HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Fielding, J. R. Articles by Jolesz, F. A. Search for Related Content PubMed PubMed Citation Articles by Fielding, J. R. Articles by Jolesz, F. A. Social Bookmarking                      What's this?

MR-Based Three-Dimensional Modeling of the Normal Pelvic Floor in Women

Quantification of Muscle Mass

¹ All authors: Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, 75 Francis St., Boston, MA 02115.

Received May 4, 1999; accepted after revision August 9, 1999.

 
Address correspondence to J.R. Fielding.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. Our objective was to use a combination of axial MR source images and three-dimensional (3D) models to describe the anatomy of the normal pelvic floor in young nulliparous women and to measure the volume of the levator ani.

SUBJECTS AND METHODS. Ten healthy nulliparous female volunteers (average age, 27 years) underwent T2-weighted MR imaging of the pelvis. Three-dimensional color-coded models of the pelvic bones and organs and the three major components of the levator ani—puborectalis, iliococcygeus, and coccygeus—were created. Source images were used to measure muscle width and signal intensity and to identify ligamentous structures. Using 3D models, we measured the volume of the levator ani, the angle of the levator plate, the posterior urethrovesical angle, and the distance of the bladder neck from the symphysis pubis and the pubococcygeal line.

RESULTS. In all volunteers, the signal intensity of the puborectalis exceeded that of the obturator externus. The average volume of the levator ani was 46.6 ml, the average width of the levator hiatus was 41.7 mm, and the average posterior urethrovesical angle was 143.5°. Vaginal shape in the volunteers followed no recognizable pattern.

CONCLUSION. Muscle morphology, signal intensity, and volume is relatively uniform among healthy young women.

Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Stress urinary incontinence and pelvic organ prolapse are important health issues that affect 10 million American women at a cost of approximately $10 billion annually [1]. Although significant research has been done, the cause of incontinence and prolapse remains elusive, in part because of limited knowledge of anatomy of the pelvic floor. Conventional two-dimensional MR imaging was used by several research groups to assess the anatomy of the female pelvic floor in cadavers and incontinent women [2,3,4,5,6,7]. Although not yet extensively evaluated, three-dimensional (3D) imaging has the potential advantage of quantification of muscle volume. This quantification may be valuable in the evaluation of pelvic floor disorders. Three-dimensional imaging may give a more accurate representation of the relationships among pelvic floor structures necessary for surgical planning. Our objective was to use MR-based 3D models to show and quantify the normal appearance of the female pelvic floor to better understand the anatomy of the specific structures responsible for maintaining support.

Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
We recruited 10 young (age range, 22-33 years; mean age, 27 years), nulliparous continent female volunteers from the hospital community. One woman had undergone resection of an ovarian cyst. Because this surgery was performed approximately 10 years before the MR examination and because the surgery involved an abdominal wall incision, it seemed unlikely that the pelvic floor anatomy would have been altered. The remaining women denied previous pelvic surgery. All women underwent MR imaging of the pelvis using a 1.5-T magnet (Signa 1.5; General Electric Medical Systems, Milwaukee, WI) and a pelvic phased array or torso coil wrapped around the pelvis. After discussion of possible risks and benefits, informed consent was obtained from all the women before imaging. Institutional review board approval is not required at our institution for MR imaging using standard pulse sequences. Standard two-dimensional T2-weighted images were obtained in the axial plane with the following imaging parameters: TR/TE_eff, 4200/108; phase encodes, 128; field of view, 24 cm; slice thickness, 3-mm interleaved; acquisitions, two. Because of a diminished signal-to-noise ratio, it was not possible to use a slice thickness smaller than 3 mm. Therefore, the entire sequence was repeated to adjust the slice locations to obtain interleaved contiguous images 1.5 mm thick. In most volunteers, scan time was 9 min. Depending on body size, we obtained between 60 and 100 images.

