A lung scan is any imaging test that creates pictures of the lungs to help diagnose or monitor disease. The term covers a wide range of technologies, from a standard chest CT that takes seconds to a nuclear medicine study that tracks blood flow or metabolism over an hour or more. Which scan you get depends on the clinical question your doctor needs answered, whether that is ruling out a blood clot, staging a cancer, or checking how well your lungs would function after surgery.
CT Scans and Why They Dominate Lung Imaging
Computed tomography, the workhorse of modern lung imaging, uses X-rays taken from many angles to build detailed cross-sectional images of the chest. When doctors say “lung scan” without specifying further, they usually mean a chest CT. The reason is diagnostic power. In patients with major blunt trauma, CT was significantly more effective than a standard chest X-ray at detecting lung contusions, collapsed lung segments, and fractured ribs and vertebrae.1PubMed. The use of chest computed tomography versus chest X-ray in patients with major blunt trauma For chronic lung conditions, a study comparing the two modalities found that CT delivered the correct first-choice diagnosis about three-quarters of the time, compared with just over half for a plain chest X-ray.2PubMed. Chronic diffuse infiltrative lung disease: comparison of diagnostic accuracy of CT and chest radiography
A standard chest CT scan is fast. You lie on a narrow table that slides into a short, open-ended tunnel, hold your breath for a few seconds when instructed, and you are done. The breath-hold matters more than most patients realize. Inadequate breathing maneuvers can alter what the images show, either making healthy tissue look abnormal or masking real disease, and can introduce motion blur that degrades image quality.3Radiographics. Quality initiatives. Respiratory instructions for CT examinations of the lungs: a hands-on guide Technologists will typically coach you through the breath-hold before and during the scan, so listening carefully to those instructions is one of the few things within your control.
Specialized CT Protocols
Not all chest CTs are the same. The scan settings, the use of contrast dye, and the thickness of the image slices vary depending on what is being investigated. Three specialized protocols come up most often in lung imaging.
High-Resolution CT for Interstitial Lung Disease
High-resolution CT, or HRCT, uses thinner image slices and specific reconstruction techniques to capture fine detail in the lung tissue. It is the central tool for distinguishing idiopathic pulmonary fibrosis from other forms of interstitial lung disease.4PubMed Central. HRCT evaluation of patients with interstitial lung disease: comparison of the 2018 and 2011 diagnostic guidelines The advantage of HRCT lies in its ability to pick out specific markers of fibrosis, including honeycombing (a pattern of small air-filled cysts clustered together), traction bronchiectasis (widened airways pulled open by stiffened surrounding tissue), and overall loss of lung volume.5Breathe. Imaging in the diagnosis and management of fibrosing interstitial lung diseases If your doctor suspects scarring in your lungs, HRCT is typically the first scan ordered.
CT Pulmonary Angiography for Blood Clots
When a pulmonary embolism, a blood clot lodged in the lung’s arteries, is suspected, the go-to test is CT pulmonary angiography, or CTPA. This involves injecting iodine-based contrast dye into a vein, then timing the CT scan to capture the moment that dye fills the pulmonary arteries. The result is a detailed map of blood flow that can reveal a clot. CTPA is the current standard of care for diagnosing acute pulmonary embolism, providing accurate results with rapid turnaround and simultaneously revealing other potential causes of chest pain.6PubMed Central. Imaging of acute pulmonary embolism: an update Because it is so accessible and quick, CTPA tends to be overused in some settings; clinical scoring tools help doctors decide which patients truly need the scan and which can be evaluated by simpler means first.7PubMed Central. The role of computed tomography in the diagnosis of acute and chronic pulmonary embolism
Low-Dose CT for Lung Cancer Screening
Lung cancer screening programs use a low-dose version of CT that cuts radiation exposure while still detecting small nodules. Clinical trials have shown that this approach reduces lung cancer deaths among people with significant smoking histories. A cost-effectiveness analysis modeled on the large NELSON trial found that screening provided meaningful gains in quality-adjusted life expectancy for both men and women who had ever smoked, with the benefit slightly larger for women.8PubMed. Cost-effectiveness of low-dose CT lung cancer screening among individuals that have ever smoked in Norway: A model-based analysis using NELSON trial criteria and outcomes In most countries that offer screening, eligibility depends on age and smoking history. The scan itself feels identical to a regular chest CT: lie down, hold your breath, wait a few seconds.
