Atherosclerotic aortic component quantification by noninvasive magnetic resonance imaging: an in vivo study in rabbits

In vivo magnetic resonance evaluation of associations between aortic atherosclerosis and both risk factors and coronary artery disease in patients referred for coronary angiography

BACKGROUND

Magnetic resonance imaging was recently reported to detect atherosclerotic plaques in thoracic and abdominal aortas.

METHODS

Using magnetic resonance imaging, we investigated associations of risk factors and plasma inflammatory markers with plaques in both thoracic and abdominal aortas in 102 patients undergoing coronary angiography. Associations between coronary artery disease (CAD) and aortic plaques were also evaluated.

RESULTS

Plaques in thoracic and abdominal aortas were detected in 61% and 90% of patients, respectively. Age and systolic blood pressure correlated with plaque extents in both the aortas. Serum LDL cholesterol level correlated with plaque extent in the thoracic aorta (r(s) = 0.42). The degree of smoking correlated with plaque extent in the abdominal aorta (r(s) = 0.43). In multivariate analysis, age and systolic blood pressure were associated with plaques in both the aortas. The LDL cholesterol and smoking were characteristically associated with plaques in the thoracic and abdominal aortas, respectively. Regarding inflammatory markers, fibrinogen and C-reactive protein levels correlated with total plaque extent in the aortas (r(s) = 0.50 and r(s) = 0.51). Compared with 24 patients without CAD, 78 with CAD more often had plaques in the thoracic (71% vs 29%) and abdominal (95% vs 75%) aortas. Although plaque extents in both the aortas correlated with the severity of CAD, only thoracic plaques were independently associated with CAD.

CONCLUSIONS

The thoracic and abdominal aortas may have different susceptibilities to risk factors. However, plasma inflammatory markers appear to reflect total extent of aortic atherosclerosis. Although aortic plaques are common in patients with CAD, only thoracic plaques are an independent factor for CAD.

Lipid-rich atherosclerotic plaques detected by gadofluorine-enhanced in vivo magnetic resonance imaging

BACKGROUND

MRI of specific components in atherosclerotic plaque may provide information on plaque stability and its potential to rupture. We evaluated gadofluorine in atherosclerotic rabbits using a new MR sequence that allows plaque detection within 1 hour after injection and assessed enhancement in lipid-rich and non-lipid-rich plaques.

METHODS AND RESULTS

Twelve rabbits with aortic plaque and 6 controls underwent MRI before and up to 24 hours after gadofluorine injection (50 micromol/kg). Two T1-weighted, segmented gradient-echo sequences (TFL) were compared to enhance vessel wall delineation after injection: (1) an inversion-recovery prepulse (IR-TFL) or (2) a combination of inversion-recovery and diffusion-based flow suppression prepulses (IR-DIFF-TFL). With the use of IR-TFL at 1 hour after injection, the vessel wall was not delineated because of poor flow suppression; at 24 hours after injection, the enhancement was 37% (P<0.01). IR-DIFF-TFL showed significant enhancement after versus before contrast (1 hour: 164% [P<0.005]; 24 hours: 207% [P<0.001]). At 1 hour and 24 hours after injection, the contrast-to-noise ratio was higher with the use of IR-DIFF-TFL than with IR-TFL (1 hour: 13.0+/-7.7 versus -19.8+/-10.3 [P<0.001]; 24 hours: 15.2+/-5.9 versus 11.4+/-8.9, respectively [P=0.052]). There was no enhancement in the vessel wall after gadofluorine injection in the control group. A strong correlation was found (r2=0.87; P<0.001) between the lipid-rich areas in histological sections and signal intensity in corresponding MR images. This suggests a high affinity of gadofluorine for lipid-rich plaques.

CONCLUSIONS

Gadofluorine-enhanced MRI improves atherosclerotic plaque detection. The IR-DIFF-TFL method allows early detection of atherosclerotic plaque within 1 hour after gadofluorine injection.

The selective peroxisomal proliferator-activated receptor-gamma agonist has an additive effect on plaque regression in combination with simvastatin in experimental atherosclerosis

OBJECTIVES

We sought to investigate the anti-atherogenic effects of a selective peroxisomal proliferator-activated receptor-gamma (PPAR-gamma) agonist and simvastatin, as well as their combination, over time, in a rabbit model of experimental atherosclerosis.

BACKGROUND

The PPARs are nuclear transcription factors that control a variety of cellular functions, with the potential effects required to induce plaque regression and stabilization.

