectric material having 
a relative permittivity .epsilon..sub.r =20 is used. It is readily 
apparent that the central region where substantially uniform field 
strength is produced is shortened considerably along the z-axis when the 
dielectric material is used. When axial images are produced, this 
shortened region of uniform RF field strength along the z-axis is not of 
great concern. However, when sagittal or coronal views are produced, the 
variations in RF field strength along the z-axis will appear as shading in 
the image. 
SUMMARY OF THE INVENTION 
The present invention relates to an RF coil assembly for an NMR instrument 
in which variation in the RF field strength along the z-axis are reduced 
by preventing standing waves from occurring. More particularly, the coil 
assembly includes a set of linear conductors, disposed along and around a 
central axis to define a cylindrical region of interest, a cylindrical 
shield disposed around the set of linear conductors and substantially 
concentric about the central axis, shielded return conductors connected to 
the ends of the linear conductors to form a closed loop which is an 
integer number of wavelengths in length, and drive means having two 
phase-displaced RF signal sources which are connected to said closed loop 
to produce an RF field in the region of interest which travels along the 
central axis. 
A general object of the invention is to improve the RF field homogeneity 
along the central axis. By providing shielded return paths of the proper 
length and driving the coil assembly with a polyphase RF source, the 
standing wave which is produced in prior coil assemblies is destroyed. As 
a result, the wave is caused to move along the central axis and any 
variations in its field strength in that direction are averaged out over 
time. The nuclei which are excited by the RF field thus "see" a uniform RF 
field intensity over the relatively long data acquisition period typical 
in an MRI imaging system. Shading in coronal and sagittal views of the 
patient is thus substantially reduced. 
A more specific object of the invention is to offset the increased 
inhomogeneities produced in the RF field strength along the central axis 
caused by the addition of a high permittivity dielectric material between 
the linear conductors and the shield. While such dielectric materials 
improve RF field homogeneity in the radial direction they produce standing 
waves along the central axis which vary considerably in amplitude. The 
present invention causes such waves to continuously move, or travel, in 
the direction of the central axis to average out the variations at any 
given point in the region of interest. 
The foregoing and other objects and advantages of the invention will appear 
from the following description. In the description, reference is made to 
the accompanying drawings which form a part hereof, and in which there is 
shown by way of illustration a preferred embodiment of the invention. Such 
embodiment does not necessarily represent the full scope of the invention, 
however, and reference is made therefore to the claims herein for 
interpreting the scope of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring particularly to FIGS. 1A-1C, the RF coil assembly includes a 
circular cylindrical shield 10 which is disposed concentrically about a 
central axis 11 (z-axis). Located inside the shield 10 is a set of 
longitudinal conductors 12 which are disposed concentrically around the 
central axis 11 and which extend along the length of the shield 10. There 
are sixteen linear conductors 12 which take the form of copper strips that 
are equally spaced from each other around the circumference of a circular 
cylindrical fiberglass support form 25. The shield 10 is also formed as a 
sheet of copper that is supported on the inner surface of a circular 
cylindrical fiberglass outer form 26. The shield 10 and its support form 
26 extend a short distance beyond the ends of the linear conductors 12 and 
parallel slots (not shown) are formed in the shield 10 as is well-known in 
the art to reduce audio frequency eddy currents therein due to imaging 
gradients. A dielectric material 13 is disposed in the annular space 
between the shield 10 and the set of linear conductors 12 and it has a 
relative permittivity in the range 20.ltoreq..epsilon..sub.r 40. 
Preferably, this dielectric material 13 is a water/isopropanol mixture 
contained in a thin-walled polyethylene sack (not shown), although solid 
materials may also be used. 
Each linear conductor 12 is provided with a return path to form a closed 
loop. These closed loops are formed by a set of shielded conductors 14 
that are mounted to the inner surface of the shield 10 and extend its 
entire length. In the preferred embodiment a shielded conductor 14 is 
associated with each of the sixteen linear conductors 12 and their ends 
are electrically connected together to form sixteen separate loops. 
Referring particularly to FIG. 2, the coil assembly is illustrated in part 
schematically. One of the sixteen closed loops is illustrated to 
demonstrate how the RF signal source is coupled to the coil assembly. 
First, it is a fundamental teaching of the present invention that each 
loop formed by a linear conductor 12 and shielded conductor 14 has a total 
effective length which is an integer number of wavelengths long. 
