1. principles & instruments of CT

Principles & Instruments of CT
Muhammad Arif Afridi
Lecturer in Medical Imaging
Medical Imaging Technologist (MIT) - Radiology
Master in Public Health (MPH)
Email: RFafridi@hotmail.com
Lecturer in Medical Imaging | RFafridi@hotmail.com
1

Design
• CT scanner uses a motorized x-ray source that rotates
around the circular opening of a donut-shaped structure
called a gantry.
• During a CT scan, the patient lies on a bed that slowly
moves through the gantry while the x-ray tube rotates
around the patient, shooting narrow beams of x-
rays through the body.
• Instead of film, CT scanners use special digital x-ray
detectors, which are located directly opposite the x-ray
source.
• As the x-rays leave the patient, they are picked up by the
detectors and transmitted to a computer.
2

X-Rays Production
• The x-ray tube is a special type of vacuum-sealed,
electrical diode that is designed to emit x rays.
• It is made up of two electrodes, the cathode and anode.
• To produce x rays, a filament in the cathode is charged
with electricity from a high voltage generator.
• This causes the filament to heat up and emit electron.
• Using their natural attraction and a special focusing cup,
the electrons travel directly toward the positively charged
anode.
3

X-Rays Production
• X rays are emitted indiscriminately when the
electrons strike the anode.
• The anode, which can be rotating or not, then
conducts the electricity back to the high-voltage
generator to complete the circuit.
• To focus the x rays into a beam, the x-ray tube is
contained inside a protective housing.
• This housing is lined with lead except for a small
window at the bottom. Useful x rays are able to
escape out this window, while the lead prevents the
escape of stray radiation in other directions.
4

Detectors
• Unlike other radiological devices, the detectors in a CAT scanner do
not measure x rays directly.
• They measure radiation attenuated from the body structures due to
their interaction with x rays.
• One type of detector is an ideal gas-filled detector.
• When radiation strikes one of these detectors, the gas is ionized and a
radiation level can be determined.
5

Console
• The computer is specially designed to collect and analyze input from the
detector.
• It is a large capacity computer capable of performing thousands of
equations simultaneously.
• The reconstruction speed and image quality are all dependent on the
computer's microprocessor and internal memory.
• A quick computer is particularly important because it greatly influences the
speed and efficiency of the examination.
• Since the computer is so specialized, it requires a room with a strictly
controlled environment.
• For example, the temperature is typically maintained below 68°F (20°C)
and the humidity is below 30%.
6

Console
• The operating console is the master control center of the CAT scanner.
• It is used to input all of the factors related to taking a scan.
• Typically, this console is made up of a computer, a keyboard, and
multiple monitors.
• Often there are two different control consoles, one used by the CAT
scanner operator, and the other used by the physician.
• The operator's console controls such variables as the thickness of the
imaged tissue slice, mechanical movement of the patient couch, and
other radiographic technique factors.
7

Console
• The physician's viewing console allows the doctor to view the image
without interfering with the normal scanner operation.
• It also enables image manipulation, if this is required for diagnosis
and image storage for later use.
• For this type of data storage, magnetic tapes or floppy disks are
available.
8

Design Advancement
• The design of a CAT scanner improved incrementally over time.
• The original CAT scanners utilized a thin, pencil beam of x rays and
took 180 readings, one at each degree of rotation around a
semicircle.
• The x-ray generator and detectors moved horizontally for each scan
and then were rotated one degree to take the next scan.
• Two detectors were used, so that two different images could be
generated from each scan.
• The drawback of this system was lengthy scanning times.
• A single scan could take up to five minutes.
9

Design Advancement
• Designs improved as more detectors were added and the x-ray beam
was fanned out using a special filter.
• This significantly reduced scanning time to about 20 seconds.
• The next major design improvement resulted in the elimination of the
horizontal movement of the generator and detector, making it a
rotate-only scanner.
• More detectors were added and grouped into a curvilinear detector
array.
• The detector array eventually was designed to be stationary, and the
resulting scan time was reduced to one second.
10

Raw Materials
• A wide variety of materials, such as steel, glass and plastic, are used
to construct the components of a CAT scanner.
• Some of the more specialized compounds can be found in the patient
couch, detector array, and the x-ray tube.
• The patient couch is typically made from carbon fiber to prevent it
from interfering with the x-ray beam transmission.
11

Raw Materials
• CAT scanners use X-ray technology to create three-dimensional
images of the body's internal structures.
• Images are obtained by rotating the x-ray generator and detectors
around the patient.
• This information is fed into a computer, which reconstructs images of
the body structures within its plane of focus.
12

