0:01 in this video we're going to discuss the
0:04 major components and a brief history of CT
0:05 CT
0:08 scanners when compared to Conventional
0:10 radiography CT has the advantage of
0:12 eliminating superimposition of
0:14 structures by acquiring data in what we
0:17 describe as slices in the axial plane
0:19 which can then be reconstructed into a
0:20 volumetric data set with
0:23 three-dimensional information so where
0:25 conventional radiography has us thinking
0:28 in pixels or picture elements CT has us
0:31 thinking in voxal or volume elements a
0:32 voxel is like a pixel but with
0:39 space CT also provides us with a much
0:41 greater ability to differentiate between
0:43 very small differences in density
0:46 between anatomical structures in imaging
0:48 terms this means CT has very good low contrast
0:54 resolution the first CT scan was
0:58 performed by Sir gy houndsfield in 1971
0:59 and the first generation of scanners
1:02 used a translate rotate motion where a
1:04 single detector element would be exposed
1:06 sequentially by a pencil beam while
1:09 translating widthways across the patient
1:11 then the whole system would rotate one
1:13 degree at a time and repeat the process
1:16 needless to say this took a very long
1:19 time the pencil beam was soon replaced
1:21 with a fan beam which maintained the
1:23 translate rotate movement but covered a
1:26 wider area the fan beam injured into
1:28 modern day scanners but in the late 80s
1:30 we lost the Translate rotate nonsense
1:32 and replaced it with the rotate rotate
1:34 system with which we're familiar today
1:36 where the tub and detector assembly spin
1:39 in unison on opposing sides of the
1:42 patient we've upgraded the single
1:45 slit-shaped fan to a thicker fan beam
1:48 exposing a multi row detector where the
1:50 detector instead of being a single row
1:52 of detector elements is now composed of
1:54 multiple rows of detector elements so it
1:56 looks more like a curved version of a
1:58 conventional digital radiography
2:01 detector it's just much wider than it is
2:04 long this type of system is referred to
2:06 as a third generation scanner and is the
2:14 today from the outside a CT scanner is
2:16 composed of the generator which provides
2:18 high voltage constant potential to the
2:20 X-ray tube in the same manner as
2:23 conventional x-ray system and also
2:25 governs the speed of anode
2:28 rotation the Gantry which is the
2:31 ring-shaped body of the scanner housing
2:34 the tube detector array a control panel
2:36 and laser lights to align the patient
2:39 to the table or couch which the patient
2:41 is positioned upon and which slides in
2:44 and out of the scanner along the Zed AIS
2:48 so the Ed x-axis being left to right the
2:51 Y AIS being up and down and the Z axis
2:53 being in and out of the
2:56 scanner and the computer and console
2:57 from which the technologist acquires the
2:59 scan which is in the control booth
3:05 room on the inside of the scanner is the
3:08 X-ray tube which generates x-rays the
3:10 detector array which detects the x-rays
3:11 transmitted through the
3:14 patient and the data acquisition system
3:16 which receives data from the detectors
3:18 and communicates that data to the scan
3:21 computer also how throughout the inside
3:23 of the Gantry are a technology called
3:25 slip Rings slip rings are used in many
3:27 modern Electronics requiring rotating
3:29 Parts through which an electrical
3:32 current or signal needs to pass they
3:33 function like a bearing where two
3:35 separate components can turn relative to
3:38 one another or one can turn while one
3:41 stays stationary but while turning there
3:43 is a contact surface either a smooth
3:45 ring or metal brushes which maintains
3:47 conduction of electrical current or
3:49 signal throughout the rotation without
3:51 the need for wires which would twist up
3:54 under that type of
3:56 motion this is significant because it
3:57 allowed for the continuous acquisition
3:59 of data in what's called helical
4:02 scanning where the scanner acquires data
4:04 in a spiral pattern wrapping around the
4:06 patient acquiring continuously while the
4:08 patient moves through the scanner along
4:11 the Zed axis this is in contrast to
4:13 axial scanning or step and shoot
4:15 scanning where the scanner would acquire
4:18 one rotation move the table in acquire
4:20 the next rotation move the table in
4:23 again and so on now the terminology here
4:25 can be confusing because when conducting
4:27 a helical scan we are still considering
4:29 that to be acquiring data in the axial plane
4:30 plane
4:32 in CT we're always scanning in the axial
4:35 plane but the difference between helical
4:37 scan mode and an axial scan mode just
4:39 refers to this difference of continuous
4:41 scanning in a spiral versus step and
4:49 slices there are various ways in which
4:51 the X-ray beam is optimized within the
4:53 Gantry between the tube and the patient
4:55 are metal filters which absorb low
4:58 energy x-ray photons increasing the
4:59 average Photon energy or high hardening
5:01 the X-ray
5:04 beam and one type of filter unique to CT
5:06 is the bow tie filter which is shaped
5:08 like a bow tie so is narrower in the
5:10 middle and thicker at the edges this
5:12 serves to remove a higher number of
5:14 photons at the edge of the patient where
5:16 less photons are needed while allowing a
5:18 higher number of photons to pass through
5:20 the center of the patient uh where the
5:23 additional beam penetration is
5:25 required this leads to a more
5:28 homogeneous beam exiting the patient
5:29 while reducing unnecessary RIS radiation
5:32 dose to the patient's
5:34 periphery as in conventional x-ray there
5:36 is a cator between the tube and the
5:37 patient Which shapes and defines the
5:40 edges of the X-ray beam leaving the tube
5:42 and unlike conventional x-ray there's
5:45 also a post-patient or pretector cator
5:47 which aligns to the edges of the
5:50 detector area being used again shaping
5:51 the beam before it hits the detector and
5:53 absorbing scatter radiation to improve image
5:54 image
5:57 quality also absorbing scatter we have
5:59 an anti-scatter grid again much like in
6:02 conventional radiography made up of lead
6:04 strips which absorb any x-rays which
6:05 aren't traveling straight towards the
6:08 detector since any x-rays incident at an
6:12 odd angle to the detector are scattered
6:15 x-rays unlike in conventional x-ray in
6:17 some CT systems the grid lines are
6:19 oriented in both horizontal and vertical
6:21 directions much like what nuclear
6:23 medicine technologists might call a
6:26 cator different professions different
6:28 language and the reason we can do this
6:30 is because we know that the tube and
6:32 detector will never be angled relative
6:35 to one another in CT where in x-ray
6:37 we're used to angling in One Direction
6:39 relative to the detector so we have grid
6:42 lines only pointing in that
6:44 direction the detector itself is
6:46 composed of multiple rows of detector
6:48 elements and the number of rows in The
6:50 Zed axis is the number of slices that
6:53 the scanner can acquire at once most
6:55 modern scanners have at least 64 rows
6:57 and many vendors are pushing the limit
7:00 of 256 rows or even
7:03 higher the detector is an indirect type
7:05 digital detector meaning the x-rays
7:07 incident on the detector are absorbed by
7:09 a scintillation layer and converted into
7:12 visible light this visible light hits a
7:14 photo diode which converts that light
7:16 into an analog electrical signal that
7:18 analog signal is then converted into a
7:20 digital signal by an analog to digital
7:24 converter or ADC and at this point we
7:26 have raw image data which is ready to be
7:28 communicated to our workstation and
7:34 we'll cover the course concepts of
7:36 producing an image in CT in the next
7:38 video including field of view pitch
7:40 window width and level linear
7:42 attenuation coefficients CT numbers and
7:45 hfield units if you have any questions
7:46 on the content of this video you're
7:48 welcome to comment or send me an email
