Apparatus and method for examining an object by nuclear magnetic resonance

An NMR apparatus and method in which the S/N ratio is improved in reception of FID signals. An oscillator includes a pre-amplifier, a driver amplifier, a final power amplifier and a bias control circuit. The operation modes of the final power amplifier can be changed by controlling the biasing voltage to the final power amplifier which is applied by the bias control circuit. While the selective exciting pulse is applied to the slice portion of the patient, the final power amplifier is being operated in class A i.e., high gain mode. Immediately after an application of the selective exciting pulse is accomplished, the operation class of the amplifier is switched to "C" e.g., low gain mode in order to receive FID signal from the excited slice portion by suppressing noise signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a diagnostic apparatus and method wherein 
a density distribution of a specified proton (generally, hydrogen nucleus) 
in biological tissue is measured externally from the object examined 
(i.e., a patient) in a non-invasive manner by utilizing a nuclear magnetic 
resonance phenomenon so as to obtain information for medical diagnosis. 
2. Description of the Prior Art 
Such a diagnostic apparatus is described in e.g., U.S. Pat. No. 4,254,778. 
The known nuclear magnetic resonance techniques (referred to as "NMR" 
techniques) will now be described with reference to FIGS. 1 through 5. 
A steady magnetic field is generated by an air coil C1 shown in FIGS. 1A 
and 1B, and a magnetic gradient field is generated by gradient field 
generating coils C2, C3 and C4 (FIGS. 2 and 3) assembled together with the 
air coil C1. FIG. 4 shows the fields illustrated diagrammatically in the 
side elevation in relation to a patient P. A steady field H.sub.0 
generated by the air coil C1 is superimposed in advance on a gradient 
field G.sub.z generated by the coils C2. The gradient field G.sub.z can be 
obtained by flowing reverse currents through a pair of Helmholtz coils C2 
shown in FIG. 2. This coil pair is called a "Maxwell pair". The gradient 
field G.sub.z has the same direction (z-axis) as that of the steady field 
H.sub.0 and has a zero magnetic intensity on a central plane 
(perpendicular to the z-axis) between the pair of coils C2 so that the 
absolute values of the intensities of reverse field components linearly 
increase in opposite directions from the above-described central plane 
along the z-axis (FIG. 4). The patient P is then placed in the resultant 
magnetic field. A selective exciting pulse H.sub.1 having a proper 
frequency component is applied to the patient P through a pair of 
saddle-shaped probe head coils C5. The selective exciting pulse H.sub.1 
has a center frequency of 4.258 MHz (corresponding to a magnetic field of 
1,000 gausses for a hydrogen nucleus) of a carrier wave and is obtained by 
amplitude-modulating an RF pulse by a SINC function. When the selective 
exciting pulse H.sub.1 is applied to the patient P, resonance occurs in a 
plane region (cross-sectional slice region with respect to the Z axis) 
wherein a frequency corresponding to a vector sum of the steady field 
H.sub.0 and the gradient field G.sub.z becomes equal to the frequency of 
the selective exciting pulse H.sub.1. A gradient field G.sub.R obtained by 
a sum of vector components of gradient fields G.sub.x and G.sub.y (G.sub.x 
and G.sub.y are perpendicular to each other and to G.sub.z) respectively 
generated by the gradient field generating coils C3 and C4 is applied to 
the slice region (i.e., chosen slice region) where resonance occurs. In 
this condition, when a free induction decay signal FID (referred to as 
"FID signal") is measured through the probe head coil C5, this signal 
corresponds to a signal obtained by Fourier-transforming a projection 
signal indicating a specific nucleus density distribution in the direction 
of the gradient field G.sub.R within the slice of the patient P. The 
direction of the gradient field G.sub.R can be varied within the x,y plane 
by changing the relative ratio of intensity of the field G.sub.x generated 
by the coils C3 to that of the field G.sub.y generated by the coils C4. A 
resultant free induction decay signal FID is subjected to inverse Fourier 
transformation, thereby obtaining projection signals in various directions 
in the x,y plane. By utilizing these projection signals, an image 
indicating the density distribution of the specific nucleus within the 
slice of the patient P is obtained. 
As a method which does not perform image reconstruction as described above, 
a multi-sensitive point method is proposed and disclosed in U.S. Pat. No. 
4,184,110 (1980) wherein an AC current flows through a gradient field coil 
to vibrate the gradient field, to accumulate FID signals and hence to 
extract only those signal components which are stable on the central line 
over a period of time. 
