A new ultrasound instrument for in vivo microimaging of mice

Abnormal cardiac inflow patterns during postnatal development in a mouse model of Holt-Oram syndrome

Tbx5(del/+) mice provide a model of human Holt-Oram syndrome. In this study, the cardiac functional phenotypes of this mouse model were investigated with 30-MHz ultrasound by comparing 12 Tbx5(del/+) mice with 12 wild-type littermates at 1, 2, 4, and 8 wk of age. Cardiac dimensions were measured with two-dimensional and M-mode imaging. The flow patterns in the left and right ventricular inflow channels were evaluated with Doppler flow sampling. Compared with wild-type littermates, Tbx5(del/+) mice showed significant changes in the mitral flow pattern, including decreased peak velocity of the left ventricular (LV) early filling wave (E wave), increased peak velocity of the late filling wave (A wave), and decreased or even reversed peak E-to-A ratio. The prolongation of LV isovolumic relaxation time was detected in Tbx5(del/+) neonates as early as 1 wk of age. In Tbx5(del/+) mice, LV wall thickness appeared normal but LV chamber dimension was significantly reduced. LV systolic function did not differ from that in wild-type littermates. In contrast, the Doppler flow spectrum in the enlarged tricuspid orifice of Tbx5(del/+) mice demonstrated increased peak velocities of both E and A waves and increased total time-velocity integral but unchanged peak E/A. In another 13 mice (7 Tbx5(del/+), 6 wild-type) at 2 wk of age, significant correlation was found between Tbx5 gene expression level in ventricular myocardium and LV filling parameters. In conclusion, the LV diastolic function of Tbx5(del/+) mice is significantly deteriorated, whereas the systolic function remains normal.

Thermal assessment of 40-MHz pulsed Doppler ultrasound in human eye

Tissue exposure to diagnostic pulsed Doppler ultrasound (US) can cause significant temperature rises. Temperature rise induced by US biomicroscopy (UBM) system (VS40, VisualSonics, Toronto, ON, Canada) was measured in ex vivo human and rabbit eyes with a 26-gauge K-type needle thermocouple. The operating frequency was 40 MHz with a free field I(SPTA) of 2.6 mW/cm(2) (B-mode) and 11.9 W/cm(2) (Doppler). Peak negative pressures were 5.22 MPa (B-mode) and 7.32 MPa (Doppler), resulting in MIs of 0.83 (B-mode) and 1.05 (Doppler mode). In Doppler mode, mean temperature rises of 2.27 degrees C and 1.93 degrees C were measured for the human lens and ciliary body after a 3-min insonation, vs. 2.66 degrees C for the rabbit lens. Our results indicate that US-induced temperature rise decreases with decreasing number of cycles, decreasing pulse-repetition frequency (PRF) or increased transmit attenuation, and is consistent with simple models of heating. To limit risk of temperature rises of 1 degrees C in human ciliary body, use of the maximum settings of 16 cycles (0.400 micros pulse duration), 20-kHz PRF should include 3-dB transmit attenuation, and exposure time should be limited. For insonation of the lens, exposure settings no higher than nine cycles (0.225-micros pulse duration) and 10-kHz PRF should be employed and exposure time limited to minimize risk of temperature increases of 1 degree C.

Non-invasive high-frequency vascular ultrasound elastography

Non-invasive vascular elastography (NIVE) was recently introduced to characterize mechanical properties of superficial arteries. In this paper, the feasibility of NIVE and its applicability in the context of high-frequency ultrasound imaging is investigated. First, experiments were performed in vitro on vessel-mimicking phantoms. Polyvinyl alcohol cryogel was used to create two double-layer vessels with different mechanical properties. In both cases, the stiffness of the inner layer was made softer. Radial stress was applied within the lumen of the phantoms by applying incremental static pressure steps with a column of a flowing mixture of water-glycerol. The vessel phantoms were insonified at 32 MHz with an ultrasound biomicroscope to provide cross-section sequences of radio-frequency (RF) ultrasound data. The Lagrangian speckle model estimator (LSME) was used to assess the two-dimensional-strain tensors, and the composite Von Mises elastograms were computed. A new implementation of the LSME based on the optical flow equations was introduced. Deformation parameters were estimated using an inversion algorithm. For each in vitro experiment, both layers of approximately 1 mm were distinguished. Second, the use of the method for the purpose of studying small vessels (MicroNIVE) in genetically engineered rodents was introduced. Longitudinal scans of the carotid artery were performed at 40 MHz. The in vivo results give confidence in the feasibility of MicroNIVE as a potential tool to non-invasively study the impact of targeted genes on vascular remodelling in rodents.

