System for improving the SIN ratio of a NMR signal by summing in phase cosine and/or sine components

In a nuclear magnetic resonance (NMR) diagnostic apparatus, the NMR signals from a portion of an object excited repeatedly by the same type excitation pulses are temporarily stored and the signal-to-noise (S/N) ratio thereof is improved by summing together at least two cos signal components, or two sin signal components of the NMR signals which were taken during these examination periods. An image processing circuit is used to process the NMR signals with impioned S/N ratio so as to obtain a tomographic image of the object.

BACKGROUND OF THE INVENTION 
This invention relates to a nuclear magnetic resonance diagnostic apparatus 
which utilizes the nuclear magnetic resonance phenomenon (referred to as 
"NMR" phenomenon hereinafter) so as to noninvasively measure information 
concerning the spin density and relaxation time of a specific atomic 
nucleus or a proton within a selected sectional slice plane of an object 
to be examined, e.g., a patient, for which a tomographic image is obtained 
with a high S/N ratio. 
First, the principle of an NMR apparatus will be summarized. 
Atomic nuclei are composed of protons and neutrons. It is generally 
considered that they are spinning as a whole like a top. In other words, 
an atomic nucleus of hydrogen (.sup.1 H) is comprised of one proton which 
is spinning in a manner indicated by spin quantum number 1/2 as shown in 
FIG. 1A. Also shown in FIG. 1B, since the proton holds a positive charge 
(e.sup.+), rotating nuclei of hydrogen may be considered equivalent to a 
current corresponding to the above positive charge flowing in a small 
coil. As a result, a magnetic moment .mu. occurs. In other words, a 
respective nucleus of hydrogen can be regarded as a very small magnet. In 
general, as schematically shown in FIG. 2A in ferromagnetic materials such 
as an iron, all of the very small magnets are oriented in the same 
direction, so that a macroscopic magnetization "37 M" can be observed. To 
the contrary, since each of the magnetic moments in the nucleus of 
hydrogen is oriented at random, the macroscopic magnetization cannot be 
observed as shown in FIG. 2B. If a static magnetic fields H.sub.O is 
applied to the nuclei, each of the nuclei is directed toward a 
magnetization direction of the field H.sub.O (,i.e., an energy level of 
the nucleus is quantized in the Z direction). This condition of the nuclei 
of hydrogen is displayed in FIG. 3A. As the nucleus of hydrogen has 1/2 
quantum number, the nuclei of hydrogen are divided into two energy levels, 
i.e., -1/2 and +1/2. Most of the divided hydrogen nuclei are oriented in 
the Z direction corresponding to +1/2 energy level. 
A difference between these two energy levels is given by formular (1); 
EQU .DELTA.E =.gamma.H.sub.O ( 1) 
where .gamma. is a gyromagnetic ratio (the ratio between the magnet and 
mechanical moments), h is a Plank's constant and is equal to h/2.pi.. 
Since the static magnetic field H.sub.O is being applied to each of the 
hydrogen nuclei so that a force indicated by .mu..times.H.sub.O is applied 
thereto, each of the hydrogen nuclei rotates around the Z axis at an 
angular velocity of .omega.=.gamma.H.sub.O (i.e. the Larmor angular 
velocity). Under these conditions of the nuclei of hydrogen when an 
electro-magnetic wave (normally a radiofrequency wave) having a frequency 
corresponding to the angular velocity .omega. is applied, a nuclear 
magnetic resonance occurs. As a result, the nuclei of hydrogen absorb an 
energy .gamma..multidot.H.sub.O which corresponds to the above-mentioned 
energy level difference (.DELTA.E), so that transition of the nuclei of 
hydrogen occurs to a higher energy level. Although there exist several 
kinds of nuclei in one object which have their own respective spin angular 
momentum, it is possible to pick up a resonance of a specific atomic 
nucleus only, because each of the nuclei has its specific gyromagnetic 
ratio .gamma., and each of them has different resonance frequency. 
Moreover if an amplitude of the resonant signal is measured, the density 
of the atomic nucleus in the object can be obtained. The nucleus which has 
been excited to the high energy level returns to the lower energy level 
after the occurence of the nuclear magnetic resonance in a period of time 
that is defined by a time constant (i.e. the so-called "relaxation time"). 
The relaxation time includes a spin-lattice relaxation time "T1" and a 
spin-spin relaxation time "T2". The spin-lattice relaxation time "T1" and 
the spin-spin relaxation time "T2' are such time constants that they are 
decided depending upon the combination of the composition of the object. 