In these conventional NMR apparatuses, a signal to noise ratio (referred to 
as an "S/N ratio" hereinafter) of the resultant FID signal is low, and 
this is the main reason for low spatial resolution of a tomographic image 
in medical diagnosis. Noise components mixed in the FID signal are 
summarized as follows: 
(a) external noise (e.g., automobile ignition noise, impulse noise mixed in 
the AC primary power supply (commercial power supply)); 
(b) noise from digital equipment (e.g. noise from a digital computer for 
processing image reconstruction); 
(c) noise generated in an oscillator which serves as a component part of 
the NMR apparatus to generate a selective exciting pulse; and 
(d) noise obtained in such a manner that the noise components in items (a) 
and (b) is mixed in the oscillator described in item (c) and is amplified, 
and an amplified noise component is mixed in the FID signal. 
In particular, noise in item (d) causes great degradation of the S/N ratio 
since the oscillator has a high amplification, resulting in a great 
drawback. 
OBJECT OF THE INVENTION 
It is an object of the present invention to provide an apparatus and method 
for examining an object by nuclear magnetic resonance wherein an S/N ratio 
of an oscillator for generating a selective exciting pulse, which 
conventionally greatly degrades an S/N ratio of an FID signal is improved 
to perform highly precise diagnosis. 
SUMMARY OF THE INVENTION 
These objects stated above may be accomplished by providing a diagnostic 
apparatus and method utilizing nuclear magnetic resonance techniques 
comprising: 
magnet means for applying to the object a steady magnetic field along a 
longitudinal axis thereof; 
first coil means which is arranged along said longitudinal axis and is 
energized so as to apply to the object a first gradient field, which in 
conjunction with the steady field gives a predetermined field in a 
cross-sectional slice of the object, the field direction of the first 
gradient field being parallel to that of the steady magnetic field and the 
absolute value of the field strength thereof increasing linearly in 
opposite directions from said cross-sectional slice along the longitudinal 
axis; 
probe head coil means for applying RF pulses to the cross-sectional slice 
of the object in a direction perpendicular to the longitudinal axis so as 
to excite a nucleus therein to which is being applied the predetermined 
field combined with the steady field and the first gradient field, and for 
detecting nuclear magnetic resonance signals derived from the 
cross-sectional slice of the object; 
second coil means for applying a second gradient field to the 
cross-sectional slice of the object so as to define a projection angle of 
the nuclear magnetic resonance signals, the field direction of the second 
gradient field being perpendicular to the steady magnetic field and the 
strength of the second gradient field being gradient at a right angle with 
respect to the first gradient field; 
a processing unit which receives the nuclear magnetic resonance signals 
obtained from the cross-sectional slice of the object through the probe 
head means by changing the strength of the first gradient field, and 
executes the processing operation on duration times and phase information 
of the nuclear magnetic resonance signals by use of two-dimensional 
Fourier transformation changing linearly in an opposite manner along an 
axis perpendicular to the longitudinal axis; and 
oscillating means including at least means for generating the RF pulses, 
means for amplifying the RF pulses derived from the generator means, and 
means for controlling the amplifying means during and after an RF pulse 
generator period, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before proceeding with the various types of the preferred embodiments, the 
fundamental operations of the diagnostic apparatus utilizing NMR 
techniques in accordance with the present invention will now be explained. 
The bias timings of an amplifier element constituting an amplifier section 
of an oscillator for generating a selective exciting pulse differ 
according to an oscillation mode and a nonoscillation mode of the 
oscillator. In the nonoscillation mode, an S/N ratio of a received NMR 
signal is improved utilizing nonlinear operation characteristics of the 
amplifier. 
An apparatus for examining an object by nuclear magnetic resonance 
according to an embodiment of the present invention will be described with 
reference to FIG. 6. 
