Magnetic resonance imaging of lipid deposits in human atheroma via a stimulated-echo diffusion-weighted technique

MR plaque imaging of the carotid artery

Atherosclerotic carotid plaque represents a major cause of cerebral ischemia. The detection of vulnerable plaque is important for preventing future cardiovascular events. The key factors in advanced plaque that are most likely to lead to patient complications are the condition of the fibrous cap, the size of the necrotic core and hemorrhage, and the extent of inflammatory activity within the plaque. Magnetic resonance (MR) imaging has excellent soft tissue contrast and can allow for a more accurate and objective estimation of carotid wall morphology and plaque composition. Recent advances in MR imaging techniques have permitted serial monitoring of atherosclerotic disease evolution and the identification of intraplaque risk factors for accelerated progression. The purpose of this review article is to review the current state of techniques of carotid wall MR imaging and the characterization of plaque components and surface morphology with MR imaging, and to describe the clinical practice of carotid wall MR imaging for the determination of treatment plan.

High-resolution, multicontrast three-dimensional-MRI characterizes atherosclerotic plaque composition in ApoE-/- mice ex vivo

PURPOSE

To systematically investigate intrinsic MR contrast mechanisms that would facilitate plaque characterization and quantification in the aortic root and brachiocephalic artery of ApoE-/- mice ex vivo.

MATERIALS AND METHODS

To establish unambiguous MR parameters for routinely analyzing atherosclerotic plaque ex vivo at 11.7 T, relaxation times of plaque components were quantitatively assessed. Magnetization transfer and lipid-proton three-dimensional MR imaging was investigated for visualization of collagen- and lipid-rich plaque regions, respectively. A three-dimensional multiecho sequence with a spatial resolution of 47 x 47 x 63 microm was implemented providing a variable degree of T2-weighting.

RESULTS

Relaxation time measurements showed clear tissue heterogeneity between atherosclerotic plaque components in the T2-values, but similar T1-values at 11.7 T (T1/T2 mean +/- SD; cellular plaque component: 1.2 +/- 0.3 seconds/26.3 +/- 0.4 msec; fibrofatty plaque component: 1.1 +/- 0.2 seconds/13.7 +/- 2.0 msec). The three-dimensional multiecho sequence allowed the calculation of the intrinsic proton density and T2-maps. The sum of the multiecho data provided strong T2-weighting that facilitated quantification of various components of atherosclerotic plaque in the mouse aortic root and correlated well with histology (P < 0.0001).

CONCLUSION

High-resolution MRI allows for accurate classification and quantification of atherosclerotic plaque components in the aortic root of mice.

Chemical-shift imaging utilizing the positional shifts along the readout gradient direction

In this work, we describe a method that uses the linear phase acquired during the readout period due to chemical shift to generate individual magnetic resonance (MR) images of chemically shifted species. The method utilizes sets of Fourier (or k-space) data acquired with different directions of the readout gradient and a postprocessing algorithm to generate chemical shift images. The methodology is developed for both Cartesian data acquisition and for radial data acquisition. The method is presented here for two chemically shifted species but it can be extended to more species. In this work, we present the theory, show the results in phantoms and in human images, and discuss the artifacts and signal-to-noise ratio of the images obtained with the technique.

The assessment of the vulnerable atherosclerotic plaque using MR imaging: a brief review

The study of atherosclerotic disease during its natural history and after therapeutic intervention may enhance our understanding of the progression and regression of this disease and will aid in selecting the appropriate medical treatments or surgical interventions. Several invasive and non-invasive imaging techniques are available to assess atherosclerotic disease vessels. Most of these techniques are strong in identifying the morphological features of the disease such as lumenal diameter and stenosis or wall thickness, and in some cases provide an assessment of the relative risk associated with the atherosclerotic disease. However, none of these techniques can fully characterize the composition of the atherosclerotic plaque in the vessel wall and therefore are incapable of identifying the vulnerable plaques. High-resolution, multi-contrast, magnetic resonance (MR) can non-invasively image vulnerable plaques and characterize plaques in terms of lipid and fibrous content and identify the presence of thrombus or calcium. Application of MR imaging opens up whole new areas for diagnosis, prevention, and treatment of atherosclerosis.

Atherosclerotic plaques: classification and characterization with T2-weighted high-spatial-resolution MR imaging-- an in vitro study

PURPOSE

To evaluate if T2-weighted high-spatial-resolution magnetic resonance (MR) imaging (117 microm per pixel) can help accurate classification of atherosclerotic plaques.

