Pulmonary Medicine

Multidetector computed tomography

General description of procedure, equipment, technique

Current CT technology utilizes multi-detector helical acquisition to image the chest. Multi-detector CT generates thin-section CT images with excellent spatial and temporal resolution. Using the latest detector arrays of 256 or 320 detector rows, the entire chest can be scanned in two or three seconds. Images obtained during intravenous contrast injection are optimized to assess the pulmonary arteries for pulmonary embolism, the aorta, and its branches for congenital or acquired disease, and the coronary arteries for atherosclerrotic disease.

The 1-2mm sections reconstructed from the current multi-detector CT scanners allow both high-resolution and thin-section images of the entire chest, with coronal and sagittal reconstructions that are of equal resolution to the standard axial images. Such imaging allows for better assessment of the relationship of lesions to pertinent anatomy and provides an assessment of the distribution of lung or pleural disease in the cranio-caudal, medio-lateral, and antero-posterior planes.

Three-dimensional rendering of intrathoracic structures that differ in attenuation from adjacent structures allows surface renderings of chest wall structures, enhancement of visualization of vessels like the pulmonary arteries and aorta, and precise delineation of the outer and inner walls of the trachea and main bronchi (called "virtual bronchoscopy") when a series of images of the inner margins of the airways is viewed from an endoluminal perspective, as with fiberoptic bronchoscopy.

High-resolution chest CT describes a sampling technique in which axial scans obtained throughout the chest at discrete, noncontiguous intervals are generated to assess for diffuse airway and pulmonary parenchymal abnormalities. The technique is particularly helpful in the assessment of bronchiectasis, cystic fibrosis, emphysema, and diffuse interstitial lung diseases. Serial studies can be used to assess the course of a disease and its response to treatment.

Adult patients being assessed for diffuse disease in whom radiation exposure is not associated with long-term sequelae are most often evaluated using multi-detector CT of the entire lungs during a breath hold (i.e., standard or volumetric multi-detector CT), as "high-resolution" or thin-section axial, sagittal, and coronal images can be reconstructed from the acquired data, providing additional information and facilitating comparison with prior studies to assess subtle changes.

Indications and patient selection

Indications for multi-detector CT imaging of the chest include:

  • Assessment of the solitary pulmonary nodule (Figure 1A, Figure 1B)

    Frontal chest radiograph in a patient with bilateral breast prostheses shows a focal right upper lobe density (arrow).

    CT scan at lung windows through the upper lobes shows an irregular lesion with internal lucencies indicative of an lung adenocarcinoma, proven at biopsy and subsequent resection.

  • Evaluation of pulmonary embolism (Figure 2A, Figure 2B)

    Axial image from CT pulmonary arteriogram shows bilateral acute emboli (arrows).

    Axial CT image through the heart shows right ventricular dilatation (arrow) indicative of strain resulting from the embolism.

  • Staging or follow-up of lung cancer or extrathoracic malignancy (Figure 3A, Figure 3B, Figure 3C, Figure 3D)

    Frontal chest radiograph shows a left apical lesion (arrow).

    Axial CT at lung windows through the upper lobes confirms a spiculated left apical mass (arrow) and a small contralateral nodule (arrowhead).

    Unenhanced CT shows an enlarged aortopulmonary window lymph node (arrow).

    Scan through the lung bases shows fissural and costal pleural nodules on the left (arrows). CT-PET showed hypermetabolic activity in the aortopulmonary nodes, left pleural nodules, and right apical nodule indicative of metastatic lung cancer.

  • Assessment of hemoptysis

  • Detection and characterization of emphysema or diffuse infiltrative lung disease (Figure 4A, Figure 4B, Figure 5A, Figure 5B, Figure 5C)

    Frontal chest radiograph is normal.

    Coronal CT lung windows demonstrates moderately severe upper lobe emphysema.

    Frontal chest radiograph in a patient with scleroderma shows decreased lung volumes with lower zone predominant peripheral reticulation.

    Axial CT through the lung bases shows reticular interstitial disease with traction bronchiectasis. Findings reflect UIP associated with scleroderma. Note the dilated esophagus.

    Coronal CT image through the posterior lungs shows the basilar distribution of reticulation characteristic of UIP.

  • Assessment of fever in the immunocompromised patient

  • Evaluation of the trauma patient with suspected injury to the aorta or great vessel, central airway, parenchymal, chest wall, or diaphragm

  • Detection of tracheobronchomalacia

  • Characterization and assessment of the extent of pleural fluid accumulation or pleural thickening

  • Detection and follow-up of thoracic aortic diseases, including aortic aneurysm and dissection (Figure 6A, Figure 6B)

    Axial CT image from CT aortogram shows a markedly dilated ascending aorta with an intimal flap indicative of a type A aortic dissection in a patient with Marfan’s syndrome.

