Note: Descriptions are shown in the official language in which they were submitted.
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9-874 Description
Nuclear Magnetic Resonance
Radio Frequency Antenna
Technical Field
The present invention relates to nuclear m2gne.ic
resonance imaging and more particularly to an improve
resonator for applying radio frequency pulses and receive
in low level RF signals over a region of interest.
Background Art
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In medical applications, nuclear magnetic Rosen
No can indicate variations in the distribution of
atomic substances in slices or volumes of interest with-
in a patient. Such variations can be displayed in a
wry similar to the distributions provided by a kiwi.-
iced tomography system. In a nuclear magnetic resonance
examination, magnetic and of fields rather than x-radi2;io.-
scan the body. Resonances caused by these fields are
detected as induced signals in one or more detector
coil systems. The outputs from these coils are then
stored and analyzed so that NOR distributions can be
displayed.
Techniques for producing these images are well
Nina in the art and disclosed in various printed put
cations end U.S. patents. Several proposals for ape-
tusk to utilize these procedures are embodied, for exar,?le,in U.S. Patent Nos. 4,454,474 to Young, 4,384,255 to
Yours et at, and 4,379,262 to Young.
The techniques disclosed in the above mentioner
prior art patents involve selection of a planer slice
of interest in the body and application of a strong
magnetic field gradient in a direction perpendicul2 I
the slice. This field is perturbed in a perpendicular
direction in the plane of the slice. The direction ox
the perturbation is continuously varied by a procedure
documented in the literature.
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The effect of this perturbation is to introduce a
dispersion in nuclear resonance frequencies which rev -.
to their original unperturbed state in ways ch2r2cteris- c
of the structure within the slice of interest. eye..
of this procedure for different directions can size
many signals for each slice of interest which are then
used to construct cross-sectional images descriptive o'
the internal structure of the patient slice.
The radio frequency perturbation excites the nuclei
by realigning the macroscopic magnetization or magnetic
moment within the cross-section of interest. This ratio
frequency energization is performed at the Armor frozen
cry. This frequency is related to a constant descriptive
of the nuclei making up the region of interest end the
magnetic field gradient imposed during perturbation.
Experience in NOR imaging indicates that the scan-
nine times can be reduced and special resolution of OR
images can be increased by increasing this field to
higher levels. Since the Armor frequency of a given
nuclei is directly proportional to the field strength,
this increase in field strength must be accompanied by
higher frequencies for I energization. In the prior
art this energization us accomplished with suitably
designed energization coils which generate pert~Jrbztion
fields and in some instances are also used for detecting
signals caused by resonances set up within the region
of interest.
Transmission and reception of radio frequency skis-
nets for NOR imaging requires a resonant radi2tins sac
lure, often called an of coil, meeting sever cry Thea resonant point of the structure must be high eno~c
to alloy proper tuning at the frequency OX interest on
the structure must have sufficiently high "Q" to Provide
good signal to noise performance in the receive mode.
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Generally, in small volume nuclear magnetic resow-
ante systems-an unbalanced feed system and coil configure
anion is used. A simple reactive element is used as an
impedance matching component and a second reactive eye-
mint is used in parallel with the coil structure Titan the coil to an appropriate frequency.
For large volume nuclear magnetic resonance applique-
lions, however, such as a head imaging system a balanced
coil system is preferred. This is preferable since
under sample loading the coil system will be less in-
flounced than an asymmetrical system.
As the frequency of operation is raised, however,
the effectiveness of a symmetrical matching system is
limited by a variety of factors. The reactive components
Buick unmanageably small and are also subjected to
extremely high peak voltages. The stray capacitance of
the RF coil network eventually makes it impossible to
match the network to a useful impudence. In addition
to problems in achieving proper energization frequencies,
use of larger RF coils creates problems in achieving a
uniform magnetic field over the region to be perturbed.
various prior art proposals to provide new and
improved OF energization and detection coils are discus-
sod in the literature. A publication entitled "Slotted
Tube Resonator: A new NOR probe head at high observing
frequencies" by Schneider and Dullenkopf discusses a
resonator for use at high frequencies. This work was
the first of a number of similar prior art publications
discussing NO resonator structures. Much of this work,
however, has been conducted with extremely small dime-
signal structures which do not encounter the difficulties
encountered when imaging a cross-section of a head.
The task of converting a resonator coil for use in small
structure anuses into a device suitable for NOR medical
imaging is not a straight forward extension of this
prior work.
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Disclosure of Invention
The invention is particularly suited or No iris
of a human head. A resonator having a length and Doria
of approximately 30 centimeters is constructed Jo provide
a homogeneous magnetic field in the region of the head
that does not unreasonably degrade with sample loading
and is easily tuned over a wide frequency range.
An antenna or coil arrangement constructed in accord-
ante with the invention both transmits and receives
high frequency energy in the range of 30 to 95 muggier.
