Radioaerosol ventilation imaging in ventilator-dependent patients. Technical considerations

Area of Science:

• Nuclear Medicine
• Pulmonary Imaging
• Diagnostic Radiology

Background:

• Differentiating pulmonary embolism (PE) from regional ventilatory abnormalities with reduced perfusion is challenging in ventilator-dependent patients.
• Conventional Tc-99m MAA perfusion studies alone may lead to misinterpretation of ventilation defects.

Purpose of the Study:

• To evaluate the utility of Tc-99m aerosol ventilation imaging as an adjunct to Tc-99m MAA perfusion imaging for diagnosing PE in ventilator-dependent patients.
• To assess the impact of combined perfusion-ventilation imaging on the accuracy of PE diagnosis.

Main Methods:

• Utilized a Tc-99m aerosol for regional ventilation characterization and Tc-99m MAA for perfusion studies in ventilator-dependent patients.
• Employed a simple in-house apparatus for aerosol delivery and photographic techniques, with optional computer subtraction for image analysis.
• Performed sequential perfusion-ventilation imaging, with angiography used for confirmation in high-probability cases.

Main Results:

• Perfusion imaging alone suggested PE in 8/16 patients, while combined perfusion-ventilation imaging indicated PE in only 1/8 patients.
• Two patients with abnormal perfusion but initially low suspicion for PE were found to have normal ventilation and were subsequently diagnosed with PE via angiography.
• Aerosol ventilation imaging significantly reduced false positives compared to perfusion imaging alone.

Conclusions:

• Tc-99m aerosol ventilation imaging is a valuable addition to Tc-99m MAA perfusion imaging for diagnosing PE in patients on ventilators.
• The technique requires an efficient aerosol delivery system, especially when aerosol administration follows perfusion imaging.
• Bronchial deposition due to mucus is a notable disadvantage of aerosol imaging in intubated patients compared to gas ventilation studies.