WO1994003210A1

WO1994003210A1 - Contrast agents for medical imaging

Info

Publication number: WO1994003210A1
Application number: PCT/GB1993/001667
Authority: WO
Inventors: Ian John Rowland; Martin Osmund Leach; David John Collins
Original assignee: Institute Of Cancer Research
Priority date: 1992-08-07
Filing date: 1993-08-06
Publication date: 1994-02-17
Also published as: GB9216843D0

Abstract

Contrast agent for medical diagnostic techniques comprising a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer, and a medical diagnostic method for the human or animal body comprising the administration of this contrast agent and detection thereof by a spectroscopic or imaging technique. The polymer preferably contains two or more different NMR-visible nuclei and the images of the different nuclei are superimposed. Advantageously, the polymer contains fluorine and is, for example, a fluorinated organosilicon polymer, for example polymethyl-3,3,3-trifluoropropylsiloxane. Non-fluorinated and partially fluorinated polyethers are also mentioned. The method is especially suitable for imaging the gastrointestinal tract.

Description

CONTRAST AGENTS FOR MEDICAL IMAGING

Field of the invention

The present invention relates to contrast agents suitable for medical imaging and spectroscopy. Various imaging techniques are now used in medical diagnostics, including X-rays, radioisotope imaging, nuclear magnetic resonance imaging, ultrasound imaging and computer tomography.

Nuclear magnetic resonance imaging (often referred to simply as magnetic resonance imaging, MR imaging, MRI) is a non-invasive technique which produces cross- sectional images of poorly-conducting heterogeneous systems, particularly living systems, by using the well- established methods of nuclear magnetic resonance (NMR) . NMR is the characteristic resonant absorption and emission of radio-frequency electromagnetic radiation by atomic nuclei possessing a magnetic moment. Tn vivo these nuclei are predominantly ¹H, ³¹P, ¹³C and ²³Na. The hydrogen nucleus is one of the most suitable for NMR purposes, and MR imaging is widely used as a diagnostic technique, exploiting the different proton relaxation rates, which are in turn governed by the local environ¬ ment at a molecular level.

Generally, two different relaxation processes can occur for nuclei. In the first, the excess spin energy equilibrates with the surroundings (the lattice) by spin-lattice relaxation having a spin-lattice relaxation time (or longitudinal relaxation time) -^. Secondly, there is a sharing of excess spin energy directly between nuclei via spin-spin (or transverse) relaxation, the symbol for the time of which is T₂ • The relaxation behaviour of the excited magnetism (i.e. the loss of the transverse component of magnetization, T₂ relaxation, and the recovery of the longitudinal component of magnetiza- tion, T_]_ relaxation) affect the measured signal.

T_]_ relaxation comes about by lattice motions (e.g. atomic vibrations in a solid lattice or molecular tumbling in liquids and gases) having approximately the right frequency to interact coherently with nuclear spins. T^ varies greatly, ranging typically from 200 ms to 20 s for protons in solids, and from 10^-4 to 10 sec for liquids, the overall shorter times for liquids being due to the greater freedom of molecular movement, leading to larger fluctuations of magnetic field in the vicinity of the nuclei. In tissues this relaxation depends on dipole-dipole interactions, and the T*_j_ relaxation time tends to be longer for small molecules in fluids.

T₂ relaxation is inversely proportional to spectral line width and a short T₂ relaxation time is characteris- tic of slowly tumbling or relatively strongly bound molecules which produce broad peaks in spectra. For solids T₂ is usually very short, and can range from approximately 20 μs to 20 ms, while for liquids T₂~T₁.

In biological systems, the nuclear magnetic relaxa¬ tion rates vary from tissue to tissue and, more importantly, between diseased and normal tissue. By carefully choosing the timing intervals in the irradiation pattern, it is possible, for example, to discriminate against long relaxation rates and, there¬ fore, highlight regions having short relaxation rates, or, for example, highlight tissue with a long T₂ at the expense of tissue with a short T₂.

MR imaging is of increasing clinical importance and can be used, for example, to locate and study tumours and to monitor their response to therapy, as well as to reveal other abnormalities. Although the contrast in MR and other images between different tissues can provide sufficient information for discrimination between those tissues, it is often desirable to enhance the contrast, more especially when the tissues are otherwise very similar, and a number of contrast agents have been developed for this purpose.

These may be divided into two classes: positive contrast agents, leading to enhanced signals or images, and negative contrast agents, which destroy the signals.

Positive contrast agents for MR imaging are usually paramagnetic and their effect on the nearby water molecules is to shorten the T--_ relaxation time, so that with adjustment of the imaging parameters to be sensitive to short relaxations, the image in the region of the contrast agent is bright in comparison with that from surrounding tissue; GdDTPA ( the dimeglumine salt of the gadolinium(III) complex of diethylenetriaminepentaacetic acid) is a common example.

Negative contrast agents for MR imaging are usually ferromagnetic or superparamagnetic, and have an effect over a wider region. They act by reducing T₂ relaxation times or by causing large local magnetic susceptibility gradients so as to cause rapid dephasing of the signal, leading to areas of diminished image intensity. Other negative contrast agents act by displacing imageable protons/material, thereby causing reduced signal inten¬ sity. Oral magnetic particles (OMP, Nycomed) is a common example of an oral superparamagnetic contrast agent; ferromagnetic contrast agents include magnetite (Ferro¬ magnetic contrast agents: A new approach, Renshaw, et al., Magn. Reson. Med. (1986), 3, 217-25) .

