This is about a basic idea about a CT scanner and image acquisition methods .

1. By:
T.Antony godson,
M.sc radiology and imaging technology.
(Integrated)

2. • CT uses X-rays to generate cross
sectional images of the body.
• Image is acquired by rapid rotation
of the X-ray tube 360 around the
patient body.
• It is known as Computed
Tomography or Computerized
Axial Tomography(CAT).

4. • CT was invented in 1972 by British Engineer, Sir Godfrey
Newbold Hounsfield in United kingdom.
• First clinical CT scan was installed in 1974.
• The initial system were only for head scanning due to
small gantry.
• And developed CT system with large gantry became
available in 1976 for whole body imaging.

5. • “The internal structure
of the object can be
reconstructed from
multiple projections of
the object”.

6. FIRST GENERATION -
• Dectector : One
• Type of Beam : Pencil like X-ray beam
• Tube-detector movements : Translate-Rotate
• Duration of scan : 25-30 minutes (average).

8. SECOND GENERATION -
• Detectors : Multiple(upto 30)
• Type of Beam : Fan shaped X-ray beam
• Tube-detector movement : Translate-Rotate
• Duration of scan : less than 90 seconds(average).

10. THIRD GENERATION -
• Detectors : Multiple,originally 288 and newer ones use
over 700 arranged in an arc
• Type of beam : Fan shaped X-ray beam
• Tube-detector movements : Rotate-Rotate
• Duration of scan : Approximately 5 seconds(average).

12. FOURTH GENERATION -
• Detectors : Multiple (more than 2,000) arranged in an
outer ring which is fixed
• Type of beam : Fan shaped X-ray beam
• Tube-Detector movements : Rotate-Fixed
• Duration of scan : within few seconds(average).

14. • Other CT technological developments have been adapted
to third and fourth generations including:
Helical(spiral)image acquisition
• Slip ring technology made acquisition possible.
Dual energy CT(Spiral).

15. • Nowadays multislice CT is used for imaging the internal
organs and pathological conditions.
• Spiral CT uses single row of detectors resulting in a single
slice per gantry rotation.
• Whereas,Multislice CT uses multiple detector arrays
resulting multiple slices per gantry rotation.
• The Fan shaped X-ray beam of spiral CT is replaced by
Cone shaped X-ray beam.

16. • The major advantages of multislice CT is
Improved spatial resolution,
Reduced image noise,
Faster and longer anatomical coverage,
Increased concentration of intravenous contrast.

17. SCINTILLATION DETECTORS:
• Early scintillation detectors contains scintillation crystals
and photomultiplier assemblies.
• Crystals like Sodium iodide,Cadmium tungstate were
used.
GAS FILLED DETECTORS:
• It has the same efficiency as scintillation crystals.
• Gases used are Xenon,Krypton gases.

18. • DRAWBACKS:
• Crystals cannot be packed very tightly together, so that
there may be a chance of image loss.
• Photomultipliers require separate power supply for each
other.(Require more power,area & cost).
• Development:Photomultiplier were replaced by photo
cathode assembly.

19. The 3 planes of CT are:
• Coronal plane
• Sagittal plane
• Axial (or) Transverse plane

21. • Voxel-Volume element (3D)
• Pixel-Picture element (2D)
• Field of view-Diameter of the area being scanned.

22. • A CT slice is subdivided into a matrix of upto 1024X1024
volume elements (VOXELS (3D)).
• Each voxel has been transversed by numerous X-ray
photons at the time of scan.
• And the intensity of the transmitted radiation is measured
by detectors.
• The density or attenuation value of the tissue at each
point in the slice can be calculated.

24. • Specific attenuation values are assigned to each
individual voxel.
• The viewed image is then reconstructed as a
corresponding matrix of picture elements (PIXELS (2D)).
• Each pixel is assigned a numerical value known as
CT number .

25. • Numerical value for each pixel.
• It is the average of all the attenuation values contained
within the corresponding voxel.
• This number is compared to the attenuation value of
water and displayed on a scale of systematical units
named Hounsfield units (HU) .
• This is named after Sir Godfrey Hounsfield.
• It represents about the density of the tissue.

28. • Windowing or grey-level mapping is the process in which
CT image greyscale component of an image is
manipuated via CT numbers.
• Doing this will change the appearance of the picture to
highlight particular structures.
• The Brightness of the image is adjusted by Window
level .
• The Contrast of the image is adjusted by Window width.

29. • The various windows used in Computed Tomography are,
Brain
Bone
Lung
Liver
Abdomen
Spine etc,.....
• These are the mainly used windows(parameters) in CT.

30. • Window width as the name suggests is the measure of
the range of CT numbers that an image contains.
• CONTRAST
Types of window width;
1. WIDE WINDOW :Decreased contrast range Eg:Lungs or
cortical tissue will be viewed as air and vessels side by
side.
2. NARROW WINDOW :Increased contrast range.Eg:Soft
tissues.

31. • The Window level is also known as Window center.
• It is the midpoint of the range of CT numbers displayed in
a slice.
• BRIGHTNESS
• When the Window level is decreased the CT image will be
brighter.
• When the Window level is increased the CT image will
become dark.

34. • Pitch is the dictance in millimeters that the table moves
during one complete rotation of the X-ray tube,divided by
the slice thickness(mm).
• Increasing the pitch by increasing the table speed
reduces dose and scanning time,but the image resolution
will be decreased.

36. THANK YOU