Learning Unit Template (PowerPoint)

Document Sample
Learning Unit Template (PowerPoint) Powered By Docstoc
					CT Image Quality

     Dr M A Oghabian
Medical Physics Department,
Tehran University of Medical
         Sciences
Sinogarm




           2
Reconstruction Filters




                         3
              CT Number




CT number=1000×(mpixel-mwater)/mwater
Window width & window level




                          5
6
7
8
9
10
Quality criteria for CT images
 Image Quality
 1) Spatial Resolution
 2) Density Resolution
 3) Noise
 4) Artifact
     Partial volume effect
     Beam Hardening
     Star Artifact
     Slice profile
                              12
Spatial Resolution (high contrast)




Spatial Resolution is defined as:
The number of line pairs per cm just visible in image
                                                13
            Spatial Resolution
       high contrast resolution

   The minimum size of detail visualized in the
    image with a contrast >10%.
     It is affected by:
      Reconstruction algorithm
      Detector width
      Slice thickness
      Object to detector distance
      X-ray tube focal spot size
      Matrix size.
                                             14
                  Spatial Resolution
              Low contrast resolution
   The size of detail
    that can be
    visualized when
    there is only a
    small difference in
    density
        It limited by
         noise.

                                        15
      Slice Thickness and Pixel size

   Thicker slice or bigger pixel size causes
    worse Spatial Resolution, but better
    SNR

   Thicker Slice improve Density resolution

   Higher mAs and more patient dose
    leads to better SNR

                                                16
                       Noise

Noise is obtained
  from the standard
  deviation in CT
  number in a
  region of interest
  (ROI) placed
  within the image



                               17
                    Noise
   It affects the low contrast resolution
   Noise dependents on the radiation dose
   The medical problem is: to obtain an image
    with an acceptable level of noise while keeping
    the patient dose as low as reasonably
    achievable
                 Noise =    1
                           dose




                                                18
Scatter!!




            19
Typical Scatter Survey




                         20
     CT Number Accuracy

 Measured CT number should be < ± 4 HU in
  the central ROI
 CT number of water is by definition equal to 0
 CT number depends on tube voltage, filtration,
  object thickness




                                           21
         CT number uniformity
                                         CT number of
                                         each pixel in
                                         the image of
                                         an
                                         homogeneous
                                         object should
                                         be the same
                                         over various
                                         regions

•The difference in the CT number between a peripheral and a
central region should be < 8HU
•Differences are largely due to beam hardening phenomenon
                                                    22
         CT number linearity
Linear relationship between the calculated CT number and
the linear attenuation coefficient of each element of the
object. Deviations from linearity should be < ± 5 HU




                                                   23
Artifacts




            24
Partial Volume Artifact




                          25
26
Star Artifact




                27
28
Ring Artifact




                29
The slice sensitivity profile (SSP)




                                      30
   Z-Sensitivity (Imaged slice width)




Plan view of a test object used to measure imaged slice widths for
   axial scans, to assess the accuracy of post patient collimation,
   and to calculate the geometric efficiency for the scanner
                                                          31
   Alignment of indicating lights with
   scan, coronal and sagittal planes
 Several methods can be
  used to perform these tests
The wrapped film is placed flat
   on the table and illuminated
   by the external scan plane
   light

The position of the light is
   marked on the film envelope
   and the table is moved
   automatically to the scan
   plane
                                     32
 Couch travel accuracy




To assess the distance indicator accuracy, a ruler or tape
   measure placed alongside the table, to check that the
   degree of couch movement indicated on the gantry
   agrees with the actual distance moved.           33
Axial scan incrementation
accuracy
 Verification of incrementation accuracy
  between successive axial slices can be
  achieved by placing envelope-wrapped film
  on the couch (in the isocentre plane) and
  exposing it to an axial scan sequence
 Narrow slices separated by a couch
  increment greater than 1 slice width can be
  used, and the distance between the lines on
  the film measured
                                           34
Couch travel accuracy for
helical scans
 In helical scanning, to assess imaged distance
  accuracy, a Perspex test object containing two small
  radio-opaque markers, separated by a fixed distance
  (ex:20 cm) is used.
 A helical run is planned to start at the first marker and
  to end at a distance x from the first marker
 If couch travel is accurate during the helical scan, the
  markers should be clearly seen on the first and final
  images of the series.


                                                       35
Couch travel accuracy for
helical scans




                            36
Assessment of accuracy of gantry tilt




   A film must be held vertically, so that it is parallel to the sagital plane
   Three axial exposures with 0, max superior and max inferior gantry
    tilt are made using the same film:
    The three scan planes should then be visible on the developed film
                                                                    37
   Dosimetry - CTDI in air (helical)


               Axial slice positions




               Helical scan (pitch 1)

The Computed Tomography Dose Index (CTDI) in air
  can be measured using a 10cm pencil ionization
  chamber, bisected by the scan plane at the isocentre.
                                               38
Dosimetry - CTDI in Perspex
Phantoms

      Insert to plug holes




     Head phantom            Body phantom (or annulus
                               to fit over haed phantom)


                                                     39
Dosimetry - CTDI in Perspex
Phantoms
 Central and peripheral CTDI’s are used to calculate
  weighted CTDI, CTDIw:


      n CTDI w =
                 1
                 C
                     (   1
                         3
                                      2
                           CTDI100,c + CTDI100,p
                                      3
                                                   )
 CTDIws can be compared against diagnostic
  reference levels for standard scan examinations




                                                       40

				
DOCUMENT INFO