Multidetector Computed Tomography (MDCT)

Thoraco‐abdominal aorta

• Diagnosis of congenital and degenerative aortic diseases
• Assessment of acute aortic injuries and dissections
• Evaluation of visceral arteries (coeliac, superior mesenteric, and renal arteries)
• Preoperative planning and follow up
• Tumor staging and surgical planning

Renal arteries

• Assessment of anatomy for donor transplants
• Diagnosis of renal artery stenosis in hypertensives or deteriorating renal function
• Assessment of renal arteries post‐intervention (renal artery stenting)

Peripheral arterial system

• Assessment of peripheral vascular disease
• Assessment of bypass grafts

Carotid/intracranial circulation

• Characterization of the atherosclerotic disease
• Assessment of aortic arch vessels
• Verification of internal carotid artery stenosis
• Preoperative planning of endovascular and surgical treatment of intracranial aneurysms and vascular malformations

Cardiac imaging

• Atypical chest pain
• Patients with intermediate-risk
• Young patients with a high risk for coronary disease
• Coronary artery anomalies
• Non‐invasive follow‐up following percutaneous transluminal angioplasty and stenting
• Assessment of myocardial scars, aneurysms, tumors and thrombi
• Assessment of coronary artery bypass grafts
• Assessment of the pulmonary veins before and following radiofrequency ablation

Contraindications of Multidetector Computed Tomography (MDCT)

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 craniocaudal 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, the 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 the 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 is 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 diseases 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 immunocompromised patients. 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.

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 the 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. The development of clinically significant sequelae depends on the site of extravasation and the 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 a 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. The use of prophylactic N-acetyl-cysteine in reducing the likelihood of CIN remains controversial.

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