It is known that such a method and device are suitable for determining the r.f. power of an r.f. electromagnetic pulse having a pulse angle of 90.degree. or 180.degree.. An r.f. electromagnetic pulse having a pulse angle .alpha. will be referred to hereinafter as .alpha.-pulse. For the execution of the known method for determining the r.f. power of an r.f. electromagnetic 90.degree.-pulse, a measurement cycle which includes an r.f. electromagnetic .theta..sub.o -pulse, where .theta..sub.o is, for example smaller than 90.degree., and an r.f. electromagnetic 180.degree.-pulse, said pulses being separated by an interval, is repeated a number of times, each time using a different pulse angle .theta. for the pulse angle .theta..sub.o, where .theta.=.theta..sub.o +n.DELTA..theta., wherein .DELTA..theta. is a positive angular variation when .theta..sub.o &lt;90.degree. and n=1 for the first repetition, n=2 for the second repetition etc. Because the intensity of an echo resonance signal is highest for a pulse angle of 90.degree., measurement of the intensities of the various echo resonance signals generated during the various measurement cycles enables estimation, by interpolation, of the r.f. power of an r.f. electromagnetic pulse which rotates the magnetization through 90.degree.. For the determination of the r.f. power of an r.f. electromagnetic 180.degree.-pulse, a similar method is performed; in that case a measurement cycle which includes an r.f. electromagnetic 90.degree.-pulse and an r.f. electromagnetic .theta..sub.o -pulse, where .theta..sub.o is, for example smaller than 180.degree., said pulse being separated by an interval, is repeated a number of times, where .theta.=.theta..sub.o +n.DELTA..theta.. Because the intensity of an echo resonance signal is also highest for a pulse angle of 180.degree., the measurement of the various intensities of a number of echo resonance signals similarly enables an estimation to be made by interpolation, of the r.f. power of an r.f. electromagnetic pulse which rotates the magnetization through 180.degree..
A fixed relationship exists between the pulse angle and the r.f. power of an r.f. electromagnetic pulse, i.e. the pulse angle is proportional to the square root of the r.f. power.
It is a drawback of the known method that the periods of time elapsing between the measurement cycles must be sufficiently long, that is to say much longer than the longitudinal relaxation time T.sub.1, in order to ensure that after each measurement cycle a steady state is re-established which corresponds to the magnetization when spin nuclei are in a state of thermal equilibrium. Consequently, the total experimentation time is considerable, the more so because a comparatively large number of measurement cycles must be performed in order to make a reasonably acceptable estimation of the r.f. power, and hence the pulse angle, of an r.f. electromagnetic 90.degree.-pulse or 180.degree.-pulse. Due to the long experimentation time, the known method is more susceptible to the effects of motion. Moreover, using the known method, the r.f. power of an r.f. electromagnetic pulse other than a 90.degree.-pulse or 180.degree.-pulse can be determined only if the relationship between r.f. power and pulse angle is known. As has already been stated, the pulse angle is proportional to the root of the r.f. power. However, the determination of a proportionality constant is difficult in practice; usually only a coarse approximation of the proportionality constant will be known.
It is an object of the invention to provide a magnetic resonance imaging method and device in which the total experimentation time is comparatively short and the determination of the pulse angle of an r.f. electromagnetic pulse is accurate, for accurate adjustment of an arbitrary pulse angle on the basis thereof.
A pulse sequence where five resonance signals are generated by means of three r.f. electromagnetic pulses is known per se from Physical Review, Nov. 15, 1950, Volume 80, No. 4, pages 580-594, Hahn. However, this article does not disclose a method of determining the pulse angle of an r.f. electromagnetic .alpha..degree.-pulse.