Method for determining flowing matter by means of NMR tomography

NMR tomography permits identification of flowing matter within a closed body, as there is a reduction in signal intensity as a consequence of a saturation of the magnetization in the area of a slice that has been excited for imaging, while freshly inflowing matter has not experienced this saturation and consequently provides a stronger signal. Special excitation sequences permit either virtually complete saturation of stationary tissue, and thus particularly strong emphasis of flowing matter, or excitation of stationary and flowing matter in different zones and with differing intensities, thereby enabling very precise quantitative and even directional flow measurements to be performed.

The present invention relates to a method for determining flowing matter 
within a self-contained body by means of NMR tomography, in which said 
body is subjected to selected magnetic fields and the spins which are 
present in the area of a cross-sectional plane of said body that is 
defined by a slice gradient are excited by sequences of excitation pulses, 
whose spacing is shorter than the spin-lattice relaxation time, thereby 
producing at least a partial saturation of the magnetization during the 
scanning period, which saturation in turn reduces the intensity of the 
resonance signals, and in which said resonance signals are processed in 
such a manner as to produce an image of said cross-sectional plane of said 
body in which those locations in which matter is present which has flowed 
into said cross-sectional plane from outside and is therefore less 
saturated are emphasized relative to their surroundings through their 
greater brightness as a result of the greater intensity of said resonance 
signals. 
A method of this type is known from "Investigative Radiology", No. 17, 
1982, pages 554 through 560. The known method is intended primarily to 
identify clogged blood vessels in the human body. While healthy blood 
vessels can be identified by the high signal intensity that they produce 
as a result of the flow of blood therein, clogging causes a decline in the 
flow velocity and/or constriction of the vessel, which can be identified 
by means of the correspondingly darker areas. 
Moreover, determination of the flow velocity of blood in the human body 
through observation of the time that is required to achieve the maximum 
intensity after excitation, i.e. for complete substitution of the blood in 
the observed zone, is known from "Science", No. 221, 1983, pages 654 
through 656. 
The optimum application of a method of this nature for various tasks 
necessitates optimation of various parameters. If the intent is to 
identify very narrow blood vessels and/or blood vessels with low flow 
rates, it is important to produce the greatest possible contrast. On the 
other hand, the most precise possible determination of the volume of 
liquid that flows through a vessel during a given unit of time, and not 
optimum contrast, is the important aspect in identifying flow velocity. 
Moreover, it is also necessary to identify the direction of flow of the 
matter. 
It is therefore the object of the present invention to further develop a 
method of the type described at the outset in such a manner as to permit 
any of the various requirements to be satisfied, as may be needed in the 
individual instance. 
According to the present invention., this object is solved in that between 
successive RF excitation pulses a further slice gradient is applied =nd 
said body is irradiated with at least one additional RF saturation pulse, 
the time intervals between said RF excitation pulses and said RF 
saturation pulses and/or the flip angles that are produced by said pulses, 
being appropriately selected to produce a stationary state of minimal 
magnetization in at least one zone defined by said slice gradients. 
The employment of additional RF saturation pulses in conjunction with a 
slice gradient of their own permits differing operating states to be 
produced through selection of the chronological intervals, the flip 
angles, as well as the location of the excitation, with these various 
operating states being able to be optimized for the individual 
applications. Thus, in particular, it is possible to bring those zones 
which are excited by the RF excitation pulses and those which are excited 
by the RF saturation pulses into coincidence so as to produce minimum 
magnetization in the entire excited zone, as a result of which the body's 
stationary matter supplies virtually no further signal, thus permitting 
even very minute portions of flowing matter to be properly identified. 
However it is also possible for the zone that is excited by the RF 
saturation pulses to have a greater extent perpendicular to the 
cross-sectional plane of the body than the zone that is covered by the RF 
excitation pulses, so that the zone which is excited by the RF saturation 
pulses includes the zone which is excited by the RF excitation pulses. In 
this case, too, it is again possible to produce virtually complete 
suppression of the signal of the stationary matter and, in particular, of 
the stationary body tissue, in the zone that is covered by the RF 
excitation pulses, while only partial saturation is produced in zones 
adjacent thereto. Setting the thickness of these zones permits volume 
elements to be marked and thus highly precise measurement of the flow 
velocity. If, in addition, it is ensured that a border of the zone which 
is excited by the RF excitation pulses is in coincidence with a border of 
the zone which is excited by the RF saturation pulses, the directional 
flow can also be identified, as matter that is still completely 
undisturbed will enter the observation zone that is defined by the RF 
excitation pulses from the one side, while matter that has been partially 
saturated by means of the RF saturation pulses will enter this zone from 
the other. 
