How to Interpret Your CT Angiogram Results

Introduction

A CT angiogram report contains terminology and classifications that can confuse patients unfamiliar with cardiac imaging. Stenosis percentages, plaque compositions, CAD-RADS scores, and anatomical descriptions all carry clinical meaning, but that meaning is not obvious from the technical language. Understanding your results enables informed conversations with your physician about what they mean for your health.

This article translates the medical vocabulary of CTA reports into practical terms. It explains how stenosis is measured and categorized, what different plaque types signify, and what the standardized CAD-RADS classification system communicates about your findings and recommended next steps.

Before diving into interpretation, the fundamentals article explains what CTA is and how the procedure works. The technology article addresses how scanner specifications affect image quality. Once you understand your results, the actionability article helps translate findings into treatment decisions.

How do radiologists and cardiologists categorize the severity of coronary artery blockages?

Stenosis refers to narrowing of the arterial lumen, the channel through which blood flows. Interpreters estimate the percentage of diameter reduction compared to a normal reference segment. The standard categories are: normal (0% stenosis), minimal (1-24%), mild (25-49%), moderate (50-69%), and severe (70% or greater). A complete blockage is called an occlusion.

These categories carry different clinical implications. Mild stenosis indicates plaque is present but unlikely to limit blood flow under most conditions. Moderate stenosis may or may not cause ischemia depending on plaque characteristics and downstream demand. Severe stenosis probably limits flow during exertion and often warrants further evaluation or intervention.

The categories are not precise boundaries. A 48% stenosis is clinically similar to a 52% stenosis despite falling into different categories. Measurement variability means the same lesion might be reported differently by different readers or even by the same reader on different occasions. Use the categories as general guides rather than exact thresholds.

What is the difference between mild, moderate, and severe stenosis?

Mild stenosis (25-49%) indicates atherosclerosis is present but the lumen remains relatively open. Blood flow is not significantly compromised at rest or during typical exertion. Patients with only mild stenosis are unlikely to have angina from the identified lesions. However, mild stenosis is not clinically irrelevant. It confirms atherosclerotic disease and identifies patients who may benefit from aggressive risk factor modification.

Moderate stenosis (50-69%) occupies an intermediate zone. These lesions may or may not limit blood flow depending on factors CTA cannot always determine. The functional significance of a 55% stenosis depends on plaque characteristics, vessel size, downstream territory, and collateral supply. Additional testing with CT-derived fractional flow reserve (FFR-CT) or stress imaging can clarify whether a moderate lesion causes ischemia.

Severe stenosis (70% or greater) usually indicates flow-limiting disease. Most patients with true 70% or greater stenosis experience ischemia during exertion, though compensatory mechanisms sometimes preserve flow despite anatomical narrowing. Severe stenosis often triggers referral to invasive angiography for possible revascularization. However, even severe stenosis does not mandate intervention; medical therapy effectively manages many patients with stable ischemic heart disease.

What does “50% stenosis” actually mean and how is it calculated?

A 50% stenosis means the arterial diameter at the narrowest point is half what it would be without disease. Interpreters compare the lumen diameter at the lesion to a nearby “normal” reference segment, then express the reduction as a percentage. If the reference segment measures 4 millimeters and the lesion measures 2 millimeters, stenosis is 50%.

This calculation involves assumptions that introduce imprecision. The reference segment is not always truly normal; diffuse disease may affect the entire vessel. The measurement location within the lesion affects results; the tightest point gives a higher stenosis value than adjacent areas. Cross-sectional images must be oriented perpendicular to the vessel, and oblique angles produce inaccurate measurements.

Diameter stenosis differs from area stenosis. A 50% diameter reduction corresponds to a 75% reduction in cross-sectional area because area scales with the square of the radius. Some reports describe area stenosis, which yields higher numbers for the same lesion. When reviewing results, note whether percentages refer to diameter or area.

What is the CAD-RADS scoring system and how should I interpret my score?

CAD-RADS (Coronary Artery Disease Reporting and Data System) standardizes CTA reporting. Developed by major cardiovascular imaging societies, it categorizes stenosis severity on a scale from 0 to 5 and provides management recommendations for each category. The C-CORE registry demonstrated that CAD-RADS categories predict cardiovascular events and mortality (van Rosendael, 2023).

