I.a I - Interstitial/parenchymal lung disease
I.a - Pneumonitis (ILD), acute and/or severe (may cause ARDS) I.b I - Interstitial/parenchymal lung disease
I.b - Pneumonitis (ILD) I.c I - Interstitial/parenchymal lung disease
I.c - Eosinophilic pneumonia (pulmonary infiltrates and eosinophilia) I.d I - Interstitial/parenchymal lung disease
I.d - Organizing pneumonia pattern (an area or areas of consolidation on imaging) I.e I - Interstitial/parenchymal lung disease
I.e - Acute eosinophilic pneumonia (AEP) I.f I - Interstitial/parenchymal lung disease
I.f - Acute fibrinous organizing pneumonia (AFOP) I.g I - Interstitial/parenchymal lung disease
I.g - Pulmonary fibrosis I.h I - Interstitial/parenchymal lung disease
I.h - Subclinical pulmonary infiltrates/ILD I.k I - Interstitial/parenchymal lung disease
I.k - Lung nodule or nodules I.l I - Interstitial/parenchymal lung disease
I.l - Diffuse alveolar damage (DAD) (see alsoo under IIb and XVf) I.s I - Interstitial/parenchymal lung disease
I.s - A mass or masses I.u I - Interstitial/parenchymal lung disease
I.u - Relapsing or migrating pneumonitis/pneumonia (see also Id) I.w I - Interstitial/parenchymal lung disease
I.w - Rapidly progressive ILD/pulmonary fibrosis (Hamman-Rich syndrome) I.z I - Interstitial/parenchymal lung disease
I.z - An area or areas of consolidation I.aa I - Interstitial/parenchymal lung disease
I.aa - Delayed ILD, -pneumonitis, -fibrosis I.ao I - Interstitial/parenchymal lung disease
I.ao - Pulmonary infiltrates I.at I - Interstitial/parenchymal lung disease
I.at - Amiodarone pulmonary toxicity - Amiodarone lung I.ax I - Interstitial/parenchymal lung disease
I.ax - Chronic pneumonitis I.ay I - Interstitial/parenchymal lung disease
I.ay - Unilateral interstitial lung disease II.b II - Pulmonary edema - Acute lung injury - ARDS
II.b - ARDS - Acute lung injury II.l II - Pulmonary edema - Acute lung injury - ARDS
II.l - Postoperative ARDS III.a III - Pulmonary/alveolar./airway hemorrhage/bleeding
III.a - Alveolar hemorrhage (AH), diffuse AH (DAH) III.c III - Pulmonary/alveolar./airway hemorrhage/bleeding
III.c - Hemoptysis IV.a IV - Airway involvement
IV.a - Bronchospasm - Wheezing - Asthma IV.d IV - Airway involvement
IV.d - Cough (lone) IV.r IV - Airway involvement
IV.r - Airway pigmentation (black, bluish, petechial) V.a V - Pleural and/or pericardial involvement
V.a - Pleural effusion (uni- or bilateral) (can accompany DI-LDs) V.c V - Pleural and/or pericardial involvement
V.c - Pleural thickening V.d V - Pleural and/or pericardial involvement
V.d - Pleural/pericardial effusion, ANA positive (DI lupus) V.t V - Pleural and/or pericardial involvement
V.t - The apical cap VI.a VI - Pulmonary vasculopathies
VI.a - Pulmonary embolism - Venous thrombosis/thromboembolism VI.i VI - Pulmonary vasculopathies
VI.i - Acute pulmonary hypertension VIII.a VIII - Central-large-upper airway (incl. pharyngeal-nasal) involvement
VIII.a - Angioedema (may cause UAO, asphyxia and death) IX.a IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.a - Diaphragm/inspiratory muscle weakness/paralysis (w/wo ARF) IX.d IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.d - Respiratory failure from ventilatory depression (due to neuromuscular blockade/paralysis) IX.ai IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.ai - Acute respiratory failure (hypoxemia, acidosis) from myxedema (-coma) X.d X - Systemic/Distant conditions, syndromes and reactions
