Power Doppler: an advance over colour Doppler for transcranial imaging?

In vitro model test and preliminary clinical application of a new method of ultrasonographic imaging: Vascular enhancement technology

Vascular enhancement technology (VET) is a new form of ultrasonographic technology that can optimize images by enhancing the B-mode display with information derived from power Doppler. We designed an in vitro model to evaluate the accuracy and application method of VET and to apply this technology preliminarily in vivo in the vascular periphery. An in vitro model was designed with a flow pump system to simulate blood flow in soft tissue and the intracranial vasculature. Modeling vessels were imaged by traditional B mode, color Doppler flow imaging and VET. The diameter of the various silicon tubes was measured to verify the accuracy of VET. For in vivo application, 15 normal subjects and 26 patients suspected of having carotid artery plaques and cerebrovascular disease were examined using these three image modes. The imaging effects were observed and compared. VET imaging could clarify the lumens of the modeling vessels and reduce artifacts. The caliber of three sizes of silicon tubing was also measured accurately by VET. Of 15 normal subjects, sound artifacts in large vessels were inhibited and the intermedia membrane was clearly displayed by VET. The boundaries of carotid plaques were manifested by VET with well-defined edges. Three cases of hypoechoic soft plaque on the anterior wall missed in B-mode imaging were detected by VET. Intracranial scanning with VET identified cerebral vascular disease, including cerebral stenosis, arteriovenous malformations and aneurysms. The size and shape of the focus displayed by VET coincided with that observed using digital subtraction arteriography. VET is helpful in improving detection of the boundary of vessels and visualization of the microvasculature.

Power-based versus conventional transcranial color-coded duplex sonography in the assessment of the vertebrobasilar-posterior system

BACKGROUND

Power Doppler (PD) is a new ultrasonic technique that allows improved visualization of vascular structures. The aim of our study was to compare the ability of power-based transcranial color-coded duplex sonography (p-TCCS) to conventional transcranial color-coded duplex sonography (TCCS) and contrast-enhanced transcranial color-coded duplex sonography (CE-TCCS) in the visualization of the vertebrobasilar system.

METHODS

In 41 patients without cerebrovascular diseases, we evaluated and compared identification rates of major vessels of the vertebrobasilar system and branches using both ultrasound (US) techniques. In 9 patients we performed additional CE-TCCS studies. Furthermore, the possibility of visualization of the basilar artery (BA) and the vertebral arteries (VA) over the long course was investigated.

RESULTS

TCCS and p-TCCS were equally effective at showing the VAs (74 of 82 v 80 of 82), proximal segments of the BA (37 of 41 v 41 of 41) and P1 segments of the posterior cerebral artery (PCA) (72 of 82 v 82 of 82). However, the diagnostic sensitivity of p-TCCS was significantly better for peripheral segments of the PCA (34 of 82 v 68 of 82 for P2 and 2 of 82 v 24 of 82 for P3 segments, P<.001), distal parts of the BA (25 of 41 v 38 of 41, P<.01), the anterior inferior cerebellar artery (AICA) (0 of 82 v 8 of 82, P<.0001) and the posterior inferior cerebellar artery (PICA) (13 of 82 v 34 of 82, P<.001). Furthermore, using transtemporal coronal sections, p-TCCS allowed visualization of the BA over the complete course in connection with one or both VAs in half of the patients. Except P2 and P3 segments, CE-TCCS did not increase resolution compared with p-TCCS.

CONCLUSIONS

PD offers significant advantages over color-coded sonography in imaging the BA and small-calibre vessels like the PICA, AICA, and peripheral segments of the PCA. P-TCCS in combination with TCCS increases the diagnostic sensitivity to identify vascular structures of the vertebrobasilar system.

