Lung Scan PDF

7 The Pulmonary System INTRODUCTION: THE VENTILATION–PERFUSION inhalation of a radiolabeled gas or aerosol can allow ventilation...

7 The Pulmonary System INTRODUCTION: THE VENTILATION–PERFUSION inhalation of a radiolabeled gas or aerosol can allow ventilation imaging. These ventilation (V) and perfusion (Q) examinations LUNG SCAN are the two components that make up the VQ lung scan (Figs. 7.1 Particles slightly larger than red blood cells can be radiolabeled and 7.2). Although the VQ scan is most commonly performed to and injected into a peripheral vein. After passing through the diagnose suspected pulmonary embolism, it can also be used for heart and central pulmonary arteries, they finally lodge in the other purposes, including quantitation of pulmonary function, peripheral lung capillaries, creating a map of pulmonary blood often pre– or post–lung resection or transplant surgery, and flow that can be imaged with a gamma camera. Similarly, assessment of corrective surgery on pulmonary vasculature. A B Fig. 7.1 Normal ventilation–perfusion (VQ) scan. (A) Ventilation Tc-99m diethylenetriaminepentaacetic acid (DTPA) and (B) perfusion Tc-99m macroaggregated albumin (MAA) lung scan images show homogeneous distribution and the normal gradient of increasing activity in the bases relative to the apices. (Top row, left to right): POST, posterior; LPO, left posterior oblique; L LAT, left lateral; LAO, left anterior oblique. (Bottom row, left to right) ANT, anterior; RPO, right posterior oblique; R LAT, right lateral; RAO, right anterior oblique. 125 126 PART 2 Clinical Studies A B Fig. 7.2 Normal ventilation–perfusion (VQ) scan with xenon-133 ventilation. (A) Perfusion Tc-99m macro- aggregated albumin (MAA) images show normal radiotracer distribution. (B) Normal ventilation Xe-133 left posterior oblique (left) and right posterior oblique (right) include initial breath (upper row), equilibrium images (second row), and sequential washout images (lower rows) that show rapid normal clearance without reten- tion from air trapping. PULMONARY EMBOLISM assign points based on patient history, symptoms, and phys- ical findings. Diagnosis Patients are at greatest risk of a pulmonary embolus from The clinical diagnosis of pulmonary embolus (PE) can be diffi- immobilization, recent surgery, and hypercoagulable states. The cult because of the wide range of presenting signs and symp- chance of PE is also significantly increased with a history of toms as well as the limitations of available diagnostic tests. prior PE and in the presence of deep vein thrombosis (DVT). Although correct identification and prompt treatment can sig- Among patients with symptomatic DVT, 30% to 50% result in a nificantly improve mortality rates (from approximately 30% to PE, and 70% to 90% of patients with PE have had a DVT. Preg- 10%) and help prevent recurrence, treatment regimens also nancy and hormone use are more moderate risk factors. expose patients to potential harm. Chest radiographs are frequently ordered and often identify It is important to understand the tests used to diagnose other causes for the patient’s symptoms. However, findings from PE. The accuracy of any test depends not only on its sensitiv- a PE are highly variable (Box 7.1). Serum D-dimer is sensitive ity and specificity but also on pretest probabilities according but nonspecific. Doppler ultrasound is an excellent way to non- to principles of Bayes’ theorem (i.e., a positive test result is invasively diagnose venous thrombosis in the lower extremities, more likely a true positive if pretest suspicion is high, but the making it a frequent component in the workup of possible PE. likelihood dramatically drops if suspicion is low). Therefore Further testing beyond these examinations depends on the level referring physicians should perform patient risk stratifica- of clinical suspicion. tion to assess the overall likelihood of PE before ordering an The historical imaging gold standard, pulmonary angiogra- extensive workup using validated criteria, such as the Modi- phy, is very rarely performed today. It is not only invasive but fied Wells Scoring System (Table 7.1), which objectively also requires significant facility resources and may not visualize  Chapter 7 The Pulmonary System 127 TABLE 7.1 Wells Scoring for Pretest Determination of PE Probabilitya Criteria Modified Wells (Points) Clinical signs of DVT 3.0 Recent surgery or immobilization 1.5 Heart rate >100 beats/min 1.5 Previous vascular thromboemboli 1.5 Hemoptysis 1.0 Malignancy 1.0 PE most likely diagnosis 3.0 Pregnancy 0 (not included in Wells score) CLINICAL RISK ASSESSMENT Risk Total Score (Points) PE in Wells Patients High >6 41% Moderate 2–6 16% Low 75% of the segment involved lung periphery. However, in asthma and chronic obstructive Moderate: 25% to 75% of segment pulmonary disease (COPD) or when patients cannot cooper- Small: