sampling criteria, the minimum speckle size that can be resolved must be two times
larger than the camera pixel sizeρpixel, that is,ρspeckle≥ 2 ρpixel. This is illustrated in
Fig.10.13.
Example 10.9Consider a speckle imaging system operating at 532 nm in
which the magnification of the optics is M = 10. Suppose the camera has
f/# = 4.0. (a) What is the minimum speckle size that can be resolved by the
camera? (b) What are the requirements for the pixel size?
Solution: (a) Using Eq. (10.19) yields the following speckle sizeqspeckle¼ 1 : 22 ð 1 þM)(f=#Þk¼ 1 : 22 ð 1 þ 10 Þð 4 : 0 Þð532 nm)¼ 28 : 6 lm(b) The maximum pixel size of the camera must be 28.6/2μm = 14.3μm.10.4 Optical Coherence Elastography.
Thefield ofelastographyevolved from the observation that diseases such as edema
or dropsy (an excess of wateryfluid collecting in the cavities or tissues of the body),
fibrosis, cancer, and calcification alter the elastic properties of tissues [ 35 ]. The
basis of elastography is to measure the mechanical properties of tissue, such as the
Young’s modulus, by means of optical medical imaging, ultrasound imaging, or
magnetic resonance imaging when a mechanical loading is applied. An example of
an optical medical imaging procedure is shown in Fig.10.14. Here a circular
piezoelectric transducer puts a mechanical loading on a tissue sample, and the
response to the pressure variation is monitored by means of a light beam impinging
on the tissue through the center of the transducer. This process gives rise to images
calledelastograms, which are used to examine and monitor the onset and pro-
gression of a disease through the temporal variations in mechanical properties of the
tissue. Thefirst elastographic methodologies were based on using ultrasound and
magnetic resonance imaging. Subsequently, optical elastographic techniques
Individual
speckleCamera pixelsSpeckle sizePixel sizeFig. 10.13The minimum
speckle size that can be
resolved must be two times
larger than the camera pixel
size
310 10 Optical Imaging Procedures