Transcranial harmonic power duplex sonography for the evaluation of cerebral arteries

Harmonic power-based duplex sonography is a new ultrasound method that improves the signal-to-noise ratio of extracranial vascular imaging. The authors evaluated this new method for transtemporal imaging of the basal cerebral arteries. Fundamental power-based duplex sonography (p-TCCS) and harmonic power-based duplex sonography (HI-p-TCCS) in combination with a novel perfluoropropane-containing ultrasound contrast agent (Optison) were investigated for the evaluation of the basal cerebral arteries in 12 healthy volunteers. The number of identified vascular segments and the blood flow velocities in the middle and posterior cerebral arteries were determined for p-TCCS and for two doses of Optison (0.5 and 1.5 mL) using HI-p-TCCS. Furthermore, the authors determined the time course of signal enhancement after Optison bolus injections. The results were compared using Friedman two-way ANOVA test. Significantly more arterial segments were visualized using HI-p-TCCS with enhancement of either 0.5 mL or 1.5 mL Optison (p < 0.01, each) than using p-TCCS. The spatial resolution was markedly increased with HI-p-TCCS, resulting in a striking difference in the detection of distal arterial segments and cortical and parenchymal branches. Except for the diastolic blood flow velocities (BFVs) in the M1 segment, the BFVs did not differ significantly between p-TCCS and HI-p-TCCS. Comparing HI-p-TCCS with 0.5 mL and 1.5 mL Optison, the authors found a small but significant reduction of the latency period (18.2 vs. 15.9 seconds, respectively; p < 0.01), a significant increase of the blooming phase (62.7 vs. 99.8 seconds, respectively; p < 0.0006) and a significant prolongation of the diagnostically useful signal enhancement (233.7 vs. 427.6 seconds, respectively; p < 0.004).

Imaging the basilar artery by contrast-enhanced color-coded ultrasound

Conventional transcranial color-coded real-time sonography of the vertebrobasilar system is limited by imaging problems of the distal segment of the basilar artery. Lung-stable contrast-enhancing agents may overcome this problem by enhancing the quality of Doppler signals by as much as 20%. Fourty-two patients underwent sonographic evaluation of the vertebrobasilar system before and after receiving intravenously administered galactose-based contrast-enhancing agent Levovist by transforaminal and transtemporal routes. Imaging quality was classified into five categories depending on the length of visible color-flow by transforaminal approach: 1--no signal, 2--1-9.9 mm, 3--10-19.9 mm, 4--20-29.9 mm, 5--> or = 30 mm. For transtemporal insonation, imaging quality was classified either as no color flow or sufficient color flow of the basilar tip. By unenhanced investigation, average signal length of color flow was 16 +/- 8 mm for transforaminal investigation; application of Levovist improved this value to 26.6 +/- 6 mm. For unenhanced transforminal approach, 4.8% were assigned to category 1, 11.9% to category 2, 54.8% to category 3, 23.8% to category 4 and 4.8% to category 5. After signal enhancement with Levovist, category 1 covered 0%, category 2 2.4%, category 3 7.14%, category 4 59.5% and category 5 30.9% (p < 0.001). Unenhanced transtemporal approach allowed identification of the basilar tip in 78.6% with an average length of 6.3 +/- 2 mm; contrast enhancement improved this values to 92.9% and 8.3 +/- 3.3 mm respectively (p < 0.05). The application of transpulmonary contrast-enhancing agents improves the reliability of transcranial color-coded duplex sonography of the basilar artery.

Blood flow velocities in middle cerebral artery branches after subarachnoid hemorrhage

In a prospective study, 55 patients were examined by transcranial duplex sonography (TCCS) after subarachnoid hemorrhage (SAH) to determine whether additional transcranial duplex examination on the middle cerebral artery M2 segments would aid in the examination of the MCA stem segment. The mean blood flow velocities and pulsatility index were correlated to the occurrence of delayed ischemic neurologic deficits (DIND). Out of 47 patients included, 21 did not experience any delayed deficit (group I), 15 did (group II), and in 11 the extent to which vasospasm contributed to a neurologic deficit was unclear (group III). The highest blood flow velocity and the greatest increase of mean blood flow velocity on 1 day were significantly higher in groups II and III both in M1 and in M2. In 10 patients in group II, where the onset day of DIND was known exactly, Doppler data indicating ischemia before or at the time of DIND were observed in nine. In eight patients, Doppler of the MCA stem alone would have provided enough information to recognize the risk of symptomatic vasospasm; in one patient, only the M2 Doppler gave an indication of ischemic complication. Transcranial duplex sonography may provide additional information to TCD by accurate delineation of M1/M2 vasospasm and therefore may help plan cerebral angiography and neurointerventional treatment.

