Chest Computed Tomography
GENERAL PRINCIPLES
• The cross-sectional orientation and contrast sensitivity of CT are well suited for detecting, describing, and distinguishing among diseases of the thorax.
• Common indications for a CT scan of the chest are broken into two main categories: those patients with abnormal CXR findings requiring further evaluation, and those patients with a normal CXR but with suspicion for occult disease.
Common abnormal radiographic findings prompting a follow-up CT include staging of bronchogenic carcinoma; evaluation of a nodule, mass, or opacity; and characterization of infiltrative lung disease, mediastinal, pleural, or chest wall abnormalities.
Common radiographically occult diseases include the evaluation of potential metastases, suspected aortic dissection, hemoptysis, bronchiectasis, infiltrative lung disease, endocrine abnormalities, or source of infection.
• Similar to the interpretation of CXR, it is important not to bias your interpretation of the CT scan based solely on the patient’s known clinical history. While an understanding of the clinical scenario is important to focus on specific areas of the study, careful attention must be paid to the entirety of the examination to avoid missing pertinent findings.
• Comparison to prior CT studies is also essential to both characterize the time progression of lesions, and to determine whether subtle findings truly represent pathology.
INITIAL ASSESSMENT
Ordering a CT
• Not all chest CT examinations are performed in the same way. For efficient throughput, many CT departments use a variety of protocols to scan the thorax. These protocols are created to convey to the technologists information on radiation technique, reconstruction techniques, and methods of contrast use and enhancement. The protocols are usually based on clinical scenarios, such as aortic dissection, pulmonary embolism, or interstitial lung disease, and providing a meaningful indication for the examination will help ensure that the proper protocol is used.
• There are several important aspects to be considered when preparing the patient for a CT scan, including the area to be scanned, the use of contrast, and the patient’s ability to tolerate the contrast.
Body Region
• The region of the body to be scanned should be documented, and will typically consist of a combination of the chest, abdomen, and pelvis. This decision will be made by the referring physician based on clinical context and may be adjusted by the radiologist as needed.
• Increasingly, insurance restrictions do not allow for the changing of the region to be scanned by anyone but the referring clinician. Hence, careful thought at the time of writing the order can save added work later.
• A chest CT tends to scan from the thoracic inlet through the adrenal gland.
• An abdomen CT tends to cover the dome of the diaphragm through the iliac crests.
• A pelvic CT scans from the iliac crest through the pubic symphysis.
Contrast
• The appropriate use of contrast is necessary to understand when ordering a CT study. A scan can be ordered with contrast, without contrast, and with and without contrast and is dependent on the indication for the study.
• Contrast can be administered intravenously (most common) or orally (rare for thoracic conditions). As a general rule, IV contrast is indicated for patients with suspected hilar, mediastinal, or pleural abnormalities and in patients with potential vascular abnormalities such as a pulmonary embolus. It can help distinguish lymph nodes from hilar vessels, underscore the vascular component of arteriovenous malformations, and identify the enhancing rim characteristic of empyemas.
• A noncontrast scan is generally indicated for assessing lung disease, ruling out pulmonary metastases, and for assessing nodules.
• A chest CT scan with and without contrast is typically only indicated for evaluation and differentiation of an aortic dissection or intramural hematoma, initial evaluation of pulmonary arteriovenous malformations, or characterization of a known mediastinal mass.
• There are four important considerations to understand if a patient can receive contrast: renal function, allergy, vascular access, and volume status.
• Since contrast agents are excreted by the kidneys and may cause changes in renal hemodynamics or tubular toxicity, it is important to assess renal function prior to ordering a contrast-enhanced study, as contrast may result in irreparable damage to a borderline set of kidneys.
Many centers use a serum creatinine level because of its ease in acquisition and it can be converted via a simple equation to creatinine clearance, which is an estimation of the glomerular filtration rate (GFR). Normal creatinine clearance ranges from 100 to 160 mL/min with physiologic variation by age. Generally, IV contrast should be avoided in patients with a clearance of <30 mL/min.
Of note, patients on dialysis can receive contrast media since the contrast will be filtered in their next dialysis session.
