Transcranial Doppler ultrasound (TCD) has been used to detect microembolic material. The detection of microembolic signals (MES) by TCD was achieved by placing two probes on the bilateral temporal windows using a closely fitting headband, on the patient's head. The probes detected blood flow velocity of the bilateral middle cerebral arteries (MCA) or posterior cerebral arteries (PCA) continuously through the temporal windows. Blood flow velocity of the MCA or PCA was measured for 30 min. Although the optimum criteria for MES have not been established, our tentative criteria were the following: 1) a single direction; 2) decibel threshold from the general background noise and spontaneous speckle-like intensity fluctuations of the physiological Doppler flow signal of 6 dB; 3) a short duration (under 300 ms) ; 4) an accompanying chirp or snap sound; and 5) a time difference between signals of dual gate. Because the systems available for the auto-detection of MES may pick up artifacts, close observation of the signals during examination is essential. Detection of MES by TCD is a useful method to evaluate the pathophysiology of stroke and efficacy of antithrombotic agents.

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Transcranial Doppler ultrasound (TCD) can detect circulating microembolic signals (MES) from a variety of embolic sources. We mentioned about MES detection by TCD, MES detection in patients with ischemic stroke, and tail signs (TS) . A prevalence of MES of 5.7% was found in 151 consecutive patients with ischemic stroke. TCD was useful to confirm the stroke mechanism and to predict individual prognosis in some patients. The prevalence of MES was low and TCD did not always detect MES in patients with ischemic stroke. Our attention also focused on TS, which appeared as a small reversal signal after MES on the high time resolution spectral display. MES and artifacts were examined in experimental animals to determine whether TS were specific changes associated with MES, and TS were investigated in both animal and clinical studies. TS associated with MES were observed in both animal and clinical studies, and were never associated with artifacts. These results indicate that TS are highly specific for MES and can be used as another criterion for MES identification.

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Detection of microembolic signals (MES) by transcranial Doppler is the only way to evaluate the effect of antithrombotic agents on circulating thrombi in vivo. The effect of antithrombotic agents on MES from various embolic sources was determined. First, the effect of antiplatelet agents on MES from carotid lesions was investigated. MES counts in some patients with carotid lesions decreased after antiplatelet administration. MES detection is enormously useful for evaluating antiplatelet therapy in patients with carotid lesions, since MES are detected frequently and reflect real thrombi in these patients. On the other hand, MES from artificial organs, such as mechanical valves and ventricle assisted systems, usually reflect air bubbles. Detection of MES from artificial organs does not represent the effect of anticoagulation therapy. Recently, several papers have reported a decrease in air bubbles due to oxygen inhalation. This method may make it possible to distinguish thrombi and air bubbles. MES from atrial fibrillation, the aorta, or persistent follamen ovale, are rare. MES detection was not useful for the evaluation of therapy in such patients.

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To investigate whether transcranial Doppler (TCD) monitoring can identify patients at risk of hyperperfusion following carotid endarterectomy (CEA), the hemodynamics of CEA were investigated in 10 patients by electromagnetic flowmetry, digital subtraction angiography (DSA) and stable Xe-computed tomography (CT) . The continuous measurement of flow velocity (FV) and pulsatile index (PI) in the middle cerebral artery (MCA) was performed by TCD every day, starting pre-CEA and continuing until 7 days after CEA. Cerebral circulation time (CCT) was calculated during intra-arterial DSA with morphological evaluation. Cerebral blood flow (CBF) on the MCA territory was measured by stable Xe-CT, and cerebrovascular reserve capacity (CVRC) was also calculated by acetazolamide activation. Ten patients were divided into two groups depending on their risk factors for hyperperfusion after CEA (i.e. ratio of carotid stenosis, collateral circulation, stump pressure and carotid blood flow) . Reduced CVRC and delayed CCT investigated preoperatively suggested increased cerebrovascular resistance (CVR) in the high-risk group. In the postoperative study, increased FV and PI were both observed in the high-risk group 7 days after surgery, with significant differences in the FV and PI between the high-risk group and low-risk group observed on the second day after CEA. It is important to know the conditions of CVR and CBF in order to understand hyperperfusion after CEA. The continuous measurement of PI and FV along with preoperative CCT may be a useful method to prevent severe complications caused by hyperperfusion.

