Multiple-mouse MRI with multiple arrays of receive coils

Technical note: 3D-printed MRI-compatible syringe pump

BACKGROUND

A power injector is typically employed to deliver a contrast agent to a subject (e.g., humans, animals, phantoms) during MRI, but it is costly and cannot inject more than one subject at a time. Conventional syringe pumps housing multiple syringes are typically non-MRI compatible and, thus, should be placed outside the MRI room.

PURPOSE

To develop the prototype of a cost-effective, easy-to-use, reliable syringe pump that could be deployed inside a clinical MRI bore and operated on during a scan.

METHODS

The MRI-compatible syringe pump (MSP) was designed using Solidworks 3D modeling software and fabricated using a Raise3D Pro2 Printer. The MSP was designed to infuse up to three syringes simultaneously. The injection speed was mainly controlled with different gear sets in the escapement unit and further adjusted by changing the effective hairspring length via a pinch pin. The MSP was evaluated with three gear sets (gear ratios: 0.20:1, 0.56:1, and 1.09:1) and 10 different effective hairspring lengths at each gear ratio. A video was recorded while operating MSP at each injection speed to calculate the volume injection rate of a 5-mL syringe (mL/s). The MSP was operated five times repeatedly at each injection speed, and the mean and standard deviation of the volume injection rate were calculated.

RESULTS

The volume injection rates produced by three gear ratios (0.20:1, 0.56:1, and 1.09:1) were 0.209 ± 0.003 mL/s, 0.411 ± 0.002 mL/s, and 0.625 ± 0.006 mL/s, respectively, at the full hairspring length. The injection rates of gear set 1 (gear ratio: 0.20:1) decreased from 0.273 ± 0.001 mL/s to 0.245 ± 0.001 mL/s with a decrement of 0.003 mL/s for 10 different effective hairspring lengths (r = 0.997, p < 0.001). The injection rates of gear set 2 (gear ratio: 0.56:1) decreased from 0.519 ± 0.003 mL/s to 0.469 ± 0.003 mL/s with a decrement of 0.006 mL/s (r = 0.987, p < 0.001), and that of gear set 3 (gear ratio: 1.09:1) decreased from 0.779 ± 0.012 mL/s to 0.709 ± 0.005 mL/s with a decrement of 0.007 mL/s (r = 0.963, p < 0.001). The coefficient of variation in the injection rate measurement was 0.727 ± 0.346%.

CONCLUSIONS

The MSP is a portable device that can reliably deliver a liquid agent to multiple subjects inside a clinical MRI bore during a scan.

Multiple-mouse magnetic resonance imaging with cryogenic radiofrequency probes for evaluation of brain development

Multiple-mouse magnetic resonance imaging (MRI) increases scan throughput by imaging several mice simultaneously in the same magnet bore, enabling multiple images to be obtained in the same time as a single scan. This increase in throughput enables larger studies than otherwise feasible and is particularly advantageous in longitudinal study designs where frequent imaging time points result in high demand for MRI resources. Cryogenically-cooled radiofrequency probes (CryoProbes) have been demonstrated to have significant signal-to-noise ratio benefits over comparable room temperature coils for in vivo mouse imaging. In this work, we demonstrate implementation of a multiple-mouse MRI system using CryoProbes, achieved by mounting four such coils in a 30-cm, 7-Tesla magnet bore. The approach is demonstrated for longitudinal quantification of brain structure from infancy to early adulthood in a mouse model of Sanfilippo syndrome (mucopolysaccharidosis type III), generated by knockout of the Hgsnat gene. We find that Hgsnat^-/- mice have regionally increased growth rates compared to Hgsnat^+/+ mice in a number of brain regions, notably including the ventricles, amygdala and superior colliculus. A strong sex dependence was also noted, with the lateral ventricle volume growing at an accelerated rate in males, but several structures in the brain parenchyma growing faster in females. This approach is broadly applicable to other mouse models of human disease and the increased throughput may be particularly beneficial in studying mouse models of neurodevelopmental disorders.