MATERIALS AND METHODS

Thirty human arteries and 11 carotid endarterectomy specimens from 31 patients underwent T2-weighted MR imaging (2-T magnet; repetition time, 2,000 msec; echo time, 50 msec) at room temperature. After imaging, Bouin fixative was used to fix 26 arteries, and the other 15 arteries were fixed by means of freezing. Specimens were stained with hematoxylin-eosin and safranin or Sudan lipid stain. MR images and histologic slices were classified independently by two radiologists and a pathologist, respectively, on the basis of the American Heart Association classification.

RESULTS

Results with MR imaging were the following: type I-II plaques, sensitivity of 67% and specificity of 100%; type IV-Va plaques, sensitivity of 74% and specificity of 85%; type Vb plaques, sensitivity of 90% and specificity of 100%; type Vc plaques, sensitivity of 80% and specificity of 90%. No type III plaque was diagnosed in the study. The overall kappa value was 0.68.

CONCLUSION

High-spatial-resolution MR imaging with T2 weighting alone can help accurate classification of fibrocalcic plaques (type Vb), but it is subject to limitations for the classification and analysis of other types of atherosclerotic plaques.

High resolution ex vivo magnetic resonance imaging of in situ coronary and aortic atherosclerotic plaque in a porcine model

Atherosclerotic plaque composition is central to the pathogenesis of plaque disruption and acute thrombosis. Thus, there is a need for accurate imaging and characterization of atherosclerotic lesions. Even though there is no ideal animal model of atherosclerosis, the porcine model is considered to most closely resemble human atherosclerosis. We report the feasibility of MR imaging and characterizing of atherosclerotic lesions from in situ coronary arteries and aortas in an ex vivo setting and validate this with histopathology. Coronary and aortic atherosclerosis was induced in Yucatan mini-swine (n=4) by a combination of atherogenic diet (6 months) and balloon injury. All coronary arteries were imaged ex vivo on the intact heart, preserving the curvature of their course. The aorta also underwent MR imaging. The MR images were correlated with the matched histopathology sections for both the coronary arteries (n=54) and the aortas (n=43). MR imaging accurately characterized complex atherosclerotic lesions, including calcified, lipid rich, fibrocellular and hemorrhagic regions. Mean wall thickness for the coronary arteries (r=0.94, slope: 0.81) and aortas (r=0.94, slope: 0.81) as well as aortic plaque area (r=0.97, slope: 0.90) was accurately determined by MR imaging (P<0.0001). Coronary artery MR imaging is not limited by the curvature of the coronary arteries in the heart. MR imaging accurately quantifies and characterizes coronary and aortic atherosclerotic lesions, including the vessel wall, in this experimental porcine model of complex atherosclerosis. This model may be useful for future study of MR imaging of atherosclerosis in vivo.

Characterization of atherosclerotic plaques by magnetic resonance imaging

The study of atherosclerotic disease during its natural history and after therapeutic intervention will enhance our understanding of the progression and regression of this disease and will aid in selecting the appropriate medical treatments or surgical interventions. Several invasive and noninvasive imaging techniques are available to assess atherosclerotic vessels. Most of these techniques are strong in identifying the morphological features of the disease, such as lumenal diameter and stenosis or wall thickness, and in some cases provide an assessment of the relative risk associated with the atherosclerosis. However, none of these techniques can fully characterize the composition of the atherosclerotic plaque in the vessel wall and, therefore, are incapable of identifying the vulnerable plaques. High-resolution, multi-contrast, magnetic resonance (MR) can non-invasively image vulnerable plaques, characterize plaques in terms of lipid and fibrous content, and identify the presence of thrombus or calcium. Application of MR imaging opens up whole new areas for diagnosis, prevention, and treatment (e.g., lipid-lowering drug regimens) of atherosclerosis.

