Ex vivo study of carotid endarterectomy specimens: quantitative relaxation times within atherosclerotic plaque tissues

Purpose: Previous studies reporting relaxation times within atherosclerotic plaque have typically used dedicated small-bore high-field systems and small sample sizes. This study reports quantitative T-1, T-2 and T-2* relaxation times within plaque tissue at 1.5 T using spatially co-matched histology to determine tissue constituents. Methods: Ten carotid endarterectomy specimens were removed from patients with advanced atherosclerosis. Imaging was performed on a 1.5-T whole-body scanner using a custom built 10-mm diameter receive-only solenoid coil. A protocol was defined to allow subsequent computation of T-1, T-2 and T-2* relaxation times using multi-flip angle spoiled gradient echo, multi-echo fast spin echo and multi-echo gradient echo sequences, respectively. The specimens were subsequently processed for histology and individually sectioned into 2-mm blocks to allow subsequent co-registration. Each imaging sequence was imported into in-house software and displayed alongside the digitized histology sections. Regions of interest were defined to demarcate fibrous cap, connective tissue and lipid/necrotic core at matched slice-locations. Relaxation times were calculated using Levenberg-Marquardt's least squares curve fitting algorithm. A linear-mixed effect model was applied to account for multiple measurements from the same patient and establish if there was a statistically significant difference between the plaque tissue constituents. Results: T-2 and T-2* relaxation times were statistically different between all plaque tissues (P=.026 and P=.002 respectively) [T-2: lipid/necrotic core was lower 47+/-13.7 ms than connective tissue (67+/-22.5 ms) and fibrous cap (60+/-13.2 ms); T-2*: fibrous cap was higher (48+/-15.5ms) than connective tissue (19+/-10.6 ms) and lipid/necrotic core (24+/-8.2 ms)]. T1 relaxation times were not significantly different (P=.287) [T-1: Fibrous cap: 933+/-271.9 ms; connective tissue (1002+/-272.9 ms) and lipid/necrotic core (1044+/-304.0 ms)]. We were unable to demarcate hemorrhage and calcium following histology processing. Conclusions: This study demonstrates that there is a significant difference between qT(2) and qT(2)* in plaque tissues types. Derivation of quantitative relaxation times shows promise for determining plaque tissue constituents. (C) 2012 Elsevier Inc. All rights reserved.