23. Thoracic CT

The conducting airways are a branching, cartilage-supported conduit whose CT appearance is governed by the relationship between lumen, wall, and the accompanying pulmonary artery. In health an airway runs alongside an artery of nearly identical caliber, so the bronchoarterial ratio (internal bronchial diameter divided by adjacent arterial diameter) approximates unity; values above roughly $1.0$–$1.5$ define bronchial dilatation, the structural hallmark of bronchiectasis. The pathophysiology is a self-amplifying cycle — described by Cole as the vicious vortex — in which an inciting insult (post-infectious injury, impaired mucociliary clearance in cystic fibrosis or primary ciliary dyskinesia, immunodeficiency, or traction from fibrosis) permits chronic bacterial colonization, neutrophilic inflammation, proteolytic destruction of the elastin and cartilage of the wall, and irreversible dilatation that further impairs clearance. The CT signatures follow directly from this geometry. When a dilated, thick-walled bronchus is sectioned in cross-section beside its smaller artery, the pair forms the $\textbf{signet-ring sign}$; when sectioned along its axis, the failure of normal distal tapering and the visualization of airways within $1\,\text{cm}$ of the costal pleura are diagnostic. Morphology carries etiologic weight: cylindrical bronchiectasis is least specific, varicose (beaded) bronchiectasis suggests more advanced injury or allergic bronchopulmonary aspergillosis, and cystic bronchiectasis with air-fluid levels indicates severe destruction. Upper-lobe predominance favors cystic fibrosis and ABPA (the latter often with high-attenuation mucoid impaction), central distribution favors ABPA, and a right-middle-lobe and lingular distribution with tree-in-bud nodules in an older woman is the classic Lady Windermere presentation of Mycobacterium avium complex.

Small-airway disease is inferred indirectly because bronchioles below about $2\,\text{mm}$ are below routine spatial resolution. Cellular (infectious) bronchiolitis fills and surrounds the terminal bronchioles with inflammatory exudate, producing the $\textbf{tree-in-bud}$ pattern of centrilobular branching nodules that spare the pleural surface — a finding whose differential is dominated by infection (mycobacterial, bacterial, viral) and aspiration. Constrictive (obliterative) bronchiolitis, by contrast, narrows and obliterates bronchiolar lumina without filling them, so the airways become invisible and the disease declares itself only through its physiologic consequence: regional air trapping producing $\textbf{mosaic attenuation}$, with geographic lucent zones whose vessels are attenuated. The single most important maneuver here is the expiratory acquisition; areas of air trapping fail to increase in attenuation on expiration, and a difference exceeding roughly $80$–$100\,\text{HU}$ between inspiratory and expiratory density confirms trapping. Mosaicism has a tripartite differential — small-airway disease, occlusive vascular disease (chronic thromboembolic pulmonary hypertension), and infiltrative ground-glass — and the expiratory scan plus vessel caliber discriminate among them. Tracheobronchomalacia is a dynamic diagnosis: excessive expiratory collapse of the tracheal lumen by more than $50\%$ (often with a frown-shaped or lunate cross-section) on dynamic-expiratory or cough CT defines it, and a static inspiratory scan will miss it entirely. The dominant $\textbf{failure modes}$ are therefore as much protocol as perception: omitting expiratory imaging blinds the reader to air trapping and malacia; respiratory motion mimics ground glass and bronchial wall blur; and the cognitive trap of anchoring on emphysema can cause true mosaic small-airway disease to be dismissed as background lucency. Management hinges on these distinctions — airway clearance and targeted antimicrobials for bronchiectasis, smoking cessation and occupational counseling for constrictive bronchiolitis, and consideration of stenting or tracheoplasty for severe symptomatic malacia.

