3. CT Lasers & Imaging

breast

thorax

• tissue usually extends from 2nd to 6th rib

suprasternal notch (SSN)

chest

• T2/T3 interspace

angle of louis (sternal angle)

chest

• T4/T5 vertebral level

tip of xiphoid process

chest

• approximately T10 vertebral level

vertebra prominens

back

• C7

• base of neck

apex of lung

thorax

• ANT level

  • 2-3cm SUP to clavicle

carina (bifurcation of trachea)

thorax

• T5

umbilicus

abdomen/pelvis

• L3/L4 vertebral level

superior iliac crest

abdomen/pelvis

• L4-L5

anterior superior iliac spine (ASIS)

abdomen/pelvis

• L5-S1

• used to level pelvis

pubic symphysis

abdomen/pelvis

• 8-10cm INF to ASIS

ischial tuberosity

abdomen/pelvis

• 4-5cm INF to symphysis pubis

greater trochanter

abdomen/pelvis

• ~ same level as superior border of symphysis pubis

• common landmark for prostate gland

laser alignment

• uses surface + relational anatomy landmarks to estimate location of treatment area

  • starting point for CT sim

• departments have site-specific guidelines

  • lung tumour @ T3

    • sagittal through midline

    • coronal at mid sep

    • transverse 3cm inf of SSN

  • prostate

    • sagittal through midline

    • coronal at mid sep

    • transverse at greater trochanter

  • breast in upper quadrant

    • sagittal through midline

    • coronal at mid sep

    • transverse 4cm inf of SSN

ball bearing marks (BBs)

reference marks positioned at intersection of room lasers

• small radiopaque markers

  • material dense enough to show up as white dots on CT image

• placed on patient’s skin using stickers/adhesive dots

external + internal purpose

• identify location where patient was aligned to room lasers during CT sim

  • known starting location

  • location can be reproduced in future

scout image

simple 2D lower-dose x-ray taken just before CT scan begins

• ensures scan covers correct anatomy

• ensures patient is straight, leveled, centered

  • straightness

    • see if midline structures are centered

  • level

    • compare paired anatomical structures + ensure they are at same level = not twisted/rotated

  • centered

    • assess whether patient is positioned centrally within scanner = not too far left/right

• ensures BB marks are in correct place

  • assess whether BBs are located near intended treatment area

• determines if plannes CT scan limits are appropriate

BB marks in scout image

1. set up patient to external lasers + BBs placed at intersection of lasers (1 anterior + 2 lateral reference marks)

2. couch moved 500mm into scanner → BBs to center of bore/internal laser

   1. BB marks are 500mm away from center of bore (Z axis)

      • CT scan automatically moves patient until BBs reach internal bore lasers

3. BB center to bore = indicates center point of scout image

4. scout image captures anatomy superior + inferior to this point

   1. SUP + INF margins depend on department, eg. can be 30cm sup + 30cm inf of BBs

5. scout image used to select CT scan limits → where CT scan starts + finishes

   1. scan limits depend on treatment site

scout image steps

• x-ray tube stays in fixed position (unlike CT scan)

• couch moves patient through scanner

• x-rays pass through patient to detectors

• produces single 2D x-ray image

CT scan

3D volumetric image showing anatomy in 3 dimensions → height, width, depth

• acquired after scout image confirms correct position, appropriate BBs location, scan limits

CT scan steps

• patient lies on couch + RT moves couch through bore

• gantry rotates around patient + sends kV x-ray beams through body

• detectors on opposite side catch x-rays

• computer collects + processes data to build 3D image of inside of body

x-ray interaction

x-rays interact differently w/ various tissues → creates contrast in data collected + allows computer to differentiate b/w different tissues

• bone = dense → absorbs more x-rays

  • white/light grey

• soft tissue → absorbs some x-rays

  • various shades of grey

• air (lungs) = not dense → most of x-rays pass through

  • black

atomic number influence

• body made up of different elements

• each have atomic number (Z) = # of protons in nucleus

  • high Z = massive positive charge → pulls electrons tightly to nucleus

    • harder for x-rays to pass through tightly bound electron barriers

bone x-rays

• mainly composed of calcium → high Z = 20

  • electrons tightly bound to nucleus

• x-rays struggle to pass through

• appearance = white

lung x-rays

• mainly composed of oxygen, carbon, nitrogen → low Z

  • electrons loosely bound + spread far apart

• x-rays pass through easily

• appearance = black

soft tissue x-rays

• made up of lightweight elements (O, C, N, H) → low Z

  • electrons loosely bound together but packed closely packed

• x-rays pass through easier than bone but not as easily as lungs

• appearance = grey

tattoos

small, permanent marks placed at BB locations after CT sim is complete

• ensures reproducible position each day during treatment

  • patient is set up to those points every treatment session

• when patient arrives for treatment:

  • patient is positioned on treatment couch w/ same immobilization equipment documented at CT

  • treatment room lasers aligned w/ these marks → recreates laser alignment established during CT sim

  • reproduces position established during CT sim