• It is also important to assess for a history of a reaction to contrast media when preparing a patient for CT. Patients should be specifically asked about iodinated contrast material, as many do not consider contrast a type of medication.
Shellfish allergy alone is not a contraindication to the use of IV contrast.
The severity of any reaction to contrast agents should also be characterized.
Generally, a patient with a history of itching or hives following prior contrast administration can receive premedication, whereas a patient with a history of prior serious contrast reactions such as laryngeal edema or anaphylaxis should not receive contrast despite premedication. The reactions can be somewhat idiosyncratic and tend to get more severe over time.
Premedication typically consists of a combination of corticosteroid and antihistamine. These medications may produce side effects of increased intraocular pressure or urinary retention, so a history of glaucoma or prostate enlargement should also be obtained.
• When ordering a study with contrast, the vascular access of the patient is an important and potentially limiting consideration. Although convention may vary by institution, typically central access or peripheral antecubital access with a 20-gauge line or larger is required. Specific questions about access requirements are best addressed through consultation with the radiology department.
• It is also important to remember that contrast is a bolus of fluid volume. Because of the osmolality, the contrast dose is equivalent to over 1 L of normal saline and may cause problems for patients with pulmonary edema or cardiac issues. As a general guideline, if the patient could not tolerate a 1-L bolus of saline, they should not receive IV contrast.
CT Scans and Protocols
In the modern era, all thoracic CTs are performed on helical, multidetector CT machines. The term “spiral CT,” which gained popularity as a synonym for the PE protocol CT, is not helpful, as most scans are performed helically or spirally.
High-Resolution CT
• High-resolution CT (HRCT) is a scanning protocol often used to diagnose diffuse lung diseases, bronchiectasis, emphysema, and focal lung lesions.
• HRCT does not require contrast and obtains detailed images that are comparable to gross tissue inspection.
• HRCT uses thin slices to improve resolution and view the fine details of the pulmonary parenchyma.
• In many centers, HRCT is performed in both inspiration and expiration.
Low-Dose CT
• Low-dose CT reduces the total radiation dose and is accomplished by lowering the tube current or kilovolt peak during the scan, which still results in readable images in the majority of patients.
• This type of scan is typically indicated for lung cancer screening, in children, or if multiple follow-up examinations are required.
• Low-dose CT may be limited by parameters such as patient size.
Other Protocols
Various protocols also exist for the evaluation of pulmonary embolus, aortic dissection, and thoracic aorta pathology. The appropriate use of these protocols is best clarified through consultation with a radiologist.
Preparing the Patient
• Patients can be hesitant about CT scans, which usually stem from lack of knowledge about the radiation dose and specifics of the procedure. It is helpful to relate the radiation exposure to that of natural background radiation, where one conventional chest CT is approximately equal in exposure to 3 years of natural background radiation.
• It is also helpful to explain to patients that the scan can be interrupted or terminated at any time if problems arise and that they will be able to communicate with the radiographer in the control room through an intercom.
• Claustrophobic patients may find it helpful to close their eyes during the examination.
• Patients should also be aware of the need for controlled breathing throughout the study, as this reduces image noise due to diaphragmatic movement.
• All clothing with zippers and all metallic objects should be removed to prevent confusion when interpreting the image.
• Patients should be made NPO 4 hours prior to their scheduled scan. IV contrast material can occasionally be proemetic. Four hours allow the stomach to be cleared of contents so the risk of aspiration can be reduced.
GENERAL APPROACH TO CT INTERPRETATION
• When starting the CT analysis, prior imaging studies and the clinical history should be examined to focus the interpretation and the differential diagnosis.
• The use of prior imaging to aid in the interpretation of the current chest CT cannot be stressed enough, as it is extremely useful in determining the time course of certain lesions and bringing subtle abnormalities to light.
• The characterization of lesions into acute or chronic is also essential to narrow the differential diagnosis and help rule out malignancy.
Window Levels and Window Width
• CT has much better contrast discrimination than a standard CXR. Levels of CT attenuation that are often able to be differentiated include (from dark to light): air, fat, fluid, muscle, enhancing organ, bone, and metal. The density levels of these items are assigned values known as a hounsfield unit (HU). The density of water is arbitrarily set to a value of 0 HU and the scale increases/decreases with corresponding radiodensity.
