Modelling of the Ultrasound Return from Albunex Microspheres                                         

Sverre Holm                                              
Marte Myhrum                                             
Lars Hoff                                              
Vingmed Sound AS, Research Department, Vollsveien 13 C, N-1324 Lysaker, Norway                      
Faculty of Mathematics and Natural Sciences, University of Oslo, Norway                      
Nycomed Imaging AS, P.O. Box 4220, Torshov, N-0401 Oslo, Norway                            


Abstract                                                                              
A mathematical model that predicts the changes to the ultrasound frequency spectrum           
after passing through human tissue and Albunex (a registered trademark of 
Molecular Biosystems Inc., San Diego, CA) microspheres is proposed. Changes in                
backscattered intensity and mean frequency of the reflected signal can be estimated           
as a function of imaging geometry, ultrasound frequency, and microsphere                      
concentration and size distribution.                                                          
An important result is that the frequency shifts and the intensity variations are caused      
both by the microspheres in the path between the transducer and the region of interest,       
and by the reflection properties of the contrast agent in the region of interest.             
The model enables one to explain and predict clinically observed intensity effects            
such as the shadow effect in the right ventricle, and the fact that received intensity        
increases with concentration up to a certain point where it falls off.                        
The effect of the removal of the larger microspheres in the lungs is so significant that      
concentrations that give positive frequency shifts with increasing concentration in the       
right ventricle will give negative shifts in the left ventricle. The frequency shift is only  
a good indicator of concentration for ultrasound frequencies below 4 MHz, while the           
intensity is very sensitive to Albunex microsphere concentration and imaged depth    
also at the higher frequencies.                                                               
It is also found that the periodic variation in heart muscle thickness during the heart       
cycle will affect the intensity and mean frequency of the backscattered ultrasound            
observed in the left ventricle significantly, and in such a way as to give them a             
periodic variation over the heart cycle.                                                      


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