A nuclear medicine (NM) lung scan, most commonly called a V/Q scan, is an imaging test that uses small amounts of radioactive tracers to map how air and blood move through your lungs. It remains one of the frontline tools for diagnosing pulmonary embolism (blood clots in the lungs) and plays a growing role in screening for chronic lung-vessel disease, planning lung surgery, and detecting abnormal heart shunts. Despite being developed decades ago, the test keeps evolving, and modern three-dimensional versions have dramatically improved its accuracy compared to the flat images that gave the scan a somewhat shaky reputation in earlier eras.
What Happens During the Scan
The name “V/Q” stands for ventilation and perfusion, the two halves of the test. In the ventilation phase, you breathe in a radioactive gas or aerosol so that a gamma camera can record where air reaches inside your lungs. In the perfusion phase, a small dose of technetium-99m-labeled macro-aggregates of albumin (Tc-99m-MAA) is injected into a vein. These tiny particles temporarily lodge in the smallest blood vessels of the lungs, letting the camera capture where blood is flowing.1SpringerLink / Annals of Nuclear Medicine. Ventilation/perfusion lung scintigraphy: what is still needed? A review considering technetium-99m-labeled macro-aggregates of albumin Comparing the two images side by side is the whole point: if air reaches a region but blood does not, that mismatch strongly suggests a clot is blocking the vessel.
From a patient standpoint, the test is straightforward. You sit or lie under a gamma camera, inhale a ventilation agent for a few minutes, and then receive an intravenous injection. Imaging typically takes 15 to 30 minutes depending on the technology used, and there is no contrast dye of the kind that can trigger kidney problems or allergic reactions in CT scans. The radioactive particles break down quickly, and the tracers clear from the body within hours to a day.
Planar Versus SPECT Imaging
One of the biggest sources of confusion about lung scans is that the older, two-dimensional (“planar”) version and the newer three-dimensional (“SPECT”) version perform very differently, yet both still get referred to as a V/Q scan. That distinction matters a lot. In a head-to-head comparison, SPECT achieved a sensitivity of 100% and a specificity of 87%, while planar imaging managed only 64% sensitivity and 72% specificity for pulmonary embolism.2PubMed. Comparison of V/Q SPECT and planar V/Q lung scintigraphy in diagnosing acute pulmonary embolism Another study put SPECT at 97% sensitivity and 94% accuracy versus 76% and 81% for planar.3Journal of Nuclear Medicine. Tomographic Imaging in the Diagnosis of Pulmonary Embolism: A Comparison Between V/Q Lung Scintigraphy in SPECT Technique and Multislice Spiral CT
Across the published literature, SPECT sensitivities range from 80% to 100% with specificities from 93% to 100%, and the fraction of scans that come back as “indeterminate” (the frustrating neither-positive-nor-negative result) drops below 5%, compared to far higher rates with planar imaging.4Journal of Nuclear Medicine. V/Q Scanning Using SPECT and SPECT/CT That indeterminate category was historically the biggest complaint about lung scans, and SPECT has largely resolved it. If your hospital still performs only planar V/Q, the test you are getting is a meaningfully less accurate version of what current technology can deliver.
How Results Are Interpreted
Radiologists reading a V/Q SPECT study look for mismatched defects, regions where perfusion is reduced or absent but ventilation appears normal. The size and number of those mismatches drive the diagnosis. Research testing various interpretation criteria found that the best diagnostic performance came from using a threshold of at least one segmental mismatch or two subsegmental mismatches, which yielded a sensitivity of 92% and a specificity of 91%.5Journal of Nuclear Medicine. V/Q SPECT Interpretation for Pulmonary Embolism Diagnosis: Which Criteria to Use? Clinical probability also matters. If your doctor already considers pulmonary embolism unlikely before the scan, a negative result drops the chance of a missed clot to about 1%. If clinical suspicion is high and the scan is positive, the probability of actual embolism climbs above 90%.6Journal of Nuclear Medicine. V/Q SPECT Interpretation for Pulmonary Embolism Diagnosis: Which Criteria to Use?