The disclosed resonator includes a cylindrical base of
a diameter suitable for enclosing the human head and a
metallic foil coupled to the base and forming the antenna
structure with a resonant frequency of about 30 to I
megahertz. The self resonant frequency of the structure
is well in excess of 100 megahertz. This resonant struck
lure includes a pair of diametrically opposed ark
electrical conductors with each conductor subtending an
arc of between 75 and 85 degrees. Short circuiting
conductors interconnect these conductors at one end and
wing strips extend circumferential from each of the
conc~c.ors at the other end of the resonator. Enrages-
lion signals are applied to the resonator through conduct
live 'eel skis which interconnect the wing strips.
The arrangement between feed strips, wing strips
and conductor strips causes uniform magnetic field ~lthi~
the region of interest, i.e. the head. In a preferrer
embodiment, the conductor strips are circumferentic i;
spaced parallel conductor strips with each strip inquiries-
in in cross-sectional area from its center towards
each end. This configuration of the conductor Sue us
enhance_ tune uniformity o' the magnetic 'to`, w-
affecting the tunability of the structure.
An interface circuit is preferably coupled between
the disclosed resonator and a standard 50 ohm input
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cable. The resonator also acts as a pick up coil so
that resonances within a patient slice of-lnterest incus
electrical signals which are detected, amplified, end
utilized in constructing an NOR image- To achieve Roy-
impedance matching and resonance the disclosed resonators coupled to the 50 ohm input cable through three adjust.-
able capacitors and a half wavelength Boolean coaxial
cable.
The resonator is a quarter wavelength antenna which
can ye easily tuned and matched in a transmit mode of
operation and effectively coupled to a preamplifier
for generating output signals for use in NO imaging.
From the above it should be appreciated that one object
of the invention is a antenna structure suitable for of
signal generation and reception at high frequencies
with a geometry large enough for head imaging. Other
objects, advantages and features of the invention will
Buick better understood when a detailed description of
a preferred embodiment of the invention is described in
conjunction with the accompanying drawings.
Brief Description of the Drawings
inure 1 is a perspective view of an NOR imaging
station.
Figure 2 is a perspective view of a resonator Poe
for providing of signals in the vicinity of a patient's
head.
Figure 3 is a top plan view of a foil con inured
to form the Figure 2 resonator.
Figure 4 is a schematic circuit diagram OX the
resonator showing use of three adjustable capacitor
for tuning and impedance matching.
Figure 5 is a schematic showing an entire I
coil system for both transmitting and receiving resonance
signals from within a region of interest.
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Figure 6 shows a filter for reducing transmitter
noise. I/
Figures 7-9 are graphical representations showing
magnetic field uniformity in the region encircled by
the antenna.
Best Modifier Carrying out the Invention
Turning now to the drawings and in particular issuer
1, an imaging station for an NOR scanner 10 is disclosed.
The scanner 10 includes a large encircling magnet 12
for generating magnetic fields of between 1.5 and 2
Tussle within a patient aperture 14. Shown positioned
in proximity to the magnet 12 is a patient couch 16
having a headrest 18. The patient is positioned on the
couch in a prone position and then moved into the patient
aperture 14 for NOR scanning.
During a head scan a probe coil or resonator 20 is
moved on rollers 22 so that the patient's head is post-
toned within the coil 20. In accordance with techniques
well known in the nuclear magnetic resonance imaging
art, the magnet 14 is energized to produce a strong
magnetic field having a gradient to selectively choose
2 slice or region of patient interest. With the probe
coil 20 encircling the patient's head, the coil is ever-
gibed with a high frequency (between 30 and 95 megahertz)
signal which sets up-a time varying magnetic field in
the region of interest. Various techniques are known
within the art for pulsing the probe coil in ways to
produce meaningful resonance information which can be
utilized in NOR imaging. The particular configuration
of the disclosed coil 20 allows high frequency enters zap
lion necessary to cause resonance of the spin soys e, a
the high r,agne,ic fields generated by the masse' 12.
At such high frequencies, the disclosed prove 20 produces
uniform magnetic fields which do not exhibit undue Q
degradation with sample loading.
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Turning now to Figures 2-4, details of the construe-
lion of the probe coil are discussed. A cylindrical
base 30 formed from an acrylic material forms a surface
to which a metallic foil can be affixed. The base 30
has physical dimensions such that a patients head can
be inserted within the base and the probe coil 20 ever-
gibed in conjunction with generation of the high strength
magnetic field. Two copper foil resonator sections 32
having a thickness of .0635 millimeter are affixed to
an outer surface of the base 30 in a configuration shown
in Figure 2. The foils are self adhesive with a backing
layer which is removed prior to application to the base
30. One of the foil sections 32 is shown in plan view
prior to mounting to the substrate 30 in Figure 3. The
physical dimensions of this foil are shown in that figure.
The thickness of the foil is chosen to be approxi-
mutely seven spin depths at the resonant frequency.
Use of this thickness causes the resonator 20 to be
essentially transparent to the high strength field grad-
tents generated by the magnet 12. This minimizes thegener~tion of eddy currents within the foil by this
high strength magnetic field gradient which would be
undesirable since the induced eddy currents would produce
their awn magnetic field in addition to the desired
homogeneous of field.