Referring to FIG. 6, an air coil assembly has four air coils 1 (e.g., 
electromagnetic coils). The air coil assembly generates a uniform steady 
field H.sub.0 in the same manner as the air coil C1 shown in FIGS. 1A and 
1B. Reference numeral 2 denotes an energizing source for the 
electromagnetic coils 1. Coils 3-1 and 3-2 constitute a first coil 
assembly (e.g., Helmholtz coil pair) for generating a linear gradient 
field G.sub.z applied along a direction (z-axis) of the uniform steady 
field H.sub.0. Reference numeral 4 denotes an energizing source for the 
coils 3-1 and 3-2. A current flowing through each coil is controlled by a 
processing device 11 to be described later. Reference numeral 5 denotes a 
second coil assembly for generating a linear gradient field G.sub.x,y in 
the x-Y plane in a direction perpendicular to the z-axis. The second coil 
assembly 5 comprises the same saddle-like coils as the coils C3 and C4 
(FIG. 3). Reference numeral 6 denotes an energizing source for the second 
coil assembly 5. The energizing source 6 is controlled by the processing 
device 11 in the same manner as the energizing source 4. Reference numeral 
7 denotes an oscillator for generating a selective exciting pulse; 8, a 
bridge-type receiver; and 9, a probe head coil. The probe head coil 9 
comprises the same coils as the coils C5 (FIG. 5). Reference numeral 10 
denotes an amplifier for amplifying the FID signal. The processing device 
11 converts the FID signal to a digital signal. The processing device 11 
further records and accumulates the digital signals and Fourier-transforms 
the accumulated signals. The processing device 11 also controls the 
energizing sources 4 and 6. Reference numeral 12 denotes a display device 
for displaying the image data obtained by the processing device 11. 
The operation of the apparatus for examining an object by nuclear magnetic 
resonance will now be described. 
As shown in FIGS. 7A and 7B, the energizing source 2 supplies power to the 
electromagnetic coils 1 to generate the uniform steady field H.sub.0. The 
uniform steady field H.sub.0 is applied to the patient P. The energizing 
source 4 then supplies power to the coils 3-1 and 3-2 to generate the 
gradient field G.sub.z along the z-axis. A selective exciting pulse H1 is 
then applied from the oscillator 7 to the patient P through the 
bridge-type receiver 8 and the probe head coil 9. The center frequency of 
the selective exciting pulse H1 is preset to be a value f.sub.0 
corresponding to the steady field H.sub.0, and the frequency difference 
from the selective exciting pulse H1 is set to be .DELTA.f. Therefore, a 
cross-sectional slice portion between two flat surfaces corresponding to 
.+-.G.sub.z determined by .pi..DELTA.f=.gamma..DELTA.G.sub.z with respect 
to a surface on which the magnetic field G.sub.z is zero is selected and 
excited. When the selective exciting pulse is cut off, the gradient field 
(this gradient field is given as the gradient field G.sub.x along the 
x-axis by way of simplicity) within the x-Y plane is generated by the coil 
assembly 5 energized by the energizing source 6, and is applied, as shown 
in FIG. 7C. The FID signal shown in FIG. 7D is detected and measured 
through the probe head coil 9. The FID signal is received by the receiver 
8 and is amplified by the amplifier 10. The amplified FID signal is then 
converted by the processing device 11 to a digital signal. The subsequent 
digital signals are accumulated a predetermined number of times (e.g., 
2.sup.5 =32). The accumulated digital signals are processed by Fourier 
transform or the like, such that a projection image along the x-axis of 
the plane excited as described above can be obtained. 
When opposing currents having the same intensity flow through the coils 3-1 
and 3-2 which serve as a Helmholtz pair for generating the gradient field 
G.sub.z, the intensity of the gradient field G.sub.z becomes zero at the 
mid-point between the coils 3-1 and 3-2, so that the selected slice 
surface is a surface including the mid-point, as indicated by a curve Ra 
in FIGS. 8A and 8B. However, when a current flowing through the coil 3-2 
is decreased, and a current flowing through the coil 3-1 is increased, the 
zero intensity point is shifted toward the coil 3-2, as indicated by a 
curve Rb (alternate long and short dashed curve) in FIGS. 8A and 8B. 
Conversely, when a current flowing through the coil 3-2 is greater than 
that through the coil 3-1, the zero intensity point is shifted toward the 
coil 3-1, as indicated by a curve Rc (dotted curve) in FIGS. 8A and 8B. 
The energizing source 4 is controlled by the processing device 11 to 
sequentially shift the selected slice, so that projection signals are 
obtained so as to correspond to the positions thereof along the z-axis, 
thereby forming a scanogram. The scanogram is then displayed at the 
display device 12. 
As previously mentioned, the apparatus according to the present invention 
is characterized by an oscillator 7 for generating the selective exciting 
pulse. The detailed configuration of the oscillator 7 will be described 
hereinafter. 
