Can transcranial Doppler discriminate between solid and gaseous microemboli? Assessment of a dual-frequency transducer system.

BACKGROUND AND
PURPOSE: Transcranial Doppler ultrasound can reliably detect both gaseous and solid cerebral emboli. However, conventional equipment is unable to discriminate between gaseous and solid emboli. This is a major limitation in situations in which the 2 coexist, because they may have very different clinical relevance. Recently, a novel Embo-Dop system, using insonation at 2 ultrasound transducer frequencies, has been developed. An initial study with a small sample size suggested it provided excellent discrimination. We performed a validation study in subjects with embolic signals of known nature.
METHODS: Gaseous embolic signals were obtained in 7 patients with known patient foramen ovale by intravenous injection of agitated saline injections. Solid embolic signals were obtained in patients with symptomatic carotid stenosis (N=23). Discrimination of the 2 using the Embo-Dop system dual-frequency system was assessed. It was compared with discrimination using embolic signal maximum intensity with an intensity threshold.
RESULTS: One hundred forty-five solid embolic signals were recorded from carotid stenosis patients. Seventy-three were classified as solid and 72 as gaseous by the Embo-Dop system. Six hundred forty-eight gaseous embolic signals were recorded from 7 patients with patent foramen ovale. Six hundred twenty-five were classified as gaseous and 23 as solid. This gave a sensitivity of 50.3% and specificity of 96.5% for detecting solid embolic signals. Discrimination was better than using a simple intensity threshold.
CONCLUSIONS: The Embo-Dop dual-frequency system allows better discrimination than a simple intensity threshold but it is not accurate enough for use in clinical or research studies. Further work is needed to develop reliable clinical systems for discrimination of emboli.