Chapter 5

Thromboembolic Disease

Daniel F. Worsley and Philip S. Wells Introduction

Pulmonary embolism (PE) is a relatively common and potentially fatal disorder for which treatment is highly effective and improves patient survival. The diagnosis of acute PE requires an interdisciplinary team approach and may be difficult because of nonspecific clinical, laboratory and radiographic findings. The incidence of venous thromboembolism is approximately 1 in 1,000 per year. Approximately 10% of patients with PE die within one hour of the event. For those patients who survive beyond the first hour following PE, treatment with heparin or thrombolytic agents are both effective therapies. The mortality in patients with PE who are untreated has been reported to be as high as 30%. In contrast, the correct diagnosis and appropriate therapy significantly lowers mortality to between 2.5% and 8%. Although anticoagulant therapy is effective in treating PE and reducing mortality, it is not without some risk. The prevalence of major hemorrhagic complications has been reported to be as high as 10-15% among patients receiving anticoagulant therapy. Therefore, the accurate and prompt diagnosis of PE is not only essential to prevent excessive mortality but also to avoid complications related to unnecessary anticoagulant therapy.

Technical Considerations in Lung Scanning

Perfusion Lung Scanning

The agent of choice for perfusion imaging is 99mTc-labeled macroaggregated albumin (99mTc-MAA). Technetium-99m MAA particles range in size from 10-150 ^m with over 90% of injected particles measuring between 10-90 ^m. The injection of labeled particles should be performed with the patient in the supine position to limit the effect of gravity on regional pulmonary arterial blood flow. Following the intravenous administration of 99mTc-MAA, particles are mixed within the heart and then lodge within precapillary arterioles in the lungs. The usual administered activity is between 74-148 MBq (2-4 mCi) bound to 0.5-2.0 mg of human serum albumin. The distribution of particles within the lungs is proportional to regional pulmonary blood flow at the time of injection. Approximately 200,000-500,000 particles are injected during a routine clinical perfusion lung scan. The blockage of pulmonary precapillary arterioles by 99mTc-MAA is transient, and the biological half-life within the lung ranges between 2-6 hours. In pediatric patients, patients with right to left shunts, pulmonary hypertension or those who have undergone pneumonectomy or single lung transplantation, the number of particles injected should be reduced. A minimum of 60,000 particles is required to obtain an even distribution of activity

Nuclear Medicine, edited by William D. Leslie and I. David Greenberg. ©2003 Landes Bioscience.

Lao Perfusion Image
Figure 1. Normal 99mTc-MAA perfusion images. (RPO-right posterior oblique; LPO-left posterior oblique; RAO-right anterior oblique; LAO-left anterior oblique).

within the pulmonary arterial circulation and avoid potential false positive interpretations.

When performing perfusion scintigraphy, at least six views of the lungs should be obtained. These include anterior, posterior, right and left anterior oblique and posterior oblique views (Fig. 1). Additionally, right and left lateral views may be helpful in selected cases. Animal studies have demonstrated that perfusion imaging will detect greater than 95% of emboli which completely occlude pulmonary arterial vessels greater than 2 mm in diameter. In spite of this sensitivity, the perfusion scan may underestimate some perfusion abnormalities. For example, a perfusion defect limited to the medial basal segment of the right lower lobe is completely surrounded by normal lung; consequently a perfusion defect in this segment will not be detected on planar perfusion imaging.

Ventilation Lung Scanning

Perfusion scintigraphy is sensitive but not specific for diagnosing pulmonary diseases. Virtually all parenchymal lung diseases (including tumors, infections, chronic obstructive pulmonary disease [COPD] or asthma) can cause decreased pulmonary arterial blood flow within the affected lung zone. Ventilation imaging was combined with perfusion scintigraphy to improve the diagnostic specificity for PE based upon the principle that PE characteristically cause abnormal perfusion with preserved ventilation (mismatched defects) (Fig. 2). In contrast, parenchymal lung disease most often causes both ventilation and perfusion abnormalities in the same lung region (matched defects) (Fig. 3). Conditions in which the ventilation abnormality appears larger than the perfusion abnormality (reverse mismatch) include airway obstruction, mucous plug, airspace disease, atelectasis or pneumonia (Fig. 4). Patients

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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