Dedicated low-field MRI in mice

Low-Field Magnetic Resonance Imaging: Its History and Renaissance

ABSTRACT

Low-field magnetic resonance imaging (MRI) systems have seen a renaissance recently due to improvements in technology (both hardware and software). Originally, the performance of low-field MRI systems was rated lower than their actual clinical usefulness, and they were viewed as low-cost but poorly performing systems. However, various applications similar to high-field MRI systems (1.5 T and 3 T) have gradually become possible, culminating with high-performance low-field MRI systems and their adaptations now being proposed that have unique advantages over high-field MRI systems in various aspects. This review article describes the physical characteristics of low-field MRI systems and presents both their advantages and disadvantages for clinical use (past to present), along with their cutting-edge clinical applications.

Design and 3D-printing of MRI-compatible cradle for imaging mouse tumors

BACKGROUND

The ability of 3D printing using plastics and resins that are magnetic resonance imaging (MRI) compatible provides opportunities to tailor design features to specific imaging needs. In this study an MRI compatible cradle was designed to fit the need for repeatable serial images of mice within a mouse specific low field MRI.

METHODS

Several designs were reviewed which resulted in an open style stereotaxic cradle to fit within specific bore tolerances and allow maximum flexibility with interchangeable radiofrequency (RF) coils. CAD drawings were generated, cradle was printed and tested with phantom material and animals. Images were analyzed for quality and optimized using the new cradle. Testing with multiple phantoms was done to affirm that material choice did not create unwanted image artifact and to optimize imaging parameters. Once phantom testing was satisfied, mouse imaging began.

RESULTS

The 3D printed cradle fit instrument tolerances, accommodated multiple coil configurations and physiological monitoring equipment, and allowed for improved image quality and reproducibility while also reducing overall imaging time and animal safety.

CONCLUSIONS

The generation of a 3D printed stereotaxic cradle was a low-cost option which functioned well for our laboratory.

Low-field permanent magnets for industrial process and quality control

In this review we focus on the technology associated with low-field NMR. We present the current state-of-the-art in low-field NMR hardware and experiments, considering general magnet designs, rf performance, data processing and interpretation. We provide guidance on obtaining the optimum results from these instruments, along with an introduction for those new to low-field NMR. The applications of lowfield NMR are now many and diverse. Furthermore, niche applications have spawned unique magnet designs to accommodate the extremes of operating environment or sample geometry. Trying to capture all the applications, methods, and hardware encompassed by low-field NMR would be a daunting task and likely of little interest to researchers or industrialists working in specific subject areas. Instead we discuss only a few applications to highlight uses of the hardware and experiments in an industrial environment. For details on more particular methods and applications, we provide citations to specialized review articles.

In vivo preclinical low-field MRI monitoring of tumor growth following a suicide-gene therapy in an orthotopic mice model of human glioblastoma

PURPOSE

The aim of this study was to monitor in vivo with low field MRI growth of a murine orthotopic glioma model following a suicide gene therapy.

METHODS

The gene therapy consisted in the stereotactic injection in the mice brain of a modified vaccinia virus Ankara (MVA) vector encoding for a suicide gene (FCU1) that transforms a non toxic prodrug 5-fluorocytosine (5-FC) to its highly cytotoxic derivatives 5-fluorouracil (5-FU) and 5'-fluorouridine-5'monophosphate (5'-FUMP). Using a warmed-up imaging cell, sequential 3D T1 and T2 0.1T MRI brain examinations were performed on 16 Swiss female nu/nu mice bearing orthotopic human glioblastoma (U87-MG cells). The 6-week in vivo MRI follow-up consisted in a weekly measurement of the intracerebral tumor volume leading to a total of 65 examinations. Mice were divided in four groups: sham group (n=4), sham group treated with 5-FC only (n=4), sham group with injection of MVA-FCU1 vector only (n=4), therapy group administered with MVA-FCU1 vector and 5-FC (n=4). Measurements of tumor volumes were obtained after manual segmentation of T1- and T2-weighted images.

RESULTS

Intra-observer and inter-observer tumor volume measurements show no significant differences. No differences were found between T1 and T2 volume tumor doubling times between the three sham groups. A significant statistical difference (p<0.05) in T1 and T2 volume tumor doubling times between the three sham groups and the animals treated with the intratumoral injection of MVA-FCU1 vector in combination with 2 weeks per os 5-FC administration was demonstrated.

CONCLUSION

Preclinical low field MRI was able to monitor efficacy of suicide gene therapy in delaying the tumor growth in an in vivo mouse model of orthotopic glioblastoma.