Partitioning k-space for cylindrical three-dimensional rapid acquisition with relaxation enhancement imaging in the mouse brain

Three-dimensional rapid acquisition with relaxation enhancement (RARE) scans require the assignment of each phase encode step in two dimensions to an echo in the echo train. Although this assignment is frequently made across the entire Cartesian grid, collection of only the central cylinder of k-space by eliminating the corners in each phase encode dimension reduces the scan time by ~22% with negligible impact on image quality. The recipe for the assignment of echoes to grid points for such an acquisition is less straightforward than for the simple full Cartesian acquisition case, and has important implications for image quality. We explored several methods of partitioning k-space-exploiting angular symmetry in one extreme or emulating a cropped Cartesian acquisition in the other-and acquired three-dimensional RARE magnetic resonance imaging (MRI) scans of the ex vivo mouse brain. We evaluated each partitioning method for sensitivity to artifacts and then further considered strategies to minimize these through averaging or interleaving of echoes and by empirical phase correction. All scans were collected 16 at a time with multiple-mouse MRI. Although all schemes considered could be used to generate images, the results indicate that the emulation of a standard Cartesian echo assignment, by partitioning preferentially along one dimension within the cylinder, is more robust to artifacts. Samples at the periphery of the bore showed larger phase deviations and higher sensitivity to artifacts, but images of good quality could still be obtained with an optimized acquisition protocol. A protocol for high-resolution (40 μm) ex vivo images using this approach is presented, and has been used routinely with a success rate of 99% in over 1000 images.

Multi-port-driven birdcage coil for multiple-mouse MR imaging at 7 T

In ultra-high field (UHF) imaging environments, it has been demonstrated that multiple-mouse magnetic resonance imaging (MM-MRI) is dependent on key factors such as the radiofrequency (RF) coil hardware, imaging protocol, and experimental setup for obtaining high-resolution MR images. A key aspect is the RF coil, and a number of MM-MRI studies have investigated the application of single-channel RF transmit (Tx)/receive (Rx) coils or multi-channel phased array (PA) coil configurations under a single gradient coil set. However, despite applying a variety of RF coils, Tx (|B₁⁺ |)-field inhomogeneity still remains a major problem due to the relative shortening of the effective RF wavelength in the UHF environment. To address this issue, we propose a relatively smaller size of individual Tx-only coils in a multiple birdcage (MBC) coil for MM-MRI to image up to three mice. We use electromagnetic (EM) simulations in the finite-difference time-domain (FDTD) environment to obtain the |B₁ |-field distribution. Our results clearly show that the single birdcage (SBC) high-pass filter (HPF) configuration, which is referred to as the SBC_HPF , under the absence of an RF shield exhibits a high |B₁ |-field intensity in comparison with other coil configurations such as the low-pass filter (LPF) and band-pass filter (BPF) configurations. In a 7-T MRI experiment, the signal-to-noise ratio (SNR) map of the SBC_HPF configuration shows the highest coil performance compared to other coil configurations. The MBC_HPF coil, which is comprised of a triple-SBC_HPF configuration combined with additional decoupling techniques, is developed for simultaneous image acquisition of three mice. SCANNING 38:747-756, 2016. © 2016 Wiley Periodicals, Inc.

Geometric decoupling of a mouse array coil using a dual plane pair design with crisscrossed return paths and custom mounting fixture

An element design for receive array coils that decouples from the transmit coil without external active detuning is presented for magnetic resonance imaging (MRI) of mice. The array element uses a crisscrossed geometry on the return paths to reduce the current induced by the transmit coil. Without the need for an external active detune network, the proposed method simplifies the construction of MRI coil systems and also mitigates problems in space-limited MRI applications. In addition, an adaptable scissor-jack-like fixture is presented that allows the receive array to move parallel to the transmit coil to maintain the decoupling condition while maintaining close contact with varying sizes of mice.

