The document provides an overview of the basic principles of CT scanning. It discusses key differences between radiography and CT imaging, including that CT produces cross-sectional images without overlaying structures. It describes the CT scanning process where an X-ray beam passes through the patient and is measured to construct voxel values and pixels. CT numbers represent attenuation coefficients measured in Hounsfield units. The document also covers topics like polychromatic beams, volume averaging effects, raw vs image data, and different scan modes like step-and-shoot and helical scanning.

1. Chapter 1: BASIC PRINCIPLES OF CT

2. Radiography vs. CT
• Radiography
– Structures are
superimposed on
film (Viewed through
underlying /
overlying structures)
– Thin highly-
attenuating objects
appear to be same
density as thicker
low-attenuating
object.
Patient
X-ray
Beam
Thin dense
object
Thick less
dense
object
Film

3. – Terminology
– tomography used in 1920, “body section
radiography” and “stratigraphy” (from stratum,
meaning “layer”.
– In 1935 the word tomo used meaning section.
– In 1937 transverse axial tomography used.
– older scanning systems is a computerized
axial tomography(CAT scan).
– Newer model scanners offer options in more than
just the transverse plane. Therefore, the word
“axial” has been dropped from the name of
current CT systems.
– the primary image is “topogram”
(Siemens),“scout” (GE Healthcare), or
“scanogram” (Toshiba).

4. • The continuous acquisition scanning can also be
called “spiral” (Siemens), “helical” (GE Healthcare),
or “isotropic” (Toshiba) scanning.
• In simplified, radiation passes through each cross
section in a specific way and is projected onto a
detector that sends signals to a computer for
processing. The computer produces images.

5. • CT
– Cross-sectional image
i.e. Image computed from beam
intensity measurements through
only slice of interest
– View anatomy without looking
through underlying / overlying
structures→ improves the
contrast
CT X-ray
Conventional
X-ray Beam
Beam

6. -Selecting a slice thickness limits the x-ray beam so
that it passes only through this volume; hence,
scatter radiation and superimposition of other
structures are greatly diminished.
CT X-ray
beam
– Uses tightly collimated
beam → minimizes
scattered radiation,
improves contrast.

7. Matrix,
Pixels
and
Voxels

8. The total number of
pixels in a matrix is the
product of the number of
rows and the number of
columns, in this case
512 × 512 (262,144)

9. Pixels & Voxels
• Pixel is 2D component of an image
• Voxel is 3D cube of anatomy
• Voxel depth
– same as slice thickness
• CT reconstruction calculates
attenuation coefficients of Voxels
• CT displays these attenuation
coefficients as gray shades of Pixels

10. - The degree to which an x-ray beam is reduced by
an object is referred to as attenuation.
-x-ray photons that pass through objects shows
black area on the image referred to
low attenuation of organ.
-X-ray which completely absorbed by an object
cannot be detected, the place on the image is
white. Referred to high attenuation of organ.
-Areas of intermediate attenuations are
represented by various shades of gray

11. -The number of the photons that interact
depends on the thickness,
density, and atomic number
of the object.
- The amount of the x-ray
beam that is scattered or
absorbed per unit thickness of
the absorber is expressed by
the linear attenuation
coefficient(µ).

12. - For example, if a 125-kVp x-ray beam is used, the
linear attenuation coefficient for water is
approximately 0.18 cm-1 (the unit cm-1 indicates
per centimeter). This means that about 18% of the
photons are either absorbed or scattered when the
x-ray beam passes through 1 cm of water.
- attenuation coefficient decreases with increasing
photon energy and increases with increasing
atomic number and density

13. -To differentiate an object on a CT image from
adjacent objects, there must be a density difference
between the two objects. An oral or intravenous
administration of a contrast agent is often used to
create a temporary artificial density difference
between objects.
-In the cases of agents that contain barium sulfate
and iodine, the material is of a higher density than
the structure. These are typically referred to as
positive agents. Low-density contrast agents, or
negative agents, such as water, can also be used.

14. CT Number = Hounsfield Units
• Where:
• ut = linear attenuation coefficient for tissue in voxel
uW = linear attenuation coefficient for water
Determines the grade of gray which is displayed in each pixel
Depends on the attenuation coefficient of the voxel
t - W)
CT # = 1000 X ------------
W

15. • Notes:
1- air and water are used for calibration of CT
number scale of the scanner
2- CT number depends on μ of the tissue
i.e. depends on : Z, Density , kv used & filtration
3- range of CT number values can be displayed is :
-1024 to + 3071
Explanation:
Pixel depth of the CT scanners usually = 12
i.e. levels of gray displayed = 2¹² = 4096

16. - attenuation less than that of water have an associated
negative number. Conversely, substances with an
attenuation greater than that of water have a
proportionally positive.

17. POLYCHROMATIC X-RAY BEAMS:
- the x-ray beam comprises photons with varying
energies. The spectrum ranges from x-ray
photons that are weak to others that are
relatively strong.
- Filtering the x-ray beam with a substance, such
as Teflon or aluminum, helps to reduce the
range of x-ray energies that reach the patient by
eliminating the photons with weaker energies
(solute Beam-hardening artifacts) .

18. Volume averaging effect
Dr.YossefGamal
• Cause: The CT number stored in each voxel
represents average attenuation coefficient in the
voxel
• Results:
- High contrast object can be seen even if it is
smaller than the pixel (small low contrast details
will not be visualized)
- thin high contrast structure will be visualized larger
than normal e.g. oblique enhanced vessel

19. Depends on: slice thickness : the thinner
the slice, the less is the volume averaging

20. Why do some scanning protocols use thicker cuts?
1) Modern scanners acquire data very quickly and have the
capability of creating slices thinner than 1 mm.
2) thinner slices result in a higher radiation dose to the
patient.
3) if the area to be scanned is large, a huge number of slices
are produced.

21. RAW DATA VERSUS IMAGE DATA:
- raw data: data acquired by the system with each scan.
- image reconstruction: The process of using the raw data to
create an image.
-The reconstruction that is automatically produced during
scanning is often called prospective reconstruction.
- The same raw data may be used later to generate new
images. This process is referred to as retrospective
reconstruction.

22. 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°.

23. Helical (Spiral) Scanning
continuous acquisition scanning mode most often called spiral
or helical scanning.
Multidetector Row CT Scanning
multiple rows of detectors, allowing data for many slices to
be acquired with each gantry rotation.

24. IMAGING PLANES
Changing the image plane
from axial to coronal is
indicated for two distinct
reasons:
The primary reason is when
the anatomy of interest lies
vertically rather than
horizontally like ethmoid
sinuses.

25. - The second indication for scanning in a different plane
is to reduce artifacts created by surrounding structures.
For this reason, the coronal plane is preferred for scanning
the pituitary gland. In the axial plane, the number of streak
artifacts and the partial volume effect are greater than in
the coronal plane.