HOW TO INTERPRET

ACT CHEST

CT of the chest plays an important role in the care of the critically ill patient.

CT is much more sensitive than CXR and can help sort out nonspecific CXR findings as well as provide valuable information that may change patient management.

Clinically occult or unsuspected abnormalities (such as a small pneumothorax or interstitial pneumonitis from drug toxicity) can be readily detected by CT.

Furthermore, contrast-enhanced CT scans are useful in the critically ill patient for excluding life-threatening conditions, such as a pulmonary embolism or aortic dissection, and aid in differentiating empyema from a lung abscess.

Finally, a chest CT may reveal unsuspected findings in the upper abdomen (such as free air, pancreatitis, or retroperitoneal hemorrhage) that would seldom be visible on plain chest radiograph.

Despite these advantages, CT studies have a few drawbacks: the need to transport potentially unstable patients

outside the closely monitored ICU

risks associated with use of iodinated IV contrast material

exposure to a substantial amount of radiation ( equivalent to 400 chest xrays)

WINDOWS

WINDOW SETTINGSThe average density of each voxel is measured in

Hounsfield Units; these units have been arbitrarily chosen so that zero is water density and −1000 is air density and +1000 for bone.

The range of Hounsfield Units encountered in the thorax is wider than in any other part of the body, ranging from aerated lung (approximately −800 HU) to ribs (+700 HU).

The operator uses two variables to select the range of densities to be viewed: window width and window center or level.

The window width determines the number of Hounsfield Units to be displayed.

Any densities greater than the upper limit of the window width are displayed as white, and any below the limit of the window are displayed as black. Between these two limits the densities are displayed in shades of gray.

The median density of the window chosen is the center or level, and this center can be moved higher or lower at will, thus moving the window up or down through the range.

The narrower the window width, the greater the contrast discrimination within the window.

No single window setting can depict this wide range of densities on a single image.

For this reason, thoracic work requires at

least two sets of images, usually to demonstrate the lung parenchyma and the soft tissues of the mediastinum.

Soft tissues, mediastinum, chest wall: center 40 HU, width 300–500 HU

Lung parenchyma: center −600 HU, width 1500 HU

HRCT: center −500 to −800 HU, width 1300–

1600 HU

HRCT is generally interpreted with the image displayed using "lung windows," as shown above, in which air is black, aerated lung is dark grey, and other structures are white.

In soft tissue windows, bone appears white; muscle, lymph nodes, and consolidated lung appear grey; and air and aerated lung appear black.

WHAT IS HRCT ?

HRCTThree factors significantly improve the spatial

resolution of CT images of the lung: narrow scan collimation, a high spatial resolution reconstruction algorithm,

and a small field-of-view

A standard HRCT examination yields approximately 30 transverse images; a protocol involving the reconstruction of 5 mm contiguous sections and 1.25 mm sections at 10 mm increments (from a MDCT volumetric set acquired with 1 mm detector collimation) would produce approximately 90 images.

In HRCT, a high spatial frequency algorithm is used which takes advantage of the inherently high-contrast environment of the lung.

The high spatial frequency algorithm (also known as the edge-enhancing, sharp, or formerly ‘bone’ algorithm) reduces image smoothing and makes structures visibly sharper, but simultaneously increases image noise.

PLEASE NOTE!!!•As HRCT's aim is to assess a generalized lung disease, the test is conventionally performed by taking thin sections 10–40 mm apart. The result is a few images that should be representative of the lungs in general, but that cover only approximately one tenth of the lungs.

•Because HRCT does not image the whole lungs (by using widely spaced thin sections), it is unsuitable for the assessment of lung cancer or other localised lung diseases.

…contd.•Similarly, HRCT images have very high levels of noise (due to thin sections and high-resolution algorithm), which may make them non-diagnostic for the soft-tissues of the mediastinum.

•Intravenous contrast agents are not used for HRCT as the lung inherently has very high contrast (soft tissue against air), and the technique itself is unsuitable for assessment of the soft tissues and blood vessels, which are the major targets of contrast agents.

