Page 225 - Adams and Stashak's Lameness in Horses, 7th Edition

Diagnostic Imaging 191

5. Primary X‐ray beam restrictor (the preferred system interpretation at the viewing station. Key factors that
is an adjustable light‐beam collimator) can be manipulated include the lookup table (LUT),
VetBooks.ir Portable, mobile, and fixed (ceiling‐suspended) X‐ray the image by altering the actual pixel values, region of
which allows the operator to change the grayscale of
6. Absence of radiation or electrical hazards
interest, and contrast, as well as other options. Once
machines are available for equine radiography. These
differ in size and capacity, from small, portable 15‐mA the study is completed, the images are sent to the view­
ing station where Digital Imaging and Communication
machines to large, fixed 800‐mA machines. Portable in Medicine (DICOM) viewing software is used for
X‐ray machines are best suited for out‐of‐hospital loca­ image display on a computer monitor. A complete dis­
tions. They are lightweight (30–55 lbs) and can be easily cussion of image processing is beyond the scope of this
stored for transport. Options for portable X‐ray chapter. More in‐depth information is available in text­
machines include both corded and battery‐operated books of diagnostic radiology physics and radiology
wireless and range in size and weight. Most manufactur­ journal articles. 9,29,31,41
ers also provide a variety of types of stands on which the
portable X‐ray machine can be suspended. This helps
reduce dose to personnel and minimize motion artifact DIGITAL RADIOGRAPHY SYSTEMS
and fatigue.
High‐frequency transformers that reduce the expo­ Computed Radiography
sure time are available for portable X‐ray machines. CR systems are similar to screen‐film systems, in
They are equivalent to a 3‐phase, 12‐pulse X‐ray gener­ which a cassette stores the latent image until processing.
ator and provide a 40%–50% increase in exposure In CR, the cassette contains an imaging plate possessing
compared with a full‐wave rectified machine. The prac­ photostimulable phosphors (PSP). After the imaging
tical application is to reduce the exposure time and still plate is exposed to X‐ray photons, the PSP change to a
obtain an adequate exposure for a diagnostic radio­ higher energy state and store the latent image until
graph. A line voltage compensator is also desirable for processing.
portable X‐ray machines. It is especially important in When the cassette is placed in the reader unit, the
field situations, where the line voltage may fluctuate imaging plate is automatically removed from the cas­
with the simultaneous use of the electrical equipment, sette and scanned by a laser beam. The laser light stimu­
and it can compensate for a line voltage drop when a lates the energy trapped in the PSP, and visible light is
long electrical extension cord is used. released from the plate. The released light strikes a pho­
Fixed X‐ray machines are usually suspended from the tomultiplier tube and is converted into an electronic sig­
ceiling. They are limited to use within a single room. nal. The magnitude of the electronic signal represents
These machines can be coupled with large transformer the degree of X‐ray attenuation of the structure that was
systems that are capable of providing high milliamper­ imaged and is also assigned a corresponding shade of
age (800–1,000 mA). Ceiling‐suspended tubes are easily gray after conversion from analog‐to‐digital format.
62
movable and can be locked into position by magnetic The data is then stored temporarily in a local hard disk.
locks while the X‐ray exposure is made. The tube should Subsequently, the imaging plate is exposed to bright
be capable of extending to the floor. X‐ray machines white light to erase any residual trapped energy before it
with high milliamperage capability have fast exposure is returned to the cassette for reuse. The whole readout
9
times, eliminating most motion artifacts when radio­ process for a 14 × 17‐in. image plate takes approximately
graphing equine limbs.
30–40 seconds. The resultant image is sent to an acqui­
29
sition computer, where post‐processing manipulations
can be performed.
Types of Digital Radiography
The separation of the readout unit in CR systems
Digital radiography can be divided into computed means that in a multi‐veterinarian practice, cassettes can
radiography (CR) or direct digital radiography (DDR). be divided between practitioners, both in the field and in
CR uses a detector panel that stores the image and the clinic to allow for simultaneous performance of mul­
then uses a separate image readout unit. In DDR, the tiple radiographic examinations. It also enables a degree
detector panel converts the X‐rays into electrical charges of wireless imaging not always afforded by digital sys­
by means of a direct readout process. DDR systems can tems since the cassettes themselves do not require wires.
be further divided into direct or indirect conversion The initial costs of a CR system are less than those of a
technologies depending on the type of X‐ray conversion DDR system, which may also make the system a more
used. Both CR and DDR systems convert the electronic attractive prospect for purchase. CR units can also be
2
signal to digital data and send it to an acquisition device. appealing in mixed animal practice because of the flex­
At this point, the digital data is considered “raw,” and it ibility in the use of plates. However, the resolution
can be further processed. achieved by these systems is often inferior to DR sys­
The term picture archiving and communication sys­ tems, which have several other advantages, outlined
tem (PACS) is used to describe a system that allows for below.
image retrieval, review, processing, interpretation, and
storage. The term “image processing” incorporates Direct Digital Radiography
multiple steps in the acquisition of a digital image
before the study is completed. During the image pro­ DDR systems have a direct readout, meaning that the
cessing, the raw image can be manipulated with the detector sends the signal straight to the computer instead
purpose of having the best image possibly available for of using a reading unit such as the CR system. The time