In abdominal magnetic resonance imaging, barium sulphate (which is also used extensively in computer tomography ("CT") investigations) has found application as a negative contrast agent, appearing hypointense in magnetic resonance images by reducing intra-luminal proton density; other negative contrast agents which also act via reduction in proton density include gas, clays and perfluorochemical emulsions; superparamagnetic compounds (iron oxides) also appear hypointense, due to T /susceptibility effects. Positive contrast agents that have found application include oils, ferric ammonium citrate, mixtures of both, and GdDTPA.

Neither type of contrast agent, however, gives completely satisfactory results in the abdomen. The available negative contrast agents do not completely negate the image in the tissue of interest, and the known positive contrast agents do not give a sufficiently bright image, providing insufficient contrast between the gastrointestinal (GI) tract and associated abnormalities. In addition, artefacts from bowel movement and respiratory-induced gut motion can produce a blurred image. This can be obviated by the use of very fast sequences that freeze motion. With slower imaging sequences, bowel movement can be reduced by adminis¬ tration of glucagon, but respiratory-induced motion of the upper GI tract remains a problem unless imaging times short enough for breath-holding can be achieved. This requires the use of imaging sequences that have a short duration compared with the respiratory cycle.

There is, therefore, considerable demand for an improved magnetic resonance image contrast agent which avoids the above problems; the ability to distinguish stomach, bowel and other parts of the GI tract clearly from neighbouring structures in MRI is essential for accurate diagnosis of intra-abdominal disease. Summary of the invention

The present invention provides the use of a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer, for the manufacture of a contrast agent for use in MR imaging or other medical diagnostic technique.

The present invention also provides a medical diagnostic method, more especially a method of MR imaging the human or animal body, wherein there is administered as contrast agent a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer.

The present invention further provides a contrast agent for medical diagnosis or medical diagnostic techniques, more especially for MR imaging, comprising a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer, optionally together with a suitable carrier or adjuvant or two or more such additives. Unlike the common contrast agents of the prior art, the contrast agents of the invention may be visualised directly; with both fluorinated and non-fluorinated contrast agents of the present invention a bright image is produced, not by the effect of the contrast agent on the surrounding water molecules, but by the imaging of the contrast agent itself.

Accordingly, the present invention also provides a method for the non-invasive determination of chemical and/or physical conditions within a whole intact human or animal body, which comprises administering a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer, and visualising this polymer directly.

The present invention further provides a method for the study of the human or animal body, for example of the GI tract, which comprises administering, as contrast agent, a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer, and detecting the contrast agent by a spectro¬ scopic or imaging technique.

The term "polymer" is used herein to denote homopolymers and copolymers, and to include oligomers (including dimers) . The polymer may have a linear or branched or cyclic structure, or may comprise two or more such moieties; it may, if desired, be cross-linked.

There may, for example, be one or more than one type of NMR-visible nucleus in the contrast agent selected, for example, from H, D, F, ^{l λ}B, ¹³C, ²⁹Si, ²³Na and P, and the different nuclei may be imaged and the images compared, preferably by superimposing one on another.

An NMR-visible ( ' imageable' ) nucleus should generally be magnetically equivalent to any of the same nuclei in the same chemical moiety, and when the same 'imageable' nuclei are contained in more than one unit within the same polymer molecule they should be in approximately the same magnetic environment. Increasing numbers of imageable monomer units within the polymer (increasing chain length) reduces the relative signals from the end groups andi from other monomers, thereby increasing the potential signal intensity. Thus, preferably, one or more NMR-visible nuclei are contained in repeated units of the polymer, although, depending on the size of the polymer, a monomer unit containing an NMR-visible nucleus need not be repeated, and, for example, the polymer may comprise two or more different monomer units each having an NMR-visible nucleus. For example, the polymer may contain two or more of H, D, ¹³C, F and P in separate monomer units, with, for example, one nucleus being sensitive to pH and another to temperature, and/or one may be sensitive to pH or temperature and another may absorb X-rays strongly.

Polymers containing fluorine are especially useful, enabling ¹⁹F NMR imaging to be carried out. Alterna- tively or, more usually, in addition, proton magnetic resonance imaging may be carried out, and anatomical information may be distinguished by superimposing ¹H images and ¹⁹F images, advantageously with different image intensity weighting. Fluorinated silicone oils have proved especially suitable as contrast agents of the present invention. Prior art

Up till now, there has been very little clinical use of F-imaging, and no use of fluorinated polymeric contrast agents for medical diagnosis of pathology or abnormal anatomy.

Thus, for example, fluorocarbons and fluorocarbon derivatives have been administered previously to humans, for example as blood substitutes, and such substances have been investigated by MR imaging (Branch et al. , Magn. Reson. Med. 2_0, 151-7 (1991) and Joseph et al., J. Comput. Assist. Tomogr. (1985) , 9, 1012-9) . For example, Joseph et al. have imaged the blood substitute perfluoro- tributylamine and have suggested that perfluorotri- propylamine may find clinical acceptance for imaging the cardiovascular system for identifying the location of vascular elements and providing information about the velocity of blood flow. Branch et al. have used trifluoromethane in imaging cerebral blood flow. Shimizu et ai. (Radiation Medicine, Vol. 9, No. 1, pages 1-8 (1991)) disclose the examination of gastrointestinal lesions by ¹⁹F-MRI using perfluorotripropylamine and perfluorotripropylamine-perfluorodecalin mixtures. However, the compounds used for these purposes were non- polymeric and in the form of aqueous emulsions. EP 118 281 A (Children's Hospital Medical Center) discloses an NMR spectroscopic method for the detection of gases in vivo by detecting a first radio-frequency signal derived from resonance of an element of the animal influenced by a gas, detecting a second radio-frequency signal derived from an element independent of the animal uninfluenced by the gas, and comparing the signals to detect the gas. The "element" may be a perfluorocarbon, for example in the form of an aqueous artificial blood composition. There is, however, no disclosure of the use of such substances for medical diagnosis detecting abnormal anatomy or pathology, and more especially no use for imaging the abdomen.