Pneumonia is the radiologic expression of an alveolar and bronchiolar inflammatory exudate, and its CT pattern is dictated by how the pathogen propagates through the secondary pulmonary lobule. Lobar pneumonia, the prototype of which is Streptococcus pneumoniae, spreads centrifugally through the pores of Kohn and canals of Lambert as edema fluid and then neutrophil-rich exudate, filling contiguous airspaces while initially sparing the airways; the result on CT is a nonsegmental, homogeneous $\textbf{consolidation}$ — opacification that obscures the underlying vessels — traversed by patent, air-filled bronchi, the $\textbf{air bronchogram}$. Attenuation is that of soft tissue (roughly $20$–$40\,\text{HU}$ before contrast); the presence of frankly low-attenuation ($<20\,\text{HU}$, often $0$ to $10$) non-enhancing regions within consolidation signals necrotizing pneumonia or evolving abscess and shifts management toward prolonged or broadened therapy and drainage. Bronchopneumonia, typified by Staphylococcus aureus, Pseudomonas, and many gram-negatives, propagates along the airways, producing patchy, multifocal, often bilateral peribronchial consolidation and centrilobular nodularity with $\textbf{tree-in-bud}$ opacities reflecting bronchiolar impaction. The atypical and viral pneumonias (Mycoplasma, respiratory viruses, Pneumocystis) injure the interstitium and incompletely fill alveoli, yielding ground-glass opacity, smooth interlobular septal thickening, and a more diffuse pattern.

Interpretation is fundamentally Bayesian, because the imaging pattern is rarely pathognomonic and the clinical context dominates the posterior. A peripheral, basal consolidation with the $\textbf{reversed-halo sign}$ (central ground glass rimmed by denser consolidation) in a subacute, antibiotic-unresponsive patient should raise the posterior probability of organizing pneumonia, which on confident recognition redirects management entirely toward corticosteroids and a search for a trigger rather than further antibiotics. The $\textbf{halo sign}$ — a nodule surrounded by ground glass representing hemorrhagic infiltration — in a profoundly neutropenic patient is, by virtue of an extremely high pretest probability, treated as angioinvasive aspergillosis until disproven, prompting empiric antifungal therapy; the later air-crescent sign marks recovery and sequestration of the infarct. Distribution sharpens the differential: a gravity-dependent posterior upper-lobe and superior lower-lobe distribution implicates aspiration; upper-lobe cavitary disease with surrounding tree-in-bud implicates reactivation tuberculosis; and perihilar bat-wing ground glass with sparing of the lung periphery in an immunocompromised host suggests Pneumocystis, which may evolve cysts that predispose to pneumothorax. The essential $\textbf{mimics}$ are non-infectious: organizing pneumonia, eosinophilic pneumonia, vasculitis, pulmonary infarction, lymphoma, and lepidic-growth adenocarcinoma can each masquerade as a focal consolidation, and the discriminator is temporal — true infection should improve over days to weeks, so consolidation that persists or migrates beyond six weeks mandates a deliberate hunt for these alternatives, classically with CT-guided or bronchoscopic sampling. The $\textbf{failure modes}$ couple technique and cognition: dependent atelectasis and respiratory motion mimic basal consolidation and dissolve on prone or repeat imaging; contrast timing alters the conspicuity of necrosis; and the most consequential cognitive errors are satisfaction of search, in which an obvious lobar consolidation halts the read before an occult nodule or effusion is found, and premature closure, in which a confident label of pneumonia forecloses the slowly resolving malignancy beneath it.

Ground-glass opacity (GGO) is a precise descriptive term, not a diagnosis: it denotes a hazy increase in lung attenuation that, unlike consolidation, does not obscure the underlying bronchovascular margins. Its mechanistic substrate is deliberately nonspecific because several distinct processes reduce the air-to-tissue ratio within the secondary lobule short of complete airspace filling — partial alveolar filling by fluid, pus, blood, or cells; thickening of the alveolar walls and interstitium; partial collapse; and increased capillary blood volume. This is why GGO sits at the center of so many differentials and why its interpretation must be anchored to acuity, distribution, and accompanying findings rather than to the opacity alone. Acute diffuse GGO in a dyspneic patient is dominated by hydrostatic or permeability edema, diffuse alveolar damage (the histologic correlate of ARDS), diffuse alveolar hemorrhage, and acute infection including viral pneumonitis; the presence of smooth interlobular septal thickening superimposed on GGO produces the $\textbf{crazy-paving}$ pattern, which narrows toward alveolar proteinosis, edema, hemorrhage, and certain pneumonias. Subacute or chronic GGO instead suggests hypersensitivity pneumonitis, the nonspecific interstitial pneumonia and organizing pneumonia patterns, eosinophilic pneumonia, drug toxicity, and — critically for nodule management — lepidic-pattern adenocarcinoma.

