Introduction
Imaging is used to supplement the clinical and biochemical assessment of children and adolescents presenting with gynecological abnormalities. A key concern is to avoid the use of ionizing radiation in these young patients and consequently ultrasound (US) and magnetic resonance imaging (MRI) are the most commonly used imaging modalities. Ultrasound is used in the vast majority of cases as first-line imaging assessment supplemented by MRI when necessary.
Principles of Ultrasound and MRI
What Is Ultrasound?
Ultrasound (US) is a diagnostic technique that uses sound waves at high frequency to generate images of internal organs. In the pediatric gynecological patient, US examination is performed transabdominally (TAS), using a full bladder as the ultrasonic window. Good views cannot always be achieved if the bladder is under-filled, overlying bowel gas is present, extensive pelvic adhesions fix loops of bowel to pelvic organs, or complex bladder and bowel anomalies are present.
TAS is usually performed using a curvilinear probe with varying frequencies depending on the age of the patient; 7.5 MHz transducers are used in neonates and 5 MHz transducers in children and adolescents, facilitating better views of deeply pelvic organs. In sexually active adolescents, transvaginal ultrasound (TVS) is the preferred route.
Transperineal ultrasound (TPS) is useful where there is a transverse vaginal septum. Transrectal scans (TRS) should be considered in virgin adolescents when determining the nature of the pelvic abnormality is critical for further management, and diagnosis cannot be made using other diagnostic modalities [1].
Ultrasound is a widely available, simple, and inexpensive diagnostic modality, which makes it ideal for the initial assessment of both pediatric and adult gynecological patients. It is operator dependent and the diagnostic performance is determined by locally available expertise. In optimal circumstances ultrasound provides sufficient information to solve most routine diagnostic problems in pediatric gynecology.
What Is MRI?
Magnetic resonance imaging is a method of creating images by placing the patient in a strong magnetic field, commonly around 1.5 Tesla, and pulsing the body with radiofrequency (RF) waves. The body is transparent to RF waves, so they can pass through the body tissue and interact with it. MRI images protons (hydrogen atoms) and in the human body the vast majority of these are in water, with a smaller number in fat. The absorption of energy from the RF waves is determined by the nature of the molecules that the protons are bound to and their environment. The energy is re-emitted by the body and the pattern reflects the type of tissue the protons are in, and their position in space. Consequently a picture of the body can be built up in two or sometimes three-dimensional slices, reflecting water and fat distribution and their environment within the body tissue.
MRI uses non-ionizing low-energy radiation and therefore is safer than CT scans (which use higher energy X-rays), when used in a controlled fashion. Some people cannot have an MR scan. For example, some brain implants and cardiac pacing wires are not suitable for MR scanning and can move, be damaged, or generate local heat.
Babies, children, and adolescents can be assessed with MRI. Most can be scanned with just some preliminary preparation from radiographers and play specialists. A few patients cannot tolerate the noise or being inside the scanner; they may need sedation or general anesthesia. Babies can be fed, wrapped, and mildly sedated, if indicated, for scans.
An advantage of MRI is that the images are generally easier for clinicians to interpret compared to ultrasound. Repeat scans can produce equivalent images facilitating easier follow-up.
The key to high-quality MRI studies is multiplanar imaging with appropriate scan planes, sequences, high-resolution images of the pelvis, and generally the use of gadolinium contrast agent. Images are vastly improved by the use of buscopan/glucagon to suppress bowel motion artifact, and propellor sequences that reduce movement artifacts.
By the time patients are imaged with MRI, there is usually a provisional diagnosis made clinically and by diagnostic ultrasound; MRI is generally used to provide confirmatory and supplementary information. For example, it might be used to confirm and stage tumors, to confirm a diagnosis of ovarian torsion, and to fully describe the abnormalities of urogynecological congenital anomalies.
Normal Pelvic Anatomy
The uterus and ovaries undergo changes in size and morphology during childhood and adolescence affecting their appearance.
The Uterus
During the neonatal period, the uterus is enlarged and the endometrium appears as a clear echogenic stripe because of the influence of in-utero stimulation by maternal hormones. In 25 percent of neonates, a small amount of fluid is present within the endometrial cavity. The uterus is usually “spade-shaped,” measures approximately 3.5 cm in length and the cervix is larger than the fundus (ratio 2:1). As circulating maternal hormones decline, after three months the uterus regresses to around 2.5–3 cm in length with equal corpus-cervix ratio, and it can be tubular.
The uterus gradually increases in length to 4.5 cm between ages 2 and 8. From puberty onward, the uterus develops the adult pear shape and measures 5–8 cm. The corpus-cervix ratio approaches 3:1 and the endometrium demonstrates fluctuations in thickness and echogenicity in response to the menstrual cycle. Color Doppler usually detects blood flow within the myometrium but little or no flow in the endometrium [2].
On MRI, the postpubertal adolescent uterus shows a three-layered corpus and a one- or two-layered cervical stroma. There is a T2 high-signal center that represents the endometrial stripe. Surrounding this is the junctional zone, and superficial to this is the myometrium. The cervical stroma can consist of a single low-signal layer, with a very small high-signal center due to the endocervix. However, the stroma can also consist of a low-signal ring around the endocervical canal with an intermediate signal layer superficial to it, very similar to and blending with the myometrial layer of the corpus.