    Coronal scan demonstrates the fusiform ascending aortic dilatation with intraluminal flap.

  • Screening for lung cancer in a defined high-risk population

Contraindications

Two concerns may contraindicate chest CT: radiation exposure and contrast administration.

Standard chest CT is associated with the patient's exposure to an effective dose of 4-5 milliSieverts (mSv), while a low-dose study generates an exposure of 1-2 mSv. High-resolution chest CT, which samples only about 10-15 percent of the chest, is associated with significantly lower effective radiation doses than standard, volumetric multi-detector CT does. The radiation dose is of particular concern for younger patients, especially young females, as exposure of the breasts to ionizing radiation is associated with a small but definable increased risk of breast cancer.

Nephrotoxicity secondary to administration of iodinated contrast agents is a significant concern in patients with pre-existing renal dysfunction, particularly those with diabetes-associated chronic kidney disease. Pre-study estimation of the glomerular filtration rate (GFR) to assess for risk of contrast administration is routine in most radiology departments. When such risk is noted, preventative measures to reduce the likelihood of contrast-induced nephrotoxicity may be implemented, or consideration may be given to an alternative imaging procedure, such as sonography or MRI.

A history of contrast allergy, significant atopic history, or prior anaphylactic reaction to intravenous contrast warrant prophylactic administration of corticosteroids and anti-histamines at least six hours prior to contrast administration. Alternatively, another form of imaging should be considered.

Details of how the procedure is performed

Multi-detector chest CT scans are usually performed with the patient supine and arms raised above the shoulders. A scanogram or "scout view" is obtained in order to set the cranio-caudal extent of the scan and to determine the reconstructed field of view for diagnostic images.

Study exposure settings are based on the patient's weight, with doses for smaller patients and children reduced to the lowest levels that provide diagnostic-quality images. For scans performed with intravenous contrast, placement of an 18-20-gauge intravenous catheter in the antecubital fossa is preferred. When intravenous contrast is used, the scans are timed to begin when a threshold value of attenuation is detected via monitoring, low-dose scans done through the pulmonary artery (for CT pulmonary angiography) or aorta (for evaluation of the aorta and great vessels).

All current multi-detector CT scans are reconstructed in axial, sagittal, and coronal planes (Figure 6A, Figure 6B) and sent to PACS workstations for interpretation and system-wide access. Using a thin-section data set, specialized CT technologists may generate three-dimensional reconstructions, which are particularly useful for depicting the complex anatomy of aortic diseases and tracheobronchial pathology. The three-dimensional, reconstructed images are placed onto the PACS to be viewed and archived.

Interpretation of results

Chest CT examinations are typically interpreted by thoracic radiologists or radiologists experienced in cross-sectional imaging. Many radiologists have moved to a systematic analysis and reporting method to provide a comprehensive interpretation of scans. These components include:

  • Assessment of the technical adequacy of the scan, including analysis of overall image quality, evaluation for respiratory motion, and assessment of contrast-enhancement of vessels.

  • Identification of support and monitoring devices, tubes, lines, and catheters, including their appearance and position.

  • Analysis of the chest wall, including bony and soft tissue structures. Different window settings provide for evaluation of the bones (wide window width) and soft tissues (narrower window widths).

  • Evaluation of the heart, pericardium, and great vessels. In studies performed to evaluate for pulmonary embolism, assessment includes detection of both acute and chronic emboli.

  • Mediastinal examination, including lymph nodes, trachea and central bronchi, and esophagus.

  • Evaluation of the airways and lungs, including evaluation of focal lung disease (e.g., nodules, masses), air-space and interstitial diseases, cystic lung disease, and emphysema.

  • Assessment of the pleural space, including observation for pneumothorax, and the detection of pleural thickening, fluid, calcification, or masses.

  • Analysis of visible upper abdominal structures, including a search for free intraperitoneal air and observation of hepatic, splenic, gastric, duodenal, colonic, adrenal, pancreatic, upper abdominal aortic, and upper renal pathology.

Performance characteristics of the procedure (applies only to diagnostic procedures)

The accuracy of multi-detector chest CT has been evaluated in the detection of a variety of processes. The technique is more sensitive than chest radiography in the detection of lung nodules. Multi-detector CT aortography has greater than 95 percent sensitivity in detecting traumatic aortic injury, and it is superior to conventional radiography in detecting parenchymal lung injury, chest wall injury, and traumatic diaphragmatic rupture.