For example, values of those relaxation times for the normal tissue are 
different from that for the malignant tumor. 
Although the above description will cover only hydrogen-1, it is obvious 
that similar measurements can be applied to other atomic nuclei having 
spin angular momentums different from that of hydrogen-1. For example in 
the normal chemical analysis, nuclei of flourine-19, of phosphorus-31 and 
carbon-13 are utilized. 
As described hereinbefore in detail, since the density and relaxation times 
of the specific atomic nucleus are measured by utilizing the NMR 
phenomenon, chemical information of this nucleus can be obtained. 
It should be noted that the NMR signals introduced in the present 
specification involve echo pulses, or echo pulse signals and also free 
induction decay signals (referred to as "FID signals" hereinafter). The 
following embodiments will involve only the echo pulse signals. 
There is known "a spin echo method" as one of measuring methods for 
utilizing these echo signals. According to this spin echo method, an 
"echo" signal of the NMR signal is measured after 2.tau. time periods by 
using 90.degree.-.tau.-180.degree. -2.pi.-180.degree.-2.pi.-180.degree. 
pulse series, ".tau." being a predetermined wait time. It is understood 
that angles of 90.degree. and 180.degree. of the applied pulses are 
determined by the following equation (2) under the strength of the applied 
magnetic field and the applied time of the pulse "tp"; 
EQU .theta.=.gamma.H.sub.1 tp[rad] (2) 
As is well known, there is a Nuclear Magnetic Resonance-Computerized 
Tomographic Apparatus (referred to as "NMR-CT apparatus") in which using 
this echo signal, a distribution of the spin density of a specific atomic 
nucleus in a certain imaginary slice of the object is processed in the a 
computer so as to reconstruct a tomographic image of the slice. According 
to a recent development in this technical field, a phase detection 
technique is newly introduced in order to utilize frequency information of 
the detected echo signals. However there are still difficulties that the 
echo signals are very weak, and random noises caused by the receiver 
channels and the object are superimposed to the above-described very weak 
echo signals, resulting in a low signal-to noise (S/N) ratio. Consequently 
there exists an extreme difficulty in that only pure signals induced by 
the NMR phenomenon are selectively detected. 
It is therefore an object of the present invention to provide an NMR 
diagnostic apparatus in which an S/N ratio of the NMR signal induced by 
the nuclear magnetic resonance phenomenon can be improved. 
SUMMARY OF THE INVENTION 
Those objects and other features of the invention may be accomplished by 
providing a nuclear magnetic resonance diagnostic apparatus comprising: 
means for applying a static magnetic field to the object under 
observation; signal transmitter means for exciting that object to generate 
a nuclear magnetic resonance (NMR) signal from a planar portion of that 
object; means for detecting those NMR signals; means for temporarily 
storing that detected NMR signal and for improving the signal-to-noise 
(S/N) ratio thereof by undertaking a summation of a plurality of signal 
components of the detected NMR signal; and means for processing the 
detected NMR signal, the S/N ratio of which is improved, so as to obtain a 
tomographic image of that portion of the object. 
In one preferred embodiment, the signal transmitter means excites an 
observed portion of the object identially for a plurality of successive 
examination periods, the detecting means detects the NMR signals from that 
portion of the object during successive examination periods, and the S/N 
ratio improving means operates such that at least one of the cosine signal 
components or one of the sine signal components which were taken during 
one of the examination periods is summed with one of the cosine signal 
components or one of the sine signal components which were taken during 
one of the examination periods, respectively. 
In an alternative preferred embodiment the signal-to-noise ratio improving 
means operates such that at least two of the cosine components or two of 
the sine signal components taken during one examination period are summed 
together with each other, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before preceeding with the various preferred embodiments, a principle of 
the NMR diagnostic apparatus and methods in accordance with the present 
invention will be explained. 
The basic idea of the NMR diagnostic apparatus and methods is as follows: 
An NMR signal particularly, an echo signal, derived from a probe head coil 
is processed in a phase detection circuit, especially a quadrature 
detector. 
After temporarily storing the detected NMR signal into adequate memory 
means, this detected signal is processed in an S/N ratio improving 
circuit. 
