Study of ultrasound stiffness imaging methods using tissue mimicking phantoms Kavitha Manickam a,⇑ , Ramasubba Reddy Machireddy a , Suresh Seshadri b a Biomedical Engineering Group, Department of Applied Mechanics, IIT Madras, Chennai 600036, India b Mediscan Systems, Chennai 600004, India article info Article history: Received 11 June 2013 Received in revised form 23 August 2013 Accepted 30 August 2013 Available online 12 September 2013 Keywords: Ultrasound stiffness imaging Tissue mimicking phantom Acoustic characterization Elastic characterization Elastography abstract A pilot study was carried out to investigate the performance of ultrasound stiffness imaging methods namely Ultrasound Elastography Imaging (UEI) and Acoustic Radiation Force Impulse (ARFI) Imaging. Specifically their potential for characterizing different classes of solid mass lesions was analyzed using agar based tissue mimicking phantoms. Composite tissue mimicking phantom was prepared with embedded inclusions of varying stiffness from 50 kPa to 450 kPa to represent different stages of cancer. Acoustic properties such as sound speed, attenuation coefficient and acoustic impedance were characterized by pulse echo ultrasound test at 5 MHz frequency and they are ranged from (1564 ± 88 to 1671 ± 124 m/s), (0.6915 ± 0.123 to 0.8268 ± 0.755 db cm -1 MHz -1 ) and (1.6110 6 ± 0.127 to 1.76  10 6 ± 0.045 kg m -2 s -1 ) respectively. The elastic property Young’s Modulus of the prepared samples was measured by conducting quasi static uni axial compression test under a strain rate of 0.5 mm/min upto 10 % strain, and the values are from 50 kPa to 450 kPa for a variation of agar concentration from 1.7% to 6.6% by weight. The composite phantoms were imaged by Siemens Acuson S2000 (Siemens, Erlan- gen, Germany) machine using linear array transducer 9L4 at 8 MHz frequency; strain and displacement images were collected by UEI and ARFI. Shear wave velocity 4.43 ± 0.35 m/s was also measured for high modulus contrast (18 dB) inclusion and X.XX m/s was found for all other inclusions. The images were pre processed and parameters such as Contrast Transfer Efficiency and lateral image profile were computed and reported. The results indicate that both ARFI and UEI represent the abnormalities better than conven- tional US B mode imaging whereas UEI enhances the underlying modulus contrast into improved strain contrast. The results are corroborated with literature and also with clinical patient images. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Ultrasound B-mode imaging is a popular and most widely used method for imaging breast, thyroid, prostate and human abdomi- nal organs like kidney, spleen and liver. Even though it is used as a screening tool in cancer diagnosis, it is poor at distinguishing cancerous tissue from soft tissue. Basically there are two types of cancer tissue namely benign and malignant depending on whether or not they can spread by invasion and metastasis. Benign lesions are those that cannot spread out by invasion. They grow only lo- cally and they can be cured by suitable therapy where as malignant tumor invades neighboring cells, enter into blood vessels, lympha- tic system and metastasize to different sites. For distinguishing be- nign and malignant lesions, conventional ultrasound techniques use B-mode image shape features like lesion margin irregularity, shadowing, microlobulation and wider than taller orientation. However these features are often found to be overlapping, which decreases the reliability of B-mode in classification of lesions [1]. This leads to invasive biopsies to confirm the presence of cancer which causes patient discomfort and unnecessary anxiety. On the other hand, pathological cancerous changes of tissues are highly correlated with changes in stiffness [2]. Abnormalities such as be- nign and malignant cancer lesions could be identified based on their stiffness properties; benign tumors are generally around 2– 3 times stiffer than normal tissues and deform more for an applied compression. But malignant tumors are harder than surrounding tissues and show less deformation. Thus they can be distinguished by stiffness contrast than acoustic contrast and this property is used in stiffness imaging. In recent decades, there has been an increasing need in assessing the stiffness properties like Young’s modulus E and shear modulus G of tissues. In isotropic materials, the ratio of longitudinal deformation (strain) in response to an applied longitudinal force (stress) is known as Young’s modulus (E) of elasticity. The shear modulus (G) relates transverse strain to transverse stress. A number of stiffness imaging modalities are being developed and they are based on applying a mechanical excitation to tissues of interest and measuring tissue deformation. 0041-624X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultras.2013.08.018 ⇑ Corresponding author. Tel.: +91 9444023139. E-mail address: kavitharunkumar@gmail.com (K. Manickam). Ultrasonics 54 (2014) 621–631 Contents lists available at ScienceDirect Ultrasonics journal homepage: www.elsevier.com/locate/ultras