METHODS

Atherosclerosis was induced in rabbits (n = 37) by the combination of double-balloon injury and a nine-month high-cholesterol (HC) diet. The rabbits were randomized into a continued HC diet, a normal chow (NC) diet, NC plus simvastatin, NC plus PPAR-gamma agonist, and NC plus simvastatin plus PPAR-gamma agonist. All rabbits underwent magnetic resonance imaging (MRI) at randomization and after six months of treatment and were then sacrificed for histopathologic study.

RESULTS

All groups had a similar vessel wall area by MRI (8.45 +/- 0.65 mm(2), p = NS between groups) at randomization. Significant progression was seen in the HC diet group (15 +/- 4%, p < 0.01). In the NC and NC plus PPAR-gamma agonist groups, progression was abolished (-2.5 +/- 3% and -4.5 +/- 5%, respectively; p = NS). The NC plus simvastatin and NC plus simvastatin plus PPAR-gamma agonist groups had significant plaque regression (-12 +/- 4% [p < 0.05] and -22 +/- 4% [p < 0.01], respectively). Regression was independent of plasma lipid levels. All NC groups had similar lipid profiles at the end of treatment. Histopathologic analysis of the NC groups showed a decreased macrophage content and matrix metalloproteinase activity and an increased smooth muscle cell/collagen content of lesions.

CONCLUSIONS

Our data indicate that normalization of plasma lipid levels abolishes progression of atherosclerosis. Simvastatin elicits regression of atherosclerotic lesions, and the combination simvastatin plus PPAR-gamma agonist has additive regression effects on plaque. This is paralleled by structural alterations in plaque composition, which may increase plaque stability. These observations support the beneficial effects of statins on atherosclerosis and show additional anti-atherogenic benefits of combining a PPAR-gamma agonist with simvastatin.

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.

Effective blood signal suppression using double inversion-recovery and slice reordering for multislice fast spin-echo MRI and its application in simultaneous proton density and T2 weighted imaging

PURPOSE

To design a multislice double inversion-recovery fast spin-echo (FSE) sequence, with k-space reordered by inversion time at slice position (KRISP) technique, to produce black-blood vessel wall magnetic resonance imaging (MRI).

MATERIALS AND METHODS

In this sequence, central k-space sampling for each slice is required at inversion time (TI) of the blood signal. To fill the entire k-space, the peripheral lines are obtained less or greater the TI and using a rotating slice order. Blood flow signal suppression was first evaluated using a phantom. Simulation studies were used to investigate FSE image quality. The final sequence was then applied to the rabbit abdominal aorta MRI at 4.7 T.

RESULTS

In the flow phantom study, artifacts from slow-flowing water were substantially reduced by the KRISP technique; residual water spins were dephased by the strong phase-encoding gradient required for peripheral k-space. These dephased spins flowed into the slice plane where the center of k-space was being acquired at the TI of the flowing water signal. Multislice black-blood MR images were successfully obtained in the rabbit abdomen using the sequence with the k-trajectory optimized by the simulation study.

CONCLUSION

The KRISP technique was effective both in multislice double inversion-recovery FSE and in blood signal suppression.

Detection of atherosclerotic plaque with Gadofluorine-enhanced magnetic resonance imaging

BACKGROUND

The purpose of this study was to visualize atherosclerotic plaques independently of luminal narrowing using T1-weighted contrast-enhanced MRI.

METHODS AND RESULTS

Eight Watanabe heritable hyperlipidemic (WHHL) rabbits, aged 9 to 18 months, and 8 age-matched controls (New Zealand White rabbits) underwent MRI of the aortic arch before and up to 48 hours after injection of 100 micromol/kg Gadofluorine (Schering AG). Additionally, 8 WHHL rabbits were examined with Magnevist (Schering AG). A half-Fourier acquisition single-shot turbo-spin-echo (HASTE) sequence and a T1-weighted inversion-recovery turbo fast, low-angle shot sequence were used for data acquisition. Immediately after the MR examination, the animals were killed, the aorta was stained with Sudan red, and ex vivo imaging of the stained aortic specimens was performed. Additionally, gadolinium concentrations in plaque (Sudan-positive) and normal (Sudan-negative) aortic wall segments were measured. Plain MR imaging revealed no plaques in the aortic arch in either animal group. Enhancement occurred in the aortic wall of all WHHL rabbits examined with Gadofluorine but not in the vessel wall of animals examined with Magnevist and the control group. Sudan red staining demonstrated multiple plaques in the aortic arch of all WHHL rabbits. Ex vivo imaging demonstrated that the area of hyperenhancement matched the area of plaques stained with Sudan red. The gadolinium concentration was 7+/-5 nmol/g for normal aortic wall of the control group and 368+/-30 nmol/g for aortic wall with plaque in WHHL.

