MR examining apparatus of heart beat synchronous type

A magnetic resonance examining apparatus of heart beat synchronous type in which one heart beat period is substantially equally divided to provide a plurality of phases, and at least some of those phases are selected as imaging phases so as to obtain nuclear magnetic resonance images. In the apparatus, spin excitation is effected at all the phases including those where nuclear magnetic resonance images are obtained and those where nuclear magnetic resonance images are not obtained, so that all the phases have the same longitudinal relaxation time after spin excitation. Therefore, nuclear magnetic resonance signals free from variations of the longitudinal relaxation time can be obtained to provide clear nuclear magnetic resonance images at anyone of the plurality of phases.

BACKGROUND OF THE INVENTION 
This invention relates to a magnetic resonance examining apparatus of heart 
beat synchronous type, and more particularly to an apparatus of the kind 
described above in which a nuclear magnetic resonance signal (which will 
be referred to hereinafter as an NMR signal) is obtained in synchronism 
with a heart beat of a subject to be examined, and a nuclear magnetic 
resonance image (which will be referred to hereinafter as an NMR image) is 
formed on the basis of the NMR signal. 
An NMR image is formed from an NMR signal obtained in each of a plurality 
of times of spin excitation in a subject being examined. A clear NMR image 
cannot be obtained because the position where an NMR signal is generated 
in each time of spin excitation differs depending on parts of the heart 
moving due to a heart beat. Therefore, it has been a common practice to 
obtain an NMR signal by effecting spin excitation in synchronism with a 
heart beat. 
In the art of imaging by scanning in synchronism with a heart beat, an 
electrocardiograph is usually used as a synchronous detector. FIG. 1 shows 
a general heart beat waveform recorded on the electrocardiograph. In FIG. 
1, the period of the P and Q waves corresponds to a period 17 of 
contraction of the atrium, the period of the QRST waves corresponds to a 
period 18 of contraction of the ventricle, and the period between the end 
of the T wave and the beginning of the P wave corresponds to an expansion 
period 19. 
The R wave has a highest peak, and this peak is usually used as a trigger 
for starting a signal read sequence of examination. 
However, according to such a manner of scan imaging, spin excitation is 
effected only once for each heart beat which takes a period of time of 
about 0.8 to 1.3 sec, and the resultant data is read or fetched. 
Therefore, a method called a multiphase imaging method well known in the 
art from the disclosure of, for example, "Journal of NMR Medicine", Vol. 
6, September, 1986, Page 119 is now employed. According to this method, 
the R wave of a heart beat waveform is used as a trigger, and, after a 
predetermined time from this R wave, spin excitation is repeatedly 
effected a plurality of times at intervals of a predetermined repetition 
time A from the delay time as shown in FIG. 2 so as to obtain a plurality 
of NMR signals. Thus, an NMR image, that is, a phase image is obtained at 
each phase of spin excitation. 
In the method of multiphase imaging described above, spin excitation is 
effected at the interval of the repetition time A at each phase where the 
NMR image is obtained. However, no spin excitation is effected at the 
phase where the NMR image need not be obtained. Therefore, the value of a 
repetition time B, in which no NMR image is obtained, differs from that of 
the spin-excitation repetition time A, and this means that the spin 
excitation is not continuously effected at a constant period. As a result, 
the state of recovery of longitudinal relaxation after the spin excitation 
at each phase is not uniform, and the NMR signals generated from the 
subject at individual phases will differ from each other, resulting in 
different contrasts of the NMR images obtained at individual phases. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a magnetic resonance 
(MR) examining apparatus of heart beat synchronous type which is used to 
obtain multiphase images in synchronism with the heart beat of a subject 
being examined and in which the state of recovery of longitudinal 
relaxation after spin excitation can be made uniform at individual phases 
so that clear NMR images having a good contrast can be formed. 
According to the MR examining apparatus of heart beat synchronous type of 
the present invention, the length of time of one heart beat period is 
substantially equally divided, and spin excitation is effected at both of 
a phase where an NMR image is obtained and a phase where no NMR image is 
obtained thereby making uniform the state of recovery of the longitudinal 
relaxation after the spin excitation at individual phases, so that NMR 
signals having uniform conditions can be obtained when the NMR images at 
selected ones of all the phases are to be formed. 
According to one aspect of the apparatus of the present invention, when the 
period of spin excitation is preselected, an integer closest to the value 
obtained by dividing one heart beat period by the period of spin 
excitation is found, and the heart beat period is divided by this integer 
to determine the period of spin excitation. Then, the number of phase 
images to be obtained and the number of imaging phases are determined, and 
NMR images are measured. 
