Background
Computed tomography (CT) was founded based on developments
in two fields: x-ray imaging and computing. X-rays were discovered in 1895 and quickly
became an established medical tool. Tomography was being developed in the 1930s,
enabling the visualization of sections though the body. By the 1960s, several independent
researchers had worked on cross-sectional imaging, which culminated in
Hounsfield's development of a CT scanner. Image data were acquired from
multiple x-ray transmissions through the object under investigation, and the
computer used the data to reconstruct the image.1
Historical Overview
The first clinical CT scan was performed in October, 1971 at
Atkinson Morley's Hospital in London. The patient, a woman with a suspected
frontal lobe tumor, was scanned with a prototype scanner, developed by Godfrey
Hounsfield and his team at EMI Central Research Laboratories in Hayes, west
London. The scanner produced an image with an 80 x 80 matrix, and each scan
took about 5 minutes. By contrast, modern CT scanners can produce images with a
1024 x 1024 matrix, acquiring data for a slice <0.3 seconds.1 CT
scanning is often used to image complex fractures, especially involving the
joints, as the process can reconstruct areas of interest in multiple planes.
Fractures, ligament injuries and dislocations can be readily recognized with a
0.2 mm resolution.
Description
The essential components of a CT system are: a circular
scanning gantry housing the x-ray tube and image sensors, a table for the
patient, an x-ray generator, and a computerized data processing unit. The
patient lies on the table and is placed inside the gantry. The x-ray tube is
rotated 360 around the patient, taking multiple images from different angles
while the computer collects data and formulates an axial image, or virtual
“slice.” Each cross-sectional image represents a slice of body tissue between
0.1 and 1.5 cm thick.
The CT computer software converts the x-ray beam attenuation
of of the tissue into a CT number (Hounsfield units) by comparing it with the
attenuation of water, which is designated as 0 (zero). The table below provides
a list of attenuation ranges by different bodily tissue types:
|
Tissue Type
|
Attenuation Range (H units)
|
|
Air
|
−400 to −1,000
|
|
Fat
|
−60 to −100
|
|
Water
|
0
|
|
Body fluid
|
+20 to +30
|
|
Muscle
|
+40 to +80
|
|
Trabecular bone
|
+100 to +1,0000
|
|
Cortical bone
|
+1,000
|