NORMAL CHEST ANATOMY

ONCT CHEST & HRCT

Knowledge of the lung anatomy is essential for understanding HRCT.

The secondary lobule is the basic anatomic unit of pulmonary structure and function.

Interpretation of interstitial lung diseases is based on the type of involvement of the secondary lobule.

It is the smallest lung unit that is surrounded by connective tissue septa.

It measures about 1-2 cm and is made up of 5-15 pulmonary acini, that contain the alveoli for gas exchange.

Secondary lobules. The terminal bronchiole in the center divides into respiratory bronchioli with acini that contain alveoli. Lymphatics and veins run within the interlobular septa

Centrilobular area in blue (left) and perilymphatic area in yellow (right)

In this fresh autopsy lung, the black pigmentationmarks the centriacinar portion of the lobule.Anthracotic particles (environmental soot) isdeposited in the respiratory bronchioles. Notice theoff-axis location of the respiratory bronchiolescompared to the core bronchovascular structure.

CT OF

NORMAL HILATwo-millimetre collimation images have been obtained through the hilar structures during contrast medium enhancement and displayed on lung windows (L-500, W 1500).

(A)Section just below the tracheal carina at the origin of the right upper lobe bronchus, immediately posterior to the upper lobe vein (v).

(B) Section through level of right main pulmonary artery (RPA) and bronchus intermedius (curved arrow). Note the tongue of lung that contacts the left main bronchus between the aorta (A) and the left lower lobe artery (straight arrow). Note also that the right lung contacts the posterior wall of the bronchus intermedius as it extends into the azygo-oesophageal recess.

(C) Section through the level of the middle lobe bronchus (long arrow) at the point of origin of the bronchus to the superior segment of the right lower lobe (arrow). Note that the middle lobe bronchus separates the right lower lobe artery from the right superior pulmonary vein as it enters the left atrium (LA). The lung contacts the posterior wall of the right lower lobe bronchus as it extends into the azygo-oesophageal recess.

(D) Section through the level of the inferior pulmonary veins (arrows). At this level the lower lobe arteries have bilaterally divided into basal segmental divisions and are therefore narrower than 1 cm in diameter.

CT OF

NORMAL MEDIASTINUM

Five 1-cm thick sections have been selected to show the important anatomical features

A.Ao = ascending aorta, Ao arch = aortic arch, AV = azygos vein, D. Ao = descending aorta, IA = innominate artery, LA = left atrium, LCA = left carotid artery, LIV = left innominate vein, LPA = left pulmonary artery, LSA = left subclavian artery, MPA = main pulmonary artery, N = normal lymph node, OES = oesophagus, RA = right atrium, RIV = right innominate vein, RPA = right pulmonary artery, RVO = right ventricular outflow tract, SPV = superior pulmonary vein, SVC = superior vena cava, T = trachea.

A.Ao = ascending aorta, Ao arch = aortic arch, AV = azygos vein, D. Ao = descending aorta, IA = innominate artery, LA = left atrium, LCA = left carotid artery, LIV = left innominate vein, LPA = left pulmonary artery, LSA = left subclavian artery, MPA = main pulmonary artery, N = normal lymph node, OES = oesophagus, RA = right atrium, RIV = right innominate vein, RPA = right pulmonary artery, RVO = right ventricular outflow tract, SPV = superior pulmonary vein, SVC = superior vena cava, T = trachea.

A.Ao = ascending aorta, Ao arch = aortic arch, AV = azygos vein, D. Ao = descending aorta, IA = innominate artery, LA = left atrium, LCA = left carotid artery, LIV = left innominate vein, LPA = left pulmonary artery, LSA = left subclavian artery, MPA = main pulmonary artery, N = normal lymph node, OES = oesophagus, RA = right atrium, RIV = right innominate vein, RPA = right pulmonary artery, RVO = right ventricular outflow tract, SPV = superior pulmonary vein, SVC = superior vena cava, T = trachea.

