Biomedical Engineering Reference
In-Depth Information
metabolic or physiologic interactions between the agent and the tissue. Emission
of nuclear particles from nuclear substances introduced into the body is utilized in
the evaluation.
8.4
Basics of Imaging
8.4.1 Image Acquisition
Biomedical images are evaluated by a human analyst. For this purpose, the interac-
tions of the energy source should be presented in a form so that the eye can dis-
tinguish the intended observation. Acquisition implies recording the interaction of
the energy source and the sensing of the environment. Sensing can be accomplished
either passively or actively. Sensing is passive when the radiation originates in the
surrounding environment, is reflected or absorbed and re-emitted from objects,
and then sensed by eyes, or cameras. Sensing is termed active when the radiation is
artificially introduced into the environment as an integral part of the vision system
(e.g., X-ray, sonar, and radar). In both cases, basic elements in the image acquisi-
tion system (Figure 8.5) include detectors, signal processors, display devices such as
films and cathode ray tube (CRT) monitors, and storage devices such as hard disks.
The first stage of any imaging system is detecting raw image data and transforming
it into a form suitable for digital, optical, or biological processing. Detectors help
in registering the radiation behind the object that bears the information of inter-
est. The interaction in the detector in most cases is absorption, either by a directly
converting semiconductor or by a scintillator followed by a light sensor (e.g., a
photodiode). Many types of detectors are used based on the type of energy source
used. X-ray imaging as an example is discussed later, but other detectors specific to
an imaging technique are described under those medical devices.
The conventional method of acquiring the image is adapted from photogra-
phy—the screen/film detector—that is, to capture the emitted 3D distributed radia-
tion by a normal photographic film in a 2D projection. For example, when X-rays
collide with atoms of phosphorous compounds (scintillators) embedded in a glass
plate, the electrons in the compound are raised to higher energy levels causing the
phosphorous plate to glow, the dark regions being zones of high X-ray absorption
(i.e., this forms a positive image). The phosphor screen converts the X-ray to visible
light. Alternately, the glass plate is replaced by a photographic plate. The transmit-
ted X-ray photons expose the photographic film, which is physically separate from
Figure 8.5
Basic components in biomedical image acquisition.