Nuclear Medicine Scans
Nuclear medicine lung scans work differently from CT. Instead of shooting X-rays through you from the outside, they involve introducing a small amount of radioactive tracer into your body and then using a specialized camera to detect where that tracer ends up. The two main types are ventilation-perfusion scans and PET-CT.
Ventilation-Perfusion Scans
A ventilation-perfusion scan, commonly called a V/Q scan, evaluates both airflow and blood flow in the lungs. For the ventilation part, you breathe in a radioactive gas or aerosol. For the perfusion part, a radioactive tracer is injected into a vein and travels to the lung’s blood vessels. A gamma camera captures images from both steps, and doctors compare the two to look for mismatches: areas where blood flow is blocked but ventilation is normal, a hallmark of a pulmonary embolism. V/Q scans remain a commonly performed study in nuclear medicine, with roles in both acute and chronic pulmonary embolism evaluation.9Radiographics. Ventilation-Perfusion Scan: A Primer for Practicing Radiologists
The test takes longer than a CT, often 30 to 60 minutes, and requires you to lie still under the camera. It does not use iodine contrast, which makes it an option for people with iodine allergies or kidney problems. V/Q scans also play a role in surgical planning. Before a lung cancer patient undergoes surgery to remove a lobe or more, doctors need to predict how well the remaining lung will function. Perfusion scintigraphy and its more advanced version, SPECT/CT, can estimate post-operative lung function with strong accuracy: predicted values correlate closely with actual post-operative measurements.10PubMed. The use of SPECT in preoperative assessment of patients with lung cancer11Journal of Nuclear Medicine. Head-to-Head Prospective Comparison of Quantitative Lung Scintigraphy and Segment Counting in Predicting Pulmonary Function in Lung Cancer Patients Undergoing Video-Assisted Thoracoscopic Lobectomy
PET-CT for Cancer
PET-CT combines a metabolic tracer, most commonly a radioactive sugar called FDG, with a standard CT scan. Cancer cells tend to consume more sugar than normal tissue, so they light up on the PET portion. The CT portion provides the anatomical map. Together, PET-CT offers both structural and metabolic information about lung nodules and known cancers.12PubMed. Molecular imaging of pulmonary nodules It is now a mainstay for initial lung cancer staging and for investigating suspicious lung nodules found on other scans.13Diagnostic and Interventional Imaging. FDG PET-CT for solitary pulmonary nodule and lung cancer: Literature review
The procedure takes longer than most other lung scans. After the FDG injection, you wait about an hour for the tracer to distribute through your body, then lie still in the scanner for roughly 20 to 30 minutes. You are usually asked to fast for several hours beforehand and to avoid strenuous exercise the day prior, because both can affect how the tracer distributes. Diabetic patients may need to adjust insulin or meal timing to keep blood sugar in a usable range for the test.
Radiation Exposure Across Different Lung Scans
Every scan that uses X-rays or radioactive tracers delivers some radiation. A plain chest X-ray delivers very little. CT delivers considerably more, and PET-CT carries the highest combined dose because it adds both the CT radiation and the tracer’s own emissions. In a dose-monitoring study of pediatric patients, the mean effective dose per exam was about 0.04 mSv for a standard X-ray, roughly 1 mSv for a CT, and around 8.4 mSv for the CT component of a PET-CT alone, with the PET tracer adding further exposure on top of that.14PubMed Central. Comparison of effective radiation doses from X-ray, CT, and PET/CT in pediatric patients with neuroblastoma using a dose monitoring program Adult doses for a standard diagnostic chest CT typically fall in a similar range, though they vary with the scanner, protocol, and the patient’s body size. Low-dose CT screening protocols deliberately push the dose lower.
These numbers are worth knowing for context, but they are not reasons to refuse a scan your doctor recommends. The radiation from a single diagnostic CT is small relative to the natural background radiation you receive over a year. The risk from a missed diagnosis almost always outweighs the tiny added cancer risk from diagnostic imaging. Where radiation awareness matters most is in patients who need repeated scans over time, such as people undergoing long-term cancer surveillance or those with chronic lung disease requiring periodic follow-up.