Other fluoro compounds used in clinical MR imaging, computer tomography and sonography are perfluoroalkyl bromides and iodides, which have been tested as GI contrast agents, but apparently only with proton imaging (Mattrey et al., Am. J. Roentgenol., 148. 1259-1263

(1987) and Mattrey, 152, 247-252 (1989)) ; in the former, the authors state that the agents used are free of protons, thus causing no MR signal, and argue that a decrease in signal intensity is more advantageous than increase in signal produced by paramagnetic solutions, and in the latter Mattrey states that fluorine "is the next best nucleus (after hydrogen) for MR application", but notes that acceptability and potential utility in vivo of ¹⁹F imaging of perfluorooctyl bromide has not been documented. Moreover, such compounds are non- polymeric.

Sucrose polyester has also been used as an oral contrast agent for MR imaging of the GI tract (Ballinger et al. , Magnetic Resonance in Medicine, H, 199-202, (1991)). Bellinger et al. nowhere suggest that the sucrose polyester itself could be the only source of signal. This material has a structure very different from the contrast agents of the present invention: sucrose polyester (a dietary fat substitute) is formed from esterification of up to eight fatty acid chains to a sucrose molecule, whereas contrast agents of the present invention have a polymeric structure, which is par¬ ticularly well suited to MR imaging by virtue of the mobility of its molecules and intensity of images produced.

Silicone oils have been imaged previously within breast implants, and WO 91/14457 (The Victoria University of Manchester) discloses the study of internal body tissues using a proton-rich organo-silicon polymer, more especially polydimethylsiloxane. However, there is no disclosure of the use of ¹⁹F spectroscopy or imaging or of the use of fluorinated compounds; nor is there a disclosure of the use of any non-silicon-containing polymer or of the use of any polymer with more than one NMR-nucleus.

Accordingly, there has been no disclosure of a medical diagnostic method which comprises MR spectroscopy or imaging using a fluorinated and/or non-silicon- containing polymeric contrast agent of the present invention, nor has there been any disclosure of MR imaging of different nuclei in polymers and superimposing the images obtained. Brief description of drawings Figure 1 shows the measured T-^ and T₂ relaxation times of polydi ethylsiloxanes with differing viscosities;

Figure 2 shows the effect of mixing two poly¬ dimethylsiloxanes (18 mPas and 378 mPas viscosities) on the longitudinal and transverse relaxation parameters;

Figure 3a shows the ¹⁹F NMR spectrum in CDC1₃ of a polymethyl-3 , 3 , 3-fluoropropylsiloxane FS 1265 (Trade Mark) having viscosity at 25°C of 300 ctsk;

Figure 3b is an expansion of the spectrum of Figure 3a;

Figure 4a shows a -'-H spin echo image of a rat approximately four hours after administration of FS1265 (10000 ctsk viscosity) ;

Figure 4b shows the equivalent fluorine spin echo image;

Figure 4c shows the combined proton and fluorine image;

Figure 5 shows a combined proton and fluorine FLASH gradient echo image of a rat after administration of FS1265 (10000 ctsk viscosity);

Figure 6 shows an in_. vivo multi-spin echo determina¬ tion (PHAPS) of the ¹⁹F transverse relaxation time of FS1265 (300 ctsk viscosity) within the GI tract of a rat;

Figure 7 shows a maximum intensity projection (0°) fluorine image (FLASH 3D gradient echo sequence) of a rat after administration of FS1265 (300 ctsk viscosity) ; Figure 8 shows an in vitro multi-spin echo deter¬ mination (PHAPS) of the ¹⁹F transverse relaxation time of mixed cyclic 3,3,3-trifluoropropylmethylsiloxane (CF₃CH₂CH₂SiCH₃-0-)_n, n = 3-4;

Figure 9 shows an in vitro multi-spin echo ¹H determination (PHAPS) of the transverse relaxation times of cyclic derivatives of dimethylsiloxane. Tube A contains polydi ethylsiloxane (18mPas) , Tube B contains decamethylcyclopentasiloxane and tubes C, D & E contain octamethylcyclotetrasiloxane; Figure 10 shows an in vitro multi-spin echo deter¬ mination (PHAPS) of the ¹⁹F transverse relaxation time of polyperfluoroethylene glycol dimethacrylate. Detailed description of the invention

In comparison with contrast agents previously used for the gastrointestinal tract, the contrast agents of the present invention provide a very strong signal, even when the concentration of the agent is diluted by the presence in the GI tract of water and air. Polymeric structures are particularly suitable for optimisation of properties: not only may modifications to the basic structure be made, but changes in physical properties of the polymer, such as viscosity, and changes in the extent of cross-linking/branching as well as chemical modifica¬ tion, for example by metal co plexation, particle suspension or emulsification, may be used, together with selection of appropriate MR imaging techniques, to provide optimum imaging processes. Increasing the number of monomer units within the polymer, for example, will also increase the viscosity and reduce the relaxa¬ tion times. Moreover, the absence of significant natural fluorine in the body makes the use of fluorine-containing contrast agents and fluorine imaging especially advan¬ tageous, avoiding the potential for confusion of contrast agent with anatomical signal experienced with proton magnetic imaging. It is therefore possible to obtain clearly delineated and unequivocal images of the structures/tissues containing the contrast agent, and also to obtain these with rapid imaging techniques, which reduces the problems of artefacts from peristalsis or respiratory movement.

