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The Evolution of CT Scanner Detectors

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                <p>CT scanners are designed for imaging of the internal
structures of the body. They provide detailed anatomical information by
utilizing the principle that different types of tissue structures scanned
are displayed in the image as different shades of grey. Intravenous or
oral contrast media may be used to further enhance differentiation
between tissues.</p>
<p>The basic components of a CT scanner are an x-ray tube and an arc
banana of detectors or a flat panel, mounted on a gantry with a circular
aperture. Along the patient long axis (Z) there are many rows of these
arcs of detectors, giving rise to the term multi-slice CT.</p>
<p>Multi-detector CT is also a commonly used term. The extent of patient
coverage by the detector rows currently ranges from 12mm to 160mm in
length, depending on the CT scanner model.</p>
<p>CT scanner technology has advanced rapidly in recent years, moving to
more efficient and stable detectors, more refined engineering and data
acquisition systems and electronics, and faster computers.</p>
<p>These CT scanner developments have been largely directed towards
faster scanning of further lengths of the patient, using finer slices. As
a result, CT scanners have evolved from a slice-by-slice diagnostic
imaging system into a truly volumetric imaging modality, where images can
be reconstructed in any plane without loss of image quality. This has
lead to the increased use of multi-planar and 3D display modes in
diagnosis.</p>
<p>However, it is also important to recognize that the performance of CT
scanners in practice depends on the trade-off between image quality and
radiation dose. As a result, each system should also be assessed in terms
of clinical performance, with close observation of the radiation dosage
utilized.</p>
<p>Generally, multi-slice scanners cover the patient volume between 20
and 40mm in length per rotation. The latest diagnostic multi-slice CT
scanners can image patient volumes of up to 160mm per rotation.</p>
<p>The length of the detector array of the CT scanners determines the
number of rotations needed to cover the total scan length, and
consequently, the overall scan time. The ability to scan a given length
with fewer rotations also helps to minimize head load on the x-ray tube,
thereby allowing the scanning of longer lengths.</p>
<p>Detector arrays of the CT scanners are broadly divided into two types:
fixed and variable. Fixed arrays have detectors of equal z-axis dimension
over the full extent of the array, while with variable arrays; the
central portion comprises finer detectors. With variable arrays, the
total scan time for a given length for the finest slice acquisition is
longer, because the z-axis coverage is reduced.</p>
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<p>All CT scanners with greater than 64-slice acquisition have fixed
array.</p>
<p>Complete coverage of an organ offers advantages for both dynamic
perfusion and cardiac studies. The z-axis detector array lengths on the
current 64-slice scanners, of up to 40 mm, are adequate to cover these
organs in only a few rotations. A coverage length of 160mm usually allows
complete organ coverage in a single rotation, so the function of the
whole organ can be monitored over time.</p>
<p>The evolution of CT scanner designs reflect different strategies to
accommodate future developments and allow for production costs. There is
also some small dose saving where larger detector elements are used on
the lower slice category scanners.</p>
<p>Spatial resolution is the ability of the CT scanners to image an
object without blurring. It is often described as the sharpness of an
image. It may be quoted as the smallest object size able to be
distinguished, and as such, is evaluated using high-contrast test objects
where signal-to-noise level is high and does not influence
perception.</p>
<p>Modern CT scanners should be capable of achieving isotropic
resolution: a z-axis resolution that is equal to or approaching the scan
plane resolution, as this is essential for good-quality, multi-planar and
3D reconstructions.</p>
<p>It is helpful to remember that the cost of high-spatial resolution of
CT scanners is either in the high image noise or in a high-patient
radiation dose when the tube current is raised to reduce the image
noise.</p>
<p>Contrast resolution of CT scanners is the ability to resolve an object
from it surroundings, when the CT numbers are similar. It is sometimes
referred to as low-contrast detectability. The ability to detect an
object is dependent on its contrast, the level of image noise and its
size. Contrast resolution is usually specified as the minimum size of
object of a given contrast difference, that can be resolved for a
specified set of scan.</p>
<p>The temporal resolution of CT scanners is defined as the time taken to
acquire a segment of data for image reconstruction.</p>
<p>In CT scanners, temporal resolution is usually considered in the
context of cardiac scanning. The goal in cardiac CT is to minimize image
artifacts due to the motion of the heart. This can be achieved using ECG-
gating techniques, and imaging the heart during the period of least
movement in the cardiac cycle, resulting in temporal resolution
requirements of very short periods, as compared with the heart cycle.</p>
<p>There is an optimum combination of pitch, gantry rotation time, and
number of segments for each given heart rate.</p>
<p>CT scanner detectors capture the radiation beam from the patient and
convert it into electrical signals, which are subsequently converted into
binary coded information for onward transmission to a computer system for
further processing.</p>
<p>CT scanners detectors must be capable of responding with extreme speed
to a signal, without lag, must quickly discard the signal, and prepare
for the next. They must also respond consistently and be small in size.
CT scanners detectors should have high capture efficiency, high
absorption efficiency and high conversion efficiency. These three
parameters are called the detector dose efficiency.</p>
<p>The capture efficiency is how well the detectors receive photons from
the patient. It is primarily controlled by detector size and the distance
between detectors.</p>
<p>Absorption efficiency is how well the detectors convert incoming x-ray
photons. It is primarily determined by the materials used, as well as the
size and thickness of the detector.</p>
<p>Conversion efficiency is determined by how well the detector converts
the absorbed photon information to a digital signal for the computer.</p>
<p>In recently manufactured CT scanners, the entire array of detectors
consists of groupings of detectors, with each group known as a detector
module, which is plugged into a motherboard unit of the detection
system.</p>
<p>Flat-panel detectors have been developed for use in radiography and
fluoroscopy, with the defined goal of replacing standard x-ray film,
film-screen systems and image intensifiers by an advanced solid state
sensor system. Flat-panel detector technology offers high dynamic range,
dose-reduction, and fast digital conversion - yet keeping to a compact
design. It appears logical to employ the same design for CT scanners, as
well.</p>
<p>The use of flat-panel detectors for CT scanners provides a very
efficient way of x-ray detection and acoustics. Flat-panel detectors
provide high-spatial resolution. However, there are also some
disadvantages: relatively lower dose efficiency, smaller fields or view
and lower temporal resolution.</p>
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