X.d - Lupus - Lupus syndrome (see also Vd) X.f X - Systemic/Distant conditions, syndromes and reactions
X.f - Anaphylaxis-Anaphylactoid reaction (can be fatal) X.q X - Systemic/Distant conditions, syndromes and reactions
X.q - Systemic inflammatory response XI.c XI - Miscellaneous
XI.c - Pleuritic chest pain XI.f XI - Miscellaneous
XI.f - Bluish, pseudocyanotic skin discoloration XI.bq XI - Miscellaneous
XI.bq - Platypnea-orthodeoxia XII.a XII - Cardiovascular involvement / toxicity
XII.a - Left or biventricular dysfunction/failure XII.c XII - Cardiovascular involvement / toxicity
XII.c - Pericardial effusion (w/wo tamponade) XII.e XII - Cardiovascular involvement / toxicity
XII.e - Takotsubo (stress) cardiomyopathy XII.l XII - Cardiovascular involvement / toxicity
XII.l - Cardiac arrhythmias or dysrhythmias (AF, VT, VF, TdP) XII.n XII - Cardiovascular involvement / toxicity
XII.n - Cardiovascular collapse - Cardiogenic shock - Hypotension XII.p XII - Cardiovascular involvement / toxicity
XII.p - QTc prolongation XII.t XII - Cardiovascular involvement / toxicity
XII.t - Torsades de pointe XII.af XII - Cardiovascular involvement / toxicity
XII.af - Coronary vasospasm - Vasospastic angina XII.au XII - Cardiovascular involvement / toxicity
XII.au - Heart failure (thyrotoxicosis-, thyroid storm-mediated) XV.a XV - Pathology
XV.a - Path: NSIP-cellular pattern (see also Ia, Ib) XV.b XV - Pathology
XV.b - Path: Eosinophilic pneumonia (subacute or acute) (see also Ic) XV.c XV - Pathology
XV.c - Path: Organizing pneumonia (OP/BOOP) pattern (see also Id) XV.d XV - Pathology
XV.d - Path: Acute fibrinous organizing pneumonia (AFOP-pattern) (see also If) XV.f XV - Pathology
XV.f - Path: Diffuse alveolar damage (DAD-pattern) (see also IL) XV.g XV - Pathology
XV.g - Path: Alveolar hemorrhage (see also IIIa) XV.h XV - Pathology
XV.h - Path: NSIP-fibrotic pattern XV.i XV - Pathology
XV.i - Path: Pneumocyte atypia (reactive epithelial cells) (a.k.a. the "Napoleon Hat" sign) XV.j XV - Pathology
XV.j - Path: Pulmonary fibrosis (UIP-pattern) XV.k XV - Pathology
XV.k - Path: Desquamative interstitial pneumonia (DIP-pattern) XV.m XV - Pathology
XV.m - Path: Lymphoid hyperplasia (including nodular- or a lymphocytic interstitial pneumonia pattern) XV.o XV - Pathology
XV.o - Path: Endogenous lipoid pneumonia (phospholipidosis) XV.r XV - Pathology
XV.r - Path: Smudged geographic necrosis XV.ao XV - Pathology
XV.ao - Path: Pleuritis, pleural fibrosis XV.bt XV - Pathology
XV.bt - Path: Histiocytic proliferation XVI.b XVI - Imaging
XVI.b - Imaging: Ground-glass opacities (GGO) / shadowing XVI.i XVI - Imaging
XVI.i - Imaging: An area or areas of involvement with a recognizable anatomic distribution XVI.j XVI - Imaging
XVI.j - Imaging: Wandering (migratory) pulmonary opacities XVI.n XVI - Imaging
XVI.n - Imaging: Intralobular septal thickening - Crazy paving XVI.s XVI - Imaging
XVI.s - Imaging: An area or areas of involvement with high attenuation numbers or metallic density XVI.w XVI - Imaging
XVI.w - Imaging: Lung nodule or nodules XVI.aa XVI - Imaging
XVI.aa - Imaging: Nodules, mass or masses with a central area of low attenuation (see also under XVIaa) XVI.ab XVI - Imaging
XVI.ab - Imaging: Cavitating/cavitary lung nodule, mass or nodules (see also Iq, XIs, XIIi, XVIaa and XVIIp) XVI.ay XVI - Imaging
XVI.ay - Imaging: Asymmetrical, predominantly unilateral involvement XVI.az XVI - Imaging
