Visualization of the cerebral circulation using three-dimensional transcranial power Doppler ultrasound imaging

Effect of Skull Porous Trabecular Structure on Transcranial Ultrasound Imaging in the Presence of Elastic Wave Mode Conversion at Varying Incidence Angle

With the advancement of aberration correction techniques, transcranial ultrasound imaging has exhibited great potential in applications such as imaging neurological function and guiding therapeutic ultrasound. However, the feasibility of transcranial imaging varies among individuals because of the differences in skull acoustic properties. To better understand the fundamental mechanisms underlying the variation in imaging performance, the effect of the structure of the porous trabecular bone on transcranial imaging performance (i.e., target localization errors and resolution) was investigated for the first time through the use of elastic wave simulations and experiments. Simulation studies using high-resolution computed tomography data from ex vivo skull samples revealed that imaging at large incidence angles reduced the target localization error for skulls having low porosity; however, as skull porosity increased, large angles of incidence resulted in degradation of resolution and increased target localization errors. Experimental results indicate that imaging at normal incidence introduced a localization error of 1.85 ± 0.10 mm, while imaging at a large incidence angle (40°) resulted in an increased localization error of 6.54 ± 1.33 mm and caused a single point target to no longer appear as a single, coherent target in the resulting image, which is consistent with simulation results. This first investigation of the effects of skull microstructure on transcranial ultrasound imaging indicates that imaging performance is highly dependent on the porosity of the skull, particularly at non-normal angles of incidence.

Editorial review: pediatric 3D ultrasound

Three-dimensional ultrasound is an established diagnostic imaging technique in many specialties. However, in neonates, infants and children three-dimensional ultrasound still is underutilized, partially due to time constraints for post-processing and restricted availability, of devices as well as dedicated pediatric transducers. Also reimbursement issues still need to be addressed. This editorial review presents more or less established pediatric three-dimensional ultrasound applications with proven diagnostic benefit as well as potential future applications of three-dimensional/four-dimensional ultrasound in infants and children, aiming at enhancing research and promoting practical use of three-dimensional ultrasound in relevant pediatric conditions. Particularly, applications in neonatal neurosonography, ultrasound of the urogenital tract as well as some other small part and miscellaneous queries are highlighted. Additional other potential and future indications are discussed briefly, also mentioning restrictions and potential future developments. In summary, three-dimensional ultrasound holds some potential to widen sonographic diagnostic capabilities throughout childhood and hopefully will be increasingly investigated and introduced into clinical practice provided respective equipment and pediatric three-dimensional/four-dimensional ultrasound transducers become available.

Potential role of 3DUS in infants and children

The objective of this review is to discuss basic technical aspects as well as potential pediatric applications of three-dimensional ultrasound (3DUS). Different 3DUS-techniques are already commercially available. Most commonly, a transducer-integrated motor drives a scan-head to acquire a volumetric dataset, which is then reconstructed and viewed using various post-processing techniques. It has been proved feasible to apply 3DUS with pediatric transducers in typical pediatric investigations. Based on our own experiences and on review of the literature, 3DUS can be successfully used in infants and children for brain, spine, cardiac, urinary tract (particularly assessment of hydronephrosis, and virtual cystoscopy), and female genital (e.g., for assessment of uterine malformations) US, and for various other applications, particularly in small parts. Power Doppler data can be integrated, allowing 3DUS-angiography. Thus, 3DUS promises to become a useful adjunct for imaging children, particularly as it enhances ultrasound by offering additional, previously inaccessible planes, rendering options and surface assessments. 3DUS should be increasingly exploited, thus hopefully helping reduce the need for more invasive or burdening (e.g., ionizing radiation) investigations in children.

Three-dimensional ultrasonography-based virtual cystoscopy of the pediatric urinary bladder: a preliminary report on feasibility and potential value

OBJECTIVE

The purpose of this study was to validate the feasibility and potential of 3-dimensional ultrasonography (3DUS)-based virtual cystoscopy in the pediatric urinary bladder.

