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The Use of Diagnostic
Ultrasound in Canine Reproduction |
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The human ear can detect sound waves of certain frequencies; higher frequencies are outside this range. These higher frequency sound waves may be directed into a small beam of sound; the principle of diagnostic ultrasound. The ultrasound beam is produced by small crystals which are housed within a transducer or probe. These crystals both produce the sound (by expanding or contracting when an electric current is applied to them) and receive the returning sound beam and convert it back into an electrical signal. The electrical impulses produced by the returning sound are then converted into a picture on a small screen. The picture produced on the screen is referred to as an image7. The ultrasound crystals produce many rapid pulses of ultrasound which allow images to be continually updated on the screen so that movement of organs can be seen. This is called real-time ultrasonography and is now the most commonly used method. When a beam of ultrasound is directed into the body from an ultrasound machine the sound travels through tissue until it meets an obstruction. At this point some sound continues on its original path, whilst some is reflected back to the ultrasound machine. The speed of sound within the body is almost constant; therefore by measuring the time taken for the echo to return to the machine, the distance between the ultrasound machine and the tissue obstruction can be calculated. This calculation is done by complicated electronics within the machine and the information is displayed on the ultrasound screen. Ultrasound pictures are black and white with varying shades of grey. The brightness of the image is related to the degree of sound reflection. This type of machinery is called brightness (or B-mode) ultrasound. If most of the ultrasound beam is reflected back to the transducer the images appear white on the screen. When little of the sound is reflected the images appear darker shades of grey or black. Very dense structures such as bone reflect nearly all of the sound beam and produce bright white images. Gases such as air (found in the lungs and in some areas of the gastrointestinal tract) do not allow the transmission of ultrasound and also appear white on the ultrasound screen. Conversely most fluids allow sound to be easily transmitted; they appear black on the ultrasound screen. The amount of fluid within an organ therefore affects its appearance when it is examined with ultrasound. There are several types of ultrasound machine. Most machines used in small animal practice have mechanical sector transducers. These have their crystals situated on a small head which moves. The picture produced by this type of transducer is wedge shaped. Linear array transducers are more commonly used in large animal practice and consist of crystals arranged in a line. The picture produced by this type of transducer is rectangular in shape. Ultrasound transducers also vary in the frequency of sound which they emit. High frequency transducers (7.5MHz) produce excellent detail but the ultrasound is unable to penetrate deep into the body. These transducers are most useful for small superficial structures, and for small dogs and cats. Low frequency transducers (3MHz) are able to penetrate deep into the body but do not have such good image definition. These transducers may be used for examining larger animals such as sheep and horses. Medium frequency transducers (5MHz) are most commonly used to examine dogs since they produce good image definition and have sufficient penetration for most purposes. Most images are displayed as white on black pictures which represent a thin slice through the patient. It is important to remember that the picture is only a two dimensional slice through a three dimensional structure. For example if one considers slicing a loaf of bread, the cross section of bread is square in shape; however the loaf itself is not cuboidal in shape. The pictures produced by the ultrasound machine may be observed during the examination. However, it is often best to save images on a video recorder so that they can be reviewed later, or to take copies of images using a Polaroid camera or video printer. What is involved? The great advantage of ultrasound is that it is not invasive and does not involve the use of ionising radiation. The procedure is therefore safe for the patient and operator. It is not necessary to leave the room during the investigation as is the case with radiography and animals can be reassured and/or restrained during the examination. It is rarely necessary to anaesthetise animals for ultrasound examination. In most cases animals can be lightly restrained, and in the case of pregnancy diagnosis it may be most convenient to examine the bitch in the standing position. A close contact is necessary between the ultrasound transducer and the patient's skin. Air does not allow the transmission of sound so it is important to clip the patient's coat (as this traps air), to clean the skin, and to apply an acoustic gel which allows ultrasound transmission. It is possible to manage without clipping the animal's coat but this invariably results in poorer images and may make examination of certain areas of the body very difficult. Most animals are examined standing or laying on their side or back. After preparing the patient the ultrasound transducer is placed onto the animal's skin and moved across the body to image structures under the transducer. Examination of the bitch Ultrasound may be used to image the uterus of most bitches. This is most useful for the diagnosis