Chapters 29_30 Computed Tomography by linxiaoqin

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									  Chapter 27 Digital Radiography
• With conventional radiography, the end
  result in a film that can not be adjusted for
  contrast and density.
• The film requires processing and storage.
  Film processing is prone to problems.
• It can not be seen by someone else
  without mailing or carrying the film to the
  other person.
        Digital Radiography
• Storage space is costly as is the film
  processor and chemicals.
• Conventional radiography has relatively
  poor contrast resolution due to scatter
  radiation.
• Many of these problems can be solved by
  digital radiography.
    Conventional Radiography
• Conventional
  Radiography has film
  as the image
  receptor.
• Film must be
  processed
• Views on a view box
• Stored in a File Room
        Conventional Radiography
• Computed Radiography
  replaces:
    –   The film,
    –   Processor
    –   View Box
    –   File Room
•   With a electronic detector
•   A detector reader
•   Computer monitor
•   CD or DVD or hard disc
             Image Matrix
• The x-rays form a latent image on the
  image detector .
• The latent image is then electronically
  developed by the computer into a matrix of
  numerical values called a matrix.
• The Image Matrix refers to a layout of cells
  in rows and columns. Each cell
  corresponds to a location on the image.
              Image Pixel
• Each cell is called a pixel (picture
  element).
• In Digital X-ray Imaging, the value of the
  pixel determines the pixel brightness. The
  value is relative and defines the image
  contrast.
• In CT the pixel represents the CT number
  or Houndsfield Unit (HU).
              Image Matrix
• The size of the image matrix is determined
  by the characteristics of the imaging
  equipment and the capacity of the
  computer.
• Matrixes may be operator selectable.
• For the same Field of View (FOV) spatial
  resolution will be better with a larger image
  matrix.
Matrix & Resolution
• This picture illustrated the
  improvement in resolution
  with larger matrix size.
• DR spatial resolution is
  equal to the FOV divided
  by the matrix.
• The resolution on a 14” x
  17” image would be less
  than a 8” x 10” image with
  the same matrix.
 Dynamic Range
• The range of values
  which a system can
  respond is called the gray
  scale range or dynamic
  range.
• This can be called the bit
  depth.
• For radiography, CT and
  MRI images a 12 bit
  range is required.
• 212 (0-4095 shades of
  gray
 Dynamic Range
• The greater the
  dynamic range mean
  there can be better
  contrast resolution.
• The dynamic range of
  a digital system is
  limited only by the
  capacity of the
  computer and the
  software.
 Dynamic Range
• A wide dynamic range
  provides more latitude in
  exposure.
• Recent research has
  shown that higher kVp
  and lower mAs can be
  used with DR.
• This will lower patient
  exposure.
• Wide latitude means less
  retakes.
           Digital Radiography
• Among the first digital
  radiographs was the
  scout film taken with
  computed tomography.
  This is called scan
  projection radiography
  or SPR.
• A fan beam is used with
  the CT detectors and the
  patient is moved to record
  the image.
          Digital Radiography
• Since modern C T scanners have Pre- patient
  and Post-patient collimators, the image is
  virtually free of scatter radiation. This improved
  the image contrast.
• Problem: Poor spatial resolution. The more
  detectors there are per degree of fan bean, the
  better the spatial resolution.
• Long exposure as patient translation is involved.
  Conventional radiography has exposure times in
  milliseconds.
  Area Beam versus Fan Beam
• Conventional radiography uses an area
  beam so the image is produced in
  milliseconds.
• Fan Beam DR uses several seconds.
• The problem with area beam in Digital
  Radiography is designing the image
  receptor that will retain the rapid response
  time required.
      Computed Radiography
• Directly acquired DR images using a
  photostimulable phosphor is called
  computed radiography.
• The image receptor resembles a
  conventional intensifying screen in a
  cassette. Exposure is made with
  conventional equipment
     Computed Radiography
• Speed is similar to a 200 to 250 speed
  screen film system. (More exposure than
  our current screen film systems)
• The latent images consists of valence
  electron stored in high energy traps.
