**5. Conclusion**

372 Practical Applications in Biomedical Engineering

acquisition adjustments and/or image subtractions.

**Cardiac Index Mouse**

failure).

and human myocardium.

**4. Future perspectives** 

**4.1. Molecular imaging** 

As an extension of the development and use of such techniques (DENSE, tagging, HARP) has been the tremendous value for regional cardiac functional quantification and direct applicability to transgenic mice and in pathological states (myocardial infarction, heart

Such approaches are, however, associated with a number of limitations and drawbacks, including the necessity for use of complex algorithms and laborious data post-processing (tagging, HARP, and DENSE), the inherently low spatial resolution for strain quantification (tagging vs. DENSE), the T1-tag dependence, the low SNR performance of DENSE (often with a temporal decreasing dependence following ECG-triggering), and the necessity to eliminate the anti-echo and free-induction decay signals in DENSE through proper

**(n=9)** 

Myocardial Mass - 153.1±37.2 g rEF 38.0±25.0 % 52.2±15.8 % Wall Thickness 27.9±22.4 % 42.6±21.8 % Wall Motion 2.8±3.0 % 4.8±2.4 % **Table 2.** Summary of mean global and regional cardiac mechanical functional indices (±sd) of murine

In the new era of molecular imaging, MRI faces major challenges in accomplishing detection of molecular probes with increased sensitivity and specificity, comparable to other diagnostic techniques (such as PET/SPECT). Currently, 10-100 μmolar sensitivity is attained by MRI in contrast to the established nano- and picomolar sensitivity of PET and SPECT. While a new generation of contrast agents is anticipated to extend current limits shedding new light into cellular and molecular mechanisms, current efforts focus primarily on stem cell technology,

Advances in the biology of stem cells have evoked great interest in cell replacement therapies for the regeneration of heart tissue after myocardial infarction. Despite the initial

cellular tracking, and construction of hybrid PET/MRI systems, as discussed below.

*Regenerative techniques – Stem cell technology and cellular tracking* 

EF 50.7±3.7 % 64.4±2.4 % EDV 45.4±11.0 μl 117±32.7 ml ESV 22.5±6.2 μl 46.5±10.3 ml SV 22.9±5.4 μl 83±15.4 ml CO 1.4±0.3 ml/min 5.0±0.9 l/min

**Human (n=8)** 

> In the short-lived period of mouse cardiac MRI of the past 15 years, tremendous strides have been made for image-based phenotyping of the cardiovascular system. Such were realized in terms of the scalability of equipment, ease of handling and maintaining animals under proper physiological homeostatic conditions, adaptation of conventional imaging techniques, and inception of new, fast imaging acquisition schemes that have revolutionized cardiac, image-based phenotyping with efficient, high-throughput imaging protocols.

> In conclusion, despite the usefulness, practicality, and low costs associated with the study of the mouse, important genetic, developmental, morphological, and cardiovascular system differences exist between mouse and man especially as such are unmasked in pathological conditions or under stress. Physiological results indicate that the optimal ISO anesthetic regimen for mouse studies under anesthesia is approximately 1.5% v/v, yielding stable MAP and HR values comparable to those observed in the animal's conscious state, with a minute-tominute variability in MAP and HR of ≤11%. Based on such recordings, the optimal FiO2 appears to be 50%. The additional use of N2O was associated with higher and more stable values of MAP and HR (at a mixture of 25–50% O2 and 75–50% N2O). Arterial pH values are within the physiological range and varied between 7.20 and 7.43. ISO anesthesia at 1.5% v/v is also associated with mild hyperglycemia (+47%) whereas insulin levels remain unaltered. The protocol described for physiological studies of mice under anesthesia has the potential for high reproducibility in diagnostic modalities, including MRI, microCT, ultrasound, and microPET. This regimen can be useful in phenotypic screening and pharmacological studies of cardiac function in mice and can facilitate the transfer of such work to noninvasive imaging platforms, with tremendous potential for both basic science and translational research.

> Basic MRI studies of murine global cardiac structure and function under optimal physiological conditions, in combination with PCA and other image processing techniques,

can identify modal components of shape variability and disseminate components of global mechanical motion. Such atlases can be population-based instead of single-subject based and can serve as a powerful reference tool for morphological and functional inter-strain mouse studies, complementary to current ongoing efforts for image-based phenotyping that target the cardiovascular system. Based on constructed morphometric maps and atlases using principal component analysis in C57BL/6J, it is found that in probabilistic atlases, a gradient of probability exists for both strains in longitudinal locations from base to apex. Based on the statistical atlases, differences in size (49.8%), apical direction (15.6%), basal ventricular blood pool size (13.2%), and papillary muscle shape and position (17.2%) account for the most significant modes of shape variability for the left ventricle of the C57BL/6J mice. Correspondingly, for DBA mice, differences in left ventricular size and direction (67.4%), basal size (15.7%), and position of papillary muscles (16.8%) account for significant variability. These data reason in favor of existing variability in the apical location in both strains, a direct consequence of the heart's effort to re-establish the position of the apex in a consistent manner at end-diastole. Additionally, higher variability exists in DBA mice in the location of the papillary muscles, as well as in the epicardial areas of the left ventricle.

