Hey, everybody,
and welcome to the first video in a series on
digital radiography. In this video, we will focus on
the components of the cassettes used in computed radiography
and how they work. Computed radiography, or CR,
is a form of digital imaging that uses a cassette based
system similar in many ways to conventional film
based radiography. A thin, active layer within
a protective cassette records and stores the X-ray
energy as an unprocessed image, much like the film in
traditional radiography when exposed to X-ray photons. Just like with film imaging,
this initial exposure results in only what we
will call a latent image and still needs to be processed
into a usable radiograph. In computed radiography,
this processing is done by a plate
reader that will be discussed in another video. So now that we have
the basics, let's take a close look at the
components of the CR cassettes. The CR cassettes have
protective outer cases and a photo stimulable phosphor
called a PSP plate made up of five key components. These components are the
protective layer, phosphor layer, conductive or
anti-static layer, support layer or the base, and reflective
or light shielding layer. These cassettes can
come in different sizes and are typically constructed
of hard plastics, light metals, and carbon fibers. The outer protective
cassette only serves as a protective
barrier and has no bearing on image production. The protective layer
protects the phosphor layer, which is the active
component of the plates. The conductive layer serves
to ground the imaging plate and reduce electrostatic charge. The reflective layer serves
to direct the emitted light in the imaging plate reader. The phosphor layer
or active layer is the most important layer
within the imaging plate. The phosphor layer,
like the film in traditional radiography,
is where the latent image will be produced. The phosphor layer is composed
of extremely small particles that have the ability to both
store and release energy. The most common phosphors are
barium fluorohalide bromides and iodides with
europium activators. It is these two components,
the europium activated barium fluorohalide
bromides and iodides, that will work together to capture
the X-ray energy photons and create our latent
radiographic image. X-ray photon energy is
captured in the phosphor layer by energy transfer
through a process called photoelectric absorption. This is a key interaction
between X-ray photons and matter. So here's what happens. When the X-ray
photon strikes one of the atoms in
the phosphor layer, it ejects an inner shell
electron and is absorbed. This energy from the
absorbed X-ray photon excites the ejected electron,
making it a photoelectron. During the photoelectric
absorption, the number of photoelectrons
produced in the phosphor layer is proportional to the
number of X-ray photons that interact with it. These photoelectrons
then interact with other nearby electrons
within the phosphor, exciting them from a low energy
valence band to a high energy conduction band. It's in the gap
between the conduction band and the valence band
that the europium activators in the imaging play act
as electron traps known as F-centers. They trap and store the
high energy electrons, and it is this
stored energy that houses the information
that makes up the latent radiographic
image or stored X-ray image. The number of
electrons trapped is proportional to the
number of X-ray photons that interact with
the imaging plate. The latent image remains
stored within the individual phosphors, until being
released by a laser light within the plate reader. In summary, computed radiography
is an X-ray imaging system, which utilizes
cassettes to capture latent radiographic images and
process those latent images into a digital radiograph. The cassettes are made
up of several layers along with an active
phosphor layer, where the latent
image is produced. X-ray photons interact
with the phosphor atoms through a process called
photoelectric absorption and excites the outermost
shell electrons. The excess energy
created in this process is stored within
the imaging plate, creating the latent image.