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W H Mess

Publications and source records attributed to W H Mess.

13 recordsLinked to original sources
PubMed2005

The depth of microembolic signal direction change corresponds with vessel anatomy.

BACKGROUND: Analysis of microembolic signals (MES) suggests a change of flow direction (CFD). The aim of the present study was to relate MES direction in an amplitude plot, based on the radiofrequent (RF) signal, to the vascular anatomy as seen with transcranial color-coded duplex (TCCD). METHODS: In 5 patients undergoing heart valve surgery or aortic arch replacement, preoperatively TCCD of the distal part of the internal carotid artery and the middle and anterior cerebral arteries on the right side was performed to determine potential depths of changes in flow direction. Peroperatively, a transcranial pulsed Doppler (TCD) monitoring probe was fixed over the right temporal bone. A customized RF-based system, connected to the TCD device, captured and stored the MES. Off-line, the color-coded amplitude of the clutter-filtered RF signals was plotted as a function of time (sample interval 0.17 ms) and depth (sample interval 0.05 mm). RESULTS: A total of 313 MES were recorded in 4 patients with 66 MES (21%) showing a CFD. All MES with CFD could be assigned to maximally three different depth values, six out of eight CFD depth values as seen with the RF analysis were within 1 mm from a turn in flow direction as estimated with TCCD. CONCLUSIONS: A CFD of MES occurred at a very limited number of depths and corresponded mostly with the intracranial vascular anatomy, namely a turn of the flow direction in the intracranial vessels as observed with TCCD.

Carotid Artery, Internal↗
PubMed2003

Feasibility and reliability of on-line automated microemboli detection after carotid endarterectomy. A transcranial Doppler study.

OBJECTIVES: recently, a new algorithm for transcranial Doppler (TCD) ultrasound detection of microembolic signals (MES) was developed. In the present study, we investigated its on-line performance in TCD monitoring after carotid endarterectomy (CEA) and assessed off-line its accuracy in detecting MES. MATERIALS AND METHODS: first, the feasibility of MES detection in TCD monitoring after CEA in a routine clinical setting was evaluated in 50 patients. Second, to test the reliability of the software a 2-h digital audio study tape was made and analysed by the algorithm and five human experts. The "gold standard" was defined as the agreement between human experts: a MES was considered to be present if at least three human observers agreed. RESULTS: TCD monitoring for emboli detection after CEA was well tolerated by the patients and could be performed reliably. In the study tape, the human gold standard detected 107 MES, with 93 MES having an intensity of > or =7 dB. The software detected 81 and 77 MES, respectively. Using the 7 dB intensity threshold, the software had no false positives and 16 false negatives. The kappa value between the human gold standard and the software was 0.91, the proportion of specific agreement was 0.83. CONCLUSIONS: the tested algorithm provides a reliable method for automated on-line microemboli detection after CEA. This makes monitoring of the effectiveness of antiplatelet agents in the prevention of stroke after CEA more practicable.

Algorithms↗
PubMed2000

A new algorithm for off-line automated emboli detection based on the pseudo-wigner power distribution and the dual gate TCD technique.

Research on microembolic signals (MES) using the dual-gate technique has shown promising results, when the time difference (Deltat) of a MES in two sample volumes (SVs) placed serially has been measured manually. On the other hand, the computerized discrimination of MES and artefacts has been reported not to be superior to algorithms based on a single SV. Therefore, a dataset containing MES as well as four types of artefacts was made to test a preliminary version of a new algorithm for automated emboli detection. We monitored 20 patients during carotid endarterectomy (n = 17) and heart surgery (n = 3). Two transcranial Doppler (TCD) signals with a partial overlap of the SVs were recorded online and analysed off-line with an algorithm based on three consecutive steps: 1. Is there an intensity increase in both channels (64-point FFT; 50% overlap)? 2. What is the expected time difference (Deltat), with the velocity measured in channel 1 as the calculation basis? 3. What is the 'exact' Deltat (pseudo-Wigner power function)? Two human experts decided whether a signal was a MES or belonged to one of the four artefact groups. Of a total of 97 MES, 28% (n = 27) could not be detected in the distal channel. Thus, 72% (n = 70) of the MES were present in both channels and could be analysed based on the abovementioned criteria. Of these 70 MES, 87% (n = 61) were correctly identified off-line. We assessed artefact rejection for four different types of artefacts: changes of TCD settings, probe movement, low flow artefacts and electrocautery. The reliability of artefact rejection was 98% for setting changes (n = 382), 96% for probe movement (n = 477) and 98% for low flow artefacts (n = 91), but only 68% for electrocautery (n = 264). These preliminary results are promising, but need careful interpretation: 28% of the MES were not detectable in the distal SV, probably due to a poor signal-to-noise ratio (SNR) and anatomical restrictions. Electrocautery signals were insufficiently rejected. However, even an artefact rejection of 96% can be insufficient if the number of MES is very small compared to the number of artefacts.