CAD-RADS 0 indicates no plaque or stenosis. CAD-RADS 1 indicates minimal stenosis (1-24%). CAD-RADS 2 indicates mild stenosis (25-49%). CAD-RADS 3 indicates moderate stenosis (50-69%). CAD-RADS 4 indicates severe stenosis: 4A for 70-99% stenosis and 4B for left main greater than 50% stenosis or three-vessel disease with greater than 70% stenosis. CAD-RADS 5 indicates total occlusion.

Each CAD-RADS category carries recommended management. Categories 0-2 typically suggest no further testing and preventive therapies. Category 3 may warrant functional testing or FFR-CT to assess ischemia. Categories 4-5 typically prompt consideration of invasive angiography. Modifiers indicate presence of stents (S), grafts (G), vulnerable plaque (V), or non-diagnostic segments (N).

What is the difference between calcified plaque, soft plaque, and mixed plaque?

Calcified plaque appears bright white on CT because calcium absorbs X-rays strongly. It develops when atherosclerotic deposits accumulate calcium over time. Calcified plaque is considered more stable than soft plaque, though heavily calcified lesions can still cause events and create imaging challenges because the brightness obscures the adjacent lumen.

Soft plaque (also called non-calcified plaque) appears darker on CT. It includes fibrous tissue, lipid-rich material, and cellular debris. Soft plaque is considered less stable than calcified plaque because lipid-rich deposits can rupture, triggering thrombosis and acute coronary syndromes. The PROSPECT study using intravascular ultrasound identified that lipid-rich plaques with thin fibrous caps were most likely to cause future events (Stone et al., 2011).

Mixed plaque contains both calcified and non-calcified components. Most atherosclerotic lesions in patients with established disease show mixed composition. The ratio of calcified to non-calcified material varies, and some plaques have a predominantly calcified core with a rim of soft plaque while others show the opposite pattern.

Why does plaque composition matter beyond the degree of blockage?

Plaque composition predicts risk independently of stenosis severity. A 40% stenosis composed primarily of soft plaque may be more dangerous than a 60% stenosis that is heavily calcified. Soft plaque can rupture, exposing thrombogenic material to blood flow and triggering acute coronary syndrome. Calcified plaque, while still capable of causing events, is generally more stable.

CT angiography can identify high-risk plaque features beyond simple composition. Low-attenuation plaque (very dark on CT, indicating lipid-rich content) correlates with rupture-prone lesions. Positive remodeling, where the artery expands outward to accommodate plaque, associates with vulnerability. Advanced CT analysis can detect high-risk plaques and identify vulnerable patients for more aggressive treatment (Antoniades and West, 2021).

For patients with non-obstructive disease, plaque composition affects management recommendations. Someone with diffuse soft plaque may warrant more intensive lipid-lowering therapy than someone with scattered calcifications despite similar stenosis grades. Plaque information helps individualize treatment intensity.

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What are “vulnerable” or “high-risk” plaque features and can CT angiogram detect them?

Vulnerable plaque refers to atherosclerotic deposits at high risk of rupture. Pathologically, these lesions have thin fibrous caps overlying lipid-rich necrotic cores. They often show positive remodeling and inflammatory cell infiltration. When vulnerable plaques rupture, thrombosis occurs and may cause myocardial infarction or sudden death.

CT angiography can detect some features associated with vulnerability. Low-attenuation plaque, positive remodeling, spotty calcification, and the napkin-ring sign (a thin rim of high attenuation surrounding a low-attenuation core) all correlate with increased risk. Photon-counting CT shows promise for improved plaque characterization and may enhance detection of high-risk features (Mergen et al., 2022).

However, CT cannot definitively identify which specific plaque will rupture. Many plaques with high-risk features never cause events, while some events arise from plaques lacking obvious vulnerability markers. High-risk plaque features appropriately intensify medical therapy but do not mandate intervention for non-obstructive lesions.

How does the location of a blockage affect its clinical significance?

Proximal lesions in the left main coronary artery or proximal left anterior descending artery jeopardize large amounts of heart muscle. A 70% stenosis of the left main coronary artery is more dangerous than a 70% stenosis in a small diagonal branch because the territory at risk differs dramatically. Left main disease receives special attention in CAD-RADS classification and clinical guidelines.

The left anterior descending artery supplies the anterior wall and much of the interventricular septum. Proximal LAD stenosis threatens a substantial proportion of the left ventricle. The right coronary artery supplies the inferior wall and often the posterior septum. The circumflex supplies the lateral wall. Each vessel’s significance depends on dominance patterns and individual anatomy.