After the MR imaging was completed, the images were electronically transferred to a workstation (Sun Microsystems, Mountain View, CA) for production of 3D models. On average, 70 axial images were used to form each model. The data were first segmented into anatomically significant components including bones, bladder, urethra, vagina, uterus, rectum, obturator internus, and the three major components of the levator ani (puborectalis, iliococcygeus, and coccygeus) using manual editing (Fig. 1A,1B). It was not possible to identify and segment the pubovaginalis or puboanalis muscles individually; therefore, they were included in the puborectalis muscle. The iliococcygeus could be identified on axial images in all but a few patients in whom examination of both axial and reconstructed coronal images was required. Each model required 10 hr to complete segmentation.

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —32-year-old healthy female volunteer. Axial T2-weighted MR image of inferior portion of healthy female pelvis. Segmentation markings are manually drawn to surround areas of interest. Width of levator hiatus is measured at level of transverse urethral ligament (straight arrow). Note asymmetry of left and right aspects of puborectalis muscle, outlined in green (curved arrow).

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —32-year-old healthy female volunteer. Axial T2-weighted MR image slightly cephalad to A shows coccygeus (arrow) and iliococcygeus (arrowhead) muscles, which must be identified on each slice.

From these images, 3D renderings of the pelvic viscera and supporting muscles and bones were reconstructed with the marching-cubes algorithm and a surface-rendering method [8] (Fig. 2). We used a surface-rendering method rather than a volume-rendering method because the latter offered no advantage in the identification of target structures and required significantly more time. Our method of slice-by-slice outlining of polygons did not use a numeric threshold value that may alter measurements. The final results were viewed on a workstation with graphics acceleration and 3D slicer software (developed in house) allowing visualization and measurement of source images and models simultaneously (Fig. 3A,3B).

View larger version (101K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —28-year-old healthy female volunteer. Oblique coronal three-dimensional model shows female pelvis Color rendering of pelvic floor muscles allows accurate delineation of morphology and volume assessment o levator ani, which includes puborectalis, iliococcygeus, and coccygeus. Keyhole shape indicates normal sepa ration of vagina and rectum and intact perineal body.

View larger version (110K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —24-year-old healthy female volunteer. Color scheme: white = bones, pink = levator ani, yellow = bladder and urethra, blue = vagina, green = uterus, gray = rectum. Dorsal lithotomy view of three-dimensional (3D) model shows healthy female pelvis. Model was created with software developed in house. This software allows super-imposition of gray-scale source images on 3D models in corresponding anatomic locations. View mimics that used by gynecologists when examining patients or performing surgery. Arcus tendineus fasciae (arrowheads), known by gynecologists as white line of pelvis, is commonly disrupted in women with pelvic organ prolapse. Boundaries of levator hiatus are delineated by stars.

View larger version (85K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —24-year-old healthy female volunteer. Color scheme: white = bones, pink = levator ani, yellow = bladder and urethra, blue = vagina, green = uterus, gray = rectum. Lateral view of same 3D model as in A shows pubococcygeal line drawn between tip of coccyx and inferior aspect of symphysis pubis. In healthy women, bladder base (arrow) rests above pubococcygeal line, and levator plate (arrowheads) is parallel to pubococcygeal line. Descent of bladder base or caudal inclination of levator plate indicates pelvic floor laxity.

Two radiologists reviewed each case in consensus. Source images were used to determine width of the levator hiatus, width and signal intensity of the puborectalis, signal intensity of the obturator externus, vaginal shape (H shape, flattened, or asymmetric), and presence or absence of the lateral pubovesical ligaments. All measurements were made at the level of the transverse urethral ligament, a thin low-signal band located just anterior to the mid portion of the urethra. The width of the puborectalis muscle was measured at its midpoint in the axial plane to the right and left of the vagina. The angle of the levator plate, distance from the bladder neck to the symphysis and the pubococcygeal line, posterior urethrovesical angle, and volume of the levator ani were measured using 3D models. All measurements were treated as continuous data. The relationships between body mass index and the remaining variables were assessed with correlation coefficients and simple linear regression analyses.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
High-quality source images were obtained and models generated in all 10 volunteers (Table 1). The mean body mass index, calculated with the formula of weight (kg) / height (m)², was 21.2 kg / m². The mean width of the levator hiatus, measured at the level of the transverse urethral ligament, was 41.7 ± 4.7 mm. The right side of the puborectalis was consistently thinner than the left, with a mean thickness of 2.2 ± 0.5 mm versus 4.4 ± 0.7 mm. The average signal intensity of the puborectalis muscle was 45.1 ± 11.2 compared with 30.0 ± 5.5 for the obturator externus muscle. The volume of the combined coccygeus, puborectalis, and iliococcygeus was 46.6 ± 5.9 ml (range, 39.4-57.7 ml).

