Introduction to computed tomography imaging technique

1. MEDICAL IMAGING
COMPUTED TOMOGRAPHY
IMAGING TECHNIQUE
BY
DR RAMY ABDLATY

3. CONTENTS
• What is Computed Tomography?
• How CAT scan works?
• Scanning procedure
• Risks of CT/ CAT

4. WHAT IS COMPUTED TOMOGRAPHY (CT/ CAT)?
• Computed Tomography (CT) imaging is also known as "CAT scanning" (Computed Axial
Tomography). Tomography is from the Greek word "tomos" which means "slice" or "section" and
“graphy” means "describing". Therefore its is accepted for evaluation of the entire body.
• CT was invented in 1972 by British engineer Godfrey Hounsfield of EMI Laboratories, England, and
independently by South African born physicist Allan Cormack of Tufts University, Massachusetts.
The first clinical CT scanners were installed between 1974 and 1976.
• CT uses X-rays and RF electromagnetic radiation. The CT images are obtained in the axial plane
with the help of computer. CT can capture images of the body in the sagittal and coronal planes.
• The first CT scanner took several hours to acquire the raw data for a single scan or "slice" and took
days to reconstruct a single image from this raw data. The modern CT collect up to 4 slices in 350
ms and reconstruct a 512 x 512-matrix image in less than a second, and the entire chest (forty 8
mm slices) can be scanned in 5:10 seconds.

5. CT SCAN FOR BRAIN

6. CT VS. CONVENTIONAL X-RAY
• A conventional X-ray image is a shadow: You shine a
"light" on one side of the body, and a piece of film on
the other side registers the silhouette of the bones.
• If a larger bone lies between the X-ray machine and a
smaller bone, the larger bone may cover the smaller
bone on the film. To see the smaller bone, you need to
turn your body or move the X-ray machine.
• Therefore conventional radiography suffers from the
collapsing of 3D structures onto a 2D image.
• In a CAT scan machine, the X-ray beam moves all around
the patient, scanning from hundreds of different angles.
The computer takes all this information and puts
together a 3-D image of the body. Doctors can even
examine the body one narrow slice at a time to pinpoint
specific areas.

7. COMPARISON BETWEEN X-RAY AND CT
Aspect of Comparison X-ray shadowgraphs Computed Tomography (CT)
Availability Common and available Less common
Resolution Moderate High
Image dimensions Only 2-dimensional images 2-dimensional & 3-dimensional
images
Cost Cheap Expensive
Time of imaging Short Intermediate
Reason of imaging Used primarily to check broken bones
fractures and early diagnosis of pneumonia
( an infection that inflames the air sacs in
lungs. The air sacs may fill with fluid or
causing cough with phlegm, fever, chills, and
difficulty breathing)
Used primarily to diagnose
problems in soft organ and tissues.
It is also used to check the
cartilage between backbone parts.
X-ray dose Low High

8. COMPARISON BETWEEN X-RAY, CT, MRI, MRA, PET
X-Ray: shows bone/skull only. Does not show the brain.
Best used to detect if there are bone fractures.
CT: a quick test. Shows brain but detail not great. Shows if
any larger bleed, stroke, lesions, or masses.
MRI: a long test. Shows brain and detail is great. Shows
smaller bleeds, stroke, lesions, or masses.
MRA: (Magnetic resonance angiogram) shows the flow of
blood in the vasculature system of the brain. If there is
vessel narrowing or blockage this test would show it.
PET scan: shows how active different parts of the brain is.
An active brain uses sugar as energy and pet scan detects
how much sugar is being used by lighting up and turning
different colors.

9. CONVENTIONAL X-RAY DISADVANTAGES
CT Advantages:
1- CT overcomes the
superimposition of the human
body structures
2- CT has a high contrast in the
produced images of the human
body compared to conventional X-
ray
3- CT, due to high image contrast,
differentiates between various
tissues in the human body

10. CT PRINCIPLE OF OPERATION
1- the internal structure of the
object (organ/ tissue) can be
reconstructed from multiple
projections of the object.
2- the mathematical concept of CT
was developed by Radon in 1917.
3- This concept provided an
evidence that image of unknown
object could be produced if
multiple number of projections
have been taken throughout the
object.

11. CAT SCANNING PROCEDURE
1. The CT machine is like a donut. The patient lies down on a platform,
slowly moving through the machine, where the X-ray tube is
mounted on a movable ring. The ring also supports an array of X-ray
detectors opposite to the X-ray tube.
2. A motor turns the ring so that the X-ray tube and the detectors
revolve around the body. Each full revolution scans a narrow,
horizontal "slice" of the body. The control system moves the platform
farther into the hole so the tube and detectors can scan the next
slice.
3. The CT computer uses sophisticated mathematical techniques to
construct a 2-D 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. This process continues
until the desired number of slices is collected.

