Use of ECG-gated computed tomography, echocardiography and selective angiography in five dogs with pulmonic stenosis and one dog with pulmonic stenosis and aberrant coronary arteries.

Pulmonic stenosis (PS) is the most common congenital cardiac disease in dogs. Boxers and English bulldogs are among the most commonly affected breeds and also commonly associated with an aberrant coronary artery (CA). If an aberrant CA is suspected and balloon valvuloplasty indicated, an intra-operative angiography is recommended prior to the procedure. ECG-gated computed tomography (CT) can be used to screen for CA anomalies in a quick and minimally-invasive way (preventing side effects associated with selective catheter angiography) and allowing early planning of the procedure. The aim of this case series was to report CT findings associated with PS diagnosed by echocardiography. Our database was retrospectively searched for cases of dogs with PS diagnosed by echocardiography, where an ECG-gated CT was performed. A total of six cases were retrieved: all were diagnosed with severe PS. Four dogs had concurrent congenital defects: two dogs had a patent ductus arteriosus, one dog had a ventricular septal defect and an overriding aorta, one dog had an aberrant CA. Detailed CT findings of all cases were reported, including one case of a patent ductus arteriosus and an overriding aorta not identified by transthoracic echocardiography. In addition, an abnormal single left coronary ostium, with a pre-pulmonic right CA was described. In conclusion, despite echocardiography remaining the gold standard for diagnosis and assessment of PS, ECG-gated-CT angiography is a complementary diagnostic method that may provide additional relevant information, shorten surgery/anaesthesia time and reduce the amount of radiation to which the clinician is subjected.

Derivation of a respiration trigger signal in small animal list-mode PET based on respiration-induced variations of the ECG signal.

BACKGROUND: Raw PET list-mode data contains motion artifacts causing image blurring and decreased spatial resolution. Unless corrected, this leads to underestimation of the tracer uptake and overestimation of the lesion size, as well as inaccuracies with regard to left ventricular volume and ejection fraction (LVEF), especially in small animal imaging. METHODS AND RESULTS: A respiratory trigger signal from respiration-induced variations in the electro-cardiogram (ECG) was detected. Original and revised list-mode PET data were used for calculation of left ventricular function parameters using both respiratory gating techniques. For adequately triggered datasets we saw no difference in mean respiratory cycle period between the reference standard (RRS) and the ECG-based (ERS) methods (1120 ± 159 ms vs 1120 ± 159 ms; P = n.s.). While the ECG-based method showed somewhat higher signal noise (66 ± 22 ms vs 51 ± 29 ms; P < .001), both respiratory triggering techniques yielded similar estimates for EDV, ESV, LVEF (RRS: 387 ± 56 µL, 162 ± 34 µL, 59 ± 5%; ERS: 389 ± 59 µL, 163 ± 35 µL, 59 ± 4%; P = n.s.). CONCLUSIONS: This study showed that respiratory gating signals can be accurately derived from cardiac trigger information alone, without the additional requirement for dedicated measurement of the respiratory motion in rats.

Application of the compressed sensing technique to self-gated cardiac cine sequences in small animals.

PURPOSE: Self-gated cine sequences are a common choice for cardiac MRI in preclinical applications. The aims of our work were to apply the compressed sensing technique to IntraGateFLASH cardiac MRI studies on rats and to find the maximum acceleration factor achievable with this technique. THEORY AND METHODS: Our reconstruction method extended the Split Bregman formulation to minimize the total variation in both space and time. In addition, we analyzed the influence of the undersampling pattern on the acceleration factor achievable. RESULTS: Our results show that acceleration factors of up to 15 are achievable with our technique when appropriate undersampling patterns are used. The introduction of a time-varying random sampling clearly improved the efficiency of the undersampling schemes. In terms of computational efficiency, the proposed reconstruction method has been shown to be competitive as compared with the fastest methods found in the literature. CONCLUSION: We successfully applied our compressed sensing technique to self-gated cardiac cine acquisition in small animals, obtaining an acceleration factor of up to 15 with almost unnoticeable image degradation.

