GB2182449A - Nuclear magnetic resonance imaging apparatus - Google Patents

Abstract

A phantom for use in assessing the performance of an NMR imaging apparatus which enables quantitative assessments of spatial resolution under different conditions of contrast to be made comprises four wedge shaped enclosures 33-39 each containing a material of different known response, for example solutions of different concentration. In use an imaging slice extends through the enclosures in the plane of the drawing and cut-off resolution is indicated by the distance from the wide end of each enclosure at which the image of that enclosure disappears; such distance is assessed from inspection of the displayed images or more accurately from an inspection of computed brightness pixels along the length of the enclosures. <IMAGE>

Description

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

In order to assess the performance of such apparatus use is normally made of a so-called phantom, that is to say, an qbject so constructed that an assessment of apparatus performance can readily be made from a visual inspection of the image of the object produced by the apparatus and/or quantitative analysis of data produced by the apparatus for use in presentation of the image. Such data typically comprises the brightness value of each pixel of the image produced by the apparatus, pixel brightness being a function of the total NMR signal detected by the apparatus as arising from the part of the object being imaged corresponding to that pixel.

A known form of NMR phantom comprises an enclosure made of a material which produces an effectively zero NMR response when imaged by the apparatus. Within the enclosure there may be disposed various suitably shaped members of the same or another zero NMR response material, the whole of the remainder of the interior of the enclosure being filled with a material of known NMR response. The material of known NMR response is required to have a highly homogeneous distribution of protons of the kind which are to be excited to resonance in use of the apparatus. Where, as is usual, the protons to be excited are hydrogen protons, a suitable material of known NMR response for use in an NMR phantom is a copper sulphate solution or manganese chloride solution or other solution containing free hydrogen ions.

Since NMR imaging apparatuses are normally required to image a planar slice of an object, thereby to provide a cross-sectional view of the object in the plane of the slice, or in the case where imaging of a volume of an object is required, a plurality of parallel spaced planar slices disposed within the volume, NMR imaging phantoms typical have a tubular enclosure, the phantom being disposed in use so that the apparatus under test displays images of one or more planar slices of the phantom extending parallel to the plane ends of the enclosure.

The shaped members of the phantom, if present, are designed with a view to assessment of various parameters of the image. For example, to assess the spatial resolution of the image the shaped members typically comprise a number of spaced rods of different diameters extending parallel to the axis of the enclosure, and/or a series of plates disposed in planes parallel to the axis of the enclosure, the plates being of different thicknesses and different spacings.

To assess the image under different conditions of contrast, different regions of the phantom enclosure may be of different internal length i.e. so as to contain different depths of the material of known NMR response.

However, there does not exist at the present time a form of NMR imaging phantom which enables a satisfactory quantitative assessment of spatial resolution under different conditions of contrast in the image, e.g. under different conditions of proton density in the object being imaged.

It is an object of the present invention to provide an NMR imaging phantom and method of using such a phantom which provides this facility.

In accordance with a first aspect the invention provides an NMR imaging phantom comprising a plurality of enclosures each of which contains a material of different known NMR response, the enclosures being shaped so as each to provide a length of a said material of continuously decreasing width in a plane intersecting the enclosures.

Preferably each enclosure is shaped to provide a length of said material of width decreasing linearly to zero.

Said enclosures are suitably wedge-shaped.

Preferably said enclosures are identically shaped.

The invention also provides a method of using a phantom according to the third aspect of the invention to assess the spatial resolution of an NMR imaging aparatus under different conditions of contrast.

Thus in accordance with a second aspect of the invention there is provided a method of assessing the spatial resolution of an NMR imaging apparatus under different conditions of contrast comprising; disposing in an examination chamber of the NMR apparatus a phantom according to the first aspect of the invention; positioning the phantom and operating the apparatus so as to obtain an image of a planar slice of said phantom parallel to said plane intersecting said enclosures; and determining for each enclosure the distance from the wider end of that enclosure at which the image of that enclosure disappears.

One NMR phantom in accordance with the invention and a method according to the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a sectional plan view of the phantom; and Figure 2 is a perspective view of one of four enclosures constituting the phantom of Fig. 1.

Referring to Figs. 1 and 2, the phantom comprises four identically shaped and sized hollow wedge-shaped enclosures 33, 35, 37 and 39. Each of the enclosures is made of clear plastics material having a substantially zero NMR response. Each enclosure is filled with a material, 41, 43, 45 or 47, e.g. a solution, having a known NMR response, the materials in the different enclosures having different NMR responses.

The triangular ends of the enclosures, (see 49 in Fig. 2) are suitably substantially in the form of isosceles triangles having a smallest angle of about 3 .

The filling material is suitably a solution of manganese chloride, the concentration of the solutions lying in the range 1.00 x 1017Mn++ions/millilitre to 1.00 x 1015 Mn++ions/millilitre. In one embodiment used in practice solutions of concentrations 5.02x 10'6, 1.56x1017, 2.61x1017 and 5.23 x 10'7Mn++ions/millilitre respectively were used for the four enclosures.

In use of the phantom the four enclosures are disposed in the examination chamber of an NMR apparatus under test, the enclosures being disposed so that when the apparatus is operated to image a slice, the slice extends through each enclosure in a direction parallel to the triangular ends of each enclsoure.

The enclosures are suitably arranged in spaced pairs, as shown in Fig. 1, and may be secured to the outside of another phantom, e.g. to the curved wall of the phantom of Figs. 1 to 4 of United Kingdom Patent Specification GB-A-2,155,187, with their triangular ends parallel to the lid 3 and the base of the body portion 1 of the phantom shown in that specification.

A quantitative assessment of the spatial resolution of the apparatus is obtained from the images of the four enclosures. Cut-off resolution is indicated by the distance from the wider end of each enclosure at which the image of that enclosure disappears. Thus a figure for cut-off resolution is obtained at each of four values of the contrast ratio. It will be appreciated that cut-off resolution may be more accurately assessed from an inspection of the computed brightness values for pixels along lines extending along the lengths of the enclosures, than by observation of displayed images of the enclosures.

Claims (8)

1. An NMR imaging phantom comprising a plurality of enclosures each of which contains a material of different known NMR response, the enclosures being shaped so as each to provide a length of a said material of continuously decreasing width in a plane intersecting the enclosures.

2. A phantom according to Claim 1 wherein each said enclosure is shaped to provide a length of said material of width decreasing linearly to zero.

3. A phantom according to Claim 1 or Claim 2 wherein said enclosures are wedgeshaped.

4. A phantom according to any one of Claims 1 to 3 wherein said enclosures are of substantially identical shape.

5. An NMR imaging phantom according to Claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

6. A method of assessing the spatial resolution of an NMR imaging apparatus under different conditions of contrast in an image comprising: disposing in an examination chamber of the NMR imaging apparatus a phantom comprising a plurality of enclosures each of which contains a material of different known NMR response, the enclosures being shaped so as each to provide a length of a said material of continuously decreasing width in a plane intersecting the enclosures; positioning the phantom and operating the apparatus so as to obtain an image of a planar slice of said phantom parallel to said plane intersecting said enclosures; and determining for each enclosure the distance from the wider end of that enclosure at which the image of that enclosure disappears.

7. A method according to Claim 6 wherein determination of said distance at which each image of an enclosure disappears is carried out from inspection of computed data as opposed to observation of a display of an image of the phantom.

8. A method according to Claim 6 substantially as hereinbefore described with reference to the accompanying drawings.