according to the concepts of the present invention , various nanosized particles containing manganese ( ii ) ions in the crystal lattice have been developed for use as mri contrast agents . more specifically , the contrast agents are manganese ( ii ) hexacyanoferrate compounds having the formulas mn 2 [ fe ii ( cn ) 6 ], a 2 mn 3 [ fe ii ( cn ) 6 ] 2 . nh 2 o , amn [ fe iii ( cn ) 6 ]. nh 2 o where a = li , na , k , nh 4 or tl and mn 3 [ fe iii ( cn ) 6 ] 2 . nh 2 o where n = 0 or 1 - 20 . the compounds generally have the same crystal structure , that is a faced - centered cubic lattice ( space group fm 3 m ) and the unit cell parameter a = 10 ± 1 angstroms as set forth in fig2 . referring to fig2 , due to the strong ligand - field effect and simultaneous coordination of the cn − group to both iron and manganese ions in this extended 3d coordination network structure , both metal ions and cn − ligand are completely locked in the lattice positions and generally cannot be released from the compound . a result is that very low amounts of mn 2 + ions are released and thus the compounds there are considered stable and have very low toxicity . it has also been found that such compounds have a long blood circulation half life that allows a longer time window for imaging studies . blood circulation half lives of the contrast agents of the present invention generally range from about 0 . 1 to 2 hours , desirably from about 0 . 25 to about 2 . 0 hours and preferably from about 0 . 5 to about 2 . 0 hours . moreover , the concentrations of the manganese contrast agents in water that can be utilized for application to an animal such as a human being for an mri analysis are amounts generally from about 1 micromole to about 150 millimoles , desirably from about 10 micromolar to about 100 millimolar and preferably from about 25 micromolar to about 50 millimolar per liter of solution . the manganese ( ii ) hexacyanometallate nanoparticles are made using conventional methods known to the art and to the literature , having diameters generally from about 4 to about 500 nm , desirably from about 6 to about 200 nm and preferably from about 8 to about 100 nm . the particle diameter size is important in that it results in long circulation times in the blood stream before it is removed by the body . in contrast thereto , very small diameter sizes such as less than 2 nm or less than 1 nm are avoided since they are readily removed from the human body and have a short residence time therein , for example less than 20 minutes that is unacceptable for use as a mri contrast agent . the manganese contrast agents of the present invention are adapted to be applied to the body as dispersed nanoparticles in a solvent such as water stabilized by a hydrophilic coating comprising a carboxylic acid or a hydrophilic biocompatible polymer , or both . the hydrophilic coating acts to make the otherwise insoluble manganese ( ii ) nanoparticles dispersible in water , and thus promoting water stability of such nanoparticles while providing a protection shell against nanoparticle aggregation and precipitation . suitable carboxylic acids include , but are not limited to , common carboxylic acids such as acetic acid , oxalic acid , citric acid , tartaric acid , adipic acid , gluconic acid , and other mono -, di -, tri - or polycarboxylic acids . suitable hydrophilic biocompatible polymer used for coating to prolong blood circulation times , reduced biological toxicity , and particle solution stability against aggregation and precipitation include , but are not limited to , polyethylene glycol ( peg ), chitosan , dextran , e . g ., polymers of glucose having number average molecular weights up to 200 , 000 , and polyvinylpyrrolidone ( pvp ). the manganese contrast agent aqueous solutions are generally stable in acidic to neutral solutions with a ph value from about 1 to about 7 . 5 , desirably from about 2 . 5 to about 7 . 5 , and preferably from about 3 . 5 to about 7 . 