Comparative Study of Power-Based Versus Mean Frequency-Based Transcranial Color-Coded Duplex Sonography in Normal Adults
Background and Purpose Power-based transcanial color-coded duplex sonography (p-TCCD) is a new ultrasonic method that has advantages compared with frequency-based TCCD (f-TCCD), since it is essentially independent of the angle of insonation, not subject to aliasing, and has a better signal-to-noise ratio. The purpose of this study was to evaluate the ability of p-TCCD to visualize flow in cerebral parenchyma and to compare the advantages, limitations, and reliability of velocity measurements of p-TCCD versus f-TCCD in the major basal cerebral arteries of normal subjects.
Methods Two investigators performed 15 p-TCCD and 15 f-TCCD studies in 30 normal subjects with adequate ultrasonic windows. Each investigator did a p-TCCD or f-TCCD study in every patient, and each was blinded in every case to the results of the other. Peak systolic (Vs) and end-diastolic (Vd) velocities were determined in the anterior, middle, and posterior cerebral, basilar, and vertebral arteries. The reliability of p-TCCD velocimetry was evaluated by calculating both the correlation coefficient (r) of the difference between p-TCCD versus f-TCCD measurements and the coefficient of variation (CV), defined as the difference between the mean values for p-TCCD and f-TCCD divided by the mean values for f-TCCD measurements, expressed as percent.
Results p-TCCD did not display flow in cerebral parenchyma but depicted arteries with a course perpendicular to that of the ultrasound beam and showed good reliability of velocity measurements in all examined arteries: for Vs, r was .84 to .93 (P<.001) and CV was 7.7% to 10.8%; for Vd, r was .87 to .90 (P<.001) and CV was 10.3% to 13.7%. The lack of directional and velocity information and tissue motion artifacts were unimportant limitations in p-TCCD.
Conclusions Compared with f-TCCD, p-TCCD had no important advantages but had several unimportant limitations in a study of normal adults with adequate ultrasonic windows.
Mean frequency-based TCCD is a noninvasive method for imaging of the major basal cerebral arteries.1 2 3 4 This technique has shortcomings such as angle dependency, aliasing, and a tendency for noise to overwhelm low-velocity signals.5 6 7 8 Accordingly, ultrasonic depiction of flow within normal cerebral parenchyma is not feasible even with the use of contrast media.1 2 3 4 9 10 Power Doppler sonography is a new ultrasonic method that overcomes these shortcomings, since it is essentially independent of the angle of insonation, not subject to aliasing, and has a better signal-to-noise ratio.5 6 7 8 Bude et al5 have recently shown that power Doppler sonography is effective in depicting normal intrarenal vasculature. The present study was performed to assess the ability of p-TCCD to visualize flow in cerebral parenchyma and to evaluate the advantages, limitations, and reliability of velocity measurements of p-TCCD versus f-TCCD in the study of the major basal cerebral arteries of normal adults with adequate ultrasonic windows.
Subjects and Methods
Subjects were 30 normal persons (mean±SD age, 42±16 years; range, 19 to 79 years; 15 women, 15 men) with no cerebrovascular risk factors and no history of cerebrovascular disease. They had adequate ultrasonic windows and normal findings at extracranial ultrasonographic studies. All subjects were examined by two experienced investigators (C.S. and R.W.B.). To avoid differences related to interrater disagreement, each investigator performed 15 f-TCCD and 15 p-TCCD studies. Every subject was examined by one examiner and then the other. Each sonographer was blinded to the findings of the other.
The extracranial cerebral arteries were examined using an Acuson 128 XP/10 equipped with a 5.0/7.0-MHz linear scan. The intracranial cerebral arteries were studied using an Acuson 128 XP/10 equipped with a 2.0/2.5-MHz 90° sector scan. Color Doppler images and pulsed-wave Doppler spectra were obtained using 2.0 MHz, and B-mode imaging was performed using 2.5 MHz. Doppler energy output had a maximal in situ intensity of 271 mW/cm2 I-SPTA (spatial peak time, average intensity) corresponding to 123 W/cm2 I-SPPA (spatial peak pulse, average intensity).
The following color Doppler image-enhancement parameters were used. For f-TCCD studies, the level of temporal persistence that smooths color Doppler information over time due to temporal averaging was low. For p-TCCD studies, the highest persistence level was used, since it might enhance areas of low flow energy. The smallest color Doppler gate size was used for p-TCCD and f-TCCD studies because all patients had optimal insonation windows. For evaluation of the ability of p-TCCD to detect flow in cerebral parenchyma, larger gate sizes were used also, since larger gate sizes translate into better sensitivity. The color Doppler filter setting 1 was used for f-TCCD and p-TCCD studies. The color Doppler filters are combinations of enhancements, including Acuson’s “multivariate motion discrimination.”
