Chapter 30: Physiologic and Metabolic Assessment of Pulmonary Disorders Using Conventional Imaging Techniques and Positron Emission Tomography

INTRODUCTION

Since the mid-1950s, use of radiopharmaceuticals has made it possible to assess a variety of pulmonary disorders. In 1955, ¹³³Xe was introduced for the study of regional pulmonary ventilation.¹ Shortly thereafter, it became possible to evaluate regional pulmonary blood flow using inhaled carbon dioxide containing radioactive ¹⁵O² or intravenous injection of ¹³³Xe dissolved in saline solution.³ In 1964, intravenous injection of ¹³¹I-macroaggregated albumin made it feasible to obtain perfusion scans of the lungs.⁴ Although these techniques rapidly gained wide acceptance as tests of regional abnormalities in ventilation and pulmonary blood flow, the main practical application was as a ventilation–perfusion lung scan (V̇/Q̇ scan) in the diagnostic evaluation of patients with suspected pulmonary embolism. Over the years, the role of nuclear medicine in respiratory medicine expanded to include such indications as preoperative assessment of lung function, mucociliary clearance, alveolar-capillary membrane permeability, inflammatory lung disease, and lung cancer. The last two indications were facilitated by the more widespread availability of positron emission tomography (PET) and integrated PET/CT (computed tomography) that has provided powerful tools to aid in the diagnosis, staging, and management of patients.

RADIOPHARMACEUTICALS AND TECHNIQUES IN VENTILATION–PERFUSION LUNG SCANNING

Perfusion lung scans are routinely utilized to examine patients with suspected pulmonary embolism. The overall principle is simple: radiolabeled microembolic particles are injected intravenously and transported to the pulmonary vasculature where they lodge uniformly and reversibly in the smallest blood vessels proportional to the perfusion. If a part of the vasculature is blocked by an embolus, a corresponding, often wedge-shaped, downstream area of the lung devoid of perfusion and injected radiolabeled microemboli appears photopenic on subsequent gamma camera examination.

Perfusion imaging is sensitive but, unfortunately, not specific for diagnosing pulmonary embolism. Virtually all lung diseases (including tumors, infections, asthma, and chronic obstructive pulmonary disease) may cause decreased pulmonary arterial blood flow in the affected lung zones. Therefore, combined use of perfusion and ventilation studies improves the diagnostic specificity of lung scanning for pulmonary embolism (Fig. 30-1). Pulmonary embolism almost always causes abnormal perfusion, while ventilation is preserved (mismatched defects) (Fig. 30-2). In contrast, in parenchymal pulmonary disorders, decreased ventilation and perfusion are noted in the same lung region, as perfusion is reduced secondary to ventilation disturbances due to regional hypoxic pulmonary vasoconstriction (matched defects).⁵ Sometimes ventilation abnormalities appear even larger than the perfusion abnormality (reverse mismatch), for example, due to airway obstruction, mucus plug, atelectasis, or pneumonia.⁶