A Method and Parameters for the Analysis of Renal Function by External Scintillation Detector Technic

Abstract

The iodopyracet renogram (i.e., the graphic record of kidney excretion) has been a useful clinical tool, but its utility in evaluation of renal function is limited because of hepatic interference. With the advent of more selective agents, as ortho-iodohippuric acid I131 (OIHA)2, hepatic uptake has been minimized (1). It is now possible to compare right and left renal curves with greater confidence, and theoretically to produce quantitative values related to renal function. With such values it should be possible to define limits of normal function. Methods of Renogram Analysis The analysis of the renogram can be approached in four possible ways: I. A Visual Comparison of the Curves: In this method the curves are compared as to height and shape by observation. The advantage of the method is that the analysis can be completed quickly and requires only a trained interpreter. The disadvantages are: (a) that a rather broad experience in renogram interpretation is required; (b) that interpretation is difficult when only one kidney is functioning; (c) that the height of the complex and its visual presentation are affected by the distance, the dose, the time constant, and the chart recorder speed. II. A Comparison with Group Envelopes: With this method, several normal curves are plotted and an envelope drawn which includes all these curves. The advantage of this method is that results may be analyzed quickly. The disadvantages are: (a) that all curves must be standardized to the same height; (b) that the method does not allow complete description of normal function; (c) that quantitation of the degree of abnormality is not possible; (d) that individual standardization is required with different equipment. III. Angles and Tangents: This procedure utilizes the slopes of increase and decrease (measured as an angle or its tangent) as a measure of the rapidity with which the kidney is concentrating and excreting the radioactive material. The curves must be extrapolated to the same height with either method. Figure 1 shows a typical renogram. The maximum height obtained was c. However, the “standard height” chosen is c + d. Angle 1 measures the secretory slope and angle 2, the excretory slope of the original curve. These may be extended to the “standard height” by drawing them as shown. Angle 1 + 2 represents the secretory slope while angle 3 + 4 represents the excretory slope. The values may also be expressed as the tangents of the angles. To extend these to the standard heights the tangents of angles 1 and 3 must be multiplied by (c + d)/C.3 The advantage of this method is that it affords some descriptive indication as to degree of normality or abnormality. The disadvantages are: (a) that it is difficult to draw a straight line which closely represents a curve of an exponential nature; (b) that variability between machines will affect normal values.