The system has the purpose to lessen the magnetic field disturbance,
almost static or slowly unsteady (frequency band <100 Hz) that can interest NMR
Tomographs placed near moving metal masses, electric cables, either DC or Ac, etc.
and could therefore compromise their proper operation.
Background of art
The new-generation Tomographs (active shield) characterised by a very
low dispersed magnetic field and very fast acquisition sequence are very sensitive
to external magnetic field changes that, if they arise during the acquisition phases
could cause alterations of image. Presently there are no commonly used magnetic
field probes that can perceive slight variations of B, in the order of tenths of
Gauss, in the presence of a very high background field, in the order of 15.000 Gauss,
such as the one generated by a NMR Tomograph.
This technical and manufacturing limitation of B field probes has
not allowed to use the classic closed loop compensation systems that can act directly
on the machine disturbances and that represent the state of the art for active systems
for compensation of magnetic field for other applications.
Changes in the B magnetic field with respect to a background value
(represented for example by the terrestrial magnetic field) may represent a problem
for all the scientific and/or technical machines that require to be plunged into
a constant field in order to operate correctly. At present, the solution of this
problem is given by the passive shielding by means o ferromagnetic materials and
by active systems for compensation of the disturbances.
When using the passive systems, a ferromagnetic shield is placed between
the source of disturbances and the machine to be protected. The function of this
shield is to channel inside it the flow lines of the undesired magnetic field dissipating
them into the volume to be protected.
In lessening the variations of B that interest NMR Tomographs, this
solution raises serious problems. The lessening levels required to that the disturbances
are not harmful for the machine can be obtained with acceptable thickness only by
means o highly permeable materials such as mumetal or permalloy which, besides being
costly, have low saturation levels. Such materials, inserted in the high dispersed
magnetic field of the Tomograph, may saturate and become practically transparent
to the disturbances to be shielded.
In the active compensation systems magnetic field disturbances are
lessened by means of an equal and contrary field generated by appropriate coils
placed according to the Helmoutz configuration.
At present there exist compensation systems used to lessen the magnetic
field of disturbances on electronic microscopes and other laboratory devices.
In such applications, characterised by a low dispersed magnetic field
of the machine , the probe that can perceive the field of disturbances may be placed
near the sensitive element of the machine to be protected. Such positioning of the
probe enables the use of the classic scheme of retroaction control, thus remarkably
simplifying both machine operation and system set up. This solution cannot be used
for NMR Tomographs , because at present there are no magnetic field probes that
can perceive variations of milliGauss over a backgrouund dispersed field of 10.000Gauss.
In order to overcome this limitation it has therefore been decided to place the
probe at a distance from the magnet that may ensure background field values compatible
with the required sensitivity of sensor.
The compensation systems at present on the market, moreover, for generating
the correction field use coils placed at the corners of the room containing the
machine to be protected. This is once again an effective measure in the case of
compensation systems for machines that have not a high dispersed magnetic field,
but cannot always be used in the case of NMR Tomographs. The magnetic field generated
inside these diagnostic systems must be confined within a controlled volume, for
security reasons concerning persons and machines, and in many cases this requires
the placement of adequate ferromagnetic shields on the walls of the test room. In
presence of these shields, in case the magnetic field correction coils should be
placed in the corners of the room, just near ferromagnetic materials, the B field
flow would be conveyed in the high permeability materials, with a significant distortion
of the correction field.
The proceedings of the 8th annual meeting of the SMRM, on p. 968,
disclose a system for the compensation of external magnetic field instabilities
in MRI which comprises a fluxgate magnetometer probe installed at 1.5 m from the
centre of the main magnetic field and operated in zero-lock mode. The signal from
the magnetometer controls the compensation current in a Helmholtz coil wound around
the magnet. The Earth's magnetic field and the fringe field of the MRI magnet at
the probe position are offset with a small coil wound around the probe.
DE-A-195 10 142 discloses a similar system in which a fluxgate probe
is located above the upper yoke of a C-shaped open MRI magnet and is shielded from
the stray field of the magnet by means of an auxiliary coil. Filtering means are
provided for making sure that the temporal variation of the compensation magnetic
field equals the temporal variation of the external magnetic field disturbances
inside the examination volume.
US-A-5 278 503 discloses an open loop control system for the cancellation
of external magnetic field disturbances in which induction coils are used as field
JP-A-05-300896 also discloses an open loop control system in which,
however, a field sensor is used which is actively shielded from both the fringe
field of the MRI magnet and the field generated by the compensation coils.
Disclosure of invention
The present invention is defined in claim 1. The subject system is
completely automatic and it carries out compensation of the external magnetic field
by means of an open loop control through a correction curve defined during the setting
The operation principle of this systems is the following: in presence
of a B noise field of disturbances read by a magnetic field probe placed at an adequate
distance from the magnet, the system automatically generates according to a transfer
function (B nois ?I corr), an Icorr current (t) that circulates in the correction
coils placed according to the Helmoutz configuration, so as to cancel the disturbances
on the Tomograph (Btom).
