Topic 8: . Nuclear medicine diagnostics of respiratory system: perfusion and ventilation scintigraphy. Radiopharmaceuticals. Scintigraphic diagnosis of pulmonary emboli and DD with other pulmonary diseases. Application of SPECT-CT.
Nuclear medicine imaging of the respiratory system evaluates:
- Pulmonary blood flow (perfusion)
- Pulmonary ventilation (air distribution)
- Ventilation-perfusion relationship (V/Q relationship)
Types of Lung Scintigraphy
There are two main studies:
- 1. Perfusion scintigraphy (Q scan)
- 2. Ventilation scintigraphy (V scan)
These are usually performed together as a:
Ventilation/Perfusion (V/Q) Scan
Normal Physiology
For normal gas exchange, every alveolus requires:
- Adequate ventilation (air)
- Adequate perfusion (blood)
This relationship is called the: Ventilation/Perfusion Ratio (V/Q ratio)
Normal V/Q \approx 0.8
If either ventilation or perfusion is impaired,
↓
Gas exchange decreases.
PERFUSION SCINTIGRAPHY
Perfusion scintigraphy evaluates pulmonary blood flow.
It demonstrates whether blood reaches every part of the lungs.
Principle
After intravenous injection,
the radiopharmaceutical reaches:
Right heart
↓
Pulmonary arteries
↓
Pulmonary capillaries
The particles become temporarily trapped within pulmonary capillaries in proportion to regional blood flow.
Therefore:
Normal blood flow
↓
Normal tracer uptake
Reduced blood flow
↓
Reduced tracer uptake
↓
Cold defect
Radiopharmaceutical
Most commonly used: Tc-99m Macroaggregated Albumin (MAA)
Characteristics:
- Particle size: approximately 10–90 μm
- Trapped temporarily in pulmonary capillaries
- Does not obstruct pulmonary circulation because only a very small fraction of capillaries is occupied
- Cleared gradually by enzymatic breakdown
Procedure
- 1. Patient lies supine.
- 2. Tc-99m MAA injected intravenously.
- 3. Gamma camera images are obtained.
Usually:
- Anterior
- Posterior
- Right lateral
- Left lateral
- Right posterior oblique
- Left posterior oblique
Modern departments often perform SPECT or SPECT/CT.
Normal Perfusion Scan
Shows:
- Uniform tracer uptake
- Symmetrical lungs
- No segmental defects
Abnormal Perfusion Scan
Areas with reduced blood flow show
↓
Reduced tracer uptake
↓
Cold defects
Clinical Applications
- Pulmonary embolism
- Chronic thromboembolic disease
- Preoperative lung function assessment
- Congenital pulmonary vascular abnormalities
Contraindications
Relative contraindications:
- Severe pulmonary hypertension
- Right-to-left cardiac shunt (particle number may need to be reduced)
- Pregnancy (consider alternative imaging depending on clinical scenario)