A whole-lung mismatch, where one entire lung shows little or no blood flow while ventilation stays intact, is a rarer finding. Although it can be caused by a massive clot, it has a specific and limited set of other possible causes that radiologists need to consider.7PubMed Central. Acquired Whole-lung Mismatched Perfusion Defects on Pulmonary Ventilation/Perfusion Scintigraphy
Reading V/Q Scans in Patients with COPD
Chronic obstructive pulmonary disease creates patchy ventilation defects on its own, which can muddy the picture. An older landmark study found that 60% of COPD patients received an intermediate-probability reading on planar V/Q scans, compared to 21% of patients without COPD, making definitive interpretation harder.8PubMed. The diagnosis of acute pulmonary embolism in patients with chronic obstructive pulmonary disease Even so, the test was not useless: those few COPD patients who did get a high-probability reading all had confirmed clots, and low-probability or normal scans were just as reliable as in the general population.9PubMed. The diagnosis of acute pulmonary embolism in patients with chronic obstructive pulmonary disease A later study confirmed that while more nondiagnostic results can be expected with COPD (46% versus 21%), the scan still has value as a screening tool.10American Journal of Respiratory and Critical Care Medicine. Diagnosing Acute Pulmonary Embolism: Effect of Chronic Obstructive Pulmonary Disease on the Performance of D-dimer Testing, Ventilation/Perfusion Scintigraphy, Spiral Computed Tomographic Angiography, and Conventional Angiography
Adding a low-dose CT to the V/Q SPECT protocol (creating what is called V/Q SPECT/CT) helps considerably here. In COPD patients, the added CT made the scan interpretation more confident in about 10% of cases and frequently turned up extra findings such as emphysema, lung cancer, or heart failure that a V/Q study alone would have missed.11Seminars in Nuclear Medicine. Lung Scintigraphy in COPD
Screening for Chronic Thromboembolic Pulmonary Hypertension
CTEPH is a form of high blood pressure in the lungs caused by old, organized blood clots that never fully dissolved. It is treatable and sometimes curable with surgery, but it has to be caught first, and that is where the V/Q scan truly outperforms CT angiography. A normal V/Q scan essentially rules out CTEPH, whereas a normal CT angiogram does not.12Journal of Nuclear Medicine. Ventilation–Perfusion Scintigraphy Is More Sensitive than Multidetector CTPA in Detecting Chronic Thromboembolic Pulmonary Disease as a Treatable Cause of Pulmonary Hypertension The V/Q scan is the preferred screening method for CTEPH because of this superior sensitivity.13PubMed Central. The Role of Lung Ventilation/Perfusion Scan in the Management of Chronic Thromboembolic Pulmonary Hypertension
When researchers optimized the diagnostic threshold, they found that 2.5 or more segmental mismatched perfusion defects gave 100% sensitivity and roughly 95% specificity for CTEPH, and 95% of confirmed CTEPH patients had more than four segmental mismatches.14PubMed Central. Lung Ventilation/Perfusion Scintigraphy for the Screening of Chronic Thromboembolic Pulmonary Hypertension (CTEPH): Which Criteria to Use? In other words, a clean V/Q scan is extremely reassuring, and multiple large mismatches should trigger further investigation.
Planning Lung Surgery
Before removing part or all of a lung for cancer or other disease, surgeons need to know how much breathing capacity a patient will retain afterward. A V/Q scan can quantify what percentage of total lung function each region contributes, letting clinicians predict postoperative lung function. Good correlation has been observed between predicted values from either ventilation scanning with xenon-133 or perfusion scanning with Tc-99m-MAA and the actual lung function measured after surgery.15PubMed Central. Prediction of postoperative pulmonary reserve in lung resection patients
One study found that ventilation scanning alone provided the best correlation between predicted and actual postoperative values, even outperforming combined approaches recommended by some guidelines. However, the scan tended to underestimate what a patient could actually achieve after surgery, which means some patients might be unnecessarily denied a potentially curative operation if the numbers are taken at face value.16PubMed. Ventilation-perfusion scintigraphy to predict postoperative pulmonary function in lung cancer patients undergoing pneumonectomy That is a critical nuance: a borderline result on a preoperative lung scan should prompt discussion, not automatic surgical cancellation.