• Chest CT scans will often load with multiple series to view. These series are typically broken up by windowing technique or the use of contrast.
Since the human eye is unable to differentiate between the 2000 shades of gray that can be seen on a CT scan, windowing is used in areas of the body with similar density.
Windowing narrows the HU pixel range that will be displayed (decreases the potential number of shades of gray) so that each shade can be differentiated easier by the human eye. With fewer shades of gray to be displayed, contrast between the fine tissue details is maximized. Areas with density above the designated window range will appear white, and those with density below the range will appear black. The zero point or center of the window range is also adjusted and is analogous to the brightness of the image.
• Routine window settings for chest CT include one for the lung parenchyma, bone, and the mediastinum.
Basic Anatomy
• Identification of the correct anatomical structures must first start with understanding of patient positioning.
• When viewing a CT scan, imagine that you are standing at the patient’s feet looking toward the head as he or she lies supine on a table. This way the patient’s left side is on image right, and right side is on image left.
• The patient is also supine, so the vertebral column is at the bottom of the image and the chest wall is at the top.
• There may be circumstances where an image is taken in the prone position, where image right is the patient’s right and image left is the patient’s left. The top of the image is the vertebral column, and the bottom is the chest wall.
• Knowledge of the positioning is not only important for identifying anatomy, but also aids in distinguishing gravity-related changes (i.e., dependent atelectasis) from pathologic findings such as inflammation or fibrosis.
• Images can also be reconstructed in the coronal and sagittal planes. The coronal plane is as if you are looking at the patient from the front, and the sagittal plane is as if you were looking at the patient from the left side.
• Differentiating an expiratory CT from an inspiratory CT may also be required, and is best done through inspection of the shape of the trachea on corresponding levels. In the expiratory CT, the membranous portion of the trachea will flatten so that the trachea does not resemble an “O” as it does during inspiration.
• The identification of normal structures on the chest CT is required to be able to identify any abnormal structures or pathology. This is best carried out by grouping structures into the mediastinum, hila and lungs, pleura, chest wall, and diaphragm.
Mediastinal Anatomy
• The mediastinum is the tissue compartment situated between the lungs, bounded anteriorly by the sternum and posteriorly by the spine.
• Superiorly, structures are identified with reference to the trachea. The esophagus lies posterior to the trachea at this level, and the great arterial branches of the aorta lie anterior and lateral to the walls of the trachea. At this level the great arterial branches will be seen from anatomic right to left as the innominate artery (brachiocephalic artery), left carotid artery, and left subclavian artery.
• Anterior to these great arterial vessels will be the great veins with the left brachiocephalic vein coursing across the mediastinum as the most anterior great vessel.
• The thyroid gland can also be identified caudally near the level of the thoracic inlet. Because of its iodine content, the thyroid is usually very bright.
• In the subaortic mediastinum, the aorta, superior vena cava, pulmonary arteries, and lymph node groups are important to identify. Usually, the aortic arch is easily identified crossing from the anterior to posterior mediastinum lateral to the trachea. The superior vena cava is seen anterior and to the anatomic right of the trachea.
• The thymus may also be seen anterior to the aortic arch and posterior to the sternum. Other notable structures at this level are the main pulmonary arteries and the azygous vein, which can be seen passing over the right main bronchus and emptying into the superior vena cava.
• Important lymph node groups to identify and assess for enlargement or pathology include the paratracheal chain, subcarinal nodes, and aortopulmonary window nodes. Lymph nodes with a short axis >1 cm are considered enlarged, with an exception in subcarinal nodes where >1.3 cm is considered enlargement.
• The paracardiac mediastinum includes the chambers of the heart and origins of the great vessels. The main pulmonary artery can be seen arising most anterior and rising from the right ventricle. It can be followed to its split into left and right pulmonary branches.