In the S/N ratio improvement circuit, for example, a first cosine signal 
component (referred to as "cos signal") and also a first sine signal 
component (referred to as "sin signal") of the detected NMR signals which 
were taken in a first examination period at the given slice of the object 
and temporarily stored in the memory means, are summed in phase by second 
ones which were taken in a second examination period at the same slice as 
in the first examination period and temporarily stored in the memory 
means. 
In accordance with the above-described basic idea, a first description will 
now be made of four methods for improving the S/N ratio of the NMR signal 
(the echo signal in this embodiment). 
In the normal echo signal collection by utilizing a quadrature detector 
(which will be described later), signals shown in FIGS. 4A and 4B are 
obtained during one examination period. Those signals are derived in such 
a manner that the collected NMR signals are quadrature-detected based upon 
two reference signals (which will be described later) which have the same 
frequency as the resonant frequency and their own phases different from 
each other. The signal shown in FIG. 4A is a so-called "cos signal 
component", and also the remaining signal in FIG. 4B is a so-called "sin 
signal component". An amplitude of the echo signal is represented in the 
vertical axis, and a time lapse is in the horizontal axis. Those signals 
can be obtained by, e.g. Carr-Purcell method, Meibroom-Gill method, or 
CPMG method. 
During one examination period of the echo signal collection, a selective 
exciting pulses series composed of 90.degree.-.tau.-180.degree. 
-2.tau.-180.degree.-2.tau.-180.degree. pulses are applied to the object. 
".tau." is given time interval. 
1. First Method 
As already described, when the echo pulse signals necessary for diagnostic 
purposes are acquired, the same selective exciting pulses are applied 
plural times to the same slice portion of the object (in this embodiment, 
3 times). As a result, a plurality of the echo pulse signal series are 
collected as shown in FIGS. 5A, 5B and 5C. For instance, the echo pulse 
signal series shown in FIG. 5A is obtained during a first examination 
period. 
It should be noted that since the signal waveforms shown in FIGS. 5A, 5B 
and 5C indicate the quadrature-detected signal waveforms of the NMR 
signals, one echo pulse signal series consists of, e.g., the cos signal 
components C1-1, C2-1 and C3-1, as well as the sin signal components S1-1, 
S2-1 and S3-1. 
In accordance with a first S/N ratio improvement method, the cos signal 
components of the echo signal series C1-1, C1-2 and C1-3 that were taken 
during each examination period, are summed together. That is, the echo 
signal series are acquired time-sequentially by applying the same exciting 
pulses to the same slice portion, so that three series of the echo pulse 
signals are subsequently obtained during three examination periods, C1-1, 
C2-1, C3-1; S1-1, S2-1, S3-1: C1-2, C2-2, C3-2; S1-2, S2-2, S3-2: C1-3, 
C2-3, C3-3; S1-3, S2-3, S3-3. Then the cos signal components of the echo 
signal series, e.g., (C1-1, C1-2 and C1-3) are simply summed (i.e., the 
summing number is three). As shown in FIG. 5A, suppose that random noises 
"N" are superimposed to all sin signal components S1-1, S2-1, . . ., S3-3 
and all cos signal components C1-1, C2-1, . . ., C3-3 during every 
examination period. So, as a result of the above-described summation, 
although for example, the summed cos signal components (C1-1, C1-2 and 
C1-3) become approximately three times larger than one of the cos signal 
components, the randam noises "N" are not multiplied three times. 
Consequently it can be readily understood that the S/N ratio of the echo 
signal components is improved by the summation according to this first 
method. 
Moreover not only the second cos signal components (C1-2, C2-2, C3-2) but 
also the third cos signal components (C1-3, C2-3, C3-3) are summed 
respectively in accordance with the first method. That is, for example the 
summation is carried out for the second cos signal components; C1-2, C2-2, 
and C3-2. 
It is obvious that this summation should be carried out for the sin signal 
components, e.g., S1-1, S1-2 and S1-3. To obtain a distribution 
characteristic of the relaxation time "T2" and the computerized 
tomographic images of the slice portion, both the cos signal components 
and the sin signal components are processed in the S/N ratio improvement 
circuit after performing the summation defined by the first method. 
2. Second Method 
In this method, all cos signal components, or all sin signal components of 
the echo signal series are summed, e.g., C1-1, C2-1, C3-1 or S1-1, S2-1, 
S3-1 which are collected during the same examination period. That is, for 
instant, the cos signal components C1-1, C2-1 and C3-1 taken during one 
examination period have different amplitudes from each other. Those signal 
components are merely summed. 
