[Assessment of the rate of successful transcranial Doppler recording through the temporal windows in Japanese with special reference to aging and sex]

Detection of moderate to severe middle cerebral artery atherosclerotic stenosis in stroke patients: Transcranial color-coded duplex sonography versus computed tomography angiography

Background

Intracranial atherosclerotic stenosis is a common cause of ischemic cerebrovascular events and is associated with a high risk of stroke recurrence. This study aimed to assess the diagnostic accuracy of transcranial color-coded duplex sonography for moderate-to-severe middle cerebral artery stenosis in stroke patients.

Methods

A retrospective analysis was carried out, including 31 patients aged ⩾18 years hospitalized for ischemic cerebrovascular event in whom middle cerebral artery stenosis ⩾30% was identified on computed tomography angiography. Transcranial color-coded duplex sonography findings were compared to the degree of stenosis blindly identified on the computed tomography angiography used as the reference method.

Results

Overall, 27 patients had M1 stenosis and the other 4 had M2 stenosis. To detect M2 stenosis ⩾ 50% and ⩾ 70%, stenotic to pre-stenotic ratio ⩾ 2 and ⩾ 3 had a sensitivity of 100%, respectively. To detect M1 stenosis ⩾ 70%, peak systolic velocity ⩾ 300 cm/s had a sensitivity of 53.8% and specificity of 85.7% with area under the receiver-operating characteristic curve of 0.753 (95% confidence interval: 0.568-0.938; p = 0.026), and stenotic to pre-stenotic ratio ⩾ 3 had a sensitivity of 84.6% and a specificity of 78.6% (area under the curve = 0.854; 95% confidence interval: 0.707-1; p = 0.002). Middle cerebral artery/anterior cerebral artery velocity ratio < 0.7 had a sensitivity of 57.1% and specificity of 90% to detect dampened pre-stenotic flow in middle cerebral artery secondary to downstream M1 stenosis ⩾ 70% (area under the curve = 0.800; 95% confidence interval: 0.584-1; p = 0.040).

Conclusion

This study showed that stenotic to pre-stenotic ratio ⩾ 3 was more sensitive than peak systolic velocity ⩾ 300 cm/s to screen M1 stenosis ⩾ 70%. Middle cerebral artery/anterior cerebral artery ratio < 0.7 was a good indirect sign to detect dampened pre-stenotic flow due to M1 stenosis ⩾ 70%.

Simulation study on the effects of cancellous bone structure in the skull on ultrasonic wave propagation

The transcranial Doppler method (TCD) enables the measurement of cerebral blood flow velocity and detection of emboli by applying an ultrasound probe to the temporal bone window, or the orbital or greater occipital foramina. TCD is widely used for evaluation of cerebral vasospasm after subarachnoid hemorrhage, early detection of patients with arterial stenosis, and the assessment of brain death. However, measurements often become difficult in older women. Among various factors contributing to this problem, we focused on the effect of the diploe in the skull bone on the penetration of ultrasound into the brain. In particular, the effect of the cancellous bone structure in the diploe was investigated. Using a 2D digital bone model, wave propagation through the skull bone was investigated using the finite-difference time-domain (FDTD) method. We fabricated digital bone models with similar structure but different BV/TV (bone volume/total volume) values in the diploe. At a BV/TV of approximately 50–60% (similar to that of older women), the minimum ultrasound amplitude was observed as a result of scattering and multiple reflections in the cancellous diploe. These results suggest that structural changes such as osteoporosis may be one factor hampering TCD measurements.

Association of Newly Found Asymptomatic Intracranial Artery Stenosis and Ideal Cardiovascular Health Metrics in Chinese Community Population

