GB2056088A - Improvements in or relating to nuclear magnetic resonance systems - Google Patents

Abstract

The invention is concerned with mapping RF absorption in a body in a medical nuclear magnetic resonance (NMR) imager. The absorption is detected by taking two NMR images by any suitable imaging method, such that the RF pulses for the two images are different but related so that, in the absence of RF absorption, they produce the same image. The two images are composed e.g. by taking their difference. This should be zero but if absorption has occurred the difference picture is a distribution of that absorption. In an alternative arrangement the difference between the images is non-zero but uniform in the absence of absorption.

Description

SPECIFICATION Improvements in or relating to nuclear magnetic resonance systems The present invention relates to systems for providing images of the distribution of a quantity, in a chosen region of a body, by nuclear magnetic resonance (NMR) techniques.

Nuclear magnetic resonance is well known for the analysis of materials, particularly by spectroscopy. Recently it has been proposed that the technique be applied more widely including the medical examination of patients. Such imaging systems provide distributions, of, for example, water content or relaxation time constants in sectional slices or volumes of the patients, which are similar to, but of different significance from, the distributions of x-ray absorption coefficients provided by computerised tomographic (CT) systems.

Several NMR imaging systems have been proposed, for example as described in co-pending patent applications 22291/78, 22292/78, 22293/78,22295/78 and 7921183. Such systems operate by applying suitable combinations of magnetic fields, including an RF field causing nuclei to precess, to the body being examined and detecting induced currents in one or more detector coil systems. Such RF fields have associated problems including the production of eddy currents from absorption of RF energy and a consequent loss of precession rate within the area enclosed by the currents.

Although absorption is small enough not to pose severe problems for frequencies of 4MHz (1000 Oe field) it becomes a problem at 58MHz and is very significant at 1012MHz.

For this and other reasons it is beneficial to be able to determine the distribution of loss of RF energy. It is one object of this invention to provide a method of evaluating RF absorption.

According to the invention there is provided a method of determining the distribution of absorption of RF energy in a body being examined in a nuclear magnetic resonance imaging apparatus which includes the application of RF pulses to induce resonance, the method including obtaining at least two images of the same region of the body, each image appropriate to an RF pulse having a different field integral and evaluating the relationship between the NMR signals for corresponding element in the at least two images to provide an image of the said distribution.

The invention embraces an apparatus having means for carrying out the said steps.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the single figure of the accompanying drawing which shows in block diagrammatic form an apparatus for putting it into effect.

For the examination of a sample of biological tissue NMR primarily relates to protons, (hydrogen nuclei) of water molecules in the tissue although other nuclei, for example those of tritium, fluorine or phosphorus, can be analysed.

The nuclei each have a nuclear magnetic moment and angular momentum (spin) about the magnetic axis. If then a steady magnetic field is applied to the sample the protons align themselves with that field, many being parallel thereto and some being antiparallel so that the resultant spin vector is parallel to the field axis.

Application of an additional field H1, which is an RF field at the Larmor frequency for the nuclei being examined at the local magnetic field value, causes resonance at the said frequency so that energy is absorbed in the sample. The resultant spin vectors of protons in the sample then rotate from the magnetic field axis towards a plane orthogonal thereto. The RF field is generally applied as a pulse and if) H,dt for that pulse is sufficient to rotate the resultant spin vectors through 7r/2 into that plane the pulse is termed a 7r/2 pulse. This applies similarly for other angles of rotation so that a 0 pulse is defined by achieving rotation through an angle 0.

On removal of the H, field the equilibrium alignments re-establish themselves with a time constant T1, the spin-lattice relaxation time. In addition a proportion of the absorbed energy is reemitted as a signal, centred at the resonant frequency, which can then be detected by suitable coils. This resonance signal decays with a time constant2 and the emitted energy is a measure of the nuclear distribution in the sample.

As so far described the detected resonance signal relates to the entire sample. For imaging purposes it is usual effectively to obtain individual resonance signals for elemental samples in a slice or volume of the patient. The effect is produced generally by causing different elemental samples to resonate at different frequencies and using some form of frequency analysis. A preferred method is by application of a field gradient a Hz (Gz= aZ simuitaneously with the H, pulse to select resonance preferentially in a cross-sectional slice.

