1. 
   

   What are the advantages of noncontrast computed tomography (CT) compared with other CT studies?

   
   
2. 
   

   What are the indications for noncontrast CT with thin-section reconstruction?

   
   
3. 
   

   When is the use of IV contrast with CT imaging mandatory?

   
   
4. 
   

   What are the limitations of positron emission tomography (PET)-CT?

   
   
5. 
   

   What are the main indications for cardiac-CT?

   
   
6. 
   

   What are the Fleischner Society Guidelines recommendations concerning follow-up of incidental solid pulmonary nodules?

   
   
7. 
   

   What are the disadvantages of cardiac magnetic resonance with late gadolinium enhancement?

   
   
8. 
   

   Complementary to two-dimensional echocardiography, transesophageal echocardiography (TEE) is able to provide superior visualization of what cardiac structures?

   
   
9. 
   

   What calcium score would preclude contrast computed tomography angiography (CTA)?

The overwhelming majority of advanced chest imaging for hospitalized patients is performed by CT, with ultrasound, MRI, and nuclear medicine reserved for specific situations. The evolution of disease processes in the chest over time is central to the diagnostic process necessitating integration between modalities in choosing comparison studies. With the assistance of an experienced radiologist, serial bedside chest radiographs can provide physiologic and pathologic information that may not be available from more advanced imaging that reflects only a single moment in time.

The chief complaint should guide decisions about the extent of medical imaging necessary for the proper diagnosis and treatment of the acute illness. The radiologist reads the patient’s body and disease processes much as the clinician completes the history and physical examination with a checklist that will identify both the truly incidental and unrelated findings as well as separate seemingly unrelated findings that complete the picture of the acute illness.

Many patients who require hospitalization for successful care of their acute illness have underlying medical conditions and chronic disease processes. Preexisting heart disease, lung disease, and systemic disease findings help to develop the personalized differential diagnosis for the reporting of the imaging studies whether obtained as radiographs, CT, MRI or any other modality. Diabetes, collagen vascular diseases, chronic obstructive pulmonary disease, atherosclerosis, and suppression of the immune system can lead the radiologist to different conclusions about the significance of particular findings in an individual patient.

Advanced imaging during hospitalization that risks complications in an acutely ill patient, provides suboptimal imaging requiring additional studies, and unnecessarily increases length of stay should be minimized. This chapter will focus on the abnormalities that most frequently require advanced imaging for diagnosis and the common incidental findings that require mandatory follow-up post discharge.

Chest radiography images how lung function (gas exchange and perfusion of oxygenated blood) changes over time. Blood flow is greater to the lower lobes than the upper lobes, and greatest in the right lower lobe, while oxygenation is greater in the upper lobes. Because blood flow is greater to the lung bases, particularly the right lower lobe, hematogenous spread of infection and tumor likely begin in the lung bases. Pulmonary emboli also occur more often in lower lobes. Oxygen-loving mycobacterium organisms prefer the lung apices. Warm cigarette smoke rises most directly to the apical segment of the right upper lobe, not surprisingly causing the right upper lobe to be the most frequent site of primary lung cancer.

The deposition of calcium in lung parenchyma is also based upon physiology stemming from the diametrically opposed oxygenation of the apices and perfusion of the bases that result in a pH gradient from 7.52 in the lung apices to 7.39 in the lung bases. In the setting of hypercalcemia, as in renal failure, serum calcium will be precipitated in the lung apices. This abnormal “metastatic” deposition of calcium into normal tissues, reversible with correction of the hypercalcemia, only occurs in a base environment.

