Chest imaging

[Lizzy Liz]

Chest imaging studies include

X-rays
Computed tomography (CT)
Magnetic resonance imaging (MRI)
Nuclear scanning
Ultrasonography
Positron emission tomography (PET) scanning

(See also Medical History and Physical Examination for Lung Disorders and Respiratory System.)
X-rays of the chest are almost always done when doctors suspect a lung or heart disorder. Other imaging tests are done as needed to provide doctors with specific information to make a diagnosis.
Chest x-rays are routinely taken from the back to front. Usually a view from the side is also taken. Chest x-rays provide a good outline of the heart and major blood vessels and usually can reveal a serious disorder in the lungs, the adjacent spaces, or the chest wall, including the ribs. For example, chest x-rays can show most pneumonias, lung tumors, chronic obstructive pulmonary disease, a collapsed lung (atelectasis), and air (pneumothorax) or fluid (pleural effusion) in the pleural space. Although chest x-rays seldom give enough information to determine the exact cause of the abnormality, they can help a doctor determine whether and which other tests are needed to make a diagnosis.
Computed tomography (CT) of the chest provides more detail than a plain x-ray. With CT, a series of x-rays is analyzed by a computer, which then provides several views in different planes, such as longitudinal and cross-sectional views. During CT, a substance that can be seen on x-rays (called a radiopaque contrast agent) may be injected into the bloodstream or given by mouth to help clarify certain abnormalities in the chest. High-resolution CT and helical (spiral) CT are more specialized CT procedures. High-resolution CT may reveal more detail about lung disorders. Helical CT can provide three-dimensional images. Generally, CT scans are done after a person takes a deep breath (inhales). Sometimes, CT images are obtained after people both inhale and exhale to better look at small airways.
CT angiography uses a radiopaque contrast agent injected into an arm vein to produce images of blood vessels, including the artery that carries blood from the heart to the lungs (pulmonary artery). Currently, CT angiography is usually done instead of nuclear lung scanning to diagnose blood clots in the pulmonary artery (pulmonary embolism).
Magnetic resonance imaging (MRI) also produces highly detailed pictures that are especially useful when a doctor suspects blood vessel abnormalities in the chest, such as an aortic aneurysm. However, MRI takes longer to do and is more expensive than CT. Also, the resolution of MRI is lower than CT for diagnosing abnormalities in the lungs, and therefore MRI is not frequently used for chest imaging. Unlike CT, MRI does not use radiation.
Ultrasonography creates a picture from the reflection of sound waves in the body. Ultrasonography is often used to detect fluid in the pleural space (the space between the two layers of pleura covering the lung and inner chest wall). Ultrasonography can also be used for guidance when using a needle to remove the fluid. Bedside ultrasonography is sometimes done to diagnose pneumothorax. Endobronchial ultrasonography (EBUS) can be used together with bronchoscopy to help guide doctors when they need to obtain a sample of lung tissue to look for cancer (needle biopsy). In this case, the ultrasound probe is located on the bronchoscope to obtain images from inside the airways.
Nuclear lung scanning can be useful in detecting blood clots in the lungs (pulmonary emboli) but has largely been replaced by CT angiography to diagnose this disorder. However, nuclear lung scanning may be done when CT angiography is not possible because a person has kidney disease or allergies to the contrast agents used in CT. Nuclear lung scanning also may be used during the preoperative evaluation of people having part of their lung removed to treat lung cancer or severe chronic obstructive pulmonary disease (COPD) to see how well the rest of the lungs are functioning. Nuclear lung scanning uses minute amounts of short-lived radioactive materials to depict the flow of air and blood through the lungs. Usually, the test is done in two stages. In the first stage (lung perfusion scan), a radioactive substance is injected into a vein, and a scanner creates a picture of how it is distributed throughout the blood vessels of the lung. If the perfusion scan is abnormal, a second stage is necessary (lung ventilation scan); the person inhales a radioactive gas, and a scanner creates a picture of how the gas is distributed throughout the lungs. This procedure enables doctors to determine whether the remaining lung is able to absorb enough oxygen.
Pulmonary artery angiography (also called pulmonary artery arteriography) is done by injecting a radiopaque contrast agent directly into the pulmonary artery through a long, thin plastic tube (catheter) passed through a vein, into the heart and then into the pulmonary artery. After the contrast agent is injected, doctors use conventional x-rays to view the contrast agent in the lungs (angiography). Angiography has traditionally been used most often when pulmonary embolism was suspected, usually on the basis of abnormal lung scan results, and is still considered the most accurate test for diagnosing or excluding pulmonary embolism. However, currently, angiography of the pulmonary arteries is usually done instead with CT angiography because pulmonary artery angiography, involving an injection directly into the large pulmonary artery, is more invasive.
Positron emission tomography (PET) scanning may be used when cancer is suspected. This radiographic imaging technique relies on different metabolic rates of malignant (cancerous) compared with benign (noncancerous) tissues. Glucose molecules are combined with a compound that is visible using PET. These molecules are injected intravenously and accumulate in rapidly metabolizing tissue (such as in cancerous lymph nodes), making these tissues visible on PET scans. Benign growths usually do not accumulate enough molecules to be visible. PET scans are often combined with CT scans to provide two different methods to visualize lung tumors.