This invention is applicable to those systems which provide such images, including those described in the said patent applications. However the particular system used to produce an image does not form a part of this invention and will not be further described.

As mentioned hereinbefore, the absorption of RF energy, mainly in the production of eddy currents, is reflected in a loss of precession, principally within the current loops and can be measured by a determination of phase error. Thus if the H, pulse was in theory a 7r/4 pulse the precession achieved in these regions may only be ft, where 0 < 7r/4. The effect of this will appear in the image resulting from the examination in that those elements not having the expected precession will not produce the same signal.

It is proposed to produce at least two pictures using different field integrals for the respective H, pulses. These can be provided by changing the H, pulse amplitude or duration or both. For example a first picture can be obtained using iT/4 H, pulses and a second picture obtained using 37r/4 pulses (generally using a --lr/4 pulse followed by a +x pulse). Where absorption is present the first picture will correspond to a precession of, say, ft ( < 7r/4) and the second to a precession of 3fit.

The difference between the signals obtained for corresponding elements in the two pictures is then sin 0 - sin 3 0. This is of course zero if O = 7r/4 when there is no absorption.

The output signals for the two elements where O= 7r/4 - (5 are: for the first picture: sin ('r/4 - ) 21/2 (1 - (5) and for the second picture: sin (37r/4 - 3) -- 2-1/2(1 - 36).

When 6 = 0 the two elements are identical.

However the effect of a finite 6 is to increase or decrease the output and to affect the difference (or ratio). Thus if the signals for all of the elements for one picture are subtracted from those for the other image and a picture formed of differences, That picture will only show any signal where absorption has occurred.

The signals following w/4 and n/4 - n pulses respectively may give an ambiguous solution. An alternative procedure which can resolve this is to use a 7r/2 pulse to provide a picture to compare with either of the above two. For that picture the output signal should be Sin (7t/2 - 2) 1 -- 262.

In that case there will normally be a difference between the two pictures since they should be, for all elements, in the ratio 1 :2-"2. However the difference would produce a uniform picture and any image visible thereon would indicate absorption.

An alternative, non-ambiguous pair is 7r/2 and 57r/2. It will be clear that, for the two pulses of precession angles ft and fit', the condition is sin ft' = sin ft or ft' = (2n - 1 )7r - 0 or 2nrr + 3 where n is a positive integer.

Apart from those mentioned hereinbefore, other typical pulse pairs are provided by the Carr Purcell spin echo sequence, proposed for evaluating T2, the sequence pair being 7t/2, T, 7r, signal and 7t/2, T, 37t, T, signal. This has the advantage that the variable f Hdt pulse needed does not coincide with z slice selection as it does with other methods.

A further solution is found in the use of the classic T, scanning sequence. This includes a 7r pulse at the start of the echo process. In the absence of absorption replacing the pulse by a 3 pulse results in no change though in the presence of absorption the effect on the angular change is enhanced for a 3n pulse compared with a 'pulse.

This generalises for 0 = 7t or O' = (2n + 1 )7r.

In the figure there is shown an apparatus for implementing the invention. At 1 there is shown an NMR imaging apparatus indicated only by a block since it may be of any suitable type in which the duration or amplitude of the H, pulses may be varied.

A timing control 2, which may merely comprise operator switches, provides signals indicating the two chosen H, directions (or amplitudes or both). In practice read only memories in apparatus 1 will contain information for two or more H, pulses and control 2 merely needs to provide a pulse on an appropriate line, to indicate which is required, and a start pulse for each examining sequence.

As each sequence is followed, the apparatus provides signals for elements of the examined slice in known manner and these are output to a random access memory (RAM) store 3 or 4, the signals for one H, pulse being sent to store 3 and those for the other to a store 4 by a switch 5.

Switch 5 is changed by a signal from control 2 as a signal is sent to the NMR apparatus to change the H, pulse used.

On a further command pulse from control 2 the signals from stores 3 and 4 are withdrawn simultaneously for corresponding picture elements, subtracted in a subtracting circuit 6 and stored in a further RAM 7. As a result RAM 7 holds a picture showing an image of RF absorption which can be shown on a display 8 or stored or reproduced in any other desired manner.

