Method of determining a nuclear magnetization distribution of a sub-volume of an object, method of shimming a part of a steady field in which the object is situated, and magnetic resonance device for performing such a method

An MRI method for spectroscopy utilizes a sequence which includes four RF electromagnetic pulses (p1, p2, p3, p4), three of which are spatially selective to generate a resonance signal (e) from a sub-volume of an object. The phase difference between the first and the second 90.degree. excitation pulse amounts to 90.degree.. The waiting period (dt1) between the first and the second pulse (p1, p2) is chosen so that the second pulse (p2) selectively resets the nuclear spins excited by the first non-selective pulse (p1) in the longitudinal direction. The selectively reset magnetization, for example of fat, is recalled, after the dephasing of the non-reset magnetization, for example of water, by the further pulses (p3, p4). A spectrum is determined from the resonance signal (e). In a modified version in which the phases of the first and the second pulse are the same, the sequence is used for shimming a local field around the sub-volume.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method of determining a nuclear magnetization 
distribution from at least one magnetic resonance signal from a sub-volume 
of an object which is situated in a steady magnetic field, the sub-volume 
being selectively excited by means of a sequence comprising RF 
electromagnetic pulses and magnetic field gradients which are superposed 
on the steady magnetic field, resonance signals from at least a part of a 
spectrum associated with the sub-volume being suppressed. 
The invention also relates to a method of shimming at least a part of a 
steady magnetic field in which a sub-volume of an object is situated, the 
sub-volume being selectively excited by means of a sequence comprising RF 
electromagnetic pulses and magnetic field gradients which are superposed 
on the steady magnetic field. 
The invention furthermore relates to a magnetic resonance device for 
determining a nuclear magnetization distribution from at least one 
resonance signal from a sub-volume of an object, which device comprises 
means for exposing the object to a steady magnetic field and to a sequence 
of RF electromagnetic pulses and magnetic field gradients, the means being 
suitable for suppressing resonance signals from at least a part of a 
spectrum associated with the sub-volume, and means for detecting the 
resonance signal to be generated by means of the sequence. 
2. Description of the Prior Art 
A method of this kind is described in Journal of Magnetic Resonance 67, pp. 
148-155, 1986. According to such a method an object is arranged in a 
steady magnetic field which is at least substantially uniform. The object 
is first exposed to a combination of a non-selective 90.degree. excitation 
pulse, a non-selective 180.degree. refocussing pulse, and a selective 
90.degree. reset pulse. During the reset pulse a first gradient is applied 
which is superposed on the steady magnetic field. It is thus achieved that 
a magnetization which was oriented in the same direction as the steady 
field is reset, after refocussing, along the z'-axis after having been 
rotated to the transverse direction in a coordinate system x'y'z' which 
rotates at the so-called Larmor frequency and whose z'-axis coincides with 
the steady magnetic field; in other words, after application of the 
pulse/gradient combination it is achieved that longitudinal magnetization 
exists only within a slice of the object and not outside this slice. 
Outside the slice only dephased transverse magnetization will exist after 
some time. Subsequently, such a pulse gradient combination is applied two 
more times, be it with a second and a third gradient, respectively. The 
field directions of the gradients coincide with the direction of the 
steady magnetic field, the gradient directions extending perpendicularly 
with respect to one another. After the three pulse/gradient combinations, 
the magnetization in a, for example cubic sub-volume of the object will 
have been selectively longitudinally reset after having been transversally 
set. Outside the sub-volume only dephased transverse magnetization exists. 
By using so-called phase cycling, any spurious signals from outside the 
sub-volume are further suppressed. In order to obtain a magnetic resonance 
signal from the selected sub-volume by means of which, for example a 
spectrum of the sub-volume can be determined, a 90.degree. excitation 
pulse is generated. The resonance signal generated thereby is sampled and 
the spectrum is determined by means of Fourier transformation. In order to 
suppress, for example a water peak in the spectrum of the sub-volume, the 
latter 90.degree. excitation pulse may be replaced by a so-called 1-3-3-1 
composite pulse as described inter alia in Journal of Magnetic Resonance 
55, pp. 283-300, 1983, notably on page 298-299 of this article. The method 
is notably suitable for use in so-called wholebody magnetic resonance 
devices. Even though such a method offers suitable results, it is a 
drawback that a large number of RF electromagnetic pulses (at least 10) 
must be generated in order to obtain a magnetic resonance signal from a 
sub-volume of the object. 
