Gating the Heart - Blood Pool Imaging

1. Gated Cardiac
1. Multigated acquisition - May be referred to as a MUGA or CINE, and it is a planar acquisition of the LV showing myocardial contraction
2. SPECT perfusion can also be gated, usually done only on the stress images. The exception is MCV where stress and rest images are gated. The process of gating a SPECT is similar to the MUGA imaging and will be reviewed in detail next Spring
3. MUGA acquisition and the gating process will be discussed now
4. By labeling the red cells with ^99mTc and re-injecting it into the patient, gated myocardium data can be acquired as blood flows through the left ventricle of the heart
5. As the muscles of the LV contact, it pushes the labeled red cells out the aorta, and by gating, we can evaluate the function of musculature contraction
   
2. Radiopharmaceutical - there are three methods to label the red cells
1. In vivo RBC labeling - covered in the Spring
2. In vitro RBC labeling (most common)
1. 2 - 3 ml of blood is removed from the patient and labeled in vitro
2. The kit required for this procedure is called "Ultra Tag"
3. Refer to the package insert for labeling
4. Once the blood is labeled, it is then re-injected, and images are acquired
3. Modified In Vitro RBC labeling - hold for future discussion
4. The key ingredients in labeling RBCs
1. Stannous ion enters the red cell with the role of reducing ^99mTcO₄^- locking the radioisotope into the cell
5. Once the blood is labeled the; acquisition may begin


3. Gating the heart



1. First, look at the figure above, an electrocardiogram (EKG)
2. The following parts of the EKG wave or electrical activity of the myocardium are identified:

P - signal for sinus node impulse to depolarize the atria
QRS complex - electrical depolarization and contraction of the ventricles
P - recovery phase of the ventricles relaxes the contraction

3. Three leads are attached to the patient pick up the R wave so R to R intervals can be recorded. The R wave is critical and allows for cardiac gating
4. How is the heart gated?
1. The EKG signal determines the R wave and the computer sets up a series of gates/frames



2. From the above, diagram notice how the R to R interval is subdivided into five sections
3. Each section, frame, or gate is considered as a segment of time where counts are collected and stored
4. Each time the R wave is recorded, the acquisition resets itself to the first frame and data is recollected storing the counts from eahc R to R interval
5. The acquisition continues until enough counts are collected within each frame
6. Images above the EKG wave represent the data collected in each frame
7. A resting image usually contains about 400 beats or R waves. The amount of counts in 400 R's is usually enough to give adequate resolution
8. Images can then be played back in a dynamic cine mode to evaluate myocardial wall motility (see image below - "Displaying the MUGA")

Note: For demonstration purposes only five gates were used in the above diagram. Usually, the amount of gates is between 16 to 32

5. Displaying the wall motion
1. Images show wall motion of the heart, however, in reality you are actually seeing labeled RBCs moving in and out of the heart chambers
2. There are four types of wall motion to consider:

Normal contractility of all walls
Hypokinesis - one wall or section is moving slower than the rest
Akinesis - one wall or section is not moving at all when compared to the rest
Dyskinesis - one wall or section is moving in the opposite direction to the other

6. Angles of the LV that are acquired that are imaged
1. ANT
2. RAO at 10 to 15 degrees
3. LAO 30 - 45% (look for the angle of best separation of the LV)
4. Steep LAO or LEFT LAT

5. Examples of three different angles acquired in a MUGA


4. Displaying the MUGA



1. All angles are played back s in a dynamic format



2. Images can also be displayed in static mode (see the above)
3. Images can be filtered to improve image quality (discussed next semester)
4. Results of acquisition allow for the calculation of left ventricle ejection fraction (EF)


5. Calculating % EF of the LV


1. From the images being displayed above, a region of interest is drawn around the LV
2. Depending on how automated the computer software is, it may or may not be required to draw the initial ROI around the LV
3. The first frame usually represents the end-diastolic (END), which contains the maximum amount of counts
4. The computer will then calculate the activity of the LV for each frame
5. The computer will find the frame with the least amount of counts (within the ROI) and define it as end-systolic (ENS)
6. Finally, the computer will allow the user to confirm the ROI for BKG
7. BKG is subtracted from each frame and then the ejection fraction formula is applied
8. This generates a percent ejection fraction and an ejection fraction curve
9. If necessary, any or all ROI(s) can be redrawn should the observer disagree with the automated ROI locations
10. The above image shows ROIs drawn over the LV at END, ENS, and BKG



11. The computer will also display an ejection fraction curve as noted below




6. Rest and Stress Protocols (see procedures below)
1. Above is an ejection fraction curve
2. Usually, a MUGA study only involves resting images with the following angle imaged: ANT, RAO at 10 to 15 degrees, LAO 30 - 45%, and Steep LAO or LEFT LAT
3. Stress gated cardiac procedure will be covered next Spring