Nuclear: Pulmonary Flashcards
What are the usual reasons a V/Q scan is done instead of a CTPA?
- Contrast allergy
- Very low GFR
What radiotracer is used for the perfusion portion of a V/Q scan?
Tc-99m MAA (macro aggregated albumin)
Note: This is made by heat denaturation of human serum albumin and selecting a particular particle size to keep the particles trapped in pulmonary capillaries (MAA does not enter systemic circulation until it breaks down).
What is the biologic half life of Tc-99m MAA?
~4 hours
What radio tracers are used for the ventilation portion of a V/Q scan?
- Xenon 133 (radioactive gas)
- Tc-99m DTPA (radioactive aerosol)
Which portion of the V/Q scan needs to be performed first?
Ventilation
Note: This is because the radio tracers used for ventilation (Xe/DTPA) have lower energy and would be drowned out by the perfusion radiotracer (MAA).
What is the half life of Xenon-133?
- 30 seconds (biological half-life because you breath it out)
- 5.3 days (physical half life)
What are the 3 phases of the ventilation portion of a V/Q scan?
- Wash in (single maximum inspiration breath hold)
- Equilibrium (normal breathing with room air/xenon mix)
- Wash out (breathing normal air)
What radiotracers are used for a quantitative perfusion study prior to lung resection?
Xenon-133 and Tc-99m MAA
Note: You can not use Tc-99m DTPA for the ventilation portion of a quantitative perfusion scan because it will interfere with the Tc from the MAA.
V/Q scan
Right-to-left shunt (e.g. ASV, VSD, pulmonary AVM)
Note: Uptake in brain, kidneys, spleen, etc. indicated that radiotracer made it into systemic circulation via a shunt.
How big should Tc-99m MAA particles be?
10-100 micrometers
Note: If smaller than 10, the MAA won’t be trapped in pulmonary circulation and will escape to systemic circulation. If larger than 100, the particles may block arterioles.
Why might you reduce the number of Tc-99m MAA particles you use for a V/Q scan?
- Pt has fewer than normal pulmonary capillaries (e.g. pt is a child or only has 1 lung; you don’t want to block more than 0.1% of pulmonary capillaries)
- Known right-to-left shunt (to minimize blocking capillaries in the brain)
- Pulmonary hypertension
- Pregnant pt
Note: The Tc dose will be the same, just added to fewer particles.
V/Q scan
Right-to-left shunt
V/Q scan
Clumped MAA (due to inadvertent drawing of blood into the MAA syringe before injection)
V/Q scan
Normal perfusion
Prominence of the pulmonary fissures (“fissure sign”):
- Pleural effusion
- Pleural scarring/thickening
- COPD
V/Q scan
Normal ventilation study
V/Q scan
Xenon-133 uptake in the liver, suggestive of hepatic steatosis (xenon is fat soluble)
Which radiotracer is this?
Tc-99m DTPA
Note: V/Q scan with clumping in the mouth, central airways, and stomach (due to swallowing/aspirating DTPA in saliva).
How can you tell whether Xenon or DTPA was used for the ventilation portion of a V/Q scan?
DTPA will have multiple projections (xenon has a very fast washout so there is usually only time for 1 or 2 projections)
DTPA may show “clumping” in the mouth, central airways, and stomach (due to swallowed/aspirated DTPA in saliva)
You see thyroid and/or stomach on a VQ scan…
- Free technetium contamination
- Right-to-left shunt (will also have uptake in the brain)
How should you alter a V/Q scan protocol if the pt has a known right-to-left shunt?
Decrease the number of MAA particles given to the pt (from 500,000 to about 100,000)
Note: This is to minimize MAA blocking capillaries in the brain.
How should you alter a V/Q scan protocol if the pt has pulmonary hypertension?
Decrease the number of MAA particles given to the pt
How should you alter a V/Q scan protocol if the pt is a neonate?
Decrease the number of MAA particles given to the pt (from 500,000 to about 10,000-50,000)
Note: You don’t want to block more than 0.1% of pulmonary capillaries.
Suspicious for central obstructing mass (cancer, fibrosis mediastinitis, central PE), recommend a chest CT
Note: Unilateral whole lung perfusion defect with normal ventilation.
What is the grade for this V/Q scan?
Low probability for PE
Note: But recommend CT chest to look for a central obstructing mass (e.g. bronchogenic carcinoma, central PE, fibrosis mediastinitis).
How does gallium-67 localize?
It acts as an iron analog: gallium binds to lactoferrin and concentrated in areas of inflammation, infection, and rapid cell division
Note: Gallium can bind to neutrophil membranes even when those cells are dead (giving gallium some advantages over In-111 WBCs in identifying chronic infections).
How is gallium-67 produced?
In a cyclotron, by the bombardment of Zinc-68
What is the half-life of gallium-67?
3 days
What are the photopeaks for gallium-67?
- 93 keV (40%)
- 184 keV (20%)
- 300 keV (17%)
- 393 keV (5%)
Why are gallium scans usually done after 24 hours post injection?
Before 24 hours, there is usually to much background noise
What is the critical organ for gallium-67?
Colon
What is the normal distribution for gallium-67?
- Liver (highest uptake)
- Bone marrow (“poor mans bone scan”)
- Spleen
- Salivary/lacrimal glands
What scan is this?
Gallium-67
Note: Bone uptake AND liver > spleen.
Is faint breast uptake normal for gallium-67?
Yes, especially if pregnant or lactating
Is faint lung uptake normal for gallium-67?
Usually no
Note: Only normal during the first 24 hours after injection (most pts are imaged more than 24 hours after injection).
Is bowel uptake normal for gallium-67?
Yes, after 24 hours
Is bone uptake normal for gallium-67?
Yes (uptake in both the cortex and marrow)
Note: “Poor mans bone scan.”
Differential for lung uptake on a gallium-67 scan
- Infection (i.e. pneumonia)
- Atelectasis
- ARDS
- Heart failure
Why are gallium-67 scans used to evaluate for sarcoidosis?
- Helps identify active disease (scans are negative in inactive disease)
- Helps guide biopsy/lavage (if looking to prove diagnosis)
Sarcoidosis
Note: “Lambda sign.”
Gallium-67
Sarcoidosis or Sjogrens
Note: “Panda sign” can also be seen in treated lymphoma.
Gallium-67 scan in pt on chemotherapy
Pulmonary fibrosis (drug reaction to bleomycin)
Gallium-67 scan in pt taking amiodarone
Amiodarone pneumonitis
Note: Lung uptake of gallium-67.
Gallium-67 scan in pt with HIV
Think PCP (pneumocystis jirovecii pneumonia)
Note: Diffuse bilaterala pulmonary uptake.
Thallium (C) and gallium (E) scans in a pt with HIV
Kaposi sarcoma
Note: Thallium positive and gallium negative.
Gallium-67
Pneumonia
Note: Focal pulmonary uptake without parotid or nodal uptake to suggest sarcoidosis.
Malignant otitis externa
IV drug user
Think spinal osteomyelitis