Rapid high resolution three dimensional reconstruction of embryos with episcopic fluorescence image capture

One of the overarching goals in developmental biology is the elucidation of mechanisms that elaborate form and function. To this end, an accurate morphological description of embryonic development is essential. However, visualizing dynamic changes in the three-dimensional (3D) structure of the developing embryo has been a "holy grail" in the field of developmental biology. The fundamental difficulties that have hindered all efforts in 3D reconstruction using two-dimensional (2D) image stacks revolve around the seemingly intractable problems of section registration and distortion. A remarkably simple solution has come about with the development of a new technique referred to as episcopic fluorescence image capture (EFIC). With EFIC imaging, tissue autofluorescence is used to image the block face prior to cutting each section. The 2D resolution obtained is close to that achieved by histology, and such 2D image stacks can be readily reconstructed in 3D. The 3D models generated provide fine structural details with resolution unmatched by 3D reconstructions obtained with any other imaging modalities. Given the perfect registration of EFIC image stacks, another important capability provided by EFIC is digital resectioning in any plane. This provides complete flexibility in the selection of optimal virtual sectioning planes for viewing different features in a specimen, and is invaluable for analyzing dynamic changes in tissue structure in the developing embryo. The capabilities provided by EFIC for rapid high resolution 3D reconstruction together with digital resectioning make this an unparalleled tool for characterizing morphogenetic events in the developing embryo. Although our review is focused on using EFIC for studying embryonic development, it is important to note that there is no intrinsic limitation on the size of the specimen that can be analyzed by EFIC imaging. Overall, EFIC should serve as an important imaging technique that will complement other 3D imaging modalities such as MRI and optical tomography. Given the feasibility of generating EFIC image stacks using cryoembedded or polyethylene glycol (PEG)-embedded specimens, there is the possibility that EFIC may be combined with 3D RNA or protein expression profiling. Together, such studies may help further elucidate the relationship between form and function.

Making the mouse embryo transparent: identifying developmental malformations using magnetic resonance imaging

Developmental malformations are a major cause of childhood mortality and are typically characterized by lesions that allow survival of the embryo through gestation. The genetics of developmental malformations are powerfully studied by using high-throughput, phenotype-driven screens (e.g., following zebrafish or mouse mutagenesis) or by genotype-driven studies using transgenic or knockout mice. With regard to either approach, the mouse is anatomically and phylogenetically closer to humans than any other genetically tractable model organism. This is particularly important in the cardiovascular and respiratory systems, which have unique mammalian features. The identification of murine models of developmental malformations is, however, hindered by the opacity of the late gestational mouse embryo. In this review, we describe recent advances in magnetic resonance imaging that make it possible to rapidly identify malformations in the developing mouse embryo with high efficiency.

High-frequency, nonlinear flow imaging of microbubble contrast agents

It has been shown that nonlinear scattering can be stimulated from microbubble contrast agents at high-transmit frequencies (14-32 MHz). This work was extended to demonstrate the feasibility of nonlinear contrast imaging through modifications of existing ultrasound biomicroscopy linear B-scan imaging instrumentation. In this study, we describe the development and evaluation of prototype coherent flow imaging instrumentation for nonlinear microbubble imaging using transmit frequencies from 10 to 50 MHz. Phantom validation experiments were conducted to demonstrate color and power flow imaging using nonlinear 10 MHz (subharmonic) scattering induced by a 20 MHz transmit frequency. In vivo flow imaging of a rabbit ear microvessel was successfully performed. This work indicates the feasibility of performing flow imaging at high frequencies using nonlinear scattering from microbubbles.