The Ovary
The normal neonatal ovarian volume is usually around 1 ml, becoming smaller in the second year of life resulting from declining maternal hormones. Ovaries can be located anywhere along their embryological course from the lower pole of the kidney to the broad ligament. Follicles can be visualized in up to 84 percent of cases from birth to 24 months in age and in 68 percent of cases between ages 2 and 12 [3]. The ovaries remain quiescent until the age 6. Thereafter, the ovarian volume in premenarchal girls increases to 1.2–4 ml.
In postmenarchal girls ovarian volume increases to an average of 8–9 ml and typically contains a large number of antral and maturing follicles in each menstrual cycle. In addition, the ovary develops into an ovoid shape in response to circulating gonadotropins and appears located deeper into the pelvis.
The Vagina
The vagina is well seen on both ultrasound and MRI. Ultrasound is useful for the diagnosis of foreign bodies and when there is no obvious vaginal introitus. The length of the vagina can be assessed on MRI by measuring the distance between the most proximal portion of the vagina and its distal margin, which is marked by the urethral meatus [4]. The vagina is best seen on transverse images using either small field of view (FOV) transverse T2 weighted scans or post-contrast T1 images. An assessment of the size of the clitoris can be made, but this is not really essential as it is usually inspected clinically.
The Role of Imaging in Specific Clinical Problems
Prepubertal Bleeding
Ultrasound is an effective way of screening for various causes of prepubertal vaginal bleeding such as foreign bodies or tumor. In central precocious puberty, the uterine and ovarian volumes are increased due to the effect of increased gonadotropins. In these cases MRI or CT of the brain should be considered to detect intracranial causes of disease. Posttreatment ultrasound can be used for follow-up to demonstrate that the uterus and ovaries have returned to normal.
The most common cause of peripheral precocious puberty (PPP) is autonomous follicular cysts often seen on ultrasound as a unilateral follicular ovarian cyst with a “daughter cyst sign” representing an adjacent antral follicle. The stimulated uterus has a pubertal appearance. Estrogen-secreting tumors causing PPP are discussed later.
Primary Amenorrhea
The imaging investigation of primary amenorrhea follows exclusion of nonstructural causes, such as hormonal dysfunction and anorexia. The imaging pathway is determined by the presence or absence of secondary sexual characteristics and/or pain. Initial ultrasound can show whether there is an underlying anatomical cause for amenorrhea and can also be used to evaluate the renal system to identify coexisting anomalies. MRI of primary amenorrhea is used where diagnostic ultrasound is not definitive.
Painless Primary Amenorrhea
The most common cause for primary painless amenorrhea is gonadal dysgenesis. TAS will often show a normal prepubertal uterus and ovaries vary from non-visualized streak ovaries to a normal appearance. The investigation of patients for whom congenital anomalies are the cause of the primary amenorrhea is discussed later. The most common MRI diagnosed causes of primary amenorrhea are complete androgen insensitivity syndrome (CAIS) and Mayer-Rokitansky-Kuster-Hauser (MRKH) syndrome.
Painful Primary Amenorrhea
Most cases are secondary to obstructed menstruation, which can be due to an imperforate hymen or transverse vaginal septum. Ultrasound and MRI can show the level of obstruction and detect concomitant Mullerian anomalies. Real-time ultrasound can be used to guide incision of vaginal septum or cervical dilatation (Figure 3B.1).
Figure 3B.1 A transabdominal ultrasound scan on a 15-year-old girl presenting with primary amenorrhea and intermittent lower abdominal pain. On longitudinal view the vagina (V) and the uterus (U) are distended with a large amount of blood. The diagnosis of imperforate hymen was confirmed on clinical examination and at surgery.
Pelvic Pain
There are a multitude of causes of pelvic pain, and these will largely be assessed by a combination of clinical examination and ultrasound. Pain in the pediatric/adolescent female may be cyclical, non-cyclical, or acute. Gynecological causes of pelvic pain include ovarian torsion and obstructed menstruation. The latter case can be associated with primary amenorrhea, such as a transverse vaginal septum, or with menstruation, such as in patients with an obstructed hemivagina, or functioning noncommunicating unicornuate uterus (Figure 3B.2).
Figure 3B.2 A three-dimensional transvaginal ultrasound scan in a 19-year-old woman presenting with severe dysmenorrhea. The diagnosis of bicornuate uterus was made. The left uterine cornu (LC) is well developed and contains thin endometrium. The right cornu (RC) is also well formed but it is distended with blood due to an obstructed hemivagina.
Ultrasound is used as the primary imaging tool for assessing pelvic pain but MRI can sometimes help diagnose other causes such as infection, appendicitis, mesenteric adenitis, and inflammatory bowel disease.
Ovarian Torsion
Torsion can occur if there are predisposing factors such as ovarian cysts or masses, but also in the normal ovary due to excess mobility and a relatively long Fallopian tube. The US and MR features of torsion can be variable and should be combined with clinical assessment. Signs include the following:
Unilateral enlargement of the ovary with associated stromal edema (Figure 3B.3)
Ill-defined borders
Peripheral arrangement of the follicles, more prominent in prepubertal girls [4]
A twisted ovarian pedicle
In some cases, US-guided aspiration of a simple cyst torsion can be used as an immediate and interim measure for management [5].
