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MR Volumetric Measurements of the Myomatous Uterus

Posted Mar 31, 2007 Updated Apr 7, 2023
By Bonnie N. Joe MD PhD , Jongdae Suh MD , Charles F. Hildebolt DDS PhD , David M. Hovsepian MD , Brian Johnston MD , Kyongtae T. Bae MD PhD
8 min read

Rationale and Objectives

Stereology is a simple, fast method for object segmentation that involves counting the number of intersections of a randomly positioned grid over an object. The objectives of this study were to determine observer reliability in making stereologic- and ellipsoid-based measurements of uterine and leiomyoma volumes and to test the agreement between these two methods of measurement.

Materials and Methods

Two observers made uterine and dominant leiomyoma volume measurements on MR images in 30 patients using stereology and the popular ellipsoid-based technique. Stereologic volume measurements were made from high-resolution T2 images in two perpendicular planes (axial and sagittal). Ellipsoid volume was calculated by multiplying the maximal sagittal, anteroposterior, and transverse dimensions by π/6. For these measurements, interobserver reliability was tested with paired t -tests and percent differences were determined. A mean stereologic volume and a mean ellipsoid volume were determined and tested for agreement with a paired t -test. Percent differences were also calculated.

Results

Stereologic measurements demonstrated excellent interobserver reliability with 0.3% difference in mean uterine volumes ( P = .69) and 0.3% difference ( P = .81) in mean leiomyoma volumes. The ellipsoid method resulted in poorer interobserver reliability with 7% difference ( P = .01) in mean uterine volumes and 4% difference ( p = .24) in mean leiomyoma volumes. The ellipsoid method also significantly overestimated uterine volumes by 14% ( P < .01) compared with stereology.

Conclusion

Stereology provided high interobserver reliability for leiomyoma and overall uterine volume measurements and was more reliable than the ellipsoid method, which uses linear measurements. Stereology appears well suited when precise volume measurements are desired for assessing response to uterine arterial embolization treatments.

Measurements of uterine and dominant leiomyoma volumes are an integral part of the treatment evaluation of uterine artery embolization and other techniques for treating symptomatic leiomyomata. Given the increasing popularity of uterine arterial embolization treatment and the more recent addition of magnetic resonance-guided focused ultrasound as a treatment option ( ), a reliable method for volume measurement is desirable. Uterine and leiomyoma sizes are commonly assessed with the ellipsoid-based volume measurement method, i.e., the volume is estimated from linear measurements in three perpendicular directions using the equation for an ellipsoid. The reliability of this method, however, has previously been addressed in only one study, which focused on patients undergoing hormonal treatment of symptomatic leiomyomata ( ).

While direct volume measurement techniques are more time consuming than volume estimations from linear measurements, they have been shown to be more reliable ( ). One such technique is stereology (also known as point counting), a simple, fast method for object segmentation that involves counting the number of intersections of a randomly positioned grid over an object. This method has the advantage of not relying on border tracing or threshold determination. Instead, it relies on the operator’s visual perception to select all points that lie within the object of interest. Stereology, which has gained popularity in cytopathology and medical imaging analysis, has been previously shown to be a reliable and accurate method for measuring cyst and renal volumes at MRI in patients with polycystic kidney disease ( ) and in neuroimaging applications ( ). Moreover, stereology (point counting) has been shown to be faster than manual planimetry (border tracing) for measuring liver volumes at MRI ( ). Our hypothesis was that stereologic volume measurements would provide more reliable volume measurements than the commonly used ellipsoid method that estimates volumes from linear measurements. We could not address accuracy in this study because the patients did not undergo hysterectomy that would be required for a true volume determination. Thus, the objectives of this study were to determine observer reliability in making stereologic- and ellipsoid-based measurements of uterine and leiomyoma volumes and to test the agreement between these two methods of measurement.

Materials and methods

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Volume Measurement With Stereology

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Figure 1, Illustration of stereology for uterine volume measurement (applied to axial plane images). (a) A grid of points was randomly positioned over the image. (b) Points lying within the myomatous uterus (shown in green) are selected manually using a region of interest cursor. Process is repeated for each slice that images the uterus. Volume is proportional to the total number of points selected.

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Ellipsoid Measurements

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Figure 2, Example of ellipsoid method for uterine volume measurement using sagittal and axial plane T2-weighted MR images. (a) Sagittal (S) and anteroposterior (AP) uterine measurements obtained in perpendicular directions from sagittal plane images. (b) Maximum transverse (T) dimension of uterus is measured in the axial plane. Ellipsoid uterine volume = S × AP × T × π/6.

Figure 3, Ellipsoid method applied to leiomyoma measurement. (a) Maximum sagittal (S) and anteroposterior (AP) dimensions of dominant myoma obtained in sagittal plane. (b) Maximum transverse (T) measurement obtained in axial plane. Ellipsoid dominant myoma volume = S × AP × T × π/6.

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Statistical Analysis

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Results

Uterine Volume Measurement

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Figure 4, Interobserver agreement for stereology (a,b) and ellipsoid (c) uterine volume measurement mehtods. (a) Plot of stereologic measurements obtained from sagittal plane images for observer 2 versus observer 1 demonstrates excellent interobserver agreement (0.3% difference). (b) Plot of stereologic volume measurements obtained from axial plane images again demonstrates excellent agreement between observers (0.8% difference). (c) Plot of ellipsoid uterine volume measurements for observer 2 versus observer 1 demonstrates more variability. The 7% difference in measurements was statistically significant ( P = .01). All volumes given in cm 3 .

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Figure 5, Mean volumes obtained using stereology (on sagittal and axial plane images) versus ellipsoid method for uterus. Observer 1 measurements shown in light gray. Observer 2 measurements shown in black. Interobserver difference in uterine volume measurement using the ellipsoid method was statistically significant ( P = .01).

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Dominant Leiomyoma Volume Measurement

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Figure 6, Interobserver agreement for stereology (a, b) and ellipsoid (c) dominant myoma volume measurements. (a) Plot of stereologic measurements obtained from sagittal plane images for observer 2 versus observer 1 also demonstrates excellent interobserver agreement (0.3% difference). (b) Plot of stereologic volume measurements obtained from axial plane images again demonstrates excellent agreement between observers (0.1% difference). (c) Plot of ellipsoid dominant myoma volume measurements for observer 1 vs. observer 2 demonstrates more variability but the 4% difference in measurements was not statistically significant ( P = .24). All volumes given in cm 3 .

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Figure 7, Mean dominant myoma volumes obtained by observers 1 and 2 applying the stereologic method to sagittal and axial images, and by using the ellipsoid method. Observer 1 measurements shown in light gray. Observer 2 measurements shown in black. Difference between observers did not reach statistical significance for myoma volumes ( P = .24).

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Discussion

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Figure 8, Illustration of how a myomatous uterus is not well modeled by an ellipsoid. Sagittal (a) and axial (b) T2-weighted MR images demonstrate the marked irregularity of an uterus containing numerous myomas.

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Academic Radiology, Volume 14, Issue 4
Journals General Radiology
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