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Journal Club Theme of 1 June 2008: Magnetic resonance imaging in biomechanics
Medical imaging modalities such as MRI, CT and ultrasound all provide invaluable anatomic and physiological information that not only allow physicians to diagnose disease on the basis of the direct reading of images, but can also be used in the context of fluid and solid mechanics to derive additional diagnostic information. It is widely accepted that a number of diseases strongly influence the elasticity of the affected tissues. Reduced compliance of the large arteries has been proposed as an effective risk factor for cardiovascular disease and a number of tumors (thyroid, breast, prostate) are usually detected at first by touch. The concept of “palpation by imaging” has been recently proposed as a valuable tool to aid physicians in the process of detecting disease and an increasing body of literature is being produced by several groups aimed at taking this technique “from bench to bedside”. MR elastography is a phase-contrast-based technique to assess the local elasticity of internal organs and tissues that would be otherwise out of reach of the physicians’ hand. The first article I would like to draw attention to is an article by Manduca et al. in which the processing algorithms for elasticity reconstruction are presented together with a series of results obtained both in vivo and ex-vivo.
Manduca, T.E. Oliphant, M.A. Dresner, J.L. Mahowald, S.A. Kruse, E. Amronim, J.P. Felmlee, J.F. Greenleaf, R.L. Ehman, Magnetic resonance elastography: non-invasive mapping of tissue elasticity, Medical Image Analysis, 5 (2001), 237-254.
The same imaging modality that allows the reconstruction of in vivo elasticity maps is probably more widely used to visualize and quantify blood flow in arteries. This is extremely important in cardiovascular applications where the wall shear stress and other hemodynamic parameters have been found to play a key role in atherogenesis and the early stages of atherosclerotic disease. Frydrychowicz et al. have recently reported the results of an in vivo study of the hemodynamics in the iliac and proximal femoral arteries using 3D time-resolved phase contrast MRI at 3T.
Frydrychowicz, J.T. Winterer, M. Zaitsev, B. Jung, J. Hennig, M. Langer, M. Markl, Visualization of iliac and proximal femoral artery hemodynamics using time-resolved 3D phase contrast MRI at 3T, Journal of Magnetic Resonance Imaging, 25 (2007), 1085-1092.
No need to point out that both the elasticity and flow information that can be obtained using MR phase-contrast imaging constitute the basis of any realistic finite element model aimed at investigating either the hemodynamics or the stress distribution within the arterial wall.The last article I would like to mention regards the use of ultra-small super-paramagnetic iron oxide (USPIO) particles in MRI to “image” inflammation. USPIO particles are in fact taken up by activated macrophages in high-risk atherosclerotic plaques producing a T2* effect visible as a signal loss in the region of interest. The work of Howarth et al.:
S.P.S. Howarth, T.Y. Tang, R. Trivedi, R. Weerakkody, J. U-King-Im, M.E. Gaunt, J.R. Boyle, Z.Y. Li, S.R. Miller, M.J. Graves, J.H. Gillard, Utility of USPIO-enhanced MR imaging to identify inflammation and the fibrous cap: a comparison of symptomatic and asymptomatic individuals, European Journal of Radiology, (2008)
shows the potentiality of USPIO-enhanced MRI as an additional tool for patient stratification according to risk due to both its ability to identify inflammation and the related T1 effect of enhancing the fibrous cap, which is also critical for the identification of vulnerable plaques. The three articles I cited give just a glimpse of the different information that can be collected from MRI. While new and more refined CFD and FE models are being continuously developed to simulate disease-free and pathological scenarios, medical imaging techniques constitute both the primary source for physiologically realistic boundary conditions and probably the most reliable term of comparison for validation of the models themselves. In particular, the absence of ionizing radiation and the completely safe environment for the patient, make MRI “the perfect imaging technique” for clinical and biomedical research.