Method of exciting a sample for NMR tomography

In particular with 3D tomography, it is necessary to acquire a large number of individual spectra. Therefore it is advisable to reduce the time needed for the acquisition of a single spectrum without loosing too much in signal-to-noise ratio. To a significant extent, this time is determined by the relaxation time of the spins. Prior to each excitation a significant quantity of these spins have to return into their equilibrium (z-direction of the homogeneous magnetic field) in order to create a usable signal with the next excitation. For spins with long relaxation times T.sub.2 this time can be reduced by a -90.degree. pulse 12 that coincides with the center of the last spin echo 9 with appropriate application of the gradient fields. This -90.degree. pulse returns the x-y magnetization that exists in the x-y plane into the z-direction. The diagnostic relevance can be significantly increased by such a procedure.

FIELD OF THE INVENTION 
The invention concerns a method for exciting a sample in NMR tomography in 
which the sample is subjected to the homogeneous magnetic field and 
magnetic-field gradients common to FT or back-projection tomography, and 
is excited by a 90.degree. pulse followed by a train of 180.degree. 
pulses, whereby measurable nuclear induction signals, commonly known as 
spin-echos, are produced. 
DESCRIPTION OF THE PRIOR ART 
The production and measurement of NMR signals, involving the influence of 
gradients along different orthogonal axes achieved through the employment 
of common NMR-tomography techniques leads to the production of signals 
which characterize a multitude of spatial volume elements of the sample 
being examined. Through the selection of signals from particular volume 
elements, cross sectional images at arbitrary spatial locations throughout 
the sample may be produced. 
A disadvantage of NMR-tomography, especially in the case of 3-dimensional 
(3D) data sets, is that it is necessary to collect a very large number of 
individual spectra. Consequently, long examination times result. The 
repetition rate with which the individual spectrum can be taken depends on 
the fact that before each new excitation, the previous excitation must 
decay since otherwise excited nuclear spins are not located in the defined 
initial state required for the production of useful nuclear induction 
signals and diagnostically useful images. In this respect, distortions are 
largest for nuclear spins with the longest relaxation times, which, in 
turn, have the greatest diagnostic significance since they lie in the 
range of tumors, edematous tissue, demyelinated nerve fibers, cysts, etc. 
A reduction in the measurement repetition time therefore leads to images 
which are erroneous in precisely this region of long relaxation times and 
are thereby, diagnostically speaking, useless. Conversely, if one 
lengthens the measurement repetition time to optimize the nuclear 
induction signals from spins with long relaxation times, the examination 
time becomes intolerable for the patient and highly inefficient from an 
economical point of view. 
It is therefore of great importance, not only in the three dimensional case 
but also in the more conventional two dimensional measurements to employ 
the shortest possible measurement times. 
EP- 0 121 312 A2 (U.S. Pat. No. 4,579,121) describes a method wherein, 
following the excitation of a sample with a 90.degree. high frequency 
pulse, a nuclear induction signal is read out. During the 90.degree. pulse 
a so-called slice gradient is applied and later, during the acquisition of 
the signal, a read-out gradient is employed. Finally, through the 
incorporation of a 180.degree. RF pulse, again applied in the presence of 
a slice gradient, the nuclear spins of a particular slice are inverted, 
leading to the occurrence of a spin-echo. In order to improve the 
signal-to-noise ratio, one can also collect the first half, or rising 
part, of the spin-echo under the previously mentioned read-out gradient. 
Alternatively, it is possible to reduce the resultant measurement time by 
a factor of 2 by altering the read-out gradient, that is to say, going to 
a new projection step. At the center of the spin-echo formed, one can then 
irradiate a 90.degree. pulse to drive the nuclear spins back to a position 
parallel to the orientation of the magnetic field. This method is known in 
the art of NMR-spectroscopy as the DEFT (driven equilibrium Fourier 
transform) technique. Once they have been returned to the orientation of 
the magnetic field, the nuclear spins are then available for a new 
excitation by a subsequent 90.degree. pulse such as the initial one. 
Ep- 0 121 312 A2 alternatively describes a method with which, instead of 
returning the nuclear spins to a position parallel to the orientation of 
the magnetic field, one may produce and analyze additional spin-echoes 
through the use of further 180.degree. pulses. 
In U.S. Pat. No. 4,818,940 which is based on U.S. Pat. No. 4,697,148 a 
rapid acquisition method (RARE) is proposed whereby, to acquire NMR 
tomograms according to the 2D-FT-method, a sample is excited using a CPGM 
spin-echo sequence, whereby following each 180 degree pulse of the 
sequence, a phase encoding gradient is changed, such that each spin-echo 
is newly phase-encoded. The additional gradient switchings are implemented 
in a manner that ensures the preservation of the Meiboom-Gill condition. 
