Myocardial perfusion

Perfusion is the quantity of blood streaming trough one cm³ of an organ / tissue per unit time. The decrease of perfusion in some organ or in one of its region causes the decreased working of that organ or that part of it. If the drop is large and this condition is chronic then this can result in necrosis of affected tissue (infactus). This is the reason why the examination of organ’s perfusion is so important.

There are radioactive products from which the myocardium takes up a quantity proportional to the blood streaming through it. With such a product, the myocardial can be examined.

The prepared image shows a structure similar to the one in the figure below.

Figure 9.

Goris – Watson background subtraction

The judgment of left atrium myocardium’s blood supply is disturbed by inhomogeneous background activity. The Goris-Watson background subtraction gives the background activity considering the activity of tissue around myocardium.

Figure 10.

 
One should mark a convex area around the left atrium. Through an inner point two lines are drawn running parallel to axis x and y. These lines intersect the boarder of the convex area in four points. The activity in these points denoted as a, b, c and d. The (x,y) point divides the lines into two section. These arex₁, x₂ and y₁, y₂.

Following weights are composed to (x, y) point:

w_a = y₂ / y₁ + 2, w_b = x₁ / x₂ + 2, w_c = y₁ / y₂ + 2, w_d = x₂ / x₁ + 2

Then the background level in the (x, y) point calculated as:

(a w_a + b w_b + c w_c + d w_d) / (w_a + w_b + w_c + dw_d)

During background subtraction, every pixel value is set to zero outside the area and its border. Inside the area the difference is calculated and receiving a negative value it is set to zero.

Circumferential profile curve

 
At the different parts of myocardium, the activity is significantly differing after the Goris-Watson background subtraction, even if the blood supply is healthy / normal. So, a comparison between the examined heart and the healthy one is needed for the observation of the fine differences. This purpose can be achieved by the circumferential profile curve.

The center of left atrium should be marked in the image and connected with the C vertex of the left atrium. Than the maximal (average, …) value along each line started from O inside an r ring with R radius is calculated. Here R equals with the OC distance and r = c*R, where c<1 is a constant.

Figure 11.

 
φ is measured from the reflected image of C because this is the part between atrium and ventricle, where there is no myocardium.

The left ventricle is of course 3 dimensional, so myocardium is projected on it as well. Searching for the maximum value the r ring is excluded in the case of a very low perfusion to avoid receiving the value around O point as the maximum.

A standard is obtained from the circumferential profile curves of healthy hearts in order to compare it with measured ones. The yellow curve is the lower limit of a normal curve (-2SD, Figure 11.).[3]; [4]; [6] Where the current curve goes under the lower limit of the normal one, the difference appears as a spot. The horizontal width of the spot indicates the extent of the region with abnormal perfusion and the vertical extent shows the decrease of the perfusion.
Figure 12., shows the complete result image of a two phase ²⁰¹Tl labelled study - STRESS+REDISTRIBUTION - with Goris-Watson background subtraction, the “box like” ROI assignment of myocardium as well as the various circumferential profile curves together the available normal database. The double and/or multi-phase myocardial perfusion studies labelled by ²⁰¹TlCl radiopharmaceutical have to be acquired by a “quite” rigid timing protocol. First phase always is the STRESS study acquired within half an hour after the radiopharmaceutical injection (including all the directions), because the redistribution process of ²⁰¹TlCl will start and take 3÷4 hours to be finished. After the transient process is over the REDISTRIBUTION phase imaging can be started.

 
Completely similar / analogue evaluation procedure can be performed in cases of ^99mTcMIBI radiopharmaceuticals applications too. The main difference is, that in cases of ^99mTc labelled pharmaceuticals the redistribution process doesn’t exist, consequently WASHOUT parameters i.e. curves couldn’t be determined. The STRESS and REST phases can be interpreted and evaluated one by one and together. Even though STRESS and REST phases may also be acquired in reverse (first REST and then STRESS). However, it is strongly recommended to keep a delaying between the two phases at least 24h (STRESS and REST phases always should have individual and new radiopharmaceutical injection). Figure 13. below shows the evaluation result of a ^99mTcMIBI STERSS+REST study without reference database application.