Application of ultrasonography in field conditions for improving reproduction in bovines.
Despite its numerous and relevant applications, only 20% of bovine clinicians across the world and very few of us in India use ultrasound. Furthermore, many of those performing ultrasonography only use the scanner for detection of early pregnancy and research purpose in Veterinary universities across the country. The constant application of ultrasonography, can reduce or even cancel, the normal range of error of manual palpation. When an excellent clinician manually examines the ovaries, the range of error is estimated to be of 30-35%, while it can rise up to 70% when physiological and pathological conditions (except for pregnancy) are investigated. This can lead to a remarkably longer parturition conception interval in bovines, as well as a more intensive use of drugs (hormones). The range of error of an excellent reproductive clinician while examining physiological and pathological conditions of ovaries and uterus, we can expect an improvement of birth-conception interval of at least 5% in a year. This means reduction in the number of open days (each non-pregnant cow or buffalo costs around Rs.80/day after 100 days post partum), the amount of drugs used and cost of labour, leading to a total next saving of 3000-4000 INR per milking cow or buffalo. Hence, it seems clear that ultrasonography is an ideal tool for reproductive health management in bovines. So there is need to incorporate ultrasonography machine in each Veterinary Institute.
In recent years, trans-rectal ultrasonography has been used to detect early pregnancy and to determine embryo/fetal death (Kahn, 1992). In addition, reports in Veterinary medicine have shown it as a safe technique with no effects on embryo/fetus viability (Kahn, 1992; Ball and Longue, 1 994). According to recent works undertaken in last seven years in field of ultrasonography, the maximum sensitivity and negative predictive values were obtained on 30th day under field conditions or at doorstep farmers after examination of more 5000 cattle and buffaloes. The objectives of this review is help the clinician to understand the basic principles of ultrasonography and its interpretation regarding reproductive health management under field conditions or doorstep of farmers.
Basics of Ultrasonography techniques and its applications
It is important to realize that image on ultrasound screen represents a fine section of an organ, grossly resembling a weakly magnified histological cut. The probe therefore simulates passage of a knife, slicing through an organ/tissue from top to bottom. Ultrasonography thus presents a flattened two dimensional image of a finely cut section of tissue, whereas radiography is a two dimensional superimposed view of entire thickness of an animal or of a limb under observation.
Ultrasound images are rapidly renewed and images are superimposed one over other as probe moves across a tissue surface. The rapid succession of tissue section views gives impression that structures are moving like an animated cartoon. When interpreting sectional views of an organ on screen, it is essential to have a good appreciation of three-dimensional shape of organ in space.
Probe characteristics, resolutions and use
The probe is the most fragile component of the ultrasound apparatus. In Veterinary market, usual probes have a frequency ranging from 3-12 MHz. The ability of instrument to distinguish between two structures located very close together along axis of ultrasound beam is called its axial resolution. The axial resolution is best when groups of waves emitted have a short wave length. Since the number of cycles in each group of waves is set according to instrument design, the only way to shorten their length is to use a probe with a higher frequency. For example, the group of waves produced by a 7.5 MHz probe will be shorter than one produced by a 3.5 MHz probe and will provide better axial resolution.
There are two types of probes: linear and sectorial. In Veterinary science. Linear probes are preferred for trans-rectal ultrasound examination of ovaries and uterus. This probe has a set row of crystals that are selected electronically to form a rectangular image. The linear probe provides good resolution for tissues located close to the probe. Sectorial probes have one or several crystals whose position produces a beam in shape of a pie slice. The advantage of sectorial probe is that it doesn't require a large surface of contact and it scans a greater overall surface. The disadvantage is that the visual field and lateral resolution (i.e. the ability of a system to differentiate between two adjacent structures) are more restricted close to the probe. The sectorial probe is ideal for viewing small ruminant fetus by transabdominal ultrasound imaging.
The details contained in an ultrasound image (resolution) as well as depth of tissue observed depend on frequency and focalization of scanning beam. With a lower frequency, tissue penetration will be deep but resolution will be lower. A higher frequency enables better resolution, but beam attenuation will be greater and it will not penetrate the tissue as deeply.
Terminology and interpretation of images
The description of ultrasound images is based on an evaluation of shape, contour, size and position of structure being studied as well as its echogenicity, which depends on amplitude of echoes received. An echogenic structure reflects the majority of sound waves back to the probe and thus appears from white to different shades of grey on screen. An anechogenic structure does not produce echoes, instead it transmits waves on to more deeply situated tissues. An example of an anechogenic structure is follicular fluid, which appears black on screen. The terms hypoechogenic and hyperechogenic indicate respectively a decrease and increase in relative echogenicity in comparison with surrounding tissue, whereas the term isoechogenic is used to describe similar echogenicity with surrounding tissue.