Early increases in breast tumor xenograft water mobility in response to paclitaxel therapy detected by non-invasive diffusion magnetic resonance imaging

An important goal in cancer chemotherapy is to sensitively and quantitatively monitor the response of individual patients' tumors to successful, or unsuccessful, therapy so that regimens can be altered iteratively. Currently, tumor response is monitored by frank changes in tumor morphology, yet these markers take long to manifest and are not quantitative. Recent studies suggest that the apparent diffusion coefficient of water (ADCw), measured noninvasively with magnetic resonance imaging, is sensitively and reliably increased in response to successful CTx. In the present study, we investigate the combination chemotherapy response of human breast cancer tumor xenografts sensitive or resistant to Paclitaxel by monitoring changes in the ADCw. Our results indicate that there is a clear, substantial, and early increase in the ADCw after successful therapy in drug sensitive tumors and that there is no change in the ADCw in p-glycoprotein-positive tumors, which are resistant to Paclitaxel. The mechanism underlying these changes is unknown yet is consistent with apoptotic cell shrinkage and a concomitant increase in the extracellular water fraction.

Quantitation of atherosclerosis by magnetic resonance imaging and 3-D morphology operators

The objective was to ascertain whether MRI and image processing can be used to quantify atherosclerosis by measuring wall thickness in rabbit aorta. The abdominal aortas of 2 healthy and 5 atherosclerotic rabbits were examined with a gradient-echo inflow angiography sequence (2DI) and a proton density weighted turbo-spin-echo sequence (PDW). Using thresholding by four observers and 3D morphology operators, segmentation of the artery and vein lumina was performed from the 2DI sequence, and of surrounding fat and muscle from the PDW sequence. Remaining voxels adjacent to the aortic lumen were classified as vessel wall. By measuring the vessel wall volume and the lumen volume, the wall percentage was calculated. The values were significantly higher for the diseased animals than for unaffected individuals (p < 0.01). It is concluded that aortic wall thickening in atherosclerotic rabbits can be measured quantitatively by using MRI combined with 3D morphology image processing operators.

Characterization of atherosclerotic plaque components by high resolution quantitative MR and US imaging

High resolution MRI at 3 T and US imaging at 50 MHz were used for atherosclerotic plaque characterization. For 14 excised segments of human arteries, conventional MR and US images, quantitative MR T2 maps, US integrated attenuation (IA) maps, and histologic sections were produced and compared. The MR T2 and US attenuation mean values estimated in selected regions of interest were related with tissue type as identified on histologic sections. Significant distinction between media or collagen and lipid or collagen lipidic plaque was achieved with both techniques (MR: P < .001; US: P < .01). Significant distinction was obtained between media and collagen (P < .0001) and between iliac and aortic media (P < .05) with MR T2 but not with IA. MR and US native and parametric images, with different sensitivities to tissue type, provide complementary information useful for quantitative plaque characterization.

MRI and NMR spectroscopy of the lipids of atherosclerotic plaque in rabbits and humans

The early stages of atherosclerosis are characterized by the deposition of cholesteryl esters and triglycerides into the arterial wall. In the excised human atherosclerotic plaque these lipids are in a liquid-like state at body temperature and observable via MRI and NMR spectroscopy. To assess the ability of MRI to quantitatively image the lipids of atherosclerotic plaque in vivo, we have investigated eight New Zealand White rabbits fed atherogenic diets (2 weight (wt)% cholesterol, 1 wt% cholesterol + 6 wt% peanut oil, and 1 wt% cholesterol + 6 wt% com oil). Postmortem examination indicated that all rabbits developed atherosclerosis in the aorta. Except for one animal, magnetic resonance angiography showed no noticeable obstruction in the aorta. MRI was carried out in an attempt to image atherosclerotic plaque lipids directly, but no signal was detected in vivo. However, a plaque lipid signal was observed from excised tissue using a small diameter RF coil. 1H NMR spectroscopy of the atherosclerotic plaque from excised aortas indicated that the major fraction of plaque lipids in rabbits is not in a liquid state at physiological temperature and are only marginally MRI-visible compared to human plaque lipid. The differences in the MRI characteristics of rabbit and human plaque are due to differences in the fatty acid profile of the cholesteryl esters, chiefly a decrease of linoleic acid in rabbit lesions.

Water diffusion properties of human atherosclerosis and thrombosis measured by pulse field gradient nuclear magnetic resonance

