Local versus global aortic pulse wave velocity in early atherosclerosis: An animal study in ApoE-/--mice using ultrahigh field MRI.

Increased aortic stiffness is known to be associated with atherosclerosis and has a predictive value for cardiovascular events. This study aims to investigate the local distribution of early arterial stiffening due to initial atherosclerotic lesions. Therefore, global and local pulse wave velocity (PWV) were measured in ApoE-/- and wild type (WT) mice using ultrahigh field MRI. For quantification of global aortic stiffness, a new multi-point transit-time (TT) method was implemented and validated to determine the global PWV in the murine aorta. Local aortic stiffness was measured by assessing the local PWV in the upper abdominal aorta, using the flow/area (QA) method. Significant differences between age matched ApoE-/- and WT mice were determined for global and local PWV measurements (global PWV: ApoE-/-: 2.7±0.2m/s vs WT: 2.1±0.2m/s, P<0.03; local PWV: ApoE-/-: 2.9±0.2m/s vs WT: 2.2±0.2m/s, P<0.03). Within the WT mouse group, the global PWV correlated well with the local PWV in the upper abdominal aorta (R2 = 0.75, P<0.01), implying a widely uniform arterial elasticity. In ApoE-/- animals, however, no significant correlation between individual local and global PWV was present (R2 = 0.07, P = 0.53), implying a heterogeneous distribution of vascular stiffening in early atherosclerosis. The assessment of global PWV using the new multi-point TT measurement technique was validated against a pressure wire measurement in a vessel phantom and showed excellent agreement. The experimental results demonstrate that vascular stiffening caused by early atherosclerosis is unequally distributed over the length of large vessels. This finding implies that assessing heterogeneity of arterial stiffness by multiple local measurements of PWV might be more sensitive than global PWV to identify early atherosclerotic lesions.

Local arterial stiffening assessed by MRI precedes atherosclerotic plaque formation.

BACKGROUND: Atherosclerosis is known to impair vascular function and cause vascular stiffening. The aim of this study was to evaluate the potential predictive role of vascular stiffening in the early detection of atherosclerosis. Therefore, we investigated the time course of early functional and morphological alterations of the vessel wall in a murine atherosclerosis model. Because initial lesions are distributed inhomogeneously in early-stage atherosclerosis, MR microscopy was performed to measure vascular elasticity locally, specifically the local pulse wave velocity and the arterial wall thickness. METHODS AND RESULTS: Local pulse wave velocity and the mean arterial wall thickness were determined in the ascending and the abdominal aortae of ApoE(-/-) and wild-type mice. In vivo MRI revealed that baseline pulse wave velocity and morphology were similar in 6-week-old ApoE(-/-) and WT mice, whereas at the age of 18 weeks, local pulse wave velocity was significantly elevated in ApoE(-/-) mice. Significantly increased vessel wall thickness was not found in ApoE(-/-) mice until the age of 30 weeks. Histological analysis of the aortae of ApoE(-/-) and WT mice showed that increased pulse wave velocity coincided with the fragmentation of the elastic laminae in the arterial wall, which is hypothesized to induce early vascular stiffening and may be promoted by macrophage-mediated matrix degradation. CONCLUSIONS: We newly report that the assessment of local pulse wave velocity via MRI provides early information about the local progression of atherosclerosis before macroscopic alterations of the vessel wall occur.

Regional in vivo transit time measurements of aortic pulse wave velocity in mice with high-field CMR at 17.6 Tesla.

BACKGROUND: Transgenic mouse models are increasingly used to study the pathophysiology of human cardiovascular diseases. The aortic pulse wave velocity (PWV) is an indirect measure for vascular stiffness and a marker for cardiovascular risk. RESULTS: This study presents a cardiovascular magnetic resonance (CMR) transit time (TT) method that allows the determination of the PWV in the descending murine aorta by analyzing blood flow waveforms. Systolic flow pulses were recorded with a temporal resolution of 1 ms applying phase velocity encoding. In a first step, the CMR method was validated by pressure waveform measurements on a pulsatile elastic vessel phantom. In a second step, the CMR method was applied to measure PWVs in a group of five eight-month-old apolipoprotein E deficient (ApoE(-/-)) mice and an age matched group of four C57Bl/6J mice. The ApoE(-/-) group had a higher mean PWV (PWV = 3.0 ± 0.6 m/s) than the C57Bl/6J group (PWV = 2.4 ± 0.4 m/s). The difference was statistically significant (p = 0.014). CONCLUSIONS: The findings of this study demonstrate that high field CMR is applicable to non-invasively determine and distinguish PWVs in the arterial system of healthy and diseased groups of mice.

Intracellular and extracellular T1 and T2 relaxivities of magneto-optical nanoparticles at experimental high fields.