12. CAT SCANNING PROCEDURE
CT scanning can be divided into three actions
1. Scanning and data acquisition
i. X-ray generator (50:80 KW, 5:50 KHz, 80:120KVp,
80:500 mA)
ii. X-ray tube (separate cooling, focal spot = 0.6
mm, Anode heating 1:7 MHU)
iii. X-ray Filtration system (compensation filter is
used to allow only monochromatic x-ray to pass)
 Pre patient collimators: reduces the patient dose
 Post patient collimators: reduces the scattered
radiation detectors
iv. X-ray detectors
2. Image reconstruction
i. Initial back projection computation
ii. Iterative methods of computation
iii. Analytical methodology
3. Image display
i. 2-dimensional view
ii. 3-dimensional view

13. CT SCANNING PROCEDURE
1. Pre patient collimators: reduces the patient
dose
2. Post patient collimators: reduces the
scattered radiation detectors
The functions of the collimators are:
I. Decreasing the scattered X-ray radiation
produced by the tube
II. Reducing the x-ray exposure dose for the
patient
III. Improving the image quality due to using
monochromatic x-ray radiation
IV. Determining the slice width

14. CT SCANNING PROCEDURE
CT detectors:
• are responsible for gathering
information via measuring the x-
ray transmission through the
patient.
• There are two types of detectors:
1. Scintillation crystal detector
2. Xenon gas ionization chamber

15. CT SCANNING PROCEDURE
CT Display:
• The reconstructed image is displayed on
the monitor which is a digital display.
• The digital display consists of a matrix of
pixels that shows a 2-D representation of
3-D object in the human body.
• The pixel real size of the tissue is
computed by dividing the FOV by the
display matrix size (for example 512*512)
• Pixel size = FOV (mm) / Matrix size

16. GENERATION OF CT SCANNING MACHINE
1st Generation of CT:
• It has a narrow pencil beam of x-ray
• It has a single detector
• The detector is made of NaI
• The CT raw data is obtained from the
translate-rotate movements of the tube-
detector combination
• The average scan time for this generation is
5 minutes
• This generation is mainly designed for
scanning the brain

17. GENERATION OF CT SCANNING MACHINE
2nd Generation of CT:
• It has a narrow fan beam of x-ray
• It has array of detectors (5 to 30)
• The CT raw data is obtained from the
translate-rotate movements of the tube-
detector combination
• The average scan time is 30 seconds
• Fewer linear movements are needed as
there are more detectors to gather the
data

18. GENERATION OF CT SCANNING MACHINE
3rd Generation of CT:
• It has a pulsed wide fan beam of x-ray
• It has arc of detectors (600 to 900), both
scintillation and Xenon detectors can be
used
• The CT raw data is obtained from the rotate-
rotate movements of the tube-detector
motion
• The average scan time is < 5 seconds
• The disadvantage of this generation is the
presence of ring artifacts due to electronic
drift between many detectors

19. GENERATION OF CT SCANNING MACHINE
4th Generation of CT:
• It has a wide fan beam of x-ray covers the entire
patient.
• It has a complete circular array of about 1200 to
4800 stationary detectors.
• The CT raw data is obtained from the rotational
movements of the X-ray tube motion.
• The average scan time is 0.5 second or << 2 seconds
• It is designed to overcome the ring artifact
disadvantage of the 3rd generation by keeping
detector assembly stationary.
• The disadvantage of this generation is the high cost.

20. GENERATION OF CT SCANNING MACHINE
5th Generation of CT:
• It has no conventional x-ray tube, however,
it has large arc of tungsten encircles the
patient and lies opposite to the detector
ring.
• Electron beam is steered around the
patient to strike the annular tungsten
target
• The CT raw data is obtained while there is
no movement either from the tube or the
detectors.
• The average scan time is 50 msec.
• It can produce fast frame rate CT movies of
the beating heart
• It is designed specifically for cardiac
tomographic imaging.

21. COMPARISON BETWEEN GENERATIONS OF CT SCANNERS?

22. VARIOUS PARAMETERS OF CT SCAN
1.Slice
2.Matrix
3.Pixel
4.Voxel
5.CT Number
6.Windowing
7.Window Width
8.Window Level
9.Pitch

23. COMPUTED TOMOGRAPHY
1- SLICE / CUT
 The cross section portion of
the body which is scanned for
production of CT image is
called slice.
 The slice has width and
therefore volume.
 The width is determined by
width of the x-rays beam.

24. COMPUTED TOMOGRAPHY
2- SLICE CROSS SECTION
 Think like looking into the loaf of
bread by cutting into thin slices
and then viewing each slice
individually.
 Each slice shows the constituents
of tissues that it has inside.
 The various slices show the
extension of each organ/ tissue
within the body section of interest.

