GB2043914A - Improvements in or Relating to Imaging Systems - Google Patents

Abstract

In nuclear magnetic resonance (NMR) apparatus a phantom body may be used for calibration, for example to determine required sampling times such as at intervals having equal field integrals as previously proposed. A simple phantom, such as a disc produces a complex signal which is difficult to analyse to determine the required times. Here it is proposed to use a complex phantom which gives a simple signal, i.e. a substantially monotonic decreasing function. Thus the sampling times may be more readily determined. In a preferred example the NMR detectable material has a sinc<2>x density distribution in at least one direction, to give a straight line NMR time signal if sampling is correct. It is suitably an appropriately shaped water filled cavity 2 in plastic cylinder 1 having end-caps 3, 4. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Imaging Systems The present invention relates to systems for providing images of distributions of a quantity, in a chosen region of a body, by gyromagnetic resonance, in particular nuclear magnetic resonance (NMR) techniques. The invention particularly relates to the determination of sampling times for such a system.

Nuclear magnetic resonance is known for the analysis of materials, particularly by spectroscopy.

It has been suggested that the techniques be applied to medical examination to provide distributions of water content or relaxation time constants in sectional slices or volumes of patients. Such distributions are similar to, although of different significance from, the distributions of X-ray attenuation provided by computerised tomography systems.

Practical NMR systems operate by applying suitable combinations of magnetic fields to the body being examined, via coil systems, and detecting induced currents in one or more detector coil systems. A suitable sequence of pulsed magnetic fields and apparatus to operate that sequence have been devised and, together with other improvements, are disclosed and claimed in our copending applications Nos.

22291/78, 22292/78, 22293/78, 22294/78, 22295/78, and 7921183. (Serial No.

2,027,208).

In that arrangement a basic steady magnetic field Hzo is applied in a direction (z) usually parallel to the axis of the patient's body.

A further field in that direction (all fields in that direction being known as Hz) is applied as a gradient

This provides a unique total field value in a chosen cross-sectional slice of the patient A rotating RF field H1, of frequency chosen to cause resonance in the selected slice, is then applied. Thus the molecules in the selected slice, are caused to resonate. The resonance signal from the slice can then be detected. However as it is detected there is applied a further field

which is in the z-direction but has a gradient in a direction r perpendicular to z. This causes dispersion of the resonant frequencies in the rdirection and consequent dispersion of the resonance signal detected.Frequency analysis of this signal, generally by Fourier Transformation, yields a plurality of resonance signals each for a different one of a plurality of strips perpendicular to r in the chosen slice. For analysis by X-ray techniques this procedure is repeated for many different directions oft two provide a plurality of sets of signals for sets of strips in different directions. In a practical system the GR gradient field pulse is not applied as a square pulse but is another shape to suit design considerations, such as a distorted sinusoid.

In our copending application No. 22292/78 it is disclosed that it is desirable to sample the resonance signal at intervals such that the GA field integral is equal for each such interval. Such sampling is at unequal intervals of time and for that reason the technique has been identified as "non-linear sampling". In application No.

22292/78 there are disclosed arrangements for achieving this sampling. However a convenient way of determining sampling times would be by a test scan of a test object or "phantom".

Unfortunately, although such a method is inherently as accurate as the machine itself, it has proved complicated to resolve the time signal for a phantom, such as a circular disc of water, from noise which is present. It is therefore, excessively difficult to deduce the frequency of the detected signal from the distorted and noisy version obtained with a non-regular pulse profile.

It is an object of this invention to provide an arrangement by which a phantom can be used and it is another object to provide a suitable phantom.

According to one aspect of the invention there is provided a method of setting sampling times, in a nuclear magnetic resonance apparatus for examining planar sections of a body, the method comprising scanning, in at least one direction, a phantom of which the material detectable by said apparatus has a profile in that direction arranged to produce a time signal which is expected to be a predetermined substantially monotonic decreasing function, determining sampling times for the measured signal which give the predetermined monotonic function and using those times for sampling bodies other than said phantom in the said direction.

According to another aspect of the invention there is provided a phantom body, for nuclear magnetic resonance apparatus for examining planar slices of a body, the material in phantom which is detectable by said apparatus having a profile which when scanned in at least one direction produces a time signal which, in the absence of distortion, is a predetermined substantially monotonic decreasing function.

Preferably the phantom is a shaped mass of water enclosed in a plastic body.

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 accompanying drawings, of which, Figure 1 shows the shape of a phantom body giving the desired profile in one direction, Figure 2 shows the expected time signal for the phantom of Figure 1, Figure 3 shows a possible time signal provided in the presence of distortion, Figure 4a and 4b show expected and possible actual time signals respectively for a phantom producing a monotonic decreasing but not linear time signal, Figure 5 shows the construction of an actual phantom using the shape of Figure 1, Figure 6 shows in cross-section an alternative use of the shape of Figure 1, Figure 7 shows a phantom having a suitable profile in two orthogonal directions, and Figure 8 shows an NMR signal sampling system with which the invention may be used.

