Phase Contrast Magnetic Resonance Imaging in the Rat Common Carotid Artery

This method can answer key question in cardiovascular field relating to blood flow abnormality. The main advantage of this technique is that it is noninvasive. It can be done in any small animal MRI system.

The implication of this technique extend toward diagnosis of cardiovascular diseases because it can shed light on the hemodynamic alterations. Visual demonstration of this method is critical, as triggered scan are important for successful phase contrast MRI scan, but are difficult to learn and are not commonly used in routine scans. Demonstrating the procedure will be Chiu Shao-Chieh and Hsu Shih-Ting, research assistant from our center.

Prior to performing this technique, remove all magnetically susceptible objects, such as wallets, keys, and credit cards, and leave them outside of the scanner room. Following the initial dose of anesthesia, place the rat in the MRI bed in a head-first prone position, and deliver two to 3%isoflurane through a nose cone to maintain anesthesia. Monitor the animal's breathing by placing a respiratory pillow sensor under the animal's torso, and connect the sensor to a monitoring system.

Check that the rat's respiration rate is between 40 and 50 beats per minute before proceeding. For cine-gated PC-MRI acquisition, place one electrode on the right forepaw and one on the left hindpaw. Twist the electrocardiography cables together, and ensure that they are connected to the monitor.

Next, position the rat in a head holder with ear bars and a bite bar to secure the animal and to restrict its head movement. Then, turn on the warm air heating system or use gauze pads to help the rat maintain its body temperature while in the MRI. Finally, ensure that the R-wave is clear on the ECG monitor, and place the animal in the scanner.

Once the animal is found to be physiologically stable, start the MRI scans. Select the localizer sequence from the console monitor of the MRI scanner, and acquire scout images along all three orientations using any fast image acquisition sequence, such as the fast spin echo, to create coronal, axial, and sagittal images. Look at the scout images to ensure that the center of the animal's head and neck is at the center of the magnet.

If necessary, adjust the animal's position until the correct position is reached. If the animal is repositioned, repeat the scout image scans. Once the rat is properly positioned, select the time-of-flight angiogram sequence from the console monitor of the MRI scanner, and acquire a 2D time-of-flight angiogram first to ascertain the precise anatomical location of the common carotid artery.

Ensure that the saturation band is on and is placed on the top to avoid interference from venous signals. The saturation band usually comes with time-of-flight sequence. If the saturation band does not show on monitor, please notify the service person.

After locating the common carotid artery using the time-of-flight angiogram, target the image plane of the phase contrast magnetic resonance imager to the center of the same artery, and orient it such that the slice is perpendicular to the direction of blood flow. Ensure that both respiration and ECG gating are connected to the MRI system, showing the clear signal on the monitor computer, and set the trigger module to be on in the trigger mode from the console monitor of the MRI scanner. At this point, again confirm that the animal's physiological responses are stable, and verify that the gating selections are on in both the monitor computer and the console monitor of the MRI scanner.

Please remember to make sure that the gating selections are on in both monitor computer and the monitor one of MRI scanner. Next, select the sequence of PC-MRI sequence from the console monitor of the MRI scanner, and perform the gated PC-MRI scans. Repeat this process for each region of interest.

Once all of the regions have been imaged, remove the animal from the scanner and return it to its cage. Warm the animal with a heating lamp to maintain body temperature. Keep the lamp at least 15 centimeters away from the animal to prevent overheating until the animal starts to move and exhibits a response to a tail or toe pinch.

Finally, be sure to save the MRI data, and process it as described in the accompanying text protocol. A correct slice having round geometry is pivotal for ensuring the success of the PC-MRI experiment. As angulation increases, the resulting artery geometry becomes ovoid, leading to larger partial volume effects and the overestimation of blood flow.

If the slice on the right was observed, the scan should be repositioned and repeated. When correctly positioned, intra-scan reproducibility of changes in blood flow within one cardiac cycle should be high. As can be seen, blood flow reaches its maximum during the systolic phase and returns to baseline during the diastolic phase.

The blood flow in the common carotid artery in rats is age-dependent, highlighted here, in the longitudinal scans by the difference in the peak systolic flow between a two-month-old rat and one that is four months old. After watching this video, you should have a good understanding of how to apply PC-MRI to noninvasively measure the blood flow in the rat common carotid artery in under 30 minutes if performed properly. While attempting this procedure, it is important to remember to connect the gating system properly.

After its development, this technique paved the way for research in the field to explore the mechanism behind some cardiovascular disease.