Evaluation of cardiovascular flow characteristics in the 129Sv mouse fetus using color-Doppler-guided spectral Doppler ultrasound

The purpose of this study was to evaluate color- and spectral Doppler ultrasound in the establishment of normal functional cardiovascular development features in the mouse fetus. Mouse fetuses (129Sv strain) were studied in utero between embryonic day (EDs) 9.5 and 19.5. Time-velocity curves were derived from Doppler interrogation of the aorta and umbilical artery. The sample volume was accurately placed on the vessels of interest based on color-Doppler images. From these curves, the following parameters were obtained: heart rate (HR), acceleration time (AT), and deceleration time (DT). HR increased between EDs 9.5 and 19.5 from 102.9 to 303.2 b.p.m. For the other parameters, the most significant change observed was the increase of DT in the umbilical artery at the end of pregnancy, corresponding to the appearance of a diastolic flow. We report the use of a commercially available, clinical, ultrasound unit to obtain quantitative data on the cardiovascular development in the mouse fetus. These results may be useful for the recognition of in utero cardiovascular dysfunction in transgenic or knock-out fetus.

Design and fabrication of ultrafine piezoelectric composites

Making fine scale (< 20 microm) piezoelectric composites for high frequency (> 50 MHz) ultrasound transducers remains challenging. Interdigital phase bonding (IPhB), described in this paper, presents a new technique developed to make piezoelectric composites at the ultrafine scale using a conventional dicing saw. Using the IPhB technique, a composite structure with a pitch that is less than the dicing saw blade thickness can be created. The approach is flexible enough to make composites of different combination of pitch and volume ratio. Using a conventional dicing saw with a 50 microm thick blade, composite with a 25 microm pitch and a volume ratio of 61 percent are fabricated. Such a composite is suitable for fabrication of ultrasonic transducers and arrays with central frequencies of up to 85 MHz. Single element transducers working at central frequencies of 50-60 MHz were made of these composites as a mean to characterize the acoustic performance. Measurement results of the transducers show that the longitudinal electromechanical coupling coefficient is greater than 0.6 and that there are no noticeable lateral resonances in the frequency range of 55-150 MHz. Design criteria for fine scale elements are also discussed based on theoretical results from finite element analysis (FEA).

The Use of Three-Dimensional Ultrasound Micro-Imaging to Monitor Prostate Tumor Development in a Transgenic Prostate Cancer Mouse Model

Longitudinal studies of mouse cancer models required large cohorts since autopsy was the only reliable method to evaluate treatment efficacy. This paper reports the use of high-resolution three-dimensional ultrasound micro-imaging to monitor prostate tumor development in genetically engineered mice. Twenty-nine genetically engineered prostate cancer mice, including castrated and uncastrated mice, were imaged by three-dimensional ultrasound. Qualitative comparisons of three-dimensional ultrasound images with histology sections of prostate tumors demonstrate the ability of ultrasound to accurately depict the size and shape of malignant masses in live mice. The correlation coefficient of tumor diameter measurements performed in vivo with three-dimensional ultrasound and at autopsy was 0.997. Prospective tumor detection sensitivity and specificity were 91.7% and 100%. Representative exponential growth curves constructed via longitudinal ultrasound imaging indicated diameter doubling times from 10 to 37 days for four prostate tumors during an initial period of rapid progression. Three-dimensional ultrasound will likely become the micro-imaging modality most readily adopted for mouse pre-clinical trial studies.

High frequency nonlinear B-scan imaging of microbubble contrast agents

It previously was shown that it is possible to produce nonlinear scattering from microbubble contrast agents using transmit frequencies in the 14-32 MHz range, suggesting the possibility of performing high-frequency, nonlinear microbubble imaging. In this study, we describe the development of nonlinear microbubble B-scan imaging instrumentation capable of operating at transmit center frequencies between 10 and 50 MHz. The system underwent validation experiments using transmit frequencies of 20 and 30 MHz. Agent characterization experiments demonstrate the presence of nonlinear scattering for the conditions used in this study. Using wall-less vessel phantoms, nonlinear B-scan imaging is performed using energy in one of the subharmonic, ultraharmonic, and second harmonic frequency regions for transmit frequencies of 20 and 30 MHz. Both subharmonic and ultraharmonic imaging modes achieved suppression of tissue signals to below the noise floor while achieving contrast to noise ratios of up to 26 and 17 dB, respectively. The performance of second harmonic imaging was compromised by nonlinear propagation and offered no significant contrast improvement over fundamental mode imaging. In vivo experiments using the subharmonic of a 20 MHz transmit pulse show the successful detection of microvessels in the rabbit ear and in the mouse heart. The results of this study demonstrate the feasibility of nonlinear microbubble imaging at high frequencies.

Measurement of myocardial mechanics in mice before and after infarction using multislice displacement-encoded MRI with 3D motion encoding

Cardiac MRI is an accurate, noninvasive modality for assessing the structure and function of the murine heart. In addition to conventional imaging, MRI tissue tracking methods can quantify numerous aspects of myocardial mechanics, including intramyocardial displacement, strain, twist, and torsion. In the present study, we developed and applied a novel pulse sequence based on displacement-encoded imaging using stimulated echoes (DENSE) that achieves multislice coverage, high spatial resolution, and three-dimensional (3D) displacement encoding. With the use of this technique, myocardial mechanics of C57Bl/6 mice were measured at baseline and 1 day after experimental myocardial infarction. At baseline, the mean systolic transmural circumferential strain was -0.14 +/- 0.02 and the mean systolic radial strain was 0.30 +/- 0.05. Changes in circumferential and radial strains from the subepicardium to the subendocardium were detected at baseline (P < 0.05). One day after infarction, significantly reduced 3D displacements and strain were detected in infarcted and noninfarcted myocardium. Infarction also reduced normalized systolic torsion from its baseline value of 1.35 +/- 0.27 degrees /mm (R = 0.99) to 0.07 +/- 0.54 degrees /mm (R = 0.96, P < 0.05). DENSE MRI can assess the 3D myocardial mechanics of the murine heart in <1 h of scan time at 4.7 T and may be applied to studies of myocardial mechanics in genetically engineered mice.

Functional imaging of the embryonic pacemaking and cardiac conduction system over the past 150 years: Technologies to overcome the challenges

Early analyses of cardiac pacemaking and conduction system (CPCS) development relied on classic histology and visual inspection of the beating heart. Current techniques that facilitate delineation of the CPCS include the use of specific antibody markers and transgenic mouse lines specifically expressing reporter genes. Assaying the function of tiny embryonic hearts required an increase in the level of spatial and temporal resolution. Current methods for such analyses include the use of intracellular and extracellular microelectrodes, echocardiography, rapid optical imaging using fluorescent dyes, and most recently optical coherence tomography. This review will focus on methods developed to investigate the functional emergence of the embryonic cardiac conduction system. Where appropriate, the methods used to delineate the anatomic pathways will also be discussed. The combination of techniques to capture both morphological and functional data from the CPCS will further improve with continued interdisciplinary collaboration. The Supplementary Material referred to in this article can be found at the Anatomical Record website (http://www.interscience.wiley.com/jpages/0003-276X/suppmat).

