11.1 Pulmonary Ventilation & Perfusion (V/Q) Imaging

Pulmonary Ventilation & Perfusion (V/Q) Imaging

Pulmonary imaging in nuclear medicine primarily consists of ventilation and perfusion (V/Q) scintigraphy, a critical diagnostic tool used predominantly to evaluate patients for suspected pulmonary embolism (PE). This dual-phase study provides a physiological map of both airflow (ventilation) and blood flow (perfusion) within the lungs. By comparing these two distinct maps, nuclear medicine physicians can identify areas of mismatch—regions where the lung is adequately ventilated but poorly perfused—which is the classic hallmark of a pulmonary embolus.

Ventilation Imaging Radiopharmaceuticals

The ventilation phase of the study is designed to visualize the flow of air into and out of the pulmonary airways, reaching down to the alveolar level. There are two primary agents used for this purpose: Xenon-133 (Xe-133) gas and Technetium-99m (Tc-99m) DTPA radioaerosol.

Xenon-133 Gas

Xenon-133 is an inert noble gas with a physical half-life of 5.27 days and a primary gamma emission of 81 keV. Because of its low energy, Xe-133 ventilation imaging must be performed before the Tc-99m perfusion scan to avoid downscatter interference from the higher energy 140 keV photons of Tc-99m.

The standard administered activity for Xe-133 is typically 10 to 20 mCi (370-740 MBq). The procedure is uniquely dynamic and consists of three distinct phases:

1. Wash-in phase: The patient takes a single deep breath of the radioactive gas and holds it, allowing for the initial visualization of lung capacity and major airway patency.
2. Equilibrium phase: The patient breathes a mixture of Xe-133 and oxygen in a closed system for 3 to 5 minutes. This phase ensures that the gas reaches the most distal alveoli and demonstrates the total ventilated volume of the lungs.
3. Wash-out phase: The patient breathes room air and exhales the radioactive gas into a specialized trapping system (usually containing charcoal filters). This is arguably the most sensitive phase for detecting obstructive airway diseases like chronic obstructive pulmonary disease (COPD) or asthma, as affected regions will demonstrate delayed clearance (air trapping) of the radiogas.

Tc-99m DTPA Radioaerosol

An alternative to radioactive gas is the use of a radioaerosol, most commonly Tc-99m Diethylenetriaminepentaacetic acid (DTPA). To prepare this, 25 to 35 mCi of Tc-99m DTPA is placed in a nebulizer, which uses compressed air or oxygen to generate fine droplets. The patient inhales this aerosol for several minutes. Unlike Xe-133, which diffuses freely and washes out, the aerosol particles deposit on the bronchial and alveolar walls and remain in place for a sufficient time to allow for static imaging in multiple projections.

Because Tc-99m has the same 140 keV energy as the perfusion agent, the activity deposited in the lungs must be carefully controlled—usually around 0.5 to 1.0 mCi actually reaches the patient's lungs. This allows the subsequent perfusion scan (using a much higher activity of 4-6 mCi) to "overwhelm" the ventilation signal.

Perfusion Imaging with Tc-99m MAA

The perfusion phase evaluates the pulmonary arterial blood flow. The radiopharmaceutical used is Technetium-99m Macroaggregated Albumin (Tc-99m MAA). The mechanism of localization for Tc-99m MAA is capillary blockade.

Mechanism and Particle Size

When Tc-99m MAA is injected intravenously, the particles travel through the right side of the heart and into the pulmonary artery, where they become physically trapped in the first capillary bed they encounter—the pulmonary precapillary arterioles and capillaries. Because the distribution of particles is directly proportional to regional pulmonary blood flow, areas with compromised flow (e.g., due to a clot) will show absent or reduced radioactivity (a "cold" defect).

For the capillary blockade to be safe and effective, the physical size of the MAA particles must be tightly controlled. The optimal particle size is between 10 and 90 micrometers (µm), with the vast majority falling in the 10-90 µm range. Particles smaller than 10 µm may pass through the pulmonary capillaries and localize in the liver and spleen, degrading image quality and increasing non-target radiation dose. Particles larger than 150 µm pose a risk of blocking larger arterioles, potentially causing significant hemodynamic compromise.

Particle Count Considerations

The standard administered particle count for a healthy adult is between 200,000 and 700,000 particles. This provides a statistically uniform distribution without compromising a significant percentage of the pulmonary capillary bed (typically occluding less than 0.1% of the estimated 280 billion pulmonary capillaries).

However, there are critical clinical scenarios where the particle count must be significantly reduced (typically to 100,000 - 200,000 particles) to prevent life-threatening complications. These situations include:

• Pulmonary Hypertension: Patients with elevated pulmonary pressures already have a compromised vascular bed; further occlusion could trigger acute right heart failure.
• Right-to-Left Shunts: In conditions like a patent foramen ovale (PFO) or ventricular septal defect (VSD), MAA particles bypass the lungs and enter the systemic circulation, potentially lodging in the brain or kidneys. Reducing the particle count minimizes the risk of microembolic events in these critical organs.
• Pneumonectomy: Patients with only one lung have half the capillary bed, requiring a proportional reduction in particles.
• Pediatric Patients: Particle counts must be scaled down appropriately based on age and weight.

It is important to note that while the particle count is reduced in these situations, the radioactivity (in mCi) remains the same to ensure adequate image quality. This is achieved by manipulating the reconstitution volume and timing of the MAA kit.

Diagnostic Criteria: V/Q Mismatch

The diagnosis of Pulmonary Embolism relies on the comparison of the ventilation and perfusion images.

• V/Q Match: If a region shows both decreased ventilation and decreased perfusion, it is typically indicative of parenchymal disease, such as COPD, pneumonia, or atelectasis.
• V/Q Mismatch: The hallmark of a PE is a "mismatch"—a region where ventilation is normal (air is getting in), but perfusion is absent (blood flow is blocked by an embolus).

The PIOPED (Prospective Investigation of Pulmonary Embolism Diagnosis) criteria established the standards for interpreting these scans, categorizing the probability of PE into high, intermediate, low, or normal based on the size and number of mismatched defects.