Measurement of left – right shunt

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At fetal stage the lung can only get oxygen from blood, so the septum separating the right and left part of the heart is not developed, thus the blood flows freely, mixed between the two parts. This assures that both the lung and the body get fresh blood.

After birth this state is no longer desirable, so in most cases the septum closes, resulting that the pulmonary and the systemic circulations are not mixed with each other further more. However sometimes due to a developmental abnormality, the septum does not close entirely. In that case, the blood flows from the direction of that part of the heart where there is a higher pressure towards the other one. According to this, we can talk about left – right and right – left shunt. Because the systemic circulation has larger resistance, the muscles of left atrium are stronger, so the left – right shunt is more frequent. Hereinafter this is detailed below.

Measurement of left - right shunt can be performed by a simlpe parallel-hole ring-collimator mounted stationary single scintillation detector system (Figure 17.), but nowadays gamma cameras are used as usual. 2

Surprising might it be, the measurement is not done about the heart, but from a so-called peripheral (does not include large artery) lung area. Since a quite fast process has to be followed up (at least one image per second), consequently a high speed dynamic acquisition needs to be performed. More reliable result can be achieved by applying more fine i.e. faster and faster time sampling - 0.1sec ÷ 0.3sec frame image sampling -

The activity administration is special too. Intra venous bolus technique is applied. Firstly, the activity goes to the right side of the hearth, then to the lung, after it to the left side of the hearth and finally into the systemic circulation.

Figure 17.

 
The theoretical time dependence activity change would follow a gamma function, when the blood won’t return to the peripheral lung area during the first pass blood flow. Since the blood will return on time, only the start part of the curve has such a tendency. A gamma curve is fitted to the first part of the lung curve (, getting a, b, c and t0. The difference between the lung and fitted curves is calculated as well.

If the patient has a left – right shunt, than a part of the blood that goes from the lung to the left heart side gets across to the right heart side. The blood returned through the shunt appears significantly earlier in the lung than in the systemic circulation. Thanks to this, the shunt is responsible for the increment of the differential curve at the initial part it. A fitting is also made at the increasing initial part of the differential curve (.

Size of the shunt can be determined by the areas under two () fitted curves ( and ().

( The method of area calculation please see below at chapter 4.3.3. If SHUNT<1.3, then the “SHUNT” value can be considered as normal case.

Figure 18. The 16 images represent the blood flow, then a chosen frame and the defined and marked ROIs with time - activity curves (negative case /microSEGAMS© V4.275/)

 
If the injection of the activity isn’t quick enough, higher value than the real shunt (i.e. false positive) can be obtained. Consequently, a so-called reference ROI -V_CAVA (Venous Cava Superior) – has to defined near the heart for bolus quality determination. If the Full Width Half Max (FWHM) of the V_CAVA curve is too big, i.e. the V_CAVA WIDTH>3.1sec, then that bolus is extended and isn’t suitable for the shunt measurement (can be considered as technical failure). The V_CAVA curve belong to the V_CAVA ROI while the PERIFERIAL LUNG curve (SHUNT determination) belong to the LUNG ROI. Figure 19. presents the complete shunt evaluation technique including a curve fitting procedure as well.

Figure 19. Complete evaluation of LRShunt estmation - positive case /InterViewXP 3.0.77/ -