Noncontrast Chest CT

The most basic noncontrast enhanced chest CT scan is performed with breath-holding in inspiration without oral or intravenous contrast material and requires no special patient preparation. Preferred for the acutely ill patient when CT scanning is indicated, multidetector CT scanners in current use can rapidly provide many images without respiratory motion artifacts despite continuous patient breathing. The scanning room can generally accommodate ventilators and extensive support devices. The aerated lung parenchyma provides exquisite contrast for branches of the pulmonary arteries and pulmonary veins without requiring the addition of oral contrast agents. In the absence of IV contrast material, visualization of great vessels is generally adequate due to the presence of fat in the mediastinum. The noncontrast chest CT should not be used to assess hilar lymph nodes, particularly when small to borderline in size, pulmonary emboli (PE), and acute cardiovascular disease.

Noncontrast Enhanced Chest CT with Thin Section Reconstruction

Noncontrast enhanced chest CT with thin section reconstruction is the standard imaging study of intrinsic, interstitial lung disease. Expiratory scans can be added to evaluate air-trapping. A second scan during forcible expiration will best demonstrate central airway narrowing and thereby identify tracheobronchial malacia, a potential cause of difficulty weaning a patient from ventilatory support. At end-expiration, thin section reconstruction will clearly image lung parenchyma and air-trapping in small airways. Prone images to more completely evaluate the extent of interstitial lung disease are more easily performed following resolution of the acute illness.

High Resolution Computed Tomography (HRCT) uses slice thickness ≤ 2 mm with high spatial frequency reconstruction to image lung structure distal to all but the most peripheral vasculature (the secondary pulmonary lobule). HRCT images do not require contrast enhancement, which can actually confound the findings (see Table 108-1).

Contrast-Enhanced Chest CT

Contrast-enhanced chest CT examinations differ by rate of injection and delay before imaging. The delay will be slightly longer when the primary concern is for aortic dissection. Cardiac gating is also possible, and may be used in some institutions to provide a triple-rule-out scan for coronary arteries, and aorta and pulmonary arteries.

The use of IV contrast material is mandatory for CT pulmonary angiography (CT-PA). Patients who have previously had mild to moderate contrast allergy reactions can be premedicated over approximately 12 hours prior to CT-PA (See Chapter 106 Patient Safety Issues in Radiology). If the history of allergic reaction is severe, select an alternative test. Ideally, the patient should be fasting for four hours prior to the administration of IV contrast material but a nonfasting patient may have CT-PA without delay when necessary. In order to adequately opacify pulmonary arteries, the contrast material should be injected at a high velocity of approximately four cc per second, which requires a large bore, minimum 20-gauge catheter. Using a tenuous or smaller IV for the study will almost certainly result in a nondiagnostic study. PE may be seen on standard contrast-enhanced chest CT scans, so clinicians should review any recent prior CT scans with the radiologist in light of new suspicion for PE.

Thoracic PET-CT

Thoracic PET-CT uses a radionuclide that binds to a D-glucose analogue to produce FDG. Because glycolysis is up-regulated in tumor cells as well as in normal cells during anaerobic conditions, PET-CT can identify tumors when other imaging is indeterminate. PET-CT identifies tumor through inflammatory characteristics; hence, high uptake of FDG may falsely suggest malignancy in metabolically active infections and inflammatory conditions. Originally, PET-CT was used to evaluate solitary pulmonary lesions that may be cancerous. However, TB or fungal granulomas, hyperplastic normal thymic tissue in the anterior mediastinum, and brown adipose tissue at the base of the neck, supraclavicular area, or superior mediastinum in adults may avidly take up FDG. Because FDG and glucose compete for the same receptor, decreased FDG uptake may occur when there is acute hyperglycemia. It can reliably evaluate 8 mm pulmonary nodules of unclear etiology, but some well-differentiated adenocarcinomas of the lung can be very slow growing and, therefore, reveal little or no FDG-glucose avidity.

Although PET-CT does not require contrast, the dose of ionizing radiation that the patient receives from the CT scan for attenuation correction is not negligible. The CT scan performed for attenuation correction of the PET scan is not a diagnostic quality CT scan. The uncertain and incidental findings will also lead to significant additional imaging.

PET-CT is usually used for staging known cancer. This long and very expensive examination should not be undertaken when the patient cannot adequately cooperate or has acute processes that will resolve in a short interval of time.

