Abstract
Ultrasound remains one of the most routinely performed medical investigations and a mainstay of clinical decision-making rests upon the obtained results. Due to the relative proximity of the pelvic organs to the abdominal surface and easy access via the vaginal route, gynaecological scanning should be in the armamentarium of every gynaecologist. As with every gynaecological assessment, and procedures of an intimate nature, explanation to alleviate anxiety and obtain informed consent is essential. The woman should feel safe and comfortable when the procedure is performed, which relates to the equipment in the scan room, presence of a chaperone and the security of the scan room. In this chapter, we will discuss the principles of sonographic assessment of the normal female pelvis.
Introduction
Ultrasound remains one of the most routinely performed medical investigations and a mainstay of clinical decision-making rests upon the obtained results. Due to the relative proximity of the pelvic organs to the abdominal surface and easy access via the vaginal route, gynaecological scanning should be in the armamentarium of every gynaecologist. As with every gynaecological assessment, and procedures of an intimate nature, explanation to alleviate anxiety and obtain informed consent is essential. The woman should feel safe and comfortable when the procedure is performed, which relates to the equipment in the scan room, presence of a chaperone and the security of the scan room. In this chapter, we will discuss the principles of sonographic assessment of the normal female pelvis.
Indication for Scanning
As with every investigation, a clear clinical question must be asked. Ad hoc investigations often lead to incidental findings of clinically insignificant lesions, which increase the patient’s anxiety and lead to unnecessary interventions. Conversely, one could argue that such a finding could save lives due to early detection of insidiously developing malignant changes (i.e. ovarian cancer). Yearly gynaecological check-ups could serve this purpose, provided the healthcare system can support this practice.
In the UK, a great majority of gynaecological ultrasound is performed to investigate pre-existing gynaecological or surgical symptoms. Clear clinical indication for the ultrasound assessment is therefore provided. This approach allows for symptom-focused scanning. When scanning, however, the sonographer cannot only focus on the issue at hand, but must perform a thorough and complete pelvic organ assessment.
In most cases, an abdominal and transvaginal (TV) ultrasound should be performed for completion. This allows for exclusion of large masses that have developed or migrated above the pelvic brim and, due to their position, would be missed with only TV scanning.
Initialization of Transvaginal Scanning
As described in the previous chapter, this examination should be performed in a secure environment with a chaperone and after the woman has provided consent. Allergy or sensitivity to latex should be checked and accordingly a latex or non-latex probe cover is to be used. Appropriate infection control measures are taken by ensuring the probe is cleaned with disinfectant wipes. Following application of an adequate amount of sonographic gel to the probe cover, some water-based lubricant should be applied on top of the probe cover. Good amounts of lubricant allow for excellent ultrasound transduction and minimize signal loss before it reaches the tissues. Once the probe is placed at the vaginal introitus, the screen should be observed in order to determine the direction of the vaginal canal (Figure 2.1). The probe should be introduced along the vaginal canal until the fornix is reached. This allows for a smooth and pain-free introduction of the transducer. Initial optimal positioning of the tip of the transducer is the anterior fornix. If this cannot be achieved at the beginning of the examination, the probe should be withdrawn a few centimetres and, aiming upwards, reintroduced until the fornix is reached. A systematic approach with observation of appropriate landmarks, like the urethra leading to the bladder base, anterior fornix and cervix and cervical canal, with each and every scan will help to achieve smooth and accurate scanning.
The Uterus
Once the transducer is in the anterior vaginal fornix, the cervix should come into view. Using small rotational and angling movements of the transducer, the cervical canal should be visualized, allowing for identification of the sagittal cross-section of the uterus. Good contact between the tissues and the ultrasound transducer should always be achieved. The uterus should then be positioned in the middle of the screen. The image should be enlarged to minimize the amount of noise, i.e. bowel and adipose tissue located in the pouch of Douglas (POD), and optimize the view for analysis and description (Figure 2.2).
(a) Uterus occupies one-third of the screen and any details are difficult to detect
(b) The image is too enlarged, cutting off part of the uterine fundus and thus not allowing for assessment of the entire organ
(c) The image is optimized, with the uterus occupying the majority of the screen and the pouch of Douglas still visible.
Uterine measurements should include the length of the cervix and measurements of the uterine corpus in three dimensions. The cervical muscular layer should appear homogeneous and isoechoic, with myometrial appearance. Within the endocervical canal, depending on the stage of the cycle, hyperechoic normal glandular lining may be visualized. In the proliferative phase of the cycle, hypoechoic watery mucous within the endocervical canal can be visualized. In nulliparous women, the total uterine length is approximately 7 cm, with an increase to over 9 cm in multiparous women. In the same populations of women, the cervical length measures approximately 2.9 (± 0.5) and 3.7 (± 0.6) cm, respectively [1] (Figure 2.3).
Anterior and posterior uterine walls should be similar in diameter and should have a similar and homogeneous echotexture. Doppler signal should show uniformly distributed blood vessels traversing from the myometrium to the endometrium (Figure 2.4). Application of gentle pressure allows demonstrating the sliding sign and allows for identification of significant pelvic adhesions.