Distal lesions in small branches affect less myocardium and may not warrant intervention even when severely stenotic. A 90% stenosis in a tiny distal branch might be observed while a 70% stenosis in the proximal LAD prompts aggressive management. Location must be considered alongside stenosis severity when determining clinical significance.

What does it mean if multiple vessels are affected versus a single vessel?

Multivessel disease indicates atherosclerosis has affected more than one coronary territory. The standard classification describes one-vessel, two-vessel, or three-vessel disease based on involvement of the LAD territory, circumflex territory, and right coronary artery territory. Multivessel disease carries worse prognosis than single-vessel disease with equivalent maximum stenosis.

The pattern of disease affects treatment decisions. Patients with multivessel disease, particularly those with diabetes, may derive greater benefit from bypass surgery than from stenting. Single-vessel disease is more often amenable to percutaneous intervention or medical management alone. The ISCHEMIA trial demonstrated that outcomes relate to both stenosis severity and disease extent (Reynolds et al., 2021).

Total plaque burden across all vessels also carries prognostic information. Even without severe stenosis in any single location, diffuse non-obstructive disease throughout the coronary tree indicates elevated cardiovascular risk. Some scoring systems quantify total plaque burden to capture this dimension of disease beyond focal stenosis severity.

How do age and sex affect what is considered “normal” versus “abnormal” on CT angiogram?

Atherosclerosis increases with age, so plaque presence is more common and less surprising in older patients. A 70-year-old with mild scattered calcifications has age-appropriate disease. A 40-year-old with the same findings has premature atherosclerosis warranting aggressive risk factor modification. The clinical response to identical images differs based on patient age.

Sex affects both disease prevalence and presentation. Women develop coronary artery disease later than men on average but have worse outcomes once disease is present. Women more often have non-obstructive disease and microvascular dysfunction that CTA cannot detect. A woman with normal coronary arteries on CTA may still have angina from microvascular disease that requires different evaluation.

Reference ranges for coronary calcium scoring explicitly account for age and sex. A calcium score of 100 represents different percentiles for a 50-year-old woman versus a 70-year-old man. CTA stenosis interpretation does not have formal age-sex adjustments, but clinical context appropriately influences management recommendations.

Can CT angiogram detect blockages that are too small to cause symptoms?

CTA routinely detects non-obstructive plaque that does not limit blood flow or cause angina. This is a feature, not a limitation. Identifying patients with subclinical atherosclerosis enables preventive intervention before symptoms develop. Many patients discovered to have mild or moderate stenosis on CTA will never experience cardiac symptoms if appropriately treated.

Studies of zero calcium score patients demonstrate that CTA can identify non-calcified plaque invisible on calcium scoring alone (Lee et al., 2013). Even among young symptomatic patients, CTA detects substantial disease in those whose calcium scores suggest low risk (Feuchtner et al., 2021). This enhanced sensitivity represents CTA’s advantage over simpler testing.

Detecting non-obstructive disease raises questions about appropriate management. Patients often find it unsettling to learn they have coronary artery disease when they feel fine. Yet this knowledge motivates lifestyle changes and medication adherence that may prevent progression. The actionability article addresses how to respond to non-obstructive findings.

What findings on CT angiogram do not indicate coronary artery disease?

Coronary arteries may appear abnormal for reasons other than atherosclerosis. Myocardial bridging occurs when a portion of a coronary artery tunnels through heart muscle rather than coursing on the surface. The bridged segment may appear compressed during systole but opens during diastole when coronary flow occurs. Most bridging is clinically insignificant.

Coronary artery anomalies are congenital variants in how arteries arise or course. Some anomalies are benign incidental findings. Others, particularly anomalous origins where an artery passes between the aorta and pulmonary artery, carry risk of sudden death and require cardiology evaluation. CTA excels at characterizing anomalous coronary anatomy.

Coronary artery ectasia describes diffuse arterial dilation that may occur with or without atherosclerosis. Aneurysmal dilation can result from past Kawasaki disease or connective tissue disorders. These findings require interpretation in clinical context rather than assuming atherosclerotic coronary artery disease.

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What are common incidental findings on cardiac CT and how significant are they?