The focal subsolid nodule is where GGO acquires its sharpest prognostic stakes, and here interpretation is governed by the Fleischner Society guidance and, in the screening setting, by Lung-RADS. A subsolid nodule is either a pure ground-glass nodule or a part-solid nodule containing both ground-glass and soft-tissue-attenuation components; the part-solid lesion carries the highest malignancy risk of any nodule morphology because the solid component corresponds to the invasive, architecture-destroying portion of an adenocarcinoma, whereas pure ground glass often reflects in-situ or minimally invasive lepidic growth. The Fleischner thresholds operationalize this biology: a solitary pure ground-glass nodule below $6\,\text{mm}$ requires no routine follow-up, whereas one $\geq 6\,\text{mm}$ is followed at $6$–$12$ months and then periodically, because indolent lepidic lesions may be stable for years; a part-solid nodule $\geq 6\,\text{mm}$ warrants short-interval ($3$–$6$ month) follow-up and, if the solid component reaches or exceeds $6\,\text{mm}$, is treated as suspicious for invasive adenocarcinoma and triaged toward tissue sampling or resection. The key quantitative driver is growth of the solid component and the emergence of solidity within a previously pure ground-glass lesion, which marks the transition to invasion. The $\textbf{Bayesian}$ point is that the same morphology means different things at different pretest probabilities: a transient ground-glass nodule in a febrile patient is most likely infectious or hemorrhagic and should resolve on a short-interval scan, sparing the patient an invasive workup, whereas a persistent part-solid nodule in a smoker is adenocarcinoma until proven otherwise. The $\textbf{failure modes}$ are instructive. Technically, expiration, dependent atelectasis, and mosaic perfusion all simulate GGO, and thin-section ($\leq 1.5\,\text{mm}$) inspiratory imaging is required to distinguish true GGO from these artifacts; reconstruction kernel and dose strongly affect the conspicuity of faint solidity. Cognitively, the dominant errors are dismissing a faint persistent ground-glass nodule as scarring (premature closure) and, conversely, over-aggressively biopsying transient inflammatory GGO that a brief interval scan would have resolved.

The interstitial lung diseases are a heterogeneous family unified by injury to the pulmonary interstitium — the connective-tissue scaffold of alveolar walls, interlobular septa, and the peribronchovascular sheath — and high-resolution CT (HRCT) is the principal noninvasive instrument for resolving them into actionable categories. The diagnostic logic, codified in the 2018 ATS/ERS/JRS/ALAT guideline for idiopathic pulmonary fibrosis and its 2022 update, is to assign the HRCT to one of four pattern tiers whose probability of underlying usual interstitial pneumonia (UIP) histology decreases stepwise, because that probability determines whether surgical lung biopsy is required and whether antifibrotic therapy is appropriate. The defining lesion of UIP is irreversible fibrosis with architectural distortion. Its CT signature is $\textbf{honeycombing}$ — clustered, stacked, subpleural cystic airspaces of $3$–$10\,\text{mm}$ with shared thick walls — accompanied by $\textbf{traction bronchiectasis and bronchiolectasis}$ (airways pulled irregularly open by the surrounding scar) and reticulation, all distributed in a subpleural and basal-predominant gradient that worsens caudally. A pattern showing this distribution with honeycombing is classified as $\textit{UIP}$; the same distribution and traction bronchiectasis without definite honeycombing is $\textit{probable UIP}$; subpleural basal reticulation that does not meet these criteria is $\textit{indeterminate}$; and features such as upper-lobe or peribronchovascular predominance, extensive ground glass, mosaic attenuation with air trapping, cysts, nodules, or consolidation point to an $\textit{alternative diagnosis}$.

