The diagnostic relevance of colour Doppler artefacts in carotid artery examinations

Factors Influencing Long-Term Blood Flow in Extracranial-to-Intracranial Bypass for Symptomatic Internal Carotid Artery Occlusive Disease: A Quantitative Study

BACKGROUND

Maintaining the patency of extracranial-to-intracranial (EC-IC) bypass is critical for long-term stroke prevention. However, reports on the factors influencing long-term bypass patency and quantitative assessments of bypass patency are limited.

OBJECTIVE

To quantitatively evaluate blood flow in EC-IC bypass using four-dimensional (4D) flow magnetic resonance imaging (MRI) and investigate factors influencing the long-term patency of EC-IC bypass.

METHODS

Thirty-six adult Japanese patients who underwent EC-IC bypass for symptomatic internal carotid or middle cerebral artery occlusive disease were included. We examined the relationships between decreased superficial temporal artery (STA) blood flow volume and perioperative complications, long-term ischemic complications, patient background, and postoperative antithrombotic medications in patients for whom STA flow could be quantitatively assessed for at least 5 months using 4D flow MRI.

RESULTS

The mean follow-up time was 54.7 ± 6.1 months. One patient presented with a stroke during the acute postoperative period that affected postoperative outcomes. No recurrent strokes were recorded during long-term follow-up. Two patients died of malignant disease. Seven cases of reduced flow occurred in the STA, which were correlated with single bypass (P = .0294) and nonuse of cilostazol (P = .0294). STA occlusion was observed in 1 patient during the follow-up period. Hypertension, age, smoking, dyslipidemia, and diabetes mellitus were not correlated with reduced blood flow in the STA.

CONCLUSION

Double anastomoses and cilostazol resulted in long-term STA blood flow preservation. No recurrence of cerebral infarction was noted in either STA hypoperfusion or occlusion cases.

Reflections on the Ultrasound Mirror Image Artifact

A key assumption in ultrasound is that the reflected echoes that return to the transducer travel in a straight line. Mirror image artifacts occur when the transmitted pulse and returning echo reflect off of a highly reflective interface (an acoustic mirror) and change direction before returning to the transducer, thereby breaking this assumption. Mirror image artifacts are seen throughout the body on gray scale, color Doppler, power Doppler, and spectral Doppler. They may closely duplicate the true structure in shape and echo strength, may distort the true structure, may appear weaker than the true structure, or may appear on images that do not simultaneously show the true structure. If not properly recognized, mirror image artifacts can be misinterpreted as true pathology and lead to additional unnecessary tests and potentially harmful interventions.

Assessment of carotid artery ultrasonography in the presence of an acoustic shadow artifact

Background

B-mode and Color Doppler ultrasonography (CDUS) are the methods of choice for screening and determining the degree of Carotid artery stenosis. The evaluation of stenosis with calcification may be hampered by a common CDUS artifact known as acoustic shadow (AS). Our objective was to assess the change in reliability of CDUS readings in the presence of an AS artifact.

Methods

Single center retrospective observational study. Included were patients with either an AS artifact or high-grade stenosis (defined by peak systolic velocity (PSV) > 240 cm/s) demonstrated in CDUS, and had a CT angiography (CTA) done within 6 months of the sonographic exam. All subjects were identified through the Tel-Aviv Sorasky medical center (TASMC) CDUS unit registry from which clinical information was extracted. CDUS images were manually reviewed grading AS magnitude. All CTAs were reviewed and reconstructed for accurate assessment of percent stenosis and were used as gold standard.

Results

The study cohort included 227 consecutive patients (corresponding with 454 internal carotid arteries) meeting inclusion criteria. 43.2% of the arteries (n = 195) had an AS artifact present on CDUS, regardless of percent stenosis, with a large artifact present in 6.7% arteries (n = 30). Older age was significantly related to the presence of AS artifact (p < 0.001). In the study cohort as a whole there was a strong correlation between percent stenosis on CTA and PSV values (Pearson’s r 0.672, p < 0.001) regardless of AS existence. The CDUS sensitivity and specificity for predicting severe stenosis were 82 and 73% respectively. The presence of a small AS slightly diminished the correlation between CDUS and CTA results without compromising CDUS reliability. A large AS severely affected the correlation between CDUS and CTA exams (Pearson’s r = 0.24, p = 0.27) and reduced CDUS reliability with a sensitivity and specificity of 62%.

