Ultrasound contrast agents and process for the preparation thereof

1 . An injectable aqueous suspension of microbubbles filled with a biocompatible gas and comprising a stabilizing layer predominantly comprising a phospholipid, wherein at least 10% of the total volume of gas contained in the microbubbles is contained in microbubbles with a diameter of 1.5 μm or less.

2 . The aqueous suspension according to claim 1 wherein at least 25% of the total volume of gas contained in the microbubbles is contained in microbubbles with a diameter of 1.5 μm.

3 . The aqueous suspension according to claim 1 wherein at least 50% of the total volume of gas contained in the microbubbles is contained in microbubbles with a diameter of 1.5 μm

4 . The injectable suspension according to claim 1 wherein at least 70% of the total volume of gas contained in the microbubbles is contained in microbubbles with a diameter of 1.5 μm

5 . The aqueous suspension according to claim 1 wherein said microbubbles have a D V50 /D N ratio of about 2.00 or lower.

6 . The aqueous suspension according to claim 1 wherein said microbubbles have a D V50 /D N ratio of about 1.80 or lower.

7 . The aqueous suspension according to any one of the preceding claims obtainable by a method which comprises the steps of:

a) preparing an aqueous-organic emulsion comprising i) an aqueous medium including water, ii) an organic solvent substantially immiscible with water, iii) an emulsifying composition of amphiphilic materials comprising more than 50% by weight of a phospholipid and iv) a lyoprotecting agent;

b) lyophilizing said emulsified mixture, to obtain a lyophilized matrix comprising said phospholipid;

c) contacting said lyophilized matrix with a biocompatible gas; and reconstituting said lyophilized matrix by dissolving it in a physiologically acceptable aqueous carrier liquid.

8 . A method for diagnostic imaging comprising administering to a subject a contrast-enhancing amount of an aqueous suspension of any one of claims 1 to 6 and imaging at least a part of said subject.

9 . A method for diagnostic imaging comprising administering to a subject a contrast-enhancing amount of an aqueous suspension of any one of claims 1 to 6 and imaging at least a part of said subject wherein said imaging comprises insonating said subject by means of an ultrasound device generating an ultrasound wave with a predetermined transmit frequency, from which a corresponding resonance size of microbubbles is determined, and administering a contrast agent comprising gas-filled microbubbles having a narrow size distribution and a mean size close to half the resonance size.

10 . A method for preparing a lyophilized matrix which, upon contact with an aqueous carrier liquid and a gas, is reconstitutable into a suspension of gas-filled microbubbles stabilized predominantly by a phospholipid, said method comprising the steps of:

a) preparing an aqueous-organic emulsion comprising i) an aqueous medium including water, ii) an organic solvent substantially immiscible with water comprising, dispersed therein, an emulsifying composition of amphiphilic materials comprising more than 50% by weight of a phospholipid and iv) a lyoprotecting agent;

b) lyophilizing said emulsified mixture, to obtain a lyophilized matrix comprising said phospholipid.

11 . The method according to claim 10 wherein the lyoprotecting agent is dissolved in the aqueous carrier.

12 . The method according to claim 10 wherein the lyoprotecting agent is polyethylenglycol.

13 . A method for preparing an injectable contrast agent comprising a liquid aqueous suspension of gas-filled microbubbles stabilized predominantly by a phospholipid, which comprises the steps of:

preparing a lyophilized matrix according to the method of any one of the preceding claims 10 to 12 ;

contacting said lyophilized matrix with a biocompatible gas; and

reconstituting said lyophilized matrix by dissolving it into a physiologically acceptable aqueous carrier liquid, to obtain a suspension of gas-filled microbubbles stabilized predominantly by said phospholipid.