CT Quality and Safetyweb by fanzhongqing

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									CT Quality and Safety




                        1
Name the 4 factors affecting
     image quality?




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Factors Affecting Image Quality
   Four main factors contributing to image
    quality are: Spatial resolution, Contrast
    resolution, Noise, and Artifacts

   Diagnostic imaging has superior spatial
    resolution compared to CT
     Howdo we measure spatial resolution?
     What is the range for plane film?
                                                4
Image Resolution –
 (how sharply is the image seen)
CR & DR                          CR 2 - 5 lp/mm
 4000 x 4000
                                 RAD 3-6 lp/mm
 image only as good a
  monitor*                       DR 3 - 5 lp/mm
                                 CT 8 to 32 lp/cm
   525 vs 1000 line             IMAGE APPEARS
   more pixels = more            SHARPER BECAUSE
    memory needed to store        CONTRAST CAN BE
   resolution dependent on       ADJUSTED BY THE
    pixel size
                                  COMPUTER –
                                 (DIFFERENCES IN
                                  DENSITY)
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  TABLE 31-2 Typical window settings
• CT exam Width Center(level)   • CT exam Width Center(level)
• Brain     190      50         • Liver       175    45

• Skull    3500      500        •   Mediastinum325 50
                                •   Lung 2000 500
• Orbits 1200        50         •   Spinal cord 400 50
                                •   Spine 2200 400
• Abdomen400 35
                                • Merrill's pg 313

                                                            8
Technologist Determines Image
Quality Factors

   The technologist choice of slice thickness,
    focal spot size, display FOV, technique
    selection, pitch and reconstruction
    algorithm will effect the resultant image
    quality



                                                  9
Spatial Resolution
   The amount of blurring in an image.

   Affecting technical factors: focal spot size,
    slice thickness, display FOV, matrix and
    reconstruction algorithm.
     Detector width is the most significant factor
     MTF/limiting factor


                                                      10
Contrast Resolution
   The ability to differentiate between
    differences in density with in an image.

   Tissues with density differences of less
    than 0.5% can be distinguished with CT.

   What contributes to this?


                                               11
Contrast Resolution
   Affecting technical factors: Slice thickness,
    reconstruction algorithm, image display
    and x-ray beam energy
     Patient size and detector sensitivity also have
      a direct effect




                                                    12
Noise
 Random variations in photon detection
 Affecting technical factors: Matrix size,
  slice thickness, x-ray beam energy and
  algorithm
     Scatter   and patient size also contribute




                                                   13
Artifacts
   Unwanted densities with in an image, not
    representing actual patient anatomy.




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Data Acquisition
   Of the 3 major system components where
    is the data acquisition selected?

   Scan time: usually preselected as part of
    the programmed protocol, but can be
    altered. Alterations may change image
    quality

                                                19
Scan Diameter
   Scan FOV – area being scanned

   Display FOV – area to be displayed on the
    monitor.




                                            20
Changes to the
    FOV




                 21
Dynamic scanning
   After contrast injection different structures
    enhance at different rates.

   As contrast moves through the different
    phases; arterial, venous, equilibrium,
    delay, additional imaging can be
    diagnostically useful.

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Axial, Helical/Spiral,
Multi detector row CT, Dual source
   Axial = point and shoot
   Helical = continuous tube rotation and
    table movement. High speed volume
    scanning
   Multi detector = 2 to 320 slices per
    revolution
   Dual source = two tubes, two sets of
    detectors                                28
Axial Scanning




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Helical Scanning




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Pitch




        31
Pitch = 1.5




              32
Pitch = 0




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SPIRAL/HELICAL CT
   Helical CT = Continuous Tube and Table
    movement
     Pitch
     .5= more information more radiation
     2 = less information less radiation




                                             34
  How do we obtain images between
slices when the pitch is higher than 1?




                                      35
Multi-detector Imaging (MDCT)
      collects more raw data




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• This multi-detector array contains 64 rows
  of 1824 individual detectors, each 0.6 mm
  wide (116,736 detectors)




                                           38
Beam Geometry
 Fan – The traditional beam geometry, it is
  opened along the width of the patient.
  Tube is rotated to the z-axis.
 Multi-slice (cone) – The beam has to be
  “opened” up, head to foot, to cover the
  additional detectors.



