Analysis of Dynamic Magnetic Resonance Elastography Data - Advance Image Processing in Magnetic Resonance Imaging - page 458

2.5

2.3

2.1

1.9

1.7

1.5 50

100 150 200 250 300 350

Frequency (Hz)

FIGURE 14.7 Shear-wave velocity in specimens of porcine liver tissue obtained from 5
different animals, measured at frequencies of 75 to 300 Hz. The velocity increases sys-
tematically with frequency. The curve represents a best-fit line that accounts for viscoelastic
behavior. (From Kruse, S.A., Smith, J.A., Lawrence, A.J., Dresner, M.A., Manduca, A.,
Greenleaf, J.F., Ehman, R.L. (2000). Tissue characterization using magnetic resonance
elastography: preliminary results. Phys. Med. Biol. 45: 1579-1590.)

with frequency and temperature [37,59]. For example, Figure 14.7 shows an
example of the calculated shear stiffness of porcine liver tissue at different
mechanical frequencies. The data are well fit by a viscoelastic model
(Equation 14.8 and Equation 14.9). Because of the lack of metabolic activity,
homeostasis, and in situ preloading in specimens, the observed mechanical prop-
erties are likely to be different from those that would be measured in vivo .

14.6.3 B REAST

Various groups have reported MRE results on in vivo human breasts
[31,33,34,54,60,61]. Clear distinction has been found between fat and glandular
tissues [34,61], in rough agreement with earlier results on excised tissue, and

between normal breast tissue and carcinoma [60,61]. Figure 14.8 illustrates the

differentiation between adipose and glandular tissue in the elastogram of a normal
volunteer. Recent work [60] has included in vivo imaging of six normal volunteers
and six patients with cancer. In patients with tumors, the stiffest regions in the
elastogram corresponded to the known tumor locations and were 5 to 20 times

stiffer than normal tissue ( Figure 14.9 ). No regions of such high stiffness were

found in the normal volunteers. The stiffness values found for the tumors are
expected to be an underestimate in all cases because of the limitations of the
processing technique and the scans being 2-D instead of 3-D. Sinkus et al. [33]