METHODS

Twenty patients (age range, newborn-14 years) underwent urinary tract ultrasonography and 3DUS of the urinary bladder. From this data set, virtual cystoscopy was reconstructed for visualization of the inner bladder surface. Three-dimensional ultrasonography was compared with 2-dimensional ultrasonographic (2DUS) findings, voiding cystourethrography (VCUG) results, and reports from cystoscopy or surgery when available.

RESULTS

Three-dimensional ultrasonography was feasible in all patients. Data quality was sufficient for virtual cystoscopy without major motion artifacts. The 3DUS results matched all other findings; particularly, 3DUS superiorly visualized the ureteral ostium and the bladder neck configuration; in 5 patients, 3DUS depicted pathologically shaped ostia not detected by 2DUS. This correlated with the presence of vesicoureteral reflux on VCUG. Performing virtual cystoscopy added 1 minute to the investigation time (range, 0.5-2 minutes) and 3 minutes for postprocessing and viewing (range, 2-5 minutes).

CONCLUSIONS

Three-dimensional ultrasonography-based virtual cystoscopy is feasible in the pediatric urinary bladder without sedation. It reveals surface information not accessible by 2DUS, improving detection of pathologic conditions such as atypically shaped ureteral ostia. Three-dimensional ultrasonography-based cystoscopy may become a valuable adjunct to 2DUS of the pediatric urinary tract, improving selection criteria for further imaging such as VCUG, and potentially may help reduce the need for endoscopic cystoscopy. However, these preliminary results still have to be confirmed in prospective studies with larger patient numbers.

Potential of three-dimensional ultrasound in neonatal and paediatric neurosonography

The aim of this study was to describe the potential of three-dimensional ultrasound (3D US) in paediatric and neonatal neurosonography. The potential applications are illustrated based on our experience in 150 patients using three different 3D US techniques at two different sites. Various disease entities throughout the paediatric age have been evaluated. The potential of 3D US, including 3D US of the cerebral vessels based on colour Doppler data, is discussed based on comparison with conventional 2D US or other imaging (as available), and with regard to the literature. In our experience, 3D US is feasible in neonatal and paediatric neurosonography. It reduces imaging time, improves demonstration of complex anatomy and vasculature, and allows for evaluation of anatomy/pathology in any plane. The 3D US furthermore improves volume assessment (e.g. in hydrocephalus), and comparison with CT, MRI and during follow-up, with a potentially improved standardisation and documentation. The 3D US additionally offers an ideal modality for training and education, as the brain and the neonatal spine can be virtually rescanned at the workstation. Yet, limitations such as areas inaccessible to 2D US, limited resolution and motion artefacts have to be acknowledged. Three-dimensional US has the potential to become a valuable additional imaging tool in paediatric neurosonography.

Virtual endoscopy of the cerebral ventricles based on 3-D ultrasonography

Virtual endoscopy enables preoperative surgical planning based on "surgeons' view" information in the individual patient. In neurosurgery, magnetic resonance (MR) images are mainly used for planning of virtual neuroendoscopy (VNE). We studied the feasibility of three-dimensional (3-D) ultrasonography as the imaging modality for VNE in pediatric patients with hydrocephalus. 3-D ultrasonography data sets were obtained through the open anterior fontanelle and analyzed using perspective volume rendering, with delineation of the ventricular system for anatomical details in relation to standard ultrasonography and intraoperative anatomy, during endoscopy in two infants with hydrocephalus. VNE clarified anatomical variants seen on standard ultrasonography images, anticipated ventricular dysmorphia seen during neuroendosopy and enabled a realistic impression of an endoscopic inspection into the ventricular system of the two infants studied. Based on 3-D ultrasonography, VE enables detailed information on ventricular anatomy in pediatric patients for planning of endoscopic interventions.

