(Stroke. 2001;32:1520.)
© 2001 American Heart Association, Inc.
Original Contributions |
From the Department of Neurology (G.S.-A., V.P., K.H., L.C.), the Department of Anatomy, Histology, and Embryology (S.F., L.M.), the Department of Psychiatry (M.K.), and the Department of Stomatology (C.H.), University of Debrecen, Medical and Health Science Centre, Faculty of Medicine, Debrecen, Hungary; the Department of Neurology (G.S.-A., D.W.D., E.B.R.), University of Münster, Münster, Germany; and the Institute for Medical Informatics, Biometry and Epidemiology (M.S.), University of Munich, Munich, Germany.
Correspondence to Gernot Schulte-Altedorneburg, MD, Department of Radiology, Klinikum Augsburg, Stenglinstr. 2, D-86156 Augsburg, Germany. E-mail Gernot.SAD{at}gmx.de
| Abstract |
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MethodsSixty-six moribund neurological patients (mean age 71 years) underwent B-mode ultrasound of the CCA a few days before death. During autopsy, carotid specimens were removed in toto. Carotid arteries were ligated and cannulated for injection of a hydrophilic embedding material under standardized conditions. The carotid bifurcation was frozen and cut manually in 3-mm cross slices. Digital image analysis was carried out to determine the diameter and the cross-sectional area of the frozen slices of the CCA. IMT was assessed by light microscope. Ultrasonic and planimetric data were compared.
ResultsMean measurements of lumen diameter and cross-sectional area were 7.13±1.27 mm and 0.496±0.167 cm2, respectively, by ultrasound, and 7.81±1.45 mm and 0.516±0.194 cm2, respectively, by planimetric analysis of the unfixed redistended carotid arteries (R2=0.389 and 0.497). The mean IMT was 1.005±0.267 mm by ultrasound and 0.67±0.141 mm histologically, resulting in a mean difference of -31%.
ConclusionsTranscutaneous B-mode ultrasound provides a reliable approach for in vivo measurements of the cross-sectional area and, less exactly, of the lumen diameter of the CCA. Compared with histological results, in vivo ultrasound measurements of the IMT are systematically larger.
Key Words: carotid arteries pathology ultrasonography
| Introduction |
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Our purpose was to correlate in vivo ultrasound measurements of IMT, lumen diameter, and cross-sectional area of the CCA with the corresponding measurements obtained by subsequent gross pathology and histology.
| Subjects and Methods |
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The causes of death were determined by experienced pathologists during autopsy. Patients died from cardiopulmonary arrest (n=8), raised intracranial pressure (n=22), pulmonary embolism (n=6), pneumonia (n=25), sepsis (n=3), or pulmonary edema (n=1). For 1 patient, the cause of death remained unknown even after autopsy.
The mean time interval between ultrasound and death was 10.7 days (median 3 days).
Ultrasound
The ultrasound equipment used was an HP SONOS 2000
system (Hewlett-Packard) with a 7.5-MHz linear
transducer. Axial resolution was 0.5 mm, and lateral resolution
was 0.6 mm at the focal point at a depth of 3 cm. The 2D image
displayed 256 gray levels. Gain (55%) and compression (95%) were held
constant throughout the study. The largest magnification was used for
IMT measurements.
The measurements of lumen diameter and IMT were performed in
a longitudinal B-mode projection; cross-sectional area was measured
in axial projection while the patient was in a supine position.
Details are given in
Figure 1
. IMT was determined as the distance between
the leading edges of the inner and outer echoes of the double-line
pattern of the far artery wall
(arrow).\.
