B-Mode Ultrasound Diagnostic Methods and Applications in Canine Pregnancy

The application of ultrasound diagnostic technology in the veterinary field began in the mid-1960s, initially using A-mode and D-mode ultrasound. With the development of electronic technology and the electronics industry, it advanced to M-mode and B-mode by the mid-1970s. The B-mode ultrasound diagnostic apparatus, also known as B-ultrasound, uses brightness modulation, reflecting signal intensity through the brightness or darkness of light spots. Because B-ultrasound can perform real-time imaging of the body, display two-dimensional images of the examined area, and clearly show the dynamic changes, morphology and adjacent relationships of internal organs and tissues, as well as the internal echoes, structures, and distribution of blood vessels and other ducts of soft tissues, B-ultrasound is also called a real-time ultrasonic tomographic imaging diagnostic apparatus. Furthermore, because B-ultrasound causes no damage to the body, allows treatment of the examined area under its guidance, and has a wide range of applications, it gained the attention and widespread use of the medical community as soon as it was introduced, and has now become one of the four major medical imaging diagnostic methods. In veterinary and livestock production, B-ultrasound has likewise been widely applied; at present, whether abroad or domestically, whether in basic veterinary theoretical research, livestock production practice, or clinical veterinary application, B-ultrasound plays a very important role, and has been hailed as a revolutionary change in veterinary diagnostic technology, being applied in fields such as pregnancy monitoring, superovulation, disease diagnosis, and selective breeding and fattening monitoring. At present, one of the main uses of B-ultrasound is monitoring the uterus and fetus during animal pregnancy. Combining the author's experience, this article briefly describes the main applications of B-ultrasound in canine pregnancy monitoring.

1 Early pregnancy diagnosis

Using B-ultrasound for early pregnancy diagnosis can intuitively display pregnancy features such as the embryonic sac and embryo on the screen, with high accuracy. Depending on the breed and individual, the gestation period of dogs is 58-65 days, generally 62 days. The oocyte begins implantation 15-18 days after fertilization; generally, about 20 days after mating, the gestational sac (GS) can be detected by B-ultrasound, but there are also reports of GS being detected as early as 15 days. The B-ultrasound determination of early pregnancy in dogs is mainly based on observing the round anechoic fluid area of the GS (diameter 1-2 cm) in the uterine area on the ultrasound cross-sectional sonogram, as well as indicators such as enlargement of the uterine horn cross-section and thickening of the uterine wall. The examination method mostly involves shaving the ventral abdominal wall or both lateral abdominal walls, then using a 5 or 7.5 MHz linear array or sector scan probe to perform plain scan cross-sectional observation in three orientations: transverse, longitudinal, and oblique. When one or multiple GS anechoic areas are seen, pregnancy can be determined. However, it must also be differentiated from fluid-filled intestinal loops or uterine fluid accumulation. When both the transverse and longitudinal sections show a round anechoic fluid area with a relatively thick wall and strong echoes, it is a GS; when the transverse section is round and the longitudinal section is a strip-shaped anechoic fluid area with a relatively thin wall, it is luminal fluid accumulation. The positive accuracy of early pregnancy in dogs can reach 100% for skilled operators; the determination of negative early pregnancy in dogs must be made cautiously, because the uterine horn of dogs is very thin (generally less than 1 cm in diameter) before pregnancy and before 20 days of gestation, and the lumen is almost invisible, making it difficult to detect by B-ultrasound. When early pregnancy negativity is suspected, multiple careful re-examinations should be conducted at 23-25 days or even later in gestation; when the number of fetuses is very small, misjudgment of early pregnancy negativity is more likely to occur.

2 Monitoring ovarian development for timely mating

The ovaries of dogs are very small, generally 1.5-3 x 0.7-1.5 x 0.5-0.75 cm, and are covered by a capsule and fat that easily produce echo interference, so the position must be accurately located and carefully observed during examination. The canine ovary is mulberry-shaped, located below the 3rd or 4th lumbar vertebra, 1-4 cm behind the kidney; in multiparous dogs, the position is further posterior and downward. At 2-3 days after estrus, the ovary containing developing follicles can be observed on the B-ultrasound cross-sectional sonogram; the ovarian capsule with relatively thin covering fat is clearly visible, the outer layer under the capsule has relatively weak echoes, and the central area has patterned strong echoes; the follicles appear as multiple round anechoic areas at the periphery, with a maximum diameter of up to 13 mm. In dogs during the 1st-2nd stages of estrus (about 8 days after estrus), follicles grow slowly; entering the 3rd stage (about 10-15 days after estrus), follicles rapidly enlarge, then stop growing, at which point ovulation is imminent, making it the optimal time for mating or artificial insemination. When monitoring ovarian development by B-ultrasound, the number of mature follicles (diameter above 3 mm, protruding from the capsule) can be counted from multiple sections; when there are too few mature follicles (fewer than 5) or the number of synchronously growing follicles is small and the growth rate is slow, gonadotropins (such as FSH or PMFfGLH or HCG used in combination) can be administered appropriately to promote ovarian development and ovulation, improve the conception rate, and increase litter size.

3 Monitoring of embryonic development, number of fetuses, gestational age, and stillbirth

In pregnancy monitoring, predicting the number of fetuses is of great significance for feeding management. In late pregnancy, the nutritional level of the dam is directly related to the birth weight of the fetuses. Dividing animals into different groups according to the number of fetuses and formulating corresponding feeding plans accordingly can reduce feeding costs, ensure normal development of puppies and their growth and development after birth, and also avoid fetal asphyxiation death and pregnancy toxemia in the dam caused by improper feeding.

Using B-ultrasound, one can observe the external structures of the embryo (such as the uterus, gestational sac, placenta, fetal membranes, and umbilical cord), embryonic morphology (such as fetal outline, limbs, external genitalia, and fetal movement), and intraembryonic structures (such as fetal heartbeat, internal organs, and skeleton). In dogs, the gestational sac can be seen 15-20 days after conception, the embryonic body can be seen at 23-25 days (a strong echogenic bright mass appearing within the chorionic sac), fetal heartbeat can be observed at 21-28 days, the head and trunk can be distinguished at 26-28 days, the limbs and choroid plexus can be identified at 31-35 days, ossification begins at 35-40 days, and optic vesicles appear and internal organs can be distinguished at 40-47 days. At around 23 days, the number of embryos can be estimated based on the number of GS and embryonic bodies; fetal age can be predicted based on the echogenic structures identifiable on the B-ultrasound cross-sectional image; stillbirth, emphysematous fetus, embryonic resorption, and abortion can be identified based on fetal heartbeat and embryonic structures. Based on monitoring of the internal uterine structures during and after parturition, one can monitor delivery and determine whether parturition has ended and monitor postpartum uterine involution, etc.

4 Diagnosis of reproductive diseases

There are many reproductive diseases in dogs; common ones such as uterine pyometra and hydrometra, ovarian cysts, dystocia and abortion, tumors of the uterus, mammary glands, and testes, as well as prostatic hypertrophy and tumors, can all be definitively diagnosed or differentially diagnosed by B-ultrasound, and qualitative and quantitative analysis can be made of the location, size, and characteristics of the diseased tissue. If a black uniform hypoechoic area is found within the uterine horn, it indicates that the uterine horn contains a large amount of fluid; if the fluid contains substances resembling thick pus, it indicates uterine pyometra.