Three-dimensional intraoperative ultrasound of vascular malformations and supratentorial tumors

The benefits and limits of a magnetic sensor-based 3-dimensional (3D) intraoperative ultrasound technique during surgery of vascular malformations and supratentorial tumors were evaluated. Twenty patients with 11 vascular malformations and 9 supratentorial tumors undergoing microsurgical resection or clipping were investigated with an interactive magnetic sensor data acquisition system allowing freehand scanning. An ultrasound probe with a mounted sensor was used after craniotomies to localize lesions, outline tumors or malformation margins, and identify supplying vessels. A 3D data set was obtained allowing reformation of multiple slices in all 3 planes and comparison to 2-dimensional (2D) intraoperative ultrasound images. Off-line gray-scale segmentation analysis allowed differentiation between tissue with different echogenicities. Color-coded information about blood flow was extracted from the images with a reconstruction algorithm. This allowed photorealistic surface displays of perfused tissue, tumor, and surrounding vessels. Three-dimensional intraoperative ultrasound data acquisition was obtained within 5 minutes. Off-line analysis and reconstruction time depends on the type of imaging display and can take up to 30 minutes. The spatial relation between aneurysm sac and surrounding vessels or the skull base could be enhanced in 3 out of 6 aneurysms with 3D intraoperative ultrasound. Perforating arteries were visible in 3 cases only by using 3D imaging. 3D ultrasound provides a promising imaging technique, offering the neurosurgeon an intraoperative spatial orientation of the lesion and its vascular relationships. Thereby, it may improve safety of surgery and understanding of 2D ultrasound images.

Dynamic and three-dimensional transcranial ultrasonography of an arachnoid cyst in the cerebral convexity. Technical note

Structural imaging of the brain, such as cerebral computerized tomography (CT) and magnetic resonance (MR) imaging, is state-of-the-art. Dynamic transcranial (dTC) ultrasonography and three-dimensional (3D) transcranial color-coded duplex (TCC) ultrasonography are complementary, noninvasive procedures with the capacity for real-time imaging, which may aid in the temporary management of space-occupying lesions. A 16-year-old woman presented with recurrent tension-type headaches. A space-occupying arachnoid cyst in the cerebral convexity was demonstrated on MR images. The patient underwent an examination for raised intracranial pressure. which was performed using a standard color-coded duplex ultrasonography system attached to a personal computer-based system for 3D data acquisition. Transcranial ultrasonography was used to identify the outer arachnoid membrane of the cyst, which undulated freely in response to rotation of the patient's head (headshake maneuver). Three-dimensional data sets were acquired and, using a multiplanar reformatting reconstruction algorithm, the authors obtained high-resolution images that corresponded to the initial MR image and a follow-up cranial CT scan. No detectable differences were observed on dTC or 3D TC ultrasonograms obtained at follow-up examinations performed 9 and 28 months later. Three-dimensional TCC and dTC ultrasonography may complement conventional diagnostic procedures such as MR and CT imaging. This report represents evidence of the high resolution and good reproducibility of 3D TC methods. Ultrasonography is a mobile and inexpensive tool and may be used to improve management and therapeutic strategies for patients with space-occupying brain lesions in selected cases.

Results of carotid endarterectomy performed with preoperative duplex ultrasound assessment alone

Contrast injection cerebral angiography has been considered for several decades the "gold-standard" technique for diagnosis and operative planning of carotid disease. More recently, however, an increasing number of institutions are using duplex ultrasound as the single independent preoperative test. The objective of this investigation was to evaluate the impact of the utilization of duplex ultrasonography as the only preoperative test on the outcome of the procedure. Between 1993 and 1996, the authors performed 1,149 carotid procedures. Duplex ultrasound as the only preoperative test was employed with increasing frequency in a total of 728 cases. In 1995 and 1996, a cerebral arteriogram was performed only if duplex ultrasound was technically inadequate or questionable or showed an atypical pattern of disease. During the 4 years analyzed in this study, the number of the procedures increased from 165 in 1993 to 412 in 1996. The thirty-day mortality rate was 0.43%, and neurologic morbidity was 1.65%. According to the year in which the procedure was performed, the mortality/morbidity rates were 1.2/2.4 in 1993, 0.52/2.08 in 1994, 0.26/1.57 in 1995, and 0.24/1.21 in 1996. Indication to perform an arteriogram became very selective in 1995. Regardless of these changes in the diagnostic work-up, some degree of reduction in both 30-day mortality and neurologic morbidity was recorded. Considering a cost of 724 European Currency Units (ECU) per arteriogram, 527,072 ECU were saved in this period. In the last 4 years, duplex ultrasound has replaced arteriography as the first-choice technique for preoperative assessment of carotid disease at the authors' institution. There was definitely no detrimental effect on the clinical results that, on the contrary, improved during the same period. This policy has allowed a significant reduction in the cost of the procedure and has most likely prevented several arteriography-related complications. The authors recommend this policy to all institutions in which accurate duplex ultrasound is available.