In extreme cases it is thereby possible to generate a complete image with 
a single excitation, that is to say, with one 90.degree. pulse followed by 
a series of 180.degree. pulses. 
Through use of this technique it is possible to acquire images, of nuclear 
spins possessing long relaxation times, within examination times of less 
than 2 minutes. However, in order to achieve the desired contrast, long 
repetition times of 3-4 seconds are still required. An improved method is 
needed with which these waiting times can be substantially reduced, such 
that images depicting signal from nuclear spins with long relaxation times 
may be obtained in 10-30 seconds so that data acquisition can be completed 
in a time interval within which a person can hold his breath. 
Furthermore, an improved method, in conjunction with the RARE method is 
needed in order to facilitate the collection of a three dimensional 
spin-echo data in the range of nuclear spins with long relaxation times. 
Accordingly, it is the underlying purpose of the invention, to improve the 
above described methods in such a manner that use of reduced repetition 
times, and thereby acceptable total measurement times is possible without 
distorting the image contributions from nuclear spins with long relaxation 
times. 
BRIEF SUMMARY OF THE INVENTION 
This purpose is achieved in accordance with the invention in that each 
sequence of 180.degree. pulses is ended with a flip-back pulse (FB), by 
way of example, a -90.degree. pulse the beginning of which coincides with 
the spin-echo maximum generated by the last 180.degree. pulse, whereby the 
gradient fields are adjusted so that the nuclear spin phases at the time 
of occurrence of the 180.degree. pulses are the same, and the FB pulse is 
irradiated at a point in time when all the spin momenta are in phase. 
In accordance with the present invention, the FB pulse closing the sequence 
of 180.degree. pulses rotates the spin momenta, which are located in a 
plane perpendicular to the homogeneous magnetic field, back into the 
direction of this homogeneous magnetic field, so that no additional time 
is needed for their relaxation. Since the FB pulse coincides with the 
maximum of the spin-echo produced by the last 180.degree. pulse, and since 
the pulse sequence and gradient fields are so adjusted, nuclear spin 
momenta with long T.sub.2 which are still located in the plane 
perpendicular to the direction of the homogeneous magnetic field are in 
phase when the FB pulse occurs, so that they experience the full effect of 
the FB pulse and will be reorientated in a direction parallel to the 
magnetic field. In this manner, the waiting time following the FB pulse 
before the next acquisition of an individual spectrum can be significantly 
shortened without a significant deterioration in image quality in 
particular with respect to the diagnostically interesting region of long 
relaxation times, with the total image taking time being substantially 
reduced. 
The invention will be further described and elaborated upon through the 
following series of figures and diagrams. Shown are:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As is known in the art of NMR tomography, the sample under 
investigation--usually the human body or a part thereof--is subjected to a 
strong homogeneous magnetic field B.sub.o, the direction of which usually 
is assigned to the z-axis of a rectangular coordinate system. The magnetic 
spins of the sample are usually aligned along the z-direction due to this 
magnetic field, if there are no other external influences. The information 
necessary for image reconstruction can only be extracted from the rf 
signals created by spins rotating around z-direction in the x-y plane. 
These signals are called nuclear induction signals. Towards this end, the 
spins in the areas to be imaged have to be rotated away from the 
z-direction by appropriate irradiation with high frequency waves, 
preferably by 90.degree., into the x-y plane. 
The frequency with which spin momenta rotate in the x-y plane (Larmor 
frequency) depends on the strength of the magnetic field acting on the 
spins. Therefore it is possible to create a spatial variation of the 
Larmor frequency of the spin momenta of the sample by superimposing a 
magnetic gradient field upon the homogeneous magnetic field B.sub.o, said 
gradient field also directed parallel to the z-direction but with a 
strength which varies along a selected direction. This direction of 
variation of the magnetic field in the presence of a gradient field is 
hereby designated as the gradient direction. Accordingly, the Larmor 
frequency of spins in the sample changes along the direction of the 
gradient due to the influence of the applied magnetic fields. 
With 3D techniques all spins of the sample under investigation are excited 
as uniformly as possible. Towards this end, an rf pulse is irradiated onto 
the sample, the energy of which is sufficient to rotate the spins about an 
angle of 90.degree. into the x-y plane (90.degree. degree pulse). If this 
pulse is transmitted while the sample is subjected only to the homogeneous 
magnetic field B.sub.o, the pulse can be narrow-banded since all spins in 
the homogeneous magnetic field possess the same Larmor frequency. Such a 
narrow-banded pulse can be of a fairly long duration, so that even a small 
rf pulse amplitude is--due to the relatively long duration--sufficient to 
achieve the rotation by 90.degree.. However, if the sample is subjected to 
a gradient field during the excitation, the pulse has to be sufficiently 
broad-banded to encompass the entire range of different Larmor frequencies 
caused by the gradient field. Since a broadband pulse is of short 
duration, the application of high power pulses is necessary. 
