CT components Ct generation

1. Computed Tomography
Lecture#7

3. Introduction
• The term tomography refers to a picture (graph) of a slice (tomo).
• It is also known as computed axial Tomography (CAT) scan, which is medical technology
that uses X rays and computers to produce three-dimensional images of the human body.
• Unlike traditional X rays, which highlight dense body parts, such as bones, CT provides
detailed views of the body’s soft tissues, including blood vessels, muscle tissue, and organs,
such as the brain.
• While conventional X rays provide flat two-dimensional images, CT images depict a cross-
section of the body.
• The anatomical information is digitally reconstructed from x-ray transmission data obtained
by scanning an area from many directions in the same plane to visualize information in that
plane.

4. CT Instrumentation
• First-generation (1G) scanners are no
longer manufactured for medical imaging,
• It consists of a single source, collimated
(meaning that its beam is restricted) to a
thin line, and a single detector that move
in unison along a linear path tangent to a
circle that contains the patient.
• After making a linear scan, the source and
detector apparatus are rotated so that a
linear scan at a different angle can be
made, and soon

5. • A second-generation (2G) scanner, has
additional detectors, forming a detector
array, arranged along a line or a circle. As
in the 1G scanner, the source and detector
array move linearly in unison to cover the
field of view.
• With the 2G scanner geometry, we can
make a larger rotation after each linear
scan and thereby complete a full scan in
less time, making the 2G scanner faster

6. • third-generation (3G) scanner
has a fan-beam that covers the
image region with the source
held in a single position.
• allows for a dramatic decrease in
scan time.
• Need greater dose

7. • fourth-generation (4G) scanner
has a single rotating source with
a larger ring of stationary
detectors.

8. Fifth-generation (5G) scanners
• It is a method of improving the
temporal resolution of CT scanners.
• Because the X-ray source has to
rotate by over 180 degrees in order
to capture an image the technique
is inherently unable to capture
dynamic events or movements that
are quicker than the rotation time.
• and this allows a full set of fan-
beam
• Exposure =50 milliseconds.

9. Fifth-generation (5G) scanners
• Instead of rotating a conventional X-ray tube around the patient, the EBCT
machine houses a huge vacuum tube in which an electron beam is electro-
magnetically steered towards an array of tungsten X-ray anodes arranged
circularly around the patient.
• Each anode is hit in turn by the electron beam and emits X-rays that are
collimated and detected as in conventional CT. The lack of moving parts allows
very quick scanning, with single slice making the technique ideal for capturing
images of the heart.
• EBCT has found particular use for assessment of coronary artery calcium, a
means of predicting risk of coronary artery disease.

11. 6G: Helical CT
• A helical CT scanner consists of a
conventional arrangement of the x-
ray source and detectors (as in 3G
and 4G systems) which can
continuously rotate.
• While the tube is rotating and
acquiring projection data, the
patient table is set into motion,
sliding the patient through the
source–detector

12. • volume of raw data is generated,
from which axial images are
reconstructed using
interpolation
• slip ring technology allowed
transmission of energy to
rotating gantry without the need
of cables

13. A seventh-generation (7G) scanner
• Multiple-row detector CT (MDCT)are similar
in concept to the helical or spiral CT but there
are more than one detector ring.
• In these scanners, a ‘‘thick’’ fan-beam is
used, and multiple (axial) parallel rows of
detectors are used to collect the x-rays within
this thick fan. (Some scanners have fan
beams that are so thick they can be thought
of as cone beams.)
• The major benefit of multi-slice CT is the
increased speed of volume coverage. This
allows large volumes to be scanned at the
optimal time .

14. • The advent of helical and MDCT
has made the requirement for new
developments in data processing
even more critical.
• In particular, while a conventional
CT might have reconstructed 40
slices over a region of interest, with
helical CT and MDCT a clinician
might acquire 80–120 slices over
the same region in less time

16. SCAN MODES DEFINED
• Step-and-Shoot Scanning
• 1) the x-ray tube rotated 360° around the patient to acquire data for a
single slice,
• 2) the motion of the x-ray tube was halted while the patient was advanced
on the CT table to the location appropriate to collect data for the next slice.
• 3) steps one and two were repeated until the desired area was covered.
• The step-and-shoot method was necessary because the rotation of the x-
ray tube entwined the system cables, limiting rotation to 360°.
• Consequently, gantry motion had to be stopped before the next slice could
be taken, this time with the x-ray tube moving in the opposite direction so
that the cables would unwind.

