REVIE\N Echo enhancing •agents In ultrasound PeterCorr MBChB, FFRAD(D)SA, FRCR, MMed(UCT) Department of Radiology, University of Natal E cho enhancing micro bubble agents show great promise for blood pool imaging in diagnostic ultrasound. I Their applications include vascular enhancement of both large vessels such as the renal and carotid arter- ies as well as small arteries in the kidneys, brain and liver. They are being used as con- trast agents in hysterosalpinography, micturating cystourethrograms and as oral contrast agents. Targeted echo enhancers have been used to delineate thrombus and the endocardial lining of the heart. I These micro bubbles are a few microns in diameter and they resonate extremely well with 3-lOmhz transducers used in di- agnostic ultrasound. This resonance causes extensive cross sectional scattering and ex- cellent reflectivity. The micro bubbles are small enough to pass through the pulmo- nary capillaries allowing arterial enhance- ment via intravenous injection. There are three generations of micro bubble agents.' The first generation agent has a very short life of a few seconds. Echovist (Schering) is such an example. In second generation micro bubbles a stabi- lizing agent (palmitic acid) is added to in- crease the intravascular half life and pro- duce ultrasound detected vascular enhance- ment. Levovist (Schering) and Albunex (Nycomed) are examples." The third gen- eration of micro bubbles have greater intravascular stability and life and higher reflectivity. These include Echogen (Sonus), NUS (Nycomed) and SHU 563A (Schering).' Practical applications will be initially limited to colour flow ultrasound but the third generation agents have such signifi- cant reflectivity that they may be used with grey scale as well. Promising applications include: - transcranial imaging of cerebrovascular disease and vascular tumours such as meningiomas - examination of breast masses by evaluat- ing tumour perfusion - diagnosis of coronary artery disease by documenting perfusion defects - improved ultrasound diagnosis of liver, spleen, kidney, prostate, testicular and thy- roid tumours (Figures 1a and 1b) Figure ts: Ultrasound of the right/obe of the liver of a 60 year old man with RUO pain demonstrates an ill defined echogenic mass (arrow). 5 SA JOURNAL OF RADIOLOGY· September 1996 Figure 1b: After the injection of Levovist there is a marked intratumora/ enhancement of the mass confirming its hypervascu/arity (arrow). This was confirmed to be a hepatoma. - improved visualisation of blood flow in carotid, vertebral, renal and peripheral ar- teries Echo enhancing agents will be particu- larly useful to enhance Doppler signals which are very weak or difficult to obtain or in situations of very slow blood flow. In situations of slow flow such as critical ca- rotid stenosis, enhancement of a weak sig- nal will be very helpful to distinguish be- tween critical stenosis and total carotid oc- clusion. The first micro bubble contrast agent to be approved by the Medicines Control Council will be on the market in South Af- rica later this year. Cost may be a critical factor in their initial acceptance. References 1. U1trasonographic echo enhancing agents. Editor D. Cosgrove. Clin Rad 1996;51 :1-56( supp). 2. Fan P, Czuwala PJ, Nanada NC. Comparison of vari- ous agents in contrast enhancement of color flow doppler images: an in vitro study. Ultrasound Med Bioi lffi3;19:4547. 3. deJongh N, HoffL. Ultrasound scattering proper- ties of albun ex microspheres. UltrasOllics1993;31: 175-181. 4. Schurmann R, SchliefR. Saccharide based contrast agents. Characteristics and diagnostic potential. Radiol Med1 994;87: 15-23. 5. Unger E, Lund P, Shen D. Nitrogen Riled liposomes as a vascular US contrast agent: preliminary evaluation. Radio/oy 1992;185:453-456.