Figure 2
); 3 measurements along a 2- to 3-mm portion
of the vessel were performed and were averaged. Wall thickening over
2 mm was called a plaque. If there was a protruding plaque at this
site, this particular IMT measurement was excluded from further
evaluation. Afterward, the transducer was rotated by 90° exactly at
the site of IMT measurement, and the probe was slowly moved up just
above the bifurcation, with the vertical course of the vessel followed
precisely; when moving the transducer downstream, the sonographer kept
the angle between the probe and vessel circumference constant. On
Doppler mode, the angle between the external carotid artery (ECA)
and internal carotid artery (ICA) could be determined at the flow
divider by means of both the superimposed vertical line
(representing the ultrasound beam) and the adjustable arrow
for angle correction (representing the direction of the
Doppler sample volume). In this way, the localization of the IMT
measurement site on the CCA circumference was defined by the angle
between ECA and ICA at the flow divider and by the distance from the
tip of the flow divider (3 cm)
(Figure 1
). These landmarks enabled us to reconstruct the
position of the in vivo IMT measurement on the postmortem tissue
slices. Subsequently, IMT measurements were performed in the same way
0.9 cm below the tip of the flow divider. All measurements were
performed at the end of a heart cycle and were carried out by the same
investigator. Data were recorded onto videotape for later offline
analysis.
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Gross Pathological Evaluation and Planimetric
Analysis of Vessel Diameter and Cross- Sectional
Area
The different steps of the gross pathological
evaluation are illustrated in
Figure 3
; technical details of the procedure for
preparing and filling the arteries have been described
previously.8
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The time between death and autopsy ranged from 2 to 72 hours (mean 25.5 hours, median 15.5 hours).
The frozen section labeled -30 (ie, 30 mm below the
tip of the flow divider) was digitized by using a Hitachi HV-C20
(Hitachi Ltd) charge couple device (CCD) color camera equipped with a
zoom lens (1:1.8, 12.5 to 75 mm) connected to a PC
(Figure 4
). Total area, maximum diameter, and minimum
diameter of the vessel lumen were determined with image
analyzer software (IMAN 1.4, KFKI). With each individual
sample, a calibration bar was also digitized to determine the
magnification of the system and to convert the pixel values into
millimeters or square centimeters. The settings of the camera were kept
constant throughout the study. The minimum diameter was used for the
comparison with ultrasound. The results from ultrasound investigations
were unknown to the investigator performing the planimetric
analysis.
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Histological Preparations and
Determination of IMT
After the above-mentioned digitization, the samples
were slowly defrosted, and a dental silicon rubber (President Micro
System Jet Bite, Coltene AG) was filled into the lumen to prevent
shrinkage of the lumen. By means of the photos of the slice 3 mm
(+3) above the flow divider (providing the angle between ECA and ICA
just above the bifurcation) and 30 and 9 mm below the tip of the
flow divider (-30 and -9, respectively) (providing investigation
sites), it was possible to reconstruct at which site of the vessel
circumference the IMT had been measured sonographically. In this site,
the adventitia was marked with black ink for later orientation.
Afterward, the slices were placed for fixation in perforated plastic
boxes in a 4:1 mixture of 100% ethanol and 40% formaldehyde for 24 to
48 hours. After dehydration, the silicon rubber was carefully removed
before paraffin embedding. Transverse sections (7 to 8 µm) were cut
by microtome and were stained with hematoxylin/eosin, dimethylmethylene
blue, periodic acidSchiff stain, and Verhoeffs elastic tissue stain
combined with picrosirius
red.9 10 The IMT
was measured at those sites at which black ink could be identified in
the adventitia. IMT measurements were performed by light microscope
with the use of an object micrometer slide (Leitz) with an
accuracy of 0.01 mm. Three measurements along a 2-mm portion of
the vessel at a distance of
1 mm were averaged
(Figure 2
). Sections with severely damaged intima and/or
media at the site of the measurement were excluded. The results from
ultrasound investigations were unknown to the investigator performing
the light microscopic assessment.
Statistical Analysis
For the evaluation of lumen diameter and
cross-sectional area of the in vivo sonographic and the postmortem
planimetric measurements, the mean values and mean differences were
determined in millimeters and square centimeters, respectively. Linear
regression equations were preformed to compare IMT, diameter, and
cross-sectional area measured by either technique. The
R2
value was calculated. The values obtained by ultrasound were plotted
against those obtained by pathological investigation. Furthermore,
Bland-Altman plots were applied to illustrate the agreement between
anatomic and sonographic
measurements.11
The present study was performed according to national laws, and the patients were studied in compliance with a protocol that had previously been approved by the local ethics committee of our University Medical School.
| Results |
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The mean ultrasound and postmortem measurements for vessel
diameter and cross-sectional area are compared in
Table 1
. Correlation of sonographic and pathological
measurements resulted in
R2
values of 0.389 and 0.497 for diameter and cross-sectional area,
respectively.
