 |
| Fig.3.11b.1. Principle of absolute dynamic scintigraphy of the lungs by breathing 133 Xe in a closed circuit (max. Inspiration and expiration, equilibrium) and subsequently in an open circuit (wash-out). |
3.11b. Dynamic scintigraphy of pulmonary ventilation
Dynamic scintigraphy of pulmonary ventilation is a comprehensive examination that allows detailed and regional quantification of a number of parameters of lung respiratory function - total lung capacity, residual volume, exchange fraction of air in the alveoli (per unit lung volume) and exchange volume of air.
Study storage
Dynamic
scintigraphic examination of pulmonary
ventilation is performed using a special spirometric
breathing apparatus allowing to breathe in a closed
circuit a mixture of air and radioactive gas xenon 133 Xe - thus breathing air
"marked" 133 Xe. The patient, sitting with
his back resting on the camera detector, first breathes a mixture
of air and 133 Xe in a closed
circuit for a few minutes so as to establish an equilibrium concentration.radioactive gas (equilibrium) is
lubricated by the lung chambers and the working space of the
respiratory system; it takes about 3 minutes. The camera is set
so that both lungs are in the field of view and dynamic
acquisition is started - frame rate approx. 5sec./frame. The
patient then - still in the equilibrium stage - performs the
maximum inhale and exhalation several times (at least 2 times)
with breath holding, while the corresponding volume differences
are read on the respiratory system. This spirometrically measured
difference in volumes between max. Inspiration and
expiration will be used for quantitative (absolute -
"volume-pulse") calibration
of
scintigraphic data
in the evaluation . Then the appropriate valve is opened and the
patient breathes normal air without radioxenone, thereby
radioactive 133Xe gradually exhales from the
lungs. The rate of disappearance of 133 Xe from the lungs, the so-called
wash-out , is one of the important
measures of the performance of this organ - pulmonary
ventilation
. The accumulation of dynamic scintigraphy ends when practically
no 133 Xe (body background level) remain in the
lungs - this is usually after 5-8 minutes from the start of the
wash-out.
Study evaluation
Determination of
significant phases of ventilation
In the computer evaluation of the dynamic study thus obtained,
the areas of interest (ROI) of the left lung, right
lung and tissue background are first marked on the summation
image of the equilibrium (Fig ... a). From these areas, curves of the time course of radioactivity 133 Xe during the examination are formed .
The curve from the lungs has a typical shape (Fig.3.11b.1 top
left): at the beginning of the equilibrium , where the elevated points
corresponding to the maximum inspiration and the decreased points
corresponding to the maximum expiration are visible; followed by
a phase of exhalation 133 Xe from the lungs (wash-out) in which the
activity of the lungs decreases gradually to the level of body
background.
|
| Fig.3.11b.1. Principle of absolute dynamic scintigraphy of the lungs by breathing 133 Xe in a closed circuit (max. Inspiration and expiration, equilibrium) and subsequently in an open circuit (wash-out). |
The points and sections corresponding to the maximum inhale and exhale, equilibrium and the wash-out section are then defined on this curve (automatically with the possibility of manual intervention). By summing the images corresponding to these dynamically significant points and sections, the resulting scintigraphic images of the characteristic stages of the examination are created, and further analysis is used to construct parametric images and calculate the values ??of quantitative parameters.
Parametric images of
the regional distribution of spirometric parameters
Images corresponding to significant points and sections defined
on the time course curve of radioactivity 133 Xe in the lungs are summed , thus creating images of the lungs in
the phase of max. Inspiration, max. 2 c, d, e).
 |
| Fig.3.11b.2. Mathematical analysis of
dynamic scintigraphy of pulmonary ventilation. a) Areas of interest of the left and right lungs and background. b) Time course curves of radioactivity 133 Xe in the lungs and background. c) Equilibrium image. d) Image of lungs in max. breath (TLC distribution). e) Image of max. exhalation (residue distribution). f) Parametric picture of vital capacity. g) Parametric picture of the distribution of effective ventilation (exchange volume). h) Parametric picture of the regional distribution of the exchangeable air fraction. |
Next, we enter the
measured value of the volume
difference
between the maximum inhale and exhalation (in milliliters). Based
on this value and the difference between the charged number of
pulses in the respective images of the lung max. Inpiriu and
expirium calculated calibration
factor F between nastřádanám number of pulses
and the volume of air (the "tagged" 133 Xe) in milliliters - scintigraphic
studies thus becomes quantitative and what the absolute volumes - each impulse in each element of the
image is expressed in milliliters
of air at a
given location of the lungs.
The program then creates images of the
distribution of pulmonary volume in equilibrium, the distribution
of vital capacity of the lungs at maximum inspiration, the
distribution of residual volume at maximum expiration and the
distribution of vital capacity of the lungs - Fig.3.11b.2 c, d,
e, f. These images are quantified - the contents of individual
cells directly indicate the number of milliliters of air at a
given site of the lung - they are locally parametric images.
Then follows the construction of locally parametric
images of the regional distribution of pulmonary ventilation
. A time course curve of radioactivity 133 is generated for each point in the image matrixXe at
this point and from it the relevant ventilation quantity is
calculated according to the formulas in the lower part of
Fig.4.11b.1 - exchangeable fraction EF [% / s] and exchangeable
volume EV [ml./s], ie the amount of air that is in replace at the
appropriate location in 1 second. The parameter calculated in
this way is then stored at the point in the image from which the
analyzed curve originated. This is done for all points of the
image matrix, which gives parametric or functional
images , which is a kind of clear "map" of the
regional distribution of the dynamics of the studied process -
pulmonary ventilation (Fig.3.11b.2 g, h):
From these parametric images it is clear which parts of the lungs have better or worse functional (respiratory) ability and it is also possible to immediately determine the local values ??of the relevant quantitative ventilation parameters at each location - exchange air fraction in percent and exchange volume in milliliters per second .
 |
| Fig.3.11b.3. Regional distribution of exchange fraction in individual lung fields. |
The regional distribution of lung ventilation parameters is sometimes evaluated for individual lung segments . The areas of interest of the left and right lungs are usually divided vertically into three equally large parts, in which the average or summary values ??of the required ventilation parameters are calculated from parametric images and also expressed as a percentage of the total global value - an example is shown in Fig.3.11b .3.
