GB2160660A - Nuclear magnetic resonance (NMR) imaging - Google Patents

Abstract

A method of NMR imaging comprising: applying to a body to be examined a magnetic field (Bo); imposing on the field a first gradient Gx in a first direction together with a radio frequency magnetic field pulse B1(90 DEG ) thereby to excite nuclear magnetic spins preferentially in a first selected slice of the body including a region of interest which is a fraction only of the slice; imposing on the field a second gradient Gy in a second direction together with a radio frequency magnetic field pulse B2(90 DEG ), thereby to excite nuclear magnetic spins preferentially in a second selected slice of the body which intersects the first slice except in the fraction of the slice; imposing a third gradient - Gx on the field in the reverse direction to the first direction while at the same time imposing a fourth gradient - Gy on the field in the reverse direction to the second direction, thereby to cause rephasing and encoding of the spins in that portion of the first slice not intersected by the second slice, while allowing spins elsewhere, i.e. in the second slice, to dephase; detecting the rephased spins; repeating the above steps a number of times with different values of the fourth gradient, and processing the detected signals to construct an image of the region of interest. <IMAGE>

Description

SPECIFICATION Nuclear magnetic resonance imaging This invention relates to nuclear magnetic resonance (NMR) imaging methods and apparatus.

In one known NMR imaging method, commonly referred to as the two-dimensional Fourier transformation (2 DFT) technique, nuclear spins are first excited in a selected slice of a body under examination, a first magnetic field gradient is then temporarily applied in the plane of the slice, and finally the NMR spins are detected in the presence of a second magnetic field gradient in the plane of the slice orthogonal to the direction of the first gradient. The experiment is repeated a number of times with different values of the first gradient, and the resulting sets of detected NMR signals are processed to recover image data using a two-dimensional Fourier transformation technique.The applied gradients cause the detected signals to differ in frequency and phase according to the location of the protons producing them in the slice, the signals obtained with the different values of the first gradient together enabling these locations to be determined unambiguously and an image representing the distribution of the apparent magnetisation in the slice to be obtained.

In this technique the second of the two applied gradients produces a frequence difference across the slice in the direction of the gradient, which difference need not be limited by any physical constraint. However, the first gradient, which precedes the second gradient, can only be used to impress a phase encoding in the slice which can be retained until data is read out in the presence of the second gradient.

If aliasing of one part of the image into another is to be avoided, the slice must be encoded uniquely with the phase difference across it being 2an, where n is the intended number of pixels in the direction of phase encoding. If it is desired to expand any part of an image, for example, by compressing the n pixels into half the slice, using the same numer of different phase encoding gradients as before would result in such aliaising. The common solution is to increase the number of phase encoding gradients, but this increases the duration of the scanning procedure.

Another difficulty which arises when the region of interest is only a fraction of the width of a slice is that even if the region of interest is static, moving tissue in the region not of interest can produce artefacts in the image obtained. Also, material in the region not of interest can contribute to dephasing of the NMR signals which limits the time for which data for construction of an image can be collected.

It is an object of the present invention to provide an NMR imaging method whereby one or both of these difficulties can be overcome.

According to the present invention there is provided an NMR imaging method comprising: applying to a body to be examined a magnetic field; imposing on said field a first gradient in a first direction together with a radio frequency magnetic field pulse, thereby to excite nuclear magnetic spins preferentially in a first selected slice of the body including a region of interest which is a fraction only of said slice; imposing on said field a second gradient in a second direction together with a radio frequency magnetic field pulse, thereby to excite nuclear magnetic spins preferentially in a second selected slice of the body which intersects said first slice except in said fraction of said slice; imposing a third gradient on said field in the reverse direction to said first direction and imposing a fourth gradient on said field in the reverse direction to said second direction, thereby to cause rephasing and encoding of the spins in that portion of the first slice not intersected by the second slice, while allowing spins elsewhere, i.e. in said second slice, to dephase; detecting said rephased spins; repeating the above step a number of times with different values of said fourth gradient, and processing the detected signals to construct an image of said region of interest.

