Normal Pelvic Ultrasound and Common Normal Variants
The Normal Pelvic Ultrasound: How to Do It and What to See
Knowledge of the normal anatomy and techniques for scanning the female pelvis are essential for detecting pelvic disease. The complete pelvic sonogram is done in two parts. In most cases, these include the transabdominal followed by the transvaginal evaluation. Decades ago, the transabdominal pelvic ultrasound was performed using a full bladder to visualize the pelvic organs by pushing the bowel out of the way. Although on occasion filling the patient’s bladder may be helpful, the transabdominal scan is very effective even if the bladder is not completely distended. If the uterus is anteverted, it is typically well seen whether or not the bladder is full. A retroverted uterus may be difficult to visualize with an empty bladder; however, it will be seen well transvaginally. Hence it is no longer necessary to obligate patients to fill their bladders for pelvic ultrasound, assuming that the transvaginal scan is also performed. If for some reason the patient is to be scanned with a full bladder (e.g., if the patient declines the intravaginal scan), it is important not to overdistend the bladder, thus compressing the pelvic organs against the sacrum.
The pelvic scan should typically include a transvaginal component after the transabdominal scan unless contraindicated or declined by the patient. If the patient is not a candidate for the transvaginal approach, the scan can be performed transrectally (see technique that follows).
The technique for performing a transabdominal (TA) scan (Figure II-1):
1. The uterus is imaged both longitudinally and transversely using a 3- to 8-MHz abdominal transducer. If the bladder is full, it can be used as an acoustic window because it lies just anterior to the uterus. If the bladder is not distended, gentle pressure can be applied to the abdominal wall with the transducer to push the bowel away and narrow the distance between the probe and the pelvic organs. If the uterus is large or contains fibroids, the TA approach may provide the best view and measurements of the uterus. This perspective may even improve after the bladder is emptied. It is important to image the endometrial lining as well as ensure that the fundus of the uterus has been fully imaged. Looking just above the uterus is often necessary to detect any masses in the lower abdomen, when they are too cephalad to be visualized transvaginally.
2. The adnexa are evaluated, providing slight pressure on the lower abdominal organs to improve visibility. This gentle pressure can displace the bowel and better visualize the adnexal regions. The ovaries themselves may not be identified until the transvaginal portion of the scan; however, the overall evaluation of the right and left lower quadrants often requires a TA view in patients in whom there an ovary or mass located high in the pelvis or lower abdomen.
The technique for performing the transvaginal (TV) scan (Figure II-2):
1. Proper disinfection of the transvaginal probe is essential preceding each use, and should be performed in accordance with the standard guidelines recommended by each institution (hospital) or probe manufacturer. Following disinfection, the probe is rinsed with water to remove any residual chemicals and wiped clean. It is then inserted into a probe cover that typically contains coupling gel. Finally the tip of the probe is lubricated before its insertion into the vagina. If the scan is being done for infertility and the patient is a candidate for insemination, it is crucial not to use a gel that may be harmful to the sperm, and water or saline can be utilized in these special circumstances.
2. The TV scan is typically done with an empty bladder (except for rare instances in which the bladder itself is to be evaluated); therefore, the patient should empty her bladder just before initiating the TV scan.
3. The vaginal probe can be introduced into the vagina by a physician, a sonographer, or the patient herself, whichever is most appropriate and comfortable for the patient. In certain cases, a chaperone may be necessary, especially if the physician or sonographer is male.
The technique for performing the transrectal (TR) scan (Figure II-3):
1. After emptying her bladder, the patient is placed in the Sims position, on her side with her legs tucked toward her abdomen or maintained in a dorsolithotomy position.
2. The transvaginal transducer is prepared in the same way as for TV scan, and inserted into the rectum by the physician or sonographer. The insertion is done very slowly, with steady but gentle pressure, and under direct visualization of the rectum (sonographically), thus giving time for the sphincter to relax. One can also perform a digital rectal exam preceding the probe insertion to relax the sphincter muscle and gauge the direction of the rectum. In the Sims position, when the cervix is visualized and approximately half of the shaft of the probe is inside the rectum, the patient is then carefully turned onto her back while trying not to dislodge the probe. With the patient supine, the pelvic landmarks become similar to those seen transvaginally, and the protocol is the same.
Figure II-2 Transvaginal scan (TV). A demonstrates the transvaginal view of a normal anteverted uterus seen longitudinally. B shows a similar view of a normal retroverted uterus. Note that in both cases the body of the uterus is at right angles to the ultrasound beam. C shows an axial uterus that is hard to image well transvaginally because of its vertical orientation away from the ultrasound beam.
Protocol for Intracavitary Scan
The Vagina
To assess the vagina, it is important to visualize the entire length of the vagina from the introitus, before placing the vaginal probe inside (Figure II-4, A and B). Once the probe is inside the vagina, the vaginal walls and surrounding structures are no longer visible. An attempt to visualize them on the way out of the vagina is hampered by the introduction of air with the initial placement of the probe; hence if the clinical problem involves the vagina, it must be evaluated with a perineal scan before placing the probe inside. With the probe on the perineum, a 3-D volume can be acquired and reconstructed to show the floor of the pelvis, the anterior and posterior compartments of the pelvis (Figure II-4, B and C). This enables the practitioner to assess the relationships between the urethra, vagina, and rectum and any mass or cyst that may be present (see Vaginal Masses).
Figure II-4 The perineum. A shows a longitudinal scan of the floor of the pelvis looking from the introitus, down the length of the vagina. B and C show the 3-D volume acquisition with the axial reconstructed transverse view of the pelvic floor. The reconstructed view shows the urethra, vagina, and rectum in transverse section.
The Cervix
The appearance, size, and symmetry of the cervix are evaluated as well as the cervical canal for any polyps, fibroids or masses (Figure II-5). Color Doppler is helpful to detect abnormal blood flow or a feeder vessel if a polyp or mass is suspected. Small cysts in the wall of the cervix are usually nabothian cysts, also referred to as cervical inclusion cysts, and typically are ignored on a sonogram. Evaluating the outer contour of the cervix is important to look for implants of endometriosis along the posterior outer surface of the cervix and in the upper portion of the rectovaginal septum (see Endometriosis).
The Uterus
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