This study reports the T(1) and T(2) relaxation rates of rhodamine-labeled anionic magnetic nanoparticles determined at 7, 11.7, and 17.6 T both in solution and after cellular internalization. Therefore cells were incubated with rhodamine-labeled anionic magnetic nanoparticles and were prepared at decreasing concentrations. Additionally, rhodamine-labeled anionic magnetic nanoparticles in solution were used for extracellular measurements. T(1) and T(2) were determined at 7, 11.7, and 17.6 T. T(1) times were determined with an inversion-recovery snapshot-flash sequence. T(2) times were obtained from a multispin-echo sequence. Inductively coupled plasma-mass spectrometry was used to determine the iron content in all samples, and r(1) and r(2) were subsequently calculated. The results were then compared with cells labeled with AMI-25 and VSOP C-200. In solution, the r(1) and r(2) of rhodamine-labeled anionic magnetic nanoparticles were 4.78/379 (7 T), 3.28/389 (11.7 T), and 2.00/354 (17.6 T). In cells, the r(1) and r(2) were 0.21/56 (7 T), 0.19/37 (11.7 T), and 0.1/23 (17.6 T). This corresponded to an 11- to 23-fold decrease in r(1) and an 8- to 15-fold decrease in r(2) . A decrease in r(1) was observed for AMI-25 and VSOP C-200. AMI-25 and VSOP exhibited a 2- to 8-fold decrease in r(2) . In conclusion, cellular internalization of iron oxide nanoparticles strongly decreased their T(1) and T(2) potency.

In vivo measurement of local aortic pulse-wave velocity in mice with MR microscopy at 17.6 Tesla.

Transgenic mouse models of human diseases have gained increasing importance in the pathophysiology of cardiovascular diseases (CVD). As an indirect measure of vascular stiffness, aortic pulse-wave velocity (PWV) is an important predictor of cardiovascular risk. This study presents an MRI approach that uses a flow area method to estimate local aortic pulse-wave velocity at different sites in the murine aorta. By simultaneously measuring the cross-sectional area and the through-plane velocity with a phase-contrast CINE method, it was possible to measure average values for the PWV in the ascending and descending aorta within the range of 2.4-4.3 m/s for C57BL/6J mice (ages 2 and 8 months) and apoE-knockout mice (age 8 months). Statistically significant differences of the mean values of the PWV of both groups could be determined. By repeating CINE measurements with a time delay of 1 ms between two subsequent data sets, an effective temporal resolution of 1000 frames/s (fps) could be achieved.

Murine atherosclerotic plaque imaging with the USPIO Ferumoxtran-10.

In this study we intended to image plaque inflammation in a murine model of atherosclerosis with MRI and Ferumoxtran-10 (Sinerem, Guerbet, France). 8 apoE-/- mice were injected 500 micromol Fe/kg or 1000 micromol Fe/kg Ferumoxtran-10. 2 apoE-/- mice were injected NaCl. After a post-contrast time of 24 to 336 hours the mice were scarificed and the aortas were imaged ex vivo. All measurements were performed on a 17.6 Tesla Bruker AVANCE 750WB MR scanner (Bruker, Germany). Spin-echo sequences and gradient-echo sequences with variable TE were performed and T2* maps were generated. Prussian-blue and hematoxilin-eosin histology were obtained afterwards and iron-uptake was quantified by counting iron positive areas. 2 apoE-/- mice were imaged in vivo before and 48 hours after 1000 micromol Fe/kg. Atheroma iron uptake was not elevated after 24 hours compared to controls. 48 hours after 1000 micromol Fe/kg but not 500 micromol Fe/kg histology revealed a 1.3- fold increase in plaque iron content compared to NaCl injected mice. Normalized T2*-times decreased from 0.86+/-0.02 in controls to 0.66+/-0.15 after a dose of 500 micromol Fe/ml and 0.59+/-0.14 in mice injected with 1000 micromol Fe/Kg (p=0.038). These results translated into a mean of 122% increase in CNR, as measured by in vivo MRI. We have demonstrated that Ferumoxtran-10 is taken up by atherosclerotic plaques in untreated apoE-/- mice and this alters plaque signal properties.

In vivo comparison of atherosclerotic plaque progression with vessel wall strain and blood flow velocity in apoE(-/-) mice with MR microscopy at 17.6 T.