XVI.az - Imaging: Solitary pulmonary nodule XVI.bj XVI - Imaging
XVI.bj - Imaging: Atelectasis (a combination of increased density and volume loss. Typically lobar) XVI.bq XVI - Imaging
XVI.bq - Imaging: A pattern consistent with pleuroparenchymal fibroelastosis XVI.bt XVI - Imaging
XVI.bt - Imaging: A subsolid lung nodule or nodules XVI.bx XVI - Imaging
XVI.bx - Imaging: Waxing and waning nodules XVII.e XVII - Infections & related conditions
XVII.e - Pneumocystis jiroveci pneumonia XVIII.b XVIII - Distinctive patterns - 'Eye-catchers'
XVIII.b - Eye catcher: Electron-dense area or areas of consolidation on unenhanced chest CT XVIII.j XVIII - Distinctive patterns - 'Eye-catchers'
XVIII.j - Eye-catcher: Avid lung uptake on 18F-PET-CT XVIII.k XVIII - Distinctive patterns - 'Eye-catchers'
XVIII.k - Eye-catcher: An electron-dense liver on unenhanced CT XVIII.p XVIII - Distinctive patterns - 'Eye-catchers'
XVIII.p - Eye-catcher: An electron-dense ('white') thyroid XIX.a XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.a - BAL: An excess proportion of lymphocytes XIX.b XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.b - BAL: An excess proportion of neutrophils XIX.c XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.c - BAL: An excess proportion of eosinophils XIX.e XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.e - BAL: Stainable iron in macrophages XIX.f XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.f - BAL: Foamy macrophages XIX.m XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.m - BAL: A normal cell differential XIX.p XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.p - BAL: Lamellar bodies in AM on EM

I.y I - Interstitial/parenchymal lung disease
I.y - Progression, acceleration or exacerbation of preexisting ILD/fibrosis XVI.i XVI - Imaging
XVI.i - Imaging: An area or areas of involvement with a recognizable anatomic distribution

I.a I - Interstitial/parenchymal lung disease
I.a - Pneumonitis (ILD), acute and/or severe (may cause ARDS) I.b I - Interstitial/parenchymal lung disease
I.b - Pneumonitis (ILD) I.c I - Interstitial/parenchymal lung disease
I.c - Eosinophilic pneumonia (pulmonary infiltrates and eosinophilia) I.d I - Interstitial/parenchymal lung disease
I.d - Organizing pneumonia pattern (an area or areas of consolidation on imaging) I.e I - Interstitial/parenchymal lung disease
I.e - Acute eosinophilic pneumonia (AEP) I.f I - Interstitial/parenchymal lung disease
I.f - Acute fibrinous organizing pneumonia (AFOP) I.h I - Interstitial/parenchymal lung disease
I.h - Subclinical pulmonary infiltrates/ILD I.ad I - Interstitial/parenchymal lung disease
I.ad - Radiation recall pneumonitis I.aj I - Interstitial/parenchymal lung disease
I.aj - Shrinking lung syndrome II.b II - Pulmonary edema - Acute lung injury - ARDS
II.b - ARDS - Acute lung injury II.d II - Pulmonary edema - Acute lung injury - ARDS
II.d - Pulmonary edema, cardiogenic III.g III - Pulmonary/alveolar./airway hemorrhage/bleeding
III.g - Bleeding/hemorrhage from/around preexisting lung tumor (Localized AH) III.j III - Pulmonary/alveolar./airway hemorrhage/bleeding
III.j - Anti-GBM-related pneumorenal syndrome and DAH (Goodpasture) or flare of III.m III - Pulmonary/alveolar./airway hemorrhage/bleeding
III.m - Coagulopathy IV.a IV - Airway involvement
IV.a - Bronchospasm - Wheezing - Asthma IV.c IV - Airway involvement
