Introduction
Transvaginal sonography gives more accurate and more detailed information on pelvic organs and masses than vaginal examination. It is preferable to set an ultrasound scanner with a transvaginal probe by each gynecological examination table (Fig. 1.1).
A transvaginal probe consists of a head, shaft and a grip (Figs 1.2A to C). A straight-type probe (Figs 1.2A and B) may be easier to manipulate for obtaining a desirable tomographic image than a bending-type probe (Fig. 1.2C). A bending-type probe may be favorable to vaginosonographical-guided aspiration.
The head of the probe houses transducers and its shape is determined mainly by the scanning method, scanning angle and scanning direction (forward, obliquely forward or lateral). A mechanical sector probe (Fig. 1.3A) or a convex (curved-array) probe (Figs 1.3B and C) is used as a transvaginal probe. A phased-array probe is not used for the transvaginal approach because a transvaginal probe should have a wide scanning angle and the shape of the tip is preferable to be either hemispherical-shaped or arc-shaped.
Convex Probes
Many small transducers are aligned on the convex surface of the probe head. The principle of scanning is the same in a transabdominal convex probe.1 But the radius of the convex surface of the transvaginal probe is much smaller than that of a transabdominal probe, since a transvaginal probe must have a wide scanning angle in the narrow vagina.
A convex probe performs electronic beam focusing1 and dynamic focusing,1 and the depth of the focal zone is selectable (Fig. 1.4). There are many grating lobes2 on both sides of the main lobe from a convex probe. These grating lobes as well as side lobes cause artifacts (Fig. 1.4). A convex probe has the capability of color, power and pulsed Doppler as well as B-mode imaging (Fig. 1.5).
Mechanical Sector Probes
Figure 1.6 illustrates the principle of a mechanical sector probe. Two transducers are rotated in the head of the probe constantly by an ordinary motor in the grip of the probe or a small direct-driving motor attached directly to the transducers. Each transducer has its proper frequency and ultrasound frequency can be selected by switching the active transducer.
The tip of a mechanical sector probe is a small hemisphere, and thus a mechanical sector probe can be easily inserted into the vagina. This is especially preferable for examining virgins and old women with atrophic vaginae.4
Figures 1.2A to C: Transvaginal probes: (A) A straight-type mechanical sector probe; (B) A straight-type convex probe; (C) A bending-type convex probe (Abbreviations: G, grip; H, head; S, shaft;)
Figures 1.3A to C: Probe heads: (A) A mechanical sector probe; (B) A convex probe; (C) A convex probe for obliquely forward scanning
Figure 1.4: An image of a simple cyst obtained with a convex probe. The focal zone (depth) is indicated by a semicircle (arrow) on the left side. Grating and side lobes from the probe make many artifacts (triangles) and the inner outline of the cyst is not shown clearly
Figure 1.5: Images of an ovary obtained with a convex probe. Left, a B-mode image. Right, a power Doppler image of the same section showing blood flows surrounding the corpus luteum
The scan angle can be easily widened to more than 200°. The focal zone is fixed at a constant depth determined by the shape of the transducer and there appears no mark indicating the focal zone (Fig. 1.7). A mechanical sector probe is superior to a convex probe in B-mode image quality, mainly because there is no grating lobe (compare Figs 1.4 and 1.7).
The transducer is always in motion in a mechanical sector probe and consequently it has not the capability of color, power or pulsed Doppler which requires that the ultrasonic beams be transmitted to the same direction several times successively.
Three-dimensional Transvaginal Probes
A three-dimensional (3D) transvaginal probe is used in 3D ultrasound (Figs 1.8A to C). The head of the probe houses either a convex probe or a mechanical sector probe which swings to acquire 3D data4 as a large number of consecutive tomograms (Fig. 1.9).
Figure 1.6: Principle of the scanning of a mechanical sector probe.3 The head is filled with acoustically transparent liquid and two transducers rotate in it at high speed. One transducer is for a low-frequency and the other for a high-frequency. The ultrasonic frequency is changeable by switching the active transducer
Figure 1.7: An image of a simple cyst obtained with a mechanical sector probe (the same cyst in Figure 1.4). The focal zone is not indicated on the image because it is fixed. The inner outline of the cyst is clearer than in Figure 1.4, because a mechanical sector probe has no grating lobe and causes less artifacts
Figures 1.8A to C: Three-dimensional transvaginal probes. (A) A mechanical sector probe is housed in the head; (B and C) A convex probe is housed in the head
Figure 1.9: Principle of three-dimensional (3D) scanning. A mechanical sector probe or a convex probe housed in the probe head swings and a bunch of tomographic images is acquired automatically for 3D data set construction4
A 3D probe can also be used for B-mode imaging when 3D scanning is not activated.
A 3D image and any arbitrary section can be seen by 3D ultrasound.4 In obstetrics, a 3D image of a fetus and the standard section for crown-rump length measurement can be easily obtained regardless the position and orientation of the fetus.5 In gynecology, a coronal section of the uterus, which cannot be obtained by a conventional transvaginal probe, can be obtained easily.5
Accessories of a Transvaginal Probe
Both the head and the shaft of the transvaginal probe should be covered with a condom to prevent any possible infection from other patients. Ultrasonic jelly is appropriately applied to the probe head prior to the tight-fit placement of the condom, thus avoiding the presence of air that would prevent ultrasound transmission.
Figures 1.10A to B: (A) A needle guide and a transvaginal probe; (B) The transvaginal probe prepared with a condom, the needle guide and a long needle for aspiration
A needle guide is attached to a transvaginal probe in transvaginal ultrasound-guided follicular/cyst aspiration for ease and safety (Figs 1.10A and B).
References
- Baba K, Kinoshita K. Video: physics and equipment of ultrasound imaging. Medical View (Tokyo, Japan), 1991.
- American Institute of Ultrasound in Medicine. Recommended ultrasound terminology. AIUM. 1997.
- Baba K. Leaps of Obstetrics and Gynecology by Ultrasonography. Osaka, Japan: Nagai Shoten; 1992.
- Baba K, Okai T. Basis and principles of three-dimensional ultrasound. In: Baba K, Jurkovic D (Eds). Three-dimensional Ultrasound in Obstetrics and Gynecology. Carnforth, England: Parthenon; 1997. pp 1-19.
- Baba K, Io Y. Ultra-sonografia Tridimensional em Ginecologia e Obstetricia. In: Sao Paulo, Brazil: Roca Ltda; 2003.