CONTENTS

• Preamble: A slightly different approach
• Frontal chest radiograph algorithm
• CT algorithm
  • (1) Lines & tubes
  • (2) Right heart & pulmonary embolism
    • Contrast reflux into the IVC
    • Pericardial effusion
    • Right atrial dilation
    • Right ventricular dilation
    • Right ventricular wall thickness
    • Pulmonary artery dilation
    • Evaluation for PE
  • (3) Left heart
    • Left atrial dilation
    • Left ventricular dilation
    • Left ventricular wall thickness
    • Mitral valve calcification
    • Aortic valve calcification
    • Coronary artery calcification
  • (4) Airways
  • (5) Mediastinum & hila
  • (6) Lungs
• Lines and tubes
  • Endotracheal tube
  • Tracheostomy tube
  • Central venous catheter
  • Feeding tube
  • Chest tube
  • Intra-aortic balloon pump
  • Pulmonary artery catheter
• Questions & discussion

abbreviations used in the pulmonary section:

• AE-ILD: Acute exacerbation of ILD 📖
• AIP: Acute interstitial pneumonia (Hamman-Rich syndrome) 📖
• ANA: Antinuclear antibody 📖
• ANCA: Antineutrophil cytoplasmic antibodies 📖
• ARDS: Acute respiratory distress syndrome 📖
• ASS: Antisynthetase Syndrome 📖
• BAL: Bronchoalveolar lavage 📖
• BiPAP: Bilevel positive airway pressure 📖
• COP: Cryptogenic organizing pneumonia 📖
• CPAP: Continuous positive airway pressure 📖
• CPFE: Combined pulmonary fibrosis and emphysema 📖
• CTD-ILD: Connective tissue disease associated interstitial lung disease 📖
• CTEPH: Chronic thromboembolic pulmonary hypertension 📖
• DAD: Diffuse alveolar damage 📖
• DAH: Diffuse alveolar hemorrhage 📖
• DIP: Desquamative interstitial pneumonia 📖
• DLCO: Diffusing capacity for carbon monoxide 📖
• FEV1: Forced expiratory volume in 1 second 📖
• FVC: Forced vital capacity 📖
• GGO: Ground glass opacity 📖
• GLILD: Granulomatous and lymphocytic interstitial lung disease 📖
• HFNC: High flow nasal cannula 📖
• HP: Hypersensitivity pneumonitis 📖
• IPAF: Interstitial pneumonia with autoimmune features 📖
• IPF: Idiopathic pulmonary fibrosis 📖
• IVIG: Intravenous immunoglobulin 📖
• LAM: Lymphangioleiomyomatosis 📖
• LIP: Lymphocytic interstitial pneumonia 📖
• MCTD: Mixed connective tissue disease 📖
• NIV: Noninvasive ventilation (including CPAP or BiPAP) 📖
• NSIP: Nonspecific interstitial pneumonia 📖
• NTM: Non-tuberculous mycobacteria 📖
• OP: Organizing pneumonia 📖
• PAP: Pulmonary alveolar proteinosis 📖
• PE: Pulmonary embolism 📖
• PFT: Pulmonary function test 📖
• PLCH: Pulmonary Langerhans Cell Histiocytosis 📖
• PPFE: Pleuroparenchymal fibroelastosis 📖
• PPF: Progressive pulmonary fibrosis 📖
• PVOD/PCH Pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis 📖
• RB-ILD: Respiratory bronchiolitis-associated interstitial lung disease 📖
• RP-ILD: Rapidly progressive interstitial lung disease 📖
• TNF: tumor necrosis factor
• UIP: Usual Interstitial Pneumonia 📖

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preamble: a different approach for acute care clinicians

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• The approaches below describe algorithms for acute care clinicians in the evaluation of acutely unwell patients.
• These approaches are intentionally different from the traditional approach of radiologists. For example:
  • (1) These approaches intentionally omit items which are less important for immediate management (e.g., bony abnormalities). It is assumed that a radiologist will eventually read the film and find these abnormalities.
  • (2) There is an added focus on cardiac abnormalities within the CT scans (e.g., chamber size). Unfortunately, these findings are frequently ignored.(37675086) Acute care clinicians with proficiency in echocardiography are well positioned to understand cardiac pathophysiology, so it doesn't take much effort to apply this understanding to CT scan interpretation. 

