Amplitude modulated RF pulse generator

Device and process for the generation of amplitude-modulated radiofrequency excitation pulses. A first unit (2) delivers a radiofrequency signal of constant level and a second unit (3) delivers a modulation signal of the radiofrequency signal, formed from numerical data. At least two analog multipliers (6) are connected in cascade as to the radiofrequency signal (RF) from the first unit (2) and are supplied in parallel by the modulation signal (SM) from the second unit (3). For n multipliers (6) in cascade, with n.gtoreq.2, the modulation signal (SM) delivered to the n multipliers (6) is of the A.sup.1/n.f(t) type, A.f(t) being the desired resulting modulation signal (SMR). The second unit (3) is a memory (4) containing point by point the envelope of the modulation signal A.sup.1/n.f(t) and a digital/analog converter (5) whose output is connected to the inputs in parallel of the n analog multipliers (6) in cascade.

The present invention relates to the field of Nuclear Magnetic Resonance 
(NMR), more particularly the systems for the selective excitation of 
specimens, and has for its object a device for the generation of 
radiofrequency excitation pulses and a process using said device. 
At present, NMR experiments use more and more frequently radiofrequency 
excitation pulses of complex shape, and no longer simply square pulses. 
These radiofrequency pulses modulated in amplitude permit, for example, 
irradiating a precise portion of the NMR spectrum of the specimen, hence 
the term selective excitation. 
The known process used to define the complex envelope of an RF 
radiofrequency excitation impulse will be described in greater detail 
hereafter. 
To begin with, the desired envelope is calculated point by point, which is 
to say the amplitude of the RF impulse is defined point by point and the 
assembly of the resulting values is stored in a memory which is 
progressively read by the computer controlling the assembly of the 
process, when the NMR experiment is begun. 
The said memory is connected to a digital/analog converter which transforms 
the digital data (for example of 12 bits) from the memory, into an analog 
signal which will control a radiofrequency modulator, for example of the 
ring modulator type, permitting modulating a radiofrequency signal of 
constant level from a synthesizer by the modulation signal stored in the 
memory. 
This modulated radiofrequency signal is then amplified by a power amplifier 
before arriving at the NMR probe constituted by an excitation winding. 
At present, this process, practiced at a dynamic amplitude of 50 dB and 
using a ring modulator controlled by a digital/analog 12-bit converter, 
has the drawback of relatively insufficient relative precision for low 
value radiofrequency levels. 
Thus, the amplitude of the output radiofrequency signal varies by a 
constant interval of 1/4096 (1/2.sup.12) over all the dynamic of the 
converter; but the relative variation in dynamic height is 
(4096-4095)14096=1/4096 (0.02%), whilst below 50 dB (which is to say at 
13) the relative variation is (13-12)/13=1/13 (7.7%). 
This means that an RF pulse modulated in amplitude could be defined with a 
much greater relative precision at the highest levels, whilst at low 
levels the amplitude will be less well defined. 
Moreover, the digital/analog converter is excited over a wide dynamic range 
giving rise to relative precision variations of the conversion according 
to the amplitude values in question. 
The present invention has particularly for its object to overcome the 
recited drawbacks. 
To this end, it has for its object a device for the generation of 
radiofrequency excitation pulses modulated in amplitude, comprising 
essentially a first unit delivering a radiofrequency signal of constant 
level and a second unit delivering a modulation signal of said 
radiofrequency signal, formed from numerical information, characterized in 
that it comprises at least two analog multipliers connected in cascade as 
to the radiofrequency signal from the first unit and supplied in parallel 
by the modulation signal from the second unit. 
The invention also has for its object a process for the generation of 
radiofrequency excitation pulses modulated in amplitude for an NMR probe, 
utilizing the device mentioned above, which process is characterized in 
that it consists in producing separately on the one hand a radiofrequency 
signal of substantially constant amplitude, and, on the other hand, a 
modulation signal by attenuation of the type A.sup.1/n.f(t), in then 
carrying out n successive analog multiplications of the radiofrequency 
signal by the modulation signal, with n.gtoreq.2, and in delivering the 
resulting signal, of the RF.A.f(t) type, as the case may be after 
amplification, to the NMR probe.