Algorithms↗
PubMed2000

Somatosensory cortex responses to median nerve stimulation: fMRI effects of current amplitude and selective attention.

OBJECTIVES: The aim of this study was to localize and to investigate response properties of the primary (SI) and the secondary (SII) somatosensory cortex upon median nerve electrical stimulation. METHODS: Functional magnetic resonance imaging (fMRI) was used to quantify brain activation under different paradigms using electrical median nerve stimulation in healthy right-handed volunteers. In total 11 subjects were studied using two different stimulus current values in the right hand: at motor threshold (I(max)) and at I(min) (1/2 I(max)). In 7 of these 11 subjects a parametric study was then conducted using 4 stimulus intensities (6/6, 5/6, 4/6 and 3/6 I(max)). Finally, in 10 subjects an attention paradigm in which they had to perform a counting task during stimulation with I(min) was done. RESULTS: SI activation increased with current amplitude. SI did not show significant activation during stimulation at I(min). SII activation did not depend on current amplitude. Also the posterior parietal cortex appeared to be activated at I(min). The I(min) response in SII significantly increased by selective attention compared to I(min) without attention. At I(max) significant SI activity was observed only in the contralateral hemisphere, the ipsilateral cerebellum, while other areas possibly showed bilateral activation. CONCLUSIONS: Distributed activation in the human somatosensory cortical system due to median nerve stimulation was observed using fMRI. SI, in contrast to SII, appears to be exclusively activated on the contralateral side of the stimulated hand at I(max), in agreement with the concept of SI's important role in processing of proprioceptive input. Only SII remains significantly activated in case of lower current values, which are likely to exclusively stimulate the sensible fibres mediating cutaneous receptor input. Selective attention only enhances SII activity, indicating a higher-order role for SII in the processing of somatosensory input.

Adult↗
PubMed1999

Middle cerebral artery anatomy and characteristics of embolic signals: a dual gate computer simulation study.

In terms of microembolic signal (MES) detection, the anatomy of the middle cerebral artery (MCA) mainstem has only scarcely been considered. The vessel itself, however, could be at least partly responsible for the enormous variation when calculating the essential time difference (deltat) values of MES using the dual-gate technique. Therefore, we studied the time characteristics of MES in a computer simulation applying an anatomically realistic vessel and a dual-gate TCD approach. Three different MCA anatomies and two MES to blood intensities were simulated as well as two different sample volume settings. The MES length (proximal sample volume t1; distal sample volume t2) and deltat were calculated for different angles of insonation and sample volume depths. The calculations of the time characteristics of MES showed extreme variation, with only modest changes of the insonation angle (t1 4-34 ms; deltat 9-27 ms) or the sample volume depth (t1 7-27 ms; deltat 6-32 ms). The variation could be considerably reduced with modified TCD settings i.e., a shorter gate separation combined with a shorter receiver gate time in the distal sample volume (deltat with changing insonation angles 6-19 ms; deltat with changing insonation depths 13-17 ms). These results not only urge us to a cautious interpretation of the properties of single MES, but also contribute to an understanding of the marked deltat variation using the dual-gate technique.

Cerebral Arteries↗
PubMed1998

Preliminary report of detecting microembolic signals in transcranial Doppler time series with nonlinear forecasting.