Cardiac CT images the entire chest, and interpreters note abnormalities beyond the coronary arteries. Lung nodules appear commonly and require follow-up imaging when they exceed certain size thresholds. Most prove benign, but some represent early lung cancer detectable only because cardiac CT happened to image the lungs.

Aortic abnormalities including aneurysms, dissections, and atherosclerosis may be identified. A dilated ascending aorta may warrant surveillance imaging and blood pressure management. Aortic atherosclerosis indicates systemic vascular disease beyond the coronary arteries.

Cardiac chamber enlargement, pericardial thickening, and valve abnormalities may appear as incidental findings. These observations may prompt echocardiography for more detailed functional assessment. While not the primary purpose of coronary CTA, incidental findings sometimes identify clinically important conditions that would otherwise have gone undetected.

What is the difference between detecting plaque and determining whether it limits blood flow?

CTA provides anatomical information: the presence, location, extent, and composition of plaque. It does not directly measure blood flow. A 60% stenosis may or may not cause ischemia depending on factors including lesion length, vessel size, downstream demand, and collateral supply. Anatomy and physiology do not perfectly correlate.

Functional testing with stress imaging or fractional flow reserve measurement assesses whether stenosis limits blood flow under conditions of increased demand. A stenosis that appears moderate on CTA might cause significant ischemia, or might have no functional consequence. This discrepancy explains why some patients with severe anatomical stenosis have no symptoms while others with moderate stenosis experience angina.

CT-derived fractional flow reserve (FFR-CT) applies computational fluid dynamics to CTA images to estimate flow limitation without additional testing. This technology addresses the gap between anatomical and functional assessment. When available, FFR-CT helps determine whether a lesion of intermediate severity warrants intervention. The combination strategies article discusses integrating anatomical and functional testing.

How reliable is CT angiogram for estimating the functional significance of a blockage?

CTA overestimates stenosis severity compared to invasive angiography, particularly in heavily calcified vessels. Calcified plaque creates blooming artifact that makes lesions appear more severe than they are. This limitation is well-recognized and accounts for some false positive CTAs that show severe stenosis where catheterization reveals only moderate disease.

The functional significance of a lesion cannot be determined from anatomy alone. Studies comparing CTA stenosis estimates to invasive fractional flow reserve find that many lesions with 50-70% stenosis on CTA are not hemodynamically significant. CTA anatomy predicts which lesions might limit flow but cannot confirm functional importance without additional data.

For this reason, moderate stenosis on CTA often warrants functional assessment before proceeding to intervention. Stress testing or FFR-CT can clarify whether a borderline lesion causes ischemia. Proceeding directly from moderate CTA stenosis to catheterization risks confirming the anatomical finding without demonstrating that it matters clinically.

What does “non-obstructive coronary artery disease” mean and why does it matter?

Non-obstructive coronary artery disease refers to plaque that does not cause significant stenosis, typically defined as less than 50% narrowing. Patients with non-obstructive CAD have atherosclerosis that has not yet progressed to flow-limiting disease. This finding is not normal. It identifies patients at elevated cardiovascular risk compared to those with completely clean arteries.

Studies demonstrate that non-obstructive CAD on CTA predicts future cardiovascular events including myocardial infarction and death. The risk is lower than for obstructive CAD but meaningfully elevated above patients with no visible plaque. Total plaque burden in non-obstructive disease correlates with outcomes: more diffuse non-obstructive disease carries higher risk than focal mild disease.

Non-obstructive CAD warrants medical therapy to slow progression and prevent events. Lifestyle optimization, statin therapy, blood pressure control, and diabetes management all reduce risk in this population. The finding should prompt intensified prevention rather than reassurance that “nothing was found.” Understanding this distinction prevents the misperception that only obstructive disease matters.

Conclusion

Interpreting CT angiogram results requires understanding both the terminology and its limitations. Stenosis percentages are estimates with inherent variability, not precise measurements. Plaque composition adds prognostic information beyond stenosis severity. The CAD-RADS classification provides standardized categories with management recommendations.

Key takeaways: mild stenosis confirms disease but rarely causes symptoms; moderate stenosis may or may not limit flow and often warrants functional assessment; severe stenosis usually indicates hemodynamic significance; and non-obstructive disease is not normal and warrants preventive therapy.

The next step is translating findings into action. The actionability article addresses how CTA results should change your treatment plan. The comparison article explains when additional testing helps clarify CTA findings. Understanding both your results and the appropriate response enables productive collaboration with your care team.