As shown in FIG. 1 of the accompanying drawings, the device 1 for the 
generation of radiofrequency excitation pulses, modulated as to amplitude, 
comprises essentially a first unit 2, for example in the form of a 
radiofrequency synthesizer, delivering a radiofrequency signal RF of 
constant level (amplitude) and a second unit 3 delivering a modulation 
signal SM, adapted to modulate by attenuation said radiofrequency signal 
RF, this modulation signal being formed from digital data, particularly 
stored in a memory 4 connected to a digital/analog converter 5. 
According to the invention, the device also comprises, so as to carry out 
said modulation, at least two analog multipliers 6 connected in cascade as 
to the radiofrequency RF from the first unit 2 and supplied in parallel by 
the modulation signal SM from the second unit 3. 
The modulated signal available at the outlet of the last analog multiplier 
6 in cascade is then delivered to an NMR probe 7, as the case may be after 
amplification by a corresponding circuit 8. 
For the case in which the generating device 1 comprises n multipliers 6 in 
cascade, with n.gtoreq.2, the modulation signal SM delivered to said n 
multipliers 6 is of the A.sup.1/n.f(t) type, A.f(t) being the resulting 
desired modulation signal SMR. 
One of the inputs of each multiplier 6 is then connected to the output of 
the multiplier 6 which precedes it in the cascade or series structure, 
except the first, of which one of the inputs is connected to the output of 
the synthesizer 2, the other input of each of the n analog multipliers 6 
receiving the modulation signal SM delivered by the second unit 3. 
According to one characteristic of the invention, the second unit 3 is 
preferably constituted by a memory for containing point by point the 
envelope of the modulation signal A.sup.1/n.f(t) and by a digital/analog 
converter 5 whose output is connected to the inputs in parallel of the n 
analog multipliers 6 in cascade. 
Preferably, the analog multipliers 6 in cascade, for example two or three 
in number, consist of four quadrant wide band analog multipliers. 
The advantages obtained by the use of the generation device 1 according to 
the invention will become evident from an analysis of the values of Tables 
1, 2 and 3 hereinafter, obtained from a resulting modulation signal of 
ramp shape extending over 50 dB and defined numerically at 12 bits (4096 
points). 
TABLE 1 
______________________________________ 
Theoretical 
Memory SM in dB SMR in dB Equivalent 
(address) 
(amplitude) 
(amplitude) 
s/4096 pts of SMR 
______________________________________ 
4096 0 0 4096 
0.02% 
4095 -0.002 -0.002 4095 
/ / / / 
/ / / / 
/ / / / 
14 -49.32 -49.32 14 
7.7% 
13 -49.97 -49.97 13 
______________________________________ 
TABLE 2 
______________________________________ 
(Device according to the invention comprising two 
multipliers 6 in cascade) 
Theoretical 
Memory SM in dB SMR in dB Equivalent 
(address) 
(amplitude) 
(amplitude) 
s/4096 pts of SMR 
______________________________________ 
4096 0 0 4096 
0.004% 
4095 -0.002 -0.04 4094 
/ / / / 
/ / / / 
/ / / / 
231 -24.72 -49.95 13.027 
0.8% 
230 -25.01 -50.02 12.923 
______________________________________ 
TABLE 3 
______________________________________ 
(Device according to the invention comprising three 
multipliers 6 in cascade) 
Theoretical 
Memory SM in dB SMR in dB Equivalent 
(address) 
(amplitude) 
(amplitude) 
s/4096 pts of SMR 
______________________________________ 
4096 0 0 4096 
0.07% 
4095 -0.002 -0.006 4093 
/ / / / 
/ / / / 
/ / / / 
602 -16.65 -49.95 13.027 
0.69% 
601 -16.67 -50.01 12.938 
______________________________________ 
As these tables show, the minimum relative precision, corresponding to the 
smallest interval realizable, is 7.7% for the existing device as already 
indicated above, 0.8%[(13.027-12.923)/13.027] for the device according to 
the invention comprising two analog multipliers 6 in cascade and 
0.69%[(13.027-12.938)/13.027] for the device according to the invention 
comprising three analog multipliers 6 in cascade. 
Moreover, to define the envelope of a resulting modulation signal SMR at 50 
dB, it suffices to define the shape or envelope of the modulation signal 
SM (nth root of the envelope of the modulation signal SMR for n analog 
multipliers 6 in cascade), respectively at 25 dB (for two multipliers 6), 
16.67 dB (for three multipliers 6) or even (50/n) dB (for n multiplier 6). 
There results a substantial economy in memory space used for the storage of 
the signal modulation envelope, as well as an important gain in signal to 
noise ratio. 
Moreover, the digital/analog converter 5 works across a restricted range of 
its dynamic and in the region in which its relative precision is the 
highest (25 first decibels for two multipliers 6 in cascade). 
Moreover, the fact of raising to the square, the cube or the power n in an 
analog manner, the modulation signal SM, transposes the relative precision 
of the converter 4 to 25 dB, 16.67 dB or 50/n dB, to 50 dB for the 
resulting modulation signal SMR and therefore for the modulated 
radiofrequency signal transmitted to the NMR probe 7. 
The increase in precision of the definition of envelope shapes of complex 
modulation signals is also nicely achieved by comparing the three portions 
of the curves of FIG. 2 of the accompanying drawings, obtained for low 
values of a gaussian form of the type y=4096 exp(-x.sup.2 /2). 
The invention also has for its object a process for the generation of 
amplitude-modulated radiofrequency excitation pulses for an NMR probe 7, 
utilizing the device 1 as described above, said process consisting 
principally in producing separately, on the one hand, a radiofrequency 
signal RF of substantially constant amplitude and, on the other hand, a 
modulation signal SM by attenuation, of the A.sup.1/n.f(t) type, in then 
effecting n successive analog multiplications of the radiofrequency signal 
RF by the modulation signal SM, with n.gtoreq.2, and in delivering the 
resulting RF.SMR signal, of the RF.A.f(t) type, as the case may be after 
amplification, to the NMR probe 7 (SMR=SM.sup.n =[A.sup.1/n.f(t)].sup.n 
=A.f(t)). 
The generation and transmission of the modulation signal SM consists 
essentially in extracting the envelope of the modulation signal SM from a 
memory 4 in which it is stored point by point in numerical form, then in 
applying the numerical values successively read in said memory 4 to a 
digital/analog converter 5 and then delivering the output signal of said 
converter 5, in parallel, to one of the inputs of each of the multipliers 
6 of an assembly of n analog multipliers 6 in cascade, so as to perform n 
successive analog multiplications, preferably two or three, of the 
radiofrequency signal RF by the modulation signal SM=A.sup.1/n.f(t). 
Of course, the invention is not limited to the embodiment described and 
shown in the accompanying drawings. Modifications remain possible, 
particularly as to the construction of the various elements, or by 
substitution of technical equivalents, without thereby departing from the 
scope of protection of the invention.