SUMMARY OF THE INVENTION 
It is inter alia an object of the invention to provide a magnetic resonance 
method and device whereby, for example water-suppressed spectra of 
sub-volumes can be obtained using fewer RF electromagnetic pulses. 
A first method in accordance with the invention is characterized in that 
there is generated a first 90.degree. excitation pulse which has a first 
phase and which is non-selective, after which a second excitation pulse 
which has a second phase which differs 90.degree. from the first phase and 
which is selective is generated in the presence of a first magnetic field 
gradient, a waiting period between the first and the second pulse being 
chosen so that the suppression of said part of the spectrum is optimum and 
the first magnetic field gradient is sustained after the second excitation 
pulse until magnetic resonance signals from a further part of the spectrum 
have been dephased, after which a third, selective, excitation pulse is 
generated in the presence of a second magnetic field gradient and a 
selective refocussing pulse is generated in the presence of a third 
magnetic field gradient, the first, the second and the third field 
gradient having different gradient directions. This enables suitable 
suppression of, for example a water peak in a spectrum whereas, for 
example CH.sub.n groups are shown. By a suitable choice of the waiting 
period optimum suppression of the water peak can be achieved. The 
invention is based on the idea to expose the object to an RF 
electromagnetic field after the magnetization of the object has been 
rotated in the transverse direction and after expiration of a suitable 
waiting period during which the water magnetization vector remains 
stationary in a coordinate system which rotates at the Larmor frequency of 
water and vectors of other substances rotate due to chemical shift, the 
direction of said RF electromagnetic field coinciding with the water 
vector so that in one operation the magnetization is reset in the 
longitudinal direction with the exception of that of water, that is to say 
in the direction of the steady magnetic field, after which the water is 
transversally dephased by sustaining a gradient and by using further 
volume selection. Contrary to the described known method which utilizes 
three reset pulses, only one reset pulse is now required. 
It is to be noted that pulse sequences are known in which three excitation 
pulses are successively generated, for example from U.S. Pat. No. 
4,748,409. The three excitation pulses are 90.degree. pulses. Contrary to 
the method in accordance with the invention, however, all nuclear spins 
are dephased after the first RF pulse; moreover, only 50% of the intensity 
of the reset magnetization is obtained upon excitation thereof, while the 
method in accordance with the invention achieves substantially 100% in the 
central part of the spectrum of interest. A method as known from Fig. 3 in 
European Patent Application EP 0.304.984 which corresponds to commonly 
owned U.S. Pat. No. 4,893,080, is a version of the method known from the 
cited U.S. Pat. No. 4,748,409. Again all nuclear spins are dephased after 
the first RF pulse. Therefore, the method in accordance with the invention 
is distinct from these so-called stimulated echo methods in that it 
involves a combination of RF electromagnetic pulses and gradients so that 
an essentially different operation is obtained. 
A version of a method in accordance with the invention is characterized in 
that the waiting period is in conformity with (2n+1)/(4df), where df is a 
frequency difference in the spectrum of a resonance frequency of a 
resonance peak from the suppressed part of the spectrum and a resonance 
frequency of a frequency peak from the further part of the spectrum, and 
where n is a non-negative integer number. For example, if the resonance 
peak from the suppressed part of the spectrum is a water peak and the 
other peak is a CH peak from the central part of the spectrum of interest, 
it is achieved that the CH peak is phase-shifted 90.degree. with respect 
to the water peak, and is optimally recalled later after having been 
reset. 
A further version of a method in accordance with the invention is 
characterized in that the presaturation is performed by generating an RF 
electromagnetic pulse whose frequency contents correspond to the 
suppressed part of the spectrum, followed by a dephasing gradient. The 
water is then at least partly suppressed already before the sequence, i.e. 
inside as well as outside the sub-volume to be selected. 