This procedure may only be performed by or under supervision of operators skilled in the technique. Proper restraining facilities limiting animal movement are required to ensure operator's safety and speediness of procedure. Avoid direct sunlight on monitor by making temporary shed. Care must be taken to avoid perforation of rectum by hand or probe.
Animals are examined while restrained in a crush or directly in a shed with proper precaution. The contents of rectum are carefully emptied using a lubricated, gloved hand prior to insertion of probe. The ultrasound probe is then carefully inserted into rectum. The reproductive tract is located and probe is moved over uterine horns and body. The operator may determine pregnancy status, stage of gestation and number of fetuses.
The operator may examine the uterine fluid, embryo/fetus for viability and size and any other relevant features of tract or conceptus. The ovaries may be located and examined for presence and size of normal follicles or pathological conditions such as follicular or luteal cyst, corpora lutea or other significant structures.
Since the advent of bovine ovarian ultrasound in 1984, enormous progress has been made in our understanding of folliculogenesis and development of bovine corpus luteum. Ultrasound enables us to describe follicles less than 5 mm in size which cannot be diagnosed manually. The sonographic image of bovine ovarian follicles is characterized by anechoic, circular image. By freezing the follicle image, diameters of follicle can be measured. The average size of follicle is 10-15 mm. They reach maximum size during estrus.
Corpus lutea are recognized by size, shape and echogenic appearance due to its tissue content. Depending upon stage of estrus, corpus lutea may change in size, shape and appearance.
The most frequently encountered pathological conditions of ovaries in bovine practice are follicular and luteal cysts. Using ultrasound, the clinician can differentiate between these two of cysts and apply appropriate treatment. Exaggerated growth of a non-ovulating follicle may lead to creation of a follicular or luteal cyst. An ovary is usually considered cystic when it contains a hollow structure greater than 25mm. A follicular cyst can be differentiated from a luteal cyst by its thin wall and uniformly anechogenic follicular fluid (Ginther, 1998).
The two most frequently observed uterine pathologies in ultrasound diagnosis (by decreasing order of prevalence) are metritis complex and mucometra. In case of endometritis-pyometritis, the contents of uterus may vary in appearance as anechogenic fluid in black, to presence of echogenic material floating in a black background, to a purulent exudate that is echogenic in appearance and similar to surrounding tissue or isoechogenic. The aspect of mucometra resembles a pregnancy, except for the fact that it is impossible to view embryo or its adjacent membranes. In this situation, it is important to seek essential signs that would confirm pregnancy. The chosen criteria for a positive diagnosis may also influence the reliability of ultrasound diagnosis. For example, if the presence of anechogenic liquid (without visualization of the embryo) is accepted as the sole criteria of pregnancy, then an episode of mucometra may lead to a false positive diagnosis.
Early pregnancy diagnosis, diagnostic tool characteristics and embryonic death
Diagnosis of pregnancy is an integral part of reproductive health programs for bovines. The pregnancy examination is generally considered essential and profitable for farmers. Trans-rectal palpation has been used for past 50 years to diagnose pregnancy. This diagnosis can be made after 45-50 days in adult bovines and slightly earlier in heifers. However, it is important to note that precision of early diagnosis by palpation varies according to skill and experience of Veterinarian.
Ultrasonography is less traumatic and more precise than trans-rectal palpation for diagnosing pregnancy in bovines before 30th day. For the bovine practitioner whose activities are not strictly limited to reproduction and who wants to offer early pregnancy detection, ultrasound is a reliable and easy-to-use tool.
In general, ultrasound diagnosis of pregnancy can be made as early as 25th day after insemination. But when it is done in field conditions, it can be made as early as 30th day after insemination. From a diagnostic point of view, ultrasonography's greatest advantage for the Veterinarian is to obtain an excellent predictive value of non-pregnancy. This means that if the ultrasound examination reveals that animal is not gravid, then she truly isn't and won't calve. This excellent characteristic enables Veterinarians to suggest a plan for inducing estrus in non-gravid bovines as quickly as possible in order to proceed with a new insemination in hope of obtaining pregnancy in earliest possible interval.
Many factors affect the precision of early pregnancy diagnosis using ultrasonography. One of the most important is of course the period of pregnancy when reproductive examination is performed. There are other factors that influence accuracy of early pregnancy diagnosis. Diagnostic accuracy is also influenced by type of instrument and probe used. In bovine reproduction, where it is important to evaluate ovarian and uterine structures in detail, we prefer to use linear probes that produce frequencies of 5 to 7.5 MHz; these give a better resolution compared to probes with lower frequencies.
