Digital Radiography

1
Digital Radiography

• Chapter 27

2
Digital Fluoroscopy (DF)

• Produces a series of dynamic images in a digital
  format obtained with an area x-ray beam and an
  image intensifier

3
High Voltage Generator

• During DF the under table tube operates in the
  radiographic mode instead of the image
  intensifying mode
• Disadvantages
• Higher patient doses
• Advantages
• Better images

4
Digital Radiography (DR)

• Refers to static images produced either with a
  fan x-ray beam intercepted by a linear array of
  radiation detectors or an area x-ray beam
  intercepted by a phosphor plate or direct capture
  solid state device

5
Development of Digital

• In the 70s digital was investigated by two
  separate lines and became clinically ready in the
  80s
• Digital fluoroscopy
• Digital radiography

6
Image characteristics

• Dynamic range
• Range over which a system can respond
• Image matrix
• Layout of cells in rows and columns with each
  cell responding to a specific location in the
  image. Each cell has a certain brightness or
  intensity representation at that location.
• Each cell is called a pixel

7
Image Characteristics

• Fields of view
• The number of pixels that are available in the
  matrix
• Described as 512 x 512
• Spatial resolution in digital imaging can be no
  better than the size of a pixel

8
Image Characteristics

• Dynamic range
• The larger the dynamic range the more gradual the
  gray scale representing the range from maximal to
  minimal x-ray intensity

9
Image Characteristics

• Windowing
• Changing the image in the post processing mode
• Window level
• Identifies the types of tissue to be radiographed
• Window width
• Determines the gray scale of the tissue being
  radiographed

10
Components and Types of Digital Radiography

• Scanned projection radiography
• The first application of DR
• Uses CT gantry and a computer to generate an
  image that looks much like a conventional
  radiograph

11
X-ray tube requirements for DR

• Must have a high heat capacity
• Because detectors are not as efficient as screen
  film radiography
• It takes longer to expose an image receptor with
  a scan beam

12
Types of Detector Arrays

• Gas-filled detector
• Uses xenon gas which has a high atomic number and
  results in high photoelectric absorption
• Solid state scintillation detector
• Uses individual crystal-photodiode assemblies

13
Area Beam vs- Fan beam

• Area beam
• Uses a standard x-ray beam which exposes an area
  instead of a fan type beam
• Fan beam
• X-ray beam projected from a slit and the x-ray
  beam is scanned across the patient
• Disadvantage
• It takes longer to produce the image

14
Area Beam image acquisition

• Photostimulabe phosphor as a replacement for
  screen film
• 1st type of DR
• Also known as CR
• Solid state detectors in a matrix form
• Improved version of 1st type
• Charge coupled devices (CCD)
• Latest version of image acquistion

15
Types of Direct Capture Radiography

• 1st type
• Uses CsI scintillation phosphor coated over an
  active matrix array (AMA) composed of thin film
  transistors (TFT) of amorphous silicon photo
  diodes
• 2nd type
• Does not use a phosphor coating
• Both systems use the latent image and converts it
  into an electronic image in the form of a pixel

16
Types of Image Subtraction

• Temporal subtraction
• A mask image is created (mask mode)
• Contrast is introduced
• The mask is subtracted from the image desired
  leaving only the contrast material

17
Types of Image Subtraction

• Energy subtraction
• Uses the principles of characteristic x-ray
  production to subtract the k-edge of the lower
  and higher energy ranges leaving only the
  contrast material

18
Types of Subtraction

• Time-interval-difference mode (TID)
• Creates a new mask for each frame of viewing
• Subtracts the new mask from the new image

19
Picture Archiving and Communication System (PACS)

• Allows for acquisition, interpretation and
  storage of each medical image in digital form
  without having a hard copy film on hand.

20
Components of PACS

• Display system
• CRT monitor in which tech and docs may use to
  look at and manipulate images
• Network
• The cable or phone wire that connects the storage
  area with each display system
• Storage system
• Uses a magnetic or optical memory device.