CONCLUSIONS

Gadofluorine enhances the imaging of atherosclerotic plaques and enables improved plaque detection of even nonstenotic lesions that are not visible on unenhanced MRI.

Plaque disruption and thrombus in Ambrose's angiographic coronary lesion types

Lesion eccentricity with irregularities on coronary angiography is associated with ruptured plaques and thrombus based on postmortem and clinical angiographic studies. However, the predictive value of such angiographic markers of plaque disruption and thrombus remains to be determined in vivo. The purpose of this study was to establish whether Ambrose's angiographic coronary lesion types and other angiographic criteria predict the presence of disrupted plaques and thrombus using intracoronary angioscopy. Angioscopy was performed before angioplasty in 60 patients with various coronary syndromes and culprit lesions that were not totally occlusive. Lesions were classified angiographically according to Ambrose's criteria as concentric, type I and II eccentric, and multiple irregularities, or as complex or noncomplex, and then compared with the corresponding angioscopic findings. Disruption and/or thrombus were seen in 17 of 19 type II eccentric lesions and 21 of 23 angiographically complex lesions and had the highest positive predictive value to detect complicated atherosclerotic plaques (type II eccentric lesions: positive predictive value 89%, 95% confidence intervals 67% to 99%; complex lesions: 91%, 95% confidence intervals 72% to 99%). We conclude that Ambrose's type II eccentric stenoses and angiographically complex lesions are strongly associated with disrupted plaques and/or thrombus as assessed by angioscopy in patients and represent unstable plaque substrates.

Volume-selective 3D turbo spin echo imaging for vascular wall imaging and distensibility measurement

PURPOSE

To use a volume-selective 3D turbo spin echo (TSE) technique to image the carotid artery wall and measure distensibility.

MATERIALS AND METHODS

A high-resolution volume-selective 3D TSE sequence has been developed. Volume selection is accomplished by orthogonal gradients for the 90 degrees and 180 degrees excitations and allows a 3D volume of vessel wall to be imaged in a relatively short time. The technique has been developed to allow imaging at any defined phase of the cardiac cycle so that the vascular function and distensibility can be studied.

RESULTS

Scan efficiency is increased by the reduced phase encode field of view (FOV) (k(y) steps) by the use of selective volume excitation. Significantly more slices (k(z) phase encode steps) for vessel coverage can be acquired with the same scan time as that of the conventional sequence while maintaining signal-to-noise ratio (SNR) levels. The practical value of the technique was demonstrated on 10 normal subjects with high-resolution vessel distensibility measurements of the carotid arteries.

CONCLUSION

A volume-selective TSE method has been used for carotid artery wall imaging and measurement of distensibility in normal subjects. Larger coverage of the vessel, and therefore more information for clinical diagnostics, was achieved with the same overall scan time with an SNR comparable to that of 2D full FOV images.

New understanding, diagnosis, and prognosis of atherothrombosis and the role of imaging

Despite crucial advances in our knowledge of the pathologic mechanisms and the availability of effective diagnostic and treatment modalities, coronary atherothrombosis remains the most frequent cause of ischemic heart disease. Plaque disruption with superimposed thrombosis is the main cause of unstable angina, myocardial infarction, and sudden death. New findings have recently introduced exciting concepts that could have major impact on the treatment of the atherothrombotic disease. We will discuss the mechanisms that lead to the development of atherothrombosis and those responsible for the acute coronary syndromes, as well as some of the concepts derived from in vivo observations using new imaging technologies (eg, high-resolution magnetic resonance imaging).

A novel nonobstructive intravascular MRI coil: in vivo imaging of experimental atherosclerosis

OBJECTIVE

MRI is being used to characterize the composition of atherosclerotic plaques. However, the resolution achievable using surface radiofrequency coils is limited by the signal to noise ratio. We studied the utility of a new intravascular (IV) MRI probe for high-resolution in vivo imaging of atherosclerotic lesions.