The clinical leverage of this scheme is that, in the correct context — an older patient with progressive dyspnea and no identifiable cause after multidisciplinary evaluation — a UIP or probable-UIP HRCT pattern is now considered sufficient to diagnose IPF without surgical biopsy, sparing a morbid procedure and enabling prompt initiation of the antifibrotic agents nintedanib or pirfenidone, which slow the decline in forced vital capacity. Conversely, a pattern suggesting an alternative diagnosis redirects the entire workup: upper-lobe-predominant centrilobular ground-glass nodules with mosaic attenuation and the headcheese (three-density) sign indicate fibrotic hypersensitivity pneumonitis and prompt a search for an inciting antigen; extensive ground glass with relative subpleural sparing and traction bronchiectasis suggests nonspecific interstitial pneumonia, strongly associated with connective-tissue disease and carrying a better prognosis; and peripheral basal consolidation with the reversed-halo sign suggests organizing pneumonia. The $\textbf{Bayesian}$ structure is explicit in the guideline: the same reticular pattern is read very differently depending on whether the patient has scleroderma, a drug exposure, or a bird-fancier's history, and the multidisciplinary discussion exists precisely to fuse this pretest information with the imaging into a posterior diagnosis. The $\textbf{failure modes}$ are both technical and cognitive and are unusually consequential because the diagnosis determines whether a patient receives antifibrotics, immunosuppression, or neither. Inadequate technique is the classic trap: thick collimation, expiratory atelectasis, and respiratory motion all create dependent reticulation and ground glass that mimic fibrosis, so the absence of prone or supplementary imaging can manufacture pseudo-disease, while motion artifact can simulate honeycombing. Cognitively, the gravest error is overcalling subpleural dependent change or paraseptal emphysema as honeycombing, which can mislabel a treatable inflammatory ILD as IPF, and the converse anchoring error of attributing a UIP pattern to age when a treatable connective-tissue or hypersensitivity etiology is present.

Acute pulmonary embolism is the lodgment of thrombus, usually propagated from the deep veins of the lower extremities or pelvis, within the pulmonary arterial tree, and CT pulmonary angiography (CTPA) is the reference imaging test. The acquisition is engineered around contrast kinetics: a bolus is timed so that the pulmonary arteries opacify to a target of roughly $250$–$300+\,\text{HU}$ at the main pulmonary artery, against which non-enhancing thrombus is detected. The direct sign of acute embolism is an intraluminal filling defect — either a central, low-attenuation clot outlined by surrounding contrast (the $\textbf{polo-mint}$ sign in cross-section or the $\textbf{railway-track}$ sign in long axis) or a complete occluding defect that may expand the vessel. Acute thrombus tends to lie centrally and form acute angles with the wall, whereas chronic thromboembolic material is eccentric, forms obtuse angles, may calcify, and produces webs, bands, and abrupt vessel cutoffs with the mosaic perfusion of chronic thromboembolic pulmonary hypertension. PIOPED II established the performance of multidetector CTPA, with sensitivity on the order of $83\%$ and specificity near $96\%$, and — crucially — demonstrated that diagnostic accuracy is contingent on the clinical pretest probability: a CTPA result concordant with the Wells-based pretest assessment is highly reliable, whereas a positive scan against a low pretest probability or a negative scan against a high pretest probability has a substantially lower predictive value and should prompt reconsideration or additional testing.