A simple phantom suitable for use in an NMR apparatus, of the type described in said copending applications, is a circular disc, of water since the apparatus is arranged to determine, for example, densities of water protons. In practice the phantom would be a plastic body, since suitable plastics will not give a significant resonance signal, having an internal disc-shaped cavity filled, typically, with water.

To determine sampling times it would appear to be suitable to make a test scan at arbitrary (conveniently equal) sampling intervals and analyse the time signals to determine sampling times for which a correct scan, accurately representing the phantom, can be obtained.

However such a simple phantom does not produce a simple signal but rather a complex one, which is excessively difficult to analyse, particularly in the presence of noise.

It is now proposed to use a phantom which is of a complex shape when then produces a simple signal which is straightforward to analyse. For a phantom which should produce a simple shape of signal it is straightforward to determine the correct times to sample to produce the correct shape. The same sampling times are then suitable for scanning an arbitrary object of more complex shape.

In one example of a phantom shape in accordance with this invention, it is proposed to use a sinc 2(x) cross-section in the plane of examination sin x (where sinc x= x The shape of such a phantom is shown in a threedimensional view in Figure 1 in which X, Yand Z are orthogonal directions two of which define the examination plane and the other being an orthogonal direction with which the axis of the body being examined is generally aligned.

For satisfactory use of this phantom it must be aligned so that the GA gradient field is along the X axis. It may, however, be placed so that either the Zor Ydirections are parallel with the body axis. It will be realised that for either of these orientations the densities of the strips, for which resonance signals are to be determined, and therefore the signals themselves will have a sinc2 (x) distribution. For examination of a patient the thickness of the slice intended to be examined is generally small in the body axis direction, as determined by the gradient field Gz, although some signal is received from other parallel slices to provide noise signal. For the purposes of determination of sampling times it is useful to turn off the Gz gradient thus creating a very thick slice and receiving a strong signal.Thus the readily NMR detectable material in the phantom is ideally larger in the body axis direction than is a normal slice and in this example extends for about three inches. As for all dimensions given herein, this figure may be varied to suit other requirements; in certain circumstances it may be more useful to limit the effective thickness of the phantom to less than six inches.

Although a phantom as shown in Figure 1 will only be effective at an orientation in which the GA gradient field is along the X axis, the phantom may still, of course, be used for other directions of r by the simple expedient of rotating the phantom about the body axis to suit the appropriate direction of the GA gradient rotated.

The time signal which should be obtained by using the Figure 1 phantom, in the correct orientation with correct sampling times, is shown in Figure 2, being the Fourier Transform of sinc2(x).

If the correct non-uniform sampling is not used the time signal obtained may be as shown in Figure 3 (conveniently this may be obtained by uniform time sampling). In this case the correct times for sampling to obtain the required time signal are provided by projecting suitable intervals on the amplitude axis onto the time axis. If the curve of Figure 3 was obtained using equal sampling intervals then the suitable amplitude intervals should be equal. The required sampling times, for the GA pulse phase actually used are then a, ... .g. It will be appreciated that the ability to obtain these times by such a projection results from the fact that for a sinc2(x) crosssection phantom the expected time signal is a straight line. It is not necessary for it to be so, provided that it is substantially a monotonic decreasing function.Suitable functions include those having limited non-decreasing regions, but they must not be oscillatory.

For example if the expected time signal is the curve of Figure 4a the measured time signal using equal sampling times may be that of Figure 4b. (It is emphasized that these curves of Figure 3 and Figure 4 are drawn arbitrarily for illustration and do not relate to an actual phantom, furthermore although relatively few samples are shown here, for the sake of clarity, it will be understood that in practice sufficient samples are taken properly to establish the function). The equal sampling times which would be required if the GA pulse were rectangular are those shown at a', b' . .. g' on the time axis of Figure 4a.By projecting on to the amplitude axis (Figure 4a) and from the amplitude axis (Figure 4b) to the time axis of Figure 4b there are found the sampling times, a", b" ... g", to obtain the correct time signal for the phantom with the actual GA pulse used. These sampling times will also be correct for other examined bodies with the same GA pulse. It has been assumed, in Figure 3 and Figure 4, that the measured time signal has been obtained using equal sampling times. If unequal, but still incorrect, sampling times are used it is still possible to use the phantom of this invention to derive the correct times but the procedure is more complex.

