The overall goal of this procedure is to assess the efficacy of new treatment strategies for myocardial infarction in vivo using PET and MRI. This is accomplished by first performing an MRI examination on the animal. Next, the imaging bed with the animal is transferred to the PET scanner.
Then the mouse is subjected to PET imaging. Finally, offline data analysis of the MRI and PET scans are performed. Ultimately, results can be obtained that show the response of the heart to a putative treatment at multiple levels through MRI and PET imaging.
The main advantage of this technique over exvivo methods is that it allows a longitudinal design and is ready for translation to patients. In addition, due to the high sensitivity and specificity, this method can significantly contribute to reduction in refinement in animal research. After preparing the animal with surgical intervention to induce infarcts and anesthetizing and setting it up for drug delivery, according to the text protocol, place the mouse on the MRI bed and deliver one to 2%isoflurane in one liter per minute of oxygen.
To maintain anesthesia, maintain the animal's respiration rate between 20 and 70 breaths per minute. Center the MRI coil over the mouse heart position. Then place a respiratory pillow sensor slightly below the diaphragm to monitor respiration and insert a rectal thermometer with a pre lubricated cover slip to monitor core temperature.
Ensure that temperature is kept constant during the scans. For ECG monitoring, place an electrode on each of the anterior paws and on the left rear paw, making sure that the palm of the paw is completely open. Twist the ECG cables together to ensure that they do not form resonant circuits at the MRI Resonant frequency, which would severely corrupt the ECG signal.
When running pulse sequences, use tape to ensure that the electrodes are firmly attached to the bed. Next position a water heated blanket over the mouse. Encapsulating the monitoring and coil leads to maintain body temperature.
Then align the laser of the bed with the heart position using the anterior paw line as a landmark, use an automatic bed to position this in the magnet ISO center. Set the monitoring equipment to detect the R wave in the ECG. Adjust the thresholds for each mouse and within imaging sessions so that there is reliable triggering to carry out MRI imaging.
Begin by acquiring a pilot image. To plan the multiplanar pilot images. Identify the heart in the image most easily by its flow artifacts.
Next, acquire a fast gradient echo with five slices per orientation and a three centimeter field of view, or FOV with ECG gating turned on. Then run an ungated 3D scan centered on the heart for pet co-registration while the scan is running. Plan a four chamber view scan that will cut through the apex and the tricuspid and mitral valves showing all four chambers.
Then plan a two chamber view. That will cut through the apex and the tricuspid valve showing the left atrium and ventricle crosscheck the geometry of the two long axis views. If the slice planning is suboptimal, repeat the scan covering the whole heart plan.
A stack of short axis slices orthogonal to both the four and two chamber views. Starting from the first apical slice without blood pool until the first basal slice without any RV slices, should be equally spaced with no gaps. To perform late gadolinium enhancement For MRI imaging, increase the blank time for gating to acquire every other heartbeat.
Apply a delay time that allows the acquisition to be performed at the end diastolic phase of the ECG just before injection. Perform a low resolution version of the LGE sequence in order to check that the gating is correct and no flow Artifacts are present. Next, slowly and steadily, inject the Gato Vista solution over 15 seconds.
To achieve the best contrast, initiate LGE imaging within 15 minutes after the injection. Then for pet imaging, transfer the MRI bed to the PET scanner, leaving the receiver in place and firmly anchor it to the pet bed support. Connect the anesthetic and monitoring equipment.
Position the heart in the center of the pet field of view, moving the bed only in the axial direction. Acquire a single pass transmission scan with a germanium source. Next, using a syringe extract 10 to 30 mega borres of radioactively labeled fluoro deoxy glucose or FDG from its container.
Use a well counter to measure its activity. Start the emission scan set to acquire list mode gated PET for 45 minutes. Simultaneously inject the tracer in a total volume of 50 to 100 microliters slowly and steadily over 15 seconds.
Then use enough saline to flush the line to deliver the whole of the radioactive tracer. To measure the residual activity, place the syringe back in the well counter to perform MRI.Segmentation. Begin by loading the cine MRI images in segment version 1.9 and run the automatic tools for segmentation.
Starting from a point inside the ventricle. The automatic tools will inflate the estimated ventricle walls until the best match is found. Manually delineate the left ventricle or LV and the right ventricle or RV at n systole or ES and n diastole or ED defined respectively as the frames with the maximal and minimal global LV volume.
Manually delineate the epicardium at ES and ED for LV mass calculations. As shown here, the cardiac muscle is incompressible, so the LV mass must be consistent in ES and ED at the base of the ventricles. Use a straight line to discriminate ventricles from atria.
Identifying the angle of the slice with the help of the long axis views. Crosscheck the segmentation on the long axis views. Delineate the scar area on the LGE images to begin pet analysis and co-registration with MRI data.
Use a 3D filtered back projection algorithm to reconstruct the pet images and export the data to nifty format. Showing standard uptake values. Use the SPM mouse bulk co-registration tool to register the average pet image to the 3D MRI.
One of the advantages of using MRI is that a longitudinal design can be used to stage disease. This is especially important when evaluating novel compounds as the time course of effect may not be known as shown in this example. Variations in heart volumes due to disease progression in an animal are visible correct slice.
Geometry is crucial for the success of a cardiac MRI experiment. This figure shows representative slice planning and resulting geometry for various MRI scans. Segmentation of two MRI short axis slices can be seen in this figure.
Our cine MRI procedure achieved a precision of 4%for LV mass, 3%for EDV 5%for ESV, 2%for sv, 2%for ef, and 4%for infarct size. PET can be used to measure the binding of specific tracers throughout the body. This figure shows an FDG PET maximum intensity projection image.
As expected in an anesthetized animal, the greatest uptake is found in the heart and the brain shown Here is an example of coregistered. L-G-E-M-R-I and FDG PET scans. The enhanced areas on the MRI representing non-viable tissue match areas of reduced FDG uptake in the PET scan.
Once master this technique can be done in less than 90 minutes. If it's performed properly, using a simultaneous PET MRI system this time could be further reduced. 45 minutes.
