V/Q Scan

V/Q Scan

• Tc-MAA (technetium tagged to albumin)

• Inhaled radiotracer - shows ventilation

  • Xenon 133

    • Short half life - 30 s

    • Low energy ~80 keV

    • 3 phase - wash in, equilibrium, wash out

    • Fat soluble

      • May see uptake in the liver if it is fatty

  • Tc-99m DTPA

    • Energy 140 keV

    • Allows for multiple projections

    • Tracheal clumping

• IV radiotracer - shows perfusion

  • Tc-MAA (technetium tagged to albumin)

  • Should stay in lungs because it basically gets trapped into capillaries in lung

  • See radiotracer in brain

    • Look for shunt

• Downscatter

  • Lower energy tracers have to be used first

    • This is because the one with the higher energy will basically decay into the values of the lower energy one and won’t know what the real shit is

  • So for this, the Xenon has to be used before the Tc (the inhaled before the blood portion)

• When should radiotracer particle amount be decreased

  • Does not mean reduce the dose

  • Basically a more concentrated dose i guess

  • Basically the particles get stuck in and block off the capillaries and don’t want to block more than 0.1% of capillaries

    • So situations where there are less capillaries require less particles such as:

      • Kids

      • Lung resection

      • Pulmonary hypertension

      • Known shunt

• Normal to have

  • Perfusion slightly less at top of lungs (hence hematogenous shit goes to base of lungs)

  • Ventilation slightly less at bottom of lungs (hence airborne shit goes to top of lungs)

• PE

  • Normal ventilation

  • Wedge shaped area of perfusion decrease (typically in periphery)

  • Normal perfusion essentially excludes a PE regardless of findings on ventilation portion

• PIOPED Criteria

  • Criteria for reporting liklihood of having a PE

  • Very low = <5 %

  • Low < 20%

  • Intermediate = <80%

  • High = > 80%

• Must have a CXR before doing

• Size

  • Described as a fraction of the segment

  • Small = <25% (do not add the smalls)

  • Moderate = <75%

    • 2 moderate = 1 large

  • Large = > 75%

Air Trapping

• Will see slightly increased radiotracer uptake in the base of the lungs rather than the apex (note should normally be slightly high inhaled radiotracer in the lung apex)

• This is for the ventilation portion

• Not super obvious

Complete lack of uptake in one lung

• Complete lack of perfusion in one lung

  • DDx

    • Hilar mass

    • Hypoplastic pulmonary artery

    • Mediastinal fibrosis

  • If you see complete lack of perfusion in one lung and normal ventilation that is a cancer eating into the artery

• Complete lack of ventilation in one lung

• NOTE: Foreign body in mainstem bronchus will not cause whole lung mismatch

  • This is because there is hypoxic vasoconstriction leading to decrease in blood flow to the non-ventilated lung so the decrease in perfusion and ventilation is the same

Fissure Sign

• Fluid in fissure causes defect

• No PE present

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