FEW DIFFERENT TECHNIQUES

PRONE POSITION When early interstitial fibrosis is suspected, HRCT scans

are often performed in the prone position to prevent potential confusion with the increased opacification seen in the dependent posterobasal segments of many normal individuals scanned in the supine position.

The increased density seen in the posterior dependent lung in the supine position will disappear in normal individuals when the scan is repeated at the same anatomic level with the patient in the prone position.

The physiologic mechanism of the increased opacification in the dependent lung in normal individuals is not fully understood and has been ascribed to gravity-dependent perfusion and/or relative atelectasis of the dependent lung

Supine and prone sections. In this patient with suspected diffuse lung disease, A, the supine CT image shows bilateral subpleural opacities (arrows). B, The opacities disappear with the patient in the prone position (arrows).

EXPIRATORY CTExpiratory CT may sometimes be helpful in

differentiating between the three main causes of a mosaic pattern (infiltrative lung disease, small airways disease, and occlusive pulmonary vascular disease) which may be problematic on inspiratory CT.

A fewer number of expiratory than inspiratory HRCT sections (e.g. at 30 mm or 40 mm intervals) are usually obtained.

Although expiratory images almost invariably make

regional inhomogeneity more conspicuous, and occasionally reveal the presence of air-trapping not suspected on the inspiratory images.

Transverse CT section through the left lower lobe shows A, normal lung attenuation in inspiration and B, relatively uniform increased attenuation in expiration in a healthy individual.

Air-trapping in a patient after right lung transplantation for emphysema. A, Inspiratory CT section shows comparable attenuation of both lungs. B, Expiratory section shows attenuation increase in the transplanted right lung with small paracardiac areas of air-trapping (arrows) and absence of attenuation increase in the emphysematous left lung.

CHEST WALL INVASION

. CT findings of value in the diagnosis of chest wall invasion include the presence of the following:

Obtuse angles or pleural thickening at the point of contact between tumor and pleura

More than 3 cm of contact between tumor and the pleural surface (5 cm of contact is more specific but less sensitive)

A ratio of the tumor diameter to the length of pleural contact by the tumor exceeding 0.5 (the higher this ratio, the more specific this finding)

Invisibility of extrapleural (chest wall) fat planes at the point tumor contacts chest wall

A mass involving the chest wall Rib destruction The only definite findings include rib destruction or

chest wall mass. Otherwise, invisibility of the extrapleural fat plane (sensitivity 85%, specificity 85%) and a ratio of tumor diameter to pleura contact exceeding 0.9 (sensitivity 85%, specificity 80%) are most accurate in predicting invasion

CT findings of chest wall invasion in three patients. A. CT at three adjacent levels shows a lenticular mass having more than 3 cm of contact with the chest wall, obtuse angles at the point it contactsn the chest wall (white arrows) , and rib destruction (black arrow) . B. There is extensive pleural contact, with the length of contact (L) being 5 cm. The tumor diameter (D) equals the length of pleural contact (i.e., theirratio is 1). Normal fat is seen in intercostal spaces (small white arrows) , while these fat planes are invisible where the tumor contacts the chest wall (small black arrows) . C. Chest wall invasion by lung cancer with rib

CT findings usually regarded as indicating “definite” or “gross” mediastinal invasion and unresectability (although they are not 100% accurate) include the following:

Extensive replacement of mediastinal fat by soft-tissue mass

Mass surrounding mediastinal vessels, trachea, or esophagus

Mass resulting in obvious invasion of one of these structures

MEDIASTINAL INVASION

In lung cancer patients who do not have gross mediastinal invasion, additional CT findings may be of value in predicting that invasion of the mediastinum and mediastinal structures is present. These CT findings include the following:

Tumor contact of more than 3 cm with the mediastinum Tumor contact with more than one fourth of the

circumference of the aorta or other mediastinal structures

Obliteration of the fat planes that are normally seen adjacent to the aorta or other mediastinal structures

Compression of mediastinal structures by a mass Mediastinal pleural or pericardial thickening

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