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frontal chest radiograph algorithm

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Below is a streamlined algorithm for a bedside clinician evaluating a frontal chest radiograph in the acute inpatient context. Always follow up on the official radiology read to avoid missing subtle lesions (e.g., bony pathology, subtle nodules). For a more complete algorithm for approaching the chest radiograph, see this chapter by Drs. Klein and Rosado-de-Christenson: 📄

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1) lines & tubes

• Endotracheal tube
• Tracheostomy tube
• Central venous catheter
• Feeding tube
• Chest tube
• Intra-aortic balloon pump
• Pulmonary artery catheter

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2) apices & soft tissue

• ? Subcutaneous air
  • Suggests pneumothorax or pneumomediastinum.
  • Subcutaneous air in the neck or supraclavicular region may be the most obvious finding in many patients.
• ? Pneumothorax
  • Diagnosis of pneumothorax vs. skinfold: 📖

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3) diaphragm & costophrenic angle

• Blunting of the costophrenic angle?
  • Possibilities usually include effusion vs. atelectasis vs. consolidation.
  • Chest radiograph is frequently inadequate to differentiate these possibilities. POCUS can often rapidly sort this out.
• Subpulmonic effusion may be suggested by: 📖
  • Apex of the pseudo-diaphragm shifts upwards and laterally. On the left side, >2 cm between the pseudo-diaphragm and the stomach bubble may help demonstrate that the pseudo-diaphragm is elevated.
  • The pseudo-diaphragm appears unusually opaque (without vessels coursing underneath it).
  • Additional evidence of fluid in the fissures:
    • Fluid may extend upwards into the major fissure.
    • The minor fissure may be thickened.
    • A separate “pseudotumor” of fluid in the minor fissure may be present.
• Deep sulcus sign may be suggested by: 📖
  • Basilar lucency that is asymmetric.
  • Deepening of the lateral costophrenic sulcus.
  • Diaphragm dome and cardiac border may appear unusually sharp.
  • The anterior diaphragmatic margin may be visible. If both the anterior diaphragmatic margin and the dome of the diaphragm are visible, this creates a “double diaphragm sign.”
• Intra-abdominal free air?

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4) cardiac silhouette

• Abnormally prominent components of the cardiac silhouette may suggest dilation (e.g., involving the pulmonary arteries, or the left atrium).
• Note also that a globally enlarged cardiac silhouette may reflect a large pericardial effusion.

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5) pulmonary arteries & hila 

• Pulmonary hypertension may be suggested by:
  • Prominent right atrium along the right cardiac border (figure above).
  • The convexity of the main pulmonary artery creates a bulge below the aorta and above the heart (white arrow below). (32096946)
  • Prominent pulmonary arteries that converge into an enlarged hilum (“hilum convergence sign”).
• Hilar lymphadenopathy:
  • May be supported by the presence of paratracheal lymphadenopathy (which causes an abnormal thickening of the right tracheal stripe).

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6) airways

• Focal tracheal narrowing? May suggest:(32096946)
  • Stenosis s/p intubation.
  • Extrinsic compression (e.g., thyromegaly).
  • Focal pathologies (e.g., malignancy).
• Diffuse tracheal narrowing? May suggest:(32096946)
  • Saber-sheath trachea. 📖
  • Relapsing polychondritis.
  • Granulomatosis with polyangiitis.
  • Amyloidosis.
• Tracheal deviation? Suggests mediastinal shift (e.g., due to pneumothorax or atelectasis).

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7) lungs

• Atelectasis
• Cavitary lung disease
• Consolidation
• Cystic lung disease
• Distribution of abnormalities
• GGO (ground glass opacification)
• Nodular patterns
• Reticular patterns

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CT scan algorithm: foreword

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The algorithm below discusses a step-wise approach to interpreting chest CT scans. It focuses largely on evaluation of the heart (since other chapters focus on various other pulmonary abnormalities).

