ULTRASOUND BLOOD FLOW IMAGING IN CAROTID ARTERIES BEFORE AND AFTER ENDARTERECTOMY

Transcription

1 ULTRASOUND BLOOD FLOW IMAGING IN CAROTID ARTERIES BEFORE AND AFTER ENDARTERECTOMY G. BAMBI 1, F. GUIDI 1, S. RICCI 1, P. TORTOLI 1 M.R. CIRELLI 2, L. PEDRINI 2 1 Electronics & Telecommunications Department,University of Florence, via Santa Marta 3, Firenze, Italy 2 Unità Operativa Complessa di Chirurgia Vascolare, Ospedale Maggiore, Azienda U.S.L. Città di Bologna, Italy 1. Introduction Echo-Doppler analysis is a fundamental medical tool for the evaluation of the presence of atherosclerotic plaques in the carotid artery (Carpenter et al, 1995), since it may show the dimensions of the stenosis and its haemodynamic significance. This exam is used both for pre-operative evaluations and for post-operative check ups in atherosclerotic patients subject to surgical treatment, for it has diagnostic capabilities of significant modifications to vessel morphology. Echo-Doppler investigation currently gives quantitative information on blood flow dynamics only for a small region inside the vessel. Using the colour- or power-doppler modes, a larger region can be scanned but mainly qualitative information is obtained in this way. The applications of a unique range-doppler instrument (called multigate ) implemented at the University of Florence showed the capability of extending the quantitative Doppler spectral analysis to multiple sample volumes along an entire ultrasound scan line. Such a system consists of a PC board that digitally processes the data received from ultrasound equipment. The results are shown on the Personal Computer (PC) monitor, which displays the distribution of velocity within the investigated vessel (velocity profiles). To facilitate the use of the multigate system in a clinical setting, commercial ultrasound equipment was modified to control the multigate capabilities and display both morphological (B-mode) and haemodynamic (velocity profile) information on the same screen. This allowed medical staff to conduct the analysis using only one instrument, the echograph, as in a standard investigation. This paper presents the results of the first clinical application of such system, addressed to investigate the relation between velocity profiles and morphology in carotid arteries (Perktold et al, 1991) of atherosclerotic patients subject to surgical treatment. Following a rigorous measure protocol, flow profiles were investigated at different sites both upstream and downstream the carotid bifurcation. Such measures were carried out in two different circumstances: pre-operative conditions in 18 patients

2 and post-operative conditions in 15 patients. The two analyses were performed on the same patients for 9 cases of the total set reported above. Both of the analyses, before and 8-days-after endarterectomy (EA), were carried out during the usual medical check-ups. The pre-operative evaluations showed that velocity profiles in correspondence to the stenosis were always notably distorted with respect to the shape visible in healthy conditions while in some cases the presence of the plaque could not be easily detected through a conventional sonogram related to a single sample volume. After the surgical treatment the laminar flow conditions were recovered in 5 cases while in 10 cases an irregular flow was still evident with a high variability both in time (during the cardiac cycle) and in space (along the depths). 2. Materials and methods The multigate system consists of a single card housed in a free slot of a PC. It can digitise, process and store 64 complex (range-gated) signals originating at different depths along the beam axis of an ultrasound transducer, fired in Pulsed Wave mode (PW). Powerful Digital Signal Processors (DSPs) elaborate in real-time the acquired data in such a way that the power spectral density of each of the 64 Doppler signals is produced. The results are directly transferred into the PC, where the monitor displays them in the so called spectral profile mode (Tortoli et al, 1996). A large memory buffer is also available for raw data storage. The PC runs a custom software, called G.A.S.P. (Global Acquisition and Signal Processing) that consists of an integrated shell for Microsoft Window (Microsoft Corporation, Redmond, Washington) family OS. It includes applications for post processing carried out in LabVIEW (National Instruments Corporation, Austin, Texas) and a Microsoft Visual C++ (Microsoft Corporation, Redmond, Washington) coded console for real-time data acquisition and visualisation. Processed data are displayed in real-time providing dynamic images of the distribution of Doppler frequencies along the investigated depth (spectral profiles). It is possible to extract the maximum and the mean frequencies from the spectral profiles for each depth with both being suitable for conversion to velocity. For this application, a commercial ultrasound equipment (AU3, Esaote, Florence, Italy) was expressly modified to control the multigate system from the echograph console and to display the spectral profile with the B-mode representation on the echograph monitor. An opportune command sequence allows the operator to easily switch from the normal use of the equipment to the multigate mode and back again. The controls of the multigate are set by means of the keyboard and the trackball of the AU3, and are sent to the board through a serial channel, while the information of the VGA signal are used to visualize the spectral profile on the AU3 monitor, in dual image mode (Figure 1).

3 Figure 1. Visualization obtainable on the echograph with the dual image (B-mode + multigate) modality. The right side of the figure shows the spectral profile, with depth in the vertical axis, frequency in horizontal axis and power spectral density in colour code. 24 patient with atherosclerotic problems at the carotid artery level submitted to surgical treatment were monitored using the multigate system in two different phases: in pre-operative conditions, when the evaluation of the stenosis level was performed, and in post-operative conditions, 8-days after the surgical treatment, when the results of the operation were evaluated. A specific measurement protocol was defined to obtain comparable images in different sites for all patients: starting from the level of the carotid bifurcation, the acquisitions were performed in three points in the common carotid artery (1-2-3 cm upstream the bulb) and in three points in the internal carotid artery (1-2-3 cm downstream the bulb), where the plaque is usually located. This protocol allowed to analyze a long region of the carotid, to follow the development of the velocity profile outside the stenotic region, too. 3. Experimental results The profiles acquired in correspondence to the stenosis, which is typically located in the internal carotid artery 1 cm downstream the bifurcation, had the same long, narrow and filled in shape in all pre-operative cases (Figure 2). In some cases, a double peaked profile was detected. In the same cases, the sonogram related to the central region of the vessel showed a normal waveform, compatible with a non-pathological condition (Figure 3) When the plaque was calcified, it was difficult to get the data, since the received signal was low and in many cases totally immersed in noise.