When the method according to the present invention is employed, it is 
preferable to utilize sequences in which the slice gradient that is 
activated for the duration of the RF excitation pulse is reversed at the 
end of each RF excitation pulse in order to rephase the excited spins. 
Moreover, a read gradient and, possibly, a phase gradient is activated 
after every RF excitation pulse for the duration of the reversal of the 
slice gradient. The read gradient is then reversed in order to produce a 
spin echo signal, with one RF saturation pulse being irradiated between 
every two successive RF excitation pulses. As already recited above, a 
slice gradient will then again be present, which, however, need not be the 
same as the slice gradient that was activated for the duration of the RF 
excitation pulses. 
The above discussed and other objects, features and advantages of the 
present invention will become more apparent from the following description 
thereof, when taken in connection with the practical examples shown in the 
accompanying drawings, in which

Referring now to the drawings, where like reference numerals designate like 
parts throughout the several views, it will be seen that, in the case of 
the excitation sequence that is illustrated in FIG. 1, the specimen is 
first excited by means of an RF excitation pulse .phi..sub.1 in the 
presence of a slice gradient G.sub.S. Following the end of the RF 
excitation pulse, the dephasing of the magnetization that occurred under 
the slice gradient is corrected through inversion of slice gradient 
G.sub.S. During inverted slice gradient G.sub.S, the specimen is subjected 
to an inverted read gradient G.sub.R, whose reversal at the end of slice 
gradient G.sub.S causes a spin echo signal to be produced. The variation 
of the echo signal that is obtained under read gradient G.sub.R, which is 
required for image generation, is achieved through the employment of a 
phase gradient G.sub.P, which differs from scanning cycle to scanning 
cycle and which is applied whenever slice gradient G.sub.S and read 
gradient G.sub.R are inverted. Following a period of time t.sub.a, the 
spin system is then subjected to an RF saturation pulse .phi..sub.2, with 
the specimen also being subjected to a slice gradient G.sub.S for the 
duration thereof, although slice gradient G.sub.S need not necessarily 
coincide with the slice gradient that exists during RF excitation pulse 
.phi..sub.1. Following a further waiting period t.sub.b, during which a 
gradient is applied in order to destroy the transversal magnetization 
coherence that was generated by RF saturation pulse .phi..sub.2, the 
sequence is repeated. This gradient can simply be the slice gradient for 
RF saturation pulse .phi..sub.2. After this sequence has been repeated 
only a few times, the system attains a stationary state, in which the 
magnetization in the excited zone is at a minimum. 
In the case of a single excitation, the following will apply for the 
chronological curve of z-magnetization M, which has a value of M.sub.a at 
time t=0 and whose equilibrium value is equal to M.sub.O : 
EQU M=M.sub.a exp(-t/T.sub.1)+M.sub.0 [1-exp(-t/T.sub.1)] (1) 
Moreover, the following will apply if z-magnetization M.sub.a is rotated 
about flip angle .PSI. by means of an RF pulse: 
EQU M.sub.p =M.sub.a .times.cos.PSI. (2) 
If the sequence that has been explained on the basis of FIG. 1 is employed 
repeatedly, one time after another, the following value will be obtained 
for stationary state magnetization M.sub.SS directly prior to RF 
excitation pulse .phi..sub.1 : 
##EQU1## 
In this equation, .alpha. is the flip angle of RF excitation pulse 
.phi..sub.1, while .beta. is the flip angle of RF saturation pulse 
.phi..sub.2 . 
Maximum signal suppression is achieved when M.sub.SS is as small as 
possible. Several special cases will now be investigated in which M.sub.SS 
assumes a low value, i.e. the residual magnetization is, as is desired, of 
only small magnitude. 
