15543471.ppt

Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins
Basic Principles of CT
Chapter 1

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

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)

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

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

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

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

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

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

Review
Air in the lungs is considered
a. High attenuation
b. Low attenuation
c. Intermediate attenuation

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.

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

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

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

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

Scan Modes
• Step-and-shoot
– Also called “axial”
• Helical
– Also called “spiral” or “volumetric”

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

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

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

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

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

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