Continuing Education Activity
A ventilation-perfusion (VQ) scan is a diagnostic test using radioisotopes to evaluate lung perfusion and ventilation. Its most common clinical use is in the workup of suspected pulmonary embolism. This activity reviews the use of a VQ scan in the evaluation of pulmonary embolism and other clinical scenarios.
- Review the indications for a ventilation-perfusion scan.
- Describe the procedure of administering a ventilation-perfusion scan
- Summarize the interpretation of a ventilation-perfusion scan.
- Explain the importance of collaboration and communication amongst the interprofessional team to ensure appropriate testing and improve outcomes in patients undergoing ventilation-perfusion scan.
Ventilation-perfusion scan, also referred to as lung scintigraphy, or commonly V/Q scan, is a diagnostic test utilizing radioisotopes to evaluate pulmonary ventilation and perfusion. Its most common clinical use is as a screening tool for suspected pulmonary embolism.
Lung scintigraphy requires a chest X-ray in PA and lateral view before the test. A portable PA X-ray chest is an alternative only if the patient is unable to tolerate routine X-rays. CT scan is an alternative to a chest X-ray.
The test consists of two parts, namely ventilation scintigraphy and perfusion scintigraphy. Ventilation scintigraphy usually precedes perfusion scintigraphy; ventilation scintigraphy may also be excluded if not required.
Perfusion Scintigraphy (Q)
It involves injecting a radiopharmaceutical dye 99mTc-MMA (10-100 Um particle size) intravenously. The patient lays supine during the injection to allow maximum blood flow to lung apices. The radiopharmaceutical particles then embolize in the capillaries and provide a map of pulmonary blood flow.
Afterward, lung imaging is performed using either planar imaging with a high-resolution gamma camera, SPECT or 3D imaging with SPECT/CT
Ventilation Scintigraphy (V)
This component of the test assesses air distribution in the lungs. Ventilation radiopharmaceuticals classify as gases, aerosolized liquid, and aerosolized solid particles.
- 99mTc-diethylenetriaminepentaacetic acid (DTPA) is the most commonly used (57%) radiopharmaceutical in the form of aerosolized liquid droplets and is delivered via a nebulizer. Its favorable characteristics such as Other radioactive aerosols include 99mTc–sulfur colloid, 99mTc-pyrophosphate, 99mTc–methylene diphosphonate, and 99mTc labeled carbon (Technegas). Technegas has a more uniform distribution and is not available in the United States. 
- 133Xe has a half-life of 5.3 days and is one of the most commonly used radioactive gas. The advantage of Xe is the ability to obtain single-breath, wash-in, and washout images, which makes it more sensitive for obstructive lung disease. 3Disadvantages include longer half-life compared to 81mKr and lower photopeak energy; hence ventilation scintigraphy is done prior to perfusion scintigraphy.
- 81mKr has a shorter half-life of 13s, and higher photon energy levels requiring continuous administration and allowing ventilation images obtained after perfusion images. Both image sets can be matched without repositioning the patient. A disadvantage is increased cost.
The commonest indication is for the screening of pulmonary embolism, other indications include:
- Pre-operative evaluation before lung surgery in lung carcinoma to estimate the loss of lung function
- Pre-operative evaluation and post-operative monitoring in lung transplant patients 
- Measurement of cardiac shunts
- Evaluating patients with new-onset pulmonary hypertension to detect chronic thromboembolic pulmonary hypertension or CTEPH
Normal and Critical Findings
Ventilation imaging is used in conjunction with perfusion imaging to classify perfusion defects as:
1. Matched (ventilation and perfusion imaging are concordant)
2. Mismatched (perfusion defect with a normal or relatively less abnormal ventilation defect)
3. Reverse mismatched (ventilation defect with a normal or relatively less abnormal perfusion defect)
While there are multiple etiologies of ventilation-perfusion mismatch, the most common causes include acute and chronic pulmonary embolism, a tumor obstructing an artery, and radiation therapy.
The ventilation-perfusion reverse mismatch is more often attributed to emphysema, lung cysts and lung infiltrates (pneumonia or cancer)
Diagnosis of Pulmonary Embolism
There are multiple different criteria established for the diagnosis of PE, including but not limited to:
- Modified PIOPED II criteria
- Perfusion-only modified PIOPED criteria
- Perfusion-only PISAPED criteria
Out of these, the modified PIOPED II and PISAPED criteria are more commonly used and will be described further. 
Based on modified PIOPED II criteria, the results are reported as follows: High LR, very low LR, non-diagnostic, and normal. Perfusion-only PISAPED criteria utilize perfusion imaging only and classify results as follows: PE present, PE absent, non-diagnostic. The modified PIOPED II criteria are reported according to the following findings:
- If two or more large mismatched segmental defects are present
Very low probability:
- Non-segmental perfusion defect
- Perfusion defect smaller than X-ray chest lesion
- 1-3 small segmental defects
- One or less single matched perfusion defect in the mid or upper lung
- Solitary large pleural effusion
- Stripe sign, i.e., peripheral perfusion in a perfusion defect
In comparison, PISAPED criteria are classified based on the following scan findings:
- One or more large mismatched wedge-shaped perfusion defects
- Normal or near-normal perfusion
- Non-wedge shaped perfusion defect
- Contour defect caused by the mediastinum, diaphragm, or an enlarged heart
- All other findings not classified as PE present or PE absent
Pre and Post Lung Transplant
During a pre-transplant evaluation, the perfusion defects are used for a quantified perfusion analysis which detects the more dysfunctional lung
During post-transplant monitoring, V/Q scan can monitor complications such as diagnosing VTE, assessing the functional impact of bronchial stenosis, a common complication. V/Q scan may also be used to assess air trapping that may indicate early bronchiolitis obliterans seen in rejection.
While injecting tracer intravenously, blood drawn into the syringe may become coagulated and result in hot spots. If chest X-ray shows significant abnormalities such as dense consolidation, it can result in a matching ventilation defect, perfusion defect along with an X-ray abnormality referred to as a "triple match." A triple match can result in indeterminate or non-diagnostic test results.
VQ scan is generally well tolerated. Complications or adverse effects are rare and include:
- Allergic reaction to the tracer
- Injection site reaction
Patient Safety and Education
Patients do not need to prepare before the test. A chest X-ray will be necessary within 24 hours before the test. The test involves exposure to radiation, and the total amount of exposure is relatively low. The patient will be instructed to inhale a radioactive gas in the first part and have a radioactive material injected into a vein in the second part.
Patients will need to lay still in the scanner where images are taken. The test takes about an hour to complete. The test is safely tolerated in most patients; some patients may develop redness and swelling at the injection site. Allergic reaction to the radioactive material is a rare complication that is treatable. Breastfeeding women should stop breastfeeding 1 to 2 days after the test, and to discard pumped breast milk 1 to 2 days after the test.
Using the PIOPED II criteria, the sensitivity and specificity for diagnosing PE is 85% and 93%, respectively, while using the PISAPED criteria, it is 80% and 97% and can be further improved to 97% and 91% with SPECT imaging. This is comparable to CT pulmonary angiography (CTPA), which has a sensitivity and specificity of 83% and 96%, respectively. The VQ scan is currently preferred in diagnosing PE in patients who can not receive CTPA due to renal dysfunction or iodinated contrast allergy, in pregnant and very young patients, or an outpatient setting when the probability of PE is low.