Magnetic resonance apparatus having an improved RF coil

An r.f. transmitter coil is coupled to individually or group-wise independently drivable antenna wire elements, each independently drivable element being driven by its own power amplifier via a sliding contact. An r.f. supply source is coupled to the inputs of the power amplifiers via an input network which provides a phase distribution among the r.f. signals supplied to the amplifiers for the coil to transmit with a circularly polarized r.f. field.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a magnetic resonance apparatus having a magnet 
system for generating a stationary magnetic field, a gradient magnet 
system and an rf coil system. 
2. Prior Art 
Such a magnetic resonance apparatus is known from EP 213665 corresponding 
to U.S. Pat. No. 4,737,718. The apparatus described in said Specification 
comprises a bird cage rf coil which is built up from two ring conductors 
which are connected to a number of antenna wires which are mutually in 
parallel and with a symmetry axis. With a correct choice of impedances and 
reactances a spatial homogeneous rf field in the coil is generated with a 
standing closed cosine wave on the annular conductors. 
U.S. Pat. No. 4,712,067 describes an rf coil which is built up from a 
number, for example two, of saddle-shaped coils by which, with an adapted 
mutual coupling and a correct drive, a spatial homogeneous rf field can be 
generated in a mode which is dependent on the drive and mutual orientation 
of the current conductors. In this case also current conductors which are 
mutually in parallel and with a symmetry axis constitute active antenna 
wires of the coil. 
When using such coils in the described form as transmitter coils for 
generating magnetic resonant signals in an object to be examined, the 
drawback of a low flexibility in the drive occurs in that the rf field 
within the coil is actually impressed, apart from the strength, by the 
geometry of the coil. Interfering counter fields are also often generated 
which can be compensated for only by extra power supply. 
SUMMARY OF THE INVENTION 
It is the object of the invention to avoid these drawbacks and for that 
purpose a magnetic resonance apparatus of the type mentioned in the 
opening paragraph is characterized according to the invention in that the 
rf coil system comprises a coil which has means for an individually 
controllable drive of antenna wires or subgroups of aerial wires of the 
coil. 
Since an rf coil according to the invention comprises independently 
drivable aerial wires a significantly greater extent of freedom in the 
field distribution is obtained and a significantly smaller excitation 
power often suffices. 
In a preferred embodiment the rf coil comprises a number of straight 
conductors which are situated on a cylinder surface and extend in parallel 
with each other and with a cylinder axis and which are connected on each 
side via fixed capacities with a ring conductor and to which an rf source 
can be connected via a power amplifier. In particular, an input network 
having a resistor, a capacity and an inductance, is incorporated between 
the power amplifier and a supply line from the rf source. The resistor 
hereof is connected, for example, to a voltage source for adjusting a 
quiescent current, while a capacitor, for example, is connected to an 
electrically conductive screening cylinder which surrounds the coil and 
which may also be constructed as a heat dissipating element. The power 
amplifiers which are formed, for example, by a MOS-FET transistor may be 
connected thereto via a readily heat conducting contact. 
In a further preferred embodiment an output of the power amplifiers is 
connected to the antenna wires via a preferably externally adjustable 
sliding contact. The impedance of the coil can be adapted to an object to 
be measured by means of the said sliding contacts. 
In behalf of circularly polarized rf fields, a circuit is incorporated in a 
further preferred embodiment for adjusting a phase shift, for example, 
given by the number of antenna wires, between successive antenna wires of, 
for example, a bird cage coil as described in U.S. Pat. No. 4,737,718 of a 
transversal electromagnetic (TEM) coil as described in U.S. Pat. No. 