Prenatal depiction of angioarchitecture of an aneurysm of the vein of Galen with three-dimensional color power angiography

A fetus with a large supratentorial cyst and cardiomegaly was encountered at 33 weeks of gestation. The cyst appeared as an aneurysmal, fluid-filled structure extending posteriorly with a finger-like appendage. Using color flow mapping, we disclosed rapid in-and-out blood flow with marked turbulence within the cyst. For evaluation of its blood supply and venous drainage of the vascular malformation, a three-dimensional reconstruction of the power Doppler image was conducted. The results revealed that the vascular malformation was supplied by a small contralateral aneurysm from the branches of Willis' circle, draining posteriorly into an abnormal falcine sinus and then into the superior sagittal sinus. No other fetal abnormality such as hydrocephalus or hydrops was discovered. The prenatal diagnosis of an aneurysm of the vein of Galen was made on the basis of the gray-scale, color Doppler findings, and also the angioarchitecture obtained by three-dimensional power Doppler imaging. The woman was admitted at 37 weeks of gestation due to labor onset and delivered the baby via the vaginal route without complication. Postnatal angiography and magnetic resonance imaging confirmed the diagnosis of an aneurysm of the vein of Galen, and the angioarchitecture depicted it before birth. We suggest that three-dimensional power Doppler ultrasonography may assist in the diagnosis of an aneurysm of the vein of Galen, and precisely delineate the complicated corresponding vasculature. This may guide postnatal management and predict the prognosis more accurately.

Diagnostic benefit of echocontrast enhancement for the insufficient transtemporal bone window

Echocontrast agents (ECA) are known to improve transcranial color-coded duplex (TCCD) imaging, but its diagnostic benefit in the routine clinical setting has not clearly been defined. The authors investigated the diagnostic benefit of ECA application in 54 patients with insufficient transtemporal bone window, consecutively referred to their ultrasound laboratory. According to the precontrast imaging quality, patients were assigned to three categories: A, no intracranial structures or vessel segments visible on B-mode imaging and TCCD (n = 5); and intracranial structures visible on B-mode imaging and vessel segments less than 5 mm in length (B, n = 21), or larger than 5 mm in length (C, n = 28) visible on TCCD. The effect of the echocontrast enhancement was assessed with respect to signal enhancement, imaging quality, and diagnostic confidence. In 49 out of 54 patients (91%), a significant improvement of the imaging quality was noted, enabling 43 (80%) neurovascular diagnoses of sufficient diagnostic confidence. The diagnostic ECA effect was strongly dependent on the precontrast imaging quality: upon echoenhancement, a satisfactory image quality was obtained in none of the patients of category A, as opposed to 16 (76%) and 27 (96%) patients of categories B and C, respectively. In summary, in 80% of our consecutive patient series with insufficient transtemporal bone window, application of ECA allowed for a conclusive TCCD study. Properties of the transtemporal precontrast scans are strongly predictive of the diagnostic benefit and should be taken into the decisive consideration.

Morphological and hemodynamic evaluations by means of transcranial power Doppler imaging in patients with severe head injury

The following conditions of 17 patients with severe head injury (ages 9-76; mean 37:12 focal and 5 diffuse injuries) were evaluated during acute phase (1-14 days after injury, mean 5) by transcranial power Doppler imaging (PDI), a new color Doppler ultrasound technique: a) morphological changes via temporal window, b) hemodynamic changes in major intracranial/cervical arteries based on measured angle-corrected time-averaged mean (TAM)/ peak velocities and vessel diameter (Va), and calculated pulsatility indices (PI), vessel area (Va), and flow volume (Vf = TAM x Va). a) 1) Major trunks of intracranial vessels and circle of Willis and pathological changes in frontal/temporal lobes and midbrain were finely visualized. 2) Contusional hemorrhage and cerebral contusion demonstrated irregular hyper- and hypo-echoic lesions, respectively. 3) Delayed epidural hematoma showed a hyper-echoic band. b) 1) Decreased velocities, significant PI increase, and Va increase tendency were observed in intracranial arteries. 2) Increased velocities with Vf increase but no Va decrease indicated hyperemia rather than vasospasm. 3) Va in the intracranial vessels, however, tended to increase PDI appears useful in evaluating real-time and simultaneous morphological and hemodynamic information in pathogenesis and neurointensive care of patients with severe head injury.

