11.1 Pulmonary Imaging Procedures
Key Takeaways
- Lung V/Q pairs ventilation (Xe-133 gas or Tc-99m DTPA aerosol) with perfusion (Tc-99m MAA) to evaluate PE and quantify differential function
- MAA traps mechanically in pulmonary arterioles; keep adult particle counts roughly 200,000–700,000 and reduce particles when the vascular bed is limited or right-to-left shunt is suspected
- Right-to-left shunt allows MAA to reach systemic organs (brain, kidneys)—reduce particles, obtain delayed whole-body views if ordered, and never ignore shunt risk on the requisition
- PIOPED-era teaching still drives tech-level PE protocols: match mismatched perfusion defects to ventilation and chest radiograph findings
- Planar V/Q remains common; SPECT and SPECT/CT improve localization and reduce indeterminate studies when available and ordered
11.1 Pulmonary Imaging Procedures
Quick Answer: Ventilation = Xe-133 gas or Tc-99m DTPA aerosol. Perfusion = Tc-99m MAA capillary blockade. Compare V and Q (planar ± SPECT/SPECT-CT) for PE and quantitation. Control particle number; reduce particles / caution in R-to-L shunt and reduced pulmonary vascular bed.
Domain V pulmonary items test agent choice, sequence, particle safety, and tech-level PE logic—not full radiologist PIOPED scoring fluency.
Clinical Indications
| Indication | Why nuclear medicine |
|---|---|
| Suspected PE (often when CTPA contrast is limited) | Mismatched V/Q pattern supports PE |
| Baseline / pre-op lung quantitation | Relative right vs left (and lobar) function before resection |
| Chronic thromboembolic disease follow-up | Regional perfusion defects over time |
| Right-to-left shunt evaluation (selected) | Systemic MAA foci after perfusion dose |
Correlate with chest radiograph (or CT) from the same clinical episode—interpretation frameworks assume a recent CXR.
Ventilation: Two Main Approaches
Tc-99m DTPA Radioaerosol
Tc-99m DTPA is nebulized into a fine aerosol the patient inhales through a closed system with a mouthpiece and nose clips (or mask). Particles deposit on airway and alveolar surfaces roughly proportional to regional ventilation. After adequate counts are collected, the patient is imaged in the same projections later used for perfusion.
| Item | Teaching values / notes |
|---|---|
| Typical activity loaded | Often ~25–40 mCi in the nebulizer (much less reaches the lungs) |
| Lung deposited activity | Roughly on the order of ~1 mCi class (protocol-dependent) |
| Energy | 140 keV Tc-99m window |
| Strength | Same camera settings as perfusion; multi-view planar easy |
| Weakness | Central airway hotspotting if turbulent flow, COPD, or poor technique; clumping |
Tech tips: coach slow deep breathing; keep nebulizer upright and tubing free of kinks; wipe face contamination before imaging; document poor cooperation. Heavy central deposition is technical/pathophysiologic—not a PE map by itself.
Xe-133 Gas Ventilation
Xe-133 is an inert noble gas (soft gamma ~81 keV; physical t½ ≈ 5.2 days). Classic phases:
- Single-breath / washin — first inspiration or early washin maps ventilating lung.
- Equilibrium — rebreathing mixes gas throughout communicating airspaces.
- Washout — room air clearance; trapping (retained activity) suggests obstructive disease.
| Item | Teaching notes |
|---|---|
| Activity | Often ~5–20 mCi inhaled (lab-specific) |
| Room control | Negative pressure room preferred; charcoal trap on exhaust |
| Order relative to MAA | Often ventilation before perfusion so 81 keV is not swamped by Tc-99m downscatter—follow lab SOP |
| Trap | Saturated charcoal trap → room contamination |
Xe washout trapping is a ventilation obstruction clue; do not call it PE alone.
Perfusion: Tc-99m MAA
Tc-99m macroaggregated albumin (MAA) particles (typically ~10–90 µm) lodge temporarily in pulmonary arterioles—mechanical microembolization of a tiny fraction of the bed. Regional counts map regional pulmonary arterial blood flow at injection.
| Parameter | Typical adult teaching range |
|---|---|
| Activity | ~1–5 mCi (37–185 MBq) IV |
| Particle number | Commonly keep roughly 200,000–700,000 particles per dose |
| Injection | Supine preferred for even distribution; gentle agitation—do not draw blood into syringe and clot particles; inject slowly without filters that strip particles |
| Imaging | Anterior, posterior, laterals, and both posterior obliques (standard six- or eight-view sets per protocol) |
Particle Number Limits and Contraindications
Too many particles block an excessive fraction of the vascular bed. Reduce particle number (not necessarily activity alone—adjust concentration/volume per pharmacy) when:
- Known or suspected pulmonary hypertension / severely reduced vascular bed
- Pneumonectomy or major lung resection
- Known or suspected right-to-left shunt (particles bypass lungs → brain, kidneys, thyroid systemic foci)
- Some pediatric protocols (weight-based particles and activity)
Right-to-left shunt caution: MAA is relatively contraindicated or requires markedly reduced particles and careful medical direction. If shunt imaging is intentional, whole-body or brain/kidney views may be ordered after injection. Free Tc or poor MAA QC is a different problem (thyroid/stomach)—know both differentials.
Pregnancy / breastfeeding: follow institutional policy; perfusion-only or reduced-dose protocols may be used when PE must be evaluated.
Planar vs SPECT / SPECT-CT
Planar V/Q remains the workhorse for many PE protocols and is fully testable. SPECT (and SPECT/CT) improves contrast for segmental defects and anatomic localization; CT portion (low-dose) helps exclude alternative findings and register anatomy. Tech responsibilities: identical positioning between V and Q when possible, motion control, and correct SPECT orbit/matrix per SOP.
V/Q Quantitation
For surgical planning, draw lung ROIs (often geometric mean of anterior and posterior) to report percent contribution of each lung—and sometimes zones/lobes—to total counts. Use perfusion images (and ventilation if ordered) consistently; exclude hot artifacts and ensure no rotation error between projections.
PE Protocols and PIOPED Concepts (Tech Level)
PIOPED (and later refinements) taught probability language based on defect size, number, and V/Q relationship relative to chest x-ray:
| Pattern concept | Meaning for PE teaching |
|---|---|
| Mismatched perfusion defect (Q defect with normal V, clear CXR) | Classic support for PE |
| Matched V and Q defect | Often airway disease / non-PE |
| Triple match (V, Q, and CXR opacity same region) | Intermediate / less specific—needs physician criteria |
| Normal perfusion | Very low likelihood of clinically significant PE |
Tech role: acquire complete, labeled views; ensure ventilation and perfusion are comparable; provide CXR availability; flag motion, aerosol hotspots, and injection issues. Do not independently assign final PIOPED category on the exam unless the stem asks a pure pattern definition.
Sequence Cheat Sheet
| Step | Action |
|---|---|
| 1 | Verify indication, pregnancy status, prior lung surgery, shunt risk, CXR |
| 2 | Perform ventilation (Xe or aerosol) per lab order |
| 3 | Inject MAA with correct particle count; image perfusion |
| 4 | Add SPECT/SPECT-CT if ordered |
| 5 | Quantitate if ordered; complete worksheets |
Memorize MAA = particles + shunt caution, aerosol vs Xe strengths, and mismatch = PE teaching pattern.
A patient referred for lung perfusion scintigraphy has a known right-to-left cardiac shunt. Which technical adjustment is most appropriate?
Which statement best describes the localization mechanism of Tc-99m MAA in the lungs?
In classic PIOPED-style teaching, which scintigraphic pattern most strongly supports pulmonary embolism?