Chest imaging is a critical diagnostic tool for evaluating thoracic anomalies in anatomic structure and disease. The variety of imaging technologies available for diagnostic evaluation in the chest includes plain-film radiography, computed tomography (CT), positron-emission tomography (PET), concurrent PET/CT, and magnetic resonance imaging (MRI). These radiologic procedures are further enhanced by oral or intravenously administered contrast materials used alone or in combination. The role of radiologic imaging in thoracic surgery is likely to gain even more importance as imaging technologies provide ever more accurate means of visualization.

Patients often are referred to thoracic surgeons for evaluation and treatment of incidental findings on chest CT or radiography. These incidental findings can be fortuitous for the patient, providing the opportunity for treatment before the development of symptoms heralding advanced disease. The thoracic surgeon may choose to further the evaluation with registered PET/CT or to follow indeterminate findings over time with serial CT scans.

CT is the backbone imaging modality for preoperative evaluation. The adrenal glands are always included in routine chest CT images because this is a common site of lung cancer metastases. CT can be supplemented with PET/CT or MRI for special purposes. These modalities are often useful for problem solving. PET/CT has dramatically increased the ability of imaging to contribute to accurate preoperative staging in lung cancer, thereby setting patients on the proper treatment course from the outset. The resulting change in lung cancer staging flows from the ability of PET/CT both to recognize unsuspected distant metastases and to identify coexisting benign disease. For example, before the availability of PET/CT, inflammation in contralateral lymph nodes often was attributed erroneously to a tumor of more advanced stage and patients were not offered potentially curative resection. Adjuvant PET/CT can provide preoperative staging information capable of upstaging (30%) or downstaging (15%) disease in an individual patient.¹ In the setting of heterogeneous disease, PET/CT can be used to select the “best” biopsy site, in turn decreasing the number of biopsy specimens required to definitively classify difficult-to-identify cancers such as diffuse malignant pleural mesothelioma.

MRI is less useful than PET/CT, particularly with the advent of multidetector CT scanners, which permit data to be acquired with voxels of equal dimension in all three planes, thus providing sagittal and coronal images with CT that were previously only possible with MRI. This is not to say that MRI is without advantages. MRI can sensitively differentiate tissues, including blood, by differentiating the various states of hemoglobin. In addition, fascial planes are more sharply delineated by MRI. However, MRI demonstrates calcification as a signal void and thus may be considered inferior to CT for detecting calcifications. The use of MRI for problem solving is more apt to reflect the problem under consideration than a standard approach, although standardized imaging generally is applicable for visualizing the complete thorax in patients with diffuse malignant pleural mesothelioma.

In other instances, the area of interest, such as the thoracic inlet, brachial plexus, or lung apex, will be imaged without imaging the rest of the thorax by MRI. MRI and CT are equivalent for imaging lymphadenopathy in the mediastinum. MRI is by nature less than contiguous and otherwise should be viewed as complementary to CT imaging.

CT provides detailed anatomic images of the chest in which a variety of soft tissues can be recognized, along with water, fat, and bones; the resulting basic transaxial images are the in vivo equivalent of transaxial anatomic pictures of a cadaver (Fig. 3-1). State-of-the-art multidetector CT scanners are capable of acquiring ever-increasing numbers of individual slices of data at one time. Four-detector scanners are capable of scanning the chest in approximately 20 seconds, a practical time for patient breath-holding. Readily available clinical models with the capability of producing 16 to 64 slices at one time can scan the chest in 10 seconds or less. Alternatively, very small structures can be studied using ever-smaller slice thicknesses. Development of this technology is currently focused on providing up to 256 slices at one time. Along with cardiac gating, this technology permits unprecedented in vivo evaluation of ultrastructure in the lungs as well as the heart.