Cardiac Computed Tomography (CCT)

Cardiac-computed tomography (CCT) visualizes nonstenotic calcified and noncalcified coronary plaques with a very high negative predictive value (91–100%) to rule out the presence of coronary artery disease (CAD). Excellent images may be obtained within five minutes, thereby facilitating rapid triage from the emergency department in some institutions. Its primary use in acute chest pain should be reserved for patients with an intermediate pretest probability of coronary artery disease or for those patients at increased risk for aortic dissection and segmental pulmonary embolism that may be visualized at the same time. Patients with a low pretest probability should not be subjected to the risk of radiation; patients with a high pretest probability of CAD and suspected Non-ST Elevation Myocardial Infarction (NSTEMI) or Unstable Angina (UA) would not benefit because a negative CCT would not alter the pretest probability. Cardiac CT requires the administration of contrast.

Pulmonary Embolism

The diagnosis of PE has remained difficult despite successive improvements and refinements in imaging modalities. Patients who require hospitalization for acute care, particularly with advancing age, have comorbidities that increase the pretest probability of PE. Evaluation of ventilation-perfusion scanning by PIOPED investigators suggests that more than 50% of patients who are likely to be seen by hospitalists will fall into the category most at risk for actually having PE when the result is intermediate or indeterminate. As a result, CT-PA has been widely adopted as the primary diagnostic test for PE although a normal perfusion scan still provides greater exclusion of PE.

Pulmonary emboli can be hard to detect even in lobar arteries and the detection of the smallest peripheral emboli can be difficult to put in clinical perspective. The number of vessels present in each lung and each segment within each lung is daunting for complete assessment by conventional angiography. Pulmonary emboli are easier to identify in the larger vessels in the lower lobes that are also more readily examined in the axial plane. With increasing use of coronal and sagittal reformatted images and advanced image processing, the detection of upper lobe pulmonary emboli and smaller vessels will continue to improve. An important normal function of the lungs is to trap and make harmless the much more frequently occurring subclinical microemboli that fail to pass through the capillaries in the lungs, thereby not reaching other organs, particularly the brain. At what point should this normal function be deemed clinically significant pathology requiring treatment? The existence of significant sources of additional clot may inform the decision-making process more than the current miniscule burden of clot.

The evaluation of potential PE by CT initially included pelvic and lower extremity imaging to assess for DVT. This portion of the CT-PA examination is now largely historical as it required significant additional radiation and was rarely helpful. When the value of identifying DVT as a source for future PE is important, lower extremity noninvasive imaging (LENI) with ultrasound is preferred and may also obviate the need for CT-PA in the pregnant patient. The absence of clot is less helpful in the diagnosis of acute PE because one-third or more patients will have no evidence of clot in their lower extremities despite acute PE. The normal perfusion scan remains the single best test to exclude PE when the CXR is normal.

The radiographic evolution between acute PE and infarction occurs over several days. It is not infrequent for pulmonary emboli to no longer be visible, particularly when peripheral, so that multiple subsegmental defects may elude detection several days after the acute symptoms such as pleuritic chest pain occur. It then becomes more important to consider whether any secondary signs of PE or right heart strain are present on the scan already obtained. Chronic changes that may result from prior PE include pulmonary artery wall thickening and segmental bronchiolitis obliterans with marked diminution of pulmonary vessels.

The detection of pulmonary emboli also varies due to patient factors, including respiratory factors and artifacts that can be minimized by relocating foreign bodies such ECG leads for the test. A shallower inspiratory breath-hold will maximize vessel opacification particularly in young and relatively healthy patients. Clinicians should prepare patients to expect to raise their arms and to hold their breath for the study. Respiratory motion and bolus quality often conspire to limit the exclusion of PE to central PA branches in the acutely ill patient. The interpreting radiologist should report the bolus quality and use variable display window and level settings to better visualize emboli.