IV.c - Obliterative bronchiolitis (a pattern consistent with) (see also IVn, XVx) IV.n IV - Airway involvement
IV.n - Obstructive airway dysfunction (see also IVc, XVx) IV.y IV - Airway involvement
IV.y - Eosinophilic airway disease (asthma, bronchitis, bronchiolitis) V.a V - Pleural and/or pericardial involvement
V.a - Pleural effusion (uni- or bilateral) (can accompany DI-LDs) V.m V - Pleural and/or pericardial involvement
V.m - Pleuropericarditis - Pleuropericardial effusion (ANA unknown or negative) VI.b VI - Pulmonary vasculopathies
VI.b - Pulmonary arterial hypertension VIII.t VIII - Central-large-upper airway (incl. pharyngeal-nasal) involvement
VIII.t - Vocal cord paresis/paralysis IX.a IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.a - Diaphragm/inspiratory muscle weakness/paralysis (w/wo ARF) IX.d IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.d - Respiratory failure from ventilatory depression (due to neuromuscular blockade/paralysis) IX.h IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.h - Dyspnea, unexplained otherwise IX.r IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.r - Myasthenia gravis - Myasthenic-like syndrome IX.y IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.y - A flare or relapse of preexisting myasthenia gravis IX.ab IX - Neuromuscular / CNS involvement - Disordered breathing during sleep
IX.ab - Guillain-Barré (or GB-like) syndrome (w/wo ARF) X.a X - Systemic/Distant conditions, syndromes and reactions
X.a - DRES syndrome - DRESS-like reaction X.d X - Systemic/Distant conditions, syndromes and reactions
X.d - Lupus - Lupus syndrome (see also Vd) X.f X - Systemic/Distant conditions, syndromes and reactions
X.f - Anaphylaxis-Anaphylactoid reaction (can be fatal) X.g X - Systemic/Distant conditions, syndromes and reactions
X.g - Hypersensitivity reaction (may manifest with fever and/or skin, throat or airway involvement) X.j X - Systemic/Distant conditions, syndromes and reactions
X.j - Myopathy-Myositis-Polymyositis (see also under Xba) X.k X - Systemic/Distant conditions, syndromes and reactions
X.k - Sarcoid-like granulomatosis (endo-/extrathoracic) X.m X - Systemic/Distant conditions, syndromes and reactions
X.m - Vascultis (pulmonary, extrathoracic or systemic)-ANCA status unknown or negative) X.n X - Systemic/Distant conditions, syndromes and reactions
X.n - Tumor lysis syndrome (TLS) X.s X - Systemic/Distant conditions, syndromes and reactions
X.s - Vasculitis, pulmonary (w/wo AH), extrapulmonary, systemic: ANCA-positive X.u X - Systemic/Distant conditions, syndromes and reactions
X.u - Multiple organ dysfunction/failure (MODS/MOF) X.ac X - Systemic/Distant conditions, syndromes and reactions
X.ac - Anti-GBM antibody disease (Goodpasture-like or flare of preexisting GS) X.ay X - Systemic/Distant conditions, syndromes and reactions
X.ay - Polymyalgia rheumatica XI.i XI - Miscellaneous
XI.i - Esophageal toxicity XI.av XI - Miscellaneous
XI.av - Deterioration of graft function in SOT recipients XI.ax XI - Miscellaneous
XI.ax - Esophageal stenosis/stricture XII.c XII - Cardiovascular involvement / toxicity
XII.c - Pericardial effusion (w/wo tamponade) XII.d XII - Cardiovascular involvement / toxicity
XII.d - Myocarditis (can be fulminant) XII.g XII - Cardiovascular involvement / toxicity
XII.g - Coronary artery disease (acute) - Myocardial ischemia/infarction XII.n XII - Cardiovascular involvement / toxicity