Feasibility of multianimal hyperpolarized (13) C MRS

PURPOSE

There is great potential for real-time investigation of metabolism with MRS and hyperpolarized (HP) (13) C agents. Unfortunately, HP technology has high associated costs and efficiency limitations that may constrain in vivo studies involving many animals. To improve the throughput of preclinical investigations, we evaluate the feasibility of performing HP MRS on multiple animals simultaneously.

METHODS

Simulations helped assess the viability of a dual-coil strategy for spatially localized multivolume MRS. A dual-mouse system was assembled and characterized with bench- and scanner-based experiments. Enzyme phantoms mixed with HP [1-(13) C] pyruvate emulated real-time metabolism and offered a controlled mechanism for evaluating system performance. Finally, a normal mouse and a mouse bearing a subcutaneous xenograft of colon cancer were simultaneously scanned in vivo using an agent containing HP [1-(13) C] pyruvate.

RESULTS

Geometric separation/rotation, active decoupling, and use of low input impedance preamplifiers permitted an encode-by-channel approach for spatially localized MRS. A precalibrated shim allowed straightforward metabolite differentiation in enzyme phantom and in vivo experiments at 7 Tesla, with performance similar to conventional acquisitions.

CONCLUSION

The initial feasibility of multi-animal HP (13) C MRS was established. Throughput scales with the number of simultaneously scanned animals, demonstrating the potential for significant improvements in study efficiency.

A throughput-optimized array system for multiple-mouse MRI

MRI is a versatile tool for the systematic assessment of anatomical and functional changes in small-animal models of human disease. Its noninvasive nature makes it an ideal candidate for longitudinal evaluations of disease progression, but relatively long scan times limit the number of observations that can be made in a given interval of time, imposing restrictions on experimental design and potentially compromising statistical power. Methods that reduce the overall time required to scan multiple cohorts of animals in distinct experimental groups are therefore highly desirable. Multiple-mouse MRI, in which several animals are simultaneously scanned in a common MRI system, has been successfully used to improve study throughput. However, to best utilize the next generation of small-animal MRI systems that will be equipped with an increased number of receive channels, a paradigm shift from the simultaneous scanning of as many animals as possible to the scanning of a more manageable number, at a faster rate, must be considered. This work explores the tradeoffs between the number of animals to scan at once and the number of array elements dedicated to each animal, to maximize throughput in systems with 16 receive channels. An array system consisting of 15 receive and five transmit coils allows acceleration by a combination of multi-animal and parallel imaging techniques. The array system was designed and fabricated for use on a 7.0-T/30-cm Bruker Biospec MRI system, and tested for high-throughput imaging performance in phantoms and live mice. Results indicate that up to a nine-fold throughput improvement of a single sequence is possible compared with an unaccelerated single-animal acquisition. True data throughput of a contrast-enhanced anatomical study is estimated to be improved by just over six-fold.

Functional and morphological cardiac magnetic resonance imaging of mice using a cryogenic quadrature radiofrequency coil

Cardiac morphology and function assessment by magnetic resonance imaging is of increasing interest for a variety of mouse models in pre-clinical cardiac research, such as myocardial infarction models or myocardial injury/remodeling in genetically or pharmacologically induced hypertension. Signal-to-noise ratio (SNR) constraints, however, limit image quality and blood myocardium delineation, which crucially depend on high spatial resolution. Significant gains in SNR with a cryogenically cooled RF probe have been shown for mouse brain MRI, yet the potential of applying cryogenic RF coils for cardiac MR (CMR) in mice is, as of yet, untapped. This study examines the feasibility and potential benefits of CMR in mice employing a 400 MHz cryogenic RF surface coil, compared with a conventional mouse heart coil array operating at room temperature. The cryogenic RF coil affords SNR gains of 3.0 to 5.0 versus the conventional approach and hence enables an enhanced spatial resolution. This markedly improved image quality – by better deliniation of myocardial borders and enhanced depiction of papillary muscles and trabeculae – and facilitated a more accurate cardiac chamber quantification, due to reduced intraobserver variability. In summary the use of a cryogenically cooled RF probe represents a valuable means of enhancing the capabilities of CMR of mice.

