The idea of using soft tissue mechanical properties
to diagnose disease
occurred to ancient Greek physicians more than 2000 years ago.
Hippocrates and colleagues reportedly invented manual palpation as a
means for detecting occult breast tumours before the advanced phase of
this disease negated the effectiveness of surgical treatment. Simple
palpation is still used today for early cancer detection of the prostate
and breast. We now know that tissues stiffen as some tumours form and
grow because of inflammation and desmoplasia, a dense cellular reaction
specific to malignant breast lesions with highly cross-linked
collagenous fibres. The development of elasticity imaging is driven, in
part, by the need to improve the detection and differentiation of early
malignant disease. However, elasticity imaging can also provide important
new information in other clinical examinations, including visualization
of myocardial dynamics to assess tissue viability following ischaemia and
skeletal muscle force generation. Methods and applications of these
topics are addressed in the following twenty papers.
The approaches to elasticity imaging vary widely but always involve the
application of medical imaging technologies - often ultrasound and
magnetic resonance because of their high sensitivity to small tissue
movements - to track natural and applied deformations. We see from
the
papers in this special issue that elasticity is a term that applies to a
broad range of parametric imaging for describing spatial and temporal
variations in tissue viscoelasticity.
Static methods apply ultrasound or magnetic resonance signals in
procedures that are best described as palpation by remote sensing. They
are considered static because the data acquisition time (1/frame rate)
is much faster than the tissue deformation rate. The same signal
processing concepts involved in measuring velocity vectors in
applications from radar tracking to blood flow imaging are used to
estimate local displacement fields from echo signals recorded while
straining body surfaces or vessel lumen mechanically or by radiation
force. From displacement estimates, images of strain (elastograms),
viscosity or stimulated acoustic emission are formed. The parameter
selected for display in an image depends on the diagnostic task and the
measurement geometry. Several papers in this issue discuss control of
tissue movement, signal processing for parameter estimation and their
combined effects on errors and image quality.
Dynamic methods are for imaging tissues strained at rates equal to or
greater than the acquisition frame rate. Some methods estimate the
distribution of shear moduli from images of low-frequency acoustic shear
waves propagating in the body. These methods, referred to as
sonoelasticity and magnetic resonance elastography, have been used to
detect lesions and assess force generation in skeletal muscle. Also,
planar tagged MR imaging is an exciting approach to the evaluation of
cardiac dynamics that visualizes strain and strain rate during the
cardiac cycle. Methods and applications of dynamic elasticity imaging
are also presented.
Clearly, most of the approaches described in this issue are targeted
toward clinical medicine. Each has strengths and weakness that vary with
applications. However, many of these same ideas may be scaled down in
size to study cell mechanics and mechano-transduction (two exciting new
areas of basic research at the frontier of molecular biology), functional
genomics and systems engineering. Perhaps the most promising aspect of
these investigations is the interdisciplinary nature, which, in the true
spirit of biomedical engineering, teaches us the value of research teams
with expertize in physiology, biomechanics, signals and systems,
radiation physics and medicine. We look forward to the progress these
new methods will bring to clinical and basic biomedical research, and
what they will teach us about complex biological systems and disease
processes.