Simultaneous noncontrast angiography and intraplaque hemorrhage (SNAP) imaging for carotid atherosclerotic disease evaluation

Abstract

A simultaneous noncontrast angiography and intraplaque hemorrhage (SNAP) MR imaging technique is proposed to detect both luminal stenosis and hemorrhage in atherosclerosis patients in a single scan. Thirteen patients with diagnosed carotid atherosclerotic plaque were admitted after informed consent. All scans were performed on a 3T MR imaging system with SNAP, 2D time-of-flight and magnetization-prepared 3D rapid acquisition gradient echo sequences. The SNAP sequence utilized a phase sensitive acquisition, and was designed to provide positive signals corresponding to intraplaque hemorrhage and negative signals corresponding to lumen. SNAP images were compared to time-of-flight images to evaluate lumen size measurements using linear mixed models and the intraclass correlation coefficient. Intraplaque hemorrhage identification accuracy was evaluated by comparing to magnetization-prepared 3D rapid acquisition gradient echo images using Cohen's Kappa. Diagnostic quality SNAP images were generated from all subjects. Quantitatively, the lumen size measurements by SNAP were strongly correlated (intraclass correlation coefficient = 0.96, P < 0.001) with those measured by time-of-flight. For intraplaque hemorrhage detection, strong agreement (κ = 0.82, P < 0.001) was also identified between SNAP and magnetization-prepared 3D rapid acquisition gradient echo images. In conclusion, a SNAP imaging technique was proposed and shows great promise for imaging both lumen size and carotid intraplaque hemorrhage with a single scan.

Figure 1

(a)The pulse sequence of the SNAP sequence. The IR pulses are slab-selective with the slab perpendicular to the flow direction; each arrow between IR pulses represents a gradient-echo measured with a repetition time TR; α and θ are the flip angles for the image acquisition and phase corrections, respectively. TI is the delay between the IR pulse and the center of the gradient echo acquisition train; T_gap is the time gap between the IR pulse and the first gradient echo acquisition and T_ex is the time between the last gradient echo acquisition and the next IR. IRTR is the duration between the two consecutive IRs. For better visualization, T_gap and T_ex were not drawn proportionally. (b) The signal recovery curve of three main components (IPH, vessel wall, blood) in a whole IRTR. TI=500ms is the optimal value that provides the best contrast among the three.

Figure 2

Sample SNAP images and different viewing options. The original contrast of SNAP acquired cross-sectionally with both positive and negative values is shown in (a); the negative contrast (b) and positive contrast (c) SNAP images can be generated to evaluate lumen and IPH lesions. The color-coded SNAP image (d), which is by nature fully registered, can be used to jointly evaluate both risk factors. 3D MIP images (e) are also used to help facilitate the identification and evaluation of lesions. In this image, red-color labeled IPH lesion can be easily identified (arrow) and the dashed line indicates the location where images (a–d) were acquired. Notice the SNAP images (a–d) still represent good image quality although they were acquired at the peripheral region of the coil (notice the signal drop on e).

Figure 3

SNAP sequence optimization. Panel (a) shows the optimization of TI and FA: the total contrast ξ maximized when TI=500ms and FA=11o. Panel (b) shows the theoretical signal curves of IPH wall and lumen at different TIs. As seen in this plot, when the optimal TI of 500ms (arrow) is used, the lumen signal remains negative while IPH presents a strong positive signal.

Figure 4

SNAP comparison with TOF and MP-RAGE for lumen and IPH delineation. The negative portion of SNAP agrees well with TOF for lumen delineation while the positive portion of SNAP agrees well with MP-RAGE for IPH detection. SNAP was found to provide consistently higher IPH-wall contrast when compared to the MP-RAGE.

Figure 5

SNAP with histology confirmation. 3D MIP images of the MRA-portion (a), IPH-portion (b) and color-coded joint view (c) of the SNAP images. Both IPH and luminal MRA were nicely delineated throughout the 160mm coverage of bilateral carotid arteries. Even small branches of the carotid artery, high-risk features like ulceration (Arrows) and high-level stenosis (Arrowheads) were visualized. On cross-sectional reformatted images (d), both IPH and luminal shapes were confirmed by the matched Mallory’s trichrome histology slides.

Figure 6

Lumen size measurements comparison between SNAP and TOF. A very high agreement (ICC=0.96) between the two approaches were found.

Figure 7

(a) A Bland-Altman plot comparing lumen size measurements between SNAP and TOF revealed a bias between the two measurements and the dotted lines indicate the limits of agreement around the bias. SNAP was found to measure 3.4% larger lumen sizes than TOF. (b) Sample SNAP and TOF images demonstrated the relative underestimation of lumen size by TOF (left), compared to SNAP (right). As arrow indicated, the internal carotid artery is missing on the TOF image, but well delineated on SNAP image. The jugular vein (arrowhead) is also invisible on TOF due to venous flow saturation. *: External Carotid Arteries; **: Vertebral Arteries.