The contrast agent used according to the present invention is generally inert and stable, and more especially is physiologically inert and is stable under physiological conditions. Thus, when administered, the contrast agents should have acceptable toxicity, which may be an intrinsic property or mediated by suitable encapsulation. If used as an oral contrast agent, as well as being stable at body temperature, the polymer should preferably be inert to fluids present in the GI tract. There should be no significant absorption of such material through the gastrointestinal wall, and the material is generally non-toxic to the patient. If used for injection, or if the polymer is soluble enough to be absorbed into the GI tract, the polymer should of course be physiologically tolerable in the amount utilised, and may, for example, be targeted to specific tissue. The polymeric structure may increase the stability of the individual monomeric units to biological processes, but the bonds formed during the polymerisation reaction may themselves be susceptible to in vivo reactions. This may be used advantageously to monitor enzyme levels by measuring changes in relaxation rates or other parameters. Depending on the technology available, the polymer may be fluid (gas or liquid) or a solid. For assessment of gut permeability, a fluid rather than a solid material would have the advantage of more ready absorption. The polymer should of course have appropriate relaxation parameters. Current imaging technologies with liquids require T₂ values above 10 or 20 ms. Preferably, therefore, a contrast agent according to the present invention has a T₂ value >10 ms, for example >20 ms; values >30 ms, for example >40 ms, should be mentioned. There is no minimum placed on the T*_j_ value, and indeed a short T-_j_ relaxation time (for example < 600 ms, preferably < 500 ms) is preferred with current technology.

Monomer units may each contain one or more than one type of NMR-visible nucleus, for example F and H; F and D; F, H and D; H and D. NMR-visible nuclei may be present once or more than once in each such unit.

Preferably the structure of the polymer is such as to enable a single resonance for a given nucleus to be obtained in the imaging process. Thus, preferably all the NMR-visible nuclei or all the NMR-visible nuclei of any one kind are chemically equivalent.

One or more repeated units may be arranged sequentially, and advantageously all or substantially all the molecule comprises repeated units in a straight chain or cyclic arrangement. Long chain structures may be employed to minimise the proportion of end groups in the molecule and hence their effect on the image. Cyclic structures are also useful to provide equivalence; mono- or multi-cyclic compounds are possible, including bridged rings and cage structures, and macrocyclic rings are included. Crown ethers and Buckminster fullerenes and related compounds, including fluorinated derivatives, should also be mentioned.

Thus, in one embodiment, only one repeated unit is present and the molecule comprises a cyclic arrangement of identical repeated units or contains a single long chain of identical repeated units, providing a single resonance.

In a different embodiment there may be two different units that are repeated, at least one of which contains an NMR-visible nucleus. Thus, for example, repeated units of one kind or chains of two or more sequentially arranged repeated units of one kind may be separated by other units. These other units may, for example, themselves be repeated in the molecule sequentially and/or alternating with the first type of repeated unit or repeated unit chain. Alternatively or in addition, those other units may be non-NMR-visible in the imaging process used for the first kind of repeated units. For example, one repeated unit may contain fluorine and no hydrogen, or fluorine and hydrogen together; the other may contain hydrogen and no fluorine. An advantageous arrangement is, for example, as follows:

-(R) _nl- (T-i)_ml-(R)_n2-(T₂)_m2-(R) _n3- (T₃)_m3- where preferably n₁=n₂=n₃; m₁=m₂=m₃ and T₁=T₂=T₃

Advantageously, a contrast agent of the present invention is a siloxane polymer, for example a fluoro- alkylsiloxane polymer, advantageously a perfluoroalkyl- methylsiloxane polymer, a polyether, for example an alkylene oxide polymer, for example poly(fluoroethylene glycol) , or a fluorinated or non-fluorinated hydrocarbon polymer; copolymers of such compounds should also be considered. Such classes of homo- and co-polymers are generally inert, exhibit low toxicity and are relatively insoluble in water, and they provide contrast agents having suitable relaxation properties.

Thus, for example, possible repeating units include, for example, those of the general formula - Si(R₁) (R₂)0 - in which each of R-^ and R₂, which are the same or dif¬ ferent, represents a hydrocarbon group, for example a C-^-Cg-alkyl group, for example methyl or t-butyl, a vinyl or methyl- vinyl group, or a longer-chain alkyl group, e.g. a C₇ to C₂₂-alkyl group, for example as found in naturally occurring products; or a hydrocarbon group, more especially a C₂-C₆-alkyl group or a longer-chain alkyl group, e.g. a C₇ to C₂₂- alkyl group as mentioned above, substituted by one or more fluorine atoms at other than the α-carbon atom.

A fluorinated group R and/or R preferably has 3 or more carbon atoms, the β-carbon atom also being unsub¬ stituted by fluorine, and preferably the fluorine is present at the <- -carbon atom or in a C(CF₃)₃ group at the end of the chain. Preferably a fluorinated group R and/or R₂ is a fluorinated alkyl group of the general formula

-(CR₃R₃)_m-R₄ where

R₃ represents a hydrogen atom or methyl group, m represents a number >2 and R₄ represents a fluorinated ( C^-C^ ) -alkyl group, preferably CF₃ , CH F, CHF₂ or C(CF₃)₃. When such units contain long chain alkyl or fluorinated alkyl groups, these chains may be long enough for the imaging of nuclei within the chains, and the resultant compound is a branched chain polymer.

Other repeating units have, for example, phenyl and phenyl derivatives within the repeating unit; for example the repeating unit may be

^CH2^CH2 " Si(Me)₂ - 0 - Si(Me)₂

where n represents 0 or 4.