This concordance principle is the heart of the $\textbf{Bayesian}$ workflow that governs whether CTPA is even performed. In a patient meeting all PERC criteria with a low gestalt probability, the pretest probability is low enough that the harms of imaging outweigh its yield and no testing is indicated. In Wells low-or-intermediate-probability patients, an age-adjusted D-dimer or the YEARS algorithm raises the threshold for imaging and safely excludes embolism without CTPA in a large fraction, reducing radiation and contrast exposure. Only when the posterior probability remains meaningful does CTPA add value, and there its high sensitivity drives the negative likelihood ratio low enough that a high-quality negative study confidently excludes embolism. Once embolism is confirmed, CT immediately informs prognosis and management by assessing the right ventricle. Acute pressure overload dilates the RV, and a $\textbf{right-ventricular-to-left-ventricular short-axis diameter ratio}$ $> 1.0$, leftward bowing of the interventricular septum, reflux of contrast into the inferior vena cava and hepatic veins, and main-pulmonary-artery enlargement are CT markers of RV strain that, together with biomarkers, identify intermediate-high-risk patients who warrant closer monitoring or consideration of reperfusion. The clot burden itself matters less than this physiologic consequence, though saddle and large central emboli more often produce strain. The $\textbf{failure modes}$ are protocol-dependent and high-stakes. The dominant technical pitfall is $\textbf{transient interruption of the contrast bolus}$, in which a deep inspiration draws unopacified blood from the inferior vena cava and dilutes pulmonary arterial contrast, mimicking emboli or rendering the study nondiagnostic; respiratory and cardiac motion blur the subsegmental arteries; beam-hardening streak from dense contrast in the superior vena cava degrades the right main and upper-lobe arteries; and partial-volume averaging at branch points simulates defects. Cognitively, satisfaction of search after finding a large central clot causes additional emboli and incidental findings to be missed, and the isolated subsegmental filling defect is a genuine zone of uncertainty — with imperfect interobserver agreement and an unsettled treatment threshold — where overcalling artifact as clot can commit a patient to anticoagulation without benefit.

The pleural space is a near-virtual cavity between visceral and parietal layers, and its diseases are read on CT through two variables: the attenuation of its contents and the morphology of its lining. A pleural effusion is detected as a dependent, crescentic collection that layers posteriorly on supine imaging, and its Hounsfield value is the first discriminator of cause. A simple transudate (heart failure, cirrhosis, nephrotic syndrome) measures near water, roughly $0$–$15\,\text{HU}$, and is typically free-flowing and bilateral; an exudate is more variable and often higher in attenuation. Frank hyperattenuation reframes the differential sharply: a layering fluid-fluid level with a dependent attenuation of $35$–$70\,\text{HU}$ (the hematocrit effect) indicates a $\textbf{hemothorax}$, mandating attention to trauma, anticoagulation, or a bleeding pleural metastasis, whereas very high attenuation can also reflect prior talc pleurodesis or, rarely, a cholesterol effusion. Empyema — infected pleural fluid — declares itself not by attenuation alone but by morphology: with intravenous contrast the inflamed, thickened, enhancing visceral and parietal pleura envelop the loculated collection to produce the $\textbf{split-pleura sign}$, frequently accompanied by increased attenuation and stranding of the adjacent extrapleural fat and by gas locules that, in the absence of recent instrumentation, imply gas-forming organisms or a bronchopleural fistula. The split-pleura sign and loculation are decisive because they move management from simple drainage toward early tube thoracostomy, intrapleural fibrinolytics, and surgical decortication.

The morphology of the pleural lining separates benign from malignant disease, and the governing criteria derive from Leung's classic analysis. Features that substantially raise the posterior probability of malignant pleural disease are $\textbf{nodular pleural thickening}$, $\textbf{circumferential (rind-like) thickening}$, parietal pleural thickening exceeding $1\,\text{cm}$, and involvement of the mediastinal pleural surface; their specificity for malignancy is high, so any one of them in a patient with a known primary or an unexplained exudate should drive pleural biopsy. Malignant mesothelioma, characteristically related to asbestos exposure after a long latency, produces precisely this nodular, circumferential, mediastinal-surface rind, often with ipsilateral volume loss that fixes the mediastinum and encases the lung; metastatic adenocarcinoma (lung, breast) and pleural lymphoma produce similar nodularity. These must be distinguished from $\textbf{benign asbestos-related pleural disease}$ — discrete pleural plaques, frequently calcified, favoring the parietal pleura along the diaphragm and posterolateral chest wall while sparing the costophrenic angles — which is a marker of exposure rather than a malignancy and should not be over-called. The $\textbf{Bayesian}$ logic is that the same modest pleural thickening is interpreted very differently in a patient with an asbestos history and weight loss (raising mesothelioma) than in an asymptomatic patient with bilateral calcified plaques (benign exposure). The $\textbf{failure modes}$ blend technique and cognition. Supine acquisition causes effusions to layer posteriorly where they can be mistaken for ascites; the diaphragm itself is the discriminator, since pleural fluid lies posterior to the diaphragmatic crus and outside the bare area of the liver. Beam-hardening and partial-volume averaging at the lung-pleura interface simulate thickening, and dependent atelectasis compressed by an effusion can mimic a pleural mass until contrast enhancement and re-expansion clarify it. The dominant cognitive error is satisfaction of search — terminating the read at a large effusion without scrutinizing the pleural surface for the nodularity that changes the diagnosis from benign to malignant.