• The exposed cassette is placed in
  processor that removes the plate from the
  cassette.
      Computed Radiography
• Inside the processor,
  the plate is exposed
  to a high intensity
  laser to produce the
  manifest image.
• The image is read by
  a photomultiplier
  tube and digitized.
      Computed Radiography
• The digitized image
  can be viewed on a
  computer monitor or
  printed with a laser
  printer.
• The plate is erased
  for reuse.
      Computed Radiography
• The spatial resolution is not as good as
  film but the contrast resolution is better
  because the image contrast and density
  can be adjusted during post processing.
  Called Windowing.
• The latitude is exceptional so patient
  exposure may be reduced due to a
  reduction in retakes.
     Computed Radiography
• The plates are reused but eventually will
  need to be replaced.
• There is no need for film or film
  processing. Dry chemical film printers may
  be used similar to CT or MRI.
• The system can be added to conventional
  radiography machines.
• Images stored on CD, DVD or hard disc.
      Charge Coupled Device
• CCD’s are
  photosensitive silicon
  chips that are rapidly
  replacing television
  cameras in
  fluoroscopy.
• The CCD's can be
  placed in a linear
  array to view a CsI
  phosphor.
   Direct Capture Radiography
• Computed Radiography has been around for
  about 20 years. In the late 1990’s two new
  approaches appeared for direct capture of the
  image.
• Both use an active matrix array of thin film
  transistors that results in an image receptor the
  size of a conventional cassette.
• As soon as the exposure is completed, the
  image is digitized directly from the x-ray
  machine. No cassettes! No Film! No Processor!
   Direct Capture Radiography
• Requires a special x-ray machine
  designed with a built in electronic receptor.
• There are two major designs:
  – Cesium Iodide scintillation phosphors coated
    over amorphous silicon photodiodes.
  – A thin layer of amorphous selenium on an
    active matrix array.
               CsI Phosphor
• Offers good dose
  efficiency.
• Best spatial resolution
  is 5 lp/mm.
• Resolution is limited
  by the pixel size.
        Amorphous Selenium
• Improved spatial
  resolution due to no
  spreading of the light
  found in the a-Se
  design.
• Improve dose
  efficiency is seen in
  the Csl phosphors.
         Thin Film Transistors
• Both designs use a
  thin film transistors
  and an active matrix
  to store the electronic
  image.
• The image is
  constructed in the
  computer one pixel at
  a time.
  Digital Radiography Potential
• The future of conventional radiography lies
  in digital imaging. Some problems must be
  addressed first.
• Resolution must be improved. Image
  spatial resolution is equal to use of the
  large focal spot and 400 speed film.
• Cost must come down. In a hospital where
  film budgets are over $700,000 per year,
  DR will save money.
 Digital Radiography Potentials
• Current basic computed radiography
  systems cost about $45,000 to $100,000.
  At about $2.00 per sheet, it takes a lot of
  film to equal the cost of CR.
• DR systems cost from $70,000 to
  hundreds of thousands of dollars.
• In hospitals, being able to quickly send
  images to specialists and reduced storage
  costs are important factors.
  Digital Radiography Potential
• Hospitals will also have significant labor
  saving costs since file room personnel and
  dark room staff are no required.
• The image can be checked seconds after
  it is taken so patient through put is also
  improved for added time savings for the
  technologist and doctor .
         Digital Radiography
• Special computer monitors capable of
  2048 x 2048 are required for optimum
  digital radiographic images.
• The industry established standards for
  digital imaging called DICOM.
• Agfa and Kodak have a long film system
  similar to the 14” x 36 used for full spine
  radiographs. We have this in our clinic.
         Digital Advantages
• The Darkroom and Film Storage Files can
  be eliminated. Is will save hundreds of
  square feet of office space.
• The patient films can be stored on a CD in
  the patient record or left on a hard drive.
• If the doctor takes many retakes due to
  exposure factors, there can be a reduction
  in exposure for the patient.