17. SCAN MODES DEFINED
• Helical (Spiral) Scanning
• Many technical developments of the 1990s allowed for the
development of a continuous acquisition scanning mode most often
called spiral or helical scanning.
• Key among the advances was the development of a system that
eliminated the cables and thereby enabled continuous rotation of the
gantry.
• This, in combination with other improvements, allowed for
uninterrupted data acquisition that traces a helical path around the
patient.

18. Volume Data Sets
• A major advantage of spiral/helical scanning it that it produces a
continuous data set extending over some volume of the patient's
body.
• The data set is not broken up into slices as with the scan/step slice
acquisition method.

19. SCAN MODES DEFINED
• Multidetector Row CT Scanning
• The first helical scanners emitted x-rays that were detected by a single
row of detectors, yielding one slice per gantry rotation.
• This technology was expanded on in 1992 when scanners were
introduced that contained two rows of detectors, capturing data for
two slices per gantry rotation.
• Further improvements equipped scanners with multiple rows of
detectors, allowing data for many slices to be acquired with each
gantry rotation.

21. CT image formation
• The formation of a CT image is a distinct three phase process.
• The scanning phase produces data, but not an image.
• The reconstruction phase processes the acquired data and forms a digital
image.
• The visible and displayed analog image (shades of gray) is produced by the
digital-to analog conversion phase.
• There are adjustable factors associated with each of these phases
that can have an effect on the characteristics and quality of the
image.

22. CT System Designs
•Basic Concepts and Definitions
•Gantry Geometries
•X-ray Tubes, and Filters
•Detector Arrays

23. Gantry
• The gantry is the ring-shaped part of the CT scanner.
• It houses many of the components necessary to produce and detect x-rays
• Gantries vary in total size as well as in the diameter of the opening, or aperture.
• The range of aperture size is typically 70 to 90 cm.
• The CT gantry can be tilted either forward or backward as needed to
accommodate a variety of patients and examination protocols. The degree of tilt
varies among systems, but ±15° to ±30° is usual. The gantry also includes a laser
light that is used to position the patient within the scanner.
• Control panels located on either side of the gantry opening allow the
technologist to control the alignment lights, gantry tilt, and table movement. In
most scanners, these functions may also be controlled via the operator’s console.
• A microphone is embedded in the gantry to allow communication between the
patient and the technologist throughout the scan procedure.

25. Slip Rings
• Early CT scanners used recoiling system
cables to rotate the gantry frame.
• This design limited the scan method to the
step-and-shoot mode and considerably
limited the gantry rotation times
• Current systems use electromechanical
devices called slip rings.
• Slip rings use a brush like apparatus to
provide continuous electrical power and
electronic communication across a rotating
surface. They permit the gantry frame to
rotate continuously, eliminating the need to
straighten twisted system cables.

26. Generator
• High-frequency generators are currently used in CT.
• They are small enough so that they can be located within the gantry.
• CT generators produce high kV (generally 120–140 kV) to increase the
intensity of the beam, which will increase the penetrating ability of
the x-ray beam and thereby reduce patient dose.
• High kV settings also help to reduce the heat load on the x-ray tube
by allowing a lower mA setting.

27. X-ray Source
• rotating anode tube.
• Tungsten, with an atomic number of 74, is often used for the anode target
material because it produces a higher-intensity x-ray beam.
• This is because the intensity of x-ray production is approximately
proportional to the atomic number of the target material.
• CT tubes often contain more than one size of focal spot; 0.5 and 1.0 mm
are common sizes. Just as in standard x-ray tubes, because of reduced
small focal spots in CT tubes produce sharper images (i.e., better spatial
resolution), but because they concentrate heat onto a smaller portion of
the anode they cannot tolerate as much heat.
• So Cooling mechanisms are included in the gantry to reduce the effect of
heat

28. filter
• Compensating filters are used to shape the x-ray beam. They reduce
the radiation dose to the patient and help to minimize image artifact.
• Filtering the x-ray beam helps to reduce the range of x-ray energies
that reach the patient by removing the long-wavelength (or
“soft”)(low energy ) x-rays. These long-wavelength x-rays are readily
absorbed by the patient, therefore they do not contribute to the CT
image but do contribute to the radiation dose to the patient.
• In addition, creating a more uniform beam intensity improves the CT
image by reducing artifacts that result from beam hardening.