Conclusion

The presence of a large AS severely degrades the accuracy of the routine CDUS measurements. In these cases, the patient should be referred to a CDUS exam including doppler-measurement of periorbital arteries and intracranial arteries in addition to other imaging modalities such as CTA or MRA in order to assess future stroke risk.

A comparative study of color Doppler imaging and contrast-enhanced ultrasound for the detection of ulceration in patients with carotid atherosclerotic disease

OBJECTIVES

To evaluate the diagnostic accuracy of color Doppler imaging (CDI) and contrast-enhanced ultrasound (CEUS) for diagnosing carotid ulceration, having multi-detector computed tomography angiography (MDCTA) as the reference method.

METHODS

Patients with carotid disease referred for ultrasound (US), either due to the occurrence of neurovascular symptoms or for screening purposes, were included in this study if at least one plaque causing moderate (50-69%) or severe (70-99%) internal carotid artery stenosis was detected. Carotid US with CDI technique, CEUS, and MDCTA were performed in all patients, investigating the presence of ulceration. The agreement between modalities was evaluated using kappa statistics.

RESULTS

The study population included 54 patients (median age 62 years, inter-quartile range 16.2) and 66 carotid arteries. The mean degree of stenosis was 68.5% (SD 12.2%) while 47.1% of plaques were symptomatic. MDCTA characterized 28.8% of plaques as smooth, 45.5% irregular, and 24.3% ulcerated. Flow reversal was detected with CDI in 65.5% of ulcerations, while swirling of the microbubbles and neovessels adjacent to the ulcer were detected with CEUS in 17.64%. The agreement for ulceration diagnosis was moderate between CDI and CEUS (kappa 0.473) and between CDI and MDCTA (kappa 0.473) and very good between CEUS and MDCTA (kappa 0.921). The sensitivity, specificity, and positive and negative predictive values of CDI for the diagnosis of ulceration were 41.2%, 97.95%, 87.5%, 82.8% respectively, while CEUS respective measures were 94.1%, 97.95%, 94.1%, and 97.95%.

CONCLUSION

CEUS outperformed CDI in terms of agreement with MDCTA and diagnostic accuracy for the diagnosis of ulcerated carotid plaque.

KEY POINTS

• Superficial ulceration is a significant feature of carotid plaque vulnerability. • Color Doppler imaging has the potential to demonstrate carotid plaque ulceration but is characterized by limited sensitivity and moderate agreement with the reference method of multi-detector computed tomography angiography. • Contrast-enhanced ultrasound outperforms color Doppler imaging in terms of sensitivity for the detection of carotid plaque ulceration and in agreement with the reference method of multi-detector computed tomography angiography.

Imaging of the ulcerated carotid atherosclerotic plaque: a review of the literature

Abstract

Carotid atherosclerotic disease constitutes a major modern health problem whose diagnosis primarily relies on imaging. Grading of stenosis has been long used as the main factor for risk stratification and guiding of management. Nevertheless, increasing evidence has shown that additional plaque characteristics such as plaque composition and surface morphology play an important role in the occurrence of symptoms, justifying the term “vulnerable plaque”. Carotid plaque surface characteristics either in the form of surface irregularities or ulceration represent an important factor of vulnerability and are associated with the occurrence of neurologic symptoms. The delineation of the carotid plaque surface can be performed with virtually all imaging modalities including ultrasound, contrast-enhanced ultrasound, multi-detector computed tomography angiography, magnetic resonance angiography and the traditional reference method of angiography. These techniques have shown varying levels of diagnostic accuracy for the identification of ulcerated carotid plaques or plaque surface irregularities. As a consequence and given its high clinical significance, radiologists should be familiar with the various aspects of this entity, including its definition, classification, imaging findings on different imaging modalities and associations. The purpose of this review is to present the current literature regarding carotid plaque ulcerations and present illustrative images of ulcerated carotid plaques.