Suitable cyclic polymers contain, for example, the units

-Si(CH₂CH₂CF₃)₂0-, -(CF₃CH₂CH₂)Si(CH₃)0- or -Si(CH₃)₂)0- arranged, for example, in a 6 to 12-membered, for example

6 to 10-membered, ring; that is, having 3 to 6, for example 3 to 5, monomer units.

Polymers containing more than one repeating unit, preferably in a regular repeat, should also be mentioned. Examples of repeating units are

CH₂CH₂CF₃ CH-3 - Si - O - Si - O -

C IH₂CH₂CF₃ CIH₃ CH CHpCF-, CH₃ i I

- si - o - si - o - i I

CH-> CH-,

CH₂CH₂CF₃ CH₂CH₂CH₃ - Si - 0 - Si - O -

I I

CH₂CH₂CF₃ CH₂CH₂CH₃

Alkylene oxide polymers, especially ethylene oxide polymers, optionally containing one or more fluorine atoms in the repeated units, are also possible. Examples of repeating units are

-(CF₂)_m-0- ( = 2 or 3) , -C(CH₃)₂-0-, -CH₂CF₂-0- ,

CH₃

I -CH-CF₂-0- and -CH₂CF₂CH₂-0-; more than one such repeating unit may be present, preferably in a regular repeat; a cyclic structure may, if desired, be used. Examples of these polyethers are perfluoropolyethylene glycols. Corresponding thio compounds should also be mentioned.

Fluorinated and non-fluorinated hydrocarbon poly- mers and acrylates, including methacrylates, especially partially fluorinated polymers of these kinds having fluorinated and non-fluorinated repeating units, for example having (i) CF₂ units and (ii) CH₂ or C(CH₃)₂ units, are also possible, arranged, for example, in a regular sequence so the polymer comprises a chain of -(CF₂)_n -(CH₂)_n - units where n-^ and n₂ are each prefer¬ ably 1, to optimise the fluorine content and reduce coupling between local fluorine atoms.

In general, J-coupling between neighbouring fluorine nuclei (i.e. those nuclei not attached to the same carbon or other atom) will tend to complicate the NMR resonances and therefore the images. This can be overcome by suitable techniques, but, for simplicity, J- coupling is desirably kept to a minimum. Proton coupling to the fluorine resonances should also desirably be minimised for imaging purposes. However, the use of double resonance techniques to transfer magnetisation to influence or observe nuclei at frequencies other than the transmitted frequency may use any homo- or hetero- nuclear coupling advantageously.

Corresponding deuterated compounds may also be used. J-coupling can be reduced by deuteration adjacent to the carbon or other atom with the fluorines attached.

Examples of deuterated repeating units include those of the formulae

CD₃ CHoCDpCF-*. CH->

I I I

- Si - O - - Si - O - - Si - O - i I l CD₃ CH₂CD₂CF₃ CD₂CH₂CF₃

- CD₂CH₂ - and - CD₂CF₂ - and - CD₂CF₂ - O -. The polymer may, if desired, contain functional groups, for example at the ends of the molecule. Incorporation of copolymers with functional groups within the chain or at branch points should also be mentioned. The end groups of the silicone polymers may also be important for labelling with agents such as gadolinium or radioisotopes. Additionally, carboxylate end groups on lower molecular weight silicones may impart more favourable water solubility/miscibility. These various polymers are known or may be prepared, for example, by methods known per se, for example as described by Holle, H.J. & Lehnen, B.R. , Preparation and characterization of polydimethylsiloxanes with narrow molecular weight distribution, Europ. Poly. J. (1975) , 11, 663-7. Polymerisation reactions are well known to those skilled in the art.

Known polymers used in the present invention as contrast agents, however, may differ from commercially available material; for example purity is important for use as a contrast agent, but not necessarily so for other uses. In addition, specific monomer units may be included to aid emulsification, the polymers may be labelled with radioisotopes, may be encapsulated or a sulphate derivative incorporated in order to bind barium. Commercially available fluorinated polymers of the above kind include fluorosilicone fluids, for example perfluoroalkylmethylsiloxanes, for example polymethyl- 3 ,3 , 3-trifluoropropylsiloxanes, of the formula

polydimethylsiloxane (PDMS) of the formula

and copolymers of fluoroalkylsiloxanes with dimethyl- siloxanes, for example 50% methyl-3 , 3 , 3-trifluoro- propylεiloxane and 50% dimethylsiloxane copoly er, and dimethylsiloxane-alkylene oxide block copolymers.

Fluorosiloxanes are known to be relatively inert in vitro; see, for example, "Effects of emulsification, purity and fluorination of silicone oil on human retinal pigment epithelial cells", Friberg, T.R. , et al., Invest. Ophthamol. Vis. Sci. (1991) , 32, 2030-4.