During the examination to confirm pregnancy, it is also important to evaluate embryo's viability by paying particular attention to fetal heartbeat. It is generally visible at centre of embryo starting at day 25th of pregnancy, appearing as a twinkling light with a frequency that varies between 140 and 160 beats per minute (Kastelic et al., 1988). Another important sign of normal embryonic development is the appearance of umbilical cord between uterus and embryo between 40-45th day of pregnancy. Starting on 45th day, we may also observe the first movements of fetus (Kastelic et al., 1988). Finally, presence of a homogeneous, anechogenic liquid in sufficient quantities within amnios is another way of confirming that viable fetal development is progressing normally. When the presence of many echogenic particles are noted within uterine liquid, the Veterinarian should question the viability of embryo and wonder if embryonic death has occurred. At this time, it is important to review various signs of embryonic vitality such as heartbeat, fetal movement as well as its length in relation to stage of pregnancy. The presence of a large amount of debris within amnionic fluid is generally resultant of embryonic degeneration.
Early pregnancy diagnosis using ultrasonography shows uterine lumen containing a variable amount of anechogenic liquid produced by embryo. It is possible that between 25th and 27th days of pregnancy there is little accumulation of liquid within uterus, which could give impression of nonpregnancy and false negative diagnosis at that early stage of a true pregnancy. The gravid uterus at such an early stage needs to be examined with great attention before the practitioner is able to confirm its status. To avoid diagnostic errors, strongly recommend that practitioners do not initiate pregnancy exams with ultrasound before 29th day post-insemination under field conditions. At 25th day, bovine embryo measures approximately 0.5-0.6 cm in length, with a relatively straight shape that modifies gradually into a C-shape by approximately 30th day post-insemination (Kastelic et al., 1988). Sometimes it is difficult to locate the embryo within the small amount of liquid present before 30th day of pregnancy. As well, the young embryo is located very close to uterine wall and can be hidden behind endometrial folds. A careful examination of anechogenic zone (where there is liquid), should enable the practitioner to discover the embryo near the folds for an early diagnosis of pregnancy. Starting at 30th day, the amount of liquid is generally sufficient to distend the folds and move embryo towards centre of anechogenic zone of accumulated liquid within uterus. As well, it is now possible to detect an echogenic band (specular reflection) around the embryo. The specular reflection of this membrane represents the amnion; it is especially evident between 30th and 60th days of pregnancy (Kastelic et al., 1988). Placentomes of 0.5 cm (0.2 inches) in length may be visible beginning on 35th day and are located close to the embryo. Starting on 40th day, we can easily visualize forelimb and hind buds.
The embryo's age may be determined by plotting its length on a quadratic growth curve. The fetus growth is more rapid after 50th day of pregnancy than at any preceding period (Kastelic et al., 1988). Many ultrasound equipments come with a feature that enables the Veterinarian to measure the length of embryo and to estimate its age. The distance between the top of head and rump of fetus (crown-rump length, or CRL) is the most frequently used measurement in determining the age of embryo. For example, a 30th to 35th day old embryo measures between 1 and 1.5 cm and an embryo of 40th days measures between 2 and 2.5 cm.
Ultrasonography techniques assist in diagnose and differential pregnancy from pathological conditions such as fetal mummification, maceration, mucometra, pyometra, follicular cyst, luteal cyst, hydroallantois and hydroamnions according to their physio-pathology. On the other hand, the most restrictive factors of ultrasonography are that its efficiency is always dependent on expertise of the operator (Fig. 1-12).
Ultrasonography technique is very easy, safe, economical and practical way to confirm reproductive status either pregnant or non-pregnant from 25th day of gestation in bovines. The ultrasonography is used as a diagnostic as well as applied / research tool. In Veterinary imaging world, it is hard to beat ultrasound as it is safe, offer wide range of applications and cost-effectiveness. Ultrasonography becomes the preferred diagnostic imaging modality of 21st century in clinical and research field in animal reproduction. The scanner will never replace the Vet, but can be a simple, clear tool that helps him to diagnose and reduce the range of errors.
Kahn, W. (1992). Ultrasonography as a diagnostic tool in female animal reproduction. Anim Reprod Sci. 28: 1-10.
Ginther, O.J. (1998). In; Ultrasonic Imaging and Animal Reproduction: Cattle. Equiservices Publishing. Cross Plains, WI, USA :29-58.
Hanzen, C. and Delsaux, B. (1987). Use of transrectal B-mode ultrasound in early pregnancy in cattle. Vet Rec. 121: 201-02.
Kastelic, J.P., Curran, S., Pierson, R.A. and Ginther, O.J. (1988). Ultrasonic evaluation of bovine conceptus. Theriogenology 29: 39-54.
A.B. Mali (1)
Livestock Development Officer
Department of Animal Husbandry
(1.) Corresponding author.
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|Date:||Jan 1, 2015|
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