Chest Radiography in Cardiovascular Disease

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The chest radiograph (CXR) has endured, despite all advances in technology, due to its instantaneous capture of patient health as a snapshot in time. Like photographic snapshots, CXRs can also be collected over time to tell the story of disease in a particular patient and provide valuable information at the bedside when a patient is critically ill. The CXR is the first medical imaging for a wide variety of medical conditions, including suspected cardiovascular disease ( Table 17.1 ). ^,

TABLE 17.1

American College of Radiology (ACR) Appropriateness Criteria for Cardiac Diseases Algorithms for Disease Group Specific Workup Using Multiple Imaging Modalities and Appropriate Use for Specific Imaging Modalities

From American College of Radiology. Appropriateness criteria table, for cardiac diseases. ( https://acsearch.acr.org/list ). Accessed 3/11/2021.

Presentation of Symptoms and Initial Imaging	Specific Appropriateness Criteria
Asymptomatic Patient	At Risk for Coronary Artery Disease (CAD)
Acute chest pain	Possible acute coronary syndrome (ACS)
	High probability of CAD
	Low probability of CAD
	Suspected aortic dissection
	Suspected pulmonary embolism
Blunt chest trauma	Suspect cardiac injury
Chronic chest pain	High probability of CAD
	Low to intermediate probability of CAD
Heart failure	New-onset and chronic heart failure
Adult congenital heart disease	Known or suspected congenital heart disease
Use of chest radiograph (CXR)	Routine CXR
	Portable CXR

Overview

Understanding how CXRs are made will provide an appreciation for how the best radiographs can be obtained under the most difficult of situations. Understanding the imaging process increases the value of CXRs at the bedside in the intensive care unit (ICU) where there may not be availability of immediate consultation from a radiologist. The American College of Radiology maintains evidence-based guidelines that are reviewed and updated annually by leaders in radiology and other specialties.

Though partnership between clinicians and radiologists supports high-quality patient care while conserving patient exposure to ionizing radiation, keeping the dose “as low as reasonably achievable (ALARA)” is the principle that guides the creation of every CXR.

The appearance of the heart and lungs on CXRs can be very specific for some disease processes, including congenital and acquired heart diseases, based on anatomy. Chest radiography is frequently the first imaging study ordered and may be available before the first visit with a cardiologist. Anatomic structure identification is enhanced by common calcifications and radiopaque devices that supply additional landmarks within regions of radiographically undifferentiated tissue. Fully utilizing the lateral view along with the frontal posteroanterior (PA) view, when available, maximizes the information available from CXR. Approach to the evaluation of the CXR and uses for CXR in patient care will be the central components of this chapter. The first principles presented will provide a foundation for approaching CXRs in patients with diseases beyond the examples in this chapter.

PA and Lateral CXR

Magnification of structures follows the inverse square law by which magnification increases as the square of distance. With distance, sharp edges become less sharp. These two directly related principles guide the making of the standard CXR examination including PA and lateral views, made in the erect position for adults able to stand. For the PA radiograph, the anterior chest wall is placed as close as possible to the detector, thereby minimizing magnification of the heart. Despite this, the left lung is less well imaged than the right lung due to the presence of the heart primarily in the left hemithorax. This is balanced by providing a slight advantage to the left lung on lateral radiograph by placing the receptor against the left side of the patient, providing the sharpest depiction of vessels and other structures in the left lung. The inverse square law thus results in magnification of the right hemithorax and right ribs compared with the left hemithorax and left ribs, as shown in Figures 17.1 and 17.2 . Thus, when a normal CXR is made, the left lung and ribs will be projected within the right hemithorax. This is an important means by which pathology is localized. The patient’s arms are positioned for both views to minimize the overlap of scapula with lungs. The lungs are positioned where sensors will best account for the difference in tissue density, thereby making an image that is correctly exposed for both heart and lungs.

FIGURE 17.1, Normal standard two-view chest radiograph.

FIGURE 17.2, PA and lateral chest x-ray in a patient with bioprosthetic aortic and mitral valves.

The CXR is obtained at relatively high 120 to 140 kV to deliberately decrease obscuration of structures by the ribs and other bones encasing the thorax. It is for this reason that CXR may not identify calcifications, particularly larger calcifications. The radiation dose from one standard adult CXR is approximately 0.1 mSV. This is equivalent to 10 days of natural background radiation. These figures are convenient for comparing radiation dose from other imaging studies such as computed tomography (CT). The radiation exposure while on a plane flying at 40,000 feet is approximately 30% higher than our background radiation at sea level. This can be used to reassure a patient who becomes distressed about radiation exposure when many serial CXRs are required. Radiology department professional staffs include radiation physicists who can also assist with calculating patient-specific doses of ionizing radiation.

Portable Chest Radiographs

The ICU is a much more complex environment in which to perform chest radiography. Safety of ICU personnel and logistics for making the best possible images benefit from partnership between radiology technologists, nurses, and respiratory therapists to secure support lines away from regions of interest. The portable CXR machine provides a lower maximum dose of radiation and may lose capability if it is battery powered as battery level decreases toward a level at which it must be recharged. This type of unit can be most desirable when ICU outlets are in short supply. ^,

Beyond Standard Radiographs

A hybrid type of exam is frequently performed for patients in the emergency department and for patients on stretchers. The frontal radiograph is made in the anteroposterior (AP) projection like a portable CXR while using the standard radiography equipment. If the patient can sit straight up on the stretcher, a lateral view can be made. Using the radiography room in the radiology department results in a near-standard CXR. This can be especially valuable for obese patients for whom portable CXR may be inadequate due to tube limitations regarding radiation dose. When a patient needs to be flat, the standard radiography cross-table lateral will be superior to the same examination performed at the bedside in the ICU. This can be important when a lateral view is required for localization of malpositioned support line. While it takes more effort to organize the trip to the radiology department, considered use of this strategy can improve patient care, decrease cumulative radiation dose, and decrease delay in optimizing patient care.