The technical parameters of CT scanning can be altered to improve the visualization of different tissues. CT of the chest is performed with a small focal spot using kilovolts between 100 and 120 and milliamperes between 20 and 200 or more. Reconstructed images include contiguous 5-mm images with soft tissue smoothing for visualization of the heart, great vessels, and mediastinal structures. These are generally referred to as mediastinal images and are displayed with a window level of 25 to 40 Hounsfield units (HU) and a relatively narrow window width, such as 360 HU. In addition, separate images with edge enhancement for optimizing visualization of lung ultrastructure are displayed with a window level of -600 HU and a wide window width of 1500 to 2000 HU.

Hounsfield units derive their name from the developer of the CT scanner, Nobel laureate Sir Godfrey N. Hounsfield. The scale arbitrarily assigns water the attenuation value of 0, air -1000, and bone up to +1000. These numerical values of normalized x-ray attenuation define the gray scale of all CT images. The display windows highlight various structures based on the relationships between the underlying fundamental gray scale and the composition of various tissues in the body.

Intravenous iodinated contrast material is used commonly to provide optimal delineation of vascular structures, particularly when they lie in close proximity to the pathologic entity. Thus lung cancer staging is performed most often with intravenous contrast. NPO conditions should be instituted 4 hours before the examination to minimize nausea and vomiting. In many instances, patients with a history of contrast material allergy may be imaged with MRI instead of CT. When it is mandatory to use CT in a patient who has had a prior contrast material reaction, pretreatment with oral steroids and Benadryl can be considered. Oral contrast material is rarely used because air and fat often provide adequate contrast for identifying gastrointestinal tract structures.

Increasing concern for nephrogenic systemic fibrosis has led to the institution of reduced-dose regimens for patients with impaired renal function. Half the standard dose of contrast material is used for patients with an estimated glomerular filtration rate of between 30 and 60 mL/min/1.73 m²; intravenous contrast material should not be given to a patient with an estimated glomerular filtration rate less than 30 mL/min/1.73 m². In the setting of impaired renal function, as with contrast material allergy, consulting the radiologist before the study will ensure that the best possible study is selected for the given patient. Even with normal renal function, it is advisable to separate examinations that require administration of intravenous contrast material by at least 24 hours.

Varying the section thickness sometimes can improve visualization. The thinnest section that can be obtained is directly related to the size of the focal spot with which the scan was performed, currently providing images as thin as 0.6 mm. Images of 1 to 2 mm reconstructed with an edge-enhancing algorithm are still the most commonly provided thin-section images of lungs. These high-resolution CT (HRCT) images can be derived from the same data acquisition on multidetector scanners. Interspersed HRCT images permit visualization of the lung parenchyma and pleura and are most helpful for evaluating diffuse diseases such as emphysema and bronchiectasis. Contiguous or overlapping thin-section images are used for studying small nodules. These are appropriate for evaluating the features of a given nodule when reconstructed using a lung algorithm and identifying fat and calcification when reconstructed using a soft tissue algorithm.

Edge enhancement produces artifacts that can be mistaken for calcification. Thus, evaluating for the presence of calcifications should be performed using mediastinal soft tissue reconstruction. A second caveat must be offered when looking for calcified pulmonary nodules. Since contrast material may give small vessels a dense appearance very similar to calcification, non–contrast-enhanced CT may be preferable in the setting of prior granulomatous infection. Nodule surveillance guidelines published by the Fleischner Society in November 2005 can help to minimize the number of CT examinations performed, particularly for very small nodules in patients at low risk for lung cancer² (Table 3-1). In December 2013, the Fleischner Society issued on update focused on the management of subsolid pulmonary nodules detected on CT to complement the 2005 recommendations for incidentally detected solid pulmonary nodules. Since peripheral adenocarcinomas account for the most common type of lung cancer, and there is evidence of increasing frequency, developing a standardized approach to interpreting and managing subsolid nodules remains an important goal.³ The American Association for Thoracic Surgery has also published guidelines for the surgical management of pulmonary nodules in lung cancer survivors and other high-risk patients who undergo lung cancer screening with low-dose CT.⁴

Specialized CT examinations are performed according to disease-specific algorithms. Interstitial lung disease is evaluated on 1- to 2-mm thick images obtained without contrast material. Increasingly, these thin-section HRCT images are obtained from volumetric data acquired from a single breath-hold and reconstructed retrospectively at specified intervals. Radiologists generally will evaluate HRCT images in conjunction with standard renderings of the volumetric CT data set. It is very important to obtain and view HRCT images with the proper field of view. Reducing the size of the image reduces the information available from the images.

