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					Introduction to Computerized Tomography


          Dr / Ibrahim Alshikh
References for the present lecture
Historical perspective
CT scanner components
Generations of CT machine
CT number
Window level and width
Use of contrast medium
References for the present lecture
           Historical perspective
Godfrey Hounsfield:
built the first CT scanner
in 1972
Alan Cormack:
devised the mathematical computations (1964)
by which an image could be reconstructed
from the data generated from the CT scanner
   They were awarded the 1979 Nobel Prize for Medicine
                 Computed tomographic
                  scanner components
                                                 Gantry :
               Gantry                         (can tilt up to 30o)
                                              Detector array
                                              Patient support couch
                                              X-ray source
                          Aperture
                                                 Computer
                                              Data acquisition time
                      (Table)
  Patient support couch                       Reconstruction time
Pitch = Table feed per rotation (not per sec)    Operating console
                    Slice thickness
        Crystal scintillation detector (CsI, CdWO4)
                    -100%; can’t pack together
        Gas-filled detector (Xe or Xe/Krypton)
                    -50% efficiency; pack together           Refs. 2, 3
First – generation machine
   Translate-rotate scanner


                     Only accommodate a human
                     head
                     Scan time for a single slice:
                     6 min
                     (4.5 min for image
                     acquisition,
                     1.5 min for image
                     reconstruction)




                The first scanner:
                A 13 mm slice with 3 line pairs/cm
                spatial resolution
                and used an 80x80 image matrix
            Second– generation machine
                  Translate-rotate scanner
The second scanner: drawback         Can accommodate the
Linear arrangement of detectors:     whole body
Detectors in the middle of array     Scan time for a single
were a different distance from       slice: 20 second
the radiation source than those
at the ends         Increase the
scatter radiation and degrade
the image quality

                                             Detectors
                                                         Gantry
                                                         Gantry
                                                         Aperture




                                    X ray tube
Third-generation CT-scanner

         X-ray source

   Fan beam




   (Curved) Detector array
         Third – generation machine
              Rotate-rotate scanner
The third scanner:                   Can accommodate the
Contain  30 detectors and           whole body
cover between 30o~60o with           Scan time for a single
a single projection                  slice:  one second


                       Detectors

                       Gantry         Curved detectors solve the
                        Gantry        differential
                        Aperture      magnification problem of
                                      linear detectors
                                      Ring artifact- if a single
                        X-ray tube
                       X ray tube     detector in the array was
                                      defective
         Fourth – generation machine
                Rotate-only scanner

The fourth scanner:               Can accommodate the
Detector array consisted of       whole body
several thousand elements &       Scan time for a single
provided 360o of coverage –       slice: < one second
avoid the ring artifact


                      Detectors

                     Gantry       Patient ‘s radiation dose
                      Gantry
                      Aperture    is increased


                      X ray
                      tube
           Conventional CT machine-
           4th generation
                                 Angulation

  X-ray tube                                  Gantry


                       90
                       o




270o


                      Table
Detector       180o   movement

                                 Aperture
                       Slices for conventional CT

In conventional CT, a series of equally
spaced is required sequentially through a
specific region, e.g. the head.

There is a short pause after each section                          X-ray tube
in order to advance the patient table to
the next preset position.                             Gantry
The section thickness & overlap/                                       Rotation
intersection gap are selected at the outset.

The raw data for each image level is
stored separately.                             Step-wise
                                               table
The short pause between sections allows        movement
the conscious patient to breathe without
causing major respiratory artifacts
                                                                     3rd scan level
                                                                    2nd scan level
                                                               1st scan level
Ref. 3
         Fourth – generation machine
                                 modification
        Helical or Spiral CT
Principal difference: patient couch moves
continuously during image taking
This movement produces image data for
a portion of a spiral
Scan time is further decreased by
increasing the pitch – affect image quality
Development of slip ring allows for continuous
movement of X-ray source – scan time is further
decreased because X-ray source can rotate faster
without the heavy cables
                        Slices for spiral CT
In spiral CT, images are acquired
continuously while the patient table is
advanced through the gantry.                               X-ray tube
The x-ray tube describes an apparent      Imaging Gantry
helical path around the patient.          volume
If table advance is coordinated with
the time required for a 3600 rotation
(pitch factor) data acquisition is
complete and uninterrupted