XII.n - Cardiovascular collapse - Cardiogenic shock - Hypotension XII.s XII - Cardiovascular involvement / toxicity
XII.s - Heart block (bundle branch- or AV-) XII.ai XII - Cardiovascular involvement / toxicity
XII.ai - Cardiotoxicity XII.ax XII - Cardiovascular involvement / toxicity
XII.ax - Necrotizing myocarditis (w/o evidence for CAD) XV.c XV - Pathology
XV.c - Path: Organizing pneumonia (OP/BOOP) pattern (see also Id) XV.d XV - Pathology
XV.d - Path: Acute fibrinous organizing pneumonia (AFOP-pattern) (see also If) XV.g XV - Pathology
XV.g - Path: Alveolar hemorrhage (see also IIIa) XV.x XV - Pathology
XV.x - Path: Acute/subacute bronchiolitis (see also IVc, IVi) XV.bb XV - Pathology
XV.bb - Path: Airway inflammation XV.cv XV - Pathology
XV.cv - Path: Peribronchiolar granulomas XVI.i XVI - Imaging
XVI.i - Imaging: An area or areas of involvement with a recognizable anatomic distribution XVI.v XVI - Imaging
XVI.v - Imaging: Centrilobular micronodules (can be diffuse) XVI.w XVI - Imaging
XVI.w - Imaging: Lung nodule or nodules XVI.ab XVI - Imaging
XVI.ab - Imaging: Cavitating/cavitary lung nodule, mass or nodules (see also Iq, XIs, XIIi, XVIaa and XVIIp) XVI.af XVI - Imaging
XVI.af - Imaging: Lung cysts or bullae (see also XVI ah/bf) XVI.ai XVI - Imaging
XVI.ai - Imaging: A 'tree-in-bud' pattern XVII.a XVII - Infections & related conditions
XVII.a - Respiratory tract infection incl. pneumonia XVII.c XVII - Infections & related conditions
XVII.c - Aspergillosis, pulmonary (invasive, allergic [ABPA], or mycetoma) XVII.g XVII - Infections & related conditions
XVII.g - Tuberculosis (pulmonary, pulmonary, extrapulmonary or disseminated). Reactivation or de novo XVII.aj XVII - Infections & related conditions
XVII.aj - Bordetella bronchiseptica pulmonary infection XIX.a XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.a - BAL: An excess proportion of lymphocytes XIX.b XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.b - BAL: An excess proportion of neutrophils XIX.c XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.c - BAL: An excess proportion of eosinophils

I.b I - Interstitial/parenchymal lung disease
I.b - Pneumonitis (ILD) I.g I - Interstitial/parenchymal lung disease
I.g - Pulmonary fibrosis I.j I - Interstitial/parenchymal lung disease
I.j - Exogenous lipoid pneumonia (subacute, acute) I.k I - Interstitial/parenchymal lung disease
I.k - Lung nodule or nodules I.s I - Interstitial/parenchymal lung disease
I.s - A mass or masses I.z I - Interstitial/parenchymal lung disease
I.z - An area or areas of consolidation I.ap I - Interstitial/parenchymal lung disease
I.ap - Paraffinoma - Lipidoma (ELP in the form of a nodule or mass) II.b II - Pulmonary edema - Acute lung injury - ARDS
II.b - ARDS - Acute lung injury III.c III - Pulmonary/alveolar./airway hemorrhage/bleeding
III.c - Hemoptysis IV.w IV - Airway involvement
IV.w - Plugs/casts in the airways X.e X - Systemic/Distant conditions, syndromes and reactions
X.e - Autoimmunity-Autoimmune conditions (+ANA, +anti-ds-DNA, +ANCAs, other auto-Abs) XIII.a XIII - Neoplastic conditions
XIII.a - Lung cancer XV.e XV - Pathology
XV.e - Path: ILD with a granulomatous component (see also Im) XV.h XV - Pathology
XV.h - Path: NSIP-fibrotic pattern XV.p XV - Pathology
XV.p - Path: Exogenous lipoid pneumonia XV.q XV - Pathology
XV.q - Path: Foreign body deposits/granulomatous reaction XVI.a XVI - Imaging