Contrast Dye and Kidney Safety
Several lung scans, particularly CTPA and some standard chest CTs, require injecting iodine-based contrast dye into a vein. The contrast makes blood vessels and certain tissues much easier to see. Most people tolerate it well, but there is a well-known risk to the kidneys called contrast-induced nephropathy, especially in patients who already have reduced kidney function. Prevention centers on generous hydration, using the lowest effective dose of contrast, and checking kidney function with a blood test before and after the procedure.15PubMed Central. Side effects of radiographic contrast media: pathogenesis, risk factors, and prevention Limiting the contrast dose itself is considered the single most effective way to reduce the risk of kidney injury.16European Heart Journal. Contrast-induced kidney injury: mechanisms, risk factors, and prevention
If you have known kidney disease, your doctor will weigh whether the clinical need for contrast outweighs the risk, and may hydrate you with IV fluids before and after the scan. You will also be asked about allergies to iodine or prior reactions to contrast media, since allergic-type reactions, while uncommon, do occur and can range from mild hives to more serious events. These are managed with pre-medication if you have a history of reactions.
MRI and Ultrasound as Radiation-Free Alternatives
MRI uses magnetic fields and radio waves instead of X-rays. For the lungs specifically, MRI has limitations: air does not produce a strong MRI signal, so the fine detail within the lung tissue is harder to capture compared with CT. Where lung MRI shines is in evaluating whether a tumor has invaded the chest wall or in characterizing masses in the central chest (the mediastinum), thanks to its superior soft-tissue contrast.17PubMed Central. MRI of pulmonary nodules: technique and diagnostic value MRI is also used when radiation must be avoided entirely, such as in some pediatric patients or in situations requiring frequent repeat imaging.
Lung ultrasound occupies a different niche. It cannot see deep into the lungs the way CT can, but it excels at the bedside in acute situations. Over the past two decades, point-of-care lung ultrasound has changed how certain respiratory conditions are diagnosed in emergency departments and intensive care units.18PubMed Central. Clinical research on point-of-care lung ultrasound: misconceptions and limitations A physician can hold a small probe against your chest and immediately check for fluid around the lungs, a collapsed lung, or signs of pneumonia. The test involves no radiation, no contrast, no breath-holding, and no waiting for an appointment. Its main limitation is that it depends heavily on the operator’s skill and cannot replace CT for deeper or more complex questions.
Emerging Techniques
One of the more promising research tools is hyperpolarized xenon MRI. In this technique, you inhale xenon gas that has been specially magnetized, then undergo an MRI. The gas provides a signal from inside the airways that conventional MRI cannot achieve. Researchers have used it to quantify regional lung damage from radiation therapy, tracking changes in ventilation, membrane gas uptake, and red blood cell gas transfer on a per-dose basis.19PubMed Central. Quantifying Regional Radiation-Induced Lung Injury in Patients Using Hyperpolarized (129)Xe Gas Exchange Magnetic Resonance Imaging This kind of functional detail is far beyond what a standard CT can show. For now, it remains largely a research tool available at specialized centers, but it hints at a future where lung scans assess not just what the lungs look like but how well each region is actually functioning.
Lung Scans During Pregnancy
Suspected pulmonary embolism is one of the more common reasons a pregnant woman might need a lung scan. Both CTPA and V/Q scans involve radiation, so the choice between them is not straightforward. A review of the evidence concluded that CTPA, especially with modern dose-reduction techniques, actually delivers a lower radiation dose to the fetus than a V/Q scan, while also offering broader diagnostic capability.20PubMed Central. Imaging for suspected pulmonary embolism in pregnancy—what about the fetal dose? A comprehensive review of the literature However, CTPA does deliver a higher radiation dose to the mother’s breast tissue, which is more sensitive during pregnancy.
When a V/Q scan is used in pregnancy, a two-step protocol is often recommended. Perfusion-only imaging is done first, using a reduced tracer dose. Because pregnant women tend to have healthy lungs without underlying disease, a normal perfusion pattern is usually enough to rule out a clot, and the ventilation step can be skipped entirely.21PubMed Central. Computed tomography pulmonary angiography versus ventilation-perfusion lung scanning for diagnosing pulmonary embolism during pregnancy: a systematic review and meta-analysis This minimizes the total radiation. The ventilation portion is added only if the perfusion images are abnormal. Both approaches are considered safe and the fetal radiation dose from either scan is well below the threshold associated with harm, but the choice between them is typically individualized based on the patient’s chest X-ray findings, allergies, and kidney function.