                                          Continous
                                          table
This technique is helpful when data       movement
are reformatted to create other 2D
views: sagittal, oblique, coronal or 3D
                      Summaries of generation of
                           CT machines
Third                                                            Fourth
generation CT                                                    generation CT
scanner:                                                         scanner:
Both the X-ray                                                   The X-ray tube
tube & detector                                                  rotates within
array rotate                                                     a stationary
around the                                                       ring of the
patient                                                          detectors
1st generation CT                                               2nd generation CT
            Pencil beam                                                  Multiple
                                                                         pencil beam




                                                                          Multiple
    Single detector                                                       detectors

Spiral CT: The X-ray tube & detectors move in a continuous spiral motion around
the patient as the patient moves continuously into the gantry in the direction of
the red solid arrows
       Advantage of spiral technique
                  Conventional CT

   Liver




   Spiral CT


  Advantage of spiral technique
Lesions smaller than the conventional thickness of a
slice can be detected
Small liver metastases (7) will be not being included
in the section
The metastases would appear in reconstructions
from the dataset of the helical technique
   Attenuation Coefficient

      X-ray source

Fan beam

                          attenuation
                          coefficients
        (Attenuation)

                            Grey scale


        Attenuation         Hounsfield
        coefficient
                              Scale
(Curved) Detector array      (CT no.)
Attenuation coefficient and CT no.
  for biological tissues at 60 keV
            Attenuation
            Coefficient
Tissue         (cm-1)    CT Number (HU)
Different gray scale of substances under CT
Different Gray scale of substances under CT
        CT No.
Front
                 Frontal sinus
                 mastoid air cells
                 CT number -1000Hu


                 CT number -80Hu


             soft tissue,CT number 40Hu,


             CT number800-1000Hu
           Window level (center) & width
Window level: CT no; Width: range between 2 CT no
  Modern equipment has a capacity of 4096 gray tones,
  which represent different density levels in HUs. (The
  density of water was arbitrarily set a 0 HU and that of
  air at -1000 HU)

      Monitor can display a maximum of 256 gray tones
      Human eye is able to discriminate only ~20
      Densities of human tissues extend over a fairly
  narrow range (a window) of the total spectrum (10-
  90HU), it is possible to select a window setting to
  represent the density of the tissue of interest
             Window level and width
    Density levels of different types of tissues
The mean dentistry level of the window should be set as
close as possible to the density level of the tissue to be
examined.

The lung, with its high air content, is best examined at a
low HU window setting Lung window

Bone require an adjustment to high levels   Bone window


The width of the window influences the contrast of the
images: the narrower the window, the greater the contrast
                   Lung window




If lung parenchyma is to be examined, e.g. when scanning for
nodules, the window center will be lower at about -200HU, &
the window width (2000HU). Low density pulmonary structures
can be much more clearly differentiated
                   Brain window




Density values of gray & white matter differ only slightly. The
brain window must be very narrow (80-100HU-> high contrast)
and the center must lie close to the mean density of cerebral
tissue (35HU) to demonstrate these slight differences
                    Bone window




Bone window should have a much higher center, at about +300HU,
and a sufficient width of ~1500HU
Metastases in the occipital bone would only be visible in the
appropriate bone window Bone window
but not in the brain window Brain window
Brain is invisible in the bone window: small cerebral metastases
would not be detected
         Window level (center) & width
    Density levels of different types of tissues

Density levels of almost all soft-tissue organs lie within
a narrow range between 10 and 90 HUS

The only exception is the lung and this requires a special
window setting (lung window)

For hemorrhage
Density level of recently coagulated blood lies about
30HU above that of fresh blood.

This density drops again in older hemorrhages or
liquefied thromboses.
                                                         Ref. 3
          Oral administration of contrast media
Without contrast medium (CM), it is difficult
to distinguish between the duodenum (130) & Before
the head of the pancreas (131, right figure) & CM
other parts of the intestinal tract (140) would
also be very similar to neighboring structures

After an oral CM, both the duodenum & the
pancreas can be well delineated (Figures
below)
  After
  CM
      Summaries
Knowing basic knowledge of CT:
Historical perspective
CT scanner components
Generations of CT machine
CT number
Window level and width
Use of contrast medium

				
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