For pulsed-wave Doppler velocity measurements, spectral display filters at a cutoff of 125 Hz were used.
TCCD examinations of the ACA, MCA, and PCA were performed through the temporal window with the patient in a supine position. BAs and VAs were insonated through the foramen magnum with the patient in a sitting position. Cerebral arteries were identified according to their anatomic location and the well-known criteria of Aaslid et al.11 All subjects were told to close their eyes and relax. Attention was paid to providing a calm environment.
The subjects were examined by both sonographers according to the following protocol. Initially, the examiners evaluated whether p-TCCD was able to visualize flow in the cerebral parenchyma. Flow in cerebral parenchyma means flow in arteries and veins that are located within the brain. The rationale for using the expression “parenchymatous flow” was that arteries cannot be distinguished from veins by power Doppler sonography on the basis of the direction of flow. To visualize flow in cerebral parenchyma, the color gain was manipulated until “noise” first became apparent in the image background and until it first began to exceed the single-color background of electronic noise of power Doppler sonography images.5 In addition, color Doppler image parameters were adjusted as described above. Thereafter Vs and Vd for the ACA, MCA, PCA, BA, and VA and the corresponding insonation angles were determined using pulsed-wave Doppler sonography with both the frequency-based and power-based techniques, as reported previously.12 Finally, the examiners determined whether the following predictable differences of p-TCCD versus f-TCCD represented an unimportant or important advantage or disadvantage.5 6 7 8 We defined an advantage or disadvantage as unimportant when it had no impact on the identification of a major basal cerebral artery and as important when it had an impact on the identification of a major basal cerebral artery. Power Doppler sonography (1) may enable the depiction of cerebral vessels that are tortuous or perpendicular to the plan of insonation, (2) gives no directional and velocity information and is not subject to aliasing, and (3) is very sensitive to tissue motion, resulting in so-called flash artifacts, which may obscure the power Doppler image.
The duration of every p-TCCD and f-TCCD examination was noted. We defined it as the time span from switching on the ultrasound device to the last velocity measurement.
The mean±1 SD values of Vs and Vd from all subjects observed during f-TCCD and p-TCCD examinations were calculated for the ACA, MCA, PCA, BA, and VA. Statistical evaluation for the reliability of p-TCCD velocity measurements was performed by calculating both the correlation coefficient of the difference between the values of p-TCCD versus f-TCCD examinations and the CV. The CV values were calculated as CV equals the SD of the difference between the mean values for the power mode and frequency mode divided by the mean values for the frequency mode examination, expressed as percentage.
Power Doppler sonography did not depict flow in the cerebral parenchyma. All examined major basal cerebral arteries were identified using p-TCCD and f-TCCD. Table 1⇓ shows the results of the evaluation of possible advantages and limitations of p-TCCD versus f-TCCD. Both examiners estimated that there were only unimportant advantages and limitations of p-TCCD compared with f-TCCD.
The mean±SD Vs and Vd values and the corresponding mean±SD insonation angles for the ACA, MCA, PCA, BA, and VA with p-TCCD and f-TCCD showed no significant differences (Mann-Whitney U test) and are given in Tables 2 and 3, respectively. The calculated r and CV values gave good results and are summarized in Table 4⇓.
The examination times for both investigators and both TCCD modes showed no significant differences (Mann-Whitney U test) and are reported in Table 5⇓.
Color Doppler sonography is based on the mean Doppler frequency shift reflecting the mean velocity of moving particles.1 In power Doppler sonography, the ultrasound system displays the integrated power of the Doppler signal, which is related to the number of red cells that produce the Doppler shift.5 6 7 In contrast to color Doppler sonography, power Doppler sonography is essentially angle independent, shows no aliasing, and has a better signal-to-noise ratio.5 6 7 The latter property allows the gain in power Doppler imaging to be increased greatly over the level at which noise begins to obscure color Doppler images, rendering power Doppler sonography more suited for visualization of low velocities.5 8 It has been reported recently that power Doppler sonography provides color Doppler gain increases of 10 to 15 dB.5 Our hypothesis was that p-TCCD might enable the depiction of flow in cerebral parenchyma. This would be useful for noninvasive diagnosis of perfusion deficits in patients with ischemic stroke. However, p-TCCD was not able to delineate flow in the cerebral parenchyma of normal adults with adequate insonation windows. Nondepiction of cerebral parenchyma with transtemporal insonation was probably caused by signal attenuation occurring at the bony window.13 Contrast-enhanced transcranial Doppler studies using Levovist at doses of 300 mg/mL have provided a 15- to 25-dB mean increase of Doppler signal intensity.9 14 15 16 Therefore, contrast-enhanced p-TCCD might depict intracerebral vasculature, and ultrasound studies are suggested in this regard. The absence of transforaminal p-TCCD visualization of flow in cerebral parenchyma was more predictable, since this ultrasonic window precludes the insonation of most infratentorial cerebral tissue.
