High-throughput magnetic resonance imaging in mice for phenotyping and therapeutic evaluation
Introduction
Since its beginnings in the 1970s, and the introduction of its routine clinical use in the 1980s, magnetic resonance imaging (MRI) has undergone a rapid evolution because of its non-invasive nature and unmatched flexibility with respect to image contrast in living tissue. Through the past decade, MRI has shown unique uses across almost all disease areas [1, 2•, 3, 4, 5, 6, 7]. In parallel with the development of clinical MRI, and because of increasing interest in mouse disease models, small animal MRI is now also routinely used and widely available, and is playing a critical role in medical research and therapeutics development [1, 2•, 3, 4, 5, 6, 8, 9, 10, 11]. Almost all major pharmaceutical companies rely on preclinical MRI in drug development and development/validation of clinical MRI biomarkers [7, 12, 13•]. With increasing focus on cell- and gene-based therapeutics, reliance on imaging modalities such as MRI will increase [14, 15, 16, 17•, 18].
Preclinical MRI poses greater challenges with respect to resolution and sensitivity [19, 20], than clinical MRI. In MRI, an increase in resolution, signal-to-noise ratio (SNR) or throughput, must necessarily be at the expense of at least one of the other two (Figure 1). It follows that the demands of adequate resolution and sensitivity in small animals, particularly mice, have traditionally placed undesirable limits on animal throughput, a severe limitation in drug development, where large numbers of animals and lengthy MRI protocols are commonly required. This has led to increasing interest in the realization of high-throughput preclinical MRI. This review of recent advances discusses the critical determinants of preclinical MRI throughput as well as current and future strategies for optimization, as summarized in Figure 2. Box 1 contains a glossary of MRI terms to aid the newcomer to the field.
Section snippets
Throughput in preclinical MRI
In considering MRI animal throughput, one must consider all aspects of the imaging process, including animal preparation, parameter optimization and scan time (Figure 2). Complex dosing schedules and staggering of doses for specific timing with the MRI can also impact animal throughput. While image reconstruction and post-processing also influence overall throughput, animal throughput determines the usage time on the MRI system, and critically influences experimental design, determining maximum
Hardware optimization
Advances in MRI hardware design have made the triangle of Figure 1 a less daunting prospect for the preclinical MRI scientist. Development of imaging systems with higher static magnetic field strength (> 4.7 T) [21, 22•] have increased sensitivity (which scales with field), although the high-field limitations of increased susceptibility effects, sample noise, and decreased tissue transverse relaxation times, highlight the need for additional means to enable greater throughput.
Imaging at higher
Pulse sequence optimization
Rapid MRI pulse sequences [38••, 39] have seen wide and efficient use in the clinic, but limited use in preclinical MRI due to constraints in sensitivity, the higher gradients required at mouse resolutions, as well as the confounding influence of susceptibility artifacts at higher field, and motion artifacts, that can often be overcome by breath holding in the clinic. However, recent advancements in RF coil and gradient technology have made routine use of some of these sequences in small
Advances in MRI contrast agents
MRI contrast agents increase the dynamic range of signal arising from specific tissues across a field of view (FOV) by SNR enhancement (T1 contrast) or reduction (T2 contrast). Because this can lower the limit of detection of a tissue type or lesion, throughput gains can be realized, due to increased sensitivity and/or effective spatial resolution. T1 contrast agents such as gadolinium-based molecules or particles, allow more rapid scan repetition rates due to enhanced tissue T1 relaxivity.
The
Emerging multiple mouse imaging strategies
An innovative approach to increasing throughput is to simultaneously image multiple mice in a single magnet bore [49••, 50, 51, 52, 53]. Bock, Henkelman and co-workers provided a theoretical comparison of the various approaches in terms of the time to image N mice, at a given SNR [49••]. The schemes considered are: (i) a single large RF coil and single large gradient set; (ii) a single large-gradient set, N unshielded RF coils and N receivers; (iii) a single large-gradient set, N shielded RF
Efficient animal preparation
The preparation of the animal has the potential to increase image quality and throughput in multiple ways. Preparation methods for restriction of motion are critical in rodent imaging, where even small movements can obliterate an entire scan. Commonly used breathable anesthetics often enhance undesirable breathing related motion.
Small animal respiratory-, cardiac- and temperature-monitoring systems have seen rapid development in recent years, and can provide respiratory and cardiac gating
Optimized data handling
The additional time required for image processing, analysis and modeling can significantly affect overall throughput. Expertise in software design and programming is an essential requirement for an MRI laboratory aspiring to high throughput, to maintain and augment the throughput gains realized by imaging hardware and sequence optimization. Basic image analysis often requires painstaking segmentation by manual drawing of the region of interest (ROI) by an experienced operator. Processing can
Future perspective: making clinical MRI technology a preclinical reality
Optimal throughput in preclinical MRI will only occur when hardware and software innovations already standard in clinical MRI, are successfully translated by preclinical MRI manufacturers to high-field animal systems. Although RF tuning/matching, field adjustment, shimming and pulse calibration procedures are fully automated and occur in seconds in clinical MRI systems, this technology has not been successfully translated to preclinical MRI. The multiple receiver channel systems that enable
Conclusion
With careful consideration of the basic factors that influence animal throughput in a preclinical MRI laboratory (Figure 2), high-throughput scanning for therapeutic response or phenotyping can be a reality today in many applications (Table 1). However, as MRI methodology continues to move beyond basic imaging of anatomy toward high-throughput functional and molecular imaging, innovative new hardware and software approaches will be required to match throughput demands.
The coming of age of
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The authors thank Brian D Ross, Alnawaz Rehemtulla, W Dick Leopold and Prasad Sunkara for their insight, technical advice and strong support of high-throughput in vivo imaging in preclinical drug development. This work was supported in part by NIH Grants 1PO1CA85878, and 1P50CA93990.
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