Assessment of the reliability of the determination of carotid artery lumen sizes by quantitative image processing of magnetic resonance angiograms and images

Novel methodology for 3D reconstruction of carotid arteries and plaque characterization based upon magnetic resonance imaging carotid angiography data

In this study, we present a novel methodology that allows reliable segmentation of the magnetic resonance images (MRIs) for accurate fully automated three-dimensional (3D) reconstruction of the carotid arteries and semiautomated characterization of plaque type. Our approach uses active contours to detect the luminal borders in the time-of-flight images and the outer vessel wall borders in the T(1)-weighted images. The methodology incorporates the connecting components theory for the automated identification of the bifurcation region and a knowledge-based algorithm for the accurate characterization of the plaque components. The proposed segmentation method was validated in randomly selected MRI frames analyzed offline by two expert observers. The interobserver variability of the method for the lumen and outer vessel wall was -1.60%±6.70% and 0.56%±6.28%, respectively, while the Williams Index for all metrics was close to unity. The methodology implemented to identify the composition of the plaque was also validated in 591 images acquired from 24 patients. The obtained Cohen's k was 0.68 (0.60-0.76) for lipid plaques, while the time needed to process an MRI sequence for 3D reconstruction was only 30 s. The obtained results indicate that the proposed methodology allows reliable and automated detection of the luminal and vessel wall borders and fast and accurate characterization of plaque type in carotid MRI sequences. These features render the currently presented methodology a useful tool in the clinical and research arena.

Magnetic resonance angiography shows dilatation of the middle cerebral artery after infusion of glyceryl trinitrate in healthy volunteers

Previous studies have reported dilatation of the middle cerebral artery (MCA) during acute glyceryl trinitrate (GTN)-induced headache, using imaging techniques such as transcranial Doppler (TCD), positron emission tomography (PET) and single photon emission computerized tomography (SPECT). In the present study we aimed to evaluate whether magnetic resonance angiography (MRA) may be used to examine the effect of GTN on the MCA, with respect to changes in diameter and cross-sectional area in healthy volunteers. In addition, we wanted to determine the intra- and inter-observer variation of the method. In a randomized, double blind, crossover study 12 healthy volunteers received intravenous infusion of GTN (0.5 microg/kg/min for 20 min) or placebo. Using 1.5 Tesla MRA, we recorded changes in the diameter and cross-sectional area of MCA before, during and after infusion of GTN. The MRA images were evaluated by two blinded, independent observers/neuroradiologists. The primary endpoints were the differences in the AUC for diameter and cross-sectional area of the MCA between the two experimental conditions and the intra- and inter-observer variation. The areas under the curve (AUC) of the MCA diameter and cross-sectional area were significantly greater after GTN than after placebo (P < 0.05). The intra-observer variation (day-to-day) at baseline was 8.3% and 10.9% for the two observers. The mean inter-observer variation of the cross-sectional MCA area was 15.5% and for the diameter measurements 8%. The present study shows that the MRA method gives a reliable semi-quantitative index of the vascular changes in the intra-cerebral arteries after infusion of GTN and may be useful for headache research.

Thematic review series: Patient-Oriented Research. Imaging atherosclerosis: state of the art

The ability to image obstructive arterial disease brought about a revolution in clinical cardiovascular care; the development of newer technologies that image arterial wall thicknesses, areas, volumes, and composition allows valid imaging of atherosclerosis for the first time. Development of noninvasive imaging of atherosclerosis has further led to a quantum shift in research in the field by enabling the study of asymptomatic populations and thus allowing investigators to focus on preclinical disease without the many biases associated with the study of symptomatic patients. These noninvasive investigations have broad implications for clinical care as well. Coronary angiography, computed tomographic (CT) imaging of coronary calcium, intravascular ultrasound, multidetector CT angiography, B mode ultrasound of the carotid arteries, and MRI of the carotid arteries all have unique strengths and weaknesses for imaging atherosclerosis. Certain of these techniques are extremely useful as outcome variables for clinical trials, and others are uniquely useful as predictors of the risk of cardiovascular disease. All are informative in one way or another with regard to the role of plaque remodeling and composition in disease causation. CT and MRI technology are advancing very rapidly, and research and clinical uses of these imaging modalities promise to further advance our understanding of atherosclerosis and its prevention.

MR imaging in carotid artery atherosclerosis plaque characterization

AIM

To evaluate the potential role of carotid artery atherosclerosis plaque magnetic resonance (MR) microimaging as magnetic resonance imaging (MRI) marker, ex vivo MR images were acquired at optimized parameters on 9.4T Bruker animal imager for occluded tissue resected by carotid endarterectomy (CEA) and corresponding histopathological analysis was made.

METHODS AND MATERIALS

For imaging, CEA tissues of size 2-6 cm long and 0.5-1.5 cm wide, were transferred to 15 ml co-polymer laboratory culture tubes containing either 10% formalin in phosphate buffered saline (PBS) or in 50% glycerol in PBS. Imaging protocol was set at TE=30 ms, TR=1.5 s, matrix size=265 x 512, NEX=128, slice thickness=1 mm and in-plane resolution=0.1 mm for total sample size 2.5 cm. Soon after imaging done, carotid artery tissues were cut into 5-mm segments and processed for histological section for successive 5-micrometer slices. To compare morphology of 5 mum thin CEA section with that of 1 mm MR slices, registration was obtained between histologic sections and MR slices. Contrast and magnetic resonance relaxation characteristics were analyzed.