OBJECT: At present, in vivo plaque characterization in mice by MRI is typically limited to the visualization of vascular lesions with no accompanying analysis of vessel wall function. The aim of this study was to analyze the influence of atherosclerotic plaque development on the morphological and mechanical characteristics of the aortic vessel wall in a pre-clinical murine model of atherosclerosis. MATERIALS AND METHODS: Groups of apolipoprotein E-deficient (apoE(-/-)) and C57BL/6J control mice fed a high-fat diet were monitored over a 12-week time period by high-field MRI. Multi-Slice-Multi-Spin-Echo and Phase-Contrast MRI sequences were employed to track changes to aortic vessel wall area, blood flow velocity and distensibility. RESULTS: After 6- and 12-weeks, significant changes in vessel wall area and circumferential strain were detected in the apoE(-/-) mice relative to the control animals. Blood flow velocity and intravascular lumen remained unchanged in both groups, findings that are in agreement with the theory of positive remodeling of the ascending aorta during plaque progression. CONCLUSION: This study has demonstrated the application of high-field MRI for characterizing the temporal progression of morphological and mechanical changes to murine aortic vasculature associated with atherosclerotic lesion development.

Control of susceptibility-related image contrast by spin-lock techniques.

Macroscopic magnetic field inhomogeneities might lead to image distortions, while microscopic field inhomogeneities, due to susceptibility changes in tissues, cause spin dephasing and decreasing T(2)() relaxation time. The latter effects are especially observed in the trabecular bone and in regions adjacent to air-containing cavities when gradient-echo sequences are applied. In conventional MRI, these susceptibility-related signal voids can be avoided by applying spin-echo (SE) techniques. In this study, an alternative method for the examination and control of susceptibility-related effects by spin-lock (SL) radiofrequency pulses is presented: SL pulses were applied in two different susceptibility-sensitive sequence types: (a) between the jump and return 90 degrees pulses in a 90 degrees (x)-tau-90 degrees (-x) magnetization-prepared Fast Low Angle Shot (FLASH) sequence and (b) between the 90 degrees pulse and the 180 degrees pulse in an asymmetric SE sequence. The range of Larmor frequencies used for spin locking can be determined for different B(1) amplitudes of the SL pulses, allowing control of image contrast by the amplitude of the SL pulses.

In vivo quantitative three-dimensional motion mapping of the murine myocardium with PC-MRI at 17.6 T.

This work presents a method that allows for the assessment of 3D murine myocardial motion in vivo at microscopic resolution. Phase-contrast (PC) magnetic resonance imaging (MRI) at 17.6 T was applied to map myocardial motion in healthy mice along three gradient directions. High-resolution velocity maps were acquired at three different levels in the murine myocardium with an in-plane resolution of 98 mum, a slice thickness of 0.6 mm, and a temporal resolution of 6 ms. The applied PC-MRI method was validated with phantom experiments that confirmed the correctness of the method with deviations of <1.7%. Myocardial in-plane velocities between 0.5 cm/s and 2.2 cm/s were determined for the healthy murine myocardium. Through-plane velocities of 0.1-0.83 cm/s were measured. Velocity data was also used to calculate the myocardial twist angle during systole at different slices in the short-axis view.

In vivo assessment of absolute perfusion and intracapillary blood volume in the murine myocardium by spin labeling magnetic resonance imaging.

The absolute perfusion and the intracapillary or regional blood volume (RBV) in murine myocardium were assessed in vivo by spin labeling magnetic resonance imaging. Pixel-based perfusion and RBV maps were calculated at a pixel resolution of 469 x 469 mum and a slice thickness of 2 mm. The T(1) imaging module was a segmented inversion recovery snapshot fast low angle shot sequence with velocity compensation in all three gradient directions. The group average myocardial perfusion at baseline was determined to be 701 +/- 53 mL (100 g . min)(-1) for anesthesia with isoflurane (N = 11) at a mean heart rate (HR) of 455 +/- 10 beats per minute (bpm). This value is in good agreement with perfusion values determined by invasive microspheres examinations. For i.v. administration of the anesthetic Propofol, the baseline perfusion decreased to 383 +/- 40 mL (100 g . min)(-1) (N = 17, P < 0.05 versus. isoflurane) at a mean heart rate of 261 +/- 13 bpm (P < 0.05 versus isoflurane). In addition, six mice with myocardial infarction were studied under isoflurane anesthesia (HR 397 +/- 7 bpm). The perfusion maps showed a clear decrease of the perfusion in the infarcted area. The perfusion in the remote myocardium decreased significantly to 476 +/- 81 mL (100 g . min)(-1) (P < 0.05 versus sham). Regarding the regional blood volume, a mean value of 11.8 +/- 0.8 vol % was determined for healthy murine myocardium under anesthesia with Propofol (N = 4, HR 233 +/- 17 bpm). In total, the presented techniques provide noninvasive in vivo assessment of the perfusion and the regional blood volume in the murine myocardium for the first time and seem to be promising tools for the characterization of mouse models in cardiovascular research.

High-resolution MRI with cardiac and respiratory gating allows for accurate in vivo atherosclerotic plaque visualization in the murine aortic arch.