• The superior vena cava can also be visualized as it enters the right atrium. Identification of the aortic root as it projects out of the left ventricle can be helpful since coronary arteries may be seen as they originate near the aortic valve cusps, and can be assessed for calcification. The aortic root originates between the main pulmonary artery and right atrium.
• The most posterior portion of the heart is the left atrium, and most anterior is the right ventricle. The remaining heart chambers can be identified with relation to these structures and their outflow tracts. The inferior vena cava may also be identified caudally near the diaphragm as it courses into the right atrium.
• Assessment of the retrosternal space for the internal mammary arteries and veins, and lymph nodes may aid in diagnosis.
• Enlarged vessels may indicate superior vena cava obstruction, and enlarged lymph nodes always indicate pathology (most commonly breast cancer or lymphoma).
Hila and Lung Anatomy
• The anatomy of the pulmonary hila is visualized well on CT, which aids in the diagnosis of endobronchial lesions, surrounding masses, and vascular lesions. Contrast enhancement also helps to identify a hilar mass or lymph node enlargement.
• CT evaluates these structures so well and the anatomy is relatively consistent among individuals, so it is important to identify the normal anatomy to be able to distinguish abnormal pathology.
• Vascular anatomy often follows airway anatomy, so evaluation of these structures can take place concomitantly. The anatomy of the right and left hila with a focus on airway anatomy will be reviewed here separately.
• The right hilum can be tracked as the right bronchus branches from the trachea at the level of the carina.
• The right pulmonary artery passes anterior and inferior to the bronchus at this level. The right bronchus is, therefore, known as “eparterial.”
• The right upper lobe bronchus will first be seen branching off ∼1 cm distal to the carina with the right superior pulmonary vein directly anterior to this structure. This upper lobe bronchus will further branch into anterior, posterior, and superior segmental branches.
• After the upper lobe bronchus branches, the right airway will continue as bronchus intermedius. At the lower level of bronchus intermedius, the right middle lobe bronchus arises anteriorly just caudal to the right pulmonary artery and can be followed branching into medial and lateral segments.
• Distal to the branching of the middle lobe bronchus, bronchus intermedius becomes the right lower lobe bronchi and gives rise to the superior segment and the basal segmental bronchi (anterior, medial, lateral, and posterior). These segments vary in their appearance and are not always visible on CT.
• The left hilum can also be tracked as the left main bronchus courses from the trachea at the level of the carina. The left pulmonary artery passes superior to the left main bronchus at this level and will then descend posteriorly.
• The left main bronchus takes a longer course than the right before branching, and first branches off as the left upper lobe bronchus, which courses anterolaterally from its origin.
• The left superior pulmonary veins are anteromedial to the bronchus at this level. The upper lobe bronchus further branches into a lingular bronchus (which gives rise to superior and inferior segments) and anterior and apicoposterior segments.
• The left lower lobe bronchus is relatively symmetrical with the right lower lobe bronchus, and branches into a superior segment and three basal segments (anteromedial, lateral, and posterior).
Pleural, Diaphragm, and Chest Wall Anatomy
• The pleura, diaphragm, and chest wall are visualized well on chest CT imaging, and are most efficiently evaluated with soft tissue (mediastinal) windows.
• When assessing the pleura and diaphragm, it is important to remember that the diaphragmatic space extends well below the lung bases and scans must continue all the way down to this angle to be completely assessed.
• The visceral and parietal layers of the pleura are not normally visible on CT. The parietal (superficial) and visceral (deep) layers lie internal to the ribs and the innermost intercostal muscles and are separated from these structures by a layer of extrapleural fat.
• Identification of the diaphragmatic crura is also important to avoid mistaking them with enlarged lymph nodes or masses, as they can take on a rounded appearance. The crura are tendinous structures that extend inferiorly from the diaphragm to attach to the vertebral column.
• There are several physiologic openings in the diaphragm that should also be identified. These include the aortic hiatus, esophageal hiatus, and foramen of the inferior vena cava.
The aortic hiatus is most posterior and is bounded anteriorly by the crura and posteriorly by the spine. It is usually found at vertebral level T12. The azygous and hemiazygous veins, thoracic duct, intercostal arteries, and splanchnic nerves also pass through the aortic hiatus.
