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.
The commonest indication is for the screening of pulmonary embolism, other indications include:
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:
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:
Very low probability:
In comparison, PISAPED criteria are classified based on the following scan findings:
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:
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.
|||Parker JA,Coleman RE,Grady E,Royal HD,Siegel BA,Stabin MG,Sostman HD,Hilson AJ, SNM practice guideline for lung scintigraphy 4.0. Journal of nuclear medicine technology. 2012 Mar; [PubMed PMID: 22282651]|
|||Metter D,Tulchinsky M,Freeman LM, Current Status of Ventilation-Perfusion Scintigraphy for Suspected Pulmonary Embolism. AJR. American journal of roentgenology. 2017 Mar; [PubMed PMID: 28095020]|
|||Pinho DF,Banga A,Torres F,Mathews D, Ventilation perfusion pulmonary scintigraphy in the evaluation of pre-and post-lung transplant patients. Transplantation reviews (Orlando, Fla.). 2019 Apr; [PubMed PMID: 30415913]|
|||Moore AJE,Wachsmann J,Chamarthy MR,Panjikaran L,Tanabe Y,Rajiah P, Imaging of acute pulmonary embolism: an update. Cardiovascular diagnosis and therapy. 2018 Jun; [PubMed PMID: 30057872]|
|||Waxman AD,Bajc M,Brown M,Fahey FH,Freeman LM,Haramati LB,Julien P,Le Gal G,Neilly B,Rabin J,Soudry G,Tapson V,Torbati S,Kauffman J,Ahuja S,Donohoe K, Appropriate Use Criteria for Ventilation-Perfusion Imaging in Pulmonary Embolism: Summary and Excerpts. Journal of nuclear medicine : official publication, Society of Nuclear Medicine. 2017 May; [PubMed PMID: 28461589]|