Developmental changes in integrated ultrasound backscatter from embryonic blood in vivo in mice at high US frequency

Mouse blood imaged using high-frequency ultrasound (US) is more echogenic in embryos than in adults. Studying changes in blood echogenicity in embryos may be of fundamental interest in studies on the genetic regulation of normal and abnormal blood development in mutant mice. Embryonic red blood cells (RBCs) are large and nucleated in midgestation but decrease in size and become enucleated as they mature. We therefore hypothesised that these structural alterations are responsible for variations in echogenicity of embryonic blood with gestational age and development. The objective of the current study was to quantify these structural changes in echogenicity (echo brightness) and apparent integrated backscatter (AIB) from embryonic blood at high US frequencies in vivo in mice. Results from anaesthetised pregnant mice studied using transcutaneous US showed that echogenicity of embryonic blood in the heart, aorta and umbilical cord and AIB within the heart chambers peaked at embryonic day (ED) 13.5 and then decreased progressively toward term. Between EDs 13.5 and 17.5 (near term), RBC mean cell volume decreased from 133 to 109 fL, haematocrit increased from 12 to 34%, and the percentage of nucleated RBCs decreased from 59 to 2%. Relative to younger ages, RBC nuclei at ED 13.5 were small and dense (pyknotic) which may have contributed to the peak in echogenicity and AIB at this age. To calculate the AIB, radiofrequency (RF) signals with centre frequencies of 28 MHz and 35 MHz were integrated over the 16- to 35-MHz and 21- to 42-MHz frequency range, respectively. At 28 MHz, mean apparent integrated backscatter of blood in the embryonic heart increased significantly from 0.0023 +/- 0.0004 Sr.cm(-1) (mean +/- SEM) at ED 12.5 to peak at 0.0037 +/- 0.0005 Sr.cm(-1) at ED 13.5. The mean AIB then decreased progressively with advancing gestation to 0.0002 +/- 0.0001 Sr.cm(-1) at ED 17.5. At 35 MHz, the mean AIB changed similarly with gestational age, except that values were lower than at 28 MHz at all ages. Higher attenuation of US at 35 MHz than at 28 MHz in tissue likely accounted for the lower AIB of blood insonified at 35 MHz. We speculate that developmental changes in red cell morphology are responsible for the observed changes in echogenicity and AIB of embryonic blood with gestational age in mice.

In vivo molecular and genomic imaging: new challenges for imaging physics

The emerging and rapidly growing field of molecular and genomic imaging is providing new opportunities to directly visualize the biology of living organisms. By combining our growing knowledge regarding the role of specific genes and proteins in human health and disease, with novel ways to target these entities in a manner that produces an externally detectable signal, it is becoming increasingly possible to visualize and quantify specific biological processes in a non-invasive manner. All the major imaging modalities are contributing to this new field, each with its unique mechanisms for generating contrast and trade-offs in spatial resolution, temporal resolution and sensitivity with respect to the biological process of interest. Much of the development in molecular imaging is currently being carried out in animal models of disease, but as the field matures and with the development of more individualized medicine and the molecular targeting of new therapeutics, clinical translation is inevitable and will likely forever change our approach to diagnostic imaging. This review provides an introduction to the field of molecular imaging for readers who are not experts in the biological sciences and discusses the opportunities to apply a broad range of imaging technologies to better understand the biology of human health and disease. It also provides a brief review of the imaging technology (particularly for x-ray, nuclear and optical imaging) that is being developed to support this new field.

Thermal assessment of 40-MHz ultrasound at soft tissue-bone interfaces

Tissue exposure to diagnostic ultrasound (US) can cause significant temperature rises. However, little has been reported on thermal effects of high-frequency US, and guidelines for the use of US do not necessarily apply to higher frequencies. Temperature rise induced by US biomicroscopy (UBM) was measured in phantoms containing mouse skulls and in anesthetized mice and mice post mortem, with a 50-microm K-type thermocouple. The operating frequency was 40 MHz with a free field I(SPTA) of 2.6 mW/cm(2) (B-mode) and 11.9 W/cm(2) (Doppler). Peak negative pressures were 5.22 MPa (B mode) and 7.32 MPa (Doppler), resulting in a mechanical index (MI) of 0.83 (B-mode) and 1.05 (Doppler mode). In Doppler mode, mean temperature rises of 1.80 degrees C and 1.73 degrees C were measured for proximal and distal skull phantom surfaces after a 3-min insonation. In vivo, the proximal mouse skull surface showed a mean temperature rise of 2.1 degrees C, with no statistically significant differences post mortem. Our results indicate temperature rise from insonation of bone interfaces using similar exposure parameters should not cause adverse bioeffects.