Inspiration-Expiration CXR

Inspiration-expiration CXR is infrequently ordered, although it can demonstrate diaphragmatic excursion and the range in apparent size of a particular patient’s heart. This CXR can be obtained as PA or AP views. The expiration CXR alone can be used to increase visibility of a pneumothorax, although this is infrequently performed in academic medical centers where thoracic radiologists read all CXRs using PACS (picture archive and communication system) workstation display and tools.

Frontal CXR Variations

Oblique, lordotic, and reverse lordotic views are useful for problem solving. Forty-five-degree oblique views of the chest are familiar to cardiologists as a standard plane for evaluation of the aorta. The hallmark of the 45-degree obliquity used to image the aorta is a projection of the trachea to the right of the spine. Shallow 15-degree oblique views do not shift structures significantly, just enough to differentiate between nodules, vessels, and bone findings. Shallow oblique views are superior to lordotic and reverse lordotic views for assessment of lung nodules, especially in the lung apices. Shallow obliques are also useful for differentiating between breast nipples from lung nodules. A four view CXR refers to PA, lateral, and bilateral oblique views that can often substitute for chest fluoroscopy when used to determine whether a nodule is present. This and chest fluoroscopy have largely given way to CT for these differentiations, although CT carries more significant risk of additional findings leading to additional CT scans and anxiety for the patient. They are presented here because the equivalent information and views are frequently available in the cardiac catheterization laboratory, sparing the patient additional procedures.

Chest Fluoroscopy

Chest fluoroscopy is performed at lower kV than CXRs and is capable of detecting benign patterns of calcification in lung nodules. This function has been largely replaced by CT with chest fluoroscopy now used almost exclusively for functional evaluation of the diaphragm, referred to commonly as a “sniff” test. This is best performed as an outpatient procedure and contraindicated for patients requiring mechanical ventilation. It is best at detecting unilateral diaphragmatic paralysis and limited in value for detecting bilateral diaphragmatic paralysis because it depends on the asymmetry between the normal and abnormal hemidiaphragm. The exam begins in the erect position and is repeated in the supine position, especially when normal in the erect position, in order to decrease the effectiveness of accessory muscles or respiration. Abdominal muscles in particular are unable to adequately move the hemidiaphragms in the supine position. Deep inspiration and deep expiration demonstrate the overall diaphragmatic excursion. The sniff maneuver, breathing sharply through the nose while the mouth is closed, will provoke paradoxical motion, whereby the paralyzed hemidiaphragm will rise during the special sniff maneuver. A patient report of sleep position is often revealing as sleeping on a paralyzed hemidiaphragm will tend to awaken the patient. This is because the lung that is down does the primary work of breathing. This positional difference can also be exploited with radiography.

Decubitus Views

Although decubitus position in adults is most often used for evaluation of pleural effusion size and mobility, the decubitus position in an adult will also result in the upper lung being held in inspiration. Restoration of a sharp costophrenic sulcus can confirm mobility of the pleural effusion when it shifts into the mediastinum. Paired right and left lateral decubitus views are not always needed for diagnostic purposes, but the paired images will provide inspiratory examination of both lungs in a patient who is unable to stand or cooperate in taking a deep breath.

Approach to Cxr Evaluation

A systematic approach to the evaluation of the PA-lateral CXR is greatly facilitated by placing the two views side by side. When looking at PA and lateral views together, levels are matched based on readily visible anatomy. The top of the aortic arch, the carina (at approximately the same level as the bottom of an ectatic aorta), and pulmonary venous confluence are accessible for basic orientation on the lateral view. This provides a check on any hypothesis about localization based on any frontal view. Discordance requires a new hypothesis. This strategy, combined with imaging physics and a few basic radiographic signs, derives maximum information from every CXR.

Radiographic Densities

Radiographic densities are air, fat, water, calcification, and metal. Water density is the density of a wide variety of soft tissues including fluid, muscle, and solid organs. The differentiation of fat in regions of air can also result in water attenuation for pleural and pericardial fat. Calcifications provide delineation of anatomic structures for which we do not have a radiographic difference in density. These, and an ever-increasing variety of radiopaque devices, help to provide internal anatomic landmarks in the heart on CXRs. Some, such as the intra-aortic balloon pump and with luck some replacement valves, can also reveal the phase of the cardiac cycle in which a radiograph has been obtained.

The limited differentiation of tissues requires a lexicon for CXR in order to communicate accurately the findings and the significance of the findings. The most basic of terms, cardiac silhouette must be understood to include more than just the heart. The cardiac silhouette may become enlarged by one or more chamber enlargements. A pericardial effusion surrounding the heart can also enlarge the cardiac silhouette. We expand this to cardiovascular silhouette to include the aorta, great vessels, pulmonary artery, and vascular pedicle. To this we add the entire mediastinum when we refer to the cardiomediastinal contour. Pulmonary vascular redistribution is an important term from the point of view of cardiology practice.

Interpreting CXR Pearl

The most valuable advice about looking at CXRs in medical school came to me from an elderly radiologist who said, “The answer is in the jacket.” When caring for a patient who has had many CXRs, you can often find the same findings on a prior CXR and read the radiologist’s report to support your own reading of a new CXR that has not yet been interpreted by a radiologist. It is even more valuable to expand this concept to include looking at other modalities in the now-virtual radiology jacket. The information that might be gained from ordering a CT scan may already be available from a prior examination, saving money, time, and radiation exposure.

CXR Anatomy

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