MRI compatible small animal monitoring and trigger system for whole body scanners.

Performing magnetic resonance imaging (MRI) experiments with small animals requires continuous monitoring of vital parameters, especially the respiration rate. Clinical whole-body MR scanners represent an attractive option for preclinical imaging as dedicated animal scanners are cost-intensive in both investment and maintenance, thus limiting their availability. Even though impressive image quality is achievable with clinical MR systems in combination with special coils, their built-in physiologic monitoring and triggering units are often not suited for small animal imaging. In this work, we present a simple, MRI compatible low cost solution to monitor the respiration and heart rate of small animals in a clinical whole-body MR scanner. The recording and processing of the biosignals as well as the optimisation of the respiratory trigger generation is decribed. Additionally rat and mouse in-vivo MRI experiments are presented to illustrate the effectiveness of the monitoring and respiratory trigger system in suppressing motion artifacts.

Imaging of cardiac perfusion of free-breathing small animals using dynamic phase-correlated micro-CT.

PURPOSE: Mouse models of cardiac diseases have proven to be a valuable tool in preclinical research. The high cardiac and respiratory rates of free breathing mice prohibit conventional in vivo cardiac perfusion studies using computed tomography even if gating methods are applied. This makes a sacrification of the animals unavoidable and only allows for the application of ex vivo methods. METHODS: To overcome this issue the authors propose a low dose scan protocol and an associated reconstruction algorithm that allows for in vivo imaging of cardiac perfusion and associated processes that are retrospectively synchronized to the respiratory and cardiac motion of the animal. The scan protocol consists of repetitive injections of contrast media within several consecutive scans while the ECG, respiratory motion, and timestamp of contrast injection are recorded and synchronized to the acquired projections. The iterative reconstruction algorithm employs a six-dimensional edge-preserving filter to provide low-noise, motion artifact-free images of the animal examined using the authors' low dose scan protocol. RESULTS: The reconstructions obtained show that the complete temporal bolus evolution can be visualized and quantified in any desired combination of cardiac and respiratory phase including reperfusion phases. The proposed reconstruction method thereby keeps the administered radiation dose at a minimum and thus reduces metabolic inference to the animal allowing for longitudinal studies. CONCLUSIONS: The authors' low dose scan protocol and phase-correlated dynamic reconstruction algorithm allow for an easy and effective way to visualize phase-correlated perfusion processes in routine laboratory studies using free-breathing mice.

Geometric regularization for 2-D myocardial strain quantification in mice: an in-silico study.

Myocardial strain quantification in the mouse based on 2-D speckle tracking using real-time ultrasound datasets is feasible but remains challenging. The major difficulty lies in the fact that the frame rate-to-heart rate ratio is relatively low, causing significant decorrelation between subsequent frames. In this setting, regularization is therefore particularly important to discard motion estimates that are improbable. Different regularization methods have been proposed, among which is a class of regularizers based on enforcing preset geometrical characteristics of the motion field. To date, these regularization methods have not been contrasted. The aim of this study was thus to compare the performance of different geometric regularizers in the setting of myocardial motion and strain estimation in murine echocardiography using simulated datasets. In normal models, restricting the spatial curvature of the motion fields resulted in worse radial strain estimates (mean root-mean-square [RMS] error increased from 0.06 to 0.09; p < 0.05), but better circumferential strain estimates (mean RMS error decreased from 0.035 to 0.01; p < 0.05). More accurate circumferential strain estimates were also obtained by convolving a Gaussian function with the lateral motion components (mean RMS error decreased to 0.015; p < 0.05). In infarcted models, no significant differences were found between regularized and nonregularized radial strains. However, for circumferential strain, the curvature method yielded better strain estimates in all regions (mean RMS error decreased from 0.043 to 0.015; p < 0.05), whereas the Gaussian method only improved strain assessment in the remote myocardium (mean RMS error decreased to 0.021; p < 0.05).

