V/Q scan

Ventilation/perfusion scan (V/Q scan) is a medical imaging procedure used to diagnose pulmonary embolism (PE), a condition where one or more arteries in the lungs become blocked by a blood clot. The test is a type of nuclear medicine scan that uses radioactive materials to examine airflow (ventilation) and blood flow (perfusion) in the lungs. The V/Q scan is particularly useful in patients who might have an adverse reaction to the contrast material used in a CT scan, another common diagnostic tool for PE.

Procedure

The V/Q scan consists of two main parts: the ventilation scan and the perfusion scan. During the ventilation scan, the patient inhales a radioactive gas or aerosol, which allows imaging of the distribution of air flow in the lungs. For the perfusion scan, a radioactive tracer is injected into a vein, and images are taken to show how blood flows through the lungs. The results of these scans are then compared to identify any mismatches between ventilation and perfusion, which are indicative of pulmonary embolism.

Indications

The primary indication for a V/Q scan is the suspicion of a pulmonary embolism, especially in patients who cannot undergo contrast-enhanced CT scanning due to allergies to contrast material, kidney failure, or pregnancy. It is also used in certain cases to assess the severity and extent of chronic obstructive pulmonary disease (COPD) and other types of lung diseases.

Interpretation

The interpretation of a V/Q scan involves identifying areas of the lungs that are well ventilated but poorly perfused, known as a ventilation-perfusion mismatch. A significant mismatch is highly suggestive of a pulmonary embolism. The results of a V/Q scan are typically reported as normal, low probability, intermediate probability, or high probability of pulmonary embolism, based on the presence and pattern of mismatches.

Advantages and Limitations

One of the main advantages of a V/Q scan is its ability to be performed on patients who have contraindications to iodinated contrast, which is used in CT scans. Additionally, it exposes patients to less radiation compared to a CT pulmonary angiogram (CTPA). However, the V/Q scan has limitations, including lower sensitivity and specificity compared to CTPA, and its interpretation can be affected by pre-existing lung diseases, which may also cause ventilation-perfusion mismatches.

Safety

The radioactive materials used in a V/Q scan are considered safe, and the amount of radiation exposure is low. However, as with any procedure involving radiation, it is not typically recommended for pregnant women unless absolutely necessary.