Sonographic parenchymal and brain perfusion imaging: preliminary results in four patients following decompressive surgery for malignant middle cerebral artery infarct

To investigate new methods of diagnostic transcranial sonography for brain parenchymal, vascular and perfusion imaging, we performed 3-D native tissue harmonic transcranial sonography (3D-nthTCS), 3-D transcranial color-coded duplex sonography (3D-TCCS), and "loss-of-correlation" imaging (LOC-TCCS) in four patients following early hemicraniectomy due to space-occupying "malignant" middle cerebral artery infarction (MMCAI). Three-dimensional datasets, utilizing 3D-nthTCS and 3D-TCCS, were created and up to 10 axial 2-D B-mode image planes, similar to CCT, reconstructed in each patient. Three-dimensional reconstructions of the circle of Willis documented one persistent carotid-T occlusion and three recanalizations of the MCA. LOC-TCCS, based on stimulated acoustic emission from an ultrasound (US) contrast agent, demonstrated a perfusion deficit in 2 of 3 patients, with regard to their infarcts. Concluding, 3D-nthTCS, 3D-TCCS and LOC-TCCS are promising tools for bedside monitoring, early prognosis and treatment evaluation for MMCAI in the postoperative period. Further studies should be performed to standardize these new methods and evaluate their applications through the intact calvarina.

Three-dimensional transcranial colour-coded duplex sonography of the transverse sinus

The case of a patient with partial superior sagittal sinus and partial right sided transverse sinus thrombosis, and posterior fossa venous collaterals is presented. We report on the use of venous transcranial colour-coded duplex sonography (TCCS), combined with a new three-dimensional transcranial duplex data acquisition system, which allows free hand scanning of the region of interest. TCCS with three-dimensional image reconstruction allowed a more precise spatial localization of venous flow signals in the posterior fossa and facilitated the understanding of the haemodynamics of the venous collateral network.

Dynamic and three-dimensional transcranial sonography studies of an asymptomatic, cerebral convexity arachnoid cyst

Dynamic and three-dimensional transcranial sonography (dTCS and 3D-TCCS) examinations are complementary, noninvasive methods used in the assessment and follow up of patients with cerebrospinal fluid (CSF) circulation disorders.

A 16-year-old female patient who presented with a space-occupying, cerebral convexity arachnoid cyst and recurrent tension-type headache underwent examination for raised intracranial pressure (ICP) by using a standard color-coded duplex sonography system attached to a personal computer–based system for 3D data acquisition. Conventional TCS identified the outer arachnoid membrane of the cyst, which undulated freely after short rotation of the head (“headshake maneuver”). The undulation was documented as a QuickTime movie that is included with this article. A 3D dataset was acquired and, by using a multiplanar reformatting reconstruction algorithm, the authors obtained images with excellent resolution that corresponded to an initial magnetic resonance (MR) imaging study. No differences on dTCS and 3D-TCS were detectable at follow up 9 months later, indicating normal ICP and a stable, benign condition.

The use of 3D-TCCS and dTCS ultrasonography may complement other diagnostic procedures such as MR imaging and, thus, can improve management and therapeutic strategies for patients with CSF circulation disorders. In this report the authors present evidence of the excellent fine resolution and exact reproducibility of reconstructed ultrasound image planes derived from 3D datasets and the additional biomedical information from dynamic imaging.

Three-dimensional transcranial color-coded sonography of cerebral aneurysms

BACKGROUND AND PURPOSE

The role of 2-dimensional transcranial color-coded sonography (2D-TCCS) as a diagnostic tool in cases of vascular alteration is unquestioned. The skill of the operator, however, may be responsible for some intertrial variability. The clinical value of a new, workstation-based, 3D reconstruction system for TCCS was evaluated in patients with intracranial aneurysms.