After excitation of all spins in the sample, the nuclear induction signals 
created by the excited spins are measured during the application of a read 
gradient, the frequency of these signals depending on the position of 
these spins with respect to the direction of the read gradient. Frequency 
selective detection of the nuclear induction signals can effectively 
restrict each single scan to the spins that are in a plane orthogonal to 
the read gradient. A plurality of such single scans, under variation of 
the direction of the read gradient and/or a phase encoding gradient 
permits information characteristic of a multitude of volume elements of 
the sample under investigation which is necessary for the reconstruction 
of the desired image slices. 
To keep the total scan time within reasonable limits, the time for a single 
scan has to be restricted as much as possible. However, the limits to an 
increase in repetition rate are not only apparative in nature, but also 
involve the characteristics of the sample, namely the relaxation times of 
the sample nuclear spins. Particularly in areas of diagnostic interest 
such as tumors, edemas, centers of demyelinisation, cysts and so on, the 
spins have long relaxation times so that important information is 
contained in scanning areas where precisely these spins with long 
relaxation times are located. With a high repetition rate, a relaxation of 
these spins is not possible so that these important spins are not 
completely excited when the subsequent single scan is effected, and, 
consequently, the information content of these spins is lost. 
The invention allows for a drastic reduction in the repetition time for the 
acquisition of single spectra without a significant loss of information 
with respect to spins with a large relaxation time T.sub.2. 
In FIG. 1 a procedure is shown that excites a sample in a way described 
above with a 90.degree. pulse during which a slice selection gradient 1a 
is applied. The resulting NMR signal 3 is usually not analyzed. Instead a 
series of pulses is used where the 90.degree. pulse 2 is followed by a 
series of 180.degree. pulses 4,5, and 6 that create, as is known in the 
art, echo signals 7,8 and 9, analogous to a typical series of spin echo 
pulses, e.g. Carr-Purcell or Carr-Purcell-Meiboom Gill pulses. Due to the 
T.sub.2 relaxation during the scan, the magnetization in the x-y plane 
that determines the amplitude of the echo 7,8 and 9 decays. The 
magnetization in the x-y plane in FIG. 1 is illustrated by the line 10. 
If, in FIG. 1, the series of pulses is terminated with the third 
180.degree. pulse 6, the restoration of the z-magnetization starts after 
the last spin echo 9 under the influence of the homogeneous magnetic field 
B.sub.o as illustrated by the dashed line 11 in FIG. 1. 
Nuclear induction signals of full amplitude can only be acquired if, at the 
time of excitation of the sample, a complete magnetization in z-direction 
exists, since, after application of a 90.degree. pulse, it is this 
magnetization which creates the magnetization in the x-y plane responsible 
for the NMR signals. With long T.sub.2 relaxation times, a long delay 
after the last spin echo 9 is, however, necessary in order to restore the 
complete magnetization in the z-direction. The invention can shorten this 
delay considerably by transmitting, at the time of the last echo 9, a 
-90.degree. pulse 12 to the sample in consequence of which the 
magnetization of the x-y plane spins which, at the point of time at which 
the echo pulse 9 has its maximum, are all in phase is returned into the 
z-direction, so that the -90.degree. FB pulse 12 creates a magnetization 
in z-direction equal to the residual magnetization that was present in the 
x-y plane at the time of application of the -90.degree. pulse. From this 
value, which can be of considerable size especially in case of long 
relaxation times T.sub.2, a large magnetization in z-direction can be 
achieved in a relatively short period of time as line 13 in FIG. 1 shows. 
The gradient fields Gx, Gy, and Gz of FIG. 1 are adjusted so that the spins 
do not dephase in subsequent spin-echos 7,8,9 and so that all spins are in 
phase at the time of application of the FB pulse 12. Gx is applied with 
positive and negative phase sense before and after the occurrence of the 
spin-echo 7 respectively and is absent during the application of the FB 
pulse 12 and during the last spin-echo 9. Gy straddles the spin-echo pulse 
in a time symmetric fashion and includes symmetric negative going lobes 
before and after the spin-echo signals 7,8 to limit additional unwanted 
dephasing. Gy is absent during application of the FB pulse 12 and during 
the last spin-echo 9. Gz is applied during the 90.degree. excitation pulse 
2, the 180.degree. pulses 4,5, and 6 and as well as during the FB pulse 
12. 