The above-described S/N ratio improving method will be explained with 
referenct to a flow chart shown in FIG. 6. 
As an initial preparation, a variable "i" is cleared so as to recognize the 
first echo signal, and "1" is set as an initial value, supposing that the 
cos signal components of the echo signal shown in FIG. 5A are "Cn". 
Thereafter the following sequential operation is carried out: 
Increment the variable "i" by 1. 
Secondly judge whether the cos signal peak is negative or not. If it is 
negative, invert the polarity of the cos signal. There is a case that the 
polarity of the every two echo signals is changed, depending on the phase 
of the 180.degree. pulse carrier wave. 
Thirdly judge whether the variable is equal to "1" or not (namely, judge 
whether the present cos signal component is the first one or not). If yes, 
then clear the buffer memory (not shown) for the cos signal components of 
the echo signal. If no, sum the present cos signal component by the cos 
signal component stored in the buffer memory. Then, compare the variable 
"37 i" with the number of the echo signal component ("3" in the condition 
shown in FIG. 5A). If the number is greater than the variable "i", the 
operation is still continued from the increment step with an increment of 
the variable "i". When the incremented variable "i" becomes greater than 
the number of the cos signal component, this processing flow is 
accomplished. Thereafter the results of the processed signal component are 
subjected to an examination of the spin density or an arithmetic 
processing of the relaxation time. 
Furthermore, this processing flow may be applied to not only the remaining 
cos signal components C1-2, C1-3 etc., but also the sin signal components 
S1-2, S1-3 etc. so as improve the S/N ratio thereof. 
3. Third Method 
In this method, the echo signal series as shown in FIGS. 5A, 5B and 5C are 
collected and stored in the adequate memory in advance the same as in the 
previous methods. For example if each one of the echo signal series is 
considered, e.g., the cos signal component C2-1 and the sin signal 
component S2-1 as shown in FIG. 7A and 7B respectively, the former 
component C2-1 has a line-symmetrical relation with respect to a solid 
line "X", and the latter component S2-1 has a point-symmetrical relation 
with respect to a point "Y". According to the third method, waveform 
portions of the cos signal component C2-1 which are composed by a front 
waveform portion C2-1-f and a rear waveform portion C2-1-r are summed with 
maintaining the above-described line-symmetrical relation. In other words, 
the front portion C2-1-f is added to the rear portion C2-1-r by folding 
the front portion over the rear one with respect to the symmetrical line 
X, as it were. Thereafter the summed waveform portion is averaged. As a 
result, the noise component contained in the cos signal component C2-1 can 
be relatively reduced and also the S/N ratio can be improved. 
The similar summing method is also applied to the sin signal component 
S2-1. However there is one different point that due to the 
point-symmetrical relation, the polarity of either waveform portio S2-1-f 
or S2-1-r should be inverted before the summation. 
A flow chart shown in FIG. 8 describes the third method: 
First, judge whether the input echo signal corresponds to the cos signal 
component, e.g., C2-1 or not. If yes, sum the front portion C2-1-f by the 
rear portion C2-1-r and average the results of the summation. If no, judge 
whether the input echo signal corresponds to the sin signal component, 
e.g., S2-1 or not. 
Secondly if it is the sin signal component S2-1, add the front portion 
S2-1-f to the rear portion S2-1-r having first inverted the polarity of 
the rear portion or the front portion, and then take an average of the 
summation. 
The process flow is accomplished. 
4. Fourth Method 
The fourth method is one to combine more than two methods which were 
previously described. That is, there are four combinations that the first 
and second methods, the second and third methods, the third and first 
methods, and the first, second and third methods. 
FIG. 9 shows a circuit diagram of an NMR diagnostic apparatus according to 
one preferred embodiment in which one of the above-described S/N ratio 
improving method is employed. 
The NMR diagnostic apparatus 100 is comprised of the following components. 
The object 1 such as a patient is arranged in a static magnetic field 
H.sub.O that is produced by an electromagnet (not shown), and 
simultaneously in a transmitter/receiver coil 2 which is positioned in 
such a manner that a magnetic field produced by the trasmitter/receiver 
coil 2 intersects with the static magnetic field H.sub.O at a right angle. 