What above involves that the correction is not made acquiring directly
the value of B nois field that is later correlated, during the setting phase to
the effects that the machine disturbance field (Btom) has on it in terms of variations
of the resonance frequence.
This systems is particularly adapted for the compensation of variations
of magnetic field, even triaxial on NMR Tomographs. The systems has a modular structure
that enables to move from a monoaxial to a triaxial configuration. Moreover it has
a PC interface system via RS 232 that allows it to be used also as a systems for
monitoring magnetic fields.
Such systems is essentially composed of:
- a perceiving unit
- a control unit
- a correction coil.
The perceiving unit is made of a Fluxgate-type probe and of a pre-amplification
stage, and it is intended to provide to the control unit only the field variations
with respect to a background value, due to the terrestrial magnetic field and to
the dispersed field of Tomograph. The probe can be either mono or triaxial, according
to the type of disturbance, and it is able to perceive an absolute B field value,
namely background field + disturbance.
Pre-amplification phase is intended to extract and amplify only B
field variations with respect to the background field and for this purpose it is
equipped with an offset adjuster for the definition of the background field value.
At the exit of this block we find the B field of disturbances that is adjusted to
the machine disturbances during the setting phase.
The control block is the intelligent unit that can provide with respect
to the exit signal from the perceiving block a current that circulates in the correction
coils for generating the counter-field.
The characteristic of transfer Bnoise⇒ is defined during the
setting phase when the dedicated software system is also prepared, and it is stored
in a non-volatile memory inside the unit.
The control unit hardware is fed by a 220V AC 6 it is composed of
an analogic digital converter (ADC) , a microcomputer signal processor (DSP) an
analogic digital converter (DAC) and a power stage; the systems is modular and it
can be equipped with 1 to 3 correction modules that, according to the disturbance
to be lessened can be either monoaxial, biaxial or triaxial.
The correction coils are intended to generate, when the exit current
from the control unit passes through them, the magnetic counter-field. They are
placed according to Helmoutz configuration along the Tomographs axes according to
which the disturbance have to be lessened. Therefore, there can be from a minimum
of a couple of coils to a maximum of three couples placed along three axes X,Y and
Z of the machine. Both the position and the number of coil curls are defined from
time to time according to the characteristics of the disturbances to be lessened.
Brief description of the drawings
The invention systems, from now on referred to as DIAMAGS, will be
now described with reference to a preferred form that is not restrictive of the
invention illustrated in the annexed drawings in which:
Description of a preferred embodiment of the invention.
- Figure 1 shows the block scheme of the invented system.
- Figure 2 shows a scheme of the transfer characteristics Bnoise ⇒ Icorr
- Figure 3 shows a principle configuration of a monoaxial systems according to
- Figure 4 shows a preferred scheme of the sensor pre-amplifier.
- Figure 5 shows the circuit scheme of pre-amplifier.
- Figure 6 and 7 show respectively the circuit schemes of the elaboration section
and of acquired signals.
- Figure 8 shows the preferred circuit scheme of the analogic digital converter.
- Figure 9 shows the preferred scheme of the feeder.
With reference to the above Figures, and especially to Figure 1 that
shows the block scheme of the system, the perceiving unit that includes a Fluxgate
probe and a pre-amplification stage, and it is intended to provide to the control
unit, in presence of the absolute field value (Bbackground+Bnoise) perceived by
the probe, only the field variations (Bnoise) with respect to a background value,B
background) due to the terrestrial magnetic field and to the dispersed field of
The probe of a preferred configuration has the following characteristics
- band width: 0÷3k Hz
- measuring range: ±250 u T
- output impendance: <1Ω
- calibration accuracy: ±0,5%
- internal noise: 4-6pTrms/ Hz to 1 Hz
- scaling: 40mV/µ T
- offset error: ±12nT
The probe can be monoaxial, biaxial or triaxial, according to the
type of disturbances and therefore the system configuration. The preamplifier, cascade
connected to the probe has the purpose to "extract" and amplify only the variations
of Bnoise field with respect to the background field and is equipped with an offset
adjuster for the definition of a reference tension value related to the background
field value. Offset adjustment is carried out during the setting phase of the system.
Figure 4 shows the circuit scheme of the pre-amplifier and outwards
interfaces: on one side the output of the RS232 serial Fluxgate probe and on the
other side the output towards the invention machine, which is also an RS232 device.
The pre-amplifier is equipped with a variable gain amplifier which,
in presence of the maximum selected input value generates a 5VDC input tension at
the stage of analogic-digital conversion of DIAMAGS.
Connection between pre-amplifier and DIAMAGS is made by means of a
RS232 multipolar shielded cable both for feeding and for the signals.
As indicated above, the DIAMAGS control and monitoring unit is a fundamental
section both as to innovation and as for the importance of the invented system.