Using pulsed field gradient methods combined with magnetic resonance imaging, we calculated the apparent water diffusion coefficient D in different atherosclerotic components to probe the microstructure of normal and diseased arteries by characterizing molecular motion. D was equal to 0.26 +/- 0.13 x 10(-5) cm2.s-1 in plaque lipid core, 1.45 +/- 0.41 x 10(-5) cm2.s-1 in collagenous cap, and 1.54 +/- 0.30 x 10(-5) cm2.s-1 in normal media. Water diffuses isotropically in the atheromatous core of the plaque, suggesting the absence or destruction of confining structures. The comparable diffusion coefficients in collagenous cap and normal media are consistent with similar biophysical barriers in both components. In thrombi, D varies with the aging processes (fresh thrombus, 0.72 +/- 0.11 x 10(-5) cm2.s-1; 1-week-old thrombus, 0.36 +/- 0.08 x 10(-5) cm2.s-1; old occluding thrombus, 1.33 +/- 0.33 x 10(-5) cm2.s-1), consistent with the cross-linking of the fibrin strands occurring in the early phase and the later thrombus organization. Defining an indirect index of arterial lipid infiltration, remodeling, and aging, diffusion imaging provides a new nuclear magnetic resonance characterization of atherothrombosis.

Analysis and comparison of motion-correction techniques in diffusion-weighted imaging

Motion continues to be a significant problem in MRI, producing image artifacts that can severely degrade image quality. In diffusion-weighted imaging (DWI), the problem is amplified by the presence of large gradient fields used to produce the diffusion weighting. Three correction methods applicable for correction of specific classes of motion are described and compared. The first is based on a generalised projection onto convex sets (GPOCS) postprocessing algorithm. The second technique uses the collection of navigator echoes to track phase errors. The third technique is based on a radial-scan data acquisition combined with a modified projection-reconstruction algorithm. Although each technique corrects well for translations, the radial-scan method proves to be more robust when more complex motions are present. A detailed description of the causes of MR data errors caused by rigid body motion is included as an appendix.

High resolution intravascular MRI and MRS by using a catheter receiver coil

Potentially important diagnostic information about atherosclerosis can be obtained by using magnetic resonance imaging and spectroscopy techniques. Because critical vessels such as the aorta, coronary arteries, and renal arteries are not near the surface of the body, surface coils are not adequate to increase the data quality to desired levels. A few catheter MR receiver coil designs have been proposed for imaging the walls of large blood vessels such as the aorta. These coils have limited longitudinal coverage and they are too thick to be placed into small vessels. A flexible, long and narrow receiver coil that can be placed on the tip of a catheter and will enable multi-slice high resolution imaging of small vessels has been developed. The authors describe the theory of the coil design technique, derive formulae for the signal-to-noise ratio characteristics of the coil, and show examples of high resolution cross-sectional images from isolated human aortas acquired by using this catheter coil. In addition, high resolution in vivo rabbit aorta images were obtained as well as a set of spatially resolved chemical shift spectra from a dog circumflex coronary artery.

Magnetic resonance images lipid, fibrous, calcified, hemorrhagic, and thrombotic components of human atherosclerosis in vivo

BACKGROUND

Although MRI can discriminate the lipid core from the collagenous cap of atherosclerotic lesions in vitro with T2 contrast, it has not yet produced detailed in vivo images of these human plaque components.

METHODS AND RESULTS

We imaged seven lesions from six patients who required surgical carotid endarterectomy and calculated T2 in vivo before surgery in various plaque regions. Using the same acquisition parameters, we repeated these measurements in vitro on the resected fragment and compared MR images with histology. T2 values calculated in vivo correlate with in vitro measurements for each plaque component; the in vitro discrimination we demonstrated previously with T2 contrast can therefore be performed similarly in vivo.

CONCLUSIONS

MRI is the first noninvasive imaging technique that allows the discrimination of lipid cores, fibrous caps, calcifications, normal media, and adventitia in human atheromatous plaques in vivo. This technique also characterizes intraplaque hemorrhage and acute thrombosis. This result may support further investigations that include MRI of plaque progression, stabilization, and rupture in human atherosclerosis.