For the effectiveness of the -90.degree. FB pulse in accordance with the 
invention, it is not necessary to wait for complete recovery of the 
z-magnetization after the application of the pulse. With shorter recovery 
times there will be an equilibrium NMR signal from spin momenta with long 
relaxation times that is significantly larger than the signal from spins 
with short relaxation times even after extended scan times. This 
guarantees that the information contained in the relaxation times T.sub.2 
is still sufficiently available even under utilization of rapid repetition 
rates. 
FIG. 2 more closely shows how a contrast enhancement can be achieved by the 
application of the fold-back or flip-back (FB) pulse in accordance with 
the invention. As shown in FIG. 2A for spins with long relaxation time 
T.sub.2 the magnetization 21 is much smaller than the maximum possible 
value M.sub.o of the magnetization which is represented by the line 22, 
and the amplitude of the NMR signal is accordingly small. On the other 
hand the signal from spins with short relaxation times is larger in 
amplitude since a higher magnetization 23 (FIG. 2b) is achieved. However, 
with the application of a flip-back pulse, the magnetization 24 remains 
very large, so that the spin momenta create NMR signals with large 
amplitude (FIG. 2c), whereby there is only little effect due to the FB 
pulse on spins with short relaxation times and no increase of signal (FIG. 
2d). Therefore, at the end of the scan-time there is significantly more 
signal from spin momenta in regions with long relaxation times as from 
other regions resulting in a diagnostically important increase of 
contrast. 
The method according to the invention can also be applied when a 
180.degree. inversion pulse 14 precedes the 90.degree. excitation pulse 2 
(FIG. 1). Short repetition times necessitate short inversion times. As 
shown in FIG. 3a and 3b, after application of the 90.degree. pulse, the 
magnetizations 31 and 32 in the x-y plane are quite different, but the 
z-magnetization 33 or 34 and therefore the measurable NMR signal of the 
next spectrum will be smaller for spins with long relaxation times T.sub.2 
than for spins with short relaxation times. Compared to conventional 
techniques this distorts the images and makes them diagnostically 
unusable. 
In this case the flip-back pulse helps as well as is clearly shown in FIG. 
3c and 3d. Not only is the magnetization in the x-y plane 35 or 36 
significantly larger for spins with long relaxation times than for spins 
with short relaxation times for the first acquisition spectrum, but this 
condition due to the FB pulse according to the invention persists during 
acquisition of the subsequent spectra (see curves 37,38 for the 
magnetization in z-direction). 
The resulting stationary magnetization M.sub.s can be specified and 
optimized by solving the Bloch equations. An example for the case of the 
inversion recovery method with the FB pulse gives, 
##EQU1## 
Mo=equilibrium magnetization after relaxation t.sub.c =the time the 
magnetization is in the x-y plane 
t.sub.i =inversion time 
t.sub.w =delay time between FB pulse and the following excitation pulse 
T.sub.1 =longitudinal spin lattice relaxation time 
T.sub.2 =transverse spin-spin relaxation time 
The times t.sub.i,t.sub.c, and and t.sub.w are also indicated in the second 
line of FIG. 1. The procedure according to the invention can be used with 
special fast imaging sequences, e.g. especially with the RARE technique. 
FIG. 4 shows a different phase encoding than that used in FIG. 1 as 
indicated by gradient Gx in line two of FIG. 1 and labelled by 41, 42, 43 
and 44. Herein, subsequent echo signals are subjected to different phase 
encoding gradients 41,43, whereby the respective compensation pulses 42,44 
exhibit opposite phase. In accordance with the invention, the gradient 
fields Gx, Gy, and Gz of FIG. 1 are adjusted so that the spins 
contributing to subsequent echos 7,8,9 have the same phase as in the 
preceding spin-echo, and so that all spins are in phase at the time of 
application of the FB pulse 12 as was further described above in 
connection with FIG. 1. Here, with the flip-back pulse 12 in accordance 
with the invention, the usual long recovery time necessary to achieve the 
desired contrast can be substantially reduced. 
Generally, the procedure according to the invention allows for the 
reduction of recovery time in acquiring individual spectra. In principal, 
it can be applied in all cases where a number of individual spectra have 
to be recorded, although it is of special interest for 3D tomography since 
here a large number of spectra have to be acquired and, in addition no 
further additional complications prohibit its use. The application of the 
FB pulse is more effective the shorter the actual scan-time. On the other 
hand the procedure according to the invention allows, in particular, for a 
contrast enhancement between areas with different relaxation times that 
are normally effective only after an extended scan time. That is why the 
flip-back pulse also achieves practical importance when applied at the end 
of extended spin-echo pulse sequences.