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Description  |
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BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a diagnostic apparatus employing nuclear magnetic
resonance (NMR), and more particularly, to means for preventing the image
quality from being degraded due to time dependent variations of the
primary magnetic field intensity in an NMR computerized tomographic
apparatus.
2. Description of the Prior Art
Conventional NMR diagnostic apparatus, such as NMR computerized tomographic
apparatus, generate a primary magnetic field by energizing a primary
magnetic field coil with a constant current. However, the power supply for
the primary magnetic field coil may vary in its output, or the primary
magnetic field coil may vary in its shape due to variations in the room
temperature or the temperature of the primary magnetic field coil per se,
with the result that the intensity of the primary magnetic field may vary
over a few tens ppm. When the primary magnetic field varies in intensity,
the image of a diagnosed area is liable to suffer from an artifact.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to provide an NMR diagnostic
apparatus having means for preventing artifacts from being generated due
to variations in the primary magnetic field.
According to the invention, an NMR diagnostic apparatus measures the
intensity of a magnetic field by placing an object of known properties,
such as water, in the vicinity of a subject being examined, and controls
the field intensity, corrects a computed value, or controls the reference
frequency of a phase detector, based on the measured field intensity used
as a reference.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram depicting an illustrative embodiment of the
invention comprising an NMR diagnostic apparatus.
FIG. 2 is a cross sectional view of a cradle.
FIG. 3 is a diagram showing projecting images of a subject being examined
and water filled in a pipe.
FIG. 4 is a timing chart depicting a scanning pulse sequence used in a spin
warp saturation recovery method.
FIG. 5 is a spectral plot showing varying of field strength due to drift of
temperature or field core voltage.
FIG. 6 is a diagram depicting variations of a reference frequency
.omega..sub.0.
FIG. 7 is a diagram depicting the manner in which observed data are
corrected.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Turning now to FIG. 1 there is depicted an illustrative embodiment which is
incorporated in an NMR tomographic apparatus and which comprises a
controller 1, which comprises a computer, connected to a display unit 2, a
primary magnetic field power supply 3, a sequence storage unit 4, an RF
oscillator 6, an analog-to-digital (A/D) converter 9, and a control
console 12. Primary magnetic field power supply 3 is coupled to a magnetic
assembly 13. Sequence storage unit 4 is connected to a gradient magnetic
field driver 5, a gate modulator 7, and A/D converter 9. Gradient magnetic
field driver 5 is connected to magnetic assembly 13. Gate modulator 7 is
connected to an RF power amplifier 8 which is coupled to magnetic assembly
13. RF oscillator 6 is joined to gate modulator 7 and a phase detector 10
connected to A/D converter 9. Magnetic assembly 13 is coupled to a
preamplifier 11 which is connected to phase detector 10.
Magnetic assembly 13 comprises a primary field coil, x-axis, y-axis and
z-axis gradient field coils, an RF transmission coil, an NMR signal
receiving coil and a space therein for placement of a subject to be
examined. Assembly 13 applies a primary magnetic field, gradient magnetic
fields, and RF pulses to a subject 100 (see FIG. 2) placed in the space
therein, and receives an NMR signal representing a resonance frequency and
a resonance energy level.
A cradle 14, with subject 100 supported thereon, and which is movable into
and out of the space in assembly 13, as shown by the double arrow 15, is
placed into the space in the assembly 13. A pipe (such as shown
representationally as a rectangular outer shell 20), filled with water is
attached to the underside of cradle 14. The object which is used to
measure the magnetic field intensity, and which may be water filled in the
pipe, will be described in detail hereinbelow.
Primary magnetic field power supply 3 is coupled to the primary field coil
in assembly 13, for applying the primary field to subject 100. Supply 3 is
controlled by controller 1 to control the intensity of the primary
magnetic field, as described hereinbelow in greater detail.
Sequence storage unit 4 generates a timing signal for collecting observed
data of an NMR energy level, and for controlling operation of gradient
magnetic field driver 5, gate modulator 7, and A/D converter 9. Sequence
storage unit 4 thus controls the sequence in which the gradient magnetic
fields and the RF magnetic field are generated.
Gradient magnetic field driver 5 is connected to x-axis, y-axis and z-axis
gradient coils in magnetic assembly 13 for applying the respective
gradient magnetic fields to subject 100.
