# Small computer assisted analysis of camera renograms Page: 3 of 13

r - -

of 15 minutes, or more if clinically indicated. A typical 99mTc-DTPA scinti-
photo is shown in Figure 2 with "flagged" right and left kidneys and two
background regions. The steps in the analysis are listed in Table 1 and are
discussed below.
SCAAR is based primarily on a relationship between the activity time
curve of the blood "background" and the accumulated concentration of isotope
in the functioning kidney. This expression can be determined from a two-
compartment model of the blood-kidney system which is assumed to be valid
between 45 seconds and 2 minutes 45 seconds after injection of the isotope,
i.e before any of the radioactivity leaves the kidney (1)It is stated as
follows:
d (Sk Xk (t) )
dt k pbk Xb (t) - kb Xk (t) 1.
where Sk = functional kidney (in units of mass).
X = the fractional amount of material which
is radioactive. The specific activity of
the blood, Xb, or kidney, Xk.
p = transfer rate (units of mass per unit time)
from blood to kidney (bk) and kidney to blood (kb).
During the first half minute the bolus of isotope is distributed
throughout the vascular bed. For approximately the next 2 minutes the
kidney continues to accumulate the injected material, but usually none of
the isotope is eliminated by the passage of urine to the bladder.
If the transfer rate from the kidney to the blood, pk , is negligible
(this assumption may not be true in certain types of renal Lmpairment)
equation (1) becomes
d (Sk Xk (t) )
k= pbk Xb (t) 2.
dt b
Assuming pbk is unaffected by the concentration of activity in the functional
kidney, then by integration:
Sk Xk (t)=pbk S Xb (r ) d C 3.
0
where Xk (t) = o at t = o
r" is the running value of time.
In other words, at a time t the accumulated total of activity in either
kidney is proportional to the accumulated total of blood background
activity. Usually the constant of proportionality is different for
right and left kidneys.
From the serial scintiphotos, counts from the kidney area and the
appropriate background regions are determined, and equation 3 is rewritten
in terms of measured quantities.
t
Qk (t) ' m J b ( r ) d r + c
3.

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