Five women had H-shaped vaginas, four vaginas were flat, and one was asymmetric. The lateral pubovesical ligaments extending from the urethra to the arcus tendineus fasciae (site of fusion of supporting fascia to bony undersurface of the pelvis) were visible in all 10 volunteers. The mean distance from the bladder neck to the pubococcygeal line was 21.7 ± 4.2 mm; the mean distance from the bladder neck to the symphysis was 21.5 ± 5.3 mm. In six volunteers, the levator plate was parallel to the pubococcygeal line. In the remaining four volunteers, the levator plate formed an angle with the pubococcygeal line ranging from -5° to 18°, (mean, 8.5°). The mean posterior urethrovesical angle was 143.5° ± 10°.

Because a high body mass index is associated with stress incontinence and presumed diminished pelvic floor muscle width and volume, we searched for an association between the two. Statistical analysis with linear regression revealed a moderate and negative correlation (r = -0.69) between width of the right aspect of the puborectalis and body mass index. No significant correlation between body mass index and volume of the levator ani (r = 0.12) was found.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
The 3D anatomy of the healthy female pelvic floor derived from MR images shows consistent signal intensity. Morphology of the levator ani shows small standard deviations. The morphology of the levator ani depicted in our 3D models of living women is in agreement with previous studies that used cadavers [2,3]. The increased signal intensity of the puborectalis muscle compared with that of the obturator externus may indicate more fat in the puborectalis, the muscle supporting the rectum, vagina, and bladder neck. Increased T1 signal intensity of the puborectalis muscle was associated with stress incontinence [9].

As reported by Fielding et al. [6,7], the average width of the levator hiatus at the level of the transverse urethral ligament, approximately 4 cm, is constant in the healthy female population. A widened levator hiatus that correlates with the clinical genital hiatus at our measured level was reported in association with pelvic organ prolapse [10]. Tunn et al. [11] used MR imaging, including a body coil and dual-echo T2-weighted imaging, to assess the anatomy of the female pelvic floor in 20 healthy women who were 17-63 years old. Those researchers reported the puborectalis muscle to be 5.2 mm thick to the right of the vagina, and 7.6 mm thick to the left at the level of the mid urethra. In our population of women, we found the puborectalis to be narrower at a similar level, probably because we achieved higher resolution images by using a multicoil array and because our population was more homogeneous in age and pregnancy history. We also found asymmetry of width of the puborectalis, with the right aspect consistently thinner than the left. Tunn et al. showed that at least part of this difference was caused by the chemical shift artifact. Review of our images, however, revealed little artifact. No correlation between body mass index and volume of the levator ani was found; however, limiting the power of this observation, the range of body mass index was quite narrow.

The lateral pubovesical ligaments that support the proximal urethra and bladder neck and recently described as the paraurethral ligaments by Tan et al. [12] were identified in all 10 women. The variable shape of the vagina in the volunteers was reported in previous studies to be associated with continent and incontinent women [6,7]. Other researchers reported a flattened or asymmetric vagina on axial images to be associated with loss of vaginal support and a paravaginal tear [13,14]. It seems likely that the vagina varies in shape in the healthy population.

An expected occurrence in continent women was finding the levator plate nearly parallel to the pubococcygeal line. Other researchers reported a caudal inclination of the levator plate associated with cystocele and uterine and vaginal vault prolapse [10,15]. The location of the bladder neck close to the symphysis and above the pubococcygeal line is also an expected finding in healthy women [16]. The average posterior urethrovesical angle was larger than that derived from voiding cystourethrographic studies because our measurements taken from the posterior surface of the urethra rather than the lumen generated a more obtuse angle [17]. In previous studies, some authors correlated a widened posterior urethral angle (>115°) with the presence of stress urinary incontinence, although this correlation remains an area of contention [18,19].