3 . a general procedure for preparation of nanoparticulate mn 2 [ fe ( cn ) 6 ] mri contrast agents comprises the following reactions : generally any type of known h - form ion exchange resin can be used with suitable examples including amberlite ™ ir120 h from dow chemical company , a styrene divinylbenzene copolymer with sulfonic acid groups , ag 50w - x2 from bio - rad , a cation exchange resin , and amberlyst ™ 16 west from rohm and hass , a sulfonic acid ion exchange resin . with respect to the second part of the contrast agent preparation utilizing an amine , generally the suitable organic amine compounds include , but not limited to , amines having from 3 to about 12 , and desirably from 3 to about 10 carbon atoms such as triethylamine , benzylamine , ethylenediamine , piperidine , pyridine , pyrazine , 2 , 2 ′- bipyridine and 4 , 4 ′- bipyridine , or any combination thereof . alternatively , an alkali metal hydroxide aoh or alkali metal carbonate a 2 co 3 where a = li , na , k , rb or cs can be used in the place of the organic amine . the manganese ( ii ) contrast agents of the present invention can be prepared as follows : a proper concentration , i . e . 10 − 3 to 10 3 m , of k 4 [ fe ( cn ) 6 ] was first treated with a proper amount of h - form ion exchange resin , i . e . about 1 gram to about 1 , 000 grams and desirably from about 5 to about 500 grams , to yield h 4 [ fe ( cn ) 6 ] that was then mixed with a proper concentration , i . e . 10 − 3 to 10 3 m and desirably from about 0 . 01 to about 1 . 00 m of mncl 2 in the presence of a proper amount of triethylamine , i . e . 0 . 01 gram to 10 grams and desirably from about 0 . 05 to about 5 . 00 grams , citric acid , i . e . 0 . 01 gram to 10 grams , and desirably from about 0 . 01 to about 3 . 00 grams , and pvp , i . e . 0 . 01 gram to 10 grams and desirably from about 0 . 03 to about 8 . 00 grams , to form nanoparticles of mn 2 [ fe ( cn ) 6 ] with the size ranging from 4 to about 500 nm and desirably from about 8 to about 100 nm , depending on the ratio of the reacting components . simple reaction temperatures of the first reaction range from about 0 to about 100 ° c . and desirably from about 5 to about 95 ° c . the reaction temperature with respect to the second reaction generally range from about 0 to about 100 ° c . and desirably from about 5 to about 95 ° c . the reaction conditions are generally limited by the freezing point and boiling point of water . when other manganese contrast agents other than mn 2 [ fe ( cn ) 6 ] are desired , the process is essentially similar except that the ratios of the above noted compounds are changed . for example , if mn 3 [ fe iii ( cn ) 6 ] 2 is desired , a proper concentration , i . e . 10 − 3 to 10 3 m of k 3 [ fe ( cn ) 6 ] can be treated with a proper amount of h - form ion exchange resin and allowed to react with a proper concentration , i . e . 10 − 3 to 10 3 m and desirably from about 0 . 01 to about 1 . 00 m , of mncl 2 in the molar ratio of k 3 [ fe ( cn ) 6 ]: mncl 2 to be 2 : 3 while all the other conditions are kept exactly the same as described in [ 0028 ] in order to determine the release rate of the mn 2 + ions of the manganese ( ii ) hexacyanometallate contrast agents of the present invention , nanoparticles of the formula mn 2 [ fe ( cn ) 6 ], were treated with 20 parts of a saline solution , e . g . a nacl solution having a ph of 1 , 3 , 5 , and 7 and incubated at room temperatures for 16 hours . the potential transmetallation reactions between the nanoparticles and solutions each containing the following ions : 1 mm ca 2 + , 1 mm mg 2 + , 1 mm k + or 1 mm zn 2 + ions were also studied . the results were analyzed by atomic adsorption ( aa ) and showed that the highest mn concentration found has ˜ 19 ppm , which is much less than the minimal toxic level of 0 . 1 mm , see fig3 . as apparent from fig3 , the mn 2 + release amounts were higher at lower ph levels and essentially nil when utilized with magnesium and calcium ions . moreover , the release rate of the maganese ( ii ) hexacyanoferrate with respect to mn 2 + is approximately 2 , 000 times less than the release rate of mndpdp . thus , release rates of at least about 25 , about 50 , about 100 , about 500 , or about 1 , 000 times less than the release rate of mndpdp can be readily obtained . stated in other words , the in vitro release rate of mn 2 + at a ph of about 7 is from about 10 to about 20 , desirably from about 12 to 18 , and preferably from about 14 to about 16 parts per million in water for a 24 hour time period . fig3 also shows that the release rate of manganese in the presence of other ions at a ph of 7 such as zinc , magnesium , and calcium was also extremely low . the concentrations of free cyanide ions released from manganese ( ii ) hexacyanoferrate mri contrast agents of the present invention is generally at the level of ˜ 10 ppm , which is about 10 to 15 times less than a minimum toxic level of 0 . 