Optical Imaging - Correction Techniques in Emission Tomography - page 252

FIGURE 12.6: Setup for complete angle free-space tomography, based on a
laser beam scanner and CCD camera detection. (From [33].)

this method. Two other possible approaches are the use of light of modulated
intensity, typically at frequencies of 100MHz{1GHz or the use of ultrafast pho-
ton pulses in the 100 fs{100 ps range. Correspondingly, the detection systems
can either measure changes in light attenuation and phase at different frequen-
cies or offer ultrafast detection of photon kinetics with resolutions of the order
of tenths of picoseconds or better. These time-domain and frequency-domain
methods can also independently resolve fluorescence strength and lifetime [31].

12.3.1.2 Detection
The new generation of systems developed for fluorescence tomography is
based on non-contact measurements, in which source and detector do not
come in physical contact with the tissue [31, 16, 39]. Free-space detection is
performed using charge coupled device (CCD) cameras. In a CCD camera,
electromagnetic radiation is transformed into an electrical voltage. It consists
of photoactive regions, \pixels," where incoming photons are absorbed, and
a shift register to transfer charges from the photoactive regions to a charge
amplifier where the charge is converted to a voltage. The photoactive regions
are gates, each consisting of a metal-oxide-semiconductor capacitor. Photons
that are absorbed in the semiconductor capacitor create electron-hole pairs.
The photoelectrons are then transfered in sequence from one capacitor to the
next, and finally arrive at the read-out point where the corresponding voltage
per pixel is obtained. Using this technique, a much larger and higher-quality
dataset can be obtained compared to fiber-based systems. It enables detection
of signals of arbitrary shape and placement.

360 projections 12.3.1.3

In order to achieve superior imaging performance, the imaging subject
needs to be illuminated from a large number of angles, similar to other to-
mographic techniques such as X-ray CT, PET, or SPECT. Several setups can
be thought of to obtain 360 full angular coverage projections; the imaging