The control and monitoring DIAMAGS unit that can be seen in the block system in
Figure 1 is an intelligent unit which can provide, in presence of the output Bnoise
from the perceiving block an Icorr correction current for generating the compensation
counter-field. The correction is made taking into account the transfer characteristic
Bnoise⇒Icorr shown in the annexed Figure 2, defined during the setting phase
and stored stored in a non-volatile memory inside the unit.
The unit may be moreover interfaced to a via RS232, both for adjustment
and for monitoring the disturbance by means of an adequate software especially studied
and carried out by the patent inventor.
The monitoring and control unit is characterised by a modular structure
for monoaxial, biaxial or triaxial configurations. All the hardware for acquisition,
monitoring and control relative to an axis is contained on one single card which
may be connected on bus to the mother board of the machine. Modularity of the machine
enables to create highly reliable redundant structures.
The machine can be fed by 220 or 100 V current and is equipped with
the following interfaces;
- serial port with 9PIN Cannon connection for communicating with the perceiving
- 1,5 mmq three-pole feeding cable;
- 9 pin Sub D Miniature serial port for communication with PC;
- 2 analogic output with BNC connector for monitoring the input signal to the
unit, before and after the pre-amplification stage;
- clamps for feeding both the correction and error coils;
- three LEDs, two of whom for the percentage indication of the output signal from
the perceiving block and of the correction current, while the third is for measuring
the circulating within the correction coils.
The monitoring and control unit is composed of the following block
and /or sections
The pre-amplifier whose scheme is shown in Figure 5 has the purpose
to adapt the impedance and making the output signal from the perceiving block compatible
with the ADC conversion stage. On one side it can be noted RS232 connection with
the perceiving probe, and on the other the output of the adapted signal towards
the ADC converter of the computer section. There are also two BNC outputs to the
monitor for displaying the signal perceived before and after this stage.
Figures 6 and 7 show the circuit schemes of the computer section intended
to elaborate and process the perceived signals, whose aim is to read the input analogic
values, converting them into digital signals and , referring to the setting curve,
defining the output controls in numeric form. ADC conversion is carried out by the
DMG/ADC/3.1 block, whose output is processed by the micro computer DMG/MAIN/3.1,
which, on the basis of the informations contained in EEROM, which are programmed
and stored during the setting phase if the machine defines the various digital actuations
which then go as an input to the following block. This section is moreover composed
by the display block, the DMG/VIS/3.1 micro computer and the Visual interface which
is the RS232 interface to the PC.
Figure 8 shows the circuit scheme of DAC section which represents
the actuation circuit that receives the digital controls from the previous section,
converts them into analogic signal amplifying them in order to feed correction coils.
The section includes the driving stage of another coil (error coil) which can be
used in the setting phase or in the characterisation of the magnet. The output of
the correction coil can be handled either automatically by the system during compensation
phases or manually by interfacing it with the PC during the setting phase.
The output of the error coil can instead be used manually through
one of the procedures of the setting software of the system.
The circuit scheme of this section is shown in Figure 8.
Figure 9 shows the circuit scheme of the feeding section, which provides
feeding to both the digital section (+5V) and the analogic section (+ 15V) starting
from the network tensions (220V/110Vac 50/60 Hz).
Figure 3 shows the configuration of a monoaxial DIAMAGS system, evidencing
the fundamental blocks and the insertion scheme of coils on the Tomograph.
Correction coils, crossed by the output current coming from the control
unit DIAMAGS, generate the magnetic counter-field for compensation of the disturbances.
They are placed in Helmoutz configuration along the axes of the Tomograph for which
the disturbances are to be lessened; it is therefore possible to have from a minimum
of a couple of coils to a maximum of three couples placed along three axes X,Y and
Z of the machine. Both the position and the number of coil curls are defined from
time to time on the basis of the characteristics of the disturbances to be lessened.
It is evident that for the proper operation of the system and in order to fully
exploit its potential, it is essential that the setting is properly carried out.
The setting phase has the aim to define the correction curve (or historic matrix)
which will be then automaticaly recalled by the systems in order to generate, in
presence of a disturbance Bnoise in output from the perceiving unit an Icorr current.
Since it is not possible to perceive directly the disturbance that characterises
the magnet, as no B-field probes may be found on sale that could perceive small
field (around 15.000 Gauss) the disturbances read by the probe placed at an appropriate
distance from the magnet has been related to the effects that the B-field variation
causes on the machine in terms of drift of the resonance frequence. This involves
the simultaneous perception, for a certain period, of both the variation of the
resonance frequence of the Tomograph and of the variation of B-field, observed by
the latter, through the perception unit whose position is defined during site audit.
The steps which characterise the setting procedure are the following:
- adjustment of the offset of the perceiving unit;
- correlation of the disturbance read by the perceiving unit (Bnoise) to the drift
- frequence of the magnet;
- characterisation of the system composed by the magnet and the correction coils;
- definition and storing of the correction curve Bnoise⇒Icorr.
The above mentioned setting operations are carried out according a
connecting DIAMAGS unit to a PC through the RS232 serial port by means of a dedicated
software carried out by the patent inventor.