Figure 1 shows the shape of a sinc2(x) phantom in one direction. However, as discussed before, the phantom should be of water. Figure 5 shows an actual phantom of this shape. It is formed in this example by casting in plastic a 1 5 cm long by 25 cm diameter cylinder 1, with a sinc2(x) shaped hollow centre part 2. The centre hollow 2 is filled with water and the whole closed with end disc plates 3 and 4.

As an alternative, a sinc2(x) phantom may be symmetrical as shown in cross-section in Figure 6. Either of these phantoms may be enclosed in a plastic block of any suitable shape to allow the two possible orientations discussed hereinbefore.

Although the examples shown so far have been for sinc2(x) phantoms suitable for use for scanning in one direction of GA, employing rotation to change direction, it is possible to devise phantoms suitable for scanning in many directions without rotation. To achieve this the line integral of the quantity to be measured, in each possible scan direction, must be such as to give a sinc2(x) profile to be scanned.

Fig. 7 shows a shape, similar in form to that of Figure 1, suitable to provide a phantom for scanning in two orthogonal directions and giving a sinc2(x) profile in each.

A phantom of this shape must be orientated with the body axis in direction Z, i.e. Z is perpendicular to the examined slice. However at one orientation GA can be set in either the X or Y directions. Of course the phantom must also be rotated about the Z direction to allow sampling at other orientations of GA but the required rotation will be less than for the Figure 1 shape. This phantom also may be constructed of a suitable material, such as water set in a plastic block.

Other shapes may be devised suitable for more than two directions of GA without rotation.

Although the description hereinbefore is generally in terms of a sinc2(x) profile, it is emphasized that other phantoms giving a substantially monotonic decreasing time signal are within the scope of the invention.

In operation of the NMR system described in the said copending applications the sampling times determined as required are stored in a programmable read only memory PROM) which is used thereafter to sample at these times from the start of each scan. The system is shown in simplified form in Figure 8 in which the signal received from sensing coils 5 is applied via amplifier 6 to balanced demodulators 7 and 8.

Low pass filters 9 and 10 pass the analogue signal to analogue-to-digital converters (ADO) 11 and 1 2. These are clocked at the requisite time to produce the sampled spectrum for output at 13 at which it is put to further processing including Fourier Transform. The ADC's are clocked by signals from a clock 14 which are compared in a comparator 1 5 with the output of PROM 16 to obtain the correct sampling times. These times are determined as described hereinbefore using the phantom according to the invention and stored in PROM 1 6.

Claims (16)

Claims

1. A method of setting sampling times, in a nuclear magnetic resonance apparatus for examining planar sections of a body, the method comprising scanning, in at least one direction, a phantom of which the material detectable by said apparatus has a profile in that direction arranged to produce a time signal which is expected to be a predetermined substantially monotonic decreasing function, determining sampling times for the measured signal which give the predetermined monotonic function and using those times for sampling bodies other than said phantom in the said direction.

2. A method according to Claim 1 in which the phantom is sampled at equal intervals and the function thus obtained is used to determine the required sampling times to give said predetermined function.

3. A method according to Claim 2 in which the sampling times used are projected on to the amplitude axis of the predetermined function and said projections, transferred to the amplitude axis of the measured function are further projected on to the time axis thereof to give the required sampling times.

4. A method according to any preceding claim in which the phantom is arranged to provide a sinc2x density distribution in said direction to provide a straight line for said predetermined function.

5. A method according to Claim 4 in which the phantom is arranged to provide a sinc2x density distribution in more than one direction to allow variation of the said at least one direction without moving said phantom.

6. A method of setting sampling times, in a nuclear magnetic resonance apparatus for examining planar sections of a body, the method being substantially as herein described with reference to the accompanying drawings.

7. A phantom body, for a nuclear magnetic resonance apparatus for examining planar slices of a body, the material in phantom whilst detectable by said apparatus having a profile which when scanned in at least one direction produces a time signal which, in the absence of distortion, is a predetermined substantially monotonic decreasing function.

8. A phantom according to Claim 7 having a profile in said at least one direction which gives a sinc2x density distribution in that direction.

9. A phantom according to Claim 8 having a sinc2x density distribution in more than one direction.

10. A phantom according to any of Claims 7 9 comprising a plastics form having an aperture, of the required shape, filled with said detectable material.

11. A phantom according to Claim 10 in which the detectable material is water.

12. A phantom according to either of Claims 10 or 11 in which the plastic form is of substantially cylindrical shape externally.

13. A phantom according to Claim 12 in which the cylinder is of substantially 25 cm diameter.

14. A phantom according to any of Claims 713 in which the detectable material extends for less than six inches in a direction perpendicular to said at least one direction.

1 5. A phantom according to Claim 14 in which the detectable material extends for substantially three inches in a direction perpendicular to said at least one direction.

16. A phantom body, for a nuclear magnetic resonance apparatus for examining planar slices of a body, the phantom body being substantially as herein described with reference to the accompanying drawings.