Detecting Heart and Vascular Shunts
A less well-known use of the perfusion scan is detecting right-to-left shunts, abnormal connections that allow blood to bypass the lungs. Normally, the Tc-99m-MAA particles get trapped in the lung capillaries and stay there. When a shunt exists, some particles pass straight through and end up in organs like the brain or kidneys, where they show up on imaging. One case report documented a massive 30% shunt fraction discovered this way, ultimately leading to the identification of a rare vascular anomaly in the chest.17Journal of Nuclear Medicine. Massive right-to-left shunt on nuclear lung perfusion scan leading to the finding of a rare thoracic vascular anomaly Newer methods using SPECT/CT allow more precise measurement of shunt fractions from the same perfusion data.18PubMed. New method for measuring right-left shunt fraction by quantitative analysis using SPECT/CT images
Radiation Dose and Pregnancy
Every imaging test that uses radiation raises the question of dose, and this is especially pressing during pregnancy. The breast dose from a V/Q scan is dramatically lower than from CT pulmonary angiography: typically under 1.5 milligray for the V/Q scan versus 10 to 70 milligray for CT.19PubMed. Radiation dosimetry and safety issues in the investigation of pulmonary embolism That gap is particularly relevant for younger women, for whom breast radiation carries more long-term risk. The lung dose from CT was estimated to be anywhere from 1.3 to over 10 times higher than from V/Q SPECT, and breast dose 27 to 136 times higher.20PubMed. Radiation dose comparison between V/P-SPECT and CT-angiography in the diagnosis of pulmonary embolism
Fetal dose is a separate and more complicated question. A systematic review found that both tests deliver fetal doses generally below 1 milligray, but that the specific dose varies widely depending on the imaging protocol used, and direct comparison between the two was difficult because of those protocol differences.21PubMed Central. Computed tomography pulmonary angiography versus ventilation-perfusion lung scanning for diagnosing pulmonary embolism during pregnancy: a systematic review and meta-analysis Interestingly, the pattern shifts with trimester. In the first two trimesters, CT gives 50% to 97% lower fetal doses than V/Q SPECT, but after six months the relationship reverses and V/Q SPECT delivers about 15% less fetal radiation. That is because the fetal dose from CT increases as the fetus grows larger and closer to the scanning field, while the dose from V/Q SPECT decreases.22PubMed. Pulmonary embolism in pregnant patients: Assessing organ dose to pregnant phantom and its fetus during lung imaging So the best test depends partly on gestational timing, and for the mother’s own radiation exposure, the V/Q scan consistently comes out ahead.
Ventilation Agents and Why They Matter
The ventilation half of the scan can use several different tracers, and the choice affects image quality, especially in patients with airway obstruction. Technegas, an ultrafine carbon aerosol labeled with technetium-99m, is widely used outside the United States and behaves almost identically to a true gas. Even in patients with severe airflow limitation, Technegas mimicked the regional distribution of xenon-133 gas, with a penetration index of 0.99 relative to the gas.23European Respiratory Journal. Intrapulmonary distribution of 99mtechnetium labelled ultrafine carbon aerosol (Technegas) in severe airflow obstruction That means it reaches deep into the lungs the way a real gas would, which is the whole point.