In the general population, there is a strong inverse relationship between the number of ideal cardiovascular health (CVH) metrics and the total incidence of cardiovascular diseases and stroke. However, the prevalence of ideal CVH is extremely low and there are few studies on its association with newly found asymptomatic intracranial arterial stenosis (AICAS). Therefore, we performed this prospective study to assess the relationship between the newly found AICAS and ideal CVH metrics in the Chinese community population. Seven ideal CVH metrics of 3,475 participants in the Asymptomatic Polyvascular Abnormalities Community study (APAC) conducted in China (1,962 men and 1,513 women between the ages of 45 and 75 years) were collected. Based on the occurrence of newly found AICAS, all participants were divided into the AICAS group and non-ICAS group. Prevalence of ideal CVH metrics was compared between the two groups. Logistic regression was used to estimate the association of newly found AICAS with ideal CVH metrics. The result was the number of ideal CVH metrics was strongly associated with age, gender, education levels and family income (each P < 0.0001). Among the seven CVH metrics total cholesterol (TC) was the only one showing significant difference between the newly found AICAS group and non-ICAS group in our 2 years observation. Participants with less ideal CVH metrics (≤3) were associated with significantly higher prevalence of AICAS than those with more (>3) ideal CVH metrics (OR, 1.27; P = 0.045). Furthermore, less (≤3) ideal CVH metrics was markedly associated with higher incidence of AICAS for all participants, younger participants (<60 years) (OR, 1.34; P = 0.046) and men participants (OR, 1.53; P = 0.032) after adjustment for gender, age, education level, family income and stroke history. Thus we conclude that participants with newly found AICAS have high prevalence of total cholesterol status, and Individuals with low ideal CVH metrics (≤3) are associated with significantly higher prevalence of asymptomatic ICAS, especially in high-risk population of young and men participants. Therefore, primordial prevention of stroke should also focus on those high-risk populations.

High resolution transcranial acoustoelectric imaging of current densities from a directional deep brain stimulator

OBJECTIVE

New innovations in deep brain stimulation (DBS) enable directional current steering-allowing more precise electrical stimulation of the targeted brain structures for Parkinson's disease, essential tremor and other neurological disorders. While intra-operative navigation through MRI or CT approaches millimeter accuracy for placing the DBS leads, no existing modality provides feedback of the currents as they spread from the contacts through the brain tissue. In this study, we investigate transcranial acoustoelectric imaging (tAEI) as a new modality to non-invasively image and characterize current produced from a directional DBS lead. tAEI uses ultrasound (US) to modulate tissue resistivity to generate detectable voltage signals proportional to the local currents.

APPROACH

An 8-channel directional DBS lead (Infinity 6172ANS, Abbott Inc) was inserted inside three adult human skulls submerged in 0.9% NaCl. A 2.5 MHz linear array delivered US pulses through the transtemporal window and focused near the contacts on the lead, while a custom amplifier and acquisition system recorded the acoustoelectric (AE) interaction used to generate images.

MAIN RESULTS

tAEI detected monopolar current with stimulation pulses as short as 100 µs with an SNR ranging from 10-27 dB when using safe US pressure (mechanical indices  <0.78) and injected current of ~2 mA peak amplitude. Adjacent contacts were discernable along the length and within each ring of the lead with a mean radial separation between contacts of 2.10 and 1.34 mm, respectively.

SIGNIFICANCE

These results demonstrate the feasibility of tAEI for high resolution mapping of directional DBS currents using clinically-relevant stimulation parameters. This new modality may improve the accuracy for placing the DBS leads, guide calibration and programming, and monitor long-term performance of DBS for treatment of Parkinson's disease.

Selective Mapping of Deep Brain Stimulation Lead Currents Using Acoustoelectric Imaging

We describe a new application of acoustoelectric imaging for non-invasive mapping of the location, magnitude and polarity of current generated by a clinical deep brain stimulation (DBS) device. Ultrasound at 1MHz was focused near the DBS device as short current pulses were injected across different DBS leads. A recording electrode detected the high-frequency acoustoelectric interaction signal. Linear scans of the US beam produced time-varying images of the magnitude and polarity of the induced current, enabling precise localization of the DBS leads within 0.70mm, a detection threshold of 1.75mA at 1 MPa and a sensitivity of 0.52 ± 0.07 μV/(mA*MPa). Monopole and dipole configurations in saline were repeated through a human skullcap. Despite 13.8-dB ultrasound attenuation through bone, acoustoelectric imaging was still >10dB above background with a sensitivity of 0.56 ± 0.10 μV/(mA*MPa). This proof-of-concept study indicates that selective mapping of lead currents through a DBS device may be possible using non-invasive acoustoelectric imaging.

Diagnostic TCD for intracranial stenosis in acute stroke patients: experience from a tertiary care stroke center in Karachi, Pakistan

BACKGROUND

Stroke is a common cause of morbidity and mortality around the world. Intracranial large artery atherosclerosis (ICAD) is a frequent etiology of stroke in the South Asian population. There is a need for widely available screening tools to identify patients that are at high risk of stroke due to ICAD for aggressive risk management. This study describes the experience of using the transcranial Doppler (TCD) as a screening tool for this purpose at a tertiary care hospital in a developing country.

