Acousto-Mechanical Imaging for Cancer Detection

Present paper describes a new modality of medical imaging, Acousto-Mechanical Imaging (AMI) comprising features of both ultrasonic Elasticity Imaging (Er) and Mechanical hnaging(MI) (12). The data on the stress pattern measured by the force sensor array of MI complement the strain data obtained by ultrasonic EI device. Synergy of combining these two complementary methods results in the fact that for certain applications the AMI technology has a diagnostic potential superior to that of EI and MI separately. One of the hot areas of ultrasonic medical diagnostics is Elasticity Imaging (EI) (l-9). Recently, an alternative method of imaging tissue structures in terms of their elasticity, the method of Mechanical Imaging (MI), has emerged (10-l 1). The essence of MI is the solution to an inverse problem using the data of stress patterns on the surface of tissue compressed by a force sensor array. A prototype of the device for mechanical imaging of prostate has been developed and feasibility of MI technology has been proven in recent clinical studies (10). Present paper describes a modality of medical imaging, Acousto-Mechanical Imaging (AMI) comprising features of both EI and MI (12). The data on the stress pattern measured by the force sensor array of MI complement the strain data obtained by ultrasonic EI device. Synergy of combining these two complementary methods results in the fact that for certain applications the AMI technology has a diagnostic potential superior to that of EI and MI separately. The evaluation of mechanical structure of an object requires knowledge of spatial distribution of both stress and strain. A principal object of the present work is to utilize strain and stress data obtained simultaneously for the same region of the object under the same loading conditions and, consequentIy, to evaluate the mechanical structure of an investigated tissue with less ambiguity. Ultrasonic Elasticity Imaging derives information on tissue elasticity from directly evaluated strain data and using various indirect ways to estimate necessary strain data. Ophir et al. (5) developed an EI system where information on strain was obtained in parallel with ultrasonic measurements of strain. In that system, a layer of a materia1 having a known Young’s modulus and speed of sound had been interposed between a transducer and the tissue to obtain a strain profile of the compressed tissue. The change in the time shift between the echoes received ti+om the inner and outer surfaces of the layer under compression provide information on the uniaxial deformation and respective stress in the ultrasonic transmission path through this layer. Adding to the measured data on strain profile also a complementary information on the stress in the part of the compressed system facilitates estimation of tissue elasticity. There are two shortcomings to this method. First, to be able to evaluate the stress with sufficient accuracy, the layer must be thick enough to provide measurable time shift between the echoes received from the two surfaces of the layer. This inevitably results in the losses of ultrasonic energy and distortion of the ultrasonic beam. Secondly, and most importantly, the value of the u&n&l stress is adequate only for calculating stress/strain relationships in a one-dimensional system made of infinite parallel layers. The stress in a real system with 3-dimensional inclusions is of a complex character; therefore, a single value of the normal component of stress, such as that obtained in the described method, is a very deficient description of the mechanical state of the system. The Acousto-Mechanical Imaging method is Ii-ee Tom these shortcomings. An AMI device comprises a force-sensing array made of sound transparent film inserted between an ultrasonic scanner probe and the tissue. It has to be assured that the pressure sensing array between the scanner probe and the tissue will not interfere with the scanner operation. The ultrasonic scanner probe with the attached pressure sensing array serves as a pressure exerting member causing a deformation of the tissue and creating stress and strain in the tissue portion. Figure 1 presents a functional diagram of the AMI device.