Genetically engineered mouse models provide enormous potential for investigation of the underlying mechanisms of atherosclerotic disease, but noninvasive imaging methods for analysis of atherosclerosis in mice are currently limited. This study aimed to demonstrate the feasibility of MRI to noninvasively visualize atherosclerotic plaques in the thoracic aorta in mice deficient in apolipoprotein-E, who develop atherosclerotic lesions similar to those observed in humans. To freeze motion, MR data acquisition was both ECG- and respiratory-gated. T(1)-weighted MR images were acquired with TR/TE approximately 1000/10 ms. Spatial image resolution was 49 x 98 x 300 micro m(3). MRI revealed a detailed view of the lumen and the vessel wall of the entire thoracic aorta. Comparison of MRI with corresponding cross-sectional histopathology showed excellent agreement of aortic vessel wall area (r = 0.97). Hence, noninvasive MRI should allow new insights into the mechanisms involved in progression and regression of atherosclerotic disease.

In vivo time-resolved quantitative motion mapping of the murine myocardium with phase contrast MRI.

Myocardial motion of healthy mice and mice with myocardial infarction was assessed in vivo by phase contrast (PC) cine MRI. The imaging module was a segmented fast low angle shot (FLASH) sequence with velocity compensation in all three gradient directions. To accomplish additional motion encoding, the spin phase was prepared using bipolar gradient pulses, which resulted in a linear dependence between the voxel velocity and spin phase. This method provided accurate quantification of the velocity magnitude and direction of the murine myocardium at a spatial resolution of 234 microm and a temporal resolution of about 10 ms. The acquisition was EKG-gated and the mice were anesthetized by inhalation of 1.5-4.0 vol.% isoflurane at 1.5 l/min oxygen flow. To validate the MRI measurements, an experiment with a calibrated rotating phantom was performed. Deviations between MR velocity measurements and optical assessment by a light detector were lower than 1.6%. During our study, myocardial motion velocities between 0.4 cm/s and 1.7 cm/s were determined for the healthy murine myocardium across the heart cycle. Areas with myocardial infarction were clearly segmented and showed a motion velocity which was significantly reduced. In conclusion, the method is an accurate technique for the assessment of murine myocardial motion in vivo.

In vivo assessment of absolute perfusion in the murine skeletal muscle with spin labeling MRI. Magnetic resonance imaging.

PURPOSE: To assess absolute perfusion in the skeletal muscle of mice in vivo with spin labeling magnetic resonance imaging (MRI) under normal and stress conditions. MATERIALS AND METHODS: Absolute perfusion in the skeletal muscle of 27 C57BL/6 mice was assessed in vivo non-invasively by spin labeling MRI at 7.05 T. This technique was based on the acquisition of T1 maps with global and slice-selective spin inversion in separate acquisitions. T1 mapping was performed by inversion recovery snapshot fast low angle shot imaging. To guarantee proper spin inversion within the whole mouse, a dedicated radiofrequency (RF) coil combination was constructed. A birdcage resonator was used for transmission, while detection of the MRI signal was achieved by a surface coil. RESULTS: Basal perfusion in the hindlimbs was determined to be 94 +/- 10 mL (100 g x minute)(-1) (mean +/- standard error of the mean [SEM], N = 27). This value is in good agreement with perfusion values determined by invasive techniques such as microspheres. A subgroup of six animals received a constant dose of 4 mg (kg x minute)(-1) of the vasodilator adenosine by an intraperitoneal catheter. In this case, perfusion was significantly increased to 179 +/- 56 mL (100 g x minute)(-1) (mean +/- SEM, N = 6, P < 0.02). Mean basal perfusion in this subgroup was 96 +/- 26 mL (100 g x minute)(-1). CONCLUSION: Spin labeling MRI is a well-suited technique for the in vivo assessment of absolute perfusion in the murine skeletal muscle.

Analysis of right ventricular function in healthy mice and a murine model of heart failure by in vivo MRI.

Because of its complex geometry, assessment of right ventricular (RV) function is more difficult than it is for the left ventricle (LV). Because gene-targeted mouse models of cardiomyopathy may involve remodeling of the right heart, the purpose of this study was to develop high-resolution functional magnetic resonance imaging (MRI) for in vivo quantification of RV volumes and global function in mice. Thirty-three mice of various age were studied under isoflurane anesthesia by electrocardiogram-triggered cine-MRI at 7 T. MRI revealed close correlations between RV and LV stroke volume and cardiac output (r = 0.97, P < 0.0001 each). Consistent with human physiology, murine RV end-diastolic and end-systolic volumes were significantly higher compared with LV volumes (P < 0.05 each). MRI in mice with LV heart failure due to myocardial infarction revealed significant structural and functional changes of the RV, indicating RV dysfunction. Hence, MRI allows for the quantification of RV volumes and global systolic function with high accuracy and bears the potential to evaluate mechanisms of RV remodeling in mouse models of heart failure.