Duplex ultrasound criteria for the identification of carotid stenosis should be laboratory specific

Surgical decision making for carotid endarterectomy and contemporary magnetic resonance angiography

BACKGROUND

Benefit from carotid endarterectomy (CEA) centers on patient selection and percent stenosis as determined by cerebral angiography. However, angiography remains expensive and poses risks. Validated carotid duplex ultrasonography has proven to be an accurate tool for selecting patients for CEA. However, the role of another noninvasive test-magnetic resonance angiography (MRA)-remains uncertain. Because of recent advances in MRA hardware and software, we hypothesized that clinically appropriate patients could be accurately selected for CEA based on MRA alone.

METHODS

Fifty-four carotid arteries in 29 patients (with and without symptoms) underwent both three-dimensional time-of-flight MRA (1.5 Tesla) with multiple overlapping thin slab acquisition and biplanar intra-arterial digital subtraction angiography. All patients undergoing both tests over a 24-month period were included. The majority of these patients did not undergo carotid duplex ultrasound owing to the clinical practice of the hospital's neurosurgery service. Staff radiologists interpreted each study. The accuracy of patient selection based on MRA was calculated using angiography as the standard (NASCET method). Since operative thresholds vary depending on clinical history, we considered four commonly used ranges of percent stenosis for CEA.

RESULTS

Patient selection accuracy of MRA alone was low, but increased as percent stenosis increased. Out of 10 occluded arteries by angiography, 5 were interpreted as patent with stenosis (70% to 99%) by MRA. One patent artery was misread as occluded on MRA.

CONCLUSION

Reliance solely on contemporary MRA for surgical decision making cannot be justified in view of low accuracy, which leads to high rates of error in patient selection for CEA.

A comparison of single- and dual-beam methods for maximum velocity estimation

The purpose of this study was to compare the precision and accuracy of maximum velocity estimation when the target direction is known (a string phantom), and when the target direction is unknown (a flow model of arterial stenosis with stenoses of 0-80% by area). Maximum velocity was estimated using single- and dual-beam methods. A linear-array system was used to acquire Doppler spectra from a single-beam direction. The same array was used for sequential acquisition of Doppler spectra from 2 beam directions; the velocity estimates from these were then compounded in a vector manner. The variation of estimated maximum velocity with beam-string angle over the range 40-80 degrees was 27% for conventional Doppler, 2.6% for angle correction from the edge of the array and 1.6% for the vector Doppler. In the stenosis model, for the single-beam methods, the highest frequency shift was obtained just prior to the point of minimum lumen. At this location, the variation with beam-vessel angle over the range 40-80 degrees was 35% for conventional Doppler, 7.4% for the correction factor method and 6.9% for correction from the edge of the array. For the vector method, the maximum velocity is obtained from within the poststenotic jet, the variation was 2% over the range 40-80 degrees. It is recommended that existing Doppler systems use the correction-factor method to reduce variation in measured maximum velocity. The use of the vector technique by future generations of Doppler systems may lead to angle-independent velocity estimation.

Finite beam-width ray model for geometric spectral broadening

The purpose of the study was to compare measured spectral width and maximum frequency with that predicted from ray models of geometric spectral broadening. Zero and finite beam-width models were used. Spectral data were acquired from a string phantom using two commonly-used linear array systems. Beam width and Doppler aperture sizes were measured using a needle hydrophone. The results showed that the experimentally measured data agreed best with the finite beam-width model. The zero beam-width model was in error by up to 50% for calculated spectral width, and up to 10% for maximum frequency. It is concluded that spectral width and maximum frequency are best calculated using the finite beam-width model, and that ultrasound manufacturers could improve the variation in spectral broadening measured at different locations on a single machine by adjusting the aperture size to give a constant subtended angle and beam width.

Accuracy of carotid duplex is laboratory specific and must be determined by internal audit

OBJECTIVE

To assess the accuracy of carotid duplex in a single vascular laboratory at the prediction of an angiographic 70% internal carotid artery stenosis.

DESIGN

A retrospective review of all patients who underwent both carotid duplex and angiography in a 1-year period at a vascular unit which participates in the ACST trial.

METHODS

Peak systolic velocity was used as a primary end-point in carotid duplex examinations with a PSV > 130 cm/s used as an indication for angiographic assessment. Biplanar arch aortography and selective carotid catheterisation were performed as indicated and diameter reduction calculated by the ECST method.

RESULTS

The sensitivity of 130 cm/s for the detection of a 70% stenosis was 96% and the specificity 67%. If a PSV of 250 cm/s were used the sensitivity would be only 37% and specificity 96%.

CONCLUSIONS

Applying duplex criteria from one centre to another is inappropriate. Laboratory specific audit of duplex and angiography is essential before deciding to abandon preoperative angiography for carotid disease.