METHODS

86 Patients admitted with stroke due to ICAD underwent TCD for six arteries (Right and left middle cerebral arteries, right and left anterior cerebral arteries, right and left posterior cerebral arteries) in addition to the magnetic resonance angiography (MRA) that is done routinely at the stroke center. Arteries were labeled with either <50 or >50% stenosis by TCD using two separate criteria. These findings were compared with those from the MRA which was used as the gold standard. The proportion of patients that had complete exams (all six arteries insonated by TCD) was reported. The success rate of each TCD criteria in detecting arteries with >50% stenosis was also calculated.

RESULTS

There was an attempt to visualize 516 arteries (86 patients with 6 arteries each) of which 375 (72.7%) were successfully insonated. 38 of the 86 (55.8%) patients had complete examinations. MRA reported 43 (8.3%) arteries as stenosed >50%. The TCD did not categorize any artery as stenosed using either criterion and hence failed to classify any stenosed artery correctly. The positive predictive and sensitivity was 0 for this study and the negative predictive value was 93.3%.

CONCLUSIONS

This study indicates the poor sensitivity of TCD to be a reliable screening tool for the presence of ICAD in the South Asian population in a real life clinical setting.

3-D transcranial ultrasound imaging with bilateral phase aberration correction of multiple isoplanatic patches: a pilot human study with microbubble contrast enhancement

With stroke currently the second-leading cause of death globally, and 87% of all strokes classified as ischemic, the development of a fast, accessible, cost-effective approach for imaging occlusive stroke could have a significant impact on health care outcomes and costs. Although clinical examination and standard computed tomography alone do not provide adequate information for understanding the complex temporal events that occur during an ischemic stroke, ultrasound imaging is well suited to the task of examining blood flow dynamics in real time and may allow for localization of a clot. A prototype bilateral 3-D ultrasound imaging system using two matrix array probes on either side of the head allows for correction of skull-induced aberration throughout two entire phased array imaging volumes. We investigated the feasibility of applying this custom correction technique in five healthy volunteers with Definity microbubble contrast enhancement. Subjects were scanned simultaneously via both temporal acoustic windows in 3-D color flow mode. The number of color flow voxels above a common threshold increased as a result of aberration correction in five of five subjects, with a mean increase of 33.9%. The percentage of large arteries visualized by 3-D color Doppler imaging increased from 46% without aberration correction to 60% with aberration correction.

Refraction correction in 3D transcranial ultrasound imaging

We present the first correction of refraction in three-dimensional (3D) ultrasound imaging using an iterative approach that traces propagation paths through a two-layer planar tissue model, applying Snell's law in 3D. This approach is applied to real-time 3D transcranial ultrasound imaging by precomputing delays offline for several skull thicknesses, allowing the user to switch between three sets of delays for phased array imaging at the push of a button. Simulations indicate that refraction correction may be expected to increase sensitivity, reduce beam steering errors, and partially restore lost spatial resolution, with the greatest improvements occurring at the largest steering angles. Distorted images of cylindrical lesions were created by imaging through an acrylic plate in a tissue-mimicking phantom. As a result of correcting for refraction, lesions were restored to 93.6% of their original diameter in the lateral direction and 98.1% of their original shape along the long axis of the cylinders. In imaging two healthy volunteers, the mean brightness increased by 8.3% and showed no spatial dependency.

Simultaneous bilateral real-time 3-d transcranial ultrasound imaging at 1 MHz through poor acoustic windows

Ultrasound imaging has been proposed as a rapid, portable alternative imaging modality to examine stroke patients in pre-hospital or emergency room settings. However, in performing transcranial ultrasound examinations, 8%-29% of patients in a general population may present with window failure, in which case it is not possible to acquire clinically useful sonographic information through the temporal bone acoustic window. In this work, we describe the technical considerations, design and fabrication of low-frequency (1.2 MHz), large aperture (25.3 mm) sparse matrix array transducers for 3-D imaging in the event of window failure. These transducers are integrated into a system for real-time 3-D bilateral transcranial imaging-the ultrasound brain helmet-and color flow imaging capabilities at 1.2 MHz are directly compared with arrays operating at 1.8 MHz in a flow phantom with attenuation comparable to the in vivo case. Contrast-enhanced imaging allowed visualization of arteries of the Circle of Willis in 5 of 5 subjects and 8 of 10 sides of the head despite probe placement outside of the acoustic window. Results suggest that this type of transducer may allow acquisition of useful images either in individuals with poor windows or outside of the temporal acoustic window in the field.