Teaching Points

• Plaque surface and ulceration represent risk factors for stroke in carotid disease.

• Characterisation of the plaque surface and ulcerations can be performed with every modality.

• US is the first-line modality for carotid disease and identification of ulcerations.

• The administration of microbubbles increases US accuracy for diagnosis of carotid ulceration.

• MDCTA and MRA are valuable for diagnosing ulceration and evaluating plaque composition.

Acoustic shadowing impairs accurate characterization of stenosis in carotid ultrasound examinations

OBJECTIVE

Duplex ultrasonography (DUS) has been the mainstay for diagnosing carotid artery stenosis and is often the sole diagnostic modality used prior to intervention. Highly calcified plaque, however, results in an acoustic shadow (AcS) that obscures the vessel lumen and inhibits the sonographer's ability to obtain Doppler velocity measurements. It is unknown whether DUS can accurately determine the degree of carotid stenosis in these settings.

METHODS

From July 2012 to December 2013, all patients with AcS on DUS measuring ≥5 mm in the longitudinal axis were cross-referenced with multidetector computed tomographic angiography (MD-CTA) images of the neck to define the study population. After standardizing the MD-CTA windows, percent stenosis was determined by cross-sectional area reduction using two separate previously described methods based on North American Symptomatic Carotid Endarterectomy Trial (NASCET) and European Carotid Surgery Trial (ECST) criteria. DUS waveform parameters in the internal carotid artery near the AcS were then compared with these MD-CTA measurements to determine the accuracy of DUS in characterizing the severity of carotid stenosis.

RESULTS

During this period, 8517 DUS studies were performed at the Massachusetts General Hospital, 550 of which had AcS, for an incidence of 6.45%. There were 92 lesions with a concomitant MD-CTA; however, seven were excluded because of poor study quality, because ≥6 months had elapsed between DUS and MD-CTA, or because the patient had undergone carotid reconstruction between studies. Of the 85 remaining lesions, DUS characterized 17 as severe (peak systolic velocity [PSV] >250 cm/s), 31 as moderate (PSV = 151-250 cm/s), and 37 as mild (PSV ≤150 cm/s) stenoses using PSV criteria. PSV weakly correlated with CTA-NASCET (r = 0.361; P = .004) and CTA-ECST (r = 0.306; P = .004) percent stenosis. Using PSV >250 cm/s as the predictor of >70% stenosis, and a ≥70% cutoff by both CTA-ECST and CTA-NASCET methods as the reference measure, DUS sensitivity ranged from 22.7% to 32.5%, specificity from 89.4% to 91.1%, positive predictive value from 88.2% to 76.4%, and negative predictive value from 25% to 60.2%. A subgroup analysis of lesions identified as non-severe by DUS revealed that waveforms with lower deceleration were associated with severe stenosis on CTA.

CONCLUSIONS

In the presence of AcS, DUS alone is inadequate to accurately determine the degree of carotid stenosis with sensitivity, specificity, and negative predictive values far below that needed for clinical decision-making. MD-CTA may be necessary for improved characterization of plaque in these AcS lesions. Further studies are needed to determine DUS parameters that may identify patients who should undergo further evaluation with MD-CTA to characterize the true severity of the stenosis.