If RF excitation pulse .phi..sub.1 and RF saturation pulse .phi..sub.2 are 
selected in such a manner that their flip angles .alpha.=90.degree. and 
.beta.=180.degree. , respectively, cos .alpha.=0 and cos .beta.=-1. If 
these values are employed in equation (3), the following is obtained: 
EQU M.sub.SS =M.sub.0 {1-2exp(-t.sub.b ; /T.sub.1)+exp[-(t.sub.a 
+t.sub.b)/T.sub.1 ]} (4) 
For a substance having a long spin-lattice relaxation time T.sub.1, it will 
be found that M.sub.SS =0 when t.sub.a =t.sub.b to the first 
approximation. The magnetization curve for long relaxation times T.sub.1 
is shown by curve 1 in FIG. 2. As shown by curves 2 and 3, M.sub.SS will 
still vary considerably from zero for medium-length and short relaxation 
times T.sub.1. 
Human body tissue has areas that display greatly differing relaxation times 
T.sub.1. In this case, it is therefore not always the most advantageous 
approach to make t.sub.a and t.sub.b identical in order to suppress 
z-magnetization, i.e. to locate RF selection pulse .phi..sub.2 exactly 
midway between two successive RF excitation pulses .phi..sub.1. On the 
contrary, it is then practical to set t.sub.a and t.sub.b to a minimum 
stationary signal. 
Another possibility is to set the stationary state magnetization by 
adjusting the flip angle with given pulse intervals t.sub.a and t.sub.b. 
If, for example, flip angle .alpha. of RF excitation pulse .phi..sub.1 is 
set to 90.degree., it is possible to calculate that flip angle .beta. of 
the RF saturation pulse for which M.sub.SS =0. From equation (3): 
##EQU2## 
Here, too, flip angle .beta. for complete signal suppression depends upon 
relaxation time T.sub.1, which is not, however, uniform for the entire 
specimen; consequently, through trial and error, it is necessary to find 
the value of .beta. at which maximum signal suppression is achieved with 
given t.sub.a and t.sub.b. 
Until now, it had been assumed that, in a sequence according to FIG. 1, the 
RF excitation pulses and RF saturation pulses were selected, in 
conjunction with the corresponding slice gradients, in such a manner that 
precisely the same slice of the body was scanned by both. The 
above-described adjustment of t.sub.a and t.sub.b, and thus.sub..beta., 
causes the stationary matter that is located in the area of the excited 
cross-sectional plane to supply only a very insignificant portion of the 
signal. FIG. 3 shows the state in which a slice 11 has been excited in the 
abovedescribed manner in such a way as to produce minimum magnetization, 
which means that the stationary matter supplies virtually no signal 
whatever. Slice 11 is penetrated by a vessel 12 containing a flowing 
liquid, such as blood, for example. The volume element 13, which was 
initially located in the area of slice 11, is carried out of the area of 
slice 11 by the flow in vessel 12 and is replaced by liquid which had not 
been subjected to the saturation process. Consequently, this liquid 
provides a strong signal, thereby permitting vessel 12 to be identified 
with great clarity in a cross-sectional image which reproduces slice 11 of 
the body. 
Through appropriate selection of the frequency of RF saturation pulse 
.phi..sub.2 and the strength of slice gradient G.sub.S, it is possible to 
design the zones that are scanned by the RF excitation pulse,. differently 
than those that are scanned by the RF saturation pulse. In the case which 
is illustrated in FIG. 4, RF excitation pulse .phi..sub.1 excites the 
spins in the area of a very thin slice 21, while RF saturation pulse 
.phi..sub.2 acts in the area of a much thicker slice 22. Slice 21, which 
was excited by the excitation pulse, is arranged centrally relative to 
thicker slice 22. The resonance signals that are employed for imaging 
purposes always come from slice 21, which is excited by means of RF 
excitation pulse .phi..sub.1. 