A limitation of our study is the lack of correlation of imaging findings with physical examination. A young continent nulliparous woman may have some congenital laxity of the pelvic floor support structures or even a paravaginal tear. It did not seem reasonable to subject healthy volunteers to a detailed gynecologic examination. Also, because our results correlate well with those derived from cadavers with normal anatomy, it seems unlikely that our volunteers had any significant anatomic abnormality. A second limitation of our study is the small sample size, which limits statistical power.

In the future, urinary incontinence and pelvic organ prolapse may be treated in a more sophisticated and efficacious manner. Therapies have not been optimized and many different surgical procedures and nonsurgical therapies have been reported for the treatment of these conditions. Women with intact pelvic floor support structures would respond well to behavior modification techniques and estrogen replacement therapy, whereas those with disruption of the levator ani would benefit from surgery. Rapidly improving computer hardware and software tools may soon make 3D imaging faster and more cost-effective. If such imaging becomes a reality, it could provide information such as muscle morphology, bulk, and signal intensity to guide appropriate treatment. Our measurements derived from the anatomy of healthy young women provide a baseline to which symptomatic women can be compared.

References

Top Abstract Introduction Subjects and Methods Results Discussion References

 

1. Hu TW. Impact of urinary incontinence on healthcare costs. J Am Geriatr Soc 1990;38:292-295[Medline]
2. Strohbehn K, Ellis JH, Strohbehn JA, Delancey JOL. Magnetic resonance imaging of the levator ani with anatomic correlation. Obstet Gynecol 1996;87:277-285[Abstract]
3. Frohlich B, Hotzinger H, Fritsch H. Tomographical anatomy of the pelvis, pelvic floor, and related structures. Clin Anat 1997;10:223-230[Medline]
4. Healy JC, Halligan S, Reznek RH, Watson S, Phillips RK, Armstrong P. Patterns of prolapse in women with symptoms of pelvic floor weakness: assessment with MR imaging. Radiology 1997;203:77-81[Abstract/Free Full Text]
5. Lienemann A, Anthuber C, Baron A, Kohz P, Reiser M. Dynamic MR colpocystorectography assessing pelvic-floor descent. Eur Radiol 1997;7:1309-1317[Medline]
6. Fielding JR, Versi E, Mulkern RV, Lerner MH, Griffiths DJ, Jolesz FA. MR imaging of the female pelvic floor in the supine and upright positions. J Magn Reson Imaging 1996;6:961-963[Medline]
7. Fielding JR, Griffiths DJ, Versi E, Mulkern RV, Lee M-LT, Jolesz FA. MR imaging of pelvic floor continence mechanisms in the supine and sitting positions. AJR 1998;171:1607-1610[Abstract/Free Full Text]
8. Kikinis R, Jolesz FA, Gerig G, et al. 3D morphometric and morphologic information derived from clinical brain MR images. In: Hoehne KH, Fuchs H, Pizer SM, eds. 3D imaging in medicine: algorithms, systems, applications. Berlin: Springer, 1990:441-454
9. Kirschner-Hermanns R, Wine B, Niehaus S, Schaefer W, Jakes G. The contribution of magnetic resonance imaging of the pelvic floor to the understanding of urinary incontinence. Br J Urol 1993;72:715-718[Medline]
10. Goodrich MA, Webb MJ, King BF, Bampton AEH, Compeau NG, Riederer SJ. Magnetic resonance imaging of pelvic floor relaxation: dynamic analysis and evaluation of patients before and after surgical repair. Obstet Gynecol 1993;82:883-891[Abstract/Free Full Text]
11. Tunn R, Paris S, Fischer W, Hamm B, Kuchinke J. Static magnetic resonance imaging of the pelvic floor muscle morphology in women with stress urinary incontinence and pelvic prolapse. Neurourol Urodyn 1998;17:579-589[Medline]
12. Tan IL, Stoker J, Zwamborn AW, Entius KA, Calame JJ, Lameris JS. Female pelvic floor: endovaginal MR imaging of normal anatomy. Radiology 1998;206:777-783[Abstract/Free Full Text]
13. Huddleston HT, Dunnihoo DR, Huddleston PM, Meyers PC. Magnetic resonance imaging of defects in DeLancey's vaginal support levels I, II, and III. Am J Obstet Gynecol 1995;172:1778-1784[Medline]
14. Klutke C, Golomb J, Barbaric Z, Raz S. The anatomy of stress incontinence: magnetic resonance imaging of the female bladder neck and urethra. J Urol 1990;143:563-566[Medline]
15. Ozasa H, Mori T, Togashi K. Study of uterine prolapse by magnetic resonance imaging: topographical changes involving the levator ani muscle and the vagina. Gynecol Obstet Invest 1992;24:43-48
16. Yang A, Mostwin JL, Rosenshein NB, Zerhouni EA. Pelvic floor descent in women: dynamic evaluation with fast MR imaging and cinematic display. Radiology 1991;179:25-33[Abstract/Free Full Text]
17. Pelsang RE, Bonney WW. Voiding cystourethrography in female stress incontinence. AJR 1996;166:561-565[Abstract/Free Full Text]
18. Carey MP, Dwyer PL. Position and mobility of the urethrovesical junction in continent and stress incontinent women before and after successful surgery. Aust N Z J Obstet Gynaecol 1991;31:279-284[Medline]
19. Kuzmarov IW. Urodynamic assessment and chain cystogram in women with stress urinary incontinence: clinical significance of detrusor instability. Urology 1984;24:236-238[Medline]

