Page 226 - Adams and Stashak's Lameness in Horses, 7th Edition

192 Chapter 3

lapse between exposure to image display takes less than matrix size. As the pixel size decreases and the matrix
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10 seconds, which increases the patient throughput. size increases, the monitor’s resolution improves. The
VetBooks.ir facilitate greatly the workflow in a busy practice and is between the darkest (black) shade and the brightest
dynamic range of a monitor is the luminance ratio
The ability to rapidly take multiple sequential views can
(white) shade that the monitor can display. The dynamic
more likely to result in the acquisition of high‐quality,
diagnostic images, since immediate assessment of patient range of a monitor is usually correlated to the lumi­
positioning and generator settings is possible. Two dif­ nance; thus, the higher the monitor brightness, the
ferent DDR conversion systems exist: direct and indirect higher the dynamic range.
conversion. A wide variety of Windows‐ or Apple‐based DICOM
Direct conversion systems feature a flat panel detec­ software programs are available and include free
tor containing photoconductors such as selenium (most Internet‐downloadable versions to more sophisticated
commonly used), lead iodide, lead oxide, thallium bro­ software that requires purchase or leasing contracts.
mide, or gadolinium compounds. The photoconduc­ Free versions typically are less robust and may limit the
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tors are the first layer in contact with the X‐ray photons size of study that can be downloaded. The different
that exit the patient and are responsible for converting DICOM viewing software provides the user with the
them into electrical charges. The electrical charges are ability to further manipulate and improve the image
stored in capacitors and then read row by row, and the displayed on the monitor. Features such as modifying
information is sent to the computer after analog‐to‐digi­ contrast (window and level), image sharpening, edge
tal conversion. More detailed information is available in enhancement, zoom, measurement tools, multiplanar
radiology journal articles. 29,62 reconstruction, etc. are included in most DICOM view­
Indirect conversion can be done with two technolo­ ers but differ among vendors depending on the quality
gies: charged coupled device (CCD) or indirect flat panel of the software (Figure 3.3).
detector. In a CCD, the X‐ray photons from the patient
are converted to light after striking a scintillator layer. Digital Image Storage and Transmission
The light is then usually minified with a lens to fit in the
small CCD where it is recorded and converted into elec­ The technology for digital imaging storage and trans­
trical charges. In the process of light minification, some mission is rapidly evolving, making storage and sharing
of the light photons are lost and do not reach the CCD relatively simple. Images can be stored on an on‐site
unit. This results in increased image noise compared server, and many vendors offer cloud‐based storage as
with flat panel detectors in which minification is not well. While convenient, reliance solely on cloud‐based
applied. The noise, edge distortion, and artifacts that storage requires a fast and reliable Internet connection.
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are frequently seen in CCD systems mean that this tech­ The growth of the practice of teleradiology—the elec­
nology has largely fallen out of favor. As a result, servic­ tronic transmission of images for interpretation and
ing and repairing such systems has become complex and consultation—has been enhanced via downloadable
costly. DICOM images or web‐based viewing of images. These
An indirect flat panel detector also contains a scintil­ options for image transmission are preferable to email­
lator layer, or intensifying screen, made of cesium iodide ing jpeg images, which have lower resolution and have
crystals. Other materials used in the scintillator layer to be converted back to DICOM images in order to be
include gadolinium‐based crystals. The advantage of manipulated.
cesium iodide scintillators is that the crystals can be
structured into 5–10‐μm‐wide parallel needles, which
reduce the spreading of light within the scintillator. Accessory X‐ray Equipment
Hence, the spatial resolution is increased. The high reso­ Accessory X‐ray equipment for equine musculoskel­
lution of these systems means that they are the gold etal examinations has special requirements. Good‐ quality,
standard of medical imaging. well‐maintained accessory equipment is necessary to
While initial DDR systems required wired connec­ ensure that quality radiographic examinations are per­
tions between the plate and the reader, many fully wire­ formed safely. Accessory equipment consists of grids,
less systems are now on the market. Comparison charts markers, and film‐marking systems, detector holders,
are available for the commercially available DDR sys­ padded tables for radiographic projections under gen­
tems and are a good reference source when selecting a eral anesthesia, and positioning aids. These pieces of
system for purchase. Other considerations besides cost equipment are used with conventional or digital radiog­
when selecting a system include the logistics of mainte­ raphy. Most accessory equipment is available commer­
nance, service and repair, warranty duration, and the cially, but because of the unique requirement of equine
availability of technical help in the case of software/ radiology, some equipment, such as cassette holders,
hardware malfunction. 2 padded tables, and positioning aids, may have to be
custom built locally.
Digital Image Display
Grids
A computer used in a digital image viewing station
consists of two basic parts: one or more monitors (hard­ Radiographic grids are used to decrease the amount
ware) and a DICOM viewer (software). A monitor used of scattered radiation that exposes the X‐ray film.
for digital radiography should have excellent resolution, Because scattered radiation exposes film from several
high brightness (luminance), and wide dynamic range. directions, it has the effect of decreasing image detail
The resolution of a monitor is dictated by the pixel and and contrast. Radiographs of thicker body parts, such as