CT pulmonary angiography is performed with a higher concentration of iodine, such as 370 g/100 mL, compared with 300 g/100 mL for ordinary chest CT contrast. With the more crucial timing requirement for imaging contrast material in pulmonary arteries, a test bolus or automated bolus tracking software is often used to refine the timing rather than relying on an approximation of the circulation time from the antecubital fossa to the main pulmonary artery at 20 seconds. Rendering of CT pulmonary angiographic images requires thin-section imaging and often uses a variety of special reconstructions and multiplanar images in sagittal and coronal planes. A plane for reconstruction also may be chosen to follow the axis of the pulmonary arteries at the bifurcation. It is important to remember, especially when working with patients who have hypercoagulable states owing to processes such as cancer, that pulmonary emboli may be visualized on standard CT images, such as those performed for staging. Secondary criteria, including atelectasis and pleural effusions, may be absent. In patients who have had a lung resection, particularly pneumonectomy, a common location for the accumulation of thrombus is at the site of lung resection in the terminus of the pulmonary artery stump. Thrombus in such a location may persist for long periods of time.

Three-dimensional reconstruction is performed increasingly for understanding the anatomy in relation to the function and pathology. This strategy is being used to evaluate airways for bronchomalacia and in the fitting of bronchial stents, as well as in patients in whom virtual bronchoscopy can provide visualization of a point beyond the proximal obstruction. Functional information regarding obstruction may be added to such examinations by acquiring a second CT data set at reduced dose, such as 80 mA, during expiration. Acquiring images in frank expiration to eliminate respiratory motion is a useful strategy when the suspected level of obstruction is distal, at a level such as the terminal bronchioles. Pathology in the midmediastinum with a complex relationship to the heart and great vessels may be mapped using three-dimensional reconstruction with color rendering of the images for surgical planning. Cardiac gating can eliminate confusion that may arise from cardiac motion for this purpose. Since the cardiac gating increases the dose required, it should be used for imaging only when it will provide crucial information.

Some centers evaluate nodules with dynamic imaging during and following administration of intravenous contrast material. A nodule is likely benign if enhancement is less than 10 HU. The nodule is likely malignant if enhancement is greater than 20 HU. A nodule that demonstrates enhancement of 15 HU or more is not usually a granuloma. This technique has proved to be user-dependent and therefore may yield disappointing results in centers where it is not performed commonly. It can be used when PET/CT is not available; however, and may be advantageous in regions where there is endemic granulomatous disease such as histoplasmosis.

Perioperative CT scans are performed for a variety of reasons and may or may not include the administration of intravenous contrast material. Infected pleura may enhance with contrast material and better delineate lung from pleural effusion. Evaluations related to pneumothoraces generally do not benefit from intravenous contrast material administration. Oral contrast material may be used for the evaluation of potential leaks after surgery such as esophagectomy. The oral contrast material should be water soluble and administered by a surgeon with clear purpose in choosing both the route of administration and volume of contrast material. Preliminary images performed before the administration of oral contrast material are often the most important images obtained. These images permit the detection of subtle changes such as those caused by the introduction of oral contrast material that leaks into spaces such as the pleural space. The preliminary images eliminate confusion related to a variety of sources of radiopaque materials such as surgical clips and previously administered contrast material. Barium can remain in the lung and in the pleural space permanently.