Thoracic oncology on CT spans two linked tasks: stratifying the indeterminate pulmonary nodule by its probability of malignancy, and, once cancer is confirmed, staging it to determine resectability and prognosis. Nodule stratification rests on morphology, size, growth, and pretest risk. Features that raise the posterior probability of malignancy include spiculated or lobulated margins (reflecting desmoplastic and infiltrative growth), upper-lobe location, a part-solid composition, larger size, and documented growth, whereas a smooth margin, fat attenuation (a benign hamartoma), and certain benign calcification patterns (central, laminated, popcorn, or diffuse) lower it. Solid-nodule growth is quantified by volume-doubling time; malignant solid nodules typically double in roughly $20$–$400$ days, so a solid nodule that is stable over two years or that doubles in under a month (favoring infection) is reassuring, while subsolid lesions may double far more slowly because indolent lepidic adenocarcinoma can be stable for years. Two frameworks operationalize this. For incidentally detected nodules the Fleischner Society guideline sets size-stratified follow-up intervals (with the $6\,\text{mm}$ threshold a key fulcrum and separate, more conservative pathways for subsolid lesions), and for screening of high-risk individuals Lung-RADS standardizes categories and management, building on the National Lung Screening Trial, which demonstrated a relative reduction in lung-cancer mortality of about $20\%$ with low-dose CT screening, and the NELSON trial, which confirmed a mortality benefit using volumetric, growth-based nodule management.

Staging follows the IASLC TNM system (the 8th edition, with the 9th-edition refinements now in use), which CT serves by characterizing the primary tumor, nodal disease, and metastases. The $\textbf{T}$ descriptor is driven principally by size in centimeter increments and by local invasion — of the visceral pleura, chest wall, diaphragm, mediastinal structures, or the presence of separate tumor nodules — so accurate maximal-diameter measurement on the appropriate window and careful assessment of fat planes are central. The $\textbf{N}$ descriptor is the principal determinant of resectability and is mapped to the IASLC nodal stations: N1 (ipsilateral hilar/intrapulmonary), N2 (ipsilateral mediastinal/subcarinal), and N3 (contralateral mediastinal/hilar or supraclavicular). CT's nodal criterion — a short-axis diameter exceeding $1\,\text{cm}$ — is an imperfect surrogate, because reactive nodes enlarge and micrometastases inhabit normal-sized nodes; this is the explicit $\textbf{Bayesian}$ limitation that mandates pathologic confirmation of CT-positive or PET-positive mediastinal nodes (by endobronchial ultrasound or mediastinoscopy) before denying a patient curative surgery, since the cost of an N-stage error is the difference between resection and palliation. The $\textbf{M}$ descriptor encompasses contralateral lung nodules, pleural or pericardial dissemination (M1a), and distant metastasis (M1b/M1c), with the adrenal glands, liver, bone, and brain being characteristic targets that anchor the search pattern. The $\textbf{failure modes}$ are familiar but high-consequence. Technically, respiratory motion changes measured nodule size and can blur a spiculated margin into a smooth one; reconstruction kernel and slice thickness alter both detection and the apparent solid fraction; and partial-volume averaging mismeasures small nodules, so consistent technique across follow-up studies is essential to interpret growth. Cognitively, satisfaction of search is the dominant miss mechanism — a conspicuous mass captures attention while a second synchronous primary, a small metastasis, or a malignant effusion goes unreported — and anchoring on a benign-appearing calcified nodule can mask an adjacent suspicious lesion.