25. COMPUTED TOMOGRAPHY
3- MATRIX PIXEL / VOXEL
 CT image is represented as a
matrix of numbers.
 A 2-dimensional array of numbers
arranged in rows and columns is
called matrix.
 Each square in a matrix is called a
pixel.
 Each element or number in the
image matrix represents a 3-
dimensional volume element
called voxel

26. COMPUTED TOMOGRAPHY
4- CT NUMBER
 The numbers in the CT image
matrix is called CT numbers.
 Each pixel has a number which
represents the x-ray attenuation in
the corresponding voxel of the
object.
 Air: -1000
 Water: 0
 Bone: 1000
 Blood: 40 to 70
 CSF: 15
 Fat: -50 to -100

27. COMPUTED TOMOGRAPHY
5- HOUNSFIELD UNITS (HU)
 Related to different composition and nature of
tissue.
 CT number is also known as Hounsfield Units (HU).
 CT number represents the density of the imaged
tissue.
 Different tissues have unequal CT number ranges in
HU.

28. COMPUTED TOMOGRAPHY
6- WINDOWING / WINDOW WIDTH / WINDOW LEVEL
 Windowing is a system where the CT
number range of interest spreads to
cover the full grey scale available on
the display system.
 Window width means total range of
CT no. values selected for gray scale
interpretation. It corresponds to
contrast of the image.
 Window level represents the CT no.
selected for the center of the range of
the no. displayed on the image. It
corresponds to brightness of the
image.

29. COMPUTED TOMOGRAPHY
7- PITCH
 CT detector pitch is defined as table
distance traveled in one 360° gantry
rotation divided by beam
collimation.
 For example, if the table traveled 5
mm in one rotation and the beam
collimation was 5 mm then pitch
equals 5 mm / 5 mm = 1.0. If patient
moves 10 mm during the time it
takes for the x-ray tube to rotate
through 3600, the pitch is 2.
 Increasing pitch reduces the scan
time and patient dose.

30. CAT SCAN: CONTRAST AGENTS
1. Intravenous contrast:
 It is a liquid used to enhance tissues
and organs and to outline blood
vessels, which would not be visualized
without contrast.
 Intravenous contrast is administered
through an IV, usually placed in the
arm. The contrast is injected by a
pump, which controls the rate and
amount of the injection.
 The contrast travels throughout the
body via the blood vessels. Images are
taken at different times during the
circulation of the contrast.
This example illustrates how much we can miss when forced to
perform a non-contrast CT in someone with renal failure or a contrast
allergy. The image on the left, from a triphasic CT liver, has been
performed without contrast; the liver looks a little bit nodular but
otherwise fine. On the right is an image from the arterial phase of the
CT, taken at the same position – it shows multifocal hepatocellular
carcinoma (arrows)

31. CAT SCAN: CONTRAST AGENTS
2. Oral contrast:
 It is used to outline the stomach and upper
intestine.
 There are two types of oral contrast, barium and
gastrograffin.
 Barium is somewhat chalky in texture, and is
commonly used. Orange, strawberry, vanilla
and chocolate flavors are used to make the
barium mixture palatable.
 Gastrografin is a clear liquid sometimes used in
place of barium, and has a bitter taste when
mixed with water.
 Regardless of the type of oral contrast used, it
takes time for the contrast to travel through the
body. The patient is required to drink
approximately 500 - 800 mls of oral contrast.
Roughly 45 - 60 minutes after drinking the
patient will be ready for scanning.

32. CAT SCAN: CONTRAST AGENTS
3. Rectal contrast:
 When visualizing the lower bowel, rectal
contrast is used. Barium and Gastrografin
are used to fill the rectum and lower
bowel.
 A thin tube is inserted in the rectum and
the contrast agent is administered
through the tube and into the rectum.
 It is uncomfortable and you may feel full.
Fortunately, scanning is fast, and the
discomfort will last only a few minutes.
After scanning the contrast is drained
and you will be escorted to a washroom.

33. RISKS OF CT/ CAT SCANNING
• The risk of a CT scan causing a problem is small.
1. There is a slight risk of developing an allergic reaction to the iodine contrast material. The
reaction can be mild (itching, rash) or severe (difficulty breathing or sudden shock). Death
resulting from an allergic reaction is rare. Most reactions can be controlled using
medication.
2. The contrast material used during CT scanning can cause water loss or damage to the
kidneys that may lead to kidney failure. This is a concern if you have poor kidney function. If
you have a history of kidney problems.
3. There is always a slight risk of damage from being exposed to any radiation, including the
low levels of X-rays used for a CT scan. However, the risk of damage from the X-rays is
usually very low compared with the potential benefits of the test.