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

Abstract

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 m, 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 cor-rectness 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. Magn Reson Med 55: 1058-1064, 2006. In recent years transgenic and knockout mice have gained increasing importance in cardiovascular research. Murine models of cardiovascular disease with gene overexpres-sions, mutations, or knockouts can easily be produced. An examination of the consequences of these modifications adds to our understanding of the function of certain gene products. These facts have been the driving force behind the development of suitable methods for assessing the cardiac function of mice in vivo. Echocardiography and ventriculography have proven to be feasible tools for evaluating heart function in small animals (1-5). However, over the past few years MRI has emerged as the most powerful imaging modality for measuring a broad spectrum of functional parameters and thus allowing for accurate detection of the pathophysiological state of the murine heart (6-8). Traditional cine MRI allows assessment of wall thickening and the ejection fraction. If the entire left ventricle (LV) is imaged by acquiring different slices, global cardiac functional parameters such as LV mass, end-diastolic volume (EDV), and end-systolic volume (ESV) become accessible (9). In combination with spatial modulation of magnetiza-tion (SPAMM), local functional parameters (e.g., myocar-dial motion) can be measured (10,11). By generating a grid of dark bends within an image and thus tracking the deformation of the myocardium during the heart cycle, motion and strain parameters of the myocardium can be calculated. An intrinsic drawback of this method is the large difference between spatial image resolution and the tag spacing, which limits the resolution of motion information. Recently published studies achieved a resolution of 133 267 m in the image plane by applying a tag spacing of 0.7 mm (12). With respect to the size of the murine heart (inner diameter of about 10 mm and myocardial wall thickness on the order of 1-2 mm), the tagged cine MRI approach yields restricted information about local myocar-dial motion. Aletras et al. (20) introduced a combination of SPAMM with an additional unencoding gradient module called displacement encoding wi...

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

“…Data acquisition was gated to occur during systole (i.e., when blood flow velocity is at a maximum) such that excited, fast flowing blood is not refocused during acquisition and thereby generating images with black blood contrast. Next, blood flow measurements were made in a single slice location through the ascending aorta using data generated with a phase-contrast cine (PC-cine) method consisting of a velocity compensated FLASH sequence [24]. To quantify blood flow velocity, data sets of ∼34 cine frames over one period of the cardiac cycle were acquired under three encoding conditions: two with flow-encoding and one completely flow compensated.…”

Cardiac Flow Analysis Applied to Phase Contrast Magnetic Resonance Imaging of the Heart

“…40 Further development of phase contrast MRI to produce multi-slice cine images involves an electrocardiogram synchronized time-resolved framework to allow assessment of blood-flow characteristics with high spatial and temporal resolution of a cardiovascular region of interest. 30,74 Sometimes there may be poor quality image registration due to poor respiratory control during scanning. Usually a ghosting artefact 4,41,70 appears on the images.…”

High field magnetic resonance imaging of rodents in cardiovascular research

“…The downtrades are greater field inhomogeneity and susceptibility to artifacts and higher radiofrequency (RF) power deposition in tissue-measured by the specific absorption rate (SAR) [101]. Overall, although some authors report nice image acquisitions at clinical field strength (1.5-3 T) with optimized coils [29,53,62], or at the very high field of 17.6 T [60,61], it is generally admitted that the range of 7-11.7 T offers the best trade off for most applications in small animals [48]. Systems can be either horizontally or vertically mounted.…”

“…Different studies have successfully determined myocardial velocities in mice at different time point of cardiac cycles using this technique. If first studies were using bright blood contrast [60,105,134], blood suppression has been shown to improve accuracy of measurements in further developments [43,79]. Baseline values of normal transmural wall motion pattern have been carefully established with TPM, allowing further comparison with genetically and surgically manipulated mouse models [43].…”