According to another aspect of the apparatus of the present invention, when 
the number of phase images is previously selected, one heart beat period 
is divided by the number of phase images to determine the period of spin 
excitation. Then, the number of phase images and the number of imaging 
phases are determined, and NMR images are measured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the apparatus according to the present invention 
will now be described in detail with reference to FIGS. 3 to 6. 
FIG. 3 is a block diagram of an MR imaging system to which the apparatus 
according to the present invention is applied. Referring to FIG. 3, a 
superconducting magnet 1 generates a constant static magnetic field. An 
electrode 11 for sensing the heart action is mounted on the chest of the 
body of a subject 10 placed in the internal space of the superconducting 
magnet 1 and picks up an electrical signal representing the electromotive 
force generated from the heart muscle of the subject 10. This electrical 
signal will be referred to hereinafter as a heart beat signal. The heart 
beat signal is transmitted at a radio frequency from a transmitter 12 to a 
receiver 13 connected at its output to an amplifier 15 which amplifies the 
heart beat signal. The radio frequency used for transmission is about 150 
MHz. The waveform of the heart beat signal received at the receiver 13 is 
displayed on a heart beat waveform monitor 14. The output signal of the 
amplifier 15 is applied to a counter circuit 16. 
The operation of the counter circuit 16 will be described in detail with 
reference to FIGS. 4, 5 and 6. FIG. 4 is a block diagram showing the 
detailed structure of the counter circuit 16, FIG. 5 shows waveforms 
appearing at principle parts of the counter circuit 16, and FIG. 6 is a 
flow chart of processing by a computer 7 shown in FIG. 3. Waveforms of 
output signals of principal parts of the counter circuit 16 shown in FIG. 
4 are designated by symbols A to L and shown in FIG. 5. 
The heart beat signal A amplified by the amplifier 15 shown in FIG. 3 is 
differentiated by a differentiation circuit 20 shown in FIG. 4, and a 
differentiated signal B having a waveform as shown in FIG. 5 appears from 
the differentiation circuit 20. This differentiated waveform signal B is 
applied to analog comparators 21 and 22 to be compared with threshold 
levels V.sub.TH.sup.- and V.sub.TH.sup.+ respectively. As a result, output 
signals C and D having waveforms as shown in FIG. 5 appear from the 
comparators 21 and 22 and represent the negative and positive waveform 
portions respectively of the output signal B of the differentiation 
circuit 20. The heart beat signal A is also applied to another comparator 
23 to be compared with a threshold level V.sub.TH, and an output signal E 
having a waveform as shown in FIG. 5 appears from the comparator 23. 
The signals C and E are applied to an AND gate 24, while the signals D and 
E are applied to another AND gate 25, and output signals F and G having 
waveforms as shown in FIG. 5 are applied from the AND gates 24 and 25 to 
monostable multivibrators (MM's) 26 and 27 respectively. A flip-flop (FF) 
circuit 28 connected to the MM's 26 and 27 generates an output signal I 
which rises in response to the leading edge of the signal F and falls in 
response to the trailing edge of the signal G as shown in FIG. 5. The 
signal I is applied to an AND gate 30. This signal I has a pulse width 
corresponding to the period of time between an R wave and the next R wave 
in the heart beat signal A. Therefore, clock pulses generated from a clock 
generator 29 are permitted to pass through the AND gate 30 during a period 
of time which is substantially equal to the length of time of one period 
of the heart beat signal A. A counter 31 counts the number of clock pulses 
J passed through the AND gate 30, and, after the count N is latched in a 
latch circuit 32. 
Suppose, for example, that the frequency of the clock signal generated from 
the clock generator 29 is 2 kHz. Then, when the time interval between the 
consecutive R waves is 1 sec, the number of clock pulses counted between 
the R waves is 2,000+1, provided that a measurement error of 0.5 msec 
occurs. As soon as the counting operation counting the number of clock 
pulses between the R waves is completed, an MM 33 is triggered by the 
trailing edge of the gate signal I applied from the FF circuit 28, and an 
output signal K having a waveform as shown in FIG. 5 appears from the MM 
33. As soon as the count N of the counter 31 is latched in the latch 
circuit 32 in response to the application of the signal K, a flag signal L 
having a waveform as shown in FIG. 5 appears from an FF circuit 34 to be 
applied to the computer 7. When this flag signal L is detected, the 
computer 7 reads the count N latched in the latch circuit 32. After the 
computer 7 reads the count N, the computer 7 applies a reset signal 
R.sub.D to the FF circuit 34 through a NOR gate 35, with the result that 
the FF circuit 34 generating the flag signal L is now reset. The other 
input terminal of the NOR gate 35 is used to reset the FF circuit 34 by a 
reset signal generated when the system power supply is turned on. The FF 
circuit 34 generates an output pulse signal H having a waveform inverted 
relative to that of the signal L as shown in FIG. 5. Therefore, when the 
computer 7 cannot read the count N latched in the latch circuit 32 until 
the next heart beat period is started, the signal H acts to prevent the 
counter 31 from starting to count the clock pulses, so that an incomplete 
NMR signal may not be measured. 
