Cardiac exam with ECG gating

Due to its quickness the direct imaging by isotope diagnostics of heart movement phases is impossible. The approximately periodic work of the heart makes it possible for direct imaging. The recognition of every single period is feasible based on the so-called R wave of ECG. The R wave starts the ventricular contraction. In the case of certain leads it runs up significantly higher than in the others making possible a simple automatic recognition.

Before the examination begins, the software determines the most frequent R-R time and divides it into many (n) parts according to the number of different phases required. Usually n is a certain power of two (16, 32 or 64). The program allocates memory for n images and zeros the whole area. Then the recording could begin.

The ECG’s R wave causes a interruption and the software starts to collect the impulses to the first image’s place. After the time belonged to one phase passed, and another interruption occurs and the collection continues at the next picture’s place. When the collection of the last picture finishes and no new R wave has been started, then the collection is paused until the arrival of the next wave.

Figure 30.

 
The collection can be continued during a few hundred cardiac circles with this method if the used radiopharmacon stays inside the vascular system, which assures the acceptable signal-to-noise ratio in the images.

Commonly happens that a new R wave comes significantly earlier than it is expected normally. This phenomena is called systole. An extra systole generally followed by a so-called compensatory pause.

If the data coming from cardiac cycle with compensatory pause or extra systole is collected, then it falsifies the recording. Avoiding this, the collection is written into alternate puffers. While the data is collected into one puffer, in the case of a normal cycle, the software processes the data stored in the other puffer: adds them to the previously collected data and zeros. Having an abnormal cycle, only the zeroing of the puffer happens.

Figure 31.

 
As it can be seen, not only the heart cycles belonged to a compensatory pause or to an extra systole differ from the normal (different length and amplitude) but the previous and the next ones too, since the volume of the ventricle is significantly larger than normally. Therefore, most of the recording programs give the opportunity to omit the cycles having a length significantly different from the most frequent R – R distance and, moreover, make the same with the next 1, 2… cycles.

Examination of the pumping function of left ventricle

Figure 32. Cardiac exam with ECG gating, 16 images representing the cardiac cycle

 
A radiopharmacon is applied that stays durably inside the vascular system for the examination, and the exam performed ECG gated. Projection of the mostly expanded area of the left ventricle (end diastole) is marked.

Figure 33.

 
The judgment of the activity at the left ventricle is disturbed by the background activity, so a background ROI is usually marked. The ROI near the left ventricle can be considered to be appropriate if the corresponding time-activity curve is pretty much horizontal. If the background curve indents at the middle, then it is marked too close to the ventricle. If it is raised, then the background ROI intersects into the projection of the liver or the spleen which contains also a lot of blood.

The most important parameter of the pumping function is the so-called ejection fraction (EF). EDV is the end diastolic volume (most expanded ventricle), ESV denoted end systolic volume (the most contracted ventricle). By definition, EF = 100*(EDV - ESV)/EDV%. Since these volumes are proportional to the activity measured in the projection of left ventricle at the corresponding heart phase with a good approximation, then EF = 100*(EDC - ESC)/EDC%, where EDC and ESC are the maximum and the minimum of the left ventricle activity curve after subtraction of the background.

Sometimes ESC is calculated from the projection of the left ventricle’s end systole. In that case, the difference between the ROIs of end systole and end diastole is regarded as background.

Figure 34.

 
The correct application of this method arises difficulties. As long as the left ventricle projection is marked too large, it does not cause problems, because the background correction conceals its effect. So it is enough to pay attention to mark the left ventricle large enough. The imprecision of marking the end systole’s projection causes the error of EF value: if this projection has been chosen too small, too large EF is received. However, it is extremely hard to find an objective parameter for the accurate marking