PDMS is used as an ingredient in cosmetic creams, powders and aerosols; as a mechanical or electrical fluid, as a lubricant, an anti-foam or a surfactant. Polymethyl-3 , 3 , 3-trifluoropropylsiloxane is used, for example, for foam control in solvent systems, and longer chain fluorinated silicon fluids are used, for example, as partitioning phases in gas chromatography. However, to our knowledge, fluorosiloxanes have not previously been used as contrast agents for NMR imaging in the human body. PDMS is an inexpensive, non-hazardous material with susceptibility and relaxation parameters similar to those found for aqueous protons n vivo. Polymethylsiloxane polymers are available as Dow Corning 200 Fluid (Trade Mark) in a wide range of viscosities, from 0.65 to 2,500,000 ctsk. -L and T₂ relaxation rate measurements were per¬ formed on polydimethylsiloxanes of differing viscosities using SE/IR and PHAPS imaging sequences respectively (Gowland et al., Magnetic Resonance in Medicine 12., 261, (1989), & Graumann et al., Magnetic Resonance in Medicine 3_., 707, (1986)). Spectroscopic (IR) measurements were used also to determine T . We found that the T₂ relaxa¬ tion times of PDMS shortened with increasing viscosity. In addition, the T^ relaxation times also shortened with increasing viscosity, but the effect was less pronounced (see Figure 1). PDMS's with different viscosities can be mixed to obtain blends with intermediate viscosities and to provide, in a predictable manner, different (inter¬ mediate) relaxation parameters (Figure 2) . Equally, T₂ and T-^ relaxation times for a given other polymeric structure are expected to shorten with increasing viscosity (higher molecular weight) . Thus, by utilising a chosen polymer of suitable viscosity or a mixture of polymers of the same basic structure but with different viscosities, the relaxation parameters may be optimised for use with different imaging sequences. Polymethyl-3,3,3-fluoropropylsiloxanes are available commercially, for example, with viscosities 90-150, 300, 1,000 and 10,000 ctsk, and a silanol-ter inated poly- methyl-3,3,3-trifluoropropylsiloxane is available with a viscosity of 30-50 ctsk; Dow Corning FS 1265 Fluid (Trade Mark)

(CH₃)₃Si - 0 -(-Si(CH₃) (CH₂CH₂CF₃) - O -)— Si(CH₃)₃ is available in viscosities (at 25°C) of 300, 1,000, and 10,000 ctsk.

FS 1265 has physical and chemical properties making it ideally suited to in vivo ¹⁹F magnetic resonance imaging. FS 1265 affords a single resonance (see Figure 3a) . An expansion of the frequency (x) axis is shown in Figure 3b and reveals minor peaks which are unlikely to influence image quality significantly. Spectra were obtained using a 6T Bruker Spectrospin spectrometer.

FS1265 also possesses relaxation parameters similar to those found for in vivo protons (measurement of the relaxation parameters of the commercial FS 1265 fluid having a viscosity of 300 ctsk (molecular weight 2350) gave T and T₂ values of 340 ms and 140 s respectively (25°C, 1.5T)). For use as a contrast agent of the present invention a trimethylsilanol-terminated polymethyl-3, 3, 3-trifluoro-propylsiloxane with reduced T-^ and T₂ values may be advantageous, this optimisation being achieved by blending of fluorosilicone polymers of differing viscosities or the inclusion within, or mixtures with, an appropriate relaxation agent. By suitable blending of polymers to adjust the viscosity, the in vivo transit properties of the material may be optimised to suit a particular application.

FS 1265 is a chemically inert polymeric material and is expected to exhibit low toxicity at high oral doses; over 95% of the material is eliminated via the GI tract after one week. Thus, the low toxicity associated with FS 1265 may be, in part, due to lack of significant transport/uptake from the gut. (Conversely, elimination of the material from the peritoneum may not be significant. Consequently, as for barium sulphate, non- encapsulated FS 1265 administration would not be advised if perforation of the GI tract is suspected. Encapsulation of the material may, however, reduce the potential effects of leakage into the peritoneum.)

The polymers are immiscible with water, have relatively high densities (1.25-1.30 g/cm³) and low surface tension(s) . These properties suggest that FS 1265 should pass rapidly through the GI tract without the need for peristalsis.

We have now successfully demonstrated its use as an oral contrast agent for MRI studies of the abdomen. We have used the material inter alia for ¹⁹F MR imaging of the GI tract of rats. The relatively short relaxation times and high fluorine density of the material (fluorine molar concentration « 22-25 M) permitted the use of rapid imaging techniques. 2D, 3D and maximum intensity projection images were also obtained which, when combined with proton images, clearly delineated the GI tract, enabling abnormalities or tumours to be discriminated.

Table 1 below shows ¹⁹F T-^ and T₂ values for FS 1265 polymers of different viscosities (measurement at 25°C, 1.5 T) .

Table 1

Viscosity (ctsk) No. of repeating T**L ( s) T (ms) units 300 14 306 146

1000 28 326 72

10000 89 297 < 25

We have also investigated, n vitro, poly(perfluoro- ethylene glycol) dimethacrylate

mixed cyclic 3 , 3 , 3-trifluoropropylmethylsiloxanes

n 3-4, and various cyclic dimethylsiloxanes

n = 5; n = 4; and a cyclic dimethylsiloxane of vis¬ cosity 18 mPas. The polymers were found to have suitable relaxation parameters for use in the present invention.

Polyperfluoroethylene glycol dimethacrylate was also administered intravenously and intraperitoneally to anaesthetised rats. Good quality ¹⁹F images were obtained from the compound. Intraperitoneal administra- tion of the polymer provided excellent contrast between the loops of the gut.

Relaxation properties may, if desired, be modified by formation of a metal complex or use of a paramagnetic or ferromagnetic species, thus shortening T^ and T₂ relaxation times.

When used as an oral or rectal contrast agent, the contrast enhancement by the polymer in the GI tract is limited by artefacts from bowel motion. Human studies have long shown the efficacy of glucagon in reducing bowel motion for CT and MRI studies, and glucagon may therefore also be administered in the imaging process of the present invention.

The contrast agent may be formulated with other pharmaceutically acceptable carriers and adjuvants, selected, for example, from dispersing agents, colouring agents, flavouring agents, thickening agents, preservatives and anti-bacterial agents, viscosity regulating agents, osmolality regulators and solvents, e.g. water.