RF oscillator 6, gate modulator 7, and RF amplifier 8 are arranged to apply
RF pulses to subject 100, placed in magnetic assembly 13, to give nuclear
magnetic resonance to the nuclei of atoms in subject 100. RF oscillator 6
produces an RF signal. Gate modulator 7 is responsive to the timing signal
from sequence storage unit 5 for modulating the Rf signal from RF
oscillator 6 to thereby generate RF pulses. RF power amplifier 8 amplifies
the RF pulses from gate modulator 7 and supplies the amplified Rf pulses
to the RF transmission coil in magnet assembly 13.
A/D converter 9, phase detector 10, and preamplifier 11 operate to observe
an NMR signal to collect observed data in digital form. Preamplifier 11 is
joined to the NMR signal receiving coil in assembly 13 for amplifying the
NMR signal. Phase detector 10 detects the phase of the output from
preamplifier 11 with respect to the output signal from RF oscillator 6.
A/D converter 9 serves to convert the phase detected NMR signal into a
digital signal.
Controller 1 rewrites the stored content of sequence storage unit 4 and
controls primary magnetic field power supply 3 and RF oscillator 6.
Various control sequences can be obtained by rewriting the stored content
of sequence storage unit 4. Controller 1 also effects an arithmetic
operation on the observed data to construct an image from a distribution
of resonance energy levels.
Display unit 2 displays the image of the NMR atom distribution as computed
by controller 1. Control console 12 is operated by an operator to enter
input signals provided by the operator. According to a feature of the
invention, an object for measuring the magnetic field intensity is placed
in the vicinity of the subject 100. The object may be of any given shape,
and have known properties. In the illustrative embodiment, water disposed
in a suitable container, such as a pipe, was used.
The object should be positioned to meet the following conditions:
1. It should not interfere with subject 100.
2. It should provide at least one view in which a projection of subject 100
and a projection of the field intensity measuring object are not
overlapped.
3. It should be disposed as closely to the NMR signal receiving coil as
possible.
4. It should be disposed in a uniform magnetic field.
Condition (1) is a requirement which should naturally be met by a
diagnostic apparatus. Condition (2) is required for measuring the
intensity of the primary magnetic field. The primary magnetic field
intensity cannot be measured if a projecting of subject 100 and a
projection of the field intensity measuring object are overlapped in all
views. Conditions (2) and (3) are required for providing a good
signal-to-noise ratio.
In the actual diagnostic apparatus, it is desired to search for a position
which meets the above conditions outside of the region for obtaining an
image of the subject 100, or to position the field intensity measuring
object or substance on each of both sides of cradle 14 or beneath cradle
14. If the field intensity measuring object is to be attached to cradle
14, movement of cradle 14 should be taken into account. That is, the field
intensity measuring substance should be attached to cradle 14 over its
entire length to produce a constant NMR signal without being affected by
the movement of cradle 14. It is also necessary to have accurate
information about the height and width of cradle 14.
FIG. 2 is illustrative of the cross section of cradle 14. Water 20, filling
a pipe, is disposed beneath cradle 14 and serves as the object used to
measure field intensity. The pipe (or water filled in a container) extends
over the entire length of the cradle 14.
FIG. 3 shows a projection of subject 100 and a projection of water 20, such
as filled in a pipe. As illustrated in FIG. 3, water 20, filling a pipe,
is positioned to provide at least one view in which a projection of
subject 100 and a projection of the field intensity measuring substance or
object 20 are not overlapped during one scanning cycle.
In the illustrated embodiment, the field intensity measuring substance is
of a rectangular cross section. However, the object may be of any desired
or given shape. However, where the diagnostic apparatus is used according
to the spin warp method, the signal intensity of a view is abruptly
reduced as the amount of warp is increased if the object is thick in the
direction of warp (in the direction in which spin is warped by magnetic
resonance, or in the y-direction, in FIG. 2). Thus, there are fewer views
having a signal-to-noise ratio which can be used in the measurement of the
primary magnetic field intensity. If the substance is thin in the
direction of warp, then the entire signal for measuring the primary
magnetic field intensity is reduced, resulting in a reduction of the
measuring accuracy. Thus, the optimum thickness of the field intensity
measuring object should be determined in view of the above considerations.
