A ventilation/perfusion lung scan, also called a V/Q lung scan, or ventilation/perfusion scintigraphy, is a type of
medical imaging using
scintigraphy and
medical isotopes to evaluate the circulation of air and blood within a patient's
lungs,[1][2] in order to determine the
ventilation/perfusion ratio. The
ventilation part of the test looks at the ability of air to reach all parts of the lungs, while the
perfusion part evaluates how well blood circulates within the lungs. As Q in physiology is the letter used to describe bloodflow the term V/Q scan emerged.
Perfusion deficit with matched ventilation deficit
< 20% probability of PE
Intermediate probability
Perfusion deficit that corresponds to parenchymal abnormality on chest x-ray
20% - 80% probability of PE
High probability
Multiple segmental perfusion deficits with normal ventilation
> 80% probability of PE
Decreased uptake of the inhaled radioisotope may indicate an impaired ability to breathe, airway obstruction, or possible pneumonia.
Decreased circulation of the injected MAA indicates a problem with blood flow into or within the lungs. A localized area of decreased uptake, usually in a wedge shaped (or pie shaped) configuration with normal ventilation images (mismatched defect) suggests a
pulmonary embolus or blood clot in the lungs, which leads to reduced perfusion beyond the obstruction.
Risks
Although this test uses radioactive materials, the total amount of radiation exposure is low. Typical
effective doses for a V/Q scan range from 1-5
mSv.[7][8] In order to decrease the radiation exposure in pregnant patients, the total radioactivity administered may be decreased or the ventilation phase omitted. A
CT pulmonary angiogram (CTPA) with
radiocontrast can alternatively be performed, although this can result in a greater radiation dose to the patient.[9]
Procedure
The ventilation and perfusion phases of a V/Q lung scan are performed together and may include a
chest X-ray for comparison or to look for other causes of lung disease. A defect in the perfusion images requires a mismatched ventilation defect to indicate pulmonary embolism.[8]
In the ventilation phase of the test, a gaseous radionuclides such as
xenon-133,
krypton-81m, or
technetium-99mDTPA in an aerosol form is inhaled by the patient through a mouthpiece or mask that covers the nose and mouth.[10] Ventilation imaging can also be performed using a Technegas machine which produces an aerosol of
radioactive nanoparticles, specifically carbon
nanoparticles containing
technetium-99m.[11] The perfusion phase of the test involves the intravenous injection of radioactive
technetium macro aggregated albumin (Tc99m-MAA). A
gamma camera acquires the images for both phases of the study.[12] A SPECT image can also be taken following an injection of Technetium labelled MAA. SPECT is often skipped if the patient has
pulmonary hypertension.
It is also possible to perform the scan with
positron emission tomography (PET) rather than conventional gamma camera scintigraphy.[13] This has been performed with Gallium-68 labelled carbon nanoparticles (Galligas) using a conventional Technegas machine for ventilation images, and with Gallium-68 labelled MAA (Ga68-MAA) for perfusion images. PET has multiple potential advantages including superior resolution, speed and quantification.[14][15]
^Lobov, Sergei A.; King, David W.; Knox, Karen J.; Senden, Tim J.; Stephens, Ross W. (February 2013). "Cationised radiolabelled nanoparticles for perfusion imaging of the lungs". Biomaterials. 34 (6): 1732–1738.
doi:
10.1016/j.biomaterials.2012.11.022.
PMID23218595.
^Bailey, Dale L.; Eslick, Enid M.; Schembri, Geoffrey P.; Roach, Paul J. (September 2016). "68Ga PET Ventilation and Perfusion Lung Imaging—Current Status and Future Challenges". Seminars in Nuclear Medicine. 46 (5): 428–435.
doi:
10.1053/j.semnuclmed.2016.04.007.
PMID27553468.