BIPLANE PLANIMETRY
ACCURACY OF PROSTATIC VOLUME CALCULATED BY BIPLANE PLANIMETRY
J Med Ultrasound 1997;5(Supple):31-34.
Akira Kimura1) and Kazuki Kawabe2)
1)Department of Urology, Tokyo Kyosai Hospital
2)Department of Urology, Faculty of Medicine, the University of Tokyo
Corresponding author ; Akira Kimura
Department of Urology, Tokyo Kyosai Hospital,
2-3-8 Nakameguro, Meguro-ku, Tokyo, Japan
Tel 03-3712-3151, Fax 03-3793-5582
running title ; accuracy of biplane planimetry
Abstract
Background: Recently, we proposed biplane planimetry as a new prostatic volume calculation method. The influence of the inclination of the scanning plane on this new method was tested.
Methods: Three dimensional models of the prostatic contour were constructed from transrectal ultrasonograms taken at 5-mm intervals in fifteen patients. The model was then cut by an oblique plane, yielding a new cross section. With this new cross section, volumes by biplane planimetry, by ellipsoid volume calculation, and by prolate ellipse volume calculation were calculated, and compared with the original 3D model volume.
Results: Fluctuation caused by the angle of the oblique plane was less in biplane planimetry than in ellipsoid volume calculation or in prolate ellipse volume calculation.
Conclusion: It was demonstrated that the calculated volume was less dependent on the tilt of the scanner in biplane planimetric method than in ellipsoid volume calculation or in prolate ellipse volume calculation.
Key Words: prostatic volume calculation, transrectal ultrasonography, biplane planimetry
Introduction
Estimation of the prostatic volume by transrectal ultrasonography is now widely used to evaluate a conservative treatment of prostatic cancer or hypertrophy and also to improve the specificity of prostatic specific antigen (PSA)1). Although the most accurate method for sonometrics is the multi-slice planimetric volume calculation2), the easier methods such as ellipsoid volume calculation or prolate ellipse volume calculation3,4) are more frequently used. However, the ellipsoid volume calculation has a tendency to underestimate the volume by 20%3,5), and the degree of miscalculation fluctuates markedly depending on the inclination of the transverse plane6). The prolate ellipse volume calculation is more accurate than the ellipsoid volume calculation, but errors increase as the angle between the transverse plane and the prostatic longitudinal axis moves out of perpendicular6). The influence of the tilt of the scanner on the calculated prostatic volume is called "salami effect", because the shape of the cross section alters with the scanning angle just like salami slices are of different heights depending on the inclination of the knife7).
 | Fig.1 Based on the cross and sagittal contours, a non-ellipsoidal model is created. The model is composed of the sequentially arranged copies of the cross section which are reduced so that the anteroposterior diameters (height; H) of the copies fit the contour of the sagittal section. |
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Recently, we proposed a new calculation method which we termed "biplane planimetry"8). By this new method, the prostatic contours of both cross and sagittal sections are traced. Based on the cross and sagittal contours, a non-ellipsoidal model is created. The model is composed of sequentially arranged copies of the cross section which are reduced so that the anteroposterior diameters (height; H) of the copies fit the contour of the sagittal section (Fig. 1). The areas of the copies are reduced in proportion with the square of the reduced rates of the height (H2), and so the formula for biplane planimetry is given as :
l x Amax x (Hi/Hmax)2
where l is a stepped interval of the arrangement of copies, Amax is the area of the maximum cross section, Hmax is the height of the maximum cross section, and Hi are the heights measured at certain intervals in the sagittal section where the reduced copies are arranged.
Using sonograms of the 150 patients who underwent transrectal ultrasonography, we demonstrated that the ellipsoid volume calculation had a tendency to underestimate the volume, that the prolate ellipse volume calculation was more reliable than ellipsoid volume calculation, and that the biplane planimetry was the most reliable among the three methods8).
In this study, the influence of the inclination of the transverse plane ("salami effect") on the accuracy of biplane planimetry was compared with that of the ellipsoid volume calculation and that of the prolate ellipse volume calculation.
Materials and Methods
 | Fig.2 From the serial sonograms, 3D prostatic model was created. The 3D model was then cut by an oblique plane, yielding a new cross section. With this new cross section, the volume by the biplane planimetric method was calculated.
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The three dimensional (3D ) models of the prostatic contour were constructed in fifteen patients (five normal cases, five with prostatic hypertrophy, and five with prostatic cancer) who underwent transrectal ultrasonography by Aloka SSD-60 or Toshiba SSL-51C with transrectal chair-type probe. Serial sonograms were taken at 5-mm intervals.