4,712,067.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A magnetic resonance apparatus as shown in FIG. 1 comprises a magnet system 
2 for generating a homogeneous stationary magnetic field H, a magnet 
system 4 for generating magnetic gradient fields, supply sources 6 and 8, 
for the magnet system 2 and the magnet system 4, respectively. A radio 
frequency magnet coil 10 serves to generate a radio frequency magnetic 
alternating field and for that purpose is connected to a radio frequency 
source comprising the output of r.f. amplifier 12. For the detection of 
nuclear spin resonance signals generated by the radio frequency 
transmitter field in an object to be examined the rf coil 10 may also be 
used which for that purpose is connected to a signal amplifier 14. Another 
coil, for example, a surface coil, may also be used for detection. The 
signal amplifier 14 is connected to a phase-sensitive rectifier 16 which 
is connected to a central control device 18. The central control device 18 
further controls a modulator 20 which feeds r.f. amplifier 12, the supply 
source 8 for the gradient coils and a monitor 22 for display. A 
high-frequency oscillator 24 controls both the modulator 20 and the 
phase-sensitive rectifier 16 processing the measured signals. A cooling 
device 26 with cooling ducts 27 serves for the cooling, if any, of the 
magnet coils 2 for the main field. Such a cooling device can serve water 
cooling for resistance coils or liquid nitrogen and/or helium cooling for 
high field strength, superconductive magnet coils. The transmitter coil 10 
placed within the magnet systems 2 and 4 encloses a measuring space 28 
which in an apparatus for medical diagnostic measurements is wide enough 
to comprise a patient lying on a patient table 29. So a homogeneous 
magnetic field H, cross-sections of the object-selecting gradient fields 
and a spatial homogeneous radio frequency alternating field can be 
produced in the measuring space 28. 
An rf coil according to the invention as shown in FIG. 2, in this case in 
the form of a transversal electromagnetic coil as described in U.S. Pat. 
No. 4,712,067, comprises current conductors 30 which are connected to 
electric ring conductors 34 via capacitors 32. The ring conductors 34 in 
this case have the form of flanges forming part of a cylindrical housing 
35 having a cylindrical surface 36 for screening the rf field to be 
generated by the coil from interference fields. In the embodiment shown 
the housing 35 also forms a ground potential electrode for the antenna 
wires and may also serve as a heat dissipatingg element. Antenna wire 
parts 38 are connected respectively, via power amplifiers 40 each of which 
comprises, for example, a MOS-FET transistor, to an amplitude and 
phase-controlling network 42 via preferably coaxial connection cables 44. 
Uncoupling capacitor 46 for uncoupling purposes are incorporated in the 
antenna wire parts 30'. In this manner each of the antenna wires can be 
energized individually by means of the control network 42; however, 
several antenna wires may also receive an equal supply. The supply for 
various antenna wires may also differ in phase only. The control network 
is supplied via a connection cable 48 from an rf transmitter 49 
corresponding to the rf amplifier 12 output in FIG. 1. 
FIG. 3 shows in greater detail a part of a coil as shown in FIG. 2. An 
antenna wire 30 with a pair of spaced capacitors 32, a pair of spaced ring 
conductors and a part of cylinder housing surface 36 of the coil are 
shown. A power MOS-FET 52 is connected to a part 38 of the antenna wire 30 
via a sliding contact 50. Antenna wire part 38 is coupled via a capacitor 
32 to the antennal wire 30 part 30 and, via an uncoupling capacitor 46 to 
one of the conductors 34. The wire part 30" is coupled to the other ring 
conductor 34 via a second capacitor 32. 
A supply source 54 from which a fixed voltage of, for example, 50 V can be 
applied to the antenna wire is incorporated for a fixed supply for the 
antenna wire. The MOS-FET transistor 52 is connected, via an input network 
56 which comprises a coil 58 and a capacitor 60, to the amplitude and 
phase correcting network 42 via a supply line 44 which may, for example, 
comprise a coaxial cable. A quiescent current can be applied to the 
transistor 52 via a resistor 62. 
FIG. 4 shows an example of an amplitude and phase correcting network 42. 
This is connected to amplifiers 40 as shown in FIG. 2 via the lines 44 and 
may be fed from the rf transmitter 49 via the coaxial cable 48. 
The network 42 comprises N sections 70 each having an L-C circuit 72 built 
up from an inductance 74 and two capacities 76. The number of sections 
equals the number of antenna wires or groups of antenna wires to be 
controlled individually and in practical cases is, for example, 6 to 12. 
For generating a circularly polarized rf field the amplitude to be applied 
to each of the coil wires is equal but for each of the wires mutually 
shifted in phase so that the amplitude wave hence rotates with the desired 
frequency over the antenna wires of the coil.