1. A method of contrast enhanced MR angiography of the human or non-human animal body, said method comprising administering into the vasculature thereof a contrast effective amount of a contrast agent composition comprising magnetic particles measured at 0.5 T and at 37 degree  C, having a r2/r1 ratio of no more than 5, and at a time when sufficient of said magnetic particles remain in the vasculature to provide positive contrast enhancement thereof in a T1 -weighted image while sufficient magnetic particles have been taken up into an organ of the reticuloendothelial system to provide negative contrast enhancement thereof in said T1 -weighted image, generating a T1 -weighted magnetic resonance image of at least said organ. 2. A method as claimed in claim 1 wherein said organ is the liver. 3. A method as claimed in claim 1 wherein said organ is a lymph node. 4. A method as claimed in any one of the preceding claims wherein a T2 -weighted image of at least said organ is also generated. 5. A method as claimed in any one of the preceding claims wherein said composition comprises magnetic particles having a r2/r1 ratio of no more than 5 and a second plurality of magnetic particles having a shorter blood half life than that of said first plurality, wherein said second plurality of magnetic particles has a higher a r2/r1 ratio than the r2/r1 ratio of said first plurality. 6. A method as claimed in claim 5 wherein said first plurality of magnetic particles comprises particles provided with an opsonization inhibitor or blood lifetime prolonging material, preferably polyoxyalkylene blood lifetime prolonging material. 7. A method as claimed in claim 5 wherein said first plurality of magnetic particles has a blood half time of at least 30 minutes. 8. A method as claimed in any one of the preceding claims wherein said composition comprises magnetic particles having a r2/r1 ratio of no more than 5 and which are provided with an opsonization inhibitor or blood lifetime prolonging material, preferably polyoxyalkylene blood lifetime. 9. A method as claimed in any one of the preceding claims comprising parenterally administering to said body a contrast effective amount of a first composition comprising magnetic particles, and subsequently administering into the vasculature of said body a contrast effective amount of a second composition comprising magnetic particles having a r2/r1 ratio of no more than 5. 10. A method as claimed in claim 5 wherein said first composition is administered into the lymth system of said body and as said T1 -weighed image is generated an image of at least part of said system. 11. A method as claimed in any one of the preceding claims wherein said magnetic particles are preferably composite particles comprising magnetic crystals and a biotolerable polymer wherein said biotolerable polymer is an oligo or polysaccharide, more preferably is a oxidatively cleaved starch. 12. A method as claimed in claim 11 wherein said composite particles further comprise an opsonization inhibitor or blood lifetime prolonging material, preferably a polyalkyleneoxide blood lifetime prolonging material. 13. A method as claimed in any one of the preceding claims wherein said T1 -weighted image is generated at a time from 1 minute to 24 hours, preferably up to 50 minutes after administration of said magnetic particles having an r2/r1 ratio of no more than 5. 14. A method as claimed in any one of the preceding claims wherein said T1 -weighted image is generated at a time when said positive contrast enhancement corresponds to an image intensity increase of at least 80% and said negative contrast enhancement corresponds to an image intensity decrease of at least 20%. 15. A method as claimed in any one of the preceding claims wherein the r2/r1 ratio of said magnetic particles having a r2/r1 ratio of no more than 5, is no more than 3 and preferably is no more than 2.5. 16. A method as claimed in any one of the preceding claims wherein the weight ratio of inorganic magnetic material in said first plurality to that in said second plurality is in the range 10:1 to 1:1. 17. A method as claimed in any one of the preceding claims wherein the weight ratio of inorganic magnetic material in the contrast effective amount of said second composition to that in contrast effective amount of said first composition is in the range 10:1 to 1:1. 18. A diagnostic contrast agent composition comprising magnetic particles and a psysiologically measured at 0.5 T and at 37 degree C, having a r2/r1 ratio of no more than 5 and a blood half time of up to 24 hours and a second plurality of magnetic particles having a blood half-time lower, preferably by at least 50% than that of said first plurality. 19. A method as claimed in claim 18 wherein said first plurality of magnetic particles comprises particles provided with an opsonixation inhibitor or blood lifetime plolonging material. 20. A method as claimed in claim 18 wherein said first plurality of magnetic particles comprises composite particles comprising magnetic crystals, a biotolerable polymer, and an opsonization inhibitor or blood lifetime prolonging material. 21. A method as claimed in claim 20 wherein said biotolerable polymer is an oxidatively cleaved starch and said blood lifetime prolonging material is a polyalkyleneoxide material. 22. A method as claimed in claims 18-21 wherein said magnetic particles have a mean overall particle size below 100 nm.