3.P.346 How can the variability in ultrasound measurement of intima-media thickness be reduced?

Wednesday 8 October 1997 : Posters Vascular imaging 3 .P.346

How can the variability in ultrasound measurement of intima-media thickness be reduced?

C. Schmidt, I. Wendelhag, J. Wikstrand. The Wallenberg Laboratory for Cardiovascular Research, Goteborg University, Gothenburg, Sweden The interobserver variability in measurement of intima-media thickness (IMT) was studied in three arterial regions : the common carotid artery, the carotid artery bulb, and the common femoral artery . The main aim was to investigate if the standard deviation (SD) of repeated measurements could be decreased if a mean value of the right and left arteries were used in the analyses instead of only the right artery . Furthermore, to evaluate two different reading procedures, one based on manual tracing of echo interfaces, and the other on automatic detection. Totally 50 subjects were examined with ultrasound twice on the same day by two independent laboratory technologists . The ultrasound images were analysed both in a manual tracing analysing system and in a new system based on automated detection of the echo interfaces . The lowest interobserver variability was observed when both the right and left carotid arteries were examined and analysed in the automated analysing system . The coefficient of variation (CV) was 5 .3% (SD = 0 .07 mm) for IMT of the common carotid artery, and 6 .0% (SD = 0.09 mm) for IMT of the carotid artery bulb. The CV for measurement of IMT of the common femoral artery did not improve by examination of both right and left femoral arteries . Interobserver error for two-sided examination of the common carotid artery was s = 0.058, which was significantly lower than for a one-sided examination (s = 0.076, p < 0.05) . The corresponding figures for the carotid bulb were s = 0 .060 and s = 0 .091, respectively (p < 0 .05) . This study showed that the SD of repeated measurements can be decreased by examining both the right and left carotid arteries . In the now ongoing studies in the laboratory both the right and left carotid arteries are examined but only the right femoral artery.

3 .P.347

The atherosclerotic plaque: High resolution MR imaging

J .M. Serfaty', L . Chaabane2 , A. Tabib 3 , A. Briguet 2 , P.C . Douekl . 1 Hopital de la Croix Rousse service de Radiologie 69004 Lyon ; 2Laboratoire de RMN. Methodologie et instrumentation en Biophysique, UPRES A 5012, UBCL-CNRS-CPE, Bat. 308, 43 Bd du 11 Nov. 1918, 69622 Villeurbanne Cedex; 3Hopital Cardiologique : service d'anatomo-pathologie du Pr Loire . 69 Bron, France Purpose : Is MRI able to characterize atherosclerotic plaques? Method : We have used 10 arteries (iliac femoral and carotid) that we imaged on a 2 tesla Oxford magnet . Those last were taken out postmortem, frozen at -20°C, warmed up before experimentation at surrounding temperature and imaged in NaCl . 2D Spin Echo sequences were used with T1 (TR = 600 ms ; TE = 25 ms), proton and T2 weighting (TR = 2000 ms ; TE = 25/50 ms) . Our in plane spatial resolution was 117 Am . All images were analyzed in a double blind fashion . Results: (1) Normal vascular wall: the intima was not visible at this resolution; the media was seen in 100% cases with an intermediate signal in all sequences ; the adventicia could be distinguished on 90% of the images especially on the T2 weighting images (low signal) . (2) Pathological arterial wall : 100% of the calcification were seen (no signal) ; lipid cores showed a low signal in T2 sequences, fibrosis three kinds of signals according to their water content . Conclusions: High Resolution MRI seems to be a good tool to individualize and characterize the atherosclerotic plaque . Its application in vivo should allow : 1) a better understanding of the atherosclerotic phenomena 2) the assessment of new pharmacological molecules 3) the unstable plaques detection and 4) the emergence of interventional techniques under MR control .

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(iliac vs aorta) on T2 and attenuation . Sixty-nine atherosclerotic regions were studied from twenty fresh human arteries removed at autopsy. We studied US B-scan images (resolution : 100 x 30 µm) calculated from backscattered rf signals (PVDF transducer) ; attenuation images were constructed using a multinarrow-band method with diffraction correction (resulting image resolution : 200 x 420 Am, acquisition: 30 min) . MR T2 maps were constructed from exponential fits of an 8-echo sequence (TR : 1500 ms, inter-echo: 22 ms, resol : 110 x 220 Am, acquisition: 7 min) . T2 (ms) and attenuation (dB/cm) are shown below (mean/SD) . ttest : p < 10-4 for T2s (except core-adventitia : NS), p < 10 -2 for Att.