Vector projectile imaging: time-resolved dynamic visualization of complex flow patterns

Achieving non-invasive, accurate and time-resolved imaging of vascular flow with spatiotemporal fluctuations is well acknowledged to be an ongoing challenge. In this article, we present a new ultrasound-based framework called vector projectile imaging (VPI) that can dynamically render complex flow patterns over an imaging view at millisecond time resolution. VPI is founded on three principles: (i) high-frame-rate broad-view data acquisition (based on steered plane wave firings); (ii) flow vector estimation derived from multi-angle Doppler analysis (coupled with data regularization and least-squares fitting); (iii) dynamic visualization of color-encoded vector projectiles (with flow speckles displayed as adjunct). Calibration results indicated that by using three transmit angles and three receive angles (-10°, 0°, +10° for both), VPI can consistently compute flow vectors in a multi-vessel phantom with three tubes positioned at different depths (1.5, 4, 6 cm), oriented at different angles (-10°, 0°, +10°) and of different sizes (dilated diameter: 2.2, 4.4 and 6.3 mm; steady flow rate: 2.5 mL/s). The practical merit of VPI was further illustrated through an anthropomorphic flow phantom investigation that considered both healthy and stenosed carotid bifurcation geometries. For the healthy bifurcation with 1.2-Hz carotid flow pulses, VPI was able to render multi-directional and spatiotemporally varying flow patterns (using a nominal frame rate of 416 fps or 2.4-ms time resolution). In the case of stenosed bifurcations (50% eccentric narrowing), VPI enabled dynamic visualization of flow jet and recirculation zones. These findings suggest that VPI holds promise as a new tool for complex flow analysis.

Mirror artifacts in obstetric ultrasound: case presentation of a ghost twin during the second-trimester ultrasound scan

Mirror artifacts are produced by the reflection of ultrasound waves after they propagate through a structure and encounter a strong and smooth interface capable of acting as a mirror. Ultrasound waves bounce back and forth between the mirroring interface and the reflective object and then eventually return to the transducer. The typical display of the mirror artifact consists of two similar structures separated and at similar distances from the reflective interface. We report a mirror artifact in a patient with a singleton gestation at 18 weeks. The image was interpreted as consistent with a twin gestation using transabdominal and transvaginal ultrasound. The differential diagnosis consisted of an abdominal heterotopic pregnancy. The presence of synchronized but opposite movements of both fetuses, and the blurred image of the second fetus, suggested a mirror artifact. The reflective surface was created by the interface located between a distended rectosigmoid filled with gas and the posterior uterine wall. Mirror artifacts can lead to diagnostic errors. This case illustrates how a distended rectosigmoid colon can generate an image that simulates either a twin gestation or an abdominal heterotopic pregnancy.

Duplication images in vascular sonography

OBJECTIVE

The purpose of this study was to determine the characteristics and phenomena of duplication (or mirror) images in vascular sonography.

METHODS

We retrospectively reviewed 5 cases of vascular sonography that produced duplication images performed between June 30, 2009, and January 10, 2010. The 5 vascular studies included the subclavian artery (2), subclavian vein (1), carotid artery (1), and abdominal aorta (1). The characteristics of the color flow images and spectral Doppler waveforms in the artifactual vessels and in the true vessels were analyzed. Sources and mechanisms of producing duplication images were assessed.

RESULTS

We were able to generate color flows in the artifactual vessels that were the same as in the true vessels in 2 cases. However, we were able to generate images in which the apparent flow directions in the artifactual vessels differed from the actual flow observed in the true vessels in all 5 cases by changing the orientation of the ultrasound beam relative to the true flow and the reflecting surface.

CONCLUSIONS

Duplication images result from mirror reflections producing multipath artifacts. The appearance of the flow in an artifactual vessel on color Doppler imaging strongly depends on the angle of the incident sound beam and the surface structure of the strong reflector relative to the flow in the true vessel. Alternating the scanning plane and the angle of the incident sound beam to the reflector may help distinguish a duplication artifact from a true vessel in ambiguous cases.

Mirror-Image Artifact Mimicking a Carotid Artery Dissection

Sonographic artifacts are a common occurrence in B-mode, color, and spectral Doppler. Sonographers should be aware of the potential occurrence of these artifacts so that a normal examination is not classified as abnormal. In the case presented in this article, a mirror-image artifact from the wall of the jugular vein appeared in the common carotid artery lumen, mimicking a carotid artery dissection.