  Digital Radiography Potential
• Higher end digital systems allow the operator to
  use higher kVp since contrast is not fixed to the
  kVp so additional reductions in exposure can be
  seen.
• With add-on digital systems, the time to
  download the image to the computer will need to
  be factored into the savings. Time spent
  windowing to obtain a good image also must be
  factored. Only Direct Capture is instantaneous.
  Digital Radiography Potential
• The cassette can not be used until the
  image is downloaded and cassette erased.
• Sometimes erasure fails to be complete.
• We don’t know how many time the
  cassette can be reused. Cassette are
  more prone to damage than screen film
  cassettes.
  Digital Radiography Potential
• Digital Radiography promised lower patient
  exposure. We have all heard this claim.
• Presently, this is the case, the speed of CR
  systems is about 200 so it take twice the
  exposure of a 400 speed system.
• Low dose images have objectionable noise or
  quantum mottle so operators error to the side of
  over exposure.
• There is no standard for QC for digital
  radiography as there is for film processing
  systems.
  Chapters 29 & 30 Computed
         Tomography
• Godfrey Hounsfield of EMI, LTD
  demonstrated the principle for computed
  tomography in 1970.
• Alan Cormack developed the mathematics
  used to reconstruct the CT images.
• They shared the 1982 Nobel Prize for
  physics.
      Computed Tomography
• Computed Tomography is the most
  significant development in radiology in the
  past 40 years.
• MRI and Ultrasound are also significant
  developments but they do not use x-ray to
  produce the image.
• The x-ray tube spins around the patient.
        Basic C T Principles
• Instead of film, radiation detectors
  measure the radiation attenuation as the
  beam passes through the body.
• The detectors are connected to a
  computer that uses algorithms to process
  the data into useful images that are then
  recorded on film and viewed on a
  computer monitor.
         Basic C T Principles
• Conventional
  tomography has the
  image parallel to the
  long axis of the body.
  This is referred to as
  Axial Tomography.
        Basic C T Principles
• Computed
  Tomography has the
  x-ray tube move
  across the so the
  image is called a
  transverse image or
  one perpendicular
  to the long axis of
  the body.
      Computed Tomography
         Development
• Computed tomography has gone through
  five major design advancements since
  1970
• Each development improved both scan
  time and resolution or image quality.
• Scan time have been reduced from 5
  minutes to 50 ms.
• First scanner used a very tightly collimated
  pencil beam.
  First Generation CT Scanner
• Pencil Beam
• Translate-Rotate
  Design
• 180 one degree
  images or
  translations.
• One or two detectors.
• 5 minutes scan time
Second Generation CT Scanner
• Translate-Rotate
• Fan beam collimation
  so there is more
  scatter radiation.
• 5 to 30 detectors
• 10 degrees
  /translation 18 per
  scan.
• 30 second scan times
• Faster scan time
  Third Generation CT Scanner
• Rotate-Rotate
• Fan shaped beam of
  30 to 60° for full
  patient coverage.
• Constant Source to
  detector distance due
  to curvilinear detector
  array.
  Third Generation CT Scanner
• If one detector fails, a
  ring artifact appears.
• 1 second scan times
• Superior
  reconstruction and
  resolution.
 Fourth Generation CT Scanner
• The tube rotates
  around a stationary
  ring of detectors.
• Fan beam
• Variable slice
  thickness with pre
  and post patient
  collimation.
 Fourth Generation CT Scanner
• As many as 8000
  detectors.
• 1 second scan time.
• Auto-detector
  calibration so no ring
  artifact.
• High radiation dose
  compared to earlier
  scanners.
Fifth Generation
  CT Scanner
• This is the latest
  generation of CT.
• Allows for continuous
  rotation of the tube for
  spiral CT.
• 5th Generation also
  includes two novel
  designs:
  Fifth Generation CT Scanner
• Toshiba maintains the
  same SID by
  wobbling the
  detectors.