BACKGROUND AND PURPOSE: Most algorithms used for automatic detection of microembolic signals (MES) are based on power spectral analysis of the Doppler shift. However, controversies exist as to whether these algorithms can replace the human expert. Therefore, a different algorithm was applied that takes advantage of the periodicity of the MES. This so-called nonlinear forecasting (NLF) is able to detect periodicity in a time series, and it is hypothesized that this technique has the potential to detect MES. Moreover, because of the lack of prominent periodicity in both the normal Doppler signals (DS) and movement artifacts (MA), the NLF has a potential to differentiate MES from normal blood flow variations and MA. METHODS: Twenty single MES and 100 MA were selected by 2 human experts. NLF was applied to MES and MA and compared with 200 randomly chosen DS. NLF resulted in a so-called prediction value that ranges from + 1 in signals with prominent periodicity to 0 in signals that lack periodicity. RESULTS: NLF revealed that MES are more predictable than the normal Doppler signals (prediction [MES]=0.829+/-0.084 versus prediction [DS]= -0.060+/-0.228; P<0.0001). Moreover, MES are more predictable than the MA (prediction [MA]=-0.034+/-0.223; P<0.0001). No difference in prediction could be found between DS and MA. CONCLUSIONS: This preliminary report shows that MES can be separated from DS and MA by NLF. Research is needed as to whether this technology can be further developed for automatic detection of MES.

Artifacts↗
PubMed1996

Automatic embolus detection compared with human experts. A Doppler ultrasound study.

BACKGROUND AND PURPOSE: Transcranial Doppler ultrasound (TCD) reliably detects the occurrence of microembolic signals (MES). Unfortunately, TCD monitoring is a time-consuming and mentally strenuous procedure. The purpose of this study was to assess whether automatic embolus detection software devices acting as a "stand-alone system" are able to identify MES in patients with solid cerebral microemboli. METHODS: Ten records of TCD monitoring of the middle cerebral artery in patients with symptomatic high-grade carotid artery stenosis were analyzed for the moments at which MES occurred by four observers and three automatic detection software devices (RB11 on TC2000, Pioneer Version 2.10, and Embotec). The results of the three software systems were assessed on the basic assumption that MES were present if at least three of the four observers agreed. RESULTS: The average number of 1-second periods in which MES were detected by the four observers per tape ranged from 5 to 39. The overall kappa values (and SEs) for chance-corrected interobserver agreement between the four observers ranged from .94 (.02) to .99 (.01). The agreement between the software devices and the observers was lower, with kappa values (and SEs) ranging from .18 (.17) to .93 (.07). The RB11 and Embotec systems achieved a kappa value higher than 0.4 in all tapes. The Pioneer system failed to reach a kappa value of 0.4 in three tapes. The RB11 showed a sensitivity of 70% for detecting MES, the Embotec 62%, and the Pioneer 44%. CONCLUSIONS: In patients with symptomatic high-grade carotid artery stenosis, a high degree of agreement in the detection of moments of MES can be achieved between observers. The three automatic detection software devices reached less agreement. Supervision of TCD monitoring and assessment of MES by an experienced observer is still necessary.

Carotid Stenosis↗
PubMedDate not supplied

Microembolic signal description: a reappraisal based on a customized digital postprocessing system.

The high variability in presence and signature of microembolic signals (MES), detected with transcranial Doppler sonography (TCD) in the middle cerebral artery (MCA), cannot be explained with the currently available published data. We applied customized postprocessing on the radiofrequency (RF) signal of a standard TCD system. The spatial resolution was on the order of 2 mm, depending only on the length of the ultrasound (US) burst emitted. The amplitude of clutter-filtered RF signals was color-coded and plotted as a function of time and depth (range 30 mm). Additionally, 128 point fast Fourier transforms (FFTs) (50% temporal overlap) were calculated, visualizing both the background Doppler spectrum and the MES. We evaluated 122 gaseous MES from two patients during cardiac surgery and 52 particulate MES from four patients after carotid endarterectomy. Both MES categories showed comparable properties: they appeared in the RF amplitude plot as rather straight lines of increased intensity, indicating that the velocity remained approximately the same while they passed the US beam. The velocity calculated from the amplitude plot never exceeded that of the background Doppler spectrum. Various "MES patterns" could be identified with respect to the depth range at which the MES were visible. A quarter of the gaseous MES changed their direction at a specific depth, suggesting that the MES entered a branch (e.g., an M2 artery or the anterior cerebral artery). In the FFT analysis, these MES contained both positive and negative frequencies. It is concluded that MES show consistent signature patterns in the amplitude-time plots and that the previously reported variability of MES appearance in conventional Doppler systems is an artefact caused by relatively large signal amplitudes and sample volumes.