Cutoff values discussed below represent the best available data. However, binary cutoff values have major limitations (e.g., they don't take into account the patient's size). As such, these values may help provide a rough framework, but are not intended as the final word. Ultimately, proficiency in cardiac interpretation of thoracic CT scans requires interpretation of numerous scans and subsequently comparing your impression with a contemporaneous echocardiogram.

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(1) lines & tubes

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💡 The scout film is often the fastest way to assess line and tube position on a CT scan. If there is any doubt regarding adequate placement, then cross-sectional images may usually provide definitive information.

• Endotracheal tube
• Tracheostomy tube
• Central venous catheter
• Feeding tube
• Chest tube
• Intra-aortic balloon pump
• Pulmonary artery catheter

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(2) right heart & pulmonary emboli

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contrast reflux in the IVC

• Causes of contrast reflux include: (21540221)
  • Right ventricular systolic dysfunction:
    • Acute (e.g., pulmonary embolism).
    • Chronic (e.g., pulmonary hypertension).
  • Tricuspid regurgitation.
  • Pericardial disease (constriction, tamponade).
• Performance depends on how rapidly contrast is injected:(31731905)
  • <3 ml/s (routine scan): 31% sensitive, 98% specific.
  • >3 ml/s (CT angiography): 81% sensitive, 69% specific.

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pericardial effusion?

• Density may suggest etiology:(26781150; 30725202; 30737548)
  • <~20 HU suggests a simple effusion.
  • ~20-40 HU suggests an exudative/purulent effusion.
  • >40 HU suggests a hemorrhagic effusion.

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right atrial dilation?

• Maximal transverse diameter >64 mm (♀) or >67 mm (♂). (33778499, 30240288)
  • Measure the largest diameter parallel to the tricuspid valve plane, excluding the RA appendage and coronary sinus.
  • Performance: sensitivity ~65%, specificity ~92%.

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right ventricular dilation?

• [Right ventricle > left ventricle] suggests RV dilation. The maximal dimension of both ventricles should be measured (which often will not occur within a single CT cut).
  • Limitation: May be insensitive among patients with left ventricular enlargement.
• Transverse diameter >57 mm (♀) or >60 mm (♂). (33778499, 30240288)
  • Diameter measured in a plane perpendicular to the septum.
  • Performance: sensitivity ~65%, specificity ~92%.
• Bowing of the interventricular septum may be seen in acute pulmonary embolism or chronic pulmonary hypertension.(26024596)

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right ventricular wall thickness? 

• The normal right ventricular wall is thin (up to ~3 mm) and barely noticeable on CT scan (figure above).(26024596)
• Right ventricular outflow tract thickness >6 mm may suggest chronic pulmonary hypertension.(32342182)

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pulmonary artery dilation 

• Cutoffs:
  • [Pulmonary artery > Aorta] suggests pulmonary artery enlargement.
  • Pulmonary artery diameter >3 cm suggests enlargement. (36017548)
• Significance of PA dilation:
  • PA dilation reflects both the severity and duration of pulmonary hypertension.
  • Dilation may not occur acutely. (33402372)

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evaluation for PE

• On any contrasted CT scan, evaluate for large central pulmonary emboli.
• (Follow up the official CT read for more subtle pulmonary emboli.)

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(3) left heart

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left atrial dilation?

• Maximal AP dimension >45 mm (♀) or >50 mm (♂). (33778499, 30240288)
  • Sensitivity ~50%, specificity ~95%.
  • Probably be best screening method.
• Maximal left atrial area >25-30 mm2 in an axial projection also indicates dilation (the pulmonary veins and left atrial appendage should be excluded). Some data suggests that this could have superior sensitivity, but routine use is hindered by the time required to measure it on every scan.(29530618)
• Common causes of dilated left atrium:
  • Atrial fibrillation.
  • Left ventricular failure (systolic and/or diastolic).
  • Mitral valve disease.

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left ventricular dilation? 

• Transverse diameter >55 mm (♀) or >60 mm (♂). (33778499; 30725202, 30240288)
  • Measure at the mid-ventricular level, on axial images.
  • Measure from inner wall to inner wall (figure below).
  • Sensitivity is moderate (~60%), specificity seems to be high (>95%). (33778499)
• Potential significance includes:
  • Dilated cardiomyopathy.
  • Valvular dysfunction.