For use as an oral contrast agent, for example, the polymer should advantageously be administered in an acceptable form so as to avoid any unwanted oily sensa¬ tion or taste. Macro or microencapsulation of the contrast agent, such as FS 1265, for example by inclusion in gelatin capsules, or by a suitable inert variation of the polymer or use of a different polymer may also be advantageous in reducing these effects, as well as potentially aiding the GI distribution. Encapsulation would also reduce the toxicity of polymers containing chelated gadolinium (either mixed or chemically attached) , radioisotopes, radio-opaque compounds or mixtures of each; the encapsulant may, but need not, constitute or be complexed to the polymer.

Other methods of administration are also possible, including, for example, intraperitoneal and intravascular administration. For parenteral administration an emulsion or solution in a sterile physiologically acceptable medium is preferably used, for example an isotonic aqueous solution. Gaseous contrast agents may also be administered via the respiratory tract, for example by inhalation or via a tube; aerosols should, for example, be mentioned. As will be understood in the art, oral and rectal administration of gaseous agents should also be contemplated.

The polymeric contrast agent of the invention may also be used for imaging other organs/tissues, more especially if attached to a targeting molecule. Use in imaging the kidney, the spinal canal, the genito-urinary system, the liver, the reticulo-endothelial system, joints, the lymphatic system and the eye should especially be mentioned. An example is the possibility of using oral polyethylene oxide contrast agents for imaging the liver or kidney; alternatively, these compounds may be injected.

Applications to PET, radioisotope imaging, X-ray, CT, applied potential tomography and ultrasound should also be mentioned; if required, compounds may be adapted accordingly. Complexation of a PET isotope to a polymer such as polyethylene glycol may oe used. For example, iodine may be added to a monomer within a copolymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer. The agents may be used, for example, for detection of pathology or abnormal anatomy, or, for example, may be used for general physiological measurement.

Diagnosis or other investigation may b carried out on the live or dead human or animal body. The following Examples illustrate the invention. Imaging and spectroscopic methods

Measurements were performed using a Siemens 1.5 T Magnetom imaging and spectroscopy system. and ¹⁹F in vivo images and in vivo spectra were obtained using a Hel holtz pair coil. 2D and 3D FLASH and FISP imaging sequences were found to provide optimum signal to noise ratios in the minimum imaging time. Example 1 CBH/Cbi rats were administered FS1265 liquids of 300, 1000 and 10000 ctsk viscosities by oral gavage. Animals were anaesthetised throughout the NMR experiments and were imaged for periods of up to 90 minutes. Equivalent proton and fluorine images were obtained using identical imaging sequences and parameters. In vivo spin echo images were also obtained from the 10000 ctsk material (Te 17ms, Tr 200ms, Si 4mm, Ma 128E) (Figure 4) ; images had been taken approximately four hours after administration. Figure 4(a) shows the proton image, Figure 4(b) the equivalent fluorine image and Figure 4(c) the combined proton and fluorine image. The combined image showed the presence of the fluorinated material in the caecum and suggested that the high viscosity material remains as a bolus throughout its n vivo transit. Acceptable combined proton and fluorine gradient echo (FLASH) images were obtained (Te 11ms, Tr 25ms, FI 15°, Ma 128 (half Fourier) ) within a total imaging time of less than 7 seconds (see Figure 5) ; the presence of FS1265 in the stomach was clearly shown.

Figure 6 shows the data points obtained in a multi echo PHAPS sequence (Te = 28-328 ms, Tr = 1.5 s. Si = 8mm, Ma = 128) calibrated to measure T2 relaxation times. Each point represents signal intensity acquired at different echo times. The information was obtained from fluorine images of a rat following administration of FS1265 (300 ctsk viscosity) . In this set of images, signal from the stomach was clearly evident. The extent of the fluorine signal within the rat's stomach is shown schematically, together with the region of interest from which these data points were obtained. The circle represents the relative position of a reference sample. The circle in the top right-hand corner represents the relative position of a reference sample containing FS1265 (1000 ctsk viscosity, T₂ = 74 ms) . The transverse relaxation was found to be essentially monoexponential, with a relaxation time of approximately 200 ms (see Table 2) . Addition of the proton and fluorine images provided essential anatomical detail and contrast between the GI tract and other organs/structures. As expected, in vivo longitudinal and transverse relaxation times for the 300, 1000 and 10000 ctsk viscosity materials were found to be longer than the values obtained at 25°C (see Table 2) . 3D FLASH and FISP data were also acquired from anaesthetised animals. A typical result is shown in TABLE 2:

FS1265 ¹⁹F RELAXATION TIME

MEASUREMENTS *

* relaxation times determined from the same animal are listed on the same row

** IR spectroscopic measurement

Figure 7 in the form of a maximum intensity projection. (0°) fluorine image of a rat following administration of approximately 4.5 cm³ of FS1265 (300 ctsk viscosity).

(FLASH 3D gradient echo sequence, Te = 12ms, Tr = 100ms,

Fl = 70°, SI = 2mm, Ma = 256HFE) ) .

Example 2 The following other fluorinated and non-fluorinated polymers were investigated in vitro: (a) the mixed cyclic 3, 3, 3-trifluoropropylmethylsiloxane (CF₃CH₂CH₂SiCH₃-0-)_n, n = 3-4

(b) polydimethylsiloxane (18 mPas, density 0.954 g/ml, b. pt. 95-134°C (3mm)) (c) decamethylcyclopentasiloxane (mw 370.8)

(d) octamethylcyclotetrasiloxane (mw 296.6)

(e) poly(perfluoroethylene glycol) dimethacrylate, the dimethacrylate ester of 1000 mol weight perfluoro- polyethylene glycol

CH-> CH₃

I I

CH₂=C-CO-(0-CF₂-CF₂)_n-0-CO-C=CH₂

Polymers (a) , (c) and (d) were manufactured by Hulε America Inc. , formerly known as Petrarch Ltd. , and were available from Fluorochem Limited; (b) was available from Fluka as DC200 silicon oil; (e) was available from Dajac Laboratories Inc. via Fluorochem Limited.

