A VQ scan, also known as a ventilation-perfusion scan, is a diagnostic imaging test that assesses lung function. This non-invasive procedure evaluates how air moves into and out of your lungs (ventilation) and how blood flows through your lungs (perfusion). By measuring these two aspects, the scan provides images that help diagnose various lung conditions.
Why a VQ Scan is Performed
A VQ scan is frequently ordered to diagnose a pulmonary embolism (PE), which is a blood clot in the lungs. This condition can be life-threatening if not addressed promptly. Symptoms such as sudden shortness of breath, rapid breathing, or chest pain that worsens with coughing or deep breaths often prompt a VQ scan.
The scan is particularly useful when other imaging methods, like CT pulmonary angiography (CTPA), are not suitable. For instance, a VQ scan may be preferred for individuals with kidney dysfunction, a severe allergy to the contrast dye used in CT scans, or those who are pregnant. Beyond diagnosing blood clots, a VQ scan can also evaluate lung function before surgical procedures, such as lung transplants or the removal of part of a lung. It can also assess conditions like chronic obstructive pulmonary disease (COPD) or other lung disorders.
How a VQ Scan Works
A VQ scan involves two distinct parts: the ventilation (V) scan and the perfusion (Q) scan, typically performed sequentially on the same day. During the ventilation scan, you will wear a mask or use a mouthpiece and breathe in a small amount of a harmless radioactive gas or aerosol, such as Technetium-99m DTPA. This radioactive tracer mixes with the air in your lungs, allowing a specialized camera, known as a gamma camera, to capture images of how air is distributed throughout your lungs.
Immediately following the ventilation scan, the perfusion scan is performed. For this part, a different radioactive tracer, such as Technetium-99m macroaggregated albumin, is injected into a vein in your arm. This tracer travels through your bloodstream to the small blood vessels in your lungs, allowing the gamma camera to image the blood flow. The entire imaging process takes between 30 to 60 minutes, with each part lasting about 15 minutes. The two sets of images are then compared to identify any areas where airflow and blood flow do not match, which can indicate blockages or other lung abnormalities.
Preparing for Your Scan and Understanding the Results
Preparing for a VQ scan is straightforward, with no special dietary restrictions or fasting required. Wear loose-fitting clothing without metal fasteners, or you may be asked to change into a gown. Inform your healthcare provider about any allergies, especially to contrast dyes or latex, and disclose if you are pregnant or breastfeeding. A recent chest X-ray, within 24 to 48 hours before the VQ scan, is often requested to provide additional information about your lungs.
After the scan, the radioactive tracers will naturally leave your body within a few hours to days, primarily through urine and stool. Results are interpreted by comparing the ventilation and perfusion images to identify any mismatches. A normal scan indicates proper airflow and blood flow. A “mismatch,” where an area is ventilated but not perfused, strongly suggests a pulmonary embolism. Results are categorized as normal, low probability, intermediate probability, or high probability of a pulmonary embolism. Your healthcare provider will discuss the results with you within a few days of the scan.
Safety and Important Considerations
VQ scans are considered safe, with low radiation exposure comparable to other common medical imaging tests. Allergic reactions to the tracers are rare and mild.
While there are no absolute contraindications, certain situations require careful consideration. If you are pregnant, your doctor may discuss alternative imaging options or adjust the radioactive dose to minimize fetal exposure, though VQ scans can still be performed with a reduced dose. If you are breastfeeding, you may be advised to temporarily stop for about 24 hours after the scan to prevent exposing your baby to radioactivity in breast milk. Severe lung conditions or the inability to remain still for the duration of the scan may also affect the procedure or its interpretation.