The ultrasound brain helmet: new transducers and volume registration for in vivo simultaneous multi-transducer 3-D transcranial imaging

Because stroke remains an important and time-sensitive health concern in developed nations, we present a system capable of fusing 3-D transcranial ultrasound volumes acquired from two sides of the head. This system uses custom sparse array transducers built on flexible multilayer circuits that can be positioned for simultaneous imaging through both temporal acoustic windows, allowing for potential registration of multiple real-time 3-D scans of cerebral vasculature. We examine hardware considerations for new matrix arrays-transducer design and interconnects-in this application. Specifically, it is proposed that SNR may be increased by reducing the length of probe cables. This claim is evaluated as part of the presented system through simulation, experimental data, and in vivo imaging. Ultimately, gains in SNR of 7 dB are realized by replacing a standard probe cable with a much shorter flex interconnect; higher gains may be possible using ribbon-based probe cables. In vivo images are presented, showing cerebral arteries with and without the use of microbubble contrast agent; they have been registered and fused using a simple algorithm which maximizes normalized cross-correlation.

Perioperative transorbital Doppler flow imaging offers an alternative to transcranial Doppler monitoring in those patients without a temporal bone acoustic window

Transcranial Doppler has been used to identify microembolic signals before, during and after carotid endarterectomy, but 10% to 15% of patients are reported not to have suitable temporal bone window. The aim of this study was to assess the feasibility of transorbital Doppler monitoring of patients with absent temporal bone acoustic window. Between 2005 and 2008, those patients with absent temporal bone acoustic window were assessed for a transorbital acoustic window. During the study period, 318 carotid endarterectomy were performed. In the 29 (9.1%) with absent temporal bone acoustic window, 25 (86%) had satisfactory transorbital acoustic windows, consequently only four (1.2%) of patients could not be monitored postoperatively. One patient required postoperative transorbital acoustic windows directed glycoprotein IIb/IIIa receptor antagonist infusion due to excessive carotid microembolisation to prevent stroke. This is the first description of the use of transorbital flow imaging to determine postoperative cerebral blood flow, microembolic load and to direct the use of intravenous antiplatelet agents.

Transcranial shear-mode ultrasound: assessment of imaging performance and excitation techniques

Transcranial ultrasound imaging is limited by poor acoustic windows and skull induced distortions to the beam. Shear waves in the skull have a better impedance match with longitudinal waves in water and thereby produce a more coherent focus inside the skull. This study presents work on an imaging technique that utilizes shear-wave propagation through the skull. The pulse-echo lateral distortion introduced by the skull was analyzed by imaging a point scatterer behind ex vivo human craniums at 1 MHz. Brightness images of the target obtained with either shear-mode or conventional longitudinal-mode transmission in the bone were assessed to quantify lateral resolution. As compared to longitudinal-mode transmission, it was found that the use of shear-mode resulted in improved localization along the propagation (depth) axis at the expense of degraded lateral resolution. The signal-to-noise ratio (SNR) limitations introduced by severe attenuation of shear-waves in the skull were overcome with frequency modulated (FM) coded excitations. This gain in SNR was exchanged with resolution and used for compensation of frequency-dependent attenuation in the skull, resulting in a greater than 20% improvement in lateral resolution for both modes of transcranial transmission. The results are an important step towards enhancing the quality of transcranial sonography.