A tuner 3 is connected to the transmitter/receiver coil 2 and has the 
following function to select an electromagnetic wave having a specific 
frequency from the electromagnetic waves generated by a transmitter 4 and 
to be tuned to a specific nuclei, e.g., hydrogen-1 in the object 1 by 
applying the selective exciting pulse (corresponding to the aforementioned 
electromagnetic wave having the specific frequency) to the 
transmitter/receiver coil 2. The transmitter 4 is constructed by a 
standard signal generator 4A (referred to as "SSG") and an RF 
(radio-frequency) power amplifier 4B. The RF signal containing the 
selective exciting RF pulse signal having e.g., a frequency of 4,258 MHz 
is generated from the SSG 4A and then amplified in the RF power amplifier 
4B to a given power. Precisely speaking, the above-mentioned RF pulse is 
produced in such a manner that the standard signal generated by SSG 4A is 
frequency-modulated in a frequency modulator (not shown) by pulse signals 
from a pulse generator (not shown). Accordingly those frequency modulator 
and pulse generator constitute an RF pulse generator. On the other hand, 
this RF signal is applied to a reference signal generator 7 (will be 
described later) as a reference signal. 
Then a description will now be made of a receiving system. 
A preamplifier 5 is connected to the tuner 3 so as to amplify the NMR 
signals (the echo pulse signals in this embodiment) which are received 
through the transmitter/receiver coil 2, and thereafter to apply the 
amplified echo pulse signals to two phase detectors 6A and 6B 
respectively. Those phase detectors 6A, 6B are designed to operate as the 
quadrature detector. A reference signal generator 7 is comprised of a 
phase shifter 7A and a 90.degree. phase shifter 7B so as to generate two 
reference signals whose phases are different from each other at 90.degree. 
and which have the same frequency as that of the echo signals. Those 
reference signals are applied to the phase detectors 6A and 6B 
respectively. Accordingly since the echo signal which has been amplified 
in the preamplifier 5 is separatedly supplied to those phase detectors 6A 
and 6B, the echo signal is quadrature-detected therein based upon the two 
reference signals. Thus two detected signals in an analogue form are 
independently amplified in amplifiers 8A and 8B, and thereafter filtered 
in low pass filters 9A and 9B so as to eliminate the RF signal components 
therefrom. Those filtered signal waveforms are represented in FIGS. 5A, 5B 
and 5C. The filtered analogue signals are converted by A/D converters 10A 
and 10B into corresponding digital signals. Those digitalized signals are 
input in an S/N ratio improvement circuit 11. This circuit 11 has mainly 
such two functions that the digitalized signals are temporarily stored, 
and also one of the S/N ratio improving methods as previously described is 
carried out therein. Subsequently two signals which have been improved 
with respect to their S/N ratios are supplied to an image processing 
circuit 12 wherein the spin density and the relaxation time are 
calculated. Consequently diagnostic information on the object 1 by the 
nuclear magnetic resonance phenomenon can be obtained which may be 
displayed on a monitor 13. 
The embodiments just described will now be summarized. 
That is, the echo signal series are quadrature-detected with two reference 
signals which have the frequency identical to the resonant frequency of 
the NMR signal, and whose phases are different from each other at 90 
degrees. Then those quadrature-detected signals are converted into 
respective digital signals, S/N ratios of which may be improved in the 
following stage. If those improvd signals are utilized in the NMR 
diagnostic apparatus, it can be obtained extremely high quality CT images 
of the object. 
While the invention has been described in terms of certain preferred 
embodiments, and examplified with respect thereto, those skilled in the 
art will readily appreciate that various modifications, changes, omissions 
may be conceived by those skilled in the art. 
First, in the previous embodiments, the quadrature detection circuit (7, 6A 
and 6B) was employed. As the invention is not limited to this type of the 
detection circuit, the normal phase detection circuit can be employed. In 
this case, one signal detection processing path of the detector 6A -- the 
amplifier 8A -- low pass filter 9 -- A/D converter 10A, or the detector 6B 
-- the amplifier 8B -- low pass filter 9B -- A/D converter 10B can be 
omitted. 
Secondly, an S/N ratio of the echo pulse signals was improved according to 
the above-described four methods after performing one of four methods in 
the S/N improvement circuit 11. It is also possible that the detected NMR 
signals in an analogue form are directly processed in the S/N ratio 
improvement circuit 11 in which an analogue memory such as a video tape 
recorder, or a video disc may be utilized. 
In the previous embodiments there are employed the selective exciting 
pulses such as 90.degree.-.tau.-180.degree. pulses. It is however, 
possible to introduce alternatively the normal exciting pulses to excite 
the slice of the object.