Lung Ventilation Perfusion Scan (VQ Scan) Article - StatPearls

Procedures

The patient is supposed to lie still on the table, breathing normally while the technician takes scans from different angles. The procedure is discussed in detail with the patient, as the patient's cooperation is crucial during the scanning because the patient's movements can blur the images. The duration of the whole scanning process is about 30-45 minutes, though it may take longer. The procedure involves 2 phases, which can be done simultaneously or 1 after another. One phase is ventilation, and the other is perfusion. For ventilation scans, radioactive xenon or technetium is breathed through the nebulizer via mouthpiece for a few minutes. A gamma camera is placed close to the patient, and scans at different angles are taken. Similarly, for the perfusion scan, radioactive contrast-containing technetium is given intravenously to the patient, and images are taken. V/Q scan utilizes the novel pulmonary arterial segmental anatomy as a single end-artery perfuses each segment. Each conical bronchopulmonary segment has its base towards pleural surfaces. Classically, thrombi occlude the pulmonary arteries and create characteristic wedge-shaped lobar, segmental, or subsegment defects based on the level of occlusion. According to the Society of Nuclear Medicine (SNM), before a nuclear medicine study [3], the pre-test probability of pulmonary embolism must be evaluated by using assessment tools (Well criteria, D-dimer test result).[4] A posterior-anterior and lateral chest radiograph must be done 1 hour before the study. However, chest radiographs obtained 24 hours before the V/Q scans are acceptable in patients without worsening signs and symptoms. Different products, including inert gases (81mKr, 133Xe) and radiolabelled aerosols 99mTc-DTPA and 99mTc-labelled ultra-fine dispersion of technetium-labeled carbon, are used in mapping regional ventilation.

1) Radiolabelled Aerosols (for Ventilation Scan)

• 99Tc: 99Tc-DTPA is the most widely used radionuclide with a dosage of 900 to 1,300 MBq (25 to 35 mCi) with a photopeak of 140 keV.[5] 99mTc-Technegas is preferably used in SPECT, especially in COPD patients.[6][7]
• 81mKr: with a dosage of 40 to 400 MBq (1 to 10 mCi) and with a photopeak of 190
• keV.
• 133Xe: with a dosage of 200 to 750 MBq (5 to 20 mCi) and a photopeak of 81 keV.[3]

The pulmonary clearance of 99Tc-DTPA can be used to estimate the alveolar epithelial membrane integrity. The clearance rate would increase in alveolar inflammatory conditions such as allergic or toxic alveolitis (smoking), shortening 99Tc-DTPA half-life.[8]

2) Injectable Radionuclides (for Perfusion Scan)

•  99mTc-MAA: with a dosage of 40 to 150 MBq (1 to 4 mCi).[3]

The procedure is done in 2 phases: ventilation and perfusion imaging. A disposable nebulizer delivers the 99Tc-DTPA aerosol to the lung via a mouthpiece to perform the ventilation image. The patient should be upright or supine with the nose occluded. 133Xe is preferred in assessing patients with obstructive airway disease.[3] To perform the perfusion image, the patient is instructed to cough and to take multiple deep breaths necessary before introducing the IV 99mTc-MAA. The vial containing 99mTc-MAA must be agitated, and the syringe must be inverted on withdrawal. Blood retraction into the syringe should be avoided to prevent imaging artifacts. 99mTc-MAA must be delivered slowly while the patient is breathing at their normal tidal volume and lying supine.

Imaging Protocols: Different imaging protocols are utilized; V/Q imaging with SPECT (V/Q) is a widely accepted and practiced protocol or, in rare situations, planar scintigraphy (V/Q). Sometimes, perfusion-only scanning is performed. Many institutions during the COVID-19 pandemic opted to perform perfusion-only scanning to minimize the dispersion/spread of SARS-CoV-2. V/Q may also be combined with CTPA or computed tomography. A systemic review performed on 23 prospective studies concluded that among 7000 patients in whom D-dimer assessment combined with clinical probability was inconclusive, a normal perfusion scan (Q scan) safely excluded pulmonary embolism.

1. V/Q SPECT Imaging Technique

Single-photon emission computed tomography (SPECT) obtains the image through multidetector gamma-cameras to generate 3-dimensional images. SPECT showed higher sensitivity than the planar technique.[9] If the CTPA contrast and radiation exposure are contraindicated or must be avoided, the V/Q SPECT is considered the second-line diagnostic test.[10][11] Advantages of the V/Q SPECT technique:

• Low indetermination rate and more reproducibility [12][13]
• Greater sensitivity (97%) and specificity (91%) [14]
• New and advanced analytic data processing, such as V/Q ratio qualification [15]

2. V/Q Planar Imaging Technique

Planar imaging acquisition is a 2-dimensional technique obtained through a dual-head gamma camera for ventilation and perfusion scans, respectively. Limited patient movement between the 2 scans is crucial.[9] This technique is used with at least 4 views when the V/Q SPECT is not feasible.[16]

Disadvantages of the V/Q planar technique:

• Two-dimensional images are compared with the advancing 3-dimensional V/Q SPECT.
• Inaccurate determination of lung segmental involvement after the embolic event.[17]
• Inexact determination of the degree of perfusion defects.[18]

3. V/Q SPECT/CT Imaging Technique

This technique integrates a low-dose CT scan with the functional SPECT to provide more detailed anatomic information. The CT image (without contrast) is usually taken after the perfusion scan.[10] Radiation exposure is the main disadvantage.

Advantages of SPECT/CT:

• More reliable detection of V/Q mismatch conditions than PE, such as in obstructive lung disease, external vascular compression, or neoplasm.[16]
• Better V/Q matching information due to non-embolic causes (pneumonia, pleural, or pericardial effusion). 
• Detection of pulmonary embolism cases in which V/Q is unusually matched, like PE, with an area of pulmonary infarction.[16]
• Carries the highest diagnostic accuracy.[19]

4. Combined V/Q SPECT with CTPA

It enables the radiologist to localize the clot site reported in the CT pulmonary angiography (CTPA).[10]