Accelerating cine-MR imaging in mouse hearts using compressed sensing

Purpose

To combine global cardiac function imaging with compressed sensing (CS) in order to reduce scan time and to validate this technique in normal mouse hearts and in a murine model of chronic myocardial infarction.

Materials and Methods

To determine the maximally achievable acceleration factor, fully acquired cine data, obtained in sham and chronically infarcted (MI) mouse hearts were 2–4-fold undersampled retrospectively, followed by CS reconstruction and blinded image segmentation. Subsequently, dedicated CS sampling schemes were implemented at a preclinical 9.4 T magnetic resonance imaging (MRI) system, and 2- and 3-fold undersampled cine data were acquired in normal mouse hearts with high temporal and spatial resolution.

Results

The retrospective analysis demonstrated that an undersampling factor of three is feasible without impairing accuracy of cardiac functional parameters. Dedicated CS sampling schemes applied prospectively to normal mouse hearts yielded comparable left-ventricular functional parameters, and intra- and interobserver variability between fully and 3-fold undersampled data.

Conclusion

This study introduces and validates an alternative means to speed up experimental cine-MRI without the need for expensive hardware. J. Magn. Reson. Imaging 2011. © 2011 Wiley Periodicals, Inc.

INTERCOMPARISON OF PERFORMANCE OF RF COIL GEOMETRIES FOR HIGH FIELD MOUSE CARDIAC MRI

Multi-turn spiral surface coils are constructed in flat and cylindrical arrangements and used for high field (7.1 T) mouse cardiac MRI. Their electrical and imaging performances, based on experimental measurements, simulations, and MRI experiments in free space, and under phantom, and animal loading conditions, are compared with a commercially available birdcage coil. Results show that the four-turn cylindrical spiral coil exhibits improved relative SNR (rSNR) performance to the flat coil counterpart, and compares fairly well with a commercially available birdcage coil. Phantom experiments indicate a 50% improvement in the SNR for penetration depths ≤ 6.1 mm from the coil surface compared to the birdcage coil, and an increased penetration depth at the half-maximum field response of 8 mm in the 4-spiral cylindrical coil case, in contrast to 2.9 mm in the flat 4-turn spiral case. Quantitative comparison of the performance of the two spiral coil geometries in anterior, lateral, inferior, and septal regions of the murine heart yield maximum mean percentage rSNR increases of the order of 27-167% in vivo post-mortem (cylindrical compared to flat coil). The commercially available birdcage outperforms the cylindrical spiral coil in rSNR by a factor of 3-5 times. The comprehensive approach and methodology adopted to accurately design, simulate, implement, and test radiofrequency coils of any geometry and type, under any loading conditions, can be generalized for any application of high field mouse cardiac MRI.

Accelerated cardiac magnetic resonance imaging in the mouse using an eight-channel array at 9.4 Tesla

MRI has become an important tool to noninvasively assess global and regional cardiac function, infarct size, or myocardial blood flow in surgically or genetically modified mouse models of human heart disease. Constraints on scan time due to sensitivity to general anesthesia in hemodynamically compromised mice frequently limit the number of parameters available in one imaging session. Parallel imaging techniques to reduce acquisition times require coil arrays, which are technically challenging to design at ultrahigh magnetic field strengths. This work validates the use of an eight-channel volume phased-array coil for cardiac MRI in mice at 9.4 T. Two- and three-dimensional sequences were combined with parallel imaging techniques and used to quantify global cardiac function, T(1)-relaxation times and infarct sizes. Furthermore, the rapid acquisition of functional cine-data allowed for the first time in mice measurement of left-ventricular peak filling and ejection rates under intravenous infusion of dobutamine. The results demonstrate that a threefold accelerated data acquisition is generally feasible without compromising the accuracy of the results. This strategy may eventually pave the way for routine, multiparametric phenotyping of mouse hearts in vivo within one imaging session of tolerable duration.