• Heartscan by Imatron
  used an electron
  beam instead of x-ray
  tube and 50 ms scan
  times.
  Fifth Generation CT Scanner
• Spiral CT scanners
  allow for contiguous
  or even overlapping
  data acquisition.
• As the tube spins, the
  table moves.
• On earlier units, the
  table moved between
  scans.
          Spiral C T Scanner
• Spiral CT is made
  possible by slip-ring
  technology. The tube
  can continuously
  rotate 360 degrees,
  where it must stop
  after each rotation
  with conventional CT.
          Spiral C T Scanner
• The detector array
  may contain as many
  as 14,600 detectors
  that are 1.25mm
  wide.
• This allows multiple
  slice to be made with
  one scan and more
  tissue volume to be
  imaged.
        Benefits of Spiral CT
• Less motion artifacts
• Improve lesion detection because the
  reconstructed image can be at arbitrary
  intervals.
• Reduced partial volume because of
  overlapping reconstruction intervals.
• Reduced scan time.
         Benefits of Spiral CT
• Advances in
  computer processing
  allows for multi-planar
  reconstruction and
  even 3D
  reconstruction.
Basic CT Scanner Components
• Gantry includes: the
  – Pedestal or table
  – Tube, Collimators,
    Detectors & High
    Voltage Generator
  – Mechanical Supports
• Operators Console
• Computer
Basic CT Scanner Components
• Multi-format laser
  camera using either
  dry chemical images
  or conventional laser
  film.
• Viewing station for
  radiologist (optional)
           CT Components
• Table, pedestal or couch holds patient
  and is motor driven to move the patient
  into the scanner at the correct rate and
  distance.
• X-ray tube with very high heat capacity,
  measured in millions of heat units.
        Two Collimators in CT
• Prepatient collimator
  determines slice
  thickness
• Predetector
  collimators reduce
  scatter radiation to
  improve contrast.
            Large Computer
• A very large and fast
  computer is needed
  to perform over
  250,000 calculations
  per image.
• Newer scanners use
  an array processor so
  the calculations are
  done simultaneously.
     CT Image Characteristic
• Image matrix: Original EMI format was
  80 x 80 so there were 6400 cells of
  information called pixels.
• Today the format is 512 x 512 resulting in
  262,144 pixels.
• The numerical number in each pixel is a
  CT number or Hounsfeld Number.
       CT Image Characteristic
• CT number or Hounsfeld
  Number represents the
  tissue volume in the pixel.
• Field of View (FOV)is the
  diameter of the
  reconstructed image. As
  the FOV increases, the
  size of the pixel
  increases.
• Voxel: is the square of
  the matrix times the
  thickness of the slice.
     Hounsfeld or CT Number
• The precise CT number is related to the
  attenuation of the tissue contained in the
  voxel.
• Bone = +1000
• Muscle= +50
• Lung= -200
• Air = -1000
            Image Quality
• Spatial Resolution: The motion of CT
  tends to blur the image compared to the
  actual object.
• The ability of the scanner to reproduce
  high contrast or sharp edges (edge
  response function) is measured as
  Modulation Transfer Function (MTF).
             Image Quality
• The best possible resolution is equal to the
  pixel size. In terms of line pairs, 1 would
  be two pixels.
• Items that impact spatial resolution include
  collimation, detector size and
  concentration and the mechanical gantry
  control. Much like conventional
  radiography.
             Image Quality
• Contrast Resolution: The ability to
  distinguish one soft tissue from another is
  contrast resolution. This is where CT
  excels.
• The absorption or attentation
  characteristics is affected by the atomic
  number and the mass density of the
  tissue.
          Contrast Resolution
• Conventional
  Radiography has
  relatively poor
  contrast resolution.
• CT can amplify the
  tissue characteristics
  to provide superior
  contrast resolution.
         Contrast Resolution
• The Contrast Resolution is improved
  because of the predetector collimation.
• The contrast resolution for low contrast
  tissues is limited by the size and uniformity
  of the object and the noise in the system.