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Filtration for CT beam




                         41
What if you wanted thinner more
        detailed images?




                                  42
THREE-DIMENSIONAL IMAGING

   A post-processing technique that is
    applied to the raw data to create realistic
    images of the surface anatomy to be
    visualized

   Maximum Intensity Projection (MIP),
    Shaded surface display (SSD), Volume
    rendering (VR)
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Most common filming devices
   Filming …..ha ha!     Laser printers

   PACS, DVD, Opitical Disk?




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Radiation Safety
   Compare average radiation doses from
    diagnostic x-ray to CT?

   What are your thoughts?




                                           51
      Image Gently:
ALARA* principles in pediatric
         imaging
           Sue C. Kaste, DO
 St. Jude Children’s Research Hospital
             January 2010


                                         52
California technologist faces testimony in CT
overdose case, September 18, 2009

   January 2008, 23-month-old ... fell out of bed,
    and when he could hardly
    move his head his worried parents took him to
    the emergency room at
    Mad River Community Hospital in Arcata, a
    small town ... north of San
    Francisco. Over the next 68 minutes, the toddler
    was exposed to 151
    scans ... [cervical spine]


                                                   53
Los Angeles Times October 13,
2009
   Cedars-Sinai investigated for significant
    radiation overdoses of 206 patients
    The finding prompts the FDA to issue an
    alert urging hospitals nationwide to
    review their safety protocols for CT scans.




                                              54
Dose of 1 abdominopelvic CT = __
CXRs in an adult
   1. CT less than 1 CXR
   2. CT > 1 CXR but < 10 CXR
   3. CT > 10 CXR but < 100 CXR
   4. CT = 100 to 250 CXRs
   5. CT more than 500 CXRs



                                   55
    Dose of 1 abdominopelvic CT = __
           CXRs in an adult
•    1. CT less than 1 CXR
•    2. CT > 1 CXR but < 10 CXR
•    3. CT > 10 CXR but < 100 CXR
•    4. CT = 100 to 250 CXRs
•    5. CT more than 500 CXRs




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Radiology April 2004
                       57
                  2004
• 60 million CT examinations performed per
  year in the United States
• 10% growth in number of CTs per year
• CT is the largest contributor to medical
  radiation dose in the United States.
• Currently, 11% performed in children (~ 7
  million studies per year in U.S.)

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Children at least 2x more sensitive
              (2 – 10x)




                                  59
    Typical Radiation Doses (mSv)
            (Adult patients)
•   Average annual technician dose      3.2
•   Natural background       3.5
•   Dental x-rays .09
•   BE (marrow)         8.75
•   CXR (marrow)        .01
•   Mammogram (breast)       .5 - 7.0
•   Airline passenger        .03
•   Flight crew / attendants 1.6
•   CT       < 1.0 –30 mSv

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Typical Medical Radiation Doses
        5 year-old (mSv)
• 3-view ankle           .0015 eq. cxr = 1/14
• 2-view chest .02        eq. cxr = 1
•   Tc-99m radionuclide bone scan (up to)  6.2 eq.
    cxr = 310
•   FDG PET (up to)      15.3 eq. cxr = 765
•   Fluoroscopic cystogram <.33 eq. cxr = 16
•   Chest CT       up to 3      eq. cxr = 150
•   Abdomen CT up to 5          eq. cxr = 250

                                                     62
  Technologist responsibilities
• Technologists represent the frontline of
  patient care and imaging
• Perform the needed examination Control
  exposures
• If study or imaging protocol of concern –
  ask!
• Learn and share your knowledge

                                              63
 Controlling CT radiation dose exposure

• Scan only required area
• Reduce tube output (kVp and mAs).
• Reduce exposure parameters for smaller
  patients
• Perform single phase studies
• Most pediatric conditions diagnosable with
  single phase CT
• Use breast shields for girls undergoing
  chest CT studies
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15 mAs


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8 mAs




        66
*CTDI special measurement for CT
                                   67
                 ALARA
• Image Gently



• Step Lightly




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MRI vs CT
   With the introduction of MRI the profession
    speculated if CT would become obsolete

 Advantages of CT : Metal is not affected,
  Claustrophobia, uncooperative patients,
  obese patients, fast scan times, trauma,
  more cost effective
 Both modalities are used to provide as
  much diagnostic information as possible
                                              69
CT in the Future
   CT technologist has an increased
    responsibility to understand scanning
    dynamics

   This imaging modality will continue to be a
    highly respected diagnostic tool


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Questions   Comments




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