In this case, the following is obtained for a stationary magnetization 
state: 
EQU M.sub.SS =M.sub.0 [1-exp(-(t.sub.a +t.sub.b)]/[1- cos .beta.exp(-(t.sub.a 
+t.sub.b)/T.sub.1)] (6) 
In this case, stationary state magnetization M.sub.SS will be very small 
relative to the equilibrium value M.sub.0 of the magnetization, i.e. the 
stationary state magnetization will tend toward 0 when t.sub.a +t.sub.b is 
small relative to T.sub.1. In this case, it will not be the stationary 
tissue in the area of slice 21 that is represented by weakened signals, 
but all of the matter that is located in thicker slice 22. Consequently, 
the matter that is flowing toward central slice 21 will also initially 
have a weaker signal, until the matter that is located outside of slice 22 
reaches central slice 21. By varying the thickness of slice 22, this will 
permit quantitative determination to be made of the flow velocity. FIG. 5 
illustrates how the signal strength I initially increases when matter 
flows from slice 22 to the area of slice 21 (portion 23 of the curve). The 
somewhat stronger signal continues for as long as matter from slice 22 is 
located in slice 21. When matter from outside of slice 22 then reaches 
slice 21, there is a very strong signal increase in portion 24 of the 
curve according to FIG. 5, until the signal remains constant when only 
matter that originated outside of slice 22 is located in slice 21. 
FIG. 6 illustrates an alternative in which a volume 32 which was identified 
by means of RF saturation pulse .phi..sub.2 is not arranged concentrically 
relative to a volume 31, which was identified by RF excitation pulse 
.phi..sub.1, but in such a manner in that, while each of the two volumes 
is covered one by the other, volume 32 protrudes over volume 31 on one 
side only. In this manner, greatly differing signals are created, 
depending upon whether new matter flows into slice 31 from a zone 33, 
which was not excited at all, or from thicker slice 32. The shape of the 
signal curve of that portion of an upper vessel 34 which flows in from 
slice 32 is identical to the shape of the curve of the signal according to 
FIG. 5 and is shown in FIG. 7 by curve 35. As shown by curve 37 in FIG. 7, 
on the other hand, there is an immediate, steep increase in the signal 
strength I for volumes which flow into slice 31 from excitation-free zone 
33, as is the case in a lower vessel 36 in FIG. 6. 
And, finally, the case will now also be studied in which the two zones 41, 
42 which are excited by means of RF excitation pulse .phi..sub.1 and by 
means of RF saturation pulse .phi..sub.2 are completely isolated one from 
the other, i.e. in which a non-excited slice 43 is located therebetween, 
as illustrated in FIG. 8. 
Again, the shape of the signal curve is represented by the curve that is 
shown in FIG. 9. The increase 45 of signal strength I represents the 
inflow of matter from a non-excited slice 43 into the area of an excited 
slice 41. The matter from a less strongly excited slice 42 then reaches 
slice 41, thus producing a decline in the signal, following by an increase 
again, as represented by the trapezoidal-shaped portion of the curve 46, 
which commences when, once again, as-yet unexcited matter from the other 
side of slice 42 reaches slice 41. 
FIG. 10 shows a version of an excitation sequence in which an RF saturation 
pulse .phi..sub.2 is not provided between all successive RF excitation 
pulses .phi..sub.1, but only after every third RF excitation pulse 
.phi..sub.1. And RF excitation pulses .phi..sub.1 come in such close 
succession that each follows directly behind the read phase that follows 
the preceding RF excitation pulse. Consequently, it is necessary for this 
read phase to be eliminated when an RF saturation pulse .phi..sub.2 is 
interposed, as shown in FIG. 10. Here, too, RF pulses .phi..sub.1 and 
.phi..sub.2 can have different carrier frequencies, so that, in 
conjunction with slice gradients G.sub.S that are of differing height and 
slope, they can excite slices of differing location and thickness. 
And, finally, mention should also be made of the fact that, in medical 
practice, it is readily possible to couple measurements of flowing 
substances that are performed pursuant to the method according to the 
present invention with observation of the heartbeat by having the NMR 
measurements synchronized by an EKG that is being performed. It is 
possible to derive gate signals from a patient's EKG which permit a 
plurality of echo signals having identical phase coding to be read out. At 
the next trigger, it is then again possible to read out a plurality of 
mutually identical signals which do, however, differ from the previously 
coded signals, as is necessary in order to produce EKG-synchronous flow 
images. 
The present invention has been described above on the basis of preferred 
practical examples thereof. Obviously, many modifications and variations 
of the present invention are possible in the light of the above teachings. 
It should therefore be understood that, within the scope of the appended 
claims, the present invention may be practiced otherwise than as 
specifically described. In particular, individual characteristics of the 
invention can be employed individually or in combination one with the 
other.