METHODS

Thirty patients with 30 intracranial aneurysms were investigated (8 men, 22 women; mean+/-SD age 54+/-17 years). The TCCS examinations were performed with a 2-MHz probe using the power mode. The 3D system (3D-Echotech, Germany) consisted of an electromagnet, which induced a low-intensity magnetic field near the head of the patient. A magnetic position sensor was attached to the ultrasound probe and transmitted the spatial orientation of the probe to a workstation, which also received the corresponding 2D-images from the video-port of the duplex machine. The echo contrast enhancer D-galactose (Levovist, Schering, Germany) was used in all patients to improve the signal-to-noise ratio. All patients underwent presurgical digital subtraction angiography (DSA) to demonstrate the aneurysm.

RESULTS

Twenty-nine of 30 angiographically proven intracranial aneurysms (97%) were detected by 3D-TCCS. The aneurysmal diameter estimated by DSA ranged from 3 to 16 mm (mean 7. 2+/-3.6 mm). A comparison of the 3 main diameters of each aneurysm revealed a correlation coefficient of 0.95 between DSA and 3D-TCCS. The 3D determination of the aneurysmal size by 2 experienced sonographers correlated with 0.96.

CONCLUSIONS

3D-TCCS is a new, noninvasive method to investigate intracranial aneurysms. The differentiation between artifacts and true changes of the vessel anatomy is much easier in 3D-TCCS than in conventional 2D-TCCS. The new method yields an excellent correlation with the gold standard, DSA. Because the same 3D-TCCS data can be postprocessed by different investigators, it may be possible to improve reproducibility and increase the objectivity of transcranial color-coded duplex sonography.

Effect of echo contrast media on the visualization of transverse sinus thrombosis with transcranial 3-D duplex sonography

Transcranial duplex sonography has the capacity of detecting venous flow as in the transverse sinus. During a 6-month period, 28 consecutive patients (mean age 55 y) with a clinically suspected diagnosis of cerebral sinus thrombosis were included in the study. All patients were examined using 3-D ultrasound equipment within 24 h of having undergone either venous computerized tomography (CT), venous magnetic resonance imaging (MRI) or cerebral angiography. A total of 22 healthy patients had a normal venous CT, venous MRI or cerebral angiography of both transverse sinuses. Before echo contrast enhancement, the transverse sinus could be visualized in only 2 of these 44 sinuses (22 patients). A total of 6 patients with an unilaterally missed transverse sinus in 3-D ultrasound suffered from sinus thrombosis (n = 3), hypoplasia (n = 2) or aplasia (n = 1) of the unilateral transverse sinus in neuroradiological tests. In none of the patients with an thrombosis of the transverse sinus did ultrasound contrast media application improve the visualization of the affected sinus. Our study confirms that the normal transverse sinus, insonated through the contralateral temporal bone, often cannot be visualized without the use of contrast agents. With transcranial 3-D duplex sonography, a differentiation between thrombosis, hypoplasia and aplasia of the sinus was not possible.

3-D power Doppler cerebral angiography in neonates and young infants: comparison with 2-D power Doppler angiography

The aims of this study were to evaluate the ability of 3-dimensional (3-D) power Doppler angiography (3DPDA) to depict the intracranial vasculature in infants, to compare with 2-D power Doppler ultrasonography (2DPDU), and to explore the potential clinical applications of this procedure in young infants with brain disorders. We performed 3DPDA in 27 infants. 2DPDU were completed in both sagittal and coronal directions in 12 of these patients. In the other 15, only right sagittal plane images were available for comparison. Using a grading system and with only vessels with more than half of the length demonstrated included for comparison, we compared the Doppler signals of major vessels. 3DPDA could have good visualization in more than 60% of the internal carotid artery, ophthalmic artery, pericallosal artery, callosomarginal artery, internal cerebral vein, vein of Galen, and straight sinus in the sagittal plane. 3DPDA also could have good demonstration in about 50% of basilar artery in coronal plane, and posterior communicating artery, posterior cerebral artery, and lenticulostriate artery in sagittal plane. 3DPDA was better than 2DPDU in demonstrating all the major intracranial vessels in different planes, except the anterior communicating artery. In the anterior communicating artery, neither can demonstrate more than 30%.