Normally said rephased spins are detected in the presence of a further gradient imposed on said field in a direction transverse to said first and second directions.

The invention also provides an NMR imaging apparatus arranged to carry out a method according to the invention.

One method and apparatus in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which: Figures 1 and 2 illustrate the apparatus diagrammatically; Figure 3 is a diagram illustrating the sequence of steps involved in the method; and Figure 4 is a diagram further illustrating the method.

The apparatus is for the most part of conventional form, for example, as described in UK Patent Specification No. 1,578,910 or No. 2,056,078.

The basis elements of such an apparatus are as follows:- The apparatus includes a first coil system whereby a magnetic field can be applied to a body to be examined in a given direction, normally designated the Z-direction, with a gradient in any one or more of the three orthogonal directions i.e. X, Y and Z directions.

Referring to Fig. 1, the first coil system comprises coils 1 which provide a steady uniform magnetic field Bo in the Z-direction; coils 3 which provide a magnetic field gradient Gx in the X-direction, coils 5 which provide a magnetic field gradient Gy in the Ydirection; and coils 7 which provide a magnetic field gradient Gz in the Z-direction.

In addition, the apparatus includes a second coil system whereby RF magnetic fields can be applied to the body under examination in a plane normal to the direction of the magnetic field produced by the first coil system, and whereby RF magnetic fields resulting from nuclei in the body under examination which have been excited to nuclear magnetic resonance with a spin vector component other than in the Z-direction can be detected.

The second coil system comprises a first coil arrangement comprising a pair of coils 9A and 9B for applying RF fields, and a second coil arrangement comprising coils 1 OA and lOB for detecting RF fields.

The various coils 1, 3, 5, 7 and 9A and 9B are driven by Bo, Gx, Gz and RF drive amplifiers 11, 13, 15, 1 7 and 1 9 respectively, controlled by Bo, Gxy, Gz and RF control circuits 21, 23, 25 and 27 respectively.

These circuits may take various forms which are well known to those with experience of NMR equipment and other apparatus using coil induced magnetic fields.

The circuits 21, 23, 25 and 27 are controlled by a central processing and control unit 29 with which are associated inputs and other peripherals 31, for the provision of commands and instructions to the apparatus, and a display 33.

The NMR signals detected by the coils 1 0A and 10B are applied via an amplifier 35 to a signal handling system 37. The signal handling system is arranged to make any appropriate calibration and correction of the signals, but essentially transmits the signals to the processing and control unit 29 whereby the signals are processed for application to the display to produce an image representing the distribution of an NMR quantity in the body being examined.

It will be appreciated that whilst shown separately to clarify the present description, the signal handling system 37 may conveniently form part of the unit 29.

The apparatus also includes field measurement and error signal circuits 39 which receive signals via amplifiers 41 from field probes X1, X2, Y1 and Y2 which are disposed at suitable positions in relation to the body 43 being examined, as illustrated in Fig. 2, to monitor the applied magnetic fields.

A method of operating the apparatus of Figs. 1 and 2 in accordance with the invention so as to obtain an image indicating the distribution of an NMR quantity in the spine region of a patient will now be described with reference to Fig. 3.

The patient is first positioned in the apparatus with his torso in the region to which the field Bo is applied and with his length parallel to the Z-direction, and his front to back width parallel to the Y-direction.

The steady magnetic field Bo is then applied in the Z-direction by means of coils 1, this field serving to define an equilibrium axis of magnetic alignment in the region of the patient being examined, i.e. along the Zdirection, and remaining constant throughout the examination procedure.