Figure 5
shows the scatterplots, the Bland-Altman
plots, and regression equations for both parameters.
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Comparison of Ultrasound and
Histological IMT Measurements
IMT measurements were comparable in 52 arteries at 72
sites; of the 72 sites, 36 measurements had been placed 9 mm
(-9) and 36 measurements had been placed at 30 mm (-30) below
the flow divider. Values obtained by ultrasound always turned out to be
smaller than those obtained histologically
(Table 2
), indicating a systematic
discrepancy.
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The regression equation was as follows: IMT measured histologically (mm)=0.3586xIMT measured on ultrasound (mm)+0.3095. Despite this discrepancy in size, the R2 was 0.463.
| Discussion |
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The mean CCA diameter of 7.13 mm measured by ultrasound in the present study is in agreement with the findings in previous ultrasound studies15 16 : Marosi and Ehringer15 established the average diameter of the CCA 1.5 cm below the bifurcation in 53 healthy young adults and found an average diameter of 6.7 mm. Boutouyrie et al16 measured the CCA diameter and the luminal cross-sectional area 2 cm beneath the bifurcation by means of a pulsed ultrasound echo-tracking system in 3 groups of hypertensive patients. With increasing age, they found average CCA diameters of 6.97, 7.07, and 7.64 mm, respectively. Their sonographic measurements for average luminal cross-sectional areas were 0.39, 0.40, and 0.47 cm2, respectively, which is only slightly different from our results (mean 0.496 cm2).
In the present study, values for lumen diameter and cross-sectional area obtained by ultrasound were generally smaller than those obtained by pathology. We are aware of the fact that the postmortem loss of vessel wall elasticity and the removal of the surrounding stabilizing connective tissue may have caused a minor deviation of the specimens during filling with embedding material. The ultrasound measurements had been performed at the end of a heart cycle, whereas postmortem filling of the arteries had been performed under a constant pressure of 100 mm Hg. Previous MRI and ultrasound studies17 18 on flow patterns of the CCA have shown that an increase in both relative diameter and area during systole reflects the distensibility of the artery. However, Reneman et al17 found a significantly larger relative diameter increase in subjects aged 20 to 30 years compared with subjects aged 50 to 60 years (9.6% versus 5.6%). The mean age of our study population was 71 years, suggesting an already reduced CCA distensibility. Distensibility may have accounted for some of the discrepancies found in the present study.
We found a better correlation of ultrasound and gross pathology for cross-sectional area (R2=0.497) than for lumen diameter (R2=0.389). This might be due to the slightly eccentric shape of the vessel lumen not being correctly represented by the limited number of projections obtained during longitudinal B-mode ultrasound. By contrast, axial sonography allows for measurement of the "real" contour of the whole vessel lumen, considering even small irregularities of the vessel wall. The clinical implication of this finding is that the reliable assessment of the vessel lumen with a potential narrowing requires the use of a multiplane imaging technique, such as ultrasound. In general, all R2 values were only moderate in the present study. However, the Bland-Altman plots showed that nearly all provided differences were within mean±2 SD (limits of agreement11 ) for cross-sectional area and IMT. The poorest agreement was found for lumen diameter; this finding is probably explained by the eccentric shape of the vessel lumen, as mentioned above.
In summary, transcutaneous B-mode ultrasound in vivo measurements of the CCA with a 7.5-MHz linear transducer showed good agreement with corresponding measurements obtained from standardized macroscopic postmortem findings for the determination of the cross-sectional area. Ultrasound measurements of the IMT were systematically larger than histological results; this discrepancy was presumably due to tissue fixation and processing. The agreement between both methods was poorest for lumen diameter.
| Acknowledgments |
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Received December 19, 2000; revision received March 27, 2001; accepted April 6, 2001.
| References |
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