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left ventricular wall thickness?

• <7 mm suggests LV wall thinning (e.g., due to chronic cardiomyopathy).(30725202)
• >20-25 mm septum or free wall suggests LV hypertrophy.(30725202)
  • Sensitivity is limited due to variability of cardiac cycle during the CT scan.

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mitral valve calcification 

mitral annulus calcification (MAC)

• MAC occurs in ~8% of scans (much more commonly than mitral leaflet calcification). (37675086)
• MAC may be graded qualitatively: (37675086)
  • Mild: <1/3 of the annulus involved.
  • Moderate: 1/3-1/2 of the annulus involved.
  • Severe: >1/2 of the annulus involved.
• MAC is generally asymptomatic, but it is correlated with coronary artery disease. (30737548)

mitral leaflet calcification

• Less common, potentially associated with:
  • Rheumatic heart disease.
  • Advanced renal failure.
• Mitral valve calcification correlates with mitral valve sclerosis or stenosis. (30737548)

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aortic valve calcification

• Aortic valve calcification shouldn't be confused with calcification of the aortic root (which doesn't involve the valve leaflets), mitral annulus, or coronary arteries. (33053316)
• Aortic valve calcifications may be seen in up to ~20% of scans, more often in older patients. (30737548)
• Severity may be qualitatively graded: (37675086)
  • Mild: separate, small, calcified foci.
  • Moderate: multiple, larger, calcified foci.
  • Severe: extensive, confluent calcification.
• Clinical significance:
  • (1) Moderate to severe calcification may increase the likelihood of aortic stenosis.
  • (2) Aortic valve calcification serves as a marker of atherosclerotic disease.(37675086)
• Other CT findings of aortic stenosis may include:(23395262)
  • Left ventricular hypertrophy.
  • Left atrial dilation.
  • Post-stenotic dilation of the ascending aorta (>4.1 cm).(26024596)

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coronary artery calcification

• Coronary artery calcification is not diagnostic of clinical coronary artery disease. However, coronary artery calcification may be used as a risk-stratification tool.
  • Absence of any calcium suggests a low risk of coronary artery disease. (30737548)
  • Coronary artery calcification is frequently seen with increasing age (e.g., present in 90% of men and 70% of women by age 70).
  • Substantial calcification in younger patients may suggest premature coronary artery disease (e.g., men <55 years old, or women <65 years old).(23395262)
• Severity may be defined qualitatively (figure below): (37675086)
  • Mild: Isolated flecks of calcification.
  • Moderate: Intermediate severity.
  • Severe: Continuous calcification.
• Contrast-enhanced CT scan has a slightly reduced sensitivity for coronary artery calcification, but overall it remains very good (83% sensitive) – especially for more severe calcification. (37675086)

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(4) airways

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examine proximal airways

• ? Good inspiratory effort (posterior tracheal membrane should bow outward).
• ? Proximal airway pathology.

examine distal airways

• ? Bronchiectasis (suggested if bronchus > adjacent pulmonary artery).
  • More on bronchiectasis here: 📖
• ? Secretions or mucus plugging.

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(5) mediastinum & hila

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• ? Substantial lymphadenopathy (subtle lymphadenopathy is often nonspecific, so for the acute care physician this is less important).
  • Further discussion of lymphadenopathy: 📖

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(6) lungs

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• Atelectasis
• Bronchial wall thickening
• Cavitary lung disease
• Consolidation
• Cystic lung disease
• Distribution of abnormalities
• GGO (ground glass opacification)
• Nodular patterns
• Reticular patterns

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endotracheal tube

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optimal location of the endotracheal tube (ETT)

• Ideal height above carina depends on head position (judged by position of the mandibles):
  • Neutral position: ideally 5 cm +/- 1-2 cm above carina.(Shepard 2019; 36832087; 22391269)
  • Neck flexion: ideally ~2-5 cm above carina.
  • Neck extension: ideally perhaps ~5-8 cm above the carina.
• ETT should be located around the depth of the aortic knob.
• ETT position does matter:
  • Too low: Risk of right mainstem intubation (which may cause hypoxemia and right-sided pneumothorax).
  • Too high: Risk of vocal cord damage or inadvertent extubation.
• The ETT moves lower if the neck is in flexion, or higher if the neck is in extension (“the nose goes where the hose goes”). The ETT may migrate by ~2 cm upwards or downwards.(36832087)

balloon that appears to bulge out the trachea

• If the balloon diameter is >1.5 times the diameter of the trachea, this increases the risk of tracheal injury (including tracheal rupture).(Shepard 2019)
• Differential diagnosis:
  • Cuff hyperinflation.
  • Tracheal rupture.
  • Tracheomegaly.