Once the endometrial cavity is in view, the uterus occupies approximately two-thirds of the screen. The endometrium should be measured where the upper one-third and lower two-thirds of the cavity meet, or at the thickest part. The callipers should be placed at the edges of the endometrium and should be perpendicular to the long axis of the uterus (Figure 2.5). Once the measurement is done, a comment on the endometrial pattern should be made. Endometrial pattern corresponds to the stage of the menstrual cycle, and in the context of assisted reproduction provides important information regarding endometrial receptivity.
Endometrial Physiology Recap
Endometrium is composed of two layers, the zona basalis and zona functionalis.
The zona functionalis is shed during menstruation.
During endometrial proliferation, the endometrial glands grow in length and become increasingly tortuous.
Spiral arteries increase in number and undergo remodelling in the second half of the menstrual cycle.
Endometrial growth is driven by oestrogen.
Secretory changes are caused by progesterone and lead to accumulation of intracellular glycogen.
Endometrial thickness varies from 2 mm to 20 mm throughout the menstrual cycle.
Optimal endometrial receptivity in the ART cycle has been associated with endometrial thickness of >6–8 mm and a volume of >2 cm3.
During menstruation, the endometrium is thin (<5 mm) and hyperechoic compared to the myometrium, and represents the zona basalis (Figure 2.6). Blood and sloughed endometrium may be visible within the endometrial cavity. Close and static observation of the cavity may reveal active endo-myometrial contractions and associated movement of the content, which aids to expel the content. The menstrual content can be differentiated from acquired uterine anomalies with the use of power Doppler. When a fibroid or polyp is present, Doppler signal will be present within the lesion; on the other hand, menstrual debris will be devoid of vascular patterns (Figure 2.7).
(a) Menstrual endometrium: hyperechoic endometrial stripe encompasses the mixed echogenicity cellular debris and anechoic (black) fluid most likely representing menstrual blood.
(b) Early follicular phase endometrium with a visible isoechoic stripe of endometrium.
(c) Triple-layer appearance of peri-ovulatory endometrium; small arrow = hyperechoic zona basalis; red arrow = zona functionalis; large arrow = interface between the two endometrial surfaces.
(d) Hyperechoic appearance of luteal phase endometrium.
Figure 2.7 Acquired and congenital changes within the endometrial cavity. (a) Blood clot and products of conception during a miscarriage in progress. Note the fluid level within the endometrial cavity (thin arrow) and the absence of Doppler signal within the content of the endometrial and cervical canal (large arrow). (b) and (c) Sagittal and transverse sections of the uterus with a small type 0 fibroid within the endometrial cavity (arrow); note the heterogeneous structure of the fibroid and the acoustic shadowing caused by the dense tissue. (d) Small endometrial polyp. Note the uniform structure and distortion of the midline echo; no acoustic shadowing is present. (e) Small type 0 fibroid with circumferential blood flow (arrow). (f) Endometrial polyp with a feeding vessel (arrow) arising from the posterior endometrial wall. (g) and (h) 3D images with a coronal plane of the endometrial cavity with a type 0 fibroid (g) and polyp (h). Note the differences in the texture between these lesions.
When menstruation ceases, the endometrium grows in thickness and becomes more isoechoic compared to the myometrium. At around day 8 of a 28-day cycle, a thin hyperechoic line appears at the apposition of the anterior and posterior leaf of the endometrium. This becomes more distinct as time passes. The two basal layers of the endometrium and the midline echo are hyperechoic and encompass hypoechoic zona functionalis, giving the endometrium a triple-layer appearance around the time of ovulation (Figure 2.6c) [2]. Increasing levels of progesterone shortly after ovulation cause secretory changes within the endometrium. Changes related to increasing intracellular glycogen storage cause the endometrium to gradually become hyperechoic, starting from the outside and progressing inwards towards the midline [3,4]. Hyperechoic decidual change is complete around day 19 of the cycle. Withdrawal of progesterone in non-conception cycles causes temporary spasm and dilation of spiral arteries and apoptosis of the oestrogen-dependent stromal cells [5]. This occasionally can be visualized around cycle day 26–27 on ultrasound and appears as hypoechoic irregular patches within the otherwise hyperechoic endometrium.
Throughout the menstrual cycle, the vascular patterns within the subendometrium remain constant in appearance and are homogeneously spread throughout the anterior and posterior walls of the uterus. The presence of Doppler signal within the endometrium is a good prognostic feature for pregnancy success following assisted reproductive technology (ART), as absence of vascularity observed with colour Doppler has been associated with reduced or absent pregnancies [6,7]. Observed 3D power Doppler vascularity changes throughout the cycle suggest a gradual increase in perfusion towards ovulation, with a nadir at the time of presumed window of implantation followed by a rise in Doppler signal [8]. Uterine artery Doppler assessment can be carried out as well to assess the resistance in the vascular beds and attempt to predict the chances of conception and possible pregnancy complications (Figure 2.8).
