Multi-Modal Imaging

An advanced in-vivo multi-modal imaging technology; namely, combined ultrasound, elasticity and photoacoustic imaging, capable of visualizing both structural and functional properties of living tissue, is based on the fusion of the complementary imaging modalities and takes full advantage of the many synergistic features of these systems.


1. Ultrasound Imaging

Ultrasound is sound with a frequency over 20 kHz, which is about the upper limit of human hearing. Modern medical ultrasound scanners, operating at 3-15 MHz and higher frequencies, are used for imaging nearly all soft tissue structures in the body. The ultrasound imaging is widely used, since it does not use ionizing radiation and is safe and painless for the patient. The anatomy can be studied from gray-scale B-mode images, where the reflectivity and scattering strength of the tissues are displayed. An array transducer is used for both transmitting and receiving the pulsed ultrasound field. The central frequency of the transducer is chosen to achieve best depth penetration and resolution. The mean speed of sound in the tissue investigated varies from 1446 m/s (fat) to 1566 m/s (spleen), and an averaged value of 1540 m/s is used in the scanners.1


2. Elasticity Imaging

While ultrasound imaging does not reflect changes in biomechanical properties, elasticity imaging is based on the premise that tissue pathology can be directly assessed through the quantification of tissue mechanical properties.2 Physicians have long thought that stiffness can be used as an indicator of possible cancerous lesions. The success of palpation as a diagnostic tool is the evidence of this. The ability to accurately and non-invasively quantify these values offers the possibility of not only detecting but also diagnosing and even monitoring the tissue abnormalities.

One of the approaches in elasticity imaging is based on ultrasound imaging of tissue during the externally or internally applied deformation of the object and measurement of the internal tissue motion. Ultrasound elasticity imaging technique consists of three main components: speckle or internal boundary tracking and evaluation of tissue motion, measurement of strain tensor components, and reconstruction of the spatial distribution of elastic modulus using strain images.3-4 Approaches to measure the tissue displacement include cross-correlation, sum absolute differences (SAD), and sum squared differences (SSD).


3. Photoacoustic Imaging

To utilize the differences in the optical absorption between normal tissue and cancerous cells, photoacoustic imaging can be used. Photoacoustic imaging (also called optoacoustic and, generally, thermoacoustic imaging) relies on the absorption of light, and the subsequent emission of an acoustic wave. In photoacoustic imaging, acoustic transients are generated using pulsed laser radiation. The induced photoacoustic waves are then detected and used to form an image of spatial distribution of optical absorption. There is a significant contrast between normal and deceased tissue due to preferential optical energy deposition in tumors compared to normal tissue.5 Difference in blood content is utilized in photoacoustic imaging, where an optical wavelength is chosen such that blood absorption and light penetration are optimal.


4. Combined Imaging

By integrating three complementary imaging techniques – ultrasound, elasticity and photoacoustic imaging, a combined imaging system using array transducer can be developed and built.6 The combined imaging system is practical for cancer detection, diagnosis and therapy monitoring.

Block diagram for the combined ultrasound, elasticity and photoacoustic imaging system

First, ultrasonic imaging helps to visualize the anatomy of the tissue structures, elasticity imaging detects the pathologies based on the biomechanical properties of the tumor, and photoacoustic imaging takes advantage of high optical contrast and low acoustic scattering. The combined imaging methods are complementary and together they may provide the synergistic information needed for the reliable detection and diagnosis of cancer. Second, the location of some tumors, for example breast or prostate cancer, is typically several centimeters below the skin surface. Internal deformations, needed for elasticity imaging, can be created and the surrounding tissue using free-hand surface deformations produced by the imaging probe itself. During a short, continuous deformation producing no discomfort, real-time ultrasound can capture information needed for elasticity imaging. Third, these imaging modalities utilize the same ultrasound imaging system including the array transducer. It does not require any significant modifications to existing clinical procedures. Thus, combined imaging system does not complicate any existing clinical ultrasound procedures. Finally, the system is totally non-invasive and painless for the patient. With all these advantages, real-time processing is the solution to expedite the use of this system in clinical application. Therefore, a configurable and custom-built system may be necessary.

Hardware platforms for the combined imaging system

Further readings

S. Park, S.R. Aglyamov, W.G. Scott, S.Y. Emelianov, “Strain imaging using conventional and ultrafast ultrasound imaging: Numerical analysis,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 54(5), 987-995 (2007) PDF

S. Park, S. Mallidi, A.B. Karpiouk, S. Aglyamov, S.Y. Emelianov, “Photoacoustic imaging using array transducer,” Proceedings of the 2007 SPIE Photonics West Symposium: Photons Plus Ultrasound: Imaging and Sensing, volume 6437, 14:1-7 (2007) PDF

S. Park, J. Shah, S.R. Aglyamov, A. Karpiouk, S. Mallidi, X.J. Zhang, W.G. Scott, and S.Y. Emelianov, “Integrated system for ultrasonic, photoacoustic and elasticity imaging,” Proceedings of the 2006 SPIE Medical Imaging Symposium: Ultrasonic Imaging and Signal Processing, volume 6147, 61470H1-8 (2006) PDF

S. Park, S.R. Aglyamov, W.G. Scott, and S.Y. Emelianov, “Elasticity imaging using high frame rate ultrasound imaging,” Proceedings of the 2006 IEEE Ultrasonics Symposium, 602-605 (2006) PDF

S.Y. Emelianov, S.R. Aglyamov, A.B. Karpiouk, S. Mallidi, S. Park, S. Sethuraman, J. Shah, R.W. Smalling, J.M. Rubin, W.G. Scott “Synergy and applications of ultrasound, elasticity, and photoacoustic imaging,” (invited presentation) Proceedings of the 2006 IEEE Ultrasonics Symposium, 405-415 (2006). PDF

 

References:

[1]        J. A. Jensen. Ultrasound imaging and its modeling. In: Imaging of Complex Media with Acoustic and Seismic Waves: Springer Verlag; 2002:135-165.

[2]        B. S. Garra, E. I. Cespedes, J. Ophir, S. R. Spratt, R. A. Zuurbier, C. M. Magnant, Pennanen MF. Elastography of breast lesions: initial clinical results. Radiology. 1997;202:79-86.

[3]        M. A. Lubinski, S. Y. Emelianov, O'Donnell M. Speckle tracking methods for ultrasonic elasticity imaging using short-time correlation. IEEE Trans. Ultrason., Ferroelect. Freq. Cont. 1999;46:82-96.

[4]        T. J. Hall, Zhu YN, Spalding CS. In vivo real-time freehand palpation imaging. Ultrasound Med Biol. 2003;29:427-435.

[5]        J. Folkman. What is the evidence that tumors are angiogenesis dependent. J. Natl. Cancer Inst. 1990;82:4-6.

[6]        S. Park, J. Shah, S. R. Aglyamov, A. Karpiouk, S. Mallidi, A. Gopal, H. Moon, X. Zhang, W. G. Scott, Emelianov SY. Integrated System for Ultrasonic, Photoacoustic, and Elasticity Imaging. Proceedings of the 2006 SPIE Medical Imaging Symposium: Ultrasonic Imaging and Signal Processing. 2006; 6147:61470H1-8.