The esophageal hiatus is more anterior in the diaphragm and is located in the muscular part of the diaphragm. It arises around the level of the T10 vertebrae and also contains both vagal nerve trunk branches.
The foramen of the inferior vena cava arises around the level of the T8 vertebrae and is anterior and to the right of the esophageal hiatus.
• Gross inspection of the chest wall is important to identify any abnormalities that may also be clues to the diagnosis. Knowledge of the anatomy of the axillary space is particularly helpful in identifying abnormal lymph nodes and other pathology.
• When patients are scanned with both arms by their side, the axilla is bordered by the fascial coverings of pectoralis major and minor anteriorly; the chest wall and serratus anterior medially; the latissimus dorsi, teres major, and subscapularis posteriorly; and the biceps brachii and coracobrachialis laterally.
The axillary space also contains physiologic lymph nodes, axillary vessels, and nerves such as the brachial plexus and intercostals.
Normal lymph nodes in this region can be as large as 1.5 cm in the short axis, but with the appropriate clinical context lymph nodes >1 cm may be cause for concern.
Pathologic lymph nodes are best identified by direct comparison for symmetry in the axillae.
• Inspection of the supraclavicular area, breasts, and superior sulci should also take place with a concern for enlarged lymph nodes and masses.
APPROACH FOR READING CHEST CT
• With knowledge of the key anatomy, an organized approach for evaluating the chest CT is required to identify all findings. It is important to adhere to a regimen every time a chest CT is evaluated, as obvious findings may take attention away from less obvious findings that are equally important.
• Specific evaluation of the lung parenchyma will be discussed in the following section. Inspection should begin with the transaxial images in the soft tissue window.
• Because the beginner often neglects the soft tissues of the thoracic wall, these tissues should be evaluated first, followed by the mediastinum.
• Images should then be switched to the transaxial lung window with evaluation of the lung parenchyma, pleura, and bones.
Soft Tissue Window
• Inspection of the thoracic wall will occur first in the soft tissue window. Close attention should be paid to the axilla and breasts for enlarged lymph nodes and masses.
• The mediastinum should then be evaluated for pathologic masses or anatomical abnormalities. It may be easiest to orient yourself relative to the aortic arch or trachea.
• Cranially from the aortic arch (supra-aortic mediastinum), careful attention should be paid to the presence of enlarged lymph nodes, thyroid lesions or enlargement, and vessel abnormalities. When evaluating the space caudally from the aortic arch to the superior aspect of the heart (subaortic mediastinum), focus should be paid to the aortopulmonary window, subcarinal space, and anterior aortic space for the presence of enlarged lymph nodes.
• As you extend caudally into the paracardiac mediastinum, the hilar region should be assessed for configuration and vessel caliber, lobulation, and enlargement.
• The heart should also be assessed for signs of coronary atherosclerosis or dilations, and the descending aortic space evaluated for pathologic lymph node enlargement.
• When analyzing lymphadenopathy or a mass on CT, pay attention to the location and the attenuation of the abnormality. Both will be helpful in generating differential diagnoses and will be useful in communicating with other specialists. A fatty mass in the anterior mediastinum, for example, is less likely to be malignant than one in the middle mediastinum.
Lung Window
• The lung window is very wide and allows for assessment of the parenchyma, pleura, and bones.
• The lung parenchyma should be assessed first with evaluation for the normal branching pattern and caliber of vessels along with the interlobar fissures and presence of bullae.
• Careful attention should be paid for any nodules (<3 cm), masses (>3 cm), consolidation, or infiltrate.
• The pleura should then be assessed for the presence of abnormalities such as thickening, enhancement, calcification, plaques, pleural fluid, or pneumothorax.
• Finally the bones (ribs, scapula, and vertebrae) should be evaluated for normal marrow structure, spinal stenosis, or signs of osteoarthritis such as osteophyte formation. Focal lytic or sclerotic processes and fractures should also be identified.
Basic Lung Parenchymal Patterns
• Narrowing the differential diagnosis of lung disease on CT also requires an organized schema and is best delineated by characterizing the dominant pattern, distribution within the secondary lobule, and distribution within the lung.