Echo contrast-enhanced three-dimensional power Doppler of intracranial arteries

The purpose of this study was to evaluate the potential of contrast-enhanced three-dimensional (3-D) power Doppler (CE3DPD) in the assessment of intracranial vascular structures, and to compare the results with unenhanced 3-D power Doppler (3DPD) and magnetic resonance angiography (MRA) findings. We insonated 25 patients without cerebrovascular diseases through the temporal bone window using 3DPD and CE3DPD; for comparison, 13 patients underwent MRA. Identification rates of vascular segments and of small branches of intracranial vessels were evaluated by two independent investigators blinded to MRA results. In 21 patients with adequate insonation conditions, CE3DPD significantly improved identification rates compared to 3DPD for the complete visualization of the P1 segment (80.9 vs. 19.0%, p < 0.005, P2 segment (80.9 vs. 42.8%, p < 0.05 and A1 segment (85.7 vs. 38.1%, p < 0.005). Furthermore, CE3DPD depicted, in significantly more examinations, branches of the middle (MCA) and posterior cerebral artery (PCA). Interobserver agreement was higher than 95% for the main intracranial segments and branches of the MCA, but relatively low (80.1-85.7%) for branches of the PCA. In comparison to CE3DPD, MRA identified only parieto-occipital branches of the PCA, temporal branches of the MCA, frontal branches of the anterior cerebral artery and the MCA bifurcation more frequently and accurately. In 4 patients with inadequate acoustic temporal bone windows, the application of a galactose-based microbubble suspension allowed clear 3-D visualization of almost all major intracranial vascular segments and some branches of the large arteries. In conclusion, CE3DPD is a more sensitive ultrasonic tool compared to unenhanced 3-D reconstructions. It makes 3-D ultrasound imaging of the basal cerebral circulation easier to perform and interpret, by providing an improved spatially oriented display of image position. As such, this method may increase operator diagnostic confidence level under pathologic conditions.

Power Doppler sonography: clinical applications

OBJECTIVE

Color Doppler imaging (CD) has had a great impact on ultrasonography (US). This technique depicts local flow by encoding an estimate of the mean Doppler frequency shift at a particular position in color. However, the choice of the mean frequency shift as the parameter for representing flow in color Doppler is somewhat arbitrary. Power Doppler ultrasound is a technique that encodes the power in the Doppler signal in color. This parameter is fundamentally different from the mean frequency shift. The frequency is determined by the velocity of the red blood cells, while the power depends on the amount of blood present. Providing an image of a different property of blood flow, power Doppler has shown several key advantages over colour Doppler, including higher sensitivity to flow, better edge definition and depiction of continuity of flow. In this paper we review the results of power Doppler clinical studies.

MATERIALS AND METHODS

All relevant information available in the literature on the potential clinical applications of this technique was revised to give a detailed survey.

RESULTS

The increased flow sensitivity and better vascular detailing of power Doppler have been used to detect flow presence and characteristics in vessels that are poorly imaged with conventional color Doppler. The improved depiction of tissue vasculature has shown potential advantages, especially in some areas, such as the cortex of native kidneys and renal allografts, the prepuberal testis, the infant hip and the bowel wall, in which color Doppler is not sensitive enough to detect clinically important, slow and poor flow in small vessels. In inflammatory conditions, power Doppler was valuable in depicting increased flow in vessels that are dilated because of inflammatory response. In this field, advantages have been reported in acute cholecystitis and in inflammatory states of musculoskeletal tissues. The higher sensitivity to slow flow and the improved detailing of the course of tortuous and irregular vessels made power Doppler a promising technique to image intratumoral vessels and, thereby, to ameliorate the accuracy of color Doppler in predicting the likelihood of benign versus malignant nature of nodules. Specific flow patterns, missed at color Doppler studies, have been indicated with power Doppler in some tumors of the liver and breast. In different settings, power Doppler also permitted to monitor serial blood flow changes after therapy and to display them as color intensity, allowing the observer to distinguish flow changes.