T2 Att

Core

Cap

Media (iliac)

Media (aorta)

Adv

54/3 134/5

89/6 86/14

76/9 80/18

65/9 82/17

57/5 -

This study reveals a difference between media and fibrous cap T2s, and between aortic and iliac media T2s, that was not previously reported and which may be related to proteinic composition or density. Thus 3T MRI may allow better discrimination between these components in vivo . It also indicates that both MRI and US are able to separate lipid cores from collagenous caps and media in vitro . Optimal usage of these techniques for external or intravascular imaging may depend on their respective invasiveness and accessibility . References [1] Toussaint 1996 Circulation 94:932

3 .P.349

Variability sources in carotid ultrasound studies

F. Veglial , C . Gobbil, M .G . Bond 2 , G. Gallus 3 . On behalf of The CAIUS research group; 1 Biostatistical Unit, H S . Raffaele, Milan ; 3Medical Statistics Institute, University of Milan, Italy ; 2Bowman Gray School of Medicine, Wiston-Salem North Carolina, USA Serial ultrasonographic determination of intima-media thickness (IMT) is an important non-invasive means to assess carotid atherosclerosis progression in clinical studies . To take full advantage of this methodology it is important to identify, evaluate and contain all sources of measurement variability . We analysed over 9,000 replicate IMT measurements (90% central range 0.59-1 .95 mm) taken on twelve carotid segments of 301 asymptomatic hypercholesterolemics (mean age 55 years, 51% male, 154 placebo and 147 Pravastatin treated) from the Carotid Atherosclerosis Italian Ultrasound Study (CAIUS) . Variability was estimated from variances between replicate measurements, and specific contributions of artery segments involved, wall IMT size, sonographers, videotape readers, and study centres . Wall thickness emerged as the single most important factor, with a quadratic relationship between IMT and variance (p < 0.0001) : i.e ., between 1 .0 and 1 .5 mm IMT we had a 58% increase in expected variance . A significant variability reduction (-28%, p < 0 .0001) was observed when both replicate ultrasound scans were read by the same reader, and a weaker decrease (--14%, p = 0 .04) was due to sonographer effect . Controlling for IMT size, reader and sonographer effects, variability on common and bifurcation segments was very similar, and significantly lower (-51% p < 0 .0001) than on internal carotid. As expected, a consistent variability reduction was also observed in `far' with respect to `near' walls (-26% p < 0 .0001) . A significant heterogeneity was detected among the different study centres (p < 0.0001), but it was greatly reduced when internal carotid measurements where excluded from the analysis . In conclusion, our data suggest that the most efficient estimates of IMT progression can be obtained focusing on bifurcation and common carotid, particularly on walls of intermediate thickness .

3 . P 350* ] Determination of 3D plaque geometry and composition using intravascular ultrasound 3.R348

Magnetic resonance and ultrasound imaging parameters of human aortic and iliac atherosclerotic arteries J .F. Toussaint l2 , S .L . Brida1 3 , J .S . Raynaudl, P. Fornes 2 , V. Lebon', A. Leroy-Willigl, G. Berger 3 . 1 SHFJ-CEA, Orsay; 2 Hopital Broussais ; 3 CNRS-URA 1458, Paris, France New imaging techniques need to identify unstable plaques to prevent stroke or myocardial i nfarction. MR T2 contrast characterizes lipid core and collagenous cap, the major components of plaque stability (1) . High frequency ultrasound (US) may perform similarly through backscatter analysis . We compared US attenuation (Att) and retrodiffusion maps (30-50 MHz) to MRI T2 maps (3T), and correlated both with histology . We also studied the effect of localization

D .G . Vince, K .J . Dixon, R . Shekhar, R .M . Cothren, J .F. Cornhill . The Cleveland Clinic Foundation, USA The aim of this study was to develop image analysis techniques which, when applied to intravascular ultrasound (IVUS) images, describe geometry and composition of human coronary artery plaques . 3D reconstruction of IVUS images was performed by determining the path of the IVUS transducer with the aid of biplane x-ray images acquired continuously during a manual IVUS pullback. The IVUS images were then placed along the transducer's path at the appropriate locations . 3D segmentation algorithms were used to automatically detect the luminal and medial-adventitial boundaries and the result surface rendered to reconstruct the vessel. The artery was then perfusion fixed and

11th International Symposium on Atherosclerosis, Paris, October 1997