PULMONARY NUCLEAR MEDICINE

LUNG SCAN - PERFUSION

INDICATIONS:

1. Diagnosis of pulmonary embolism.
2. Evaluation of regional pulmonary perfusion.
3. Evaluation of pulmonary perfusion after treatment with anticoagulants.
4. Pre-op evaluation of lung function for thoracotomy. In this case, the study should be ordered as quantitative.
5. Assessment of congenital pulmonary abnormalities and cardiac shunts.
6. Evaluation of regional function in acquired pulmonary disease (COPD, asthma, carcinoma, etc.).
7. In the work-up for primary pulmonary hypertension

PATIENT PREPARATION:

The patient must have a CXR within 24 hours prior to the study. The Section of Nuclear Medicine cannot order the CXR. This is the responsibility of the service and is essential for meaningful scan interpretation.

RADIOPHARMACEUTICAL:

Tc-99m macroaggregated albumin (MAA) is injected I.V. (3 mCi). This dose gives 0.8 rad to lung and 0.06 rad to total body.

SCANNING TIME:

Eight views are taken and the usual imaging time is 25 minutes.

OTHER CONSIDERATIONS:

Relative contraindications to consider:

1. Right to left intracardiac shunt.
2. Severe pulmonary arterial hypertension.
3. Allergy to human serum albumin.
4. Pregnancy. The attending physician should consult a Nuclear Medicine physician when ordering a lung scan on a pregnant patient.

RATIONALE:

The particles (Tc-99m MAA) are trapped in a small percentage (< 0.1%) of the peripheral pulmonary arterioles and capillaries; their distribution is proportional to arterial perfusion at the time of injection. Any infiltrate, atelectasis, or ventilatory change will cause decreased perfusion. Thus, an abnormal scan (defect) must be correlated with clinical, ventilatory, and radiographic findings for proper assessment. A negative (normal) perfusion study, however, virtually excludes PE. A low probability scan reading still has an associated 14% incidence of PE. A high probability scan reading has a specificity of 97%.

LUNG SCAN - VENTILATION

INDICATIONS:

1. Improve the diagnose of suspected pulmonary embolism (P.E.).
2. Evaluation of obstructive lung disease.
3. Evaluation of regional ventilatory function.
4. Pre-operative evaluation. This should be ordered as quantitative study.
5. Follow-up post therapy for P.E.

PATIENT PREPARATION:

None. However, patient cooperation is essential. The patient must be able to take in a breath and hold it for at least 20 seconds. The patient will then be asked to rebreathe into a closed system for approximately 5 minutes. These requirements generally exclude patients on ventilators from receiving this study.

RADIOPHARMACEUTICAL:

Xe-133 gas by inhalation (3 mCi/L) is used for a total dose of 20-30 mCi. The critical organ is the trachea (for a dose of 30 mCi, 6.4 rad).

SCANNING TIME:

With adequate patient cooperation, the study takes approximately 15 minutes.

RATIONALE:

A complete study includes recording 3 phases: single-breath view reflects regional ventilatory distribution (if there is maximal inspiratory effort); the washout phase is more sensitive in detecting trapping secondary to airway obstruction (COPD). Regional quantitative estimates of function can be calculated for pre-op planning when requested.

NOTE: This study may be ordered on an emergency basis with the perfusion lung scan.