Acute aortic syndromes are a spectrum of life-threatening medial pathologies — classic dissection, intramural hematoma, and penetrating atherosclerotic ulcer — and ECG-gated or fast non-gated CT angiography is the diagnostic standard because of its speed, availability, and near-complete anatomic coverage. The unifying pathophysiology is failure of the aortic media. In classic $\textbf{dissection}$, blood enters the media through an intimal tear and propagates a false lumen separated from the true lumen by an intimointimal flap; on CTA the flap is a thin curvilinear filling defect within the opacified aorta, and the diagnosis turns on identifying the true and false lumina and the extent of the dissection. The clinically decisive classification is the $\textbf{Stanford}$ scheme: type A involves the ascending aorta and is a surgical emergency because of the risk of pericardial tamponade, aortic-valve incompetence, and coronary or arch-vessel malperfusion, whereas type B spares the ascending aorta and begins distal to the left subclavian artery, and — when uncomplicated — is managed medically with aggressive impulse control, reserving thoracic endovascular aortic repair for complications such as malperfusion, rupture, or refractory pain. The $\textbf{DeBakey}$ scheme refines anatomy: type I involves ascending aorta, arch, and descending; type II is confined to the ascending aorta; and type III begins in the descending aorta. Identifying the true lumen is operationally essential because stent-grafts and the dominant outflow must address it; the true lumen is usually the smaller, continuous with the undissected aorta, surrounds the celiac axis in most cases, and may show the $\textbf{beak sign}$ at the flap's acute angle, while the false lumen is often larger, more delayed in opacification, and may contain the $\textbf{cobweb sign}$ of residual media.

The other two syndromes share the same threat with different signatures. $\textbf{Intramural hematoma}$ is a crescentic, non-enhancing high-attenuation thickening ($> 5\,\text{mm}$, frequently $60$–$70+\,\text{HU}$ on non-contrast imaging) within the aortic wall, representing hemorrhage from the vasa vasorum without a demonstrable intimal flap; it is classified and managed by the same ascending-versus-descending logic as dissection because it can progress to overt dissection or rupture, and the non-contrast acquisition is indispensable since contrast can mask its intrinsic hyperattenuation. The $\textbf{penetrating atherosclerotic ulcer}$ is a contrast-filled outpouching that extends beyond the expected intimal contour through an ulcerated atherosclerotic plaque, often with adjacent intramural hematoma, and signals high rupture risk when deep, wide, or enlarging. Across all three, the search pattern extends beyond the flap to the consequences: pericardial and mediastinal hemorrhage, periaortic and pleural blood signaling impending or contained rupture, and — a major source of mortality and morbidity — branch-vessel $\textbf{malperfusion}$, recognized when the dissection compromises the coronary, arch, visceral, renal, or iliac origins. The $\textbf{Bayesian}$ frame operates at triage: a high pretest probability (tearing interscapular pain, pulse deficit, mediastinal widening, or a markedly elevated D-dimer) justifies immediate CTA, and the aortic dissection detection risk score formalizes this stratification. The $\textbf{failure modes}$ are notorious and specifically simulate or conceal dissection. $\textbf{Cardiac-motion artifact}$ at the aortic root produces a curvilinear double contour that mimics an ascending flap — the single most common false positive — and is resolved by ECG gating or by recognizing its motion-blur character and its absence on a repeat phase; streak artifact from dense contrast in the superior vena cava and brachiocephalic vein, pulsation, and beam-hardening likewise generate pseudoflaps. Conversely, the true intimal flap of a slow-flow or thrombosed false lumen can be subtle, and the gravest cognitive errors are anchoring on a benign mimic to dismiss a real type A dissection and satisfaction of search, in which the flap is found but a critical malperfusion or contained rupture that dictates the operative plan is overlooked.