A second method in accordance with the invention is characterized in that 
there is generated a first 90.degree. excitation pulse which is 
non-selective, after which a second excitation pulse which is selective is 
generated in the presence of a first magnetic field gradient, the phases 
of the first and the second pulse being the same, a waiting period between 
the first and the second pulse being chosen so that a phase difference of 
substantially 90.degree. arises between at least two spectral components 
of a spectrum associated with the sub-volume, the first gradient being 
sustained until selective dephasing occurs, after which a third, 
selective, excitation pulse is generated in the presence of a second 
magnetic field gradient and a selective refocussing pulse is generated in 
the presence of a third magnetic field gradient, the sequence being 
repeated so many times, while varying shimming currents in coils in order 
to shim the steady magnetic field, that an optimum resonance signal is 
obtained from the sub-volume. By making the phase difference between the 
first and the second RF electromagnetic pulse 0.degree. instead of 
90.degree., the method can be made suitable for shimming the steady field 
around a sub-volume. After that the first method can be used. Thus, a 
spectrum having an even higher resolution is obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 diagrammatically shows a magnetic resonance device 1 in accordance 
with the invention, comprising transmitter means 2 and receiver means 3 
for transmitting RF electromagnetic pulses, via a transmitter/receiver 
coil 4, to an object 5 and for receiving magnetic resonance signals, 
respectively, which are generated in the object 5 by the RF 
electromagnetic pulses, which object is situated in a steady, uniform 
magnetic field. The device 1 comprises means 6 for generating the steady 
field. The means 6 comprise magnet coils 7 and, in the case of resistive 
magnets or superconducting magnets, a DC power supply 8. During operation 
of the device 1 with the object arranged within the magnet coils 7, a 
small excess of nuclear spins (of nuclei having a magnetic moment) will be 
directed in the same direction as the steady uniform field in the state of 
equilibrium. From a macroscopic point of view this is to be considered as 
a magnetization M, being an equilibrium magnetization. The device 1 also 
comprises processing means 9 which are coupled to the transmitter means 2 
and the receiver means 3, a process computer 10 which is coupled to the 
processing means 9 and the transmitter means 2, and display means 11 for 
displaying a nuclear magnetization distribution which is determined, using 
programmed means 12, from resonance signals received and demodulated by 
the receiver means 3 after signal sampling thereof (detection of resonance 
signals). The transmitter means 2 actually comprise an RF oscillator 13 
for generating a carrier signal, a modulator 14 for amplitude and/or phase 
or frequency modulation of the carrier signal, a power amplifier 15 and a 
directional coupler 16 which is coupled to the transmitter/receiver coil 
4. The transmitter/receiver coil 4 may be a coil which encloses the entire 
object 5, a coil which encloses a part of the object 5, or a surface coil. 
The RF oscillator 13 is coupled to the processing means 9 and the 
modulator 14 is coupled to the process computer 10. When excitation pulses 
are applied to the object 5 via the transmitter means 2 and under the 
control of the programmed means 12, which excitation pulses have a 
frequency contents around the Larmor frequency of, for example protons, 
magnetic resonance signals will be produced wherefrom a proton spectrum 
can be determined by the programmed means 12 by way of, for example 
Fourier transformation. The receiver means 3 for receiving the resonance 
signals comprise the directional coupler 16 and a receiver and 
demodulation unit 17. The unit 17 is, for example a double phase-sensitive 
detector whose output signals are sampled by means of a first and a second 
A/D converter 18, 19, respectively. The first and the second A/D converter 
18, 19 are coupled to the processing means 9. In the case of separate 
transmitter and receiver coils, the directional coupler 16 is absent. The 
device also comprises means 20 for generating magnetic field gradients 
which are superposed on the steady, uniform magnetic field. The means 20 
comprise gradient magnet coils 21, 22 and 23 for generating a magnetic 
field gradient G.sub.x, G.sub.y and g.sub.z, respectively, and a power 
supply unit 24 which can be controlled by the process computer in order to 
power the gradient magnet coils 21, 22 and 23 which are separately 
activatable. In the embodiment shown the arrangement in space of the 
gradient magnet coils is such that the field direction of the magnet field 
gradients coincides with the direction of the steady, uniform magnetic 
field and that the gradient directions extend mutually perpendicularly; 
this is shown in FIG. 1 by way of three mutually perpendicular axes x, y 
and z. When pulse/gradient sequences are applied to the object 5, the 
resonance signals can be used inter alia for spectroscopy, 
location-dependent spectroscopy and spectroscopic imaging. The device 1 
may also comprise shimming coils which are not shown in detail. These 
shimming coils are to be activated by the programmed means. 
