This video explains the fundamental components and historical evolution of CT scanners, highlighting their advantages over conventional radiography in producing detailed, slice-based volumetric images with superior density differentiation.
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in this video we're going to discuss the
major components and a brief history of CT
CT
scanners when compared to Conventional
radiography CT has the advantage of
eliminating superimposition of
structures by acquiring data in what we
describe as slices in the axial plane
which can then be reconstructed into a
volumetric data set with
three-dimensional information so where
conventional radiography has us thinking
in pixels or picture elements CT has us
thinking in voxal or volume elements a
voxel is like a pixel but with
space CT also provides us with a much
greater ability to differentiate between
very small differences in density
between anatomical structures in imaging
terms this means CT has very good low contrast
resolution the first CT scan was
performed by Sir gy houndsfield in 1971
and the first generation of scanners
used a translate rotate motion where a
single detector element would be exposed
sequentially by a pencil beam while
translating widthways across the patient
then the whole system would rotate one
degree at a time and repeat the process
needless to say this took a very long
time the pencil beam was soon replaced
with a fan beam which maintained the
translate rotate movement but covered a
wider area the fan beam injured into
modern day scanners but in the late 80s
we lost the Translate rotate nonsense
and replaced it with the rotate rotate
system with which we're familiar today
where the tub and detector assembly spin
in unison on opposing sides of the
patient we've upgraded the single
slit-shaped fan to a thicker fan beam
exposing a multi row detector where the
detector instead of being a single row
of detector elements is now composed of
multiple rows of detector elements so it
looks more like a curved version of a
conventional digital radiography
detector it's just much wider than it is
long this type of system is referred to
as a third generation scanner and is the
today from the outside a CT scanner is
composed of the generator which provides
high voltage constant potential to the
X-ray tube in the same manner as
conventional x-ray system and also
governs the speed of anode
rotation the Gantry which is the
ring-shaped body of the scanner housing
the tube detector array a control panel
and laser lights to align the patient
to the table or couch which the patient
is positioned upon and which slides in
and out of the scanner along the Zed AIS
so the Ed x-axis being left to right the
Y AIS being up and down and the Z axis
being in and out of the
scanner and the computer and console
from which the technologist acquires the
scan which is in the control booth
room on the inside of the scanner is the
X-ray tube which generates x-rays the
detector array which detects the x-rays
transmitted through the
patient and the data acquisition system
which receives data from the detectors
and communicates that data to the scan
computer also how throughout the inside
of the Gantry are a technology called
slip Rings slip rings are used in many
modern Electronics requiring rotating
Parts through which an electrical
current or signal needs to pass they
function like a bearing where two
separate components can turn relative to
one another or one can turn while one
stays stationary but while turning there
is a contact surface either a smooth
ring or metal brushes which maintains
conduction of electrical current or
signal throughout the rotation without
the need for wires which would twist up
under that type of
motion this is significant because it
allowed for the continuous acquisition
of data in what's called helical
scanning where the scanner acquires data
in a spiral pattern wrapping around the
patient acquiring continuously while the
patient moves through the scanner along
the Zed axis this is in contrast to
axial scanning or step and shoot
scanning where the scanner would acquire
one rotation move the table in acquire
the next rotation move the table in
again and so on now the terminology here
can be confusing because when conducting
a helical scan we are still considering
that to be acquiring data in the axial plane
plane
in CT we're always scanning in the axial
plane but the difference between helical
scan mode and an axial scan mode just
refers to this difference of continuous
scanning in a spiral versus step and
slices there are various ways in which
the X-ray beam is optimized within the
Gantry between the tube and the patient
are metal filters which absorb low
energy x-ray photons increasing the
average Photon energy or high hardening
the X-ray
beam and one type of filter unique to CT
is the bow tie filter which is shaped
like a bow tie so is narrower in the
middle and thicker at the edges this
serves to remove a higher number of
photons at the edge of the patient where
less photons are needed while allowing a
higher number of photons to pass through
the center of the patient uh where the
additional beam penetration is
required this leads to a more
homogeneous beam exiting the patient
while reducing unnecessary RIS radiation
dose to the patient's
periphery as in conventional x-ray there
is a cator between the tube and the
patient Which shapes and defines the
edges of the X-ray beam leaving the tube
and unlike conventional x-ray there's
also a post-patient or pretector cator
which aligns to the edges of the
detector area being used again shaping
the beam before it hits the detector and
absorbing scatter radiation to improve image
image
quality also absorbing scatter we have
an anti-scatter grid again much like in
conventional radiography made up of lead
strips which absorb any x-rays which
aren't traveling straight towards the
detector since any x-rays incident at an
odd angle to the detector are scattered
x-rays unlike in conventional x-ray in
some CT systems the grid lines are
oriented in both horizontal and vertical
directions much like what nuclear
medicine technologists might call a
cator different professions different
language and the reason we can do this
is because we know that the tube and
detector will never be angled relative
to one another in CT where in x-ray
we're used to angling in One Direction
relative to the detector so we have grid
lines only pointing in that
direction the detector itself is
composed of multiple rows of detector
elements and the number of rows in The
Zed axis is the number of slices that
the scanner can acquire at once most
modern scanners have at least 64 rows
and many vendors are pushing the limit
of 256 rows or even
higher the detector is an indirect type
digital detector meaning the x-rays
incident on the detector are absorbed by
a scintillation layer and converted into
visible light this visible light hits a
photo diode which converts that light
into an analog electrical signal that
analog signal is then converted into a
digital signal by an analog to digital
converter or ADC and at this point we
have raw image data which is ready to be
communicated to our workstation and
we'll cover the course concepts of
producing an image in CT in the next
video including field of view pitch
window width and level linear
attenuation coefficients CT numbers and
hfield units if you have any questions
on the content of this video you're
welcome to comment or send me an email
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