METHODS AND RESULTS

Balloon-injured Watanabe heritable hyperlipidemic rabbits served as the experimental model of atherosclerosis. The newly developed IV MRI probe is 1.3 mm in diameter and can be positioned over a guidewire. MRI was performed with both an external phased-array coil and the IV MR coil. MR observations were correlated with histopathology. After MRI, the animals were killed and analysis of agreement between MR and histopathology was performed. The IV MR coil allows aortic images to be obtained with 156x156 micro m(2) in-plane resolution versus 352x352 micro m(2) when used with the external phased-array coil. No significant motion artifacts were noted, despite the continuation of arterial blood flow during image acquisition around the IV probe. The different components of the atherosclerotic lesions (lipid core and fibrous cap) were easily identified. There was an excellent agreement between MRI with the IV coil and histopathology by simple linear regression for both the mean wall thickness (r=0.88, slope 0.82, P<0.0001) and vessel wall area (r=0.86, slope 1.08, P<0.0001).

CONCLUSIONS

The new nonobstructive design for this intravascular coil provides great promise for additional work in high-resolution MRI characterization of atherosclerotic plaques in vivo. The ability to position the probe with a guidewire allows its placement under fluoroscopic or MRI guidance, whereas its size is compatible with human coronary arteries.

MR imaging for the noninvasive assessment of atherothrombotic plaques

Imaging methods to quantify the progression and regression of atherosclerosis could play a strong role in the management of patients. High-resolution, noninvasive MR imaging may provide exhaustive 3D anatomical information about the lumen and the vessel wall. Furthermore, MR imaging has the ability to characterize plaque composition and microanatomy and therefore to identify lesions vulnerable to rupture or erosion. The high resolution of MR imaging and the development of sophisticated contrast agents offer the promise of molecular in vivo molecular imaging of the plaque. This may aid early intervention (eg, lipid-lowering drug regiments) in both primary and secondary treatment of vascular disease in all arterial beds.

Noncoronary and coronary atherothrombotic plaque imaging and monitoring of therapy by MRI

In the future, the use of imaging methods to quantify the progression and regression of atherosclerosis could play a strong role in the management of patients. High-resolution, noninvasive MRI may provide exhaustive 3-D anatomic information about the lumen and the vessel wall. Furthermore, MRI has the ability to characterize plaque composition and microanatomy and therefore to identify lesions vulnerable to rupture or erosion. The high resolution of MRI and the development of sophisticated contrast agents offer the promise of molecular in vivo molecular imaging of the plaque. This may aid early intervention (e.g., lipid lowering drug regiments) in both primary and secondary treatment of vascular disease in all arterial beds.

MRI and characterization of atherosclerotic plaque: emerging applications and molecular imaging

Noninvasive high-resolution magnetic resonance has the potential to image atherosclerotic plaque and to determine its composition and microanatomy. This review summarizes the rationale for plaque imaging and describes the characteristics of plaque by use of existing MRI techniques. The use of MRI in human disease and in animal models, particularly in rabbits and mice, is presented. Present and future applications of MRI, including real-time vascular intervention, new contrast agents, and molecular imaging, are also discussed.

In vivo noninvasive detection and age definition of arterial thrombus by MRI

OBJECTIVES

The purpose of this study was to evaluate the potential of magnetic resonance (MR) to detect arterial thrombotic obstruction and define thrombus age. BACKGROUND; Arterial thrombi underlie the clinical consequences of atherosclerosis and are not reliably detected by current noninvasive diagnostic techniques.

METHODS

Carotid thrombi were induced in swine (n = 7) by arterial injury. Serial high-resolution in vivo MR images were obtained using black-blood T1-weighted (T1W) and T2-weighted (T2W) sequences in a clinical 1.5T MR system at 6 h, 1 day and at 1, 2, 3, 6 and 9 weeks. At each time point one animal was sacrificed and the occluded carotid artery processed for histopathology. Thrombus signal intensity (SI) was normalized to that of the adjacent muscle. Thrombus age was assessed based on MR appearance by two blinded independent observers.

RESULTS

Thrombus appearance and relative SI revealed characteristic temporal changes in multicontrast-weighted MR images, reflecting histologic changes in the composition. Acute thrombus appeared very bright on the T2W images, facilitating the detection. Signal intensity was 197 +/- 25% at 6 h, peaking at 1 week (246 +/- 51%), reaching a plateau by 6 weeks (120 +/- 15%). At six weeks, complete thrombus organization was confirmed histologically. The T1W images had similar pattern with lower SI than T2W. Age definition using visual appearance was highly accurate (Pearson's chi-square with 4 df ranging from 96 to 132 and Cohen's kappa at 0.81 to 0.94). Agreement between observers was substantial (Pearson chi-square with 4 df = 91.5, kappa = 0.79).

