Imaging Principles

• CT:

  • conventional CT (aka diagnostic CT)
  • Diagnosing and evaluating treatment response
  • simulation CT
  • for radiation therapy treatment planning
  • provide data on tissue electron densities
  • HUs = pixels of differing shades of grey
  • obtains dose distribution data
  • well defined anatomy is good for image matching
  • image quality: conventional CT = simulation CT
• Conventional and simulation CT:

  • fan beam
  • 1D detector
  • 1 rotation = 1 slice of CT
  • requires multiple rotations around patient
  • unlimited scan length
  • HU = yes
• EPI: electronic portal imaging

  • MV - opposing inline imager
  • in compton region of energy absorption - images have low contrast
  • BEV images - captures true record of treatment
• OBI: on board imaging

  • KV - perpendicular to treatment head
  • photoelectric region of energy absorption - images have high contrast between tissues
  • used for reference images
  • used for CBCT - improved image quality
• CBCT - KV (via OBI)

  • cone beam (instead of fan beam)
  • 2D detector
  • 1 rotation = entire volume
  • requires single rotation around patient
  • scan length limited by detector size
  • HU = no (so it’s not used for planning)
• Orthogonal pair:

  • MV - KV
  • especially used when you need to obtain images quickly (ex. Patient is in pain)
• Image types in RT:

  • DRR = digitally reconstructed radiograph
  • treatment image
  • reference image
  • BEV = beams eye view
• DRR:

  • planar image - from volumetric CT simulation scan
  • provide 2D representation of patient position in CT sim
  • use to compare 2D daily treatment images to 3D CT sim scan
  • can be generated for reference images and BEVs
  • static or live
• Static DRR:

  • generated in external beam planning
  • image is a fixed view
  • no 6DOF couch to work with
• Live DRR:

  • generated in external beam planning
  • updated dynamically by treatment software to account for changes in pitch, roll, yaw of patient
  • used when couch has 6DOF
• Treatment image:

  • image taken on treatment unit (Linac) of the patient in treatment position
  • can be: reference image, BEV, CBCT
• Reference image:

  • image taken with collimator jaws and MLC retracted to give full view of patient's anatomy
  • used to compare patient treatment position to the CT sim (planned) position and for localizing the treatment fields
• BEV:

  • image taken at treatment angle with collimated field size and MLC in place
  • visualization of treatment field in relation to the patient's anatomy (is the field in the right location)
  • only used to confirm field placements
  • not used for image matching (field localization)
• Treatment image types:

  • 2D
  • 2D/2D
  • 3D
  • 2D/3D
• 2D image:

  • planar image
  • provide information in 2 dimensions only
  • AP: sup / inf and right / left planes
  • lateral: sup / inf and ant / post planes
• 2D / 2D: "orthogs"

  • two planar images assessed against static DRRs to give treatment field positioning information in all 3 planes
  • only allow for translational treatment field shifts (x, y, z)
  • two images taken at 90 degrees apart
• 3D:

  • volumetric images - provide 3 dimensional data of a patient
  • ex. CT, CBCT, MRI
• 2D / 3D:

  • same as 2D / 2D except treatment images are compared to live DRRs
  • allows for translational and rotational shifts (x, y, z, pitch, roll, yaw)
  • used for 6DoF treatment couches which can perform rotational shifts
• Unpaired images:

  • two orthogonal treatment images that share a common axis (sup / inf) but are not linked in the software
  • sup / inf shifts applied to one image will NOT be reflected in the other
• Paired images:

  • two orthogonal treatment images that share a common axis (sup / inf) that are linked together in the software
  • sup / inf shifts applied on one image are applied to the other

  
• Summary of RT imaging:

  A) EPI

  B) BEV

  C) BEV

  D) MV

  E) KV

  F) MVCT

  G) CBCT

  H) CT

  
• BEV exposure options:

  • Single Exposure:
  • closed
  • Double Exposure:
  • two exposures on one image
  • one exposure = closed image (of the treatment field)
  • other exposure = open image (larger field to visualize surrounding anatomy)
• BEV:

  • closed
  • image taken with collimator jaws and MLC in treatment position
  • visualize borders of the treatment field
  • open
  • image taken with MLC retracted and collimator jaws opened wider than planned field size to visualize anatomy outside of treatment field
• Image only double:

  • image with two exposures (open and closed) taken either before or after the treatment
• Before during:

  • image with open field taken before treatment and closed field taken during treatment
• During after:

  • image with closed field taken during treatment
  • open field taken after treatment
• MRI: soft tissue contrast

  • T1 - weighted: CSF = dark, fat = bright
  • T2 - weighted: CSF + fat = bright
• T1 with IV gadolinium contrast:

  • gadolinium = very bright on T1 images
  • used for looking at vascular structures and inflammation
• FLAIR = fluid suppression

  • suppressing fluid - help with visualization of abnormalities (inflammation)
  • similar to T2 weighted - except: TE and TR are very long
  • CSF = dark
  • abnormalities = bright
• MRI sequences summary:

  A) dark

  B) bright

  C) dark

  D) bright

  E) dark

  F) bright

  
• Fat Sat and STIR:

  • suppressing fat on MRI - improve visualization of pathologies or contrast
  • fat has short T1
  • methods to suppress fatty tissues:
  • chemical shift techniques (fat sat)
  • inversion recovery (STIR)
  • hybrid of chemical shift and IR
• Diffusion weighted imaging:

  • provides information regarding diffusion properties of tissues
  • pathologies can affect water distributions inter and intracellularly
  • used for acute strokes - can detect ischemic tissues