FIG. 9 shows a block diagram of the detailed circuit of the oscillator 
shown in FIG. 6. Referring to FIG. 9, reference numeral 13 denotes a 
signal generator. An output signal from the signal generator 13 is 
supplied to a preamplifier 14. An amplified signal from the preamplifier 
14 is amplified by a drive amplifier 15 and a final power amplifier 16 to 
a predetermined level. A bias control circuit 18 is connected to the final 
power amplifier 16 to control the bias of the amplifier 16. The gains of 
the amplifiers are set in such a manner that the output signals from the 
signal generator 13, the preamplifier 14, the drive amplifier 15 and the 
final power amplifier 16 have a common impedance of 50 .OMEGA., and 
voltages of about 1 V, 20 V, 150 V, and 700 V, respectively. It should be 
noted that the gain of the amplifier 16 is variable, as will be described 
later. 
In accordance with the present invention, as the FID signal can be received 
through the probe head coil 9 immediately after the trailing edge of the 
selective exciting pulse at time t.sub.F (FIG. 7), the operation point 
(e.g., A and C classes) of the final power amplifier 16 is shifted so that 
the amplification of the final power amplifier 16 is varied (with respect 
to the external and internal noise components previously described). 
In other words, while the selective exciting pulse H1 is being applied to 
the patient P, the gain of the final power amplifier 16 in the oscillator 
7 as one of the noise sources is reduced so as not to mix the internal and 
external noise components in the received FID signal at a high level by 
the receiver 8. Otherwise, the final power amplifier 16 is turned off 
during a predetermined period of the FID signal reception. The above 
operation is based on the fact that the NMR diagnostic apparatus does not 
perform a reception of the FID signal while the selective exciting pulse 
H1 is being applied to the patient P. As a result, precisely speaking, 
these noise components are mixed in the FID signal at the time of 
reception of the FID signal. However, because the levels of these mixed 
noise components are suppressed to be very low, the S/N ratio is 
considerably higher than that in the conventional apparatus. 
A practical circuit to which the principle of operation of the present 
invention is applied will be described in detail. 
FIG. 10 is a practical circuit diagram of the final power amplifier 16. The 
arrangement of this circuit is briefly described. A MOSFET (metal oxide 
semiconductor field effect transistor) 20 is employed as an amplifying 
element. A drain 22 of the MOSFET 20 is connected to an energizing source 
V.sub.0 through a two-stage output signal blocking circuit 24 (which 
includes choke coils and capacitors). A predetermined voltage is applied 
to the energizing source V.sub.0. The drain 22 is also connected to an 
output terminal 28 having an impedance of 50 .OMEGA. through an output 
matching circuit 26. This matching circuit 26 serves to obtain output 
matching with respect to the MOSFET 20 by properly adjusting two variable 
capacitors. 
A source 30 of the MOSFET 20 is grounded, and a gate electrode 32 thereof 
is connected to an input terminal 36 through an input matching circuit 34. 
As the operational principle of the input matching circuit 34 is the same 
as that of the output matching circuit 26, a detailed description thereof 
will be omitted. The input terminal 36 has an impedance of 50 .OMEGA.. The 
gate electrode 32 is also connected to a bias voltage input terminal 40 
through a biasing circuit 38. The biasing circuit 38 is arranged to apply 
a DC bias voltage from a junction-point between two series-connected 
resistors R1 and R2 to the gate electrode 32. The DC bias voltage is 
obtained by dividing a DC bias voltage applied from the bias control 
circuit 18 by the series-connected resistors R1 and R2. Capacitors are 
further arranged so as not to deliver to the external circuit (e.g., bias 
control circuit 18) the input signal (i.e., selective exciting pulse) 
supplied to the input terminal 36. 
In the circuit 16 described above, by changing, in accordance with the 
signals in FIG. 11 and the characteristics in FIG. 12, a DC bias voltage 
V.sub.GS applied from the bias control circuit 18 and applied to the gate 
electrode 22 of the MOSFET 20, the gain or amplification of the final 
power amplifier 16 can be varied. In other words, the operating classes (A 
to C, C to A etc.) of the MOSFET 20 can be varied. As the arrangement and 
operation of the bias control circuit 18 is known, a detailed description 
thereof will be omitted. 
More particularly, a drain current I.sub.D has the characteristic curve 
(FIG. 12) as a function of the gate-source voltage V.sub.GS i.e., the 
above-described DC bias voltage. When the gate-source voltage V.sub.GS is 
substantially zero, the drain current I.sub.D becomes substantially zero. 
