To develop the motion insensitive 3D technique for simultaneous measurement of in-vivo ADC and T2* in Atherosclerotic plaque to increase the quantification of high risk plaque components.

DWI has the potential to provide complementary information that will allow better discrimination of plaque components such as lipid core.¹ Iron has consistently been found in higher concentrations in atherosclerotic plaque compared to vessel tissue. It may be incorporated into hemoglobin or bound to the storage proteins ferritin and hemosiderin, both of which cause measurable changes in local magnetic field homogeneity. In previous studies, intraplaque T2* measurement distinguished symptomatic from asymptomatic plaques in patients with carotid atherosclerosis.² We have developed a motion insensitive high resolution 3D multiple echo diffusion weighted driven equilibrium Stack of Stars (3D ME-DW-DE SOS) sequence that can simultaneously measure ADC and T2* of water proton in a single scan. Figure 1 presents the 3D ME-DW-DE-SOS pulse. The DW-DE consists of an excitation pulse followed by two refocusing pulses and a tip-up pulse sequence. Two refocusing pulses were designed using three composite hard pulses. A total of four sets of diffusion sensitized bipolar gradients were applied. After DW-DE preparation, multiple echo measurements were immediately acquired using the 3D segmented SOS trajectory. After one excitation three spokes (triple echoes) were acquired. With the slice partitions acquired with a centric k-space ordering, the expected signal dependence of diffusion weighted factor b and echo time is

$$S(\triangle TE,T1,T2^{*},D)=S_{0}(T1,T2)e^{-\frac{\triangle TE}{T2^{*}}}e^{-bD}$$

The diffusion coefficients (D) were constructed using two diffusion weighted images. T2* decay, due to T2 relaxation and local field variation, was measured from the triple echo measurements with low b value and ΔTE separation. To test technique feasibility, MRI studies of five symptomatic and three asymptomatic patients with atherosclerosis were performed on a Siemens Trio 3T MRI scanner with a home built phased array carotid coils. The imaging parameters for 3D ME-DW-DE SOS were: transverse plane, FOV=152x152 mm2, 2 mm slice thickness, TE/TR = 2.05/8.0ms, bandwidths=560hz/pixel, 32 slices/slab, b =20, 450 s/mm². The resultant in-plane spatial resolution was 0.6x0.6 mm2. The total imaging time was 3 min 20 sec. 3D IR SOS images were acquired for hemorrhage detection. 3D T1 SPACE with DANTE preparation measurements were acquired before and after contrast administration. T2* and ADC maps were calculated and displayed using algorithms developed in IDL (ITT Visual Information). Mean and standard deviation values were computed using all pixels identified by ROI for T2* and ADC. Quantitative statistical comparison of ADC values from symptomatic and asymptomatic groups was conducted using unbalanced 1-way analysis of variance (ANOVA).

Three ROIs per each patient were selected in visible plaque. The mean T2* and ADC values for plaque obtained from the 8 subjects are summarized in Table 1. Symptomatic plaque had significantly lower T2* values than asymptomatic plaque (20±2.9 vs. 39±5.8ms, respectively, p<0.002). This value is close to the T2* value reported previously.² Figure 2 displays 3D MPRAGE, 3D T1w SPACE, T2* and ADC maps from a symptomatic subject with intramural hemorrhage. The ROI drawn by the red lines in the maps of Figure 2 demonstrate a typical ROI selection. In this ROI, the measured ADC and T2* values were 0.71±0.17x10^-3mm²/s and 24±2.29ms, respectively. 3D ME-DW-DE SOS can provide in-vivo T2* and ADC values in atherosclerotic plaque. We found that T2* values were reduced in symptomatic plaque, consistent with a shift to aggregate iron complexes that have greater local effects on magnetic susceptibility. The ADC values obtained from symptomatic plaque were lower than the values of asymptomatic plaque. Further study will include identifying changes in the amount, species, and chemistry of intra-plaque iron and their relationship with the changing of the diffusion properties during the course of atherosclerosis development. The SOS sequence is less sensitive to motion artifacts due to the repeated high-density sampling of the k-space center with off-resonance sensitivity.^3,4 The off-resonance sensitivity can be reduced by increased sampling bandwidth. This paper presented triple echo measurements with relatively moderate bandwidth. Multiple-echo measurement with more than three echoes and higher bandwidth may allow gradient timing calibration and multi-point fat-water separation along with T2* determination.⁵The 3D ME-DW-DE SOS can provide high resolution T2* and ADC values simultaneously, which may provide important clinical information to detect plaque progression.