Tissue characterisation of atherosclerotic carotid plaques by MRI

Carotid artery plaques with intraplaque haemorrhage or atheromatous debris have been found to be associated with an increased risk of embolic stroke. Other methods have failed to detect plaque morphology, and it is not clear whether MRI allows differentiation between prognostically and therapeutically relevant plaque types. We examined 17 carotid bifurcation plaques which had been removed in toto by MRI. For quantifying MR signal intensities (I) the contrast-to-noise ratio (CNR) was used: (ITissue-IRef)/SDRef, with normal saline (0.9%) as reference (Ref) and the standard deviation (SD) of the noise. Measurements were correlated with the histopathological appearance of "simple plaques", consisting of fibrous intimal thickening, lipid deposits and/or atheromatous tissue with cholesterol crystals, largely calcified plaques, and "complicated plaques", containing recent intramural haemorrhage or friable atheromatous debris. Significantly different mean CNR could be measured in the three plaque types on T1- and T2-weighted sequences (p < 0.00001) and using the FLASH pulse sequence with a flip angle of 15 degrees (p < 0.001). With the T1-weighted sequence simple plaques showed a CNR of 4.4 +/- 2.3, calcified plaques -4.8 +/- 2.6 and complicated plaques 15.1 +/- 4.3. Using this technique, each single plaque could be correctly classified, an unalterable prerequisite for a clinical application. To date, motion artefacts due to patient movement or insufficiently triggerable vessel pulsation in combination with relative long acquisition times (6-7 min) have limited in vivo investigations. If these problems could be overcome, MRI might become a valuable technique for studying carotid plaque morphology.

T 2 -Weighted Contrast for NMR Characterization of Human Atherosclerosis

We sought to determine whether 'H NMR images without chemical-shift selection can adequately characterize the components of human atheromatous arteries. NMR, as a nondestructive, biochemical imaging tool, has the potential to identify lipids in atherosclerotic plaques but has not yet produced detailed images of atheroma components. Using 'H NMR spectroscopy at 9.4 T, we examined microdissected components of diseased and normal arteries to determine water relaxation constants (Tl and T2) as well as the relative content of mobile lipid. Relaxation times were also measured at 1.5 and 4.7 T. Sections of arteries with atherosclerotic lesions of graded severity were imaged at 1.5 and 9.4 T. The contrastto- noise ratio (CNR) was used to assess lesion conspicuity. In the atheromatous core, the water NMR signal predominates over that of lipid (lipid-to-water ratio, 0.11). At 9.4 T, T2 is 20.2 ms for the atheromatous core, 30.1 ms for the collagenous cap, and 29.5 ms for normal media. This results in a high CNR on T2-weighted (T2w) images for atheromatous core compared with the collagenous cap and normal media. A similar contrast was measured at lower field strength. Calcifications do not generate appreciable signal due to their low water content but can be detected on Tl-weighted (Tlw) images. The water T2 contrast allows discrimination of the atheromatous lipid core from collagenous regions. The combination of Tlw and T2w sequences permits in vitro identification of the atheromatous core, collagenous cap, calcifications, media, adventitia, and perivascular fat. The discrimination of collagen fibers that overlie lipid deposits permits study of plaque protection and stability at all field strengths and may provide the basis for in vivo microscopy of human atherosclerosis. (Arterioscler Thromb Vase Biol. 1995;15:1533-1542.)

Techniques for high-resolution MR imaging of atherosclerotic plaque

Atherosclerotic cardiovascular disease is the most common cause of death in the United States. Investigation of atherosclerotic plaque morphology and composition is important because the findings may be useful in predicting prognosis or response to therapy. This study presents high-resolution magnetic resonance (MR) imaging techniques developed on a 1.5-T whole-body imager with a custom-built surface coil, for characterizing the composition and morphology of plaque removed at carotid endarterectomy. The initial comparison of MR imaging and histologic results showed good correlation. In conjunction with MR angiography, these techniques could be used in in vivo imaging to define the size, location, and contents of atherosclerotic plaque at the carotid bifurcation.

Use of a projection reconstruction method to decrease motion sensitivity in diffusion-weighted MRI

Diffusion-weighted MRI is a clinically useful technique, but its utility is compromised by high sensitivity to patient motion. Use of radial-scan data acquisition and projection reconstruction, rather than the conventional Fourier imaging method, can substantially reduce the sensitivity to global translational motion of the object. The basis of this concept and a demonstration of the technique in an animal imaging experiment are presented.

A chemical shift selective inversion recovery sequence for fat-suppressed MRI: theory and experimental validation

Fat-suppression techniques are used extensively in routine proton nuclear magnetic resonance imaging to produce images free from chemical shift artifacts and dynamic range problems. A hybrid fat-suppression sequence is studied which combines the principle of short time inversion recovery with chemical shift selective imaging. The aim of this study is to provide a theoretical understanding of the role of the sequence parameters, as well as to compare this hybrid sequence with its most closely related conventional fat-suppression techniques, namely selective pre-saturation and short time inversion recovery (STIR) imaging. The hybrid technique is shown to be robust in normal use, and more tolerant than the conventional methods to mis-settings of parameters such as inversion time, as well as tip angle and frequency bandwidth of the fat selective pulse.