The drain current I.sub.D is not responsive to disturbance noise level of 
several tens of millivolts. In this stage, the amplifier 16 is set to be a 
class "C" amplifier. Therefore, the output from the final power amplifier 
16 is set to be a level obtained by capacitance coupling of the gate-drain 
path of the MOSFET 20 and capacitance coupling caused by the layout of the 
amplifier 16. The amplification of the amplifier 16 becomes about -40 dB 
(FIG. 11D). On the other hand, because the internal noise component of the 
MOSFET 20 consists of only a leakage current (e.g., about 1 .mu.A) in the 
drain-source path, the noise component is negligible. 
When the gate-source voltage V.sub.GS is increased from 0 V to +5 V, for 
example, the bias current of the signal from the final power amplifier 16 
becomes about 4A (FIG. 12). Therefore, the operating point of the final 
power amplifier 16 changes from a class "C" amplifier to a class "A" 
amplifier. The final power amplifier 16 thus serves as a high-gain linear 
amplifier. In this case, the gain of a small signal is increased several 
tens of times as compared with that for V.sub.GS =0 (class "C" 
amplification). Although disturbance noise is also amplified, the FID 
signal need not be received during this period (time t.sub.S to t.sub.F). 
At the same time, although the internal noise is also increased during 
this period, this noise gives no influence in practice. 
The following problems are solved according to the present invention. A 
bias voltage is increased only when the selective exciting pulse is 
required to be supplied to the final power amplifier 16 during time period 
t.sub.S to t.sub.F, so that the amplifier 16 serves as a class "A" 
amplifier having high-gain linearity. During this period, although 
disturbance or external noise and the internal noise level of the output 
from the final power amplifier 16 is about several hundreds of millivolts, 
the output signal level is more than 500 V. Furthermore, during this 
period (time t.sub.S to t.sub.F), the FID signal need not be received, so 
that the external and internal noise components are negligible in 
practice. When the FID signal needs to be received and amplified after an 
application of the selective exciting pulse is stopped at time t.sub.F, 
the noise level of the output from the final power amplifier 16 serving as 
a class "C" amplifier is suppressed to a level of several microvolts. 
Therefore, the S/N ratio of the FID signal in the apparatus of the present 
invention is increased by a hundred times or more as compared to that in 
the conventional apparatus. Therefore, the apparatus of the present 
invention has an advantage in that image quality can be improved. 
While the apparatus according to the present invention has been described 
in terms of certain preferred embodiments, and exemplified with respect 
thereto, those skilled in the art will readily appreciate that various 
modifications, changes, omissions and substitutions may be made without 
departing from the spirit of the invention. 
In the above embodiments, the final power amplification changes from class 
"A" amplification to class "C" amplification. However, amplification may 
change from class "A" to "B", "B" to "C", "AB" to "C", or "AB" to "B" so 
as to improve the S/N ratio. 
In the above embodiments, the final power amplifier 16 is set by the bias 
control circuit 18 into the class "A" amplification mode during time 
period t.sub.S to t.sub.F (i.e., while the selective exciting pulse H1 is 
being generated). Moreover, before and after the selective exciting pulse 
H1 is produced, the final power amplifier 16 is set into the class "C" 
amplification mode. In other words, the amplifier 16 is operated as a 
non-linear amplifier during all periods excluding the selective exciting 
pulse generation period, thereby greatly suppressing amplification of the 
mixed noise signals. However, the amplifier 16 may be turned off during 
reception of the FID signals. In such a case, taking account of the 
spin-spin relaxation time and the spin-lattice relaxation time, this 
amplifier could be controlled under the following sequence: of class "C" 
operation (before the pulse H1 is generated), class "A" operation (while 
the pulse H1 is being generated), turn off (the FID signal collection) and 
class "C" operation (before the pulse H1 is being generated), thereby 
increasing the S/N ratio of the FID signal. 
In the above embodiments, only the bias of the final power amplifier 16 is 
variable. However, the bias of either the preamplifier 14 or the drive 
amplifier 15, or the bias of both of these amplifiers may be also variable 
to obtain a higher S/N ratio. 
The present invention may be applied not only to an NMR apparatus for 
obtaining a scanogram, but also be applied to an NMR apparatus for 
obtaining a tomograph by substituting a reconstruction device for the 
processing device.