What is the Cause of Signal Inhomogeneity at the Carotid Bifurcation During Contrast Enhanced Carotid MRA?: In Vivo and in Vitro Studies

PURPOSE: To evaluate the hemodynamic causes of signal inhomogeneity at the carotid bulb that might be misinterpreted as pathologic signal defect in carotid contrast enhanced MRA(CEMRA). MATERIALS AND METHODS: Both carotid CEMRA and fast digital subtraction angiography(DSA) were conducted on 15 patients (28 carotid arteries) and arterial phase CEMRA images were compared with fast DSA images of the same patients. A 1.5T MR imager was used. The Turbo-FLASH sequence emplayed was TR/TE/FA= 3 .2m s / 1.3m s / 35 degree. For experimental study, we utilized handmade silicon phantoms of the tortuous carotid bifurcation; these might be expected to clearly demonstrate turbulent flow at the carotid bulb. In a closed circulatory system, both CEMRA and fast DSA involved the use of these phantoms. RESULTS: During CEMRA, inhomogeneous signals of varying degrees were found at the carotid bulb in 12/28 carotid arteries. When compared with sequential DSA images, incomplete mixing of contrast agent due to turbulent flow at the carotid bulb might be responsible for this inhomogeneity. This hypothesis was reinforced by successfully reproducing signal defects at the carotid bulb from the experimental CEMRA study using carotid phantoms that showed marked turbulent flow in the same area during DSA. CONCLUSION: Incomplete mixing of contrast agent caused by turbulent flow at the carotid bulb might be responsible for the signal inhomogeneity seen on carotid CEMRA.

Ultrafast Contrast-Enhanced MR Angiography of the Carotid Artery: Time Optimization for Discrimination of theArterial from the Venous Phase

PURPOSE: To investigate the optimal delay and acquisition time for discrimination of the arterial from thevenous phase in ultrafast 3D contrast-enhanced MR angiography of the carotid artery. MATERIALS AND METHODS: Wereviewed the MR angiographic findings of 233 patients in whom carotid stenosis and cerebrovascular disease weresuspected. On the basis of delay and acquisition time they were divided into four groups. In the first three,contrast material was injected manually, and after the optimal time, mechanical injection was used for the lastgroup. On the basis of signal intensity of the carotid artery, image quality was graded in four steps.Discrimination of the arterial from the venous phase was graded in three steps, based on the degree of venousenhancement. RESULTS: The best grade of image quality was 70% in the first group, 85% in the second, and 95% inthe third. In discrimination of the arterial-venous phase, the most definite grade was 50% in the first group, 62%in the second, and 75% in the third. Between manual and mechanical injection groups, there was no significantdifference in image quality and discrimination of the arterial-venous phase. CONCLUSION: These results suggestthat for ultrafast 3D contrast-enhanced MR angiography of the carotid artery, with manual injection of contrastmaterial, 8-second delay time and 7-second acqusistion time are optimal.

An Experimental Study on the Optimization of Parameter Values for Magnetic Resonance Angiography using a Phantom Model of Ulcerated Stenotic Internal Carotid Artery

PURPOSE: It has been suggested that an ulceration or hemorrhage within an atheroma on a stenotic carotid artery is a clinically important cause of transient ischemic attack(TIA). In previous studies, due to its inherent signal loss by static or turbulent flow, magnetic resonance angiography(MRA) proved to be an unreliable methed for the evaluation of subtle changes of ulceration. To improve the detectability of the ulceration within atheroma, avascular phantom was filled with gadolinium solution of various concentrations during various MR sequences. MATERIALS AND METHODS: Several vascular phantoms made of elastic silicon mimicking an ulcerated stenotic internal carotid artery(ICA) were constructed, and gadolinium solution of different concentrations (1:1000 and 1:200 of Gd-DTPA) and distilled water were introduced into the vascular phantoms using a computerized pulsatile pump. To evaluate maximum intensity projection(MIP), multiple planar reconstruction(MPR) and source images, axial and coronal images of MRA with 2D-TOF(time of flight) and 3D-TOF were reviewed. Each image of various sequences was compared with plain X-ray films of each phantom filled with barium. RESULTS: On all MR suquences, the images of the phantom of the normal carotid bifurcation were superior to the images of ulcerated and stenotic phantoms. MPR and MIP were the optimal image for detecting and defining ulceration and stenosis. Better quality images were obtained when a higher concentration of Gd-DTPA was used and when the 3D-TOF technique instead of the 2D-TOF technique was applied. CONCLUSION: This study reveals that a combination of higher concentration gadolinium with MPR and MIP on 3D-TOF technique could be optimal for the evaluation of ulceration and/or stenosis at the bifurcation of the carotid artery.

Hemodynamic Changes on Phantoms of the Internal Carotid Arterial Stenosis: MRA, DSA and CFD

PURPOSE: The most important factor discrediting the reliability of MRAs is the overestimation of the degree of stenosis in the internal carotid artery(ICA). The purpose of this study is to evaluate the second aryhemodynamics and the cause(s) for the overestimation of the degree of variable stenotic phantoms of the carotidartery using steady-state flow on MRAs. MATERIALS AND METHODS: Using scrylic materials, normal and variable stenotic phantoms of the bifurcated carotid artery were constructed (40% and 65%). Flow patterns were evaluated with axial and coronal imaging of MRAs (2D-TOF and 3D-TOF) and DSAs of phantoms constructed from an automated closed-type circulatory system filled with 10% glucose solution. These findings were then compared with those obtained from CFD. RESULTS: 3D-TOF axial MRA of asymmetrically 40 percent stenotic phantom revealed 40 percent stenosis identical to the stenotic region of phantoms with continued poststenotic signal loss, whereas 3D-TOFzsial MRA of symmetrically 65 percent stenotic phantom showed markedly decreased signal intensity at the poststenotic segment resembling occlusion. Source image of 2D-TOF coronal MRA showed redistribution (from theinternal to external carotid artery side) of the central axis of inflow depending upon the degree of stenosis ofthe ICA ; this redistribution can be a cause of the decreased signal at the poststenotic segment, due to a reduced volume of flow through the stenotic segment. The general hemodynamics of the variable stenotic phantoms on MRA were identical to the hemodynamics on DSA and CFD. CONCLUSION: Although dephasing from turbulent flow and character of maximum intensity projection (MIP) were suggested as the main cause of the decreased poststenotic signal, our study indicated that a hemodynamically redistributed central axis of inflow and reduced flow volume through stenotic channel is one of the basic factors of the decreased signal intensity ot the poststenotic segmenton MRA.