CONCLUSIONS

Magnetic resonance imaging is a promising tool to noninvasively detect arterial thrombosis. Measurement of SI and the characteristic visual appearance of the thrombus have the potential to define thrombus age.

Progression and regression of atherosclerotic lesions: monitoring with serial noninvasive magnetic resonance imaging

BACKGROUND

Modification or stabilization of atherosclerotic plaques has been proposed as the mechanism responsible for the beneficial clinical effect of lipid-lowering therapies. An imaging modality able to quantify atherosclerotic plaque composition could potentially allow not only the identification of these vulnerable atherosclerotic lesions, but also monitoring of the effects of therapeutic interventions on plaque composition. Our aim was to monitor changes in burden and composition of atherosclerotic plaques in a rabbit model of complex atherosclerosis using serial noninvasive magnetic resonance imaging (MRI).

METHODS AND RESULTS

Aortic atherosclerotic lesions were induced in rabbits and the animals randomized to continue an atherogenic diet (atherosclerosis progression) or resume normal chow (atherosclerosis regression) for 6 months. MRI of the aorta was performed at 3 time points in each rabbit, as follows: baseline, after atherosclerosis induction (9 months old), and after atherosclerosis regression or progression (15 months old). Histopathologic correlation with MRI was performed. There was a significant (P<0.0001) reduction in atherosclerotic burden in the regression group, and an increase in the progression group. There was a significant (P<0.001) reduction in the lipidic component of plaques in the regression group, and an increase in the progression group. A small, nonsignificant increase in the fibrotic plaque components was noted in the regression group, but a significant decrease in the fibrotic composition of lesions in the progression group. A significant correlation (P<0.001) was found between MRI and histopathology for atherosclerotic burden and plaque composition.

CONCLUSIONS

These findings indicate that serial noninvasive MRI can monitor changes in atherosclerotic plaque composition under conditions of atherosclerotic progression and regression.

18F FDG uptake in the large arteries: a correlation study with the atherogenic risk factors

It has been reported that there is a high correlation between fluorodeoxyglucose (FDG) uptake in the aorta and macrophage content of atherosclerotic lesions in an experimental rabbit model. We evaluated the frequency of FDG uptake in the large arteries in relation to the atherogenic risk factors. We also investigated whether FDG uptake of the large arteries is related to clinically known coronary artery disease. The presence of FDG uptake was assessed in the abdominal aorta (AA), iliac (IA), and proximal femoral arteries (FAs) in 156 patients. Medical history of the atherogenic risk factors (age, cigarette smoking, hypertension, diabetes, high cholesterol, and obesity) and coronary artery disease (CAD) was identified for each patient. The frequency of vascular FDG uptake was compared between the patients without risk factors (Group I, 23 patients) and those with at least 1 risk factor (Group II, 133 patients). The correlation of each risk factor and known CAD with arterial FDG uptake was also assessed in the 3 different arteries. There was a significant difference in the frequency of FDG uptake between the 2 groups for the FA (22% vs 70%) and IA (30% vs 54%), but not for the AA (35% vs 53%). Among all risk factors, age was the most significant and consistent factor correlating with FDG uptake in all 3 arteries. Hypercholesterolemia also correlated consistently with FDG uptake in all 3 arteries. The correlation between the remaining risk factors and arterial FDG uptake was rather artery specific than consistent throughout all 3 arteries. A higher frequency of FDG uptake in the FA was seen in patients with CAD compared with those without CAD. Not all risk factors correlated with FDG uptake in different arteries. Among the risk factors, age and hypercholesterolemia most consistently correlated with FDG uptake in the AA, and the IA and proximal FAs. The positive correlation of arterial FDG uptake with the atherogenic risk factors suggested a promising role for FDG-PET imaging in the diagnosis of atherosclerosis and follow-up after treatment intervention.

Clinical imaging of the high-risk or vulnerable atherosclerotic plaque

The study of atherosclerotic disease during its natural history and after therapeutic intervention will enhance our understanding of disease progression and regression and aid in selecting appropriate treatments. Several invasive and noninvasive imaging techniques are available to assess atherosclerotic vessels. Most of the standard techniques identify luminal diameter, stenosis, wall thickness, and plaque volume; however, none can characterize plaque composition and therefore identify the high-risk plaques. We will present the different imaging modalities that have been used for the direct assessment of the carotid, aortic, and coronary atherosclerotic plaques. We will review in detail the use of high-resolution, multicontrast magnetic resonance for the noninvasive imaging of vulnerable plaques and the characterization of plaques in terms of their various components (ie, lipid, fibrous, calcium, or thrombus).