Characterization of ultrasound propagation through ex-vivo human temporal bone

Adjuvant therapies that lower the thrombolytic dose or increase its efficacy would represent a significant breakthrough in the treatment of patients with ischemic stroke. The objective of this study was to perform intracranial measurements of the acoustic pressure field generated by 0.12, 1.03 and 2.00-MHz ultrasound transducers to identify optimal ultrasound parameters that would maximize penetration and minimize aberration of the beam. To achieve this goal, in vitro experiments were conducted on five human skull specimens. In a water-filled tank, two unfocused transducers (0.12 and 1.03 MHz) and one focused transducer (2.00 MHz) were consecutively placed near the right temporal bone of each skull. A hydrophone, mounted on a micropositioning system, was moved to an estimated location of the middle cerebral artery (MCA) origin, and measurements of the surrounding acoustic pressure field were performed. For each measurement, the distance from the position of maximum acoustic pressure to the estimated origin of the MCA inside the skulls was quantified. The -3 dB depth-of-field and beamwidth in the skull were also investigated as a function of the three frequencies. Results show that the transducer alignment relative to the skull is a significant determinant of the detailed behavior of the acoustic field inside the skull. For optimal penetration, insonation normal to the temporal bone was needed. The shape of the 0.12-MHz intracranial beam was more distorted than those at 1.03 and 2.00 MHz because of the large aperture and beamwidth. However, lower ultrasound pressure reduction was observed at 0.12 MHz (22.5%). At 1.03 and 2.00 MHz, two skulls had an insufficient temporal bone window and attenuated the beam severely (up to 96.6% pressure reduction). For all frequencies, constructive and destructive interference patterns were seen near the contralateral skull wall at various elevations. The 0.12-MHz ultrasound beam depth-of-field was affected the most when passing through the temporal bone and showed a decrease in size of more than 55% on average. The speed of sound in the temporal bone of each skull was estimated at 1.03 MHz and demonstrated a large range (1752.1 to 3285.3 m/s). Attenuation coefficients at 1.03 and 2.00 MHz were also derived for each of the five skull specimens. This work provides needed information on ultrasound beam shapes inside the human skull, which is a necessary first step for the development of an optimal transcranial ultrasound-enhanced thrombolysis device.

Combined carotid and transcranial color-coded sonography in acute ischemic stroke

The objective of this study is to clarify whether the combination of carotid duplex sonography (CD) and transcranial color-coded sonography (TCCS) can accurately detect occlusive lesions in extra and intracranial brain arteries in acute stroke patients, using angiography as the standard. Just before angiography, we performed CD and TCCS in 40 consecutive patients within 24 h after stroke onset. We assessed 320 vessels in total, bilateral internal carotid arteries, vertebral arteries, M1 segments of middle cerebral arteries (MCAs), and P2 segments of posterior cerebral arteries (PCAs). Out of all vessels, 250 (78.1%) could be evaluated by neurosonography because 32 MCAs and 38 PCAs were excluded due to inadequate acoustic windows for TCCS. Significant occlusive lesions (>50%) were observed in 21 out of 250 vessels by neurosonography. Angiography confirmed 20 occlusive lesions as revealed by neurosonography. In the remaining 229 neurosonographically normal vessels, angiography showed no significant lesions except M2 occlusions. The accuracy, sensitivity, and specificity of neurosonography for the detection of occlusive vessels were 99.6, 100 and 99.6%, respectively. Occlusive lesions were observed in 20 of all patients by neurosonography. Nineteen of them were confirmed by angiography. The combination of CD and TCCS can make an accurate diagnosis for significant occlusive lesions in brain arteries in acute stroke patients.

Evaluation of a 1 MHz transducer for transcranial Doppler ultrasound including embolic signal detection

A 1 MHz transducer for use with transcranial Doppler ultrasound may improve the intensity and therefore the detection of embolic signals (ES) and may also reduce the number of absent acoustic windows. A series of studies was performed to investigate its potential benefits. Firstly, ES were detected using a 1 MHz and a 2 MHz transducer both in vitro and in vivo. Secondly, the time taken to identify 100 middle cerebral arteries (MCA) was recorded for both transducers and the best Doppler signal obtained was reviewed off-line and graded for quality. ES were more intense when detected with the 1 MHz compared with the 2 MHz transducer, both in vitro (p <.0001) and in vivo (p <.0001). Of the 100 MCAs studied, 81 had acoustic windows identified with both transducers. The number of acoustic windows detected with one transducer but not the other was the same for both transducers (n = 3). The time taken to identify the MCA was longer with the 1 MHz transducer (p <.0001) and the quality of the signal achieved was poorer (p <.0001). In conclusion, the 1 MHz transducer improved embolic signal intensity but the overall quality of the flow spectrum obtained was poorer with the 1 MHz than with the 2 MHz transducer. A lower frequency transducer of 1 MHz or possibly 1.5 MHz with transcranial Doppler ultrasound may improve the application of embolic signal detection but may not improve the signal for routine measurement of flow velocities.