While in the above embodiment water 20, contained in a pipe, is employed as
the field intensity measuring object, any material may be employed
provided it is of a prescribed or given shape and of known properties. One
or more field intensity measuring objects or substances may be used
together with each other.
FIG. 4 is a timing chart depicting a pulse sequence for one view scanned by
the general spin warp saturation recovery method. In a time period
t.sub.1, one sectional plane of the subject is specified by gradient
magnetic field Gz in the z direction, and spins in the subject are
selectively excited and rotated 90.degree. by an RF pulse. In a time
period t.sub.2, the phase of the spins is encoded according to a y
coordinate by the gradient magnetic field Gy in the y direction. In a time
period t.sub.3, the total of echo signals from spin frequency encoded in
the x direction by the gradient magnetic field in the x direction is
received. In a succeeding view, the intensity of the magnetic field Gy in
the y direction in time period t.sub.2 is varied to move the y coordinate
to be observed.
The amount of phase encoding (corresponding to the intensity of the
gradient magnetic field in the y axis direction) from one end to the other
of a region to be constructed as an image in the y axis direction is
called the amount of warp. By varying the amount of warp for 2n.pi. (n is
an integer) in each view, a component parallel to the x-axis of a two
dimensional Fourier transformed quantity of a density distribution of NMR
atoms in the subject can be obtained directly as observed data on the time
base. For data observed by a view in which the amount of warp is "0", the
time base data is subjected to a reverse Fourier transform to obtain
projections of subject 100 and the field intensity object onto the x axis.
FIG. 5 shows the projections of a subject and field intensity objects onto
the x axis. In this example, the field intensity measuring objects are
disposed on both sides of the subject, and hence projections 201, 202 of
the field intensity measuring objects are displayed on both sides of the
projection 200 of subject 100.
From the positions of the field intensity measuring objects and the
intensity of the gradient magnetic field Gx in the x direction, there are
obtained a central frequency .omega..sub.201A of projection 201 and a
central frequency .omega..sub.202A of projection 202 in case the intensity
of the primary magnetic field remains unchanged. These frequencies
.omega..sub.201A, .omega..sub.202A and actually measured frequencies
.omega..sub.201, .omega..sub.202 are compared to determine a variation of
the intensity of the primary magnetic field.
More specifically,
.DELTA..omega..sub.201 =.omega..sub.201 -.omega..sub.201A (1)
.DELTA..omega..sub.202 =.omega..sub.202 -.omega..sub.202A (2)
A transition .DELTA..omega. of a certain frequency due to the variation of
the primary magnetic field intensity is given by:
.DELTA..omega.=1/2(.DELTA..omega..sub.201 +.DELTA..omega..sub.202) (3)
Assuming the gyromagnetic ratio is expressed by .gamma., the transition
.DELTA..omega. of the certain frequency and the variation .DELTA.B of the
primary magnetic field intensity have the following relationship:
.DELTA..omega.=.gamma..DELTA.B (4)
Thus, the variation of the primary magnetic field intensity can also be
determined theoretically by the above procedure.
It is preferable to effect the foregoing measurement in each view in order
to detect a variation of the primary magnetic field intensity during
scanning. However, in reality, the received signal is reduced in a view
with a large amount of warp, and it is impossible to make the measurements
in each view. Thus, views with small amounts of warp are scattered during
scanning and used as magnetic field intensity measuring views, and several
primary magnetic field intensities are measured in one scanning cycle. For
the other views, interpolated values are used. Since the primary magnetic
field does not vary abruptly, the intensity thereof only needs to be
measured several times. The magnetic field intensity measuring views may
be spaced at equal or irregular intervals.
Based on the measured intensity of the primary magnetic field, the
controller 1 controls the primary magnetic field power supply 3 or
corrects the obtained data.
Correction of the obtained data will now be described. Although a basic
correcting method will be described hereinbelow, there are other
correcting methods available for various scanning methods.
Since variation of the primary magnetic field intensity is ordinarily slow,
the Fourier transform method has only small effect on the phase encoding
in many cases. Thus, variation of the primary magnetic field intensity
appears principally as a frequency shift of the observed data. Providing
the frequency difference between the Larmor frequency .omega..sub.1 given
by the primary magnetic field intensity and the reference frequency
.omega..sub.0 is given by .DELTA..omega., observed data f(t) when
.DELTA..omega.=0 and observed data f'(t) when .DELTA..omega..noteq.0 have
the following relationship:
f'(t)=f(t) exp (-j.DELTA..omega.t) (5)
Therefore, reference frequency .omega..sub.0 may be varied or the observed
data may be corrected according to the equation (5).