CONCLUSION

Although the actual role of power Doppler in changing patient management has not been assessed yet, this technique can depict flow which was previously undetectable, and thus permits an easier and more confident diagnosis in body regions where the ultrasound signal is weak because blood vessels are small.

Potential and limitations of echocontrast-enhanced ultrasonography in acute stroke patients: a pilot study

BACKGROUND AND PURPOSE

Ultrasonography (US) is a well-established method used to assess the brain-supplying arteries in the acute stroke setting. However, several technical and anatomic limitations are known to reduce its diagnostic accuracy and confidence level. Echocontrast agents (ECA) are known to improve the signal-to-noise ratio by enhancing the intensity of the reflecting Doppler signal. We undertook this prospective study to evaluate the diagnostic value of ECA in a consecutive, nonselected cohort of acute stroke patients with insufficient native US investigations.

METHODS

During a 1-year period, 25 patients were examined within 48 hours of the onset of stroke. The need for ECA was due to an insufficient transtemporal (n=18), transforaminal (n=4), or extracranial (n=3) imaging of arteries potentially involved in the ischemic event. In 12 patients, a diagnostic suspicion could natively be raised, whereas in the other 13 patients, the strongly reduced image quality did not allow for any neurovascular conclusions. Four grams of Levovist was injected at a concentration of 200 mg/mL and 400 mg/mL for the extracranial and transcranial insonations, respectively. The effect of the echocontrast enhancement was assessed with respect to (1) signal enhancement, (2) image quality, (3) final diagnostic confidence, and (4) the need for additional neurovascular imaging methods.

RESULTS

In all but one patient (96%), a strong signal enhancement was noted, leading to a moderate (n=11) or strong improvement (n=10) of the transcranial image quality. Thus in a total of 18 patients (72%), the echoenhancement provided a neurovascular diagnosis of sufficient confidence. This led to the confirmation of the previously suspected findings and disclosed three further occlusions and four stenoses of the intracranial arteries. In contrast, for the three extracranial examinations the image quality was not sufficiently improved because of persistent color artifacts derived from adjacent neck vessels. Besides the seven patients with inconclusive examinations, five patients with conclusive echoenhanced US studies (48% in total) demanded additive neurovascular imaging studies, based on the clinical decision of the attending physicians. This led to confirmation of all high-confident sonographic diagnoses.

CONCLUSIONS

In summary, in approximately three fourths of our acute stroke patients with insufficient native US investigations, echocontrast enhancement enabled a reliable neurovascular diagnosis, allowing the cancellation of additive neurovascular imaging procedures in half of our cohort. Our preliminary results suggest that ECA can reasonably support the early cerebrovascular workup in the acute stroke setting.

A flow model of cerebral aneurysms for use with power Doppler studies

It has recently been observed using power Doppler that cerebral aneurysms appear to change size through the cardiac cycle. The purpose of this study was to develop a pulsatile flow model of cerebral aneurysm expansion and to investigate whether the observations of pulsation could be confirmed using the model. The model consisted of a latex bubble glued onto the side of a latex tube. A computer controlled pump was used to generate pulsatile flow. The degree of bubble expansion was adjusted by the use of flow restrictor placed in the downstream section of the flow rig. Ultrasound images were acquired using an Acuson 128 XP/10V colour flow scanner. True expansion (maximum area/minimum area) was measured from the B-scan image using a 7 MHz probe. Three observers measured expansion of the simulated aneurysm from the power Doppler images using a 2 MHz probe. Expansion measured with the power Doppler decreased as the colour gain setting increased, and decreased as the persistence increased. The true expansion of 1.43 was comparable with the colour gain set optimally for a persistence value of 3. The model allows simple investigations of the relationship between true aneurysm expansion and that measured from the power Doppler images. Colour gain and persistence settings must be standardized in clinical studies.