Molecular imaging applications for immunology

The use of multimodality molecular imaging has recently facilitated the study of molecular and cellular events in living subjects in a noninvasive and repetitive manner to improve the diagnostic capability of traditional assays. The noninvasive imaging modalities utilized for both small animal and human imaging include positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), ultrasound, and computed tomography (CT). Techniques specific to small-animal imaging include bioluminescent imaging (BIm) and fluorescent imaging (FIm). Molecular imaging permits the study of events within cells, the examination of cell trafficking patterns that relate to inflammatory diseases and metastases, and the ability to rapidly screen new drug treatments for distribution and effectiveness. In this paper, we will review the current field of molecular imaging assays (especially those utilizing PET and BIm modalities) and examine how they might impact animal models and human disease in the field of clinical immunology.

Acoustic microscopy system: design and preliminary results

An acoustic microscopy system was designed to perform 2D imaging in the C-plane with a single-element transducer. The ultrasound transducer was fabricated by polishing bulk lithium niobate (LiNbO(3)) to the required thickness (approximately 60 or 45 micro) for the desired operating frequency (55 or 75 MHz). The polished LiNbO(3) was attached to acoustic backing and matching layers. Finally, an epoxy lens was applied and the transducer mounted in a housing. The transducer was mounted in a 3D motorized positioning stage and operated by a high-frequency pulser/receiver. Received echoes were sampled with a 2 GHz ADC card and displayed on a PC using software developed in the Matlab environment. Transducer frequency and bandwidth were measured off a steel plate positioned at the focal length. A penny was scanned initially to confirm expected performance before acquiring data from liver (n=3) and spleen (n=3) specimens. For the first probe, the peak frequency was 54.05 MHz with a -6 dB bandwidth of 6.76 MHz. The axial and lateral resolutions were estimated to be 114 and 188 microm, respectively. For the second probe, the peak frequency was measured to 82 MHz with a -6 dB bandwidth of approximately 23 MHz. The axial and lateral resolutions were estimated to be around 33 and 81 microm, respectively. C-scans of the penny clearly showed detailed structures on front and back, while the capsule and the trabecular structures of the splenic tissues could easily be separated in different layers. In conclusion, an acoustic microscopy system operating at 55-75 MHz has been constructed and the feasibility of obtaining high-resolution images of tissue specimens demonstrated.

Three-dimensional visualization and thickness estimation of aortic valve cusps using high-frequency ultrasound

High-frequency ultrasound techniques are introduced for three-dimensional imaging and thickness estimation of fresh heart valve cusps. Images of porcine aortic valve specimens were acquired within a 12 x 8 x 8 mm3 volume using a VisualSonics VS40 micro-imaging system operating at a 40 MHz centre frequency. Two image volumes were obtained from each of six left coronary cusps. One volume was acquired with the specimen submerged in distilled water and the second volume was acquired through either Hanks physiologic solution or coronary perfusion solution (CPS). The fibrosa, spongiosa and ventricularis were most readily distinguished when the specimen was imaged in distilled water. Colour thickness maps were computed from B-mode image data, and the mean and standard deviations of the thickness were determined for each cusp. In 11 of 12 trials, the image analysis algorithm yielded valid thickness estimates over greater than 98% of the region examined. Mean thickness estimates obtained with specimens submerged in Hanks solution or CPS ranged from 0.66 to 1.03 mm, and submersion in distilled water increased the mean thickness by 20-40%. This observation suggests that the cusps osmotically absorbed water. Information provided by high-frequency ultrasound is expected be valuable for characterizing the morphological properties of heart valves.