When a Lung Scan Finds Something Unexpected
One of the most common outcomes of a chest CT, particularly in screening programs, is the incidental discovery of a small lung nodule. These are typically small, round spots on the image, and the overwhelming majority are benign. The Fleischner Society, the main body issuing management guidelines for incidentally discovered nodules, updated its recommendations in 2017 with several practical changes. The minimum size threshold for routine follow-up was raised, meaning very small nodules that would previously have triggered repeat scans now get left alone.22PubMed. Guidelines for Management of Incidental Pulmonary Nodules Detected on CT Images: From the Fleischner Society 2017 Follow-up intervals are also now given as ranges rather than exact dates, allowing more flexibility based on individual risk factors.23PubMed. Updated Fleischner Society Guidelines for Managing Incidental Pulmonary Nodules: Common Questions and Challenging Scenarios
Management depends on the nodule’s size, density (solid versus partially ground-glass), and the patient’s cancer risk profile.24PubMed Central. Management of incidental lung nodules <8 mm in diameter. For many small, solid nodules in low-risk patients, no follow-up at all is recommended. Larger or suspicious nodules may warrant a repeat CT in a few months to check for growth, or further workup with PET-CT or a biopsy. Advances in bronchoscopic technology have made it possible to safely sample even peripheral nodules that are difficult to reach.25PubMed Central. Bronchoscopic biopsy of peripheral pulmonary lesions in 2020: a review of existing technologies The main takeaway if you are told you have a lung nodule: do not panic. Most are nothing, and the guidelines exist specifically to avoid unnecessary invasive procedures on harmless spots.
False Positives and Scan Limitations
No lung scan is perfect. PET-CT, for all its power in cancer evaluation, has a well-known weakness: inflammation and infection can mimic cancer. Inflammatory cells and active infections accumulate the FDG tracer just like cancer cells do.26Korean Journal of Radiology. False Positive and False Negative FDG-PET Scans in Various Thoracic Diseases In one analysis of false positives on PET-CT in lung cancer patients, inflammatory pseudotumors and tuberculosis-related masses accounted for about four out of five misdiagnosed cases.27PubMed Central. Retrospective analysis for the false positive diagnosis of PET-CT scan in lung cancer patients This is why a “positive” PET-CT is rarely the final word. It almost always leads to further testing, whether a biopsy or a follow-up scan, before treatment decisions are made.
CT scans have their own limitations. Very small nodules can be indistinguishable from lymph nodes or blood vessels on a single scan. Motion artifacts from breathing or heartbeat can blur structures or create shadows that look like abnormalities. And there is the simple challenge of reader variability: two radiologists looking at the same scan may disagree about whether a finding is concerning. This is one of the reasons follow-up intervals exist, because watching for change over time is often more informative than trying to characterize a borderline finding from a single image.
Artificial Intelligence as a Second Set of Eyes
AI tools for lung scan interpretation have been heavily investigated over the past decade, particularly for detecting and characterizing lung nodules. Several have received regulatory approval for clinical use. AI systems achieve a sensitivity in the range of 70 to 90 percent for nodule detection, which is on par with experienced radiologists, while producing fewer false alarms.28PubMed Central. Artificial intelligence in automated detection of lung nodules: a narrative review In one study focused specifically on CT, AI caught a higher proportion of nodules than radiologists did, with a sensitivity above 99 percent versus 43 percent for the human readers, though the AI flagged more false positives as a tradeoff, especially for very small nodules.29PubMed Central. Performance of Deep-learning-based Artificial Intelligence on Detection of Pulmonary Nodules in Chest CT
In practice, AI is used most effectively as a “second reader,” catching nodules a radiologist might overlook rather than replacing human judgment entirely. Despite the growing number of approved tools, integration into routine clinical workflows has been slower than the technology’s pace might suggest.30PubMed. Artificial Intelligence (AI) for Lung Nodules, From the AJR Special Series on AI Applications Challenges include standardizing how different AI models perform across different scanner brands and patient populations, determining liability when AI contributes to a misread, and establishing reimbursement pathways. For patients, the practical implication is straightforward: if your scan is read at a center that uses AI assistance, there is an extra layer of scrutiny on the images, which is generally a good thing. The final diagnostic call, though, still rests with a human radiologist.