Cerebrovascular ultrasound

Rapid progress in noninvasive ultrasound techniques has resulted in a wide variety of clinical applications for assessment of both extracranial and intracranial arterial diseases. Recent highlights in cerebrovascular ultrasound research include imaging methods for characterization of intracranial aneurysms, use of echocontrast agents for improved evaluation of acute stroke patients and transient response harmonic imaging for depiction of brain perfusion. The important role of transcranial Doppler microembolism detection in carotid endarterectomy has been defined, new approaches to noninvasive Doppler measurement of intracranial pressure are progressing, and the clinical indications for transcranial Doppler monitoring of intracranial vasospasm to prevent secondary stroke have expanded. New functional transcranial Doppler applications, which are complementary to positron emission tomography and functional magnetic resonance imaging studies, are evolving for evaluation of functional recovery after stroke; investigation of perfusion asymmetries during complex spatial tasks; assessment of hemispheric dominance in surgical candidates for epilepsy surgery; and elucidation of temporal patterns of regional neuronal activity. With increasing sophistication of cerebrovascular ultrasound methodology, it is essential that standards for data acquisition and interpretation be established. Three recent consensus meetings have provided detailed recommendations on quantification of carotid artery stenosis, on characterization of carotid artery plaques and on microembolism detection by transcranial Doppler.

Three-dimensional ultrasound imaging

The objective of this article is to provide scientists, engineers and clinicians with an up-to-date overview on the current state of development in the area of three-dimensional ultrasound (3-DUS) and to serve as a reference for individuals who wish to learn more about 3-DUS imaging. The sections will review the state of the art with respect to 3-DUS imaging, methods of data acquisition, analysis and display approaches. Clinical sections summarize patient research study results to date with discussion of applications by organ system. The basic algorithms and approaches to visualization of 3-D and 4-D ultrasound data are reviewed, including issues related to interactivity and user interfaces. The implications of recent developments for future ultrasound imaging/visualization systems are considered. Ultimately, an improved understanding of ultrasound data offered by 3-DUS may make it easier for primary care physicians to understand complex patient anatomy. Tertiary care physicians specializing in ultrasound can further enhance the quality of patient care by using high-speed networks to review volume ultrasound data at specialization centers. Access to volume data and expertise at specialization centers affords more sophisticated analysis and review, further augmenting patient diagnosis and treatment.

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.

Transcranial Doppler: state of the art

INTRODUCTION

Transcranial color Doppler sonography permits the accurate assessment of intracranial arteries. The latest Doppler units, using the color and power techniques, can show even very small flow volumes (1 x 1 mm). Low frequency (2-2.5 MHz) and very focused transducers are used in transcranial color Doppler. The skull is a very strong barrier for ultrasounds, which requires the use of some acoustic windows like some thin portions of the skull bone or some natural skull foramina. The use of echocontrast agents in color Doppler seems to increase the applications of transcranial studies.

OBJECTIVE

(1) To report on transcranial color Doppler technique and findings. (2) To assess the role of contrast agents in the visualization of intracranial vessels. (3) To define the main indications of this technique.

MATERIAL AND METHODS

The temporal, the orbital and the suboccipital are the main acoustic windows used for transcranial color Doppler studies. We use phased-array transducers (2-2.5 MHz) and, preferrably, the echocontrast agent. We examined 15 patients with severe internal carotid artery stenoses after the infusion of Levovist (Schering AG, Berlin, Germany) suspension (8 ml at 300 mg Galactose/ml, infused at 0.5 ml/s).

RESULTS

Levovist infusion permitted to depict the main intracranial vessels in all cases. The middle and the anterior cerebral arteries are shown through the temporal window. The former is the main cerebral artery, it is the easiest to identify and presents the highest peak systolic velocity. The orbital window can be used to visualize the ophthalmic artery and the internal carotid artery siphon, while the vertebral and the basilar arteries are demonstrated through the suboccipital window.