Artifacts↗
PubMedDate not supplied

Temporary occlusion of middle cerebral artery by macroembolism in carotid surgery.

Two patients are presented who during carotid endarterectomy (CEA) temporarily showed an obstruction of the middle cerebral artery (MCA) mainstem by a macroembolus resulting in cerebral ischaemia. Both cases are unusual examples of CEA and selected from a cohort of more than 1,500 operations. During surgery with general anaesthesia, brain function was monitored with computerized electroencephalography (EEG) and transcranial Doppler (TCD) ultrasonography. The simultaneous use of EEG and TCD monitoring allowed us to witness the development of intraoperative cerebral ischaemia and to relate these events to a temporary occlusion of the MCA mainstem by a macroembolus. This is the first life report that describes obstruction of a cerebral artery by arterial embolism resulting in cerebral ischaemia.

Brain Ischemia↗
PubMedDate not supplied

Functional MR imaging of the cervical spinal cord by use of median nerve stimulation and fist clenching.

BACKGROUND AND PURPOSE: Findings of blood oxygen level-dependent (BOLD) functional MR (fMR) imaging of the cervical spinal cord, obtained by using a fist-clenching motor task, have been sporadically reported. Because spinal activation by sensory stimuli has a potential at least equal to that of fist clenching, its feasibility was assessed. Whether stimulation of the median nerve could evoke an fMR imaging response at 1.5 T in the cervical spinal cord was investigated, and the response pattern was compared with that obtained by fist clenching. METHODS: A dynamic cardiac-gated T2*-weighted imaging sequence was used to quantify cervical spinal cord activation under two paradigms with different numbers of subjects. Seven subjects underwent electrical median nerve stimulation at the elbow sufficient to elicit a maximal compound muscle action potential in the flexor carpi radialis muscle. Eleven subjects performed self-paced fist clenching. Cord activation was measured in the sagittal and transverse imaging planes. RESULTS: In the sagittal view, five of seven subjects had an fMR imaging response in the lower cervical spinal cord upon median nerve stimulation, whereas seven of 11 subjects showed activation with the fist-clenching task. Within the cord, the measured fMR imaging response level was approximately 8-15% with respect to the baseline signal level. In the transverse imaging plane, significant fMR imaging responses could be measured in only two of six and six of nine subjects with median nerve stimulation or fist clenching, respectively. A consistent cross-sectional localization of the activity measured in the spinal cord was not detected, either in terms of the right and left sides or in terms of the posterior and anterior directions. CONCLUSION: In the sagittal plane, median nerve stimulation at the elbow can evoke an fMR imaging response in the lower cervical spinal cord. The activation pattern was comparable with that obtained by fist clenching. The localization of the segmental fMR imaging activation (C4 through T1) is consistent with the known functional neuroanatomy for both paradigms. In the transverse plane, reliable fMR imaging responses were obtained much less frequently, and assignment of distinct areas of the spinal cord to the stimulation methods used was not possible.

Action Potentials↗
PubMedDate not supplied

Functional MRI of the spinal cord: will it solve the puzzle of pain?

PURPOSE: Whether stimulation of the median nerve could evoke an fMR imaging response at 1.5 T in the cervical spinal cord was investigated and the response pattern was compared with that obtained by fist clenching. METHODS: Eleven subjects performed the fist-clenching task and in seven of them electrical stimulation was also applied. fMRI was performed using a T2* sensitive echo-planar imaging sequence gated by the heartbeat signal. A dynamic cardiac-gated T2*-weighted imaging sequence was used to quantify cervical spinal cord activation. Cord activation was measured in the sagittal and transverse imaging planes. RESULTS: With the fist-clenching task, activation of the spinal cord in the cervical intumescence could be seen in seven of eleven subjects in the sagittal plane and in six of nine subjects in whom axial images were acquired. When median nerve stimulation was applied, cord activation was seen in the sagittal images in five out of seven, and in the axial plane in two out of six subjects. A consistent cross-sectional localization of the activity measured in the spinal cord was not detected, either in terms of the right and left sides or in terms of the posterior and anterior directions. CONCLUSION: In the sagittal plane, median nerve stimulation at the elbow can evoke an fMR imaging response in the lower cervical spinal cord. The activation pattern was comparable with that obtained by fist clenching.

Cervical Vertebrae↗