Motor driven tuning and matching of RF coils in an NMR probe

Details Motor driven tuning and matching of RF coils in an NMR probe Motor driven tuning and matching of RF coils in an NMR probe

1999-09-16

2001-11-27

Patidar, Jay (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S320000

Reexamination Certificate

active

06323647

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to magnetic resonance. More particularly, the invention relates to tuning and matching radio frequency (RF) coils in a nuclear magnetic resonance (NMR) probe.
BACKGROUND OF THE INVENTION
Magnetic resonance may be used to analyze medical and/or chemical samples. Specifically, the diverse chemical constituents and/or the spatial distributions of such constituents of the sample may be analyzed through the application of the magnetic resonance phenomena. In general, the physical context of the invention is a NMR probe for nuclear magnetic resonance or magnetic resonance imaging. An idealized illustration is shown in FIG.
1
.
A magnet
10
having bore
11
provides a main magnetic field. In order to control the magnetic field with precision in time and direction, there are provided magnetic field gradient coils (not shown). The gradient coils are driven by gradient power supplies
16
,
18
and
20
, respectively. Additionally, other shimming coils (not shown) and power supplies (not shown) may be required for compensating residual undesired spatial inhomogenities in the basic magnetic field. An object or fluid for analysis (hereinafter “sample”) is placed within the magnetic field in bore
11
; typically, the sample is placed in a sample space of an NMR probe (not shown) and the NMR probe is placed within the bore
11
. The sample is subject to irradiation by RF power, such that the RF magnetic field is aligned in a desired orthogonal relationship with the magnetic field in the interior of bore
11
. This is accomplished through a transmitter coil
12
in the interior of bore
11
. Resonant signals are induced in a receiver coil, proximate the sample within bore
11
. The transmitter and receiver coils may be the identical structure, or separate structures.
As shown in
FIG. 1
, RF power is provided from transmitter
24
, and is amplified by an amplifier
31
and then directed via multiplexer
27
to the RF transmitter coil
12
located within the bore
11
. The transmitter
24
may be modulated in amplitude or frequency or phase or combinations thereof, either upon generation or by a modulator
26
. Transmitter and receiver coils are usually not concurrently used as such. The identical coil may be employed for both functions if so desired. Thus, a multiplexer
27
is provided to isolate the receiver from the transmitter. In the case of separate transmitter and receiver coils, element
27
, while not precisely a multiplexer, will perform a similar isolation function to control receiver operation.
The modulator
26
is controlled by pulse programmer
29
to provide RF pulses of desired amplitude, duration and phase relative to the RF carrier at preselected time intervals. The pulse programmer may have hardware and/or software attributes. The pulse programmer also controls the gradient power supplies
16
,
18
and
20
, if such gradients are required. These gradient power supplies may maintain selected static gradients in the respective gradient coils if so desired.
The transient nuclear resonance waveform is processed by receiver
28
and further resolved in phase quadrature through phase detector
30
. The phase resolved time domain signals from phase detector
30
are presented to Fourier transformer
32
for transformation to the frequency domain in accordance with specific requirements of the processing. Conversion of the analog resonance signal to digital form is commonly carried out on the phase resolved signals through analog to digital converter (“ADC”) structures which may be regarded as a component of phase detector
30
for convenience.
It is understood that these resolved data signals from the phase detector
30
may be directly stored in a storage unit
34
. The Fourier transformer
32
may, in practice, act upon a stored (in storage unit
34
) representation of the phase resolved data. This reflects the common practice of averaging a number of time domain phase resolved waveforms to enhance the signal to-noise ratio. The transformation function is then applied to the resultant averaged waveform. Display device
36
operates on the acquired data to present same for inspection. Controller
38
, most often comprising one or more computers, controls and correlates the operation of the entire apparatus.
In conducting NMR experiments, the coil
12
must be tuned to the resonant frequency of the nuclei to be observed. Additionally, the impedance of the coil
12
should be electrically matched to the impedance of the transmission line
19
which is optimally coupled through the multiplexer
27
to the receiver
28
to obtain the maximum transfer of energy and to obtain the best signal to noise ratio (SNR). To tune and match the coil
12
, conventional NMR coils have variable capacitors. Typically, at least one variable capacitor is adjusted to tune the coil to the desired resonant frequency and at least another variable capacitor is adjusted to match the impedance of the coil. To adjust the capacitance of the variable capacitors, mechanical linkages are coupled to variable capacitors in the coil.
The probe is a critical component in NMR data acquisition. Among other functions, the NMR probe provides mechanical support for the sample and coil, and the NMR probe provides electrical connections between the coil and the NMR apparatus. The NMR probe is placed into the bore
11
to position the sample and coil in a preselected position along the center of the bore
11
.
FIG. 2
illustrates the mechanical structure of one example of a contemporary NMR probe
50
. Briefly, the NMR probe
50
includes a box
52
, three tuning rods
54
a
,
54
b
and
54
c
and a pair of board levels
56
a
and
56
b
. The coil (not shown) is located above the board level
56
a
e.g; axially beyond board level
56
a
. The sample is placed within the interior volume defined by the coil and typically in the center of the coil. For example, the coil may be a simple LC circuit with variable capacitors connected to the coil (not shown). The variable capacitors are typically located on the opposite side of the board level
56
a
as the coil.
To adjust the capacitances of the variable capacitors, the tuning rods
54
a
,
54
b
and
54
c
each comprise an assembly of concentric rods
58
a
and
58
b
. The concentric rods
58
a
and
58
b
are mechanical linkages that are coupled to the variable capacitors. The inner rod
58
a
rotates to adjust one of the variable capacitors on the board level
56
a
, and the outer rod
58
b
rotates to adjust another variable capacitor on the board level
56
a
. The box
52
supports the tuning rods
54
a
,
54
b
and
54
c
and the board levels
56
a
and
56
b
. Additionally, the box
52
houses connectors to the NMR probe that link the coil to the NMR apparatus described above. Furthermore, an outside shield tube (not shown) surrounds the tuning rods
54
a
,
54
b
and
54
c
, the board levels
56
a
and
56
b
, the variable capacitors and the coil.
When NMR experiments are performed, the box
52
is positioned outside the bore
11
the of magnet
10
, and the board levels
56
a
and
56
b
are within the bore
11
. To tune and match the coil for the NMR experiment, an operator manually rotates the mechanical nubs
60
associated with each concentric rod at the base of each tuning rod. Rotating the mechanical nubs
60
rotates the respective concentric rod of the tuning rod which adjusts the capacitance of the associated variable capacitor.
One shortcoming of the contemporary NMR probe is that the manual adjustment of the mechanical nubs
60
is inconvenient and inefficient. Because the NMR probe
50
is positioned largely within the bore
11
for experiments, the mechanical nubs
60
need to be adjusted by hand at the bore
11
away from the control console and display
36
of the NMR apparatus. The manual adjustment is also time consuming and troublesome.
Thus, it is desired to develop a NMR probe that may be tuned and matched remote from the probe. It is also desired to develop a NMR probe that may be ef

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