DISCUSSION

We report the most important findings and discuss the main indications of transcranial color Doppler studies. In addition to flow presence and direction, the main indices of arterial flow can be measured thanks to contrast agent administration, namely the peak systolic velocity, the end diastolic velocity, the resistance index and the pulsatility index. A morphological assessment of the Willis circle can also be carried out with color and power Doppler. Functional studies can be performed to assess the residual autoregulatory function of the cerebral circle in the patients with internal carotid artery stenosis or occlusion. The development of intracranial collateral circles can also be studied in these patients. Moreover, the M1 segment of the middle cerebral artery and the internal carotid artery siphon can be demonstrated directly. Transcranial color Doppler is also a useful tool to detect vasospasm after subarachnoid hemorrhage and to monitor blood flow velocity in the middle cerebral artery during carotid endarterectomy. The assessment of blood supply to arteriovenous malformations and to intracranial neoplasms is another application.

CONCLUSION

With reference to internal carotid stenoses, the main applications of transcranial color Doppler are the study of intracranial vessels, of intracranial arterial stenosis, of arteriovenous malformations and of Willis circle aneurysms, as well as the monitoring of blood flow velocity during carotid endarterectomy. Echocontrast agents play an important role in the visualization of intracranial vessels.

The latest in ultrasound: three-dimensional imaging. Part II

INTRODUCTION

The three-dimensional (3D) reconstruction of ultrasound images has become a widespread option in ultrasound equipment. Specific softwares have become available and 3D reconstruction feasible since the early 1990s, particularly since 1994.

POSSIBLE CLINICAL APPLICATIONS

Several clinical applications are feasible in all parenchymatous organs (mainly the liver and prostate), hollow viscera (e.g. the bladder and gallbladder), peripheral vessels (supra-aortic trunks and limb vessels) and central (the aorta and iliac arteries) or cerebral vessels. Moreover, tumoral vessels in parenchymatous organs can be reconstructed, and even the fetus in the uterine cavity, with excellent detailing. The recent introduction of echocontrast agents and second harmonic imaging has permitted to study normal and abnormal peripheral, central and parenchymatous vessels, with similar patterns to those obtained with digital angiography. The spatial relationships between the vascular structures of the liver, kidney and placenta were studied with 3D ultrasound angiograms. The applications of this new technique include the analysis of vascular anatomy and the potential assessment of organ perfusion. THE LATEST APPLICATIONS--INTRAVASCULAR STUDIES: Some catheters with an ultrasound transducer in the tip have been tested for intravascular studies. Just like conventional transducers, they provide two-dimensional (2D) images which are then postprocessed into longitudinal 3D or volume reconstructions. The former resemble angiographic images and can be viewed 3D rotating the image along its longitudinal axis. Volume images, which are more complex and slower to obtain, can be rotated on any spatial plane and provide rich detailing of the internal vascular lumen. The clinical importance of intravascular ultrasound with 3D volume reconstructions lies in the diagnosis of vascular conditions and the assessment and monitoring of intravascular interventional procedures--e.g. to detect inaccurate deployment of intravascular stents and endoluminal grafts during the maneuver. Three-dimensional reconstructions involve geometric data assembly and volumetric interpolation of a spatially related sequence of tomographic cross sections generated by an ultrasound catheter withdrawn at a constant rate through a vascular segment of interest, resulting in the display of a straight segment. Therefore particular care is needed and there are some useful hints to avoid mistakes.

CONCLUSIONS

Three dimensional reconstructions of B-mode and color Doppler images are no longer a work in progress and their clinical importance and possible applications are both established and ever-increasing. On the other hand, independent of the different types of energy used, also computed tomography and magnetic resonance 3D reconstructions are very useful from a clinical viewpoint and they have become an established routine technique for both these methods. It is very likely that 3D volume reconstructions in ultrasound will find numerous applications in the near future. They may help to increase the diagnostic confidence and to facilitate diagnosis, intraprocedure monitoring in interventional radiology and follow-up and also to reduce the number of invasive examinations with iodinated contrast agents. This could result in cutting the cost and duration of the most expensive examinations. New, although invasive, applications can be hypothesized for intravascular or intraluminal catheters with an ultrasound transducer inside.