Etch foil segment 32 includes a shorting strip 34
and a wins strip 36. These two strips 34, 36 are inter-
connected by conductor strips 38 which are parallel to
each other and nonuniform in width along their length.
Preferably these conductor strips 38 are narrow in ye
middle and widen as they approach the shorting sir id 34
and wing strip 36. As seen most clearly in Fissure 2,
when affixed to the outside surface of the substrate
30, the two foil segments 32 contact each other at the
ends of the shorting strips 34 and define a 1 cm gay
between the ends of the wing strips 36.
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An end portion aye of each wing 36 disconnected by
a feed bar 40 having a midpoint 42 connected to an inter-
face circuit 110 (Figure 5) by copper strips (not shown).
The feed bars both energize the probe and transmit resow
5 nuance signals generated from within the patient region of interest. The feed bars 40 form a semicircle 2 cm
wide each of the same thickness as the foil and are
mounted to an acrylic substrate.
The resonator 20 is energized with a high frequency
output from a transmitter 112. A preferred transmitter
is available from Amplifier Research under Model No. 2000
Lo and produces an alternating current voltage a few
hundred volts in magnitude. In order to interface the
resonator 20 to a standard 50 ohm unbalanced transmission
line 114 a half wavelength Boolean 115 (Figures 4 and I
is utilized. The Boolean 115 is constructed from 50 ohm
coaxial cable, with a velocity factor of 0.80 or 0.66.
The total length of the Boolean is 1.875 meter using a
velocity factor of 0.80 at 64 megahertz. Since the
signal traveling in this cable is delayed by one half
wavelength the phase of the voltage at one end of the
cable is 180 shifted from the other end. Thus, the
voltages at each end of the cable are equal in amplitude
and opposite in phase. The current at an input node
116 divides equally between the load and the Boolean phzs-
in line. Thus, the resonator matching network sees a
voltage double that of the input voltage, and a query,
equal to half the applied current. This causes the
impedance at the output of the matching network to be
four times the input line impedance.
A m2tchins network 120 having three adjustable
capacitors 122, 12~, 126 is used to tune the resonator
20 and impedance match the high impedance resonator 20
with the 200 ohm balanced input. Model Number COCA 125
vacuum capacitors from ITT Jennings of San Jose, CalifOrnL2
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are preferred. Representative values of these capacitors
are 60 pick farads for the parallel capacitor 122 and
12 picofarads for the two series capacitors 124, 126.
These values are representative and are tuned to optimize
performance of the resonator 20.
To utilize the resonator 20 as both a transmitter
and receiver, a multiplex circuit 130 (Figure 5) couples
the resonator and balancing network to both the transmitter
112 and a preamplifier 132. The multiplex network
includes a plurality of diodes 134 and two quarter wave-
length cables 136, 138.
In the transmit mode the large magnitude signals
from the transmitter 112 forward bias the diodes 134.
The charter wavelength cable 136 consumes no net power
since the cable inverts the terminating impedance and
no signal from the transmitter reaches the preamplifier
132.
In a receive mode the goal is to couple induced
signals in the resonator 20 to the preamplifier 132.
these signals see a half wavelength cable since the two
qua.terw~ve cables 136, 138 act as a single half wave
cable.
A noise filter circuit 140 figure 6) couples the
transmitter 112 to the multiplexer circuit 130 and in-
eludes a plurality of diodes 142 and two quarter wave-
length cables 144, 146 which function in a way similar
to the diodes 13~ and cables 136, 138 of the multiplex
circuit 130. In the transmit mode the diodes are for-
ward biased, shorting the cable 146 to a quarter wave-
length cable 144. No net power is consumed by the cable
144. In 2 receive mode the two cables 14q, 146 act as
a single half wavelength cable. Any noise from the
transmitter is blocked since the half wavelength cable
presents a virtual short.
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The preamplifier 132 is coupled to other apparatus
known in the nuclear magnetic resonating art for convert-
in signals from the resonator into signals suitable
for imaging. The resonator 20 has an unloaded "On of
about 300 and a loaded "Q" of approximately 50. A very
good match to the 50 ohm transmission line is achieved
with reflected power levels under two percent.
Field uniformity is presented in Figures 7-9 where
a plot of variations and magnetic field strength with
position in the X, Y, and Z directions as defined in
Figure 1 are disclosed. The origin of this co-ordinate
axis is a point centered within the resonator 20 halfway
between the shorting and wing conductors. The data
presented in Figures 7-9 was generated with a probe
coil energization of 64.5 megahertz, an unloaded "Q" of
260 and a loaded "Q" of 55. The X and Y uniformity in
field is excellent and by properly positioning the resow-
atop 20 along the Z axis uniformity within a region of
interest as defined by the field gradient of the magnet
12 con be achieved.
The disclosed design fulfills all the requirements
for hush quote head imaging at field strengths of 1.5
Tussle. The operating parameters of the resonator 20,
however, should not be viewed as limiting the invention
and field strengths of 2.0 Tussle and resonance frequent
ales of 85 megahertz are possible. It is the intent
that the invention cover 211 modifications and/or alter-
anions following within the spirit or scope of the eye-
dyed claims.