The operation of the computer 7 will now be described with reference to the 
flow chart of FIG. 6. As soon as the flow is started, decision is made in 
a step 36 as to whether or not the flag signal L generated from the FF 
circuit 34 is in its "H" level. When the result of decision in the step 36 
proves that the flag signal L is in its "H" level, the count N latched in 
the latch circuit 32 is read out in a step 37. When the computer 7 reads 
the count N, decision dicision is made in a step 38 as to whether data of 
a repetition time T.sub.R or the number of required phase images is 
previously set as an input in an input unit 18 shown in FIG. 3. The term 
"repetition time T.sub.R " is used herein to indicate the case where the 
operator of the computer 7 previously sets the period T.sub.R of spin 
excitation. Also, the term "number of required phase images" is used 
herein to indicate the case where the operator of the computer 7 
previously sets the number of phase images required for imaging. When the 
result of decision in the step 38 proves that the repetition time T.sub.R 
is the input previously set by the operator, the count N is divided by the 
repetition time T.sub.R in a step 40 to compute the number of phase images 
that can be imaged, and, in a step 41, decision is made as to whether or 
not the computed number of phase images is larger than the previously-set 
number (the setting) of phase images to be imaged. When the result of 
decision in the step 41 proves that the setting is larger than the 
computed value, an error code and an input re-setting instruction are 
displayed in a step 43 on a display unit 8. On the other hand, when the 
setting of the number of phase images is smaller than or equal to the 
computed value, the repetition time T.sub.4 is corrected in a step 44 
until the number of phase images computed in the step 40 becomes an 
integer. That is, the value obtained by the computation in the step 40 is 
rounded to the nearest integer, and the count N is now divided by the 
imager to use the result of division as the timing or period of spin 
excitation. 
Then, in a step 45, the number of phase images previously selected as the 
input is set together with the number of phases required for imaging, and, 
in a step 46, an instruction is registered which instructs that spin 
excitation only is to be effected at phases other than the predetermined 
imaging phases, and no data are to be acquired at such phases. Then, in a 
step 47, a predetermined sequence of NMR measurement is executed to 
measure NMR signals. 
On the other hand, when the result of decision in the step 38 proves that 
the number of required phase images is the input previously set in the 
input unit 18, the count N is divided by the number of required phase 
images in a step 39 so as to determine the repetition time T.sub.R. 
Subsequently, the steps 44 to 47 are similarly executed to complete the 
measurement of NMR signals. 
The sequence of NMR measurement will now be described. 
After the aforementioned conditions are set in the steps 44 to 46, a 
high-frequency magnetic field generator 4, a gradient magnetic field power 
supply 3 and a gradient magnetic field coil 2 are controlled by the 
computer 7 in synchronism with the trigger signal G and according to a 
data read or fetch sequence well known in the art. As a result, a 
high-frequency receiver coil 9 effects spin excitation at the desired 
tomographic section of the heart of the subject 10. An echo signal 
generated due to the spin excitation is received by a receiver 5 and 
supplied to a data read or fetch unit 6 under control of the computer 7. 
For each of individual echo signals supplied to the data fetch part 6, a 
suitable number of integrations and Fourier transformations are carried 
out for reconstructing the image. The reconstructed image is displayed on 
the display unit 8. 
A method called a Cine-imaging method is now being increasingly employed in 
this field of art. According to this method, ten to twenty images at 
different phases between the R waves of one heart beat waveform are taken 
utilizing the method of high-speed imaging under the condition that the 
repetition time T.sub.4 is T.sub.4 &lt;T.sub.1, where T.sub.1 is the 
longitudinal relaxation time, and such images are continuously 
cinematically displayed to display the motion of the heart of a subject. 
When this cine-imaging method is utilized with the imaging apparatus 
according to the present invention, the combination can exhibit an 
especially marked effect. 
According to the present invention, a trigger signal is divided in 
synchronism with the R wave of the heart beat waveform and applied to open 
a gate during the length of time of one heart beat between the R wave and 
the next R wave. A counter counts the number of pulses spaced by a 
constant time interval, that is, the number of reference clock pulses 
permitted to pass through the gate during the above length of time. On the 
basis of the count of the counter and the repetition time of spin 
excitation or the number of phase images previously supplied as an input, 
a computer computes the optimum repetition time of spin excitation and the 
optimum number of phase images while taking into account the longitudinal 
relaxation time required after the spin excitation, so as to excite the 
spin and obtain resultant NMR signals. Therefore, clear multiphase images 
of the heart having a good contrast can be obtained