Ultra‐fast and accurate assessment of cardiac function in rats using accelerated MRI at 9.4 Tesla

Abstract

MRI can accurately and reproducibly assess cardiac function in rodents but requires relatively long imaging times. Therefore, parallel imaging techniques using a 4-element RF-coil array and MR sequences for cardiac MRI in rats were implemented at ultra-high magnetic fields (9.4 Tesla [T]). The hypothesis that these developments would result in a major reduction in imaging time without loss of accuracy was tested on female Wistar rats under isoflurane anesthesia. High-resolution, contiguous short-axis slices (thickness 1.5 mm) were acquired covering the entire heart. Two interleaved data sets (i) with the volume coil (eight averages) and (ii) with the four-element coil array (one average) were obtained. In addition, two-, three-, and fourfold accelerated data sets were generated through postprocessing of the coil array data, followed by a TGRAPPA reconstruction, resulting in five data sets per rat (in-plane voxel size 100 ؋ 100 m). The rat has been a mainstay of basic cardiovascular research for several decades. For example, rats with naturally occurring mutations such as the spontaneously hypertensive rat (1), and surgical models of myocardial infarction (2), or pressure-overload hypertrophy (3) are commonly used to study pathological processes in the heart on a structural, functional, metabolic, and molecular level. Importantly, recent descriptions of transgenic (4) and mutagenesis techniques in the rat (5) are likely to increase the need for accurate and rapid throughput cardiac phenotyping in this species.MRI has become a routine tool to noninvasively assess cardiac function in rodent models of human heart disease. This technique has been applied on MR systems with magnetic field strengths up to 11.7 Tesla (T; 6 -10), using surface-(7,9), volume-type (8,9) radio frequency (RF) coils, or a combination of both (6,10). However, the requirement for maximal spatial (Յ 200 m in-plane) and temporal (Յ 5 ms) resolution results in long imaging times (50 -90 min), and limits the usefulness of this technique, particularly in models of heart failure, which may be inherently hemodynamically unstable under general anesthesia. Technical advances in MR hardware (i.e., RF-coil arrays and receiver technology) and acquisition methods (parallel imaging techniques [PAT]) on clinical MR systems with lower magnetic field strengths have substantially reduced scan-time.The aim of our study was to implement PAT techniques (hardware and MR sequences) for experimental cardiac MRI in rats at 9.4T. We therefore developed and characterized a four-element receive-only coil array optimized for cardiac MR on rats, implemented the PAT technique TGRAPPA (11) and quantified the impact of accelerated MR on the cardiac functional and structural parameter assessment. We demonstrate for the first time that it is possible to conduct a complete cardiac functional study in rats at 9.4T in less than 3 min without losing temporal or spatial resolution and with very low variability.

Fast left ventricular mass and volume assessment in mice with three‐dimensional guide‐point modeling

“…This method is routinely used in mouse MRI studies (2,10) and may result in an overestimation of end-systolic volume in diseased hearts with asynchronous function (15). In the future, faster scanning protocols using parallel imaging (16) will reduce the variation in heart rate between slices.…”

Application of kt-BLAST acceleration to reduce cardiac MR imaging time in healthy and infarcted mice

“…Future work should include direct comparisons. Acceleration factors so far reported using parallel imaging in rodent studies are up to four for rats using a four-element coil array [14] and three for mice using an eight-element coil [15]. The combination of parallel imaging and k-space undersampling techniques is expected to lead to further reduction in scanning times, thus improving animal welfare and increasing throughput.…”

“…The additional challenges of small animal imaging include limited signal-to-noise ratio (SNR) and the difficulty of designing non-interacting RF coils for parallel acquisition at the high frequencies encountered in typical preclinical scanners (200 MHz and above). Despite these difficulties, parallel imaging in rat [14] and mouse [15] cardiac studies have recently been demonstrated by Schneider et al Ratering et al [16] have also demonstrated parallel imaging to accelerate MR angiography in mice.…”

In ovo non‐invasive quantification of the myocardial function and mass of chick embryos using magnetic resonance imaging

“…The present study used a volume resonator (72 mm diameter) to image the eggs of approximately 42 mm diameter, however sensitivity could be improved by a better match of the volume resonator diameter to the egg diameter. In rodent cardiac studies, sensitivity has been increased by using parallel imaging techniques with an appropriate coil array (29). In a similar way, a custom-designed array of coils surrounding the egg would greatly improve the sensitivity for in ovo cardiac studies.…”

“…Important advances in transgenic (30) and mutagenesis techniques (31) in rats has increased the need for high throughput screening. Currently, the rapid throughput of rodent models is limited by the time required to prepare, anaesthetise and monitor the animal (29). In this regard, the use of late stage chick embryos may have an advantage, as they require no preparation before scanning.…”