Raw Materials
• The detector array of more modern scanners uses tungsten plates, a
ceramic substrate, and xenon gas.
• Tungsten is also used to make the cathode and electron beam target
of the x-ray tube.
• Other materials found in the tube are Pyrex. glass, copper, and
tungsten alloys.
• Throughout many parts of the CAT scanner system, lead can be found,
which reduces the amount of excess radiation.
13

The Manufacturing Process
• CAT scanner manufacture is typically an assembly of various
components that are supplied by outside manufacturers.
• The following process discusses how the major components are
produced.
14

Gantry assembly components
• The x-ray tube is made much like other
types of electrical diodes.
• The individual components, including the
cathode and anode, are placed inside the
tube envelope and vacuum sealed.
• The tube is then situated into the
protective housing, which can then be
attached to the rotating portion of the
scanner frame.
15

• Various detector arrays are available for CAT
scanners.
• One type of detector array is the ideal gas-filled
detector.
• This is made by placing strips of tungsten 0.04
inch (1 mm) apart around a large metallic frame.
• A ceramic substrate holds the strips in place.
• The entire assembly is hermetically sealed and
pressure filled with an inert gas such as xenon.
• Each of the tiny chambers formed by the gaps
between the tungsten plates are individual
detectors.
• The finished detector is also attached to the
scanner frame.
16

• To create the large amount of voltage
needed to produce x rays, an
autotransformer is used.
• This power supply device is made by
winding wire around a core.
• Electric tap connections are made at
various points along the coil and
connected to the main power source.
• With this device, output voltage can be
increased to approximately twice the
input voltage.
17

Slice Thickness
• Each time the x-ray source completes one full rotation, the CT computer
uses sophisticated mathematical techniques to construct a 2D image slice
of the patient.
• The thickness of the tissue represented in each image slice can vary
depending on the CT machine used, but usually ranges from 1-10
millimeters.
• When a full slice is completed, the image is stored and the motorized bed is
moved forward incrementally into the gantry.
• The x-ray scanning process is then repeated to produce another image
slice.
• This process continues until the desired number of slices is collected.
18

Control consol and computer
• The control consol and computer are specially designed and supplied
by computer manufacturers.
• The primary model building computer is specifically programmed
with the reconstruction algorithms needed to manipulate the x-ray
data from the gantry assembly.
• The control consoles are also programmed with software to control
the administration of the CAT scan.
19

Final Assembly
• The final assembly of the CAT scanner is a custom process which
often takes place in the radiologic imaging facility.
• Rooms are specially designed to house each component and
minimize the potential for excessive radiation exposure or electric
shock.
• By following specific plans, equipment installation and wiring of the
entire CAT scanner system is completed.
20

Importance
• CT scans can be used to identify disease or injury within various regions
of the body.
• For example, CT has become a useful screening tool for detecting
possible tumors or lesions within the abdomen.
• A CT scan of the heart may be ordered when various types of heart
disease or abnormalities are suspected.
• CT can also be used to image the head in order to locate injuries,
tumors, clots leading to stroke, hemorrhage, and other conditions.
• It can image the lungs in order to reveal the presence of tumors,
pulmonary embolisms (blood clots), excess fluid, and other conditions
such as emphysema or pneumonia.
• A CT scan is particularly useful when imaging complex bone fractures,
severely eroded joints, or bone tumors since it usually produces more
detail than would be possible with a conventional x-ray.
Fractures as seen on a CT scan.
21

Tomographic Reconstruction
• Once the scan data has been acquired, the data must be processed
using a form of tomographic reconstruction, which produces a series
of cross-sectional images.
• In terms of mathematics, the raw data acquired by the scanner
consists of multiple "projections" of the object being scanned.
• These projections are effectively the Radon transformation of the
structure of the object. Reconstruction, essentially involves solving
the inverse Radon transformation.
22

• Conventional tomography results in an
image that is parallel to the long axis of
the body.
• Computed tomography (CT) produces a
transverse image.
• The main advantages of CT over
conventional radiography are in the
elimination of superimposed structures
• Ability to differentiate small differences
in density of anatomic structures.
23

Contrast Agent
• As with all x-rays, dense structures within the body—such as bone—are easily
imaged, whereas soft tissues vary in their ability to stop x-rays and, thus, may be
faint or difficult to see.
• For this reason, intravenous (IV) contrast agents have been developed that are
highly visible in an x-ray or CT scan and are safe to use in patients.
• Contrast agents contain substances that are better at stopping x-rays and, thus,
are more visible on an x-ray image.
• For example, to examine the circulatory system, a contrast agentbased on iodine
is injected into the bloodstream to help illuminate blood vessels.
• This type of test is used to look for possible obstructions in blood vessels,
including those in the heart.
• Oral contrast agents, such as barium-based compounds, are used for imaging the
digestive system, including the esophagus, stomach, and GI tract.
24

Thank you
25

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