CiteULike

Complore

Connotea

Del.icio.us

Digg

Reddit

Technorati

This article has been cited by other articles:

				L. Boyadzhyan, S. S. Raman, and S. Raz Role of Static and Dynamic MR Imaging in Surgical Pelvic Floor Dysfunction RadioGraphics, July 1, 2008; 28(4): 949 - 967. [Abstract] [Full Text] [PDF]

				J. A. Kruger, S. W. Heap, B. A. Murphy, and H. P. Dietz Pelvic Floor Function in Nulliparous Women Using Three-Dimensional Ultrasound and Magnetic Resonance Imaging Obstet. Gynecol., March 1, 2008; 111(3): 631 - 638. [Abstract] [Full Text] [PDF]

				E. Cortes, W. M. N. Reid, K. Singh, and L. Berger Clinical Examination and Dynamic Magnetic Resonance Imaging in Vaginal Vault Prolapse Obstet. Gynecol., January 1, 2004; 103(1): 41 - 46. [Abstract] [Full Text] [PDF]

				J. O. L. DeLancey, R. Kearney, Q. Chou, S. Speights, and S. Binno The Appearance of Levator Ani Muscle Abnormalities in Magnetic Resonance Images After Vaginal Delivery Obstet. Gynecol., January 1, 2003; 101(1): 46 - 53. [Abstract] [Full Text] [PDF]

				K. Singh, W. M. N. Reid, and L. A. Berger Magnetic Resonance Imaging of Normal Levator Ani Anatomy and Function Obstet. Gynecol., March 1, 2002; 99(3): 433 - 438. [Abstract] [Full Text] [PDF]

				J. R. Fielding Practical MR Imaging of Female Pelvic Floor Weakness RadioGraphics, March 1, 2002; 22(2): 295 - 304. [Abstract] [Full Text] [PDF]

				R. A. Kubik-Huch, S. Wildermuth, L. Cettuzzi, A. Rake, B. Seifert, R. Chaoui, and B. Marincek Fetus and Uteroplacental Unit: Fast MR Imaging with Three-dimensional Reconstruction and Volumetry—Feasibility Study Radiology, May 1, 2001; 219(2): 567 - 573. [Abstract] [Full Text]

This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Fielding, J. R. Articles by Jolesz, F. A. Search for Related Content PubMed PubMed Citation Articles by Fielding, J. R. Articles by Jolesz, F. A. Social Bookmarking                      What's this?