From the serial sonograms, 3D prostatic models of the 15 cases were created (Fig.2). The 3D model was then cut by an oblique plane, yielding a new cross section. With this new cross section, the volume by biplane planimetric method was calculated. Its deviation from the volume of the original 3D model was computed.
The angle of the oblique plane was changed from 45o to 135o at 15o intervals, and the deviation was calculated for each angle. This calculation yielded seven data, for each model, from which the range of fluctuation depending on the angle ("salami effect") was calculated for each model. Average ranges for normal, hypertrophy, and cancer groups were calculated by averaging the data of five models in each group.
The fluctuation range depending on the angle was also calculated for the ellipsoid volume calculation and the prolate ellipse volume calculation in the same manner.
Results
In the biplane planimetric method, fluctuation ranges in accordance with the change of the angle for five normal models were 32,25,22,19, and 11%( average being 21.8% ).
In the ellipsoid volume calculation, fluctuation ranges for the normal models were 53,39,39,35, and 32%( average being 39.6% ).
In the prolate ellipse calculation, fluctuation ranges for the normal models were 47,39,30,24, and 19%( average being 31.8% ).
The average ranges of fluctuation, in the five cases, for the three volume calculation techniques in normal group, hypertrophy group, and cancer group were calculated by summing up the data of the five cases in each group (Table1).
Table 1 Averages of fluctuation ranges in accordance with the change of the scanning angle of five cases in normal group, hypertrophy group, and cancer group are listed for three volume calculation techniques.
| | Biplane planimetry | @Ellipsoid | Prolate ellipse |
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| normal | 21.8% | @ 39.6% | 31.8% |
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| hypertrophy | 17.4% | @41.2% | 22.0% |
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| cancer | 22.4% | @41.6% | 28.2% |
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In all three groups, it was shown that the range of fluctuation was the largest in the ellipsoid volume calculation and the least in the biplane planimetric method.
Discussion
Various formulas have been used to calculate the prostatic volume from transrectal ultrasonograms. Littrup et al3) reported that the prolate ellipse volume calculation (HWL/6) was more accurate than the ellipsoid volume calculation (8A2/3H = H(4A/H)2/6 = HW2/6). Terris et al4) evaluated all of the currently used methods of transrectal sonographic volume determination and concluded that HW2/6 was the most accurate and HWL/6 was less accurate, because length measured in ultrasonograms correlated poorly with the real length. They stated that measurement of the cephalocaudal diameter (length; L) in sagittal section is technically difficult, since the point of juncture between the prostatic apex and the distal urethra is frequently poorly visualized. This is the reason why HW2/6 was more accurate than HWL/6 in their study.
Recently, we proposed biplane planimetry as a new method to calculate the prostatic volume from biplane sonograms and demonstrated that biplane planimetry showed less difference from the step-section planimetry than the prolate ellipse volume calculation or the ellipsoid volume calculation method.
Biplane planimetry uses full information obtained from biplane sections. The prolate ellipse volume calculation, however, uses only six points. Poor visualization of the juncture between the prostatic apex and the distal urethra does not yield a significant error in biplane planimetry. Because H near the margin is small, the square of H (biplane planimetry is a function of the sum of all H2) becomes too small to affect the value of biplane planimetry.
Since Terris' formula (HW2/6) uses only diameters obtained from the cross section, the calculated volume seems to fluctuate according to the tilt of the scanner. As the shape of the cross section changes with the scanning angle, this formula seems susceptible to "salami effect"7).
Using 3D models constructed from the tranrectal ultrasonogram, we herein demonstrated that the biplane planimetric method was the most resistant to "salami effect" among the three volume calculation techniques.
The accuracy of the biplane planimetry was already shown8). Accordingly, the superiority of the biplane planimetry over the ellipsoid volume calculation or the prolate ellipse volume calculation was demonstrated.
The formula of our biplane planimetry is very simple, and so it can be easily incorporated in ultrasonic consoles capable of measuring distance and area. The interval of the copy arrangement (slice thickness) can be arbitrarily set at 5mm, 1mm, or less depending on the capacity of the console.
References
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6.Kimura A, Kurooka Y, Hirasawa K, et al. Accuracy of prostatic volume calculation in transrectal ultrasonography. Int J Urol 1995;2:252-6.
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8.Kimura A, Kurooka Y, Kitamura T, et al. Biplane planimetry as a new method for prostatic volume calculation in transrectal ultrasonography. Int J Urol 1997;4:in press.