XVI.a - Imaging: Diffuse haze XVI.f XVI - Imaging
XVI.f - Imaging: Bibasilar opacities/shadowing XVI.i XVI - Imaging
XVI.i - Imaging: An area or areas of involvement with a recognizable anatomic distribution XVI.k XVI - Imaging
XVI.k - Imaging: An area or areas of consolidation XVI.n XVI - Imaging
XVI.n - Imaging: Intralobular septal thickening - Crazy paving XVI.r XVI - Imaging
XVI.r - Imaging: An area or areas of involvement with low attenuation numbers XVI.w XVI - Imaging
XVI.w - Imaging: Lung nodule or nodules XVI.z XVI - Imaging
XVI.z - Imaging: A large nodule or a mass XVI.ab XVI - Imaging
XVI.ab - Imaging: Cavitating/cavitary lung nodule, mass or nodules (see also Iq, XIs, XIIi, XVIaa and XVIIp) XVI.az XVI - Imaging
XVI.az - Imaging: Solitary pulmonary nodule XVII.h XVII - Infections & related conditions
XVII.h - Nontuberculous pulmonary mycobacterial infection/superinfection XVIII.r XVIII - Distinctive patterns - 'Eye-catchers'
XVIII.r - Eye catcher: A milky, cloudy or lumpy BAL fluid XIX.a XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.a - BAL: An excess proportion of lymphocytes XIX.h XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.h - BAL: Exogenous (oil red O-stainable) lipids in BAL macrophages XIX.j XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.j - BAL: Free exogenous lipids in supernatant XXIV.f XXIV - Veterinary medicine
XXIV.f - Veterinary: Exogenous lipoid pneumonia

I.b I - Interstitial/parenchymal lung disease
I.b - Pneumonitis (ILD) I.c I - Interstitial/parenchymal lung disease
I.c - Eosinophilic pneumonia (pulmonary infiltrates and eosinophilia) I.d I - Interstitial/parenchymal lung disease
I.d - Organizing pneumonia pattern (an area or areas of consolidation on imaging) I.h I - Interstitial/parenchymal lung disease
I.h - Subclinical pulmonary infiltrates/ILD I.o I - Interstitial/parenchymal lung disease
I.o - Focal/localized area of pneumonitis/fibrosis I.u I - Interstitial/parenchymal lung disease
I.u - Relapsing or migrating pneumonitis/pneumonia (see also Id) I.v I - Interstitial/parenchymal lung disease
I.v - Abnormal lung function/pulmonary physiology (PFTs) without necessarily imaging or clinical evidence I.z I - Interstitial/parenchymal lung disease
I.z - An area or areas of consolidation V.t V - Pleural and/or pericardial involvement
V.t - The apical cap XII.b XII - Cardiovascular involvement / toxicity
XII.b - Valvular heart disease and/or dysfunction XII.c XII - Cardiovascular involvement / toxicity
XII.c - Pericardial effusion (w/wo tamponade) XII.o XII - Cardiovascular involvement / toxicity
XII.o - Coronary artery disease (subacute, chronic) XIII.a XIII - Neoplastic conditions
XIII.a - Lung cancer XIII.m XIII - Neoplastic conditions
XIII.m - Tumor of the heart XV.c XV - Pathology
XV.c - Path: Organizing pneumonia (OP/BOOP) pattern (see also Id) XVI.i XVI - Imaging
XVI.i - Imaging: An area or areas of involvement with a recognizable anatomic distribution XIX.a XIX - Cytological, biochemical features of/in BAL, pleural fluid or FNA
XIX.a - BAL: An excess proportion of lymphocytes

I.j I - Interstitial/parenchymal lung disease
I.j - Exogenous lipoid pneumonia (subacute, acute) XVI.i XVI - Imaging
XVI.i - Imaging: An area or areas of involvement with a recognizable anatomic distribution XVI.r XVI - Imaging
XVI.r - Imaging: An area or areas of involvement with low attenuation numbers