FIG. 2 shows a sequence in accordance with the invention as a function of 
time t; the references t1 to t7 denote some instants. Under the control of 
the programmed means 12 the transmitter means 2 generate a first RF 
electromagnetic, non-selective pulse p1 at the instant t=t1. The pulse p1 
is applied to the object 5 by the transmitter/receiver coil 4, so that 
nuclear spins are excited in the object 5. The amplitude and the duration 
of the pulse p1 are such that in a coordinate system x'y'z' which rotates 
at a Larmor frequency of, for example water and whose z'-axis coincides 
with the steady field B.sub.0, a nuclear magnetization vector is rotated 
through 90.degree. with respect to the field B.sub.0, i.e. p1 is a 
so-called 90.degree. pulse. This is shown in FIG. 3A. The frequency 
contents of the pulse p1 are such that substantially all nuclear spins are 
excited; the pulse p1 is nonselective. Subsequently, at the instant t=t2 a 
second, selective 90.degree. excitation pulse is generated after a waiting 
period following the pulse p1. During the pulse p2 the means 20 generate a 
magnetic field gradient. The phase of the pulse p2 differs 90.degree. from 
that of the pulse p1. If the object contains other components in addition 
to water, for example fat, a water vector W will be stationary in the 
rotating system x'y'z' due to chemical shift, and a fat vector V will 
rotate with a difference frequency between water and fat. The phase of the 
pulse p1 is such that the direction of the RF electromagnetic field 
B.sub.1 extends perpendicularly to the y'z'-plane as shown in FIG. 3A. 
When the waiting period dt1 is chosen so that the magnetization vectors 
around the resonance frequency of the fat in the x'y'-plane are rotated 
substantially 90.degree. with respect to the water at the instant t=t2, 
these magnetization vectors will be longitudinally set due to the pulse 
p2, i.e. along the z"-axis and, because of the fact that the 
electromagnetic field B.sub.1 extends along the water vector as shown in 
FIG. 3B, this field will not have an effect on the water; in other words, 
the pulse p2 acts as a selective reset pulse. At the instant t= t3 the 
transverse magnetization still present will have been rephased inside and 
outside a slice. By sustaining the gradient G.sub.x until the instant t=t4 
(for example, for 1 ms or longer), this transverse magnetization will be 
dephased. It is achieved that longitudinal magnetization (of magnetization 
vectors having a resonance frequency around that of fat) exists 
selectively in a slice and that outside the slice only dephased transverse 
magnetization exists. Spurious echos which could arise due to field 
inhomogeneities can be suppressed by phase cycling. The selective 
longitudinal magnetization can be produced by means of a spin echo 
sequence which is indicated in FIG. 2, comprising a third, slice-selective 
90.degree. excitation pulse at the instant t=t5, in the presence of a 
magnetic field gradient G.sub.y, and a selective 180.degree. refocussing 
pulse at the instant t=t6 in the presence of a magnetic field gradient 
G.sub.z. At the instant t=t7 an echo resonance signal e arises which 
originates from a selected sub-volume of the object 5. The second and the 
third excitation pulse may be chosen to be smaller than 90.degree.; the 
signal strength of the echo resonance signal e is not optimum in that 
case. Waiting periods between p2 and p3 and between p3 and p4 are denoted 
by dt2 and dt3, respectively. After detection of the resonance signal e by 
the receiver of means 5 and after sampling of the detected signal by means 
of the A/D converters 18 and 19, for example a water-suppressed spectrum 
can be derived from the detected and sampled signal, using the programmed 
means 12, by Fourier transformation, which spectrum can be displayed by 
means of the display means 11. 