1 mm currently set forth by the epa . that is , the in vitro concentration of free cyanide ions released by the mri contrast agents of the present invention is generally about 2 to about 50 , and desirably from about 5 to about 30 times less than the current minimum toxic epa level of 0 . 1 mm of free cn − ions . these values are determined based upon the release rate of free cyanide ions in water during a 24 hour time period at room temperature , e . g . about 65 to about 85 ° f . the manganese contrast agents of the present invention thus essentially have no toxicity and are very safe for use in mri scanning . a series of proton t 1 and t 2 relaxation measurements using 500 mhz ( 11 . 7 t ) nmr were made . the results expressed as the concentration - normalized relaxivity values are r 1 = 7 . 3 mm − 1 × s − 1 and r 2 = 204 mm − 1 × s − 1 per mm of mn 2 + ions , see fig4 . these values are among the highest measured relaxivity values ever obtained for any mri contrast agent . solutions of various concentrations of mn 2 [ fe ( cn ) 6 ] nanoparticles were used for t 1 and t 2 measurements using a 7 . 0 t mri scanner . for t 1 measurements , an inversion recovery gradient echo sequence with a te = 4 ms was used . the inversion time was varied between 30 - 2000 ms . t 2 measurements were performed using a spin - echo sequence of tr of 10000 ms , and te of 10 . 6 - 340 ms , see fig5 . the results expressed as the concentration - normalized relaxivity values from these measurements are r 1 = 6 . 07 mm − 1 × s − 1 and r 2 = 117 mm − 1 × s − 1 per mm of mn 2 + ions . these results further confirmed that mn 2 [ fe ( cn ) 6 ] nanoparticles possess have relaxivity values at a medically relevant high magnetic field of 7 . 0 tesla . the in vitro t 1 ( positive contrast values ) of the mri contrast agents of the present invention have relaxivity values , i . e . r 1 , of from about 1 to about 15 , desirably from about 2 to about 15 , and preferably from about 4 or about 6 to about 14 mm − 1 · s − 1 / mm of mn + 2 ions . the t 2 ( negative contrast agents ) of in vitro relaxivity values , i . e . r 2 , is from about 50 to about 300 , desirably from about 170 to about 250 , and preferably from about 100 to about 200 mm − 1 · s − 1 / mm of mn + 2 ions . another advantage of the present invention is that the manganese mri contrast agents can be utilized in either high or low magnetic field strength such as from about 0 . 5 to about 11 tesla and desirably from about 1 . 0 to about 9 . 0 teslas . the manganese contrast agents of the present invention can be utilized where ever mri contrast agents have been utilized heretofore and the same is well known to the art and to the literature including the administration thereof . the contrast agent can be utilized with respect to various animals including pets such as dogs , cats , horses , cattle , pigs , goats , chickens , turkeys , etc . a highly preferred end use is for mri diagnosis of human beings , i . e . persons , as by various well known methods such as oral administration , intravenous injection , and the like . while in accordance with the patent statutes the best mode and preferred embodiment have been set forth , the scope of the invention is not intended to be limited thereto , but only by the scope of the attached claims .
this invention discloses a synthetic procedure for preparing nanoparticulate materials of various metal cyanide compounds containing manganese ions in the crystal lattice with the surfaces coated by a hydrophilic compound , and their use as mri contrast agents with high sensitivity , long blood circulation half lives and low toxicity at low - field and high field mr scanners .