By contrast, Tc-99m-DTPA, a liquid aerosol commonly used in the U.S. (where Technegas lacks FDA approval), produces more uneven radiotracer deposition and more central pooling, particularly in patients with obstructive disease. A head-to-head study found these differences were statistically significant and that DTPA made lung-function impairment look more severe than it actually was.24Journal of Nuclear Medicine. Ventilation–Perfusion SPECT with 99mTc-DTPA Versus Technegas: A Head-to-Head Study in Obstructive and Nonobstructive Disease For patients with COPD or asthma, the choice of ventilation agent is not a trivial detail; it can affect how confidently the scan is read.
Krypton-81m, a radioactive gas with a very short half-life, is another option used in some European centers. A comparison with Technegas in severe COPD patients found significant differences in how evenly each tracer distributed through the lungs, with all patients in the study showing Technegas “hot spots” in at least one lung that were not present with krypton.25PubMed Central. Comparison of 81m Krypton and 99m Tc-Technegas for ventilation single-photon emission computed tomography in severe chronic obstructive pulmonary disease Each agent has trade-offs in availability, cost, and image quality, and the ventilation tracer your hospital uses may depend more on geography and regulatory status than on which is theoretically best.
Pediatric Uses
V/Q scans are not limited to adults. In children, they are sometimes used to assess blood flow in congenital heart conditions, particularly pulmonary artery stenosis (narrowing of the arteries supplying the lungs). A study of pediatric patients with left pulmonary artery stenosis found that those who had undergone surgical repair showed left-lung perfusion above 31%, while the two who had not had surgery showed perfusion below 10%. Pre- and post-surgical V/Q scans documented significant improvement in blood flow to the affected lung.26Journal of Nuclear Medicine. The usefulness of ventilation/perfusion scans in children with pulmonary artery stenosis For pediatric cardiologists, the scan serves as a straightforward, quantitative way to monitor whether an intervention actually restored meaningful blood flow to a lung that was being starved of it.
PET/CT as the Next Generation
The technology that may eventually reshape V/Q imaging is PET/CT using gallium-68-labeled tracers instead of technetium-99m. PET scanners have inherently higher spatial and temporal resolution than SPECT cameras, plus better counting sensitivity. V/Q PET/CT offers the additional advantages of faster acquisition, the ability to gate images to the breathing cycle (reducing motion blur), and stronger quantitative capability for measuring regional lung function.27PubMed. PET/CT Lung Ventilation and Perfusion Scanning using Galligas and Gallium-68-MAA
A pilot study comparing gallium-68 V/Q PET/CT with conventional scintigraphy found image quality that was equal or superior and suggested the perfusion or ventilation component could be completed in three to six minutes on a current-generation PET/CT scanner, compared to 15 to 20 minutes for SPECT.28Journal of Nuclear Medicine. 68Ga PET/CT Ventilation–Perfusion Imaging for Pulmonary Embolism: A Pilot Study with Comparison to Conventional Scintigraphy Respiratory-gated, high-resolution PET/CT has also been described as having broad potential applications, from pulmonary embolism diagnosis to preoperative lung-function mapping and research into pulmonary physiology in a wide range of lung diseases.29PubMed. High-resolution imaging of pulmonary ventilation and perfusion with 68Ga-VQ respiratory gated (4-D) PET/CT The limiting factor right now is availability: gallium-68 requires a germanium/gallium generator or a cyclotron, and the technique is not yet widely adopted outside research settings.
MRI-Based Alternatives
For patients who need to avoid all ionizing radiation, or who require repeated studies over time, MRI-based ventilation and perfusion mapping is an emerging option. A technique called phase-resolved functional lung imaging can generate quantitative regional ventilation and perfusion maps without the need for ultrafast imaging sequences, making it more practical for routine scanners. Feasibility has been demonstrated in healthy volunteers as well as in patients with COPD, CTEPH, and cystic fibrosis.30PubMed. Feasibility of quantitative regional ventilation and perfusion mapping with phase-resolved functional lung (PREFUL) MRI in healthy volunteers and COPD, CTEPH, and CF patients MRI lung scanning is still largely a research tool, but its zero-radiation profile makes it especially attractive for children and pregnant women who may need serial monitoring.