Introduction:
The term anisotropism is derived from the Greek words aniso which is to be unequal or different and the verb tropos meaning to turn or direct. In the context of ultrasound this term implies that the reflective pattern may be altered by changing the angle of insonation. Sonographic anisotropism was first described by Fornage in 1987 in relation to ultrasound evaluation of normal tendons [1]. The author recognised this phenomenon as a potential pitfall encountered in the evaluation of patients presenting with suspected tendon injury [1,2].

 

Transvaginal and perineal ultrasound has been used to demonstrate the female urethra in women presenting with urinary incontinence [3-5]. Previous reports have noted that the female urethra and bladder neck appear patent on perineal and endovaginal [EV] sonography performed in women with stress urinary incontinence [SUI] [4-7]. Urethral ultrasound however, has not gained widespread acceptance in the evaluation of SUI as there is no clear distinction between continent and incontinent subjects [4,8,9,10].

 

Aim:
The aim of this study was to evaluate the effect of angle of incidence of ultrasound beam on the apparent width and patency of the female urethra and bladder neck.

 

Methods:
Seven women referred for routine pelvic ultrasound assessment from a Gynaecology Clinic (four nulliparous and three multiparous) were prospectively recruited. Each woman was questioned regarding symptoms of altered urinary continence. Translabial and EV ultrasound was performed in all cases using a linear array 6-10 MHz transducer and a 6-8 MHz EV transducer in the supine position. The appearance of the urethra was then re-assessed in all seven cases, varying the angle of the incident beam to the urethra. All women were asked to strain to assess the effect of mechanically altering the angle of the urethra in relation to the incident beam.

 

Results: 
The mean patient age was 24 years (range 22 - 43years). None of the women reported any symptoms of altered urinary continence. Translabial and EV ultrasound images obtained in the sagittal and coronal planes demonstrated the urethra as a widely patent hypoechoic funnel tapering toward the perineal surface. The bladder neck and urethral walls appeared open and unopposed in sagittal and coronal planes. The mean urethral lumen diameter (measured 1cm inferior to the bladder neck) was 5.6mm in the sagittal plane (range 4.2 - 6.5mm). Increasing the transmit power and receiver gain and reducing the median frequency produced apparent narrowing of the urethra. Altering the angle of incidence of the ultrasound beam resulted in echogenic tissue appearing within what originally appeared as an empty lumen. The bladder neck and urethral lumen appeared narrower as the angle of incidence of the beam approached 90 degrees to the long axis of the urethra. Voluntary straining resulting in alteration of the bladder neck and urethra relative to the incident beam produced a similar effect to angling the ultrasound beam. Apparent narrowing of the bladder neck and urethra was observed as the mechanical act of straining moved the urethra into a position perpendicular to the ultrasound beam.

 

Conclusion:
Our study suggests that the appearance of a patent bladder neck may be an ultrasound artefact due to anisotropism rather than true bladder neck patency.  Previous studies reporting bladder neck and urethral closure on straining may have been inaccurate as apparent urethral closure can be an artefact due to the urethral axis rotating on straining. We suggest that this effect should be taken into consideration when performing ultrasound evaluation of the female bladder neck and urethra particularly with regard to stress urinary incontinence.

 

References :

1.       Fornage BD the hypoechoic normal tendon.  A pitfall. J Ultrasound Medicine 6:19 1987

2.       Fornage BD, Rifkin MD, Ultrasound examination of tendons. Radiol Clin North Amer 1998;26:109

3.       Vannaeville G, Lenck LCH, Garcier JM et al: Contribution of imaging to the understanding of the female pelvic fasciae. Surg Radiol Anat 1992; 14: 147-154.

4.       Siegel S, Middleton JD et al: Sonography of the female urethra. AJR:170,  1998 :1269-1274.

5.       Kuo MD: Transrectal sonographic investigation of urethral and paraurethral structures in women with stress urinary incontince. J Ultrasound Med 17:311-320 , 1998.

6.       Schaer GN, Perucchini D, Munz E et al:Sonographic evaluation of the bladder neck in continent and stress incontinent women.Obstet Gynecol.1999;93(3):412-416.

7.       Mouritsen J:Techniques for imaging bladder support. Acta Obstet Gynecol Scand 1997; 166: 76: 48-49.

8.       Schaer GN, Schmid T, Peschers U et al: Intraurethral ultrasound correlated with urethral histology.Obstet Gynecol.1998;91(1):60-64.

9.       Frauscher F, Helweg G, Strasser H et al: Intraurethral ultrasound: Diagnostic evaluation of the striated urethral sphincter in incontinent females.Eur.Radiol. 8, 50-53 (1998).

10.   Peschers U, Schaer G, Anthuber C et al:Changes in vesical neck mobility following vaginal delivery.Obstet Gynecol. 1996; 88(6):1001-1006.