RESULTS

Total carotid artery area computed by MR imaging was correlated with areas determined from histologic sections (r(2)=0.989, p=0.0001). For the lumen area, the correlation between MR images and histologic area was (r(2)=0.942, p=0.0001). Relaxation times and T(2) parametric images of different plaque components were determinant for contrast resolution. Scan parameters were optimized for fibrous cap and atheroma. Scan parameters were characteristic for comparison at 1.5T and 9.4T MR imagers.

CONCLUSION

The observed correlation validated MR microimaging to assess morphological features of carotid artery plaques and contrast resolution highlighted the potential of in vivo MR imaging as non-invasive MRI marker to monitor carotid artery plaque morphometry and plaque composition.

Cerebrospinal fluid dynamics and relation with blood flow: a magnetic resonance study with semiautomated cerebrospinal fluid segmentation

RATIONALE AND OBJECTIVES

To investigate and measure temporal and amplitude aspects of blood and cerebrospinal fluid (CSF) flow waveform relations.

METHODS

A cine phase-contrast magnetic resonance imaging pulse sequence was used to measure blood and CSF flow in 16 healthy subjects aged 27 +/- 4 years. A semiautomated segmentation algorithm was developed to study CSF flow.

RESULTS

Standard deviations of the aqueductal and cervical flow measurements carried out by five observers were 1% and 4%, respectively. The peak systolic arterial flow was 1087 +/- 169 mL/min, and the peak cervical CSF flush (173 +/- 59 mL/min) occurred at 5% +/- 3% of the cardiac cycle after the internal carotid systolic peak flow. Peak aqueductal flush flow (13 +/- 5 mL/min) occurred at 21% +/- 7% of the cardiac cycle after the internal carotid systolic peak flow.

CONCLUSIONS

The CSF segmentation algorithm is reproducible. Brain expansion was quickly regulated by a major extracerebral CSF flush flow, whereas ventricular CSF made only a very small contribution.

A semi-automatic technique for measurement of arterial wall from black blood MRI

Black blood magnetic resonance imaging (MRI) has become a popular technique for imaging the artery wall in vivo. Its noninvasiveness and high resolution make it ideal for studying the progression of early atherosclerosis in normal volunteers or asymptomatic patients with mild disease. However, the operator variability inherent in the manual measurement of vessel wall area from MR images hinders the reliable detection of relatively small changes in the artery wall over time. In this paper we present a semi-automatic method for segmenting the inner and outer boundary of the artery wall, and evaluate its operator variability using analysis of variance (ANOVA). In our approach, a discrete dynamic contour is approximately initialized by an operator, deformed to the inner boundary, dilated, and then deformed to the outer boundary. A group of four operators performed repeated measurements on 12 images from normal human subjects using both our semiautomatic technique and a manual approach. Results from the ANOVA indicate that the inter-operator standard error of measurement (SEM) of total wall area decreased from 3.254 mm2 (manual) to 1.293 mm2 (semi-automatic), and the intra-operator SEM decreased from 3.005 mm2 to 0.958 mm2. Operator reliability coefficients increased from less than 69% to more than 91% (inter-operator) and 95% (intra-operator). The minimum detectable change in wall area improved from more than 8.32 mm2 (intra-operator, manual) to less than 3.59 mm2 (inter-operator, semi-automatic), suggesting that it is better to have multiple operators measure wall area with our semi-automatic technique than to have a single operator make repeated measurements manually. Similar improvements in wall thickness and lumen radius measurements were also recorded. Since the semi-automatic technique has effectively ruled out the effect of the operator on these measurements, it may be possible to use such techniques to expand prospective studies of atherogenesis to multiple centers so as to increase access to real patient data without sacrificing reliability.

Analysis of the measurement precision of arterial lumen and wall areas using high-resolution MRI

High-resolution MRI may be used to monitor the progression of human carotid atherosclerosis by measuring the lumen and wall area changes over time. The purpose of this study was to analyze the precision of quantitative measurements of lumen and wall areas. Two independent MR scans near the carotid bifurcation were conducted on eight patients within 2 weeks. The error of lumen area measurement was 6. 2%, 9.2%, and 9.7% for T(1), proton density, and T(2)-weighted images, respectively, and the error of wall area measurement was 10. 8%, 10.9%, and 12.0%. The precision of area measurement correlates strongly with image quality.

Optical detection of triggered atherosclerotic plaque disruption by fluorescence emission analysis

Fluorescence emission analysis (FEA) has proven to be very sensitive for the detection of elastin, collagen and lipids, which are recognized as the major sources of autofluorescence in vascular tissues. FEA has also been reported to detect venous thromboemboli. In this paper we have tested the hypothesis that FEA can reproducibly detect in vivo and in vitro triggered plaque disruption and thrombosis in a rabbit model. Fluorescence emission (FE) spectra, recorded in vivo, detected Russell's viper venom (RVV)-induced transformation of atherosclerotic plaque. FE intensity at 410-490 nm 4 weeks after angioplasty was significantly lower (P < 0.0033 by analysis of variance) in RVV-treated rabbits when compared to control animals with stable plaque. FE spectral profile analyses also demonstrated a significant change in curve shape as demonstrated by polynomial regression analysis (R2 from 0.980 to 0.997). We have also demonstrated an excellent correlation between changes in FE intensity and the structural characteristics detected at different stages of "unstable atherosclerotic plaque" development using multiple regression analysis (R2 = 0.989). Thus, FEA applied in vivo is a sensitive and highly informative diagnostic technique for detection of triggered atherosclerotic plaque disruption and related structural changes, associated with plaque transformation, in a rabbit model.