FIG. 6 shows the manner in which reference frequency .omega..sub.0 is
varied. As the primary magnetic field intensity varies, Larmor frequency
.omega..sub.1 varies, and reference frequency .omega..sub.0 is brought
into conformity with the Larmor frequency .omega..sub.1 during respective
magnetic field intensity measuring viewings or views as indicated by the
arrows in FIG. 6. This prevents the phase detected signal from being
affected by the variation of the primary magnetic field intensity.
For varying reference frequency .omega..sub.0, the oscillation frequency of
RF oscillator 6 is controlled so as to coincide with Larmor frequency
.omega..sub.1 obtained during the magnetic field intensity measuring view,
thus changing the reference frequency of phase detector 10.
The above correcting method is capable of correcting reference frequency
.omega..sub.0 during the timing of the magnetic field intensity measuring
views, so that reference frequency .omega..sub.0 varies in a step like
manner, as depicted in FIG. 6. Consequently, it is preferable to use many
magnetic field intensity views.
FIG. 7 shows the manner in which the observed data is corrected. The
observed data f'(t) is corrected in the following manner, using the
frequency difference .DELTA..omega. between Larmor frequency .omega..sub.1
and reference frequency .omega..sub.0 :
f(t)=f'(t) exp (j.DELTA..omega.t) (6)
Equation (6) indicates that f'(t) is frequency shifted by -.DELTA..omega..
For a variation in the magnetic field intensity from a measurement of the
primary magnetic field intensity to a next measurement thereof,
.DELTA..omega. is required to be estimated by appropriate interpolation.
FIG. 7 shows an example of linear interpolation, in which magnetic field
measuring views should be applied at such intervals that an estimated
error .epsilon..sub.i (i=1, 2, . . . ) in each interval will be kept
within a certain range. The correction of data in interval 1 is possible
after the second magnetic field measuring view, that is, at a time when
intervals 2 is being scanned. Thus, the scanning and data correction can
be effected at the same time.
The primary magnetic field applied to the subject, based on a specific
frequency, can be controled to apply relatively slow variations within a
broad frequency range, and the reference frequency of the phase detection
can be controlled to apply fast variations within a narrow frequency
range.
The observed data can be corrected independently. As the deviation of the
magnetic field becomes larger, however, a sliced plane is distorted. Thus,
data correction is preferably combined with the method in which the
reference frequency .omega..sub.0 is varied.
While the scanning according to the Fourier transform method has been
described, the present invention is also applicable to a projection
method. The NMR computerized tomographic apparatus has no moving parts,
unlike the X-ray computerized tomographic apparatus. Thus, the observing
sequence can be selected as desired providing there is a one-to-one
correspondence between the view number, and the view angle. Accordingly,
views in which a projection of the magnetic intensity measuring object and
a projection of the object are not overlapped are scattered in one
scanning cycle, and the primary magnetic field intensity is measured only
in such views, while values obtained by interpolating the measured values
are employed for the other views. The views in which the primary magnetic
field intensity is measured may be spaced at equal or irregular intervals.
The reference value for the central frequency of the projection of the
magnetic field intensity measuring object varies dependent on different
view angles, and such a variation can be easily determined by the view
angle.
The present invention is applicable irrespectively of pulse sequence such
as the saturation recovery method, the reverse recovery method, the FID
method, the spin echo method, and the like.
As described, the NMR diagnostic apparatus of the invention is capable of
measuring time dependent variations of the primary magnetic field
intensity, and hence of preventing artifacts from being generated due to
variations in the primary magnetic field intensity. The measurement of the
primary magnetic field intensity is not affected by movements of the
subject. The invention is, therefore, advantageous when applied to medical
diagnostic apparatus.
The cradle 14 is depicted in the drawing to be outside assembly 13. The
assembly 13 has a space therein for placement of the cradle 14 and subject
100 therein during examination, and at other times, the cradle 14 and
subject 100 are disposed outside of the assembly 13.
The foregoing description is illustrative of the principles of the
invention. Numerous modifications and extensions thereof would be apparent
to the worker skilled in the art. All such modifications and extensions
are to be considered to be within the spirit and scope of the invention.
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