Software

An IDL Widget for quantifying fibrosis in trichrome sections.

Cyclic variation of T1 in the myocardium

Cardiac

Standard methods of longitudinal relaxation (T1) measurements in the heart produce only one T1 map of the myocardium, usually at the end diastole. We developed a method to generate a movie of T1 maps in the myocardium during the cardiac cycle using a dual flip angle fast gradient echo technique in the steady state. The effects of non-ideal slice profile and transient steady state on the T1 measurements were evaluated by Bloch simulations. Based on these results we introduce a linear correction to the measured T1 values which was validated by phantom experiments. In vivo T1 cine maps in healthy volunteers show 70±7% drop in T1 from end diastole to end systole in the septum and a 43±13% drop in the left ventricular lateral wall. This technique could be used to assess the myocardial blood volume changes during the cardiac cycle.

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Quantification of Aortic Compliance in Mice

The Pulse Wave Velocity (PWV) and Wall Shear Stress (WSS) are well known indices of vessel compliance. However, In vivo studies with transgenic animal models of arterial diseases are still needed to elucidate the exact role played by arterial diseases in the observed variations of PWV and WSS. Conventional MR techniques of acquiring velocity data lack both spatial and temporal resolution needed to accurately calculate PWV, and WSS in small animals. We developed a radial phase contrast MR technique to measure PWV and WSS in the mouse descending aorta. Blood flow was measured at four to five axial slices along the descending aorta with 4mm gap. The normalized flow velocity at different distances (D) along the descending aorta were plotted to determine the time delay (ΔT) in arrival of the pulse wave, The pulse wave velocity was calculated from the slope of the curve D vs. ΔT by a linear fit. WSS was calculated using finite element method based on the flow pattern in flow direction.

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Download MP4 (right click, save as).

Myocardial Strain

Cardiac

MR tagging was the first widely applied non invasive method to assess myocardial strain. In MR tagging, the magnetization is spatially modulated to create a grid pattern of dark stripes across the imaging plane which deforms as the myocardium moves. 2-D tagged MR imaging can separate relative motion of points within the myocardium from the rigid body motion such as rotation and translation. Tagging has been used to assess systolic function in normal humans and patients with diseases such as hypertrophic cardiomyopathy, aortic stenosis, and coronary artery disease.

We have developed a tagging sequence on the 7 Tesla Bruker system for mouse cardiac imaging. Spatial modulation of magnetization was achieved with a pulse train with varying amplitude. Compared to DANTE with uniform RF amplitude, and well established SPAMM, this SINC like modulation of the RF pulse train produced tags with better contrast. The DANTE pulse train consists of 5 rectangular 0.1ms RF pulses with flips 10°, 30°, 50°, 30° and 10°. Cine images are acquired using an ECG gated, segmented FLASH sequence. Horizontal and vertical tag lines are acquired separately with read direction perpendicular to the tag orientation. Images are reconstructed to a 256×256 matrix and then converted to DICOM format. Tag analysis is done using HARP (Diagnosoft Plus, Diagnosoft Inc., CA, USA).

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Download MP4 (right click, save as).

Myocardial viability in Mice

Delayed myocardial enhancement MR imaging is performed after administration of paramagnetic contrast agents (Gd-DTPA) and is used for a growing number of clinical applications. In fibrosis and extracellular expansion, there is a greater extracellular space for Gd-DTPA accumulation, and the distribution kinetics are slower than normal myocardium. These two effects result in a delayed and persistently higher relative concentration of Gd in areas in the heart where extracellular tissue is abnormal. Delayed-enhanced MR has been validated for the detection of irreversible injury in myocardial infarction. Regions of myocardial late gadolinium enhancement also occur in hypertrophic cardiomyopathy, and the extent of enhancement has been associated with clinical markers of sudden death risk and progression to heart failure.

Mouse viability studies are performed in the 7 Tesla Bruker scanner. A bolus of Gd-DTPA (0.3-0.6 mmol/kg) is given intraperitonealy while the mouse is placed in the scanner bore. Delayed enhancement MR is performed using a T1 weighted cine sequence. T1 weighting is achieved by increasing the flip angle to 30°-40°. The hyper enhancement is retained in the infarcted mouse heart for 40-50 minutes reaching a peak at around 30 minutes. This long enhancement period allows a multi slice T1 weighted cine series to be acquired covering the entire left ventricle. The cine shows delayed enhancement of the lateral wall of a wild type mouse model 24 hours after coronary occlusion.

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Fat/Water Separation

Cardiac

A spin echo Dixon technique that requires only two images to separate fat and water from high resolution images in mice has been implemented on the 7 T Bruker MR scanner. The scan time is reduced by a factor of 2/3 compared to the standard 3 point technique. This reduction scan time is an advantage for large scale serial studies.