In the event that some regions may have more or less than the expected precession, the differences between the pictures may be positive or negative. Thus it may be desirable to denote zera difference by a mid range value (grey) the difference image showing excursions above or below the level.

If desired, instead of taking the difference of the two pictures, their ratio may be used.

Other arrangements for implementing the invention will be apparent to those skilled in the art.

It should be understood that for proper use of this invention a good slice-selection is important.

Suitable methods include using a sync pulse for H1, the tailored excitation method described in U.S. Patent No. 4021726 (Mansfield), or the volume method described in U.S. Patent No.

4070611 (Ernst).

In considering the full use of this method of mapping absorption, it should be remembered that mapping microwave RF absorption is equivalent to mapping permittivity and conductivity for the body. This provides pictures which are different from those of proton density but which it is believed may be clinically usefui.

Applications include study of blood vessels, including clotting and vessel wall defects.

Such examination of patients would of course require large scale apparatus such as have been proposed for medical imaging. However it will be apparent that for investigations such as those of radiation hazards smaller and simpler equipment, sized for examination of laboratory animals, would suffice.

This specification suggests the use of the pulses with rotation angles greater than 7r/2. It should, however, be noted that a practical problem arises for slice selection by simultaneous Gz gradient, since excited material at the edge of the selected slice takes all values between 0 and 0, unless suitable steps are taken to correct for this.

One practical solution to this difficulty is provided by dividing each pulse greater than Xr/2 into an unselective (U) vr component and a selective (S) component of angle required to make up the total rotation. An unselective pulse is one which is applied in the absence of the Gz slice selection gradient which normally accompanies the H1 pulse.

Applying this to the pulse sequence of 7r/4 followed by 3or/4 described hereinbefore it can be seen that a suitable practical sequence should be 7r/4 (S) for the first picture and 7 (U) with --75/4 (S) (or vice versa) for the second picture.

Similarly a pair of ir/2 followed by -3 x/2 can be given as 7r72 (S) for the first picture and - (U) with -'t/2 (S) for the second picture.

In all cases where pulses greater than 7r/2 are mentioned it should be understood that this procedure is recommended.

Claims (11)

1. A method of determining the distribution of absorption of RF energy in a body being examined in a nuclear magnetic resonance imaging apparatus which includes the application of RF pulses to induce resonance, the method including obtaining at least two images of the same region of the body, each image appropriate to an RF pulse having a different field integral and evaluating the relationship between the NMR signals for corresponding elements in the at least two images to provide an image of the said distribution.

2. A method according to Claim 1 in which the first pulse is a 0 pulse as herein defined and the second pulse is a 8' pulse as herein defined and (J' (2n - 1 )7r 0 or 0' = 2n + H where n is a positive integer.

3. A method according to claim 2 where f) = 7T/4 and f)' = v/47r.

4. A method according to Claim 2 where O- -r/2 and (I' 57r/2.

5. A method according to Claim 2 where () = and ()' (2n

6. A method according to Claim 5 where ()' 3X.

7. A method of determining the distribution of RF energy in a body being examined in an NMR imaging apparatus, the method being substantially as herein described with reference to the accompanying drawing.

8. A method of imaging the distribution of permittivity or conductivity or both in a body, including the method of determining a distribution of absorption of RF energy as claimed in any of the preceding claims.

9. A nuclear magnetic resonance apparatus including means for subjecting a body being examined to pulses of RF energy to induce resonance in a limited region thereof, means for sensing the resonance signals and means for processing the signals sensed to provide an image of said region, the apparatus further including means causing the apparatus to produce at least two images of the same region using RF pulses of different field integrals, means for comparing the two images to derive a resultant image in which amplitudes for individual elements represent absorption of RF energy.

10. An apparatus according to Claim 9 in which the means for subjecting the body to RF energy and the means causing the apparatus to produce at least two images one arranged to produce a first image with a O RF pulse as herein defined and the second image with a n' RF pulse as herein defined, O' being equal to (2n - 1 )7r - 0 or 2nr + 0 where n is a positive integer.

11. A nuclear magnetic resonance apparatus for determining the distribution of RF energy absorption in a body, the apparatus being substantially as herein defined with reference to the accompanying drawing.