Power Doppler compared to color-coded duplex sonography in the assessment of the basal cerebral circulation

Power-based transcranial duplex sonography (p-TDS) is a new promising ultrasound technique that generates intravascular color signals from the amplitude of the echo signal. The present investigation was undertaken to determine the advantages and limitations of power Doppler in the assessment of the basal cerebral circulation compared with transcranial color-coded real-time sonography (TCCS) and contrast-enhanced transcranial color-coded real-time sonography (CE-TCCS). Thirty-eight patients without cerebrovascular diseases were examined with p-TDS and TCCS, and in 11 patients CE-TCCS studies were performed. The M1 segment could be identified in 100% by both ultrasound techniques. p-TDS visualized M2 (67/70 vs 46/70, p < 0.0001), A2 (63/70 vs 46/70, p < 0.001), and P2 (67/70 vs 44/70, p < 0.0001) segments significantly more frequently and accurately compared to TCCS. The posterior communicating artery (25/70) and P3 segments (32/70) were only detectable by p-TDS and not by conventional TCCS. In comparison with CE-TCCS, p-TDS had no important advantages in the detection of intracranial vessels. In conclusion, p-TDS and CE-TCCS were superior to TCCS with regard to identification of the basal arterial circulation. Both methods permit noninvasive and reliable identification of the basal cerebral circulation.

Colour ultrasound imaging of blood flow and tissue motion

Recent years have seen the introduction of a high quality imaging modality which uses the Doppler shift for the study of blood flow and tissue motion. Colour ultrasound technology has now reached a level of maturity and it is, therefore, timely to review its features and consider how colour techniques may develop. This review concentrates on autocorrelator based colour systems. Recent developments are described including colour vector Doppler, contrast agents, 3D display, tissue vascularity assessment and volume flow measurement.

Insufficient and absent acoustic temporal bone window: potential and limitations of transcranial contrast-enhanced color-coded sonography and contrast-enhanced power-based sonography

The aim of this study was to investigate the diagnostic potential of contrast-enhanced transcranial color-coded sonography (CE-TCCS) and contrast-enhanced transcranial power-based sonography (CE-TPS) in patients with insufficient or absent acoustic bone windows (IABW). Due to temporal bone thickness, the basal cerebral circulation could not be insonated in 21 of 172 patients using unenhanced transcranial color-coded real-time sonography (TCCS) and transcranial power-based sonography. Additional CE-TCCS and CE-TPS were performed after application of 400 mg/ml galactose microbubble suspension. In both modalities, the use of echo-contrast agents allowed visualisation of the first segment of the middle cerebral artery (MCA) in all patients. The A1 segment of the anterior cerebral artery (67% in CE-TCCS; 81% in CE-TPS), P1 segment of the posterior cerebral artery (71% in CE-TCCS; 76% in CE-TPS) and the basilar artery (48% in CE-TCCS; 67% in CE-TPS) were depictable in the majority of the examinations. The M3 (5% in CE-TCCS; 33% in CE-TPS; p < 0.05), P2 (24% in CE-TCCS; 71% in CE-TPS; p < 0.005), P3 segments (0% in CE-TCCS; 43% in CE-TPS; p < 0.005) and the posterior communicating artery (5% in CE-TCCS; 33% in CE-TPS; p < 0.05) were detected in a significantly greater proportion of subjects using power Doppler. In conclusion, CE-TCCS and CE-TPS appear to be sensitive ultrasonic tools that provide reliable data regarding the basal cerebral circulation in patients with IABW. Furthermore, CE-TPS offers advantages over CE-TSSC in the identification of small-caliber arteries and vessels that run at unfavorable angels to the ultrasound beam. Both methods can overcome hyperostosis of the skull that is a major hindrance in transcranial ultrasonography, and may be helpful in the diagnosis of occlusive diseases of intracranial vessels.