A magnetic field gradient Gx along the Xdirection is then applied (see Fig. 3B) by means of coils 3, together with an RF magnetic field pulse denoted B1 (90 ) for reasons explained hereafter (see Fig. 3A). The frequency of the RF field is chosen to be at the Larmor frequency of chosen nuclei, typically hydrogen protons, in a slice 45 (see Fig. 4) of the patient's body 47 normal to the X-direction, i.e. a Y-Z plane slice extending along the length of the patient's body, and including the patient's spine 48. The slice 45 is defined by a particular magnetic field along the X-direction such that nuclei within the slice are preferentially excited.The integral of the RF pulse is such that the pulse is just sufficient to tip the spins of the excited nuclei into the X-Y plane, and is therefore referred to as a 90 pulse, the spins then precessing in the X-Y plane around the Z-axis.

The field gradient Gx is then removed and a field gradient Gy is applied in the Y-direction (see Fig. 3C) together with a further RF pulse B2 (90 ) (see Fig. 3A). The RF pulse B2 (90") differs from the B1 (90o) pulse in that it is relatively short and therefore contains a relatively broad spectrum of frequencies compared with the B1 (90 ) pulse and its centre frequency may differ as well. In consequence nuclear spins are excited in a relatively thick X-Z plane slice 51 which can be positioned as required.In the present example the pulse B2 (90 ) is tailored with respect to the applied gradient Gy so that the width of the excited thick slice 51 extends across the whole of the width of the patient's body 47 except the spine region of the patient.

The Gy field gradient is then removed and a field gradient -Gx applied in the opposite sense to the first applied gradient (see Fig.

3B). At the same time, a field gradient -Gy is also applied (see Fig. 3C). This causes rephasing of the spins in that part of the excited Y-Z slice not intersected by the excited X-Z slice, i.e. in the region of the Y-Z slice containing the patient's spine, and also serves to phase encode in known manner the rephased spins in the Y-Z slice.

Finally, the resultant of the nuclear spins in the body having a component in the X-Y plane is read-out by means of the coils 1 0A and 10B in the presence of a Z-direction frequency encoding magnetic field gradient Gz (see Fig. 3D), and the resulting detected signal is stored.

It will be appreciated that during read-out no signal is obtained due to spins within the excited X-Z slice due to the rapid de-phasing of such spins which occur after excitation.

The above described process is then repeated a number of times using a different value for the -Gy gradient each time.

The stored signals are then processed in known manner using a 2 DFT image construction technique to produce the required image of the patient's spine.

It will be understood that since no signal is obtained from the part of the patient's body corresponding to the X-Z slice, the full resolution of which the apparatus is capable may be applied across the fraction of the patient's body containing his spine. For the same reason any movement in the parts of the pa tient'swbody outside the spine region, i.e. in the heart and lung regions, will not contribute to the detected signal and will not therefore produce artefacts in the image obtained.

It will be appreciated that whilst in the particular method described above by way of example a part of a Y-Z slice through a body is imaged, the method according to the invention may be used to image part of a slice of any orientation.

Claims (5)

1. An NMR imaging method comprising: applying to a body to be examined a magnetic field; imposing on said field a first gradient in a first direction together with a radio frequency magnetic field pulse, thereby to excite nuclear magnetic spins preferentially in a first selected slice of the body including a region of interest which is a fraction only of said slice; imposing on said field a second gradient in a second direction together with a radio frequency magnetic field pulse, thereby to excite nuclear magnetic spins preferentially in a second selected slice of the body which intersects said first slice except in said fraction of said slice; imposing a third gradient on said field in the reverse direction to said first direction and imposing a fourth gradient on said field in the reverse direction to said second direction, thereby to cause rephasing and encoding of the spins in that portion of the first slice not intersected by the second slice, while allowing spins elsewhere, i.e. in said second slice, to dephase; detecting said rephased spins; repeating the above steps a number of times with different values of said fourth gradient, and processing the detected signals to construct an image of said region of interest.

2. A method according to Claim 1 wherein said third and fourth gradients are imposed at the same time.

3. A method according to Claim 1 or Claim 2 wherein said rephased spins are detected in the presence of a further gradient imposed on said field in a direction transverse to said first and second gradients.

4. An NMR imaging method substantially as hereinbefore described with reference to the accompanying drawings.

5. An NMR apparatus arranged to carry out a method according to any one of the preceding claims.