FIGS. 4A and 4B show spectra of resonance signals with suppression curves 
oc1 and oc2. FIG. 4A shows a water peak W and a fat peak V. It will be 
evident that spectra may be more complex and that the sequence shown can 
be used for spectra other than those in which water peaks and fat peaks 
occur. By a suitable choice of the waiting period dt1=(2n+1)/(4df), where 
df is the chemical shift between the water peak W and the fat peak V and n 
is an integer, non-negative number, it can be achieved that the water peak 
W and its vicinity in the spectrum are at least substantially suppressed. 
FIG. 4A shows a suppression curve oc1 for n=0. In the case of a chemical 
shift df of, for example 170 Hz, dt1 would then be approximately 1.5 ms; 
this could be too short in practice for switching inter alia gradients. 
The spectrum of interest around the fat peak V is denoted by the reference 
roi1. When dt1 is too short for the switching of gradients, n=1 may be 
chosen, and dt1 will be 4.5 ms. The suppression curve oc2 in FIG. 4B, 
however, is more complex than the suppression curve oc1 in FIG. 4A. An 
area around the fat peak in a rather flat part of the suppression curve 
oc2 may be taken as the region of interest roi2. 
FIG. 5 shows a sub-volume vp in an object 5 which has been selected by 
means of the sequence in accordance with the invention. The sub-volume vp 
is selected by the respective gradients G.sub.x, G.sub.y and G.sub.z 
during the pulses p2, p3 and p4 as an intersection of three slices, such 
as the slice s by the gradient G.sub.x. It will be evident that the 
sequence is effective for all permutations of G.sub.x, G.sub.y and 
G.sub.z. 
FIG. 6 shows presaturation in accordance with the invention. The references 
p1 and t1 denote the first pulse p1 at the instant t=t1, like in FIG. 2. 
The entire object 5 is selectively excited, for example for water by means 
of a presaturation pulse p5 at the instant t=t8. The pulse p5 may be 
selective 90.degree. pulse, such as a long Sinc pulse, having a bandwidth 
of, for example 10-20 Hz around the water. The application of a gradient G 
at the instant t=t9 will dephase the selectively excited water, so that 
the water has been presaturated at the instant t=t1. 
FIG. 7 shows a sequence adapted to spectroscopic imaging in accordance with 
the invention. Between the pulses p3 and p4 of the pulse sequence of FIG. 
2 two and three gradients G.sub.xs, G.sub.ys and G.sub.zs are applied for 
2D spectroscopic imaging and 3D spectroscopic imaging, respectively. The 
sequence is repeated for different amplitudes of G.sub.xs, G.sub.ys and 
G.sub.zs. From sampling values of the resonance signals a large number of 
VOIs (volume of interest) can be determined by means of the programmed 
means 12, or a density image of a given spectral peak can be determined. 
FIGS. 8A and 8B show that the sequence in accordance with the invention can 
be used for the shimming of a sub-volume. In that case the phases of the 
pulses p1 and p2 are chosen to be equal and the waiting period dt1 is 
chosen so that fat is actually suppressed and water is not. In FIG. 8A 
this is denoted by the suppression curve oc3 (for n=0). The device 1 then 
comprises shimming coils whereby the steady field can be locally 
influenced by variation of the currents through the shimming coils. The 
currents can be adjusted by the programmed means 12. The sequence can be 
repeated a number of times while varying the currents until an optimum 
water signal is obtained from the sub-volume. This signal can be evaluated 
by observing the Fourier transformed water signal while varying the 
currents. When the magnetic field in the sub-volume is uniform, the water 
resonance signal will relax substantially with a time constant T.sub.2, 
being the transverse relaxation time of water. When the magnetic field is 
still not uniform, transverse relaxation will take place with a time 
constant T.sub.2 * which is smaller than T.sub.2. This is shown in FIG. 
8B. 
The sequence shown can be executed using, for example, the time parameters: 
dt1=4.5 ms, dt2=5 ms, dt3=8-500 ms and a waiting period of 1-2 s after the 
resonance signal e. When dt3 is chosen to be small, the echo time will be 
short. The sequence shown is then also very suitable for spectroscopy of 
quickly moving organs such as the heart in the case of an object in the 
form of a human body.