29. filter
• Filtering shapes the x-ray beam intensity. Removing low-energy x-rays
minimizes , patient exposure and produces a more uniform beam.

30. filter

31. Collimators
• Collimators restrict the x-ray beam to a specific area , thereby
reducing scatter radiation.
• Scatter radiation reduces image quality and increases the radiation
dose to the patient.
• The source collimator affects patient dose and determines how the
dose is distributed across the slice thickness .
• The source collimator resembles small shutters with an opening that
adjusts, dependent on the operator’s selection of slice thickness.

32. Collimators
• Some CT systems also use pre-
detector collimation.
• This is located below the patient
and above the detector array.
• The primary functions of pre-
detector collimators are to
ensure the beam is the proper
width as it enters the detector
and to prevent scatter radiation
from reaching the detector.

33. detectors
• As the x-ray beam passes through the patient it is attenuated to some
degree.
• To create an x-ray image we must collect information regarding the
degree to which each anatomic structure attenuated the beam.
• In conventional radiography we used a film-screen system to record
the attenuation information. In CT, we use detectors to collect the
information

34. detectors
• the detector array comprises detector elements situated in an arc or
a ring, each of which measures the intensity of transmitted x-ray
radiation along a beam projected from the x-ray source to that
particular detector
• All new scanners possess detectors of the solid-state crystal variety.
• Detectors made from xenon gas are used in old models

35. • Pressurized xenon gas fills hollow chambers to produce detectors that
absorb approximately 60% to 87% of the photons that reach them.
Xenon gas is used because of its ability to remain stable under pressure.
• Compared with the solid-state variety, xenon gas detectors are
significantly less expensive to produce, somewhat easier to calibrate,
and are highly stable.
• A disadvantage of xenon gas is that it must be kept under pressure in an
aluminum casing. This casing causes loss of x-ray photons

36. • When a photon enters the
channel, it ionizes the xenon gas.
These ions are accelerated and
amplified by the electric field
between the plates.
• The collected charge produces
an electric current. This current
is then processed as raw data

37. Solid-state detectors
• Solid-state detectors are also called scintillation detectors because they
use a crystal that fluoresces when struck by an x-ray photon. A
photodiode is attached to the crystal and transforms the light energy
into electrical (analog) energy.
• Solid-state crystal detectors have been made from a variety of materials,
including cadmium tungstate, bismuth germinate, cesium iodide, and
ceramic.
• They absorb nearly 100% of the photons that reach them. In addition,
there is no loss in the front window, as in xenon systems. This increased
absorption efficiency

38. Solid-state detectors
• High photon absorption
Moderate photon absorption
Sensitive to temperature,
moisture

39. Detector Arrays

40. Detector Electronics
• Signals emitted from the
detectors are analog
(electric),whereas computers
require digital signals.
• The data-acquisition system, or
DAS, measures the number of
photons that strikes the
detector, converts the
information to a digital signal,
and sends the signal to the
computer..

42. The patient table
• The patient table is more than
just a place to put the patient.
• In helical scanners, it is an
integral part of the data
acquisition hardware, since it
must be moved smoothly and
precisely in synchrony with the
source and detector rotation.
• Even in single-slice scanners, the
table’s positioning capabilities
must be quite flexible.

43. dual source CT
• The dual-source CT design uses two
x-ray tubes and two corresponding
detectors positioned at 90°
• from each other.
• Siemens introduced a CT model
with dual X-ray tube and dual array
of 64 slice detectors in 2005
• Dual sources increase the temporal
resolution by reducing the rotation
angle required to acquire a
complete image, thus permitting
cardiac studies without the use of
heart rate lowering medication

44. Dual source
• have the ability to produce x-ray
photons possessing different
energies.
• Dual source is to use the dual-
energy concept to differentiate
body tissues without the
application of contrast agent