Fully automated intrinsic respiratory and cardiac gating for small animal CT.

A fully automated, intrinsic gating algorithm for small animal cone-beam CT is described and evaluated. A parameter representing the organ motion, derived from the raw projection images, is used for both cardiac and respiratory gating. The proposed algorithm makes it possible to reconstruct motion-corrected still images as well as to generate four-dimensional (4D) datasets representing the cardiac and pulmonary anatomy of free-breathing animals without the use of electrocardiogram (ECG) or respiratory sensors. Variation analysis of projections from several rotations is used to place a region of interest (ROI) on the diaphragm. The ROI is cranially extended to include the heart. The centre of mass (COM) variation within this ROI, the filtered frequency response and the local maxima are used to derive a binary motion-gating parameter for phase-sensitive gated reconstruction. This algorithm was implemented on a flat-panel-based cone-beam CT scanner and evaluated using a moving phantom and animal scans (seven rats and eight mice). Volumes were determined using a semiautomatic segmentation. In all cases robust gating signals could be obtained. The maximum volume error in phantom studies was less than 6%. By utilizing extrinsic gating via externally placed cardiac and respiratory sensors, the functional parameters (e.g. cardiac ejection fraction) and image quality were equivalent to this current gold standard. This algorithm obviates the necessity of both gating hardware and user interaction. The simplicity of the proposed algorithm enables adoption in a wide range of small animal cone-beam CT scanners.

Lung perfusion imaging in small animals using 4D micro-CT at heartbeat temporal resolution.

PURPOSE: Quantitative in vivo imaging of lung perfusion in rodents can provide critical information for preclinical studies. However, the combined challenges of high temporal and spatial resolution have made routine quantitative perfusion imaging difficult in small animals. The purpose of this work is to demonstrate 4D micro-CT for perfusion imaging in rodents at heartbeat temporal resolution and isotropic spatial resolution. METHODS: We have recently developed a dual tube/detector micro-CT scanner that is well suited to capture first pass kinetics of a bolus of contrast agent used to compute perfusion information. Our approach is based on the paradigm that similar time density curves can be reproduced in a number of consecutive, small volume injections of iodinated contrast agent at a series of different angles. This reproducibility is ensured by the high-level integration of the imaging components of our system with a microinjector, a mechanical ventilator, and monitoring applications. Sampling is controlled through a biological pulse sequence implemented in LABVIEW. Image reconstruction is based on a simultaneous algebraic reconstruction technique implemented on a graphic processor unit. The capabilities of 4D micro-CT imaging are demonstrated in studies on lung perfusion in rats. RESULTS: We report 4D micro-CT imaging in the rat lung with a heartbeat temporal resolution (approximately 150 ms) and isotropic 3D reconstruction with a voxel size of 88 microm based on sampling using 16 injections of 50 microL each. The total volume of contrast agent injected during the experiments (0.8 mL) was less than 10% of the total blood volume in a rat. This volume was not injected in a single bolus, but in multiple injections separated by at least 2 min interval to allow for clearance and adaptation. We assessed the reproducibility of the time density curves with multiple injections and found that these are very similar. The average time density curves for the first eight and last eight injections are slightly different, i.e., for the last eight injections, both the maximum of the average time density curves and its area under the curve are decreased by 3.8% and 7.2%, respectively, relative to the average time density curves based on the first eight injections. The radiation dose associated with our 4D micro-CT imaging is 0.16 Gy and is therefore in the range of a typical micro-CT dose. CONCLUSIONS: 4D micro-CT-based perfusion imaging demonstrated here has immediate application in a wide range of preclinical studies such as tumor perfusion, angiogenesis, and renal function. Although our imaging system is in many ways unique, we believe that our approach based on the multiple injection paradigm can be used with the newly developed flat-panel slip-ring-based micro-CT to increase their temporal resolution in dynamic perfusion studies.