Accordingly, a lumped element, transmission line circuit 27 is connected 
in each loop, and its variable inductors can be adjusted to make the 
effective total length of each loop an integral number of wavelengths 
long. The impedance of the circuit 27 is matched to the impedance of the 
shielded conductor 14. 
The conductor 12 and shielded conductor 14 are transmission lines which 
have different characteristic impedances. To minimize reflections at the 
interconnections of these transmissions lines, a lumped element, impedance 
matching circuit 28 is connected at each end of the linear conductor 12. 
These circuits 28 are lumped element, quarter wavelength impedance 
matching transformers whose characteristic impedance is the geometric mean 
of the characteristic impedances of the linear conductor 12 and the 
shielded conductor 14. 
Referring still to FIG. 2, to produce a wave which travels along the 
direction of the central axis 11, the loops are driven by two RF signal 
sources 17 and 18. The signal sources 17 and 18 are physically inserted 
into the loop at positions which are spaced one quarter wavelength apart 
(.lambda./4). In the preferred embodiment this spacing is accomplished 
using a lumped element quarter wavelength transmission line circuit 29 
having its characteristic impedance matched to that of the shielded 
conducter 14. In addition, the two signal sources have a phase difference 
of 90.degree. (360.degree. /4) with the result that the RF wave produced 
by each loop moves, or travels, along the loop. Within the region of 
interest inside the shield 10, the RF wave travels along the direction of 
the central axis at a uniform rate, which over a period of time, results 
in an average RF field amplitude which is homogeneous over a substantial 
portion of the region of interest. 
In the preferred embodiment a pair of RF signal sources are connected to 
each of the sixteen loops to not only produce the axially directed 
travelling wave, but to also rotate the RF field about the central axis 11 
to reduce nonhomogeneities due to eddy currents in the subject under 
study. To produce this rotation, or circular polarization, each pair of RF 
signal sources is phase shifted by an amount determined by the 
circumferential position of their associated linear conductors 12. That 
is, the RF signal sources are represented as 
EQU V.sub.0 cos(.omega.t+.theta.) 
EQU V.sub.0 sin (.omega.t+.theta.) 
where .theta. is the circumferential location in degrees of the linear 
conductor 12 associated with the RF signal sources. In the preferred 
embodiment there are sixteen equally spaced linear conductors 12 and the 
phase of successive RF signal sources around the circumference are shifted 
by .DELTA..theta.=360/16=22.5.degree.. A phase splitter circuit for 
producing the necessary RF signals is illustrated in FIG. 4 for the 
sixteen V.sub.0 cos(.omega.t+.theta.) signals. An identical circuit is 
provided for the sixteen V.sub.0 sin(.omega.t+.theta.) signals, or in the 
alternative, a quadrature hybrid splitter can be used at each output of 
the phase splitter in FIG. 4 to provide both the cosine and sine signals. 
Referring again to FIGS. 1A-1C, in the preferred embodiment of the 
invention the shielded conductors 14 are disposed within shields 30 that 
are formed on the inner surface of the cylindrical shield 10. The lumped 
circuits 27 and 28 are housed in shielded containers 31 located at one end 
of the conductors 12 and 14, and a corresponding set of shielded 
containers 32 house the lumped circuits 28 and 29 at the other end of the 
conductors 12 and 14. 
There are a number of possible variations in the RF coil assembly. Instead 
of forming a separate loop for each of the linear conductors 12, two or 
more of the linear conductors 12 can be connected in series by the 
shielded conductors to form fewer closed loops. Indeed, all of the linear 
conductors 12 can be connected in series to form a single closed loop. 
Regardless of the configuration selected, the total effective length of 
any loop must be an integral number of wavelengths long in order to create 
a resonant circuit. In addition, the return conductors of the loops must 
be shielded from the region of interest. 
While the two RF signal sources connected to each loop are 90.degree. out 
of phase and are physically inserted in the loop at a spacing of one 
quarter wavelength, other phase differences and spacings are possible. It 
is only necessary that the phase difference and the spacing be chosen so 
that the signals produced by each phase of the RF signal reinforce each 
other for a travelling wave in the preferred direction and cancel the 
travelling wave in the opposite direction. In general, the two signal 
sources 17 and 18 can have a phase difference of either plus or minus 
90.degree. and can be connected to the loop at a spacing of any odd number 
of quarter wavelengths apart.