Multi-detector CT is superior to radiography in assessing the tumor (T) and node (N) components of the lung cancer staging system (See Figure 3A, Figure 3B, Figure 3C and Figure 3D). While classification of tumor status (T) using CT is superior to positron emission tomography (PET), CT-PET is more sensitive for nodal (N) and metastatic (M) disease assessment. The sensitivity of multi-detector CT is approximately 90 percent and the specificity greater than 95 percent for the detection of pulmonary embolism.

Thin-section CT, which is the most sensitive noninvasive method for detecting emphysema (Figure 4A, Figure 4B), is particularly useful in detecting large and small airway disease when combined with expiratory imaging in patients with obstructive lung disease. Thin-section CT is more sensitive and specific than chest radiography in detecting and characterizing diffuse interstitial lung disease (Figure 5A, Figure 5B, Figure 5C).

Multi-detector CT has a role in the detection of pulmonary complications in the immunocompromised patient. Multi-detector CT angiography is the diagnostic method of choice for evaluating non-traumatic, acute aortic diseases, including aortic dissection, its variants, and ruptured aortic aneurysm (Figure 6A, Figure 6B). Multi-detector CT is superior to conventional radiography and ultrasound in the evaluation of diffuse pleural disease. Multi-detector CT is likely equivalent to MR in the evaluation of mediastinal masses.

Outcomes (applies only to therapeutic procedures)

Not applicable.

Alternative and/or additional procedures to consider

For some patients with possible lung nodules, dual-energy subtraction (DES) radiography and tomosynthesis can be useful in locating nodules at significantly lower cost and radiation dose.

MRI can be as useful as multi-detector CT in the evaluation of superior sulcus tumors and mediastinal masses and in assessment for cardiac or mediastinal invasion in some patients with central lung cancers. MRI is superior to CT in the evaluation of cardiac masses. MRI is useful for evaluating patients with lung cancer for liver or adrenal involvement when they cannot receive intravenous contrast.

CT-PET is more sensitive and slightly more specific in assessing nodal involvement in lung cancer than is MRI. CT-PET is more accurate than multi-detector CT in the detection of malignancy in solitary pulmonary nodules.

Complications and their management

Several important complications of multi-detector CT scanning are notable:

Contrast Extravasation - The rate of intravenous contrast extravasation during power injection for CT is less than 1 percent. Pain, swelling, tingling sensation, redness, and warmth may be seen. Development of clinically significant sequelae depends on the site of extravasation and volume and osmolarity of the contrast agent used. The most serious complication of contrast extravasation is compartment syndrome; skin ulceration and tissue necrosis are less common. Most contrast extravasation-related injury resolves spontaneously and without complication.

Patients who experience contrast extravasation should be observed for several hours to confirm the stability of physical findings. Hot or cold compresses and elevation of the affected extremity can be helpful in relieving symptoms. Surgical consultation should be obtained when there is concern about the potential for serious injury. Findings that should prompt surgical consultation include progressive swelling or pain, skin ulceration or blistering, loss of capillary refill, or changes in sensation in the involved extremity.

For patients who have an acute contrast reaction, the following measures are taken:

  • Urticaria - No treatment needed.

  • Facial or laryngeal edema: Oxygen by mask; epinepherine (1:1000), 0.1 - 0.3 ml (0.1 - 0.3 mg) subcutaneously or intramuscularly, if hypotension develops, epinenephrine (1:10,000), 1 - 3 ml.

  • Bronchospasm: Oxygen by mask; beta-agonist inhaler; epinephrine, via route and at dose as for laryngeal edema.

  • Hypotension and tachycardia: Leg elevation; oxygen by mask; intravenous fluid.

  • Hypotension with bradycardia (vagal reaction): Leg elevation; intravenous fluid; atropine, 0.6 - 1.0 mg intravenously, up to a total of 2 - 3 mg.

  • Hypertension, severe: Nitroglycerin, sublingual, up to 3 doses; labetalol, 20 mg intravenously every 10 minutes, up to 300 mg.

  • Seizures: Diazepam, 5 mg, or midazolam, 1 mg, intravenously.

  • Pulmonary edema: Oxygen; furosemide, 20 - 40 mg intravenously; morphine, 1 - 3 mg intravenously.

The management of contrast -induced nephrotoxicity (CIN) is focused on prevention. Patients at risk for CIN are those with pre-existing renal dysfunction, diabetes, advanced age (>70 years), dehydration, hypertension, multiple myeloma, or hyperuricemia. Provision of pre- and post-contrast hydration with intravenous fluids, with or without sodium bicarbonate, is the most important prophylactic measure. Use of prophylactic N-acetyl-cysteine in reducing the likelihood of CIN remains controversial.

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