1. Basic Principles of CT
Chapter 1

2. Introduction
The main advantages of CT over conventional radiography are
The elimination of superimposed structures
The ability to differentiate small differences in density of anatomic structures and abnormalities
The superior quality of the images

3. CT nomenclature
A feature may have a variety of names, depending on the manufacturer
For example, the preliminary image is called
Topogram (Siemens)
Scout (GE Healthcare)
Scanogram (Toshiba)
Pilot (Picker)

4. CT Defined Images are cross-sectional
An individual CT slice shows only the parts of the anatomy imaged at a particular level.
To visualize, consider a sliced loaf of bread

5. Cross-Sectional Images
Each cross-sectional slice represents a specific plane in the patient (or slice in the loaf of bread)
The thickness of the cross-sectional slice is referred to as its z axis

6. Cross-Sectional Images (cont’d)
Data that form the CT slice are further sectioned into elements called pixels
Width is x
Height is y

7. Beam Attenuation
Structures in a CT image are represented by varying shades of gray
The creation of these shades of gray is based on basic radiation principles
The degree to which an x-ray beam is reduced by an object is referred to as attenuation

8. Beam Attenuation (cont’d)
Photons that pass through objects unimpeded are represented by a black area on the image
When photons are completely absorbed by an object, the corresponding area on the image is white
Areas of intermediate attenuation are represented by various shades of gray

9. Beam Attenuation (cont’d)
The number of photons that interact depends on the object’s
Thickness
Density
Atomic number

10. Review Air in the lungs is considered High attenuation Low attenuation
Intermediate attenuation

11. Answer
b. Air has low density and does not attenuate much of the x-ray beam. Lungs are composed primarily of air, therefore, the lungs are considered low-attenuation structures.

12. Hounsfield Units Quantify the degree of beam attenuation
Also referred to as CT numbers, or density values

13. Hounsfield Units (cont’d)
In the Hounsfield Scale
0 is assigned to the density of water
–1,000 HU represents air
1,000 HU represents dense material such as bone
Values higher than 2,000 represent very dense materials, such as metallic pacemakers
Hence, there are 2,000 HU that represent naturally occurring structures

14. Volume Averaging
The process in CT by which different tissue attenuation values are averaged to produce one less accurate pixel reading
Affected by slice thickness

15. Raw Data vs Image Data Raw data are all the data collected
Also called scan data
Image data
Once the raw data have been segmented and averaged so that each pixel has an associated HU

16. Scan Modes Step-and-shoot Also called “axial” Helical
Also called “spiral” or “volumetric”

17. Directional Terms
All directional terms are based on the body being viewed in the anatomic position
Anterior/ventral
Posterior/dorsal
Inferior/caudal
Superior/cephalic
Distal
Proximal

18. Imaging Planes
Think of large sheets of glass cutting through the body in various ways
Horizontal/transverse
Vertical/longitudinal
Coronal
Sagittal
Oblique
Axial

19. Review
The imaging plane that divides the body into right and left sections is called
Sagittal
Coronal
Axial
Transverse

20. Answer
Sagittal
When the right and left sections are of equal size, the plane is referred to as mid-sagittal or median

21. CT Process
The process of CT image creation can be broken down into three general segments:
Data acquisition
Image reconstruction
Image display