Figure 8 shows a measure of the ¹⁹F signal intensity of (a) as a function of echo time in the region of interest defined by the inner circle within the outer circle (the image of a tube containing the specific material) ; imaging parameters: Te = 28-328ms, Tr = 1.5s, Si = 8mm, Ma = 128. The transverse relaxation time of the mixture was determined from the slope of the graph as 586 ms. A T_]_ image was also obtained (not shown) , providing a longitudinal relaxation time of 670ms. The polymer had suitable relaxation parameters; advantageously higher cyclic derivatives could be used.

Figure 9 shows the corresponding multi-spin echo ¹H measurements for the cyclic derivatives of dimethyl- siloxane (b) , (c) and (d) ; imaging parameters as for Figure 8 but with Te = 28-648ms. Tube A contains polydi ethylsiloxane (18mPas) , Tube B contains deca- methylcyclopentasiloxane and tubes C, D and E contain octamethylcyclotetrasiloxane. T-*L and T₂ values obtained from these samples are shown in the table below, and show the polymers to be suitable for use as contrast agents.

Material T-^ (ms) T₂ (ms)

Polydimethylsiloxane (18mPaε) 964 735

Decamethylcyclopentasiloxane 1164 990 Octamethylcyclotetrasiloxane 1300 1262

Figure 10 shows the corresponding multi-spin echo ¹⁹F measurements for (e) using the same imaging parameters as for Figure 8. The transverse relaxation time was measured as 68ms. A T*^ image was also obtained (not shown) , providing a longitudinal relaxation time of 197 ms. The polymer had very good relaxation parameters for use in the present invention.

Claims

1. A medical diagnostic method for the human or animal body, wherein there is administered as contrast agent a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer.

2. A method for the non-invasive determination of chemical and/or physical conditions within a whole intact human or animal body, which comprises administering a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer, and carrying out measurements on individual areas of the body to detect the polymer itself.

3. A method as claimed in claim 1 or claim 2, wherein the polymer contains two or more different NMR- visible nuclei.

4. A method as claimed in claim 3, wherein the polymer contains two or more of H, D and F.

5. A method as claimed in claim 3 or claim 4, wherein different NMR-visible nuclei are present in a monomer unit.

6. A method as claimed in claim 5, wherein a monomer unit contains D vicinal to F.

7. A method as claimed in any one of claims 3 to 6, wherein different nuclei are imaged and the images are superimposed.

8. A method as claimed in any one of claims 1 to 8, wherein the polymer is a fluorinated organosilicon polymer.

9. A method as claimed in any one of claims 1 to 8, wherein the polymer is a polyether.

10. A method as claimed in claim 9, wherein the polymer is a non-fluorinated or partially fluorinated polyether and H-imaging is carried out.

11. A method as claimed in any one of claims 1 to

10, wherein the polymer is a cyclic polymer.

12. A method as claimed in any one of claims 1 to

11, wherein the polymer contains different monomer units.

13. A method as claimed in claim 12, wherein the monomer units differ in respect of the identity of the NMR-visible nucleus or nuclei contained therein.

14. A method as claimed in claim 12 or claim 13, wherein the polymer contains monomer units which are repeated alternately.

15. A method as claimed in any one of claims l to 14, wherein substantially all the NMR-visible nuclei of any one kind are chemically equivalent.

16. A method as claimed in any one of claims 1 to

15, wherein the polymer contains fluorine and is used for imaging the gastro-intestinal tract.

17. A method as claimed in any one of claims 8 to

16, wherein the polymer is a polymethyl-3,3,3-trifluoro¬ propylsiloxane.

18. Use of a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer, for the manufacture of a contrast agent for use in MR imaging or other medical diagnostic technique.

19. Use as claimed in claim 18, wherein the polymer contains two or more different NMR-visible nuclei.

20. Use as claimed in claim 19, wherein the polymer contains two or more of H, D and F.

21. Use as claimed in claim 18 or claim 19, wherein different NMR-visible nuclei are present in a monomer unit.

22. Use as claimed in claim 21, wherein a monomer unit contains D vicinal to F.

23. Use as claimed in any one of claims 18 to 22, wherein the polymer is a fluorinated organosilicon polymer.

24. Use as claimed in any one of claims 18 to 23, wherein the polymer is a polyether.

25. Use as claimed in claim 24, wherein the polymer is a non-fluorinated or partially fluorinated polyether.

26. Use as claimed in any one of claims 18 to 25, wherein the polymer is a cyclic polymer.

27. Use as claimed in any one of claims 18 to 26, wherein the polymer contains different monomer units.

28. Use as claimed in claim 27, wherein the monomer units differ in respect of the identity of the NMR-visible nucleus or nuclei contained therein.

29. Use as claimed in claim 27 or claim 28, wherein the polymer contains monomer units which are repeated alternately.

30. Use as claimed in any one of claims 18 to 29, wherein substantially all the NMR-visible nuclei of any one kind are chemically equivalent.

31. Use as claimed in any one of claims 18 to 30, wherein the polymer contains fluorine and is used for imaging the gastro-intestinal tract.

32. Use as claimed in any one of claims 23 to 31, wherein the polymer is a polymethyl-3,3,3-trifluoro¬ propylsiloxane.

33. A contrast agent for medical diagnosis or medical diagnostic techniques, comprising a polymer in which an NMR-visible nucleus is contained in at least one monomer unit within the polymer, together with a suitable carrier or adjuvant or two or more such additives.