What's In An Image?

Now that you have a general understanding of the equipment you need to consider the imaging parameters, which varies and is based on the type of procedure being attempted. So let us take a look realizing that CLRS 317, 321, and 322 will cover these parameters in greater detail.

  1. Type of imaging parameters:
    2. Whenever you take an imaging remember you are taking an image of a 3-deminsional object
    1. Dynamic or flow study (2D)
      1. First pass of the heart will collect data over a short period of time (~0.1 sec/frame)as the radiopharmaceutical flows through the cardiac and pulmonary systems - Cardiac First Pass
      2. Flow study to the kidneys is broken up into two parts:
        1. First part will have a dynamic acquisition that usually ranges between 1 to 3 seconds. This defines the initial uptake of the radiopharmaceutical into the kidney
        2. Second part lasts between 20 to 30 minutes where 30 to 60 seconds per images are acquired. This evaluates the radiopharmaceutical passing through the kidney and out to the ureters - Renal Exam
      3. Another example of a flow study cans be done with a 1-minute acquisition to evaluate bile flow through the liver - Hepatobiliary imaging
    2. Static (2D)
      1. Consider what you are actually doing - taking an image of a 3-D object and placing it into 2-D space
      2. Pending the area being imaged and the amount of activity injected into the patient, this image usually has a high count density
      3. Take a look at bone scan -Bone Imaging (Spot View)
    3. Whole Body (2D)
      1. Two dimensional image that includes most or all of the body - Whole Body Bone Scan
      2. There are other soft tissue scans with other radiopharmaceuticals are also taken in this format
        1. Gallium scan is used to find infection and certain types of cancers
        2. 123I Whole body is used to look for certain types of thyroid cancer
    4. SPECT/PET (3D)
      1. Acquires three-dimensional image and displays it in 3-D space
      2. You can look at a cine where the object spins in 3D space - http://en.wikipedia.org/wiki/File:PET-MIPS-anim.gif
      3. Data is also "sliced and diced" to be displayed in 2-D format - coronal, sagittal, and/or transverse images - Look at the slices
  2. Type of collimation
    1. What are you looking for?
      1. Flow study = High count image over a short period of time
      2. Detailed information = high resolution
      3. Energy gamma related to collimation = Low - Medium - High
  3. Matrix size
    1. Amount of dots on the screen referred to as pixels
    2. Each pixel contains counts
    3. The more pixels the greater the detail in a scan (pending counts you improve resolution)
    4. The more the counts per pixel the greater the spatial resolution
    5. We'll talk about this in detail during the next two semesters
    6. Matrix size may range from 64 x 64 to 256 x 256 and 256 x 1024
  4. Setting the acquisition
    1. Time per frame on a flow study usually set to second(s) per frame
    2. Static - usually set to counts per image, but can also be set at time per image
      1. Example - Static image, bone scan, chest = 750k counts
      2. Example - Static image, bone scan, hands = 300k counts
      3. Example - Static image, bone scan, hands, set to collect for 5 minutes
  5. Whole body (static)
    1. Counts are collected as the camera moves down the patient
    2. Set at cm/minute, usually referred to as scan speed
    3. Whole body bone should be no more than 12cm per minute
    4. The slower the speed the greater the counts
    5. Most bone scans usually include the entire skeleton
  6. SPECT
    1. Camera moves around in a circle and stops every couple of degrees (~3) and collects counts for a short period of time
    2. Each stop is referred to as a slice
    3. The number of degrees the camera will cover will be between 180 (for hearts) and 360 (everything else)
  7. Acquisition of counts
    1. Most static images are acquired in counts per image, where the greater amount of counts usually means better resolution (example 1000k count image)
    2. Count density can also be used, which refers to the amount of counts per cm2
  8. Amount of images
    1. In a dynamic acquisition you want to catch the flow of the radiopharmaceutical going through the area of interest
    2. In a static image you need to get all the angles and/or areas of interest
    3. Whole body
      1. Head to Toe
      2. Head to mid-thigh or
      3. Eyes to thighs (usually seen in PET)
    4. SPECT - 32 to 128 slices pending matrix size and if you are acquiring 180 or 360 degrees
  9. Set the window
    1. Usually for 99mTc the window is set to 20% at 140 keV
    2. What might the advantage be to cut the window to 15%?
    3. The LLD and ULD are adjusted to the peak of energy gamma of interest
    4. Some radionuclides have more than one energy peak, which requires more than one window
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