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Ventilation/perfusion scan

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Title: Ventilation/perfusion scan  
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Subject: CT pulmonary angiogram, Respiratory physiology, Medical imaging, Ventilation/perfusion ratio, Lung transplantation
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Ventilation/perfusion scan

Ventilation/perfusion scan
Intervention
Normal pulmonary ventilation and perfusion (V/Q) scan. The nuclear medicine V/Q scan is useful in the evaluation of pulmonary embolism.
OPS-301 code 3-703.2

A ventilation/perfusion lung scan, also called a V/Q lung scan, 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] 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.

Contents

  • Uses 1
  • Procedure 2
  • Significance of results 3
  • Risks 4
  • See also 5
  • References 6

Uses

This test is most commonly done in order to check for the presence of a blood clot or abnormal blood flow inside the lungs (such as a pulmonary embolism or PE), although computed tomography with radiocontrast is now more commonly used for this purpose.The V/Q scan may be used in some circumstances where radiocontrast would be inappropriate, as in renal failure.[2]

A V/Q lung scan may be performed in the case of serious lung disorders such as Chronic obstructive pulmonary disease (COPD) or pneumonia as well as a lung performance quantification tool pre- and post-lung lobectomy surgery.

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.

In the ventilation phase of the test, a gaseous radionuclide such as xenon or technetium DTPA in an aerosol form is inhaled by the patient through a mouthpiece or mask that covers the nose and mouth. Ventilation imaging can also be performed using a Technegas machine which produces technetium labelled carbon nanoparticles, called Technegas. 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. 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.[3] 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.

Significance of results

Ventilation-perfusion scintigraphy in a woman taking oral contraceptives and valdecoxib with a pulmonary embolism. (A) After inhalation of 20.1 mCi of Xenon-133 gas, scintigraphic images were obtained in the posterior projection, showing uniform ventilation to lungs. (B) After intravenous injection of 4.1 mCi of Technetium-99m-labeled macroaggregated albumin, scintigraphic images were obtained, shown here in the posterior projection. This and other views showed decreased activity in the following regions: apical segment of right upper lobe, anterior segment of right upper lobe, superior segment of right lower lobe, posterior basal segment of right lower lobe, anteromedial basal segment of left lower lobe, and lateral basal segment of left lower lobe.
V/Q Scan Interpretation
Result Interpretation Significance
Normal No perfusion deficit Excludes pulmonary thromboembolism
Low probability 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.

Risks

Although this test uses radioactive materials, the total amount of radiation exposure is low. In order to decrease the radiation exposure in pregnant patients, the total radioactive dose may be decreased or the ventilation phase omitted. Computed tomography with radiocontrast can alternatively be performed, although this can result in a greater radiation dose to the patient.[4] If breastfeeding, patient must be counselled to refrain from this activity for approximately 24 hours.

See also

References

  1. ^ http://www.umm.edu/ency/article/003828.htm
  2. ^ Nigel Key; Michael Makris; Denise O'Shaughnessy; David Lillicrap (3 July 2009). Practical Hemostasis and Thrombosis. John Wiley and Sons. pp. 140–.  
  3. ^ Hofman, M. S.; Beauregard, J. -M.; Barber, T. W.; Neels, O. C.; Eu, P.; Hicks, R. J. (2011). "68Ga PET/CT Ventilation-Perfusion Imaging for Pulmonary Embolism: A Pilot Study with Comparison to Conventional Scintigraphy". Journal of Nuclear Medicine 52 (10): 1513–1519.  
  4. ^ http://www.medscape.com/viewarticle/706957
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