NUCLEAR MAGNETIC RESONANCE IMAGING METHODS

Abstract: To derive NMR imaging information an object is subjected to a static magnetic field and a sinusoidally time-varying magnetic gradient field G. A 90 DEG rf selective pulse P is applied around the time the gradient field G is zero and the subsequent free induction echoes E are detected and decoded. Additional non-selective pulses may be inserted at the times of echo peaks to refresh the echo signals. Additionally, to obtain two-dimensional information the gradient direction of the magnetic gradient field may be changed in successive cycles of the sinusoidal field.

Description: NUCLEAR MAGNETIC RESONANCE IMAGING METHODS This invention relates to NMR imaging methods and has particular application in methods of obtaining NMR imaging information from solid objects. The NMR imaging of solid materials has not been developed to the same level of sophistication as the NMR imaging of liquid and quasi-liquid materials. The principal difficulties with NMR imaging in solids arise because of the rapid rate of decay of the transverse magnetisation. In other words, the characteristic spin-spin relaxation time, usually designated T 2 , Is short. To produce sufficient spatial localisation of the selected nuclear spins 1n an object large magnetic field gradients must be superimposed, which Increases the rate of signal decay even further. In such circumstances much of the signal Is lost in the dead time of apparatus following the application of an rf excitation pulse. Also the excitation pulse itself needs to be of large amplitude to excite resonance. Furthermore, since 1n solids the characteristic spin-lattice relaxation time T 1 1s much longer than T , a considerable time interval of several T^s, must elapse before an experiment can be repeated. It is an object of the invention to provide a method 1n which the above difficulties are overcome. According to the Invention a method of obtaining NMR imaging information from a solid object comprises subjecting an object to a static magnetic field, applying a 90° rf excitation pulse to the object in the presence of a slnusoidally varying magnetic gradient field so that gradient echoes of the free induction signal are formed by successive reversals of the gradient field, and detecting the echo signals so formed. Preferably the relationship between the excitation pulse and the sinusoidally varying gradient field is such that the gradient field 1s substantially zero at the instant that the excitation pulse is applied. The excitation pulse is preferably a selective pulse. It can comprise a pulse sequence. In carry.-g out the Invention additional rf pulses can be applied at tr Instants of echo peaks. These additional pulses serve to maintain the amplitudes of the echo peaks. The rf phase of the additional pulses are preferably 1n quadrature to the rf phase of the excitation pulse. The additional pulses may be non-selective. In order that the invention may be more fully understood reference will now be made to the accompanying drawing 1n which: Figure 1 Illustrates diagrammatically a waveform of an experiment embodying the invention. Referring now to Figure 1 there 1 shown therein a time dependent magnetic gradient field G of sinusoidal shape. The power requirement to drive gradient colls to produce the sinusoidal waveform G can be reduced by Incorporating capacitance and tuning the resulting system to resonance. The cycle time of waveform G is 2τ and It passes through zero points at Instants τ and multiples thereof measur _ from a start point 0. At time 0 a90 β rf selective excitation pulse P Is applied. Pulse P Is of an appropriate frequency to excite resonant nuclei 1n a slice in an object. A free induction decay slgr.al F 1s generated. The time period over which the signal F 1s availa b le in solids before 1t decays away is extremely short. The time period 1s further reduced by the 'dead' time D of the apparatus immediately following pulse P. It is not possible to use the apparatus In time D to detect signal F. Due to the effect of the repetitive reversals of the gradient field G the initial signal F will be regenerated as a sequence of echo signal E with peaks at the time instant 2τ and multiples thereof but with progressively decreasing peak magnitudes. It will be seen that the first echo L and, of course, subsequent echoes occur well after ti- end o* the dead time L> . Furthermore since the 90° pulse P occurs wh he arrlltude of the field gradient G is at or near zero, th ower requirement for the rf pulse is a minimum. The envelope of the echo signal E contains spatially encoded information relating to the spin density distribution of resonant nuclei 1n the solid object 1n the direction of the gradient field G. However the signal must be decoded, for example by non-linear sampling, by taking Into account the time dependence of the magnetic gradient field G and Its magnitude. The peaks of amplitude of successive echoes decay with characteristic time T 2 - This decay 1s quite rapid in a solid but the decay rate can be slowed down by applying additional rf excitation pulses which act to refresh the echo signals E. These additional pulses are Inserted at the times of echo peaks and are in rf phase quadrature with the original excitation pulse P. The application of the additional pulses produces dead-time problems, obscuring part (or most) of the second half of each echo. Optimum results are achieved by collecting only the first half of each echo for data processing. An alternative procedure 1s to apply the rf pulses to coincide with alternate echoes provided the pulse separation 1s still shorter than or approximately equal to T 2 . The Intermediate echoes are used for data collection. The data from each detected echo 1s summed to Improve the signal/noise ratio. When a pulse starts at the peak of the echo the magnetisation Is spin-locked and the transverse magnetisation decay due to the field gradient will not occur until after the end of the pulse provided that the amplitude B j of the pulse 1s large compared to Gx where x Is the sample dimension 1n the direction of the field gradient G. To minimise decay due to the dipolar Interactions B 1 must be larger than the dipolar local field. The pulse length should be very much shorter than T j , the characteristic spin-lattice relaxation time in the rotating reference frame. After decoding the echo envelope to remove effects due to the time dependence of the field gradient the decoded signal is subjected to a Fourier transformation in the usual way to produce the spatial distribution of resonant nuclei from a spin-echo envelope in an assumed constant linear magnetic field gradient. The imaging information obtained by the methods described above relate to the spin distribution of the selected nuclei in the object 1n the direction of the magnetic gradient field. By changing the direction of the gradient field 1n successive cycles of that field, imaging information relating to corresponding different directions can be obtained and an entire two- dimensional Image can then be produced by projection reconstruction. Decoding of the imaging information needs to take account of the decay of the echo amplitude envelope. In this way the two-dimensional information is acquired In a time of the order of the decay time of the transverse magnetisation, which 1s typically much less than one second.

Claims: CLAIMS 1. A method of obtaining NMR imaging information from a solid object comprising subjecting an object to a static magnetic field, applying a 90° rf excitation pulse to the object 1n the presence of a sinusoidally varying magnetic gradient field so that gradient echoes of the free induction signal are formed by successive reversals of the gradient field, and detecting the echo signals so formed. 2. The method as claimed in Claim 1 in which the gradient field 1s substantially zero at the instant of application of the excitation pulse. 3. The method as claimed in either one of the preceding claims in which the excitation pulse is a selective pulse. 4. The method as claimed in any one of the preceding claims in which additional rf pulses are applied at the instants of echo peaks. 5. The method as claimed in Claim 4 in which the rf phase of the additional pulses are in quadrature to the rf phase of the excitation pulse. 6. The method as claimed in Claim 4 or Claim 5 in which the additional rf pulses are non-selective. 7. The method as claimed in any one of the preceding claims in which the gradient direction of the sinusoidally varying magnetic gradient field is changed in successive cycles thereof.

Inventors: COTTRELL STEPHEN PAUL, HALSE MORLEY ROGER, STRANGE JOHN HARVEY, COTTRELL, STEPHEN, PAUL, HALSE, MORLEY, ROGER, STRANGE, JOHN, HARVEY
Applicants: NAT RES DEV, NATIONAL RESEARCH DEVELOPMENT CORPORATION
Application year: 1990
Main classification: G01N24
IPC codes: A61B5, G01R33