Diagnostic yield of a 1-MHz transducer in evaluation of the basal cerebral arteries

The authors examined the efficacy of a 1-MHz transducer in routine ultrasound monitoring. Fifty outpatients (34 women, 16 men; mean age, 65 +/- 2) with inadequate temporal bone window for examination with 2-MHz transducers were re-examined with a 1-MHz transducer. Additionally, 50 controls (34 men, 16 women; mean age 49 +/- 2) with adequate temporal bone window were examined with both the 1- and 2-MHz transducers. Signal quality was classified as good (GQ), adequate (AQ), or poor (PQ). Finally, 30 normal controls were examined with both transducers under standardized conditions, to evaluate potential differences in velocity measurements. A total of 90 temporal windows were lacking in the 50 patients. Use of the 1-MHz transducer resulted in identification of the middle cerebral artery in 68 cases (76%; PQ, n = 13; AQ, n = 29; GQ, n = 26), the anterior cerebral artery in 51 cases (57%; PQ, n = 23; AQ, n = 17; GQ, n = 11) and the posterior cerebral artery in 46 cases (51%; PQ, n = 12; AQ, n = 21; GQ, n = 13). Examination was feasible with both transducers in the 50 controls, with minimal differences in signal quality between the two transducers. Correlation between velocity values acquired with the 1- and 2-MHz transducers was significant for all vessels examined. Best values were measured for the middle cerebral artery, followed by the anterior cerebral artery and posterior cerebral artery. The authors conclude that the use of the 1-MHz transducer increases the yield of transcranial Doppler sonography.

Technical limits in transcranial Doppler recording: inadequate acoustic windows

Transcranial Doppler (TCD) is a technique that evaluates blood flow velocity in intracranial vessels. It uses a 2-MHz probe and a Doppler signal analyzer. Absence of an acoustic window is a considerable problem for clinical utilization of TCD because cerebrovascular patients are frequently elderly. Previous reports suggest a higher prevalence of inadequate temporal acoustic window (TAW) in aged subjects and in females. A consecutive series of 624 subjects (376 males and 248 females, age range 2-86 y) were evaluated by standard TCD examination, to assess the contemporary absence of any signal corresponding to insonated basal arteries, defined as inadequate acoustic window. The rate of inadequate TAW was 8.2%, that of inadequate occipital acoustic window (OAW) was 9.0%. Prevalence of inadequate TAW was higher in females than in males, and OAW was higher in males than in females. Influence of aging on the presence of inadequate acoustic window is confirmed for temporal, but not for the occipital window. Different anatomical characteristics of the 2 regions could explain the different prevalence of TAW and OAW.

Effect of transcranial Doppler intensity on successful recording in Japanese patients

The major limitation of transcranial Doppler sonography (TCD) is the failure to obtain data for all patients. The purpose of this study was to determine in detail the effect of increasing ultrasonic acoustic intensity on the rate of successful recording of intracranial blood velocity signals. The study was performed in 239 Japanese patients using a 2-MHz range-gated, pulsed-wave TCD. The middle cerebral artery flow signals were recorded at 76, 152, 228, 304, 380, 456 and 532 mW/cm2 and the results analyzed by age, gender and intensity. The rate of successful recording showed significant increase with the ultrasonic intensity in both genders (45.7% at 76 mW/cm2 vs. 81.1% at 532 mW/cm2 in males and 29.5% vs. 60.7% in females). However, recording was only successful in 54% of aged (50-89 gamma) female patients at the highest ultrasonic intensity used. It should be possible to significantly increase TCD usefulness in an aging Japanese population by further increasing TCD acoustic intensity within safety limitation.

Rate of successful recording of blood flow signals in the middle cerebral artery using transcranial Doppler sonography

BACKGROUND AND PURPOSE

To assess the usefulness of transcranial Doppler sonography, we investigated the rate of blood flow signal recording failure in the middle cerebral artery in Japanese subjects. Furthermore, we studied the effect of increased emitted power on the rate of successful recording in some of the patients in whom recording failure had been detected at the standard transducer power of 100 mW/cm2.

METHODS

To evaluate the rate of successful recording, we measured blood flow signals in 597 patients (age range, 16 to 89 years) for screening of cerebrovascular disease by using a 2-MHz range-gated, pulsed-wave Doppler instrument at the standard transducer power. In 18 elderly patients with recording failure at the standard power, we assessed the effect of increased emitted power of 400 mW/cm2 on flow signal recording.

RESULTS

Blood flow signals were recorded in 920 (77.1%) of the 1194 middle cerebral arteries of the 597 patients studied. The rate of successful recording of bilateral middle cerebral artery flow signals (70.9%; 423 of 597 patients) decreased with age, especially in females (17.0% in women aged 70 years or older). In 12 of 18 elderly patients with recording failure at the standard power, blood flow signals could be detected at the increased emitted power of 400 mW/cm2.

CONCLUSIONS

The rate of successful recording of blood flow signals in Japanese subjects decreases with advancing age, especially in females. Increasing the emitted power markedly improves the successful recording rate.