As expected, p-TCCD also visualized flow in arterial segments that traveled at right angles to the ultrasound beam and were not detected with f-TCCD. However, both sonographers had the impression that this improvement added no relevant information. It is important to note that p-TCCD shows slight angle dependence.8 This is because flows with a mean Doppler shift close to zero are eliminated or attenuated in signal level by the soft-tissue motion canceler of the ultrasonic unit. In extracranial ultrasound studies, the sonographer may just change the position of the transducer. In TCCD, however, the aperture may be severely limited because the temporal window becomes smaller, especially in older women.13 17 Hence, the angle independence of p-TCCD may be small or nonexistent. This was not the case in this study, however.
The absence of information about flow direction in power Doppler sonography had no impact on the technique and results of the present p-TCCD investigations because the insonation angles, registered flow velocities, and time of examination were not different in p-TCCD compared with f-TCCD studies. Moreover, both sonographers had the impression that the lack of directional information was an unimportant disadvantage of p-TCCD, since the ability to identify the normal circle of Willis anatomy, and less the ability to detect flow direction, is the most important criterion for visual identification of the major basal cerebral arteries. Nevertheless, we assume that the absence of information about flow direction in p-TCCD may cause difficulties in identifying the major basal cerebral arteries and thus longer examination times for sonographers without prior experience in f-TCCD. Moreover, in patients with occlusive cerebrovascular disease, the lack of flow-direction information in the ACA may become an important limitation of p-TCCD evaluation of collateral flow through the anterior part of the circle of Willis (R.W.B., unpublished data, 1994).
The absence of flow velocity information and aliasing in the power mode were estimated to represent neither an advantage nor a disadvantage because our subjects were healthy volunteers with normal cerebral artery velocities. However, it is very likely that this will be a limitation in circumstances in which the color Doppler information helps to localize areas of high velocity for Doppler sampling.
Both investigators felt that flash artifacts were only an unimportant disadvantage of p-TCCD. Harmonic processing in contrast-enhanced power Doppler sonography may eliminate flash artifacts.18
In studying the reliability of p-TCCD for angle-corrected velocity measurements, we used f-TCCD as a reference method because several studies have shown that the intraobserver and interobserver reliability for angle-corrected f-TCCD velocimetry in the major basal cerebral arteries is good.4 12 19 End-expiratory carbon dioxide partial pressure was not measured because it did not show any significant changes during repetitive transcranial Doppler velocity measurement studies.20 21 In addition, breathing through a one-valve system might cause hypoventilation and/or provoke anxiety.22 23 Both conditions lead to an increase of cerebral blood flow and might cause erroneously high velocities in the major basal cerebral arteries. Statistical evaluation for the reliability of p-TCCD velocity measurements gave good values for r (Vs, .84 to .93; Vd, .87 to .90 [P<.001]) and CV (Vs, 7.7% to 10.8%; Vd, 10.3% to 13.7%). Therefore, p-TCCD is a reliable method for measurement of flow velocities in the major basal cerebral arteries.
The major shortcoming of transcranial ultrasound studies, the acoustic window, was not a problem in our study because all patients with insufficient bony windows were excluded to evaluate the ability of p-TCCD to detect flow in cerebral parenchyma. However, it might well be that in cases of suboptimal temporal ultrasonic windows, p-TCCD may be superior compared with f-TCCD in the detection of the major basal cerebral arteries because the better signal-to-noise ratio allows higher gain settings in power Doppler sonography.
In summary, p-TCCD provided no important advantages, but it provided several unimportant limitations compared with f-TCCD in the examination of normal adults with adequate ultrasonic windows. However, p-TCCD may be useful in patients with suboptimal ultrasonic windows in the depiction of low flow after intracranial stenoses and occlusions, and it may delineate flow in cerebral parenchyma with use of ultrasonic contrast media. It is suggested that additional studies be performed in this regard.
Selected Abbreviations and Acronyms
|ACA||=||anterior cerebral artery|
|CV||=||coefficient of variation|
|f-TCCD||=||frequency-based transcranial color-coded duplex sonography|
|MCA||=||middle cerebral artery|
|PCA||=||posterior cerebral artery|
|p-TCCD||=||power-based transcranial color-coded duplex sonography|
- Received June 22, 1995.
- Revision received August 8, 1995.
- Accepted October 10, 1995.
- Copyright © 1996 by American Heart Association
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