Gating the Heart - Blood Pool Imaging

1. Gated Cardiac
1. Multigated acquisition - May be referred to as a MUGA or CINE
2. SPECT perfusion can also be gated which is done usually on stress images. The format is similar in concept to a MUGA and will be discussed briefly


2. Radiopharmaceutical
1. In vivo labeling - covered in the Spring
2. In vitro labeling
1. 2 - 3 ml of blood is removed from the patient and labeled in vitro
2. The kit required for this is called "Ultra Tag"
3. Refer to package insert for labeling (to be discussed in detail next semester)
4. Once the blood is labeled, re-inject IV and image
3. Modified In Vitro will be discussed in 2007
4. The key ingredients to labeling
1. The stannous ion adheres to the red blood cells (RBCs)
2. After allowing the stannous ion sufficient time to tag, ^99mTcO₄^- is injected
3. Hence the ^99mTcO₄^- tags to the Sn^- that tags to the RBC
5. Once the blood is labeled the acquisition may begin


3. Gating the heart



1. First look at the figure above, which is an example of 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

3. Three leads are attached to the patient in order to pick up the R to R interval which is needed for a MUGA acquisition
4. How is the heart gated?
1. The EKG signal is gated, from each R to R interval



2. From the above diagram notice how the R to R interval is subdivided into 5 sections
3. Each section, frame, or gate is considered as a segment of time in which counts are collected and stored
4. Each time the computer records an R wave acquisition is reset to the first frame and more counts are stored within each individual frame
5. Acquisition continues until enough counts are collected in each frame
6. Images above the EKG wave represent the data collected in each frame
7. In a resting image usually 400 beats or R waves collected is considered sufficient
8. Images can then be played back in a dynamic or cine mode that identifies myocardial wall motility (see image below - "Displaying the MUGA")

Note: For demonstration purpose only, 5 gates were used in the above diagram. Usually, the amount of gates set from the R to R interval is as few as 16 or as many 32.

5. Wall motion
1. Images show wall motion of the heart, however, 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 opposite direction to the other

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


4. Displaying the MUGA



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



2. Images can also be displayed in static mode (see the above)
3. Wall motion is defined as: normal, hypo kinetic, akinetic, or dyskinetic
4. Images can be filtered to improve image quality (discussed next semester)
5. Key - To quantify or calculate 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. The very first frame represents the end-diastolic (END) image
3. Depending on how automated the computer software is, you may or may not be required to draw the initial ROI around the LV
4. Computer will then calculate the activity in each frame, inside the ROI of the LV
5. Counts are stored
6. Computer will allow the user to verify the end-systolic (ENS) image, the ROI with the least amount of counts within the LV
7. Computer will allow the user to confirm the ROI drawn for bkg
8. Computer will then subtract bkg counts from each frame to calculate EF and display the EF 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 the LV on END and ENS, plus the area where the bkg ROI is located



11. Computer then calculates the %EF using the above formula
12. Computer will also display an ejection fraction curve (image not available)


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


7. Myocardial SPECT perfusion study may also be done using a gated protocol
   1. Computer gates each step or projection of the SPECT study
   2. Usually 8 frames are gated within each projection
   3. Wall motion can be determined via wall thickening
   4. %EF can be determined via this acquisition
   5. Images are not available