Study of the Murine Cardiac Mechanical Function Using Magnetic Resonance Imaging:

I would like to thank Professors G. A. Johnson, G. Truskey, L. Hedlund, and Dr. E. Bucholz, Mr. G. Cofer, and Mr. J. Cook at Duke University, Drs. R. Balaban and A. Koretsky, and Mr. D. Despres and Dr. M. Lizak at the National Institutes of Health, Prof. D. Rueckert at Imperial College, London, Professor BA Janssen at Maastricht Cardiovascular Institute, and my fellows and students Dr. D. Perperidis, Mr. N. Aristocleous, Mr. R. Mean, Mr. S. Gkagkarelis, and Mr. S. Angeli at the University of Cyprus. I am indebted to the help and support from Remcom Inc. I am also grateful for the availability of the Image Registration Toolkit used under License from IXICO Ltd and to Dr. E. Treiber from Bruker Biospin for

I would also like to acknowledge grant support from the Research Promotion Foundation (RPF) under grants RPF/TEXNOLOGY/MHXAN/0609(BE)/05, RPF/STOXOS/0302/02, and

[1] Aletras A, Ding S, Balaban RS, Wen H. DENSE: Displacement encoding with stimulated

[2] Ali A, Dale AM, Badea A, Johnson GA. Automated segmentation of neuroanatomical structures in multispectral MR microscopy of the mouse brain. Neuroimage 2005;

[3] Axel L, Dougherty L. MR Imaging of motion with spatial modulation of magnetization.

[4] Badea C, Fubara B, Hedlund L, Johnson G. 4D micro-CT of the mouse heart. Molecular

[5] Balaban RS, Hampshire VA. Challenges in Small Animal Noninvasive Imaging. ILAR

[6] Barbee RW, Perry BD, Re RN, Murgo TP. Microsphere and dilution techniques for the determination of blood flows and volumes in conscious mice. Am J Physiol Regulatory

[7] Barbee RW, Perry BD, Re RN, Murgo JP, Field LJ. Hemodynamics in Transgenic Mice

[8] Beg MF, Helm PA, McVeigh E, Miller MI, Winslow RL. Computational cardiac anatomy

[9] Bernston GC, Bigger JT Jr, Eckberg DL, Grossman P, Kaufmann PG, Malik M, Nagaraja HN, Parges SW, Paul JP, Stone PH, Van Der Molen MW. Heart rate variability: origins,

[10] Berr SS, Roy RJ, French BA, Yang Z, Gilson W, Kramer CM, Epstein FH. Black blood gradient echo Cine magnetic resonance imaging of the mouse heart. Magn Reson in

with Overexpression of Atrial Natriuretic Factor. Circ. Res. 1994; 74:747-751.

methods, and interpretive caveats. *Psychophysiol*. , 1997; 34:623- 648.

echoes in cardiac functional MRI. J. Magnetic Resonance 1999; 137: 247-252.

**Acknowledgement** 

RPF/PROSVASI/0302/01.

**6. References** 

27:425–35.

the provision of the cardiac MR images.

Radiology 1989; 171(3):841-5.

Imaging 2005; 4(2):110-116.

Integr Comp Physiol 1992; 263:R728-R733.

using MRI. Magn Reson Med 2004; 52:1167–74.

2001; 42(3):248-262.

Med 2005; 53:1074-1079.

The Current Status, Challenges, and Future Perspectives 375

On the forefront of direct, high-field acquisitions using RF technologies with mouse cardiac MRI, the commercially available birdcage outperforms cylindrical spiral multi-turn surface coils in relative signal-to-ratio (SNR) by a factor of 3–5 times as assessed by experimental measurements, simulations, and experiments in free space, and under phantom and animal loading conditions. Nevertheless, 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 to the order of 27–167% post-mortem (cylindrical compared to flat coil), values that are by far higher than previous designs of surface coils and comparable to receiver phased array performance.

Such hardware improvements, in association with fast radial pulse sequence acquisitions, may also lead to quantification of global and regional functional parameters in various mouse strains. While morphological differences appear only to relate to increased papillary muscle variability in the DBA/2J mice, nevertheless interstrain cardiac hemodynamics, based on dynamic cardiac MRI acquisitions, do not exhibit significant differences for neither the LV nor RV in C57BL/6J or DBA/2J mice. Comparative to previous reports of global functional indices [Bucholz 2010], similar mouse and human results were also observed. The work supports the validity of the hypothesis of functional scaling in mice and humans.
