[MUSIC PLAYING] Hey, everybody. Welcome back. In this lesson,
we'll be discussing direct and indirect conversion
digital radiography, also known as DR. Direct and indirect
conversion digital radiography are different from computed
radiography in that they do not use cassettes or standalone
cassette readers. Additionally, the
image processing step follows the image
acquisition, immediately converting the radiographic
image into digital data without an operator's
involvement. Computed radiography
requires the extra step to extract the latent
image from the cassettes. So by eliminating
these extra steps, digital radiography
systems are faster and can produce
higher-quality images than computed radiography. DR acquires X-ray images
in one of two ways, either using direct conversion,
which converts X-rays to electronic signals, or
indirect conversion, which converts X-rays to visible light
and then to electronic signals. First, let's talk about
direct conversion, DR. Direct conversion DR systems
use a photo conductor made of amorphous selenium and a
thin film transistor array, otherwise known as a TFT array. The DR image receptor
is made up of a matrix of very small detector
elements or DELs. Each DEL contains a capture
element, a storage capacitor, and a TFT switch. When a direct conversion
DR system is in use, a voltage is applied to the
top surface of the detector just before the
exposure is made. The X-ray beam then
interacts directly with the amorphous selenium,
causing the selenium atoms to release electrons,
creating an electronic charge. Storage capacitors in the
DELs collect the charge. After the exposure,
the TFT switches release the electrical charges
from the individual DELs to the analog-to-digital
converter, which converts them to a digital signal used to
produce the digital image. So direct conversion
DR systems use a process which
converts X-rays directly into an electronic signal. Because direct
conversion DR systems skip the step of converting
X-rays to visible light, they have higher image
quality than other systems. Direct conversion DR systems
are mostly used in mammography. This is for two reasons. First, the higher image quality
allows for better visualization of micro-calcifications. Second, the amorphous selenium
detectors cannot be produced large enough for use
in general radiography. Next, let's talk about
indirect conversion DR. The essential difference
between direct and indirect conversion DR systems is
that indirect systems first convert the invisible
X-ray photons into visible light photons,
which are then converted to an electrical signal. Indirect conversion
DR systems can be placed in one of
two categories, one that uses charged coupled
devices, called CCDs, or one that uses TFTs. In both types of indirect
conversion DR systems, there is a scintillation
layer made up of either cesium
iodide or gadolinium. First, let's
discuss CCD systems. In the CCD system, the
scintillation layer is coupled to each
CCD sensor chip either by lenses
or fiber optics. When the X-ray photons strike
the scintillation layer, they are converted
into light photons. Because light is
emitted isotropically, light photons are spread
out in all directions, which causes blur and reduces
spatial resolution. Cesium iodide is the
preferred material for scintillation
lasers because it can be formed into very
small needles or columns. The column shape helps to
focus the light photons, which then improves the
spatial resolution of the radiographic image. After the X-ray photons are
converted into light photons, the CCD sensor chips
convert the light photons into electrical signals. Those electrical
signals are then sent through an
analog-to-digital converter which sends the
converted digital signal to a computer for processing. Now let's move on
to TFT systems, which also use the
scintillation layer just like the CCD systems, but
they have additional elements. After the scintillation
layer, there is a photodiode layer made
up of amorphous selenium and then a TFT array. Again, when X-ray photons
strike the scintillation layer, they are converted
into light photons. The photodiode layer
converts the light photons into an electrical
signal that is transferred to the TFT array. From there, the TFT array
sends the electrical signal to an analog-to-digital
converter to produce a digital
signal which is then sent to a computer for processing. Both CCD and TFT indirect
conversion DR systems are used in general-purpose
radiography, and TFT indirect conversion
DR systems are typically used in angiography and fluoroscopy. Both types of
indirect-conversion DR systems cost less than
direct-conversion DR systems. It is also easier to
repair and replace parts on indirect-conversion
DR systems. However, that extra step
of converting X-ray photons to light reduces
spatial resolution. So in summary, there
are two main types of DR systems, direct conversion
and indirect conversion. Direct conversion DR systems
directly convert X-ray photons into electronic signals. Indirect conversion
DR systems are either CCD systems, which consist of
a scintillation layer coupled with a CCD sensor chip either
by lenses or fiber optics, or TFT systems, which
consist of a scintillation layer, a photodiode
layer, and a TFT layer. Direct-conversion DR systems
have higher spatial resolution than indirect conversion DR
systems but are more expensive.