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The value of B-mode and color Doppler ultrasonography for follow-up of cases of carotid artery disease was investigated between January 1995 and January 1999. The subjects comprised 23 patients, including 22 males and one female. The mean age of the patients was 68.0 ± 6.5 years, the times of measurement ranged from 2 to 6, and the mean follow-up period was 18.0 ± 13.2 months. Two different groups were analyzed: a surgical group (n=15), including nine patients with carotid endarterectomy, three with percutaneous transluminal angioplasty, and three with extracranial-intracranial bypass, and a medical group (n=8), including six patients with cerebral infarction, one with moyamoya disease, and one with carotid artery aneurysm. Echogenicity of the plaque, stenotic rate of the carotid artery, peak flow velocity, end diastolic flow velocity, mean flow velocity, and pulsatility index (PI) in the common carotid artery (CCA), internal carotid artery (ICA) and external carotid artery were measured by B-mode and color Doppler ultrasonography. In the surgical group, both proximal PI and Δ PI (PI (CCA) -PI (ICA) ) in the CCA were lower postoperatively, but the distal PI in the ICA was higher postoperatively. One patient with severe carotid artery stenosis, which developed due to occlusion in the medical group, demonstrated an increased postoperative PI value in the CCA and ΔPI, and demonstrated a decreased postoperative PI in the ICA. ΔPI values showed more change than PI values. A follow-up study using combined B-mode and color Doppler ultrasonography is warranted to evaluate the changes in carotid plaque. In this study, ΔPI was found to be more sensitive than the other parameters.

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Twenty-one patients (mean age ± SD, 67 ± 10 years) with acute brainstem infarction who were admitted to the emergency ward were studied. The flow velocity of the vertebral arteries was measured by cervical ultrasonography 24 h after onset of infarction. Fourteen patients underwent intra-arterial digital subtraction angiography within 20 h of cervical ultrasonography. Two of four patients with basilar artery occlusion showed bilateral absence of end-diastolic vertebral flow. The other two patients with basilar artery occlusion showed absence of end-diastolic flow on one side, and a mean velocity < 18 cm/s on the other. Two of three patients with basilar artery stenosis showed a mean velocity < 18 cm/s on either side of the vertebral artery. Measurement of vertebral artery flow velocity appeared to be valuable in the emergency assessment of acute brainstem infarction.

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There have been few reports regarding the normal velocities in transcranial color flow imaging. In addition, little is known about the reproducibility of middle cerebral artery (MCA) velocity values with different types of US machines. The MCA velocities were measured in 80 healthy Japanese subjects (males 44, females 36) using the Aloka SSD 5500 and Toshiba SSA 270A. The peak systolic velocities (PSV), end diastolic velocities (EDV), mean velocities (MV) and asymmetry index (AI) of the bilateral MCA were measured. The insonation rates of the MCA were approximately the same (about 90%) with the two machines, but the PSV and EDV of the males, and the EDV of the females were not the same. The velocities with the Aloka SSD 5500 were typically more rapid than those with the Toshiba SSA 270A (P <0.05-0.001) . Rapid progress is being made in the improvement of US machines. Even if the MCA pulse wave is very weak, up-to-date US machines with a digital system can detect it very accurately. When transcranial color flow imaging (TC-CFI) is performed, the normal range of the velocities in the intracranial blood vessels should be established with individual US machines. As there were no differences in the MV and AT of the MCA with the two types of machine used in this study, the MV and AT are considered to be useful parameters in TC-CFI.

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