The PET/CT scanner combines a gamma camera and multidetector CT scanner in the same instrument, allowing the images from both examinations to be displayed together with registration to increase the identification of subtle signs of pathology. The PET scanner records the positron emissions from a radioactive tracer, that is, [¹⁸F]fluorodeoxyglucose (¹⁸F-FDG), hence the name PET. Both scans generally image from the top of the skull through the pubic symphysis. In the case of melanoma, the lower-extremity imaging is extended. The long range of PET/CT scans provides a total body examination but also permits physiologic motion over time, which creates the potential for discordance between PET and CT images. This can occur as a result of peristalsis in the gastrointestinal tract and, especially, breathing. Although a patient could not be expected to breath-hold through the entire examination, lasting minutes, a separate chest CT can be obtained during a breath-hold to improve the resolution of lung imaging. In some centers, PET/CT is a routine procedure for indications such as solitary pulmonary nodule, lung cancer, and mesothelioma.

Concurrent PET/CT imaging combines the ability to detect subtle metabolic changes through the preferential uptake of ¹⁸F-FDG by metabolically active cells responsible for the growth of abnormal cells (PET) with precise anatomic location of disease (CT), tumor, or affected tissue (Fig. 3-2). It is currently a reimbursable procedure when used for diagnosing solitary pulmonary nodules and tumor staging. The patient usually must fast for a minimum of 6 hours before the injection of ¹⁸F-FDG. In some centers, the patient may be instructed to have very specific meals at specified times before the examination to reduce cardiac uptake of ¹⁸F-FDG through saturation of receptors. The patient rests quietly for approximately 1 hour after intravenous injection of the radioisotope to permit the tracer to disperse throughout the body before imaging. Some institutions also give dilute oral contrast material to improve visualization and identification of abdominal structures. The PET and CT scans are both performed on the same scanner without moving the patient. Patients having the examination for the investigation of pathology within the chest also should have a CT of the chest during a breath-hold while in the same position. Although this scan is generally of lower quality than a diagnostic CT, such a scan will improve evaluation of the lungs significantly compared with a standard CT obtained to correct for attenuation. Intravenous contrast material is not yet a standard feature of this examination because conventional iodine contrast agents interfere with the PET scan portion of the examination.

Pulmonary nodules that measure 7 to 8 mm in diameter or greater can be evaluated reliably with PET/CT. Although it is possible for smaller pulmonary nodules and lymph nodes to demonstrate avidity for ¹⁸F-FDG, the scan requires very high metabolic activity, leaving uncertainty when a nodule is not apparently avid for ¹⁸F-FDG.

PET scans also require calibration for accuracy. The process of quantifying ¹⁸F-FDG avidity, or uptake, is complex, with extensive quality control measures. Quantification procedures vary somewhat between institutions, particularly in regard to correction and reporting of the standard uptake value (SUV). The SUV relates the activity concentration in a volume of tissue to the amount of injected dose and the patient’s body weight. The maximum SUV indicates the affinity of a pathologic process, such as a tumor, for glucose. This correlates with the aggressiveness of a tumor histologically. There is no correlation between SUV and CT attenuation measured in Hounsfield units.

The pitfalls of PET/CT scanning remain numerous at this time despite its extreme utility for thoracic surgical evaluations. Investigations of intravenous contrast material and more widespread addition of breath-hold images for the lungs, it is hoped, will lead to a diagnostic-level CT scan within a PET/CT investigation and enable radiologists to keep the radiation of patients as low as reasonably achievable. PET/CT provides the best preoperative staging currently possible. The results of the two types of scans combined may be thought of as concordant or discordant based on whether the findings can be correlated. The consistency with pathologic truth is a separate and equally important consideration. PET/CT has not replaced conventional imaging in its ability to exclude brain metastases from lung cancer. The accuracy and therefore utility of PET/CT for the brain are limited. Although brain metastasis may be found on PET/CT, the lack of a finding does not exclude metastasis. The pitfalls of PET/CT correlative imaging in bone also have proved to be a significant limitation. Metastases, depending on phase and rate of bone destruction, can be concordant or discordant, leading to a number of confusing situations, some of which would be better resolved with conventional radionuclide bone scanning. In a number of potentially confusing situations, evaluation of SUV in various locations may reconcile PET and CT findings with the pathologic processes present. Inflammation can have a very high SUV but generally will have low-to-moderate values, whereas an avid tumor will have a much higher SUV (e.g., tumor SUV of 10 and inflammatory process SUV of 3). It is also possible for tumors to have little or no avidity for ¹⁸F-FDG. Two tumors in the lung are particularly important in this regard, adenocarcinoma in situ and carcinoid. The ¹⁸F-FDG uptake of a tuberculoma can be extremely high, with an SUV as high as 20. The decision to operate must consider factors from CT and the clinical evaluation of the patient. Clarity is sometimes increased by serial PET/CT scans to watch lesions over time.