• Noise is determined by the number of x-
  rays used by the detector to make the
  image.
      Computed Tomography
           Problems
• CT scans require significantly higher
  doses of radiation compared to
  conventional radiography. Therefore the
  risks of the radiation and the benefits of
  the information gained by the scan must
  be factored when determining the need for
  Computed Tomography.
      Computed Tomography
           Problems
• If a chest x-ray is equal to the amount of
  radiation received in 10 days from our
  natural environment, a CT of the brain is
  equal to 8 months exposure and CT
  abdomen, chest or lumbar spine is equal
  to 3 years each.
• Do they mention this when they advertise
  total body CT scanning?
     Computed Tomography
          Problems
• Computed Tomography equipment are
  expensive and have high service costs.
• Computed Tomography is expensive for
  the patient or insurance. As much as
  $1,000 per exam. HMO’s require
  preauthorization
          Nuclear Medicine
• Conventional Radiology and Computed
  Tomography use x-ray.
• Nuclear Medicine uses radioactive
  compounds that are injected into the
  patient referred to as radionuclides or
  radiopharmaceuticals.
          Nuclear Medicine
• The radionuclide is used as a tracer in
  nuclear medicine studies. A tracer is a
  substance that emits radiation and that
  can be identified when placed inside the
  human body.
          Nuclear Medicine
• By detecting the tracer, information about
  the structure, function, secretion, excretion
  and volume of the target organ can be
  obtained.
• The organ imaging involves administration
  of the radionuclide to the patient either
  orally or intravenously.
           Nuclear Medicine
• Depending upon the radionuclide used, it
  will localize in a specific organ of the body
  and provide a way of identifying the
  structure and function of that organ.
• Scanning instruments detect the radiation
  produced by the radionuclide that is
  concentrated in the organ and produce an
  image that can be recorded on film or
  paper.
                 Bone Scan
• In a bone scan, the
  nuclide is
  concentrated in the
  bone blood flow.
• Hot spots can be the
  result of fractures or
  metastasis from
  cancer.
Other Nuclear Medicine Exams
• Thallium is used to study the heart and
  can detect injury from a myocardial
  infarction or evaluate the wall motion of
  the heart.
• Iodine is used to image the thyroid gland
  and to treat thyroid disease.
• In a laboratory, radionuclides can be
  added to specimens for analysis.
Other Nuclear Medicine Exams
• With x-ray, the patient is not radioactive
  after the exposure.
• With nuclear medicine in vivo exams the
  patient is radioactive after the injection and
  until the material is either excreted or
  sufficient half lives have passed. Short half
  life materials are generally used.
     Level of exposure from patient after Nuclear
       Medicine Exams in mrad/hr at skin level.
Bone scan           Gated Blood       Thallium cardiac Lung scan
20 mCi TC-99        Pool 20 mCi TC    3.5 mCi          4 mCi Tc-99
7.68 mrad           15.84 mrad        1.5 mrad            5.2 mrad
At injection time   At injection time at injection time   at injection time
5.68 mrad           11.21 mrad        1.45 mrad           3.68 mrad
3 hours             3 hours           3 hours             3 hours
3.85 mrad           7.94 mrad         1.41 mrad           2.61 mrad
6 hours             6 hours           6 hours             6 hours
1.93 mrad 12        3.97 mrad         1.34 mrad           1.31 mrad
hours               12 hours          12 hours            12 hours
0.48 mrad           0.99 mrad         .77 mrad            0.33 mrad
24 hours            24 hours          72 hours            24 hours
          Nuclear Medicine
• SPECT is a form of computed
  tomography using a radionuclide instead
  of x-ray.
• PET is Positron Emission Tomography
  uses specially formulated nuclides from a
  linear accelerator and shows the metabolic
  activity of the brain or heart. PET scanners
  are now being combined with CT
  scanners.
          Nuclear Medicine
• Nuclear Medicine is useful in evaluating
  function and blood flow.
• It has very poor spatial resolution.
End of Lecture

								
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