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tracheostomy tube

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• The tip of the tube should ideally be ~50-66% of the distance from the stoma to the carina.(36832087)

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central venous catheter

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• Exactly which catheter malpositions are clinically significant is somewhat debatable. 🌊
• Minor catheter malpositions may often be tolerated for short-term use, without catheter withdrawal or replacement (both of which carry procedural risks, including infection). These include:
  • Catheter tip in the right atrium.
  • Catheter tip in the subclavian vein (this is functionally a midline catheter).
• Catheter malpositions which require revision include:
  • Catheter looping around or kinking within the vasculature.
  • Catheter ascending upwards into the jugular vein towards the brain.
  • Catheter location within the azygos system is suboptimal, and might warrant pulling the catheter back into the superior vena cava (figure below). (22391269)
• If there is concern regarding the possibility of intra-arterial positioning (e.g., within the aorta), the most rapid and definitive approach is to perform a bubble flush test (flush 0.25 ml air into the line while imaging the right ventricle with POCUS). If microbubbles are seen within the right ventricle, this indicates that the catheter must be located somewhere within a vein (confidently excluding extra-vascular or arterial malposition).

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feeding tube

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nasogastric or orogastric Salem sump tube: break in the radiopaque line

• The break in the radiopaque line identifies the location of the most proximal hole. This break must lie distal to the gastroesophageal junction. If the most proximal hole lies in the esophagus, this may cause aspiration of medications and tube feeding.
• (Further discussion of types of feeding tubes here: 📖).

bronchial placement

• Bronchial placement may be identified if the tube veers out of the midline at the location of the carina.
• 💡 If the tube enters the bronchus, lacerates the lung, and ends up in the pleural space the final tip of the tube may appear to overlie the stomach. The key clue to misplacement is the trajectory – which will deviate from the midline at the location of the carina.

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chest tube

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surgical chest tube: break in the radiopaque line (“sentinel eye”)

• The break in the radiopaque line correlates with the location of the most proximal hole in the chest tube. This break must lie within the pleural space. If the break lies outside of the pleura that could lead to an air leak, or extravasation of tPA into peripheral tissues.

chest tube in the fissure:

• Radiological features that suggest this:
  • Lack of a gentle curve at the insertion site.
  • Horizontal or oblique upward course.
  • Course directly towards the hilum.
• Location in a fissure may or may not affect chest tube function. (26024601)

intraparenchymal chest tube is suggested by

• Hematoma and/or lung infarction.
• Bronchopleural fistula, potentially with hemothorax.
• Large, ongoing air leak.

kinking of the chest tube

• It's occasionally difficult to sort out a kink from a gentle arc that occurs in a plane that is nearly orthogonal to the plane of the chest radiograph. When in doubt, evaluate at the bedside whether the chest tube is functioning properly.

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intra-aortic balloon pump

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ideal location

• ~2 cm below the superior aspect of the aortic arch. (Shepard 2019)
• Slightly above the left main bronchus.

risks of intra-aortic balloon pump malpositioning

• Too proximal:
  • May promote ischemia to the brain and left arm.
  • Increases risk of aortic injury (e.g., dissection or pseudoaneurysm).
• Too distal:
  • Occlusion of renal and mesenteric arteries.
  • Ineffective counterpulsation.

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pulmonary artery catheter

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• The ideal position is within the right or left main pulmonary artery, or within the proximal interlobar pulmonary arteries (within <1-2 cm of the lateral edge of the mediastinum).
• As a general rule of thumb: if the catheter extends beyond the hilum of the lung, it should be retracted.(22013292)

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questions & discussion

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To keep this page small and fast, questions & discussion about this post can be found on another page here.

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