MRI uses a variety of pulse sequences to identify unique characteristics of soft tissues and fluids that cannot be detected with CT scanning. MRI can sensitively identify blood and determine the length of time it has been present based on the state of hemoglobin as it changes to deoxyhemoglobin and further degradation products. MRI can readily determine the direction of blood flow in a vessel, useful information not provided at the same level by CT and not addressed by PET/CT. The use of MRI in the chest has increased as data acquisition times have diminished, allowing breath-hold imaging of the lungs (Fig. 3-3). MRI examinations are customized to the problem being evaluated. Coils used to perform the examination not only provide improved imaging but also control technical parameters such as field of view. The bore of the available MRI scanner itself may limit the sizes of patients who can have chest MRI. Larger-bore and open scanners have decreased this limitation, but a patient may have to go to a special location to have such an examination. It is helpful to explore patient claustrophobia before ordering the examination. Patients who are concerned about having the examination often benefit from oral premedication that permits normal outpatient scanning. Of course, the standard exclusions for any MRI, such as aneurysm clip, recent surgery, and pacing devices, apply to chest MRI.

The need to visualize blood vessels, nerves, and the variety of substances, including blood, that can be found in the pleural or pericardial space serves as a guide in choosing MRI for a particular patient. As a problem-solving tool, the examination will be customized by the radiologist. In unusual situations, it is best to discuss the problem with the radiologist before ordering the examination. This will ensure adequate scanner time and the best chance that important clinical questions will be answered. Until recently, 20 mL of intravenous gadolinium contrast material was administered routinely both to identify the tissue-enhancement characteristics of many tumors and to perform magnetic resonance angiography. As with iodinated contrast material for CT scans, however, documented cases of nephrogenic systemic fibrosis have led to the restriction of contrast material administration, particularly in the setting of impaired renal function.

MRI is the primary imaging modality for thoracic outlet syndrome. For this type of evaluation, the patient is imaged with the arms up and with the arms down. The vessels and nerves of the thoracic inlet and brachial plexus region are studied using limited field of view and special blood flow techniques.

MRI is also performed routinely for diffuse malignant pleural mesothelioma. This is the most standardized chest MRI examination, and images the complete chest and upper abdomen. Intravenous gadolinium contrast material is administered to detect the enhancement of tumor masses. Of note, recent incorporation of PET/CT into mesothelioma protocols has resulted in more specific identification of sites that will yield a productive tumor biopsy than can be achieved with MRI alone. The MRI itself is more helpful for clarifying the integrity of fascial planes at the diaphragmatic and mediastinal boundaries of the tumor. Operability is determined through this combination of tests to determine unforeseen distant disease and local extension beyond the scope of extrapleural pneumonectomy (see Chapter 122).

Pancoast tumors also are imaged with MRI to best evaluate relationships between the brachial plexus and apex (see Chapter 80). Depending on lesion size and clinical considerations, the examination may be planned as a brachial plexus examination or a full field-of-view examination, as performed for mesotheliomas.

MRI is a primary tool for the noninvasive evaluation of adrenal masses. When an adrenal lesion does not exhibit the low attenuation associated with adrenal adenomas, the MRI may confirm the presence of an adenoma and obviate the need for biopsy.

Cardiac MRI, performed with cardiac gating to eliminate the motion of the heart, is also used increasingly for surgical planning in the removal of large central mediastinal masses and evaluation of structures adjacent to the heart that are not well seen on CT scans.