CTA der A.�carotis mit unterschiedlichen CT-Ger�ten

[Cerebrovascular diagnostics: imaging]

Imaging of the cerebral vasculature relies mostly on computed tomography angiography (CTA), magnetic resonance angiography (MRA) and digital subtraction angiography (DSA). Although DSA is still the gold standard, many questions can be answered with CTA and/or MRA thanks to recent technological advances. The following article describes the advantages and disadvantages of these techniques with regard to different questions. Basic principles regarding the different techniques are explained.

[CT diagnostics of carotid artery stenosis]

CT angiography can be used to evaluate carotid artery stenosis with high diagnostic safety. In contrast to the gold standard digital subtraction angiography (DSA) it is a non-invasive procedure which yields information regarding the degree of stenosis as well as information about plaque morphology. Due to multiple reconstruction planes in 3 levels in maximum intensity projections (MIP) and the possibility to perform volume rendering (VR) reconstruction more information about the extent of carotid disease can be acquired. Due to the bolus tracking technique optimum contrast medium enhancement of the carotid artery with simultaneous minimization of venous artifacts is possible.

Semi-automatic level-set based segmentation and stenosis quantification of the internal carotid artery in 3D CTA data sets

We present a new level-set based method to segment and quantify stenosed internal carotid arteries (ICAs) in 3D contrast-enhanced computed tomography angiography (CTA). Within these data sets it is a difficult task to evaluate the degree of stenoses deterministically even for the experienced physician because the actual vessel lumen is hardly distinguishable from calcified plaque and there is no sharp border between lumen and arterial wall. According to our knowledge no commercially available software package allows the detection of the boundary between lumen and plaque components. Therefore in the clinical environment physicians have to perform the evaluation manually. This approach suffers from both intra- and inter-observer variability. The limitation of the manual approach requires the development of a semi-automatic method that is able to achieve deterministic segmentation results of the internal carotid artery via level-set techniques. With the new method different kinds of plaques were almost completely excluded from the segmented regions. For an objective evaluation we also studied the method's performance with four different phantom data sets for which the ground truth of the degree of stenosis was known a priori. Finally, we applied the method to 10 ICAs and compared the obtained segmentations with manual measurements of three physicians.

Agreement of multislice CT angiography and MR angiography in assessing the degree of carotid artery stenosis in consideration of different methods of postprocessing

OBJECTIVE

We investigated the agreement of multislice computed tomography angiography (CTA) and magnetic resonance angiography (MRA) in the quantitative measurement of carotid artery stenosis. The dependency of the agreement of the chosen postprocessing procedures was also investigated.

METHODS

Fifty consecutive symptomatic patients were included in this study. In all patients, a CTA was performed with a 16-slice CT scanner. Within 30 days, the extracranial vessels were examined using a combined time-of-flight and contrast-enhanced MRA. The CT data sets were used to calculate the degree of stenosis according to the North American Symptomatic Carotid Endarterectomy Trial, European Carotid Surgery Trial, and Common Carotid methods by means of the 1-mm thick, transverse raw data (RD), a sagittal maximum-intensity projection reconstruction, and sagittal multiplanar reconstruction. In addition, a semiautomated analysis was done using a specialized postprocessing software. For all combinations of postprocessing procedures and methods of calculating the degree of stenosis, the correlation coefficient and the agreement based on Bland/Altman plots were calculated.

RESULTS

Eleven of the 100 primarily included carotid arteries could not be evaluated. The correlation coefficients for all combinations were comparable and lied in the interval between 0.932 and 0.787. The best correlation was found for the combination of RD/sagittal multiplanar reconstruction and ECST method. The evaluation of the agreement gave a systematic overestimation of CTA between 1.9% and 10.7% with a 95% confidence interval between +/-26.7% and +/-43.3%. With the semiautomated postprocessing software, additional 33 vessels could not be evaluated. The agreement of the calculated degrees of stenoses was worse than that of the planar procedures.

CONCLUSIONS

CTA and MRA had a feasible agreement in measuring the degree of stenosis of the carotid arteries. The best result could be obtained for the evaluation of the RD and the NASCET method. In this case one has to take into account a systematic overestimation of CTA of 1.9%. The combination with an additional reconstructive postprocessing procedure did not improve the result but might be useful for the radiologist to identify the location of the closest narrowing.