• The dominant pattern is assessed first with other findings serving to narrow the differential diagnosis. This dominant finding should be grouped into reticular, nodular, high attenuation, or low attenuation patterns.
Reticular Pattern
The reticular pattern displays too many lines and is usually from thickened interlobular septae.
• Smooth septal line thickening is most often due to interstitial pulmonary edema (Kerley B lines) or lymphangitic carcinomatosis. Occasionally, it may be seen with viral pneumonias.
• Nodular septal line thickening is most often due to sarcoidosis, silicosis, or lymphangitic carcinomatosis.
• Irregular septal line thickening is a finding most often seen with fibrosis (usually nonspecific interstitial pneumonia).
Nodular Pattern
With a nodular pattern, the distribution of the nodules is key to narrowing the differential diagnosis, and identifying pleural nodules can help with this process.
• If no pleural nodules are present, it is likely a centrilobular distribution, with the most likely differential diagnosis consisting of hypersensitivity pneumonitis, infection, respiratory bronchiolitis, and bronchioloalveolar carcinoma.
• The presence of a tree-in-bud pattern of irregular and often nodular branching structures most identifiable in the lung periphery can narrow this differential diagnosis to endobronchial spread of infection (usually mycobacterial or bacterial) or airway disease associated with infection (bronchiectasis, cystic fibrosis, or allergic bronchopulmonary aspergillosis [ABPA]).
• If pleural nodules are present with a random distribution, the likely differential diagnosis is miliary TB, fungal infection, sarcoidosis, or the hematogenous spread of metastases.
• Otherwise nodules are considered perilymphatic and are characteristic of sarcoidosis, silicosis, and lymphangitic carcinomatosis.
High Attenuation Pattern
• A high attenuation pattern can be characterized as ground glass opacity (GGO) or consolidation with a large degree of overlap between the two.
• GGO occurs when there is a hazy increase in lung opacity without obscuring the underlying vessels, and is broken down into acute versus chronic.
Acute GGO occurs in cases such as pulmonary edema, pneumonia, or pulmonary hemorrhage.
Chronic GGO may be due to organizing pneumonia, hypersensitivity pneumonitis, chronic eosinophilic pneumonia, alveolar proteinosis, lung fibrosis, and bronchoalveolar carcinoma.
The location of GGO in the lung is helpful in distinguishing these etiologies.
• Crazy paving is another term used to describe the distribution of GGO and occurs when it is combined with smooth septal thickening, resembling a pattern of paving stones or irregular shapes and lines. The differential diagnosis is similar to GGO. Of note, however, is the classic association of this pattern with alveolar proteinosis.
• Consolidation refers to filling in of the alveolar air spaces with loss of visualization of the pulmonary vessels.
As with GGO, the differential is very much based on the chronicity of the finding.
When chronic, one must consider atypical infection, bronchioloalveolar carcinoma, inflammatory pneumonia (organizing pneumonia or eosinophilic pneumonia), or congenital lesions (such as sequestration).
Low-Attenuation Pattern
• A low attenuation pattern occurs due to emphysema, cystic lung disease, honeycombing, or bronchiectasis.
• Cystic lung disease is defined as radiolucent areas with a wall thickness <4 mm and is most often due to pneumatoceles, honeycombing, Langerhans cell histiocytosis (LCH), lymphocytic interstitial pneumonia, or lymphangioleiomyomatosis.
• Honeycombing occurs with usual interstitial pneumonia (UIP), interstitial fibrosis, or end stage sarcoidosis.
• Bronchiectasis can occur with cystic fibrosis, ABPA, immune deficiency, or a prior infection causing focal bronchiectasis. With bronchiectasis, care should be taken to exclude a central obstructing mass.
• Mosaic attenuation refers to scattered dark (low attenuation) areas within normal lung akin to a tile mosaic. Visualization of attenuated vasculature in the darker areas helps to prevent confusion with GGO.
• In mosaic attenuation, the darker areas are abnormal secondary to:
Small airways disease (most notably bronchiolitis obliterans).
Small vessels disease (most notably chronic pulmonary embolism).
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