4 Figure 2. Typical spectral profile detected in the internal carotid artery of patients before EA. This profile was taken at the time indicated by the white line in the sonogram (upper image) corresponding to the centre of the vessel. Here aliasing is evident, due to the high velocity through the narrow vessel (diameter less than 3 mm). A B C D Figure 3. Pre-operative profiles detected at different registration sites. In the fourth image, obtained in correspondence to the more stenotic region, a narrow profile with a double horn is visible. This profile was frozen at the instant indicated on the sonogram. The examinations carried out 8 days after the EA were usually easier, due to the plaque removal. Three different surgical treatments were used for the 15 patients examined: 10 cases had an EA with a direct suture of the arteriotomy, in 4 cases the arteriotomy was closed with a polyester patch; in the last a 6 mm polytetrafluoroethylene (PTFE) graft was inserted at the level the carotid bulb.

5 In 5 cases, spectral profiles looked regular and larger than in the pre-operative condition (Figure 4), while in 10 cases some turbulences were still detected (Figure 5). (a) (b) (c) Figura 4. Post-operative check-up after TEA with direct suture and a cylinder-like carotid bulb. (a) Delineated and symmetrical profile in the common carotid artery. (b) Large and M shaped profile in deceleration 1 cm downstream the bifurcation. (c) Large and symmetrical profile in the internal carotid artery, 2 cm downstream the bifurcation. Figure 5. Post-operative check-up after TEA with patch and the carotid bulb a little elliptic. Delineated and symmetrical profile in the common carotid artery (upper images), large profile with some turbulences 1 cm downstream the bifurcation (bottom images).

6 Figure 6. Post-operative check-up after TEA with patch. Spectral profiles for the patch region, corresponding to the subsequent instants indicated in the sonogram. The main flow is directed toward left (bottom-left image), and a strong reverse flow is evident in the bottom-right image. Figure 7. Post-operative check-up after TEA with PTFE graft. Spectral profiles for the graft region, corresponding to the two peaks indicated in the sonogram. The maximum frequency is shifted toward the lower side of the profile (posterior- medial wall, bottom-left image), and toward the upper side (anteriorlateral wall, bottom-right image), respectively.

7 In 3 of the 4 cases of patch insertion back flow in the internal carotid artery was evident (Figure 6). For the patient treated with a PTFE graft, in correspondence to the initial portion of the graft a double acceleration flow was revealed. The spectral profile was skew, with the peak shifted toward the lower side of the profile (posterior- medial wall) in the first phase of acceleration, and toward the upper side (anterior-lateral wall) in the second phase (Figure 7). 4. Discussion The capability of visualizing both the spectral profile and the B-mode image on the same echograph display, and of controlling all the system by the echograph console allowed medical staff to perform the analysis using the same procedure used for conventional echo-doppler examination. The spectral profiles provided by the multigate system were highly informative. By examining them for the pre-operative condition, the geometric structure of the vessel could be estimated. For example, by looking at the spectral profiles of Figure 3, the asymmetrical shape of profile (A) and (B) suggested the presence of a flow obstruction in the lower part of the vessel, while the long and narrow profile (D) could be related with the stenosis. Comparing the angiography with the spectral results, these hypotheses were confirmed. The pre-operative analyses showed a high variability of the spectral profiles, due to the presence of different morphologies of the plaques. A common characteristic for the spectral profiles corresponding to the region of the stenosis was the narrow and extended shape. In some cases (Figure 3) a particular M shape was recognized. For some patients, an analysis using only the usual sonogram obtained with the PWmode of the echograph did not show the presence of the stenosis and revealed a quasinormal shape. The possibility of having the complete spectral profile simplified the stenosis identification. In most cases the spectral profiles of the post-operative evaluations were larger than those obtained in pre-operative conditions, due to the stenosis removal. In 10 cases turbulences were again detected, while only in 5 cases the shape was regular, corresponding to a laminar flow. The data obtained in the examinations in post-operative conditions seem to be dependent on the type of surgical treatment, but at present there is not a large enough sample of analyses to carry out statistical results. For the direct EA operation the carotid bulb maintains a regular shape, even though dimensions are reduced. For the patch insertion the bulb is enlarged and extended, due to the fact that the patch walls are more rigid than the vessel walls (Archie, 1997). The interposition of a graft also produces a rectilinear conformation of the vessel. The differences in the velocity profiles introduced by these different surgical treatments are to be investigated in the next step of this research.

8 References Archie, J.P.Jr. (1997) Geometric dimension changes with carotid endarterectomy reconstruction, J. Vasc. Surg. 25, Carpenter, J.P., Lexa, F.J., Davis, J.T. (1995) Determination of sixty percent or greater carotid artery stenosis by duplex Doppler ultrasonography, J. Vasc. Surg. 22(6), Perktold, K., Resch, M., Peter, RO (1991) Three-dimensional numerical analysis of pulsatile flow and wall shear stress in the carotid artery bifurcation, J.Biomechanics 24(6), Tortoli, P., Guidi, F., Guidi, G., Atzeni, C. (1996) Spectral velocity profiles for detailed ultrasound flow analysis, IEEE Trans. UFFC 43,