Design of efficient, broadband single-element (20-80 MHz) ultrasonic transducers for medical imaging applications

This paper discusses the design, fabrication, and testing of sensitive broadband lithium niobate (LiNbO3) single-element ultrasonic transducers in the 20-80 MHz frequency range. Transducers of varying dimensions were built for an f# range of 2.0-3.1. The desired focal depths were achieved by either casting an acoustic lens on the transducer face or press-focusing the piezoelectric into a spherical curvature. For designs that required electrical impedance matching, a low impedance transmission line coaxial cable was used. All transducers were tested in a pulse-echo arrangement, whereby the center frequency, bandwidth, insertion loss, and focal depth were measured. Several transducers were fabricated with center frequencies in the 20-80 MHz range with the measured -6 dB bandwidths and two-way insertion loss values ranging from 57 to 74% and 9.6 to 21.3 dB, respectively. Both transducer focusing techniques proved successful in producing highly sensitive, high-frequency, single-element, ultrasonic-imaging transducers. In vivo and in vitro ultrasonic backscatter microscope (UBM) images of human eyes were obtained with the 50 MHz transducers. The high sensitivity of these devices could possibly allow for an increase in depth of penetration, higher image signal-to-noise ratio (SNR), and improved image contrast at high frequencies when compared to previously reported results.

Developmental changes in left and right ventricular diastolic filling patterns in mice

Developmental changes in left and right ventricular diastolic filling patterns were determined noninvasively in isoflurane-anesthetized outbred ICR mice. Blood velocities in the mitral and tricuspid orifices were recorded in 16 embryos at days 14.5 (E14.5) and 17.5 of gestation (E17.5) using an ultrasound biomicroscope and also serially in three groups of postnatal mice aged 1-7 days (n = 23), 1-4 wk (n = 18), and 4-12 wk (n = 27) using 20-MHz pulsed Doppler. Postnatal body weight increased rapidly to 8 wk. Heart rate increased rapidly from approximately 180 beats/min at E14.5 to approximately 380 beats/min at 1 wk after birth and then more gradually to plateau at approximately 450 beats/min after 4 wk. Ventricular filling was quantified using the ratio of peak velocity of early ventricular filling due to active relaxation (E wave) to that of the late ventricular filling caused by atrial contraction (A wave) (peak E/A ratio) and the ratio of the peak E velocity to total time-velocity integral of E and A waves (peak E/total TVI ratio). Both ventricles had similar diastolic filling patterns in embryos (peak E/A ratio of 0.28 +/- 0.02 for mitral flow and 0.27 +/- 0.02 for tricuspid flow at E14.5). After birth, mitral peak E/A increased to >1 between the third and fifth day, continued to increase to 2.25 +/- 0.25 at approximately 3 wk, and then remained stable. The tricuspid peak E/A ratio increased much less but stabilized at the same age (increased to 0.79 +/- 0.03 at 3 wk). The peak E/total TVI ratio showed similar left-right differences and changes with development. Age-related changes were largely due to increases in peak E velocity. The results suggest that diastolic function matures approximately 3 wk postnatally, presumably in association with maturation of ventricular recoil and relaxation mechanisms.

Ultrasound biomicroscopy-Doppler in mouse cardiovascular development

The ability to modify the mouse genome has yielded new insights into the genetic control of mammalian cardiovascular development. However, it is far less understood how genetic factors and their consequent structural changes alter cardiovascular function, a void largely due to the lack of effective noninvasive techniques to assess function in the developing mouse cardiovascular system. In this review, we discuss the recent advances in ultrasound biomicroscopy (UBM)-Doppler echocardiography for analyzing cardiovascular function in the embryonic mouse in utero. "Cardiovascular function" encompasses broad aspects of physiology, including systolic and diastolic cardiac function, distribution of blood flow among various embryonic vascular beds, and vascular bed properties (impedance). A wide range of physiological measurements is possible using UBM-Doppler, but it is clear that the limitations of any single measurement warrant a multi-parameter approach to characterizing cardiovascular function. We further discuss the prospects for UBM-Doppler analysis of alternative vertebrate systems increasingly studied in developmental biology. The ability to correlate cardiovascular physiological phenotypes with their corresponding genotypes should lead to the elucidation of mechanisms underlying normal development, as well as embryonic disease and death.