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 High attenuation Low attenuation 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 Sagittal Coronal Axial Transverse

Answer 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: Data acquisition Image reconstruction Image display