Mediastinoscopy is useful for assessing lesions found in particular mediastinal locations, typically the right paratracheal and anterior subcarinal spaces. The lesion in this patient, however, is located well lateral of the right paratracheal space, with the right brachiocephalic vein obstructing access to the lesion, from the standard supra-cervical mediastinoscopy approach, as well. Bronchoscopy can visualize and possibly sample lesions in close proximity to the central airways, but this lesion is located several centimeters away from the trachea and could not be reached with bronchoscopic biopsy techniques. In an analogous fashion, esophageal endoscopic ultrasound is an excellent tool for visualizing and possibly biopsying lesions in close proximity to the esophagus, such as lymph nodes or masses in the left paratracheal space (2L and 4L), subcarinal space (station 7), periesophageal spaces (station 8), and aorto-pulmonary window (station 5). However, this lesion is clearly remote from the esophagus and could not be assessed with upper endoscopy. A Chamberlain procedure, or left anterior mediastinotomy, is useful for sampling left mediastinal lesions, such as those in the prevascular, left para-aortic and aorto-pulmonary window region spaces, but the lesion in this patient is remote from these locations and could not be accessed with this technique.
Dual-energy CT is a recently introduced technique that provides information regarding how substances behave when exposed to x-ray beams of different energies. Knowledge of how a particular substance behaves when exposed to x-ray beams of differing energies can provide information regarding tissue composition. Basically, x-ray beam energy is maximally absorbed when the K-edge values of the tissue exposed to the x-ray beam are close to the energy of the x-ray beam; in this case, the interaction known as the photoelectric effect is enhanced. The photoelectric effect refers to the ejection of an electron from the innermost K-shell of an atom by an incident x-ray photon, and only occurs when the incident x-ray photon has enough energy to overcome the binding energy of the K-shell electrons in the exposed tissue. The term “K-edge” refers to the spike in CT attenuation observed, owing to increased photoelectric interactions, when the energy of the x-ray beam is just greater than the K-shell binding energy of the exposed tissue. In contrast, when the K-edge of the tissue exposed to x-ray energy is remote from the energy of the x-ray beam, the atomic interaction known as Compton scatter is favored. Because K-edge values for various elements are known, exposing tissues to x-ray beams of differing energy can provide for the detection of certain substances in biological tissues, including iodine, calcium, and uric acid. In the case of iodine, dual-energy CT techniques can be employed to create “virtual unenhanced” images by subtracting iodine the blood pool of enhanced studies; in a similar vein, calcium can be subtracted from the tissues as well; removal of calcium can be useful improve vascular assessment at CTA, or to create chest radiographic images with the osseous structures “subtracted” from the images, thereby eliminating superimposition of bone over the lungs on a chest radiograph. A number of dual-energy CT applications have been developed, including renal stone composition (differentiating uric acid stones from other renal stones), adrenal nodule characterization, improved detection of hypervascular hepatic lesions, as well as detection of perfusion defects related to pulmonary embolism, among others.
The patient underwent dual-energy thoracic CT (Figures 4 and 5) using a dual-source CT scanner, with the tube potential of one tube set at 80 kV and the other at 140 kV.
Figure 4. Upper panels A-F: Coronal maximum intensity projection images from a dual-energy thoracic CT study again show destructive changes centered on the right sterno-clavicular joint, evidenced by periosteal reaction and osseous destruction and fragmentation (arrow) associated with widening of the right sterno-clavicular joint space (line in C) compared with the normal sterno-clavicular joint space on the left (arrowhead). m= manubrium. Lower panel:video of dual-energy thoracic CT.
Figure 5. Coronal dual-energy thoracic CT examination shows right sterno-clavicular joint space widening and fragmentation with clavicular head irregularity and erosion; the green-color coding indicates the presence of monosodium urate crystal deposition.
The use of differing tube potentials (energies) exploits the kV-dependent nature of CT attenuation (reflected as CT numbers expressed as Hounsfield units), and allows materials of differing atomic numbers to be distinguished. Post-processing allows materials of differing atomic number to be coded as different colors to facilitate visual recognition. Using this methodology, monosodium urate crystal deposition is detectable and was coded as a green color using the post-processing software.
Given this information, among the choices listed, which is the most likely diagnosis to explain the imaging findings? (Click on the correct answer to proceed to the seventh and final page)