of pregnancy, the assessment of foetal numbers, the assessment of foetal viability and the diagnosis of pyometra. The ovary may also be examined during oestrus; however, the information that can be gained from this procedure is limited. The non-pregnant uterus It is often difficult to find the non-pregnant uterus in the bitch especially if the bitch has never been pregnant. This is less difficult during proestrus and oestrus when the uterus becomes swollen with fluid; this fluid (oedema) makes the uterus darker in appearance. The uterus is located on top of the bladder but it may slip to one side or the other. The division of the uterine body can sometimes be seen, although the two horns may be confused with loops of small intestine. The diameter of the uterus varies during the reproductive cycle; an understanding of the normal diameter and appearance is important so that pathological conditions may be recognised. The pregnant uterus There are many reasons why bitches may be examined by ultrasound to confirm pregnancy. It may be important to diagnose pregnancy relatively early to allow adjustments to be made in feeding and routine medication regimes and is an important consideration if therapeutic drugs are to be used. Ultrasound examination may be undertaken in mid-pregnancy IN bitches that were not examined earlier or where there is concern over foetal resorption. Ultrasound may be used when it is necessary to differentiate between pregnancy and pyometra since in mid-pregnancy bitches frequently have a vaginal discharge and refuse their food. In late pregnancy bitches may be examined if they are thought to be 'overdue' or if there is concern over foetal abnormalities. Bitches which have problems during whelping may be examined to confirm whether the pups are still alive, and bitches may be examined after whelping to ensure that no pups have been retained. There is no evidence to suggest that ultrasound examination is harmful to the developing pups. There is some confusion about when ultrasound can be used to diagnose pregnancy in the hitch. This is not surprising considering the complicated events which occur around the time of ovulation. The majority of bitches whelp 63 days after ovulation. Some bitches may stand and be mated several days before ovulation. If sperm from an 'early mating' live long enough to fertilise eggs, the time from mating to whelping will be longer than 63 days, and can be up to 72 days. Some bitches, however, will stand and be mated several days after ovulation. If eggs are still present during a 'late mating' these may be fertilised. In this case the time from mating to whelping will be shorter than 63 days, and can be as little as 58 days. Therefore, although the time from ovulation to whelping is nearly the same in all hitches, the time from mating to whelping (the apparent pregnancy length) can vary between 58 and 72 days. This has important implications for the early diagnosis of pregnancy in the bitch. The size of the foetus is related to the timing of ovulation and not the timing of mating. Bitches which have been mated 'early' may be presented for ultrasound examination too soon, before foetal fluids have accumulated. These bitches may therefore be thought to be non-pregnant. Pregnancy can be detected using ultrasound as early as 19 to 22 days after ovulation. Ovulation time in most bitches is not known so that 28 days after the last mating is a suitable time to conduct an ultrasound examination. In early pregnancy the embryo is present within a small amount of fluid. The embryo, which is only 1-2 mm in diameter, appears grey on the ultrasound screen and is suspended within a fluid filled swelling (black cavity) of the uterus. A small flickering of the embryonic mass (representing the beating of the heart) can often be seen. As the embryo grows it becomes possible to recognise distinct head and body regions, and the foetal membranes become obvious. The foetal sac enlarges and the embryonic tissues become more prominent and the heart is more easily seen. At an early stage of pregnancy (28 - 35 days) it is possible to estimate the number of pups. This requires careful examination, and can be difficult even for experienced operators. It is often easiest when there are only one or two pups and is more difficult in later pregnancy when only sections of each pup are seen. The accuracy in determining the actual number decreases with larger litters; the overall accuracy is low (about 30%). Most frequently the number of pups is underestimated. In mid-pregnancy (35 - 55 days) the foetal skeleton develops and can be seen. This appears white on the ultrasound screen. The head appears circular early on but later the jaws and nasal bones can be identified. The spinal column, pelvis and the ribs can also be seen. The internal organs become more prominent in their appearance. The liver is in the front of the abdomen, being dark grey in appearance, and the fluid filled stomach is often noted just behind this. The heart and some of the great arteries and veins can also be identified. Movements of the pups are often noted and individual limbs can often be seen. In late pregnancy the bladder, which is fluid filled, may be seen. The other abdominal organs become more prominent and look more like those of a mature animal. The chambers of the heart can easily be identified and organs like the kidneys are more easily seen. Counting the number of pups at this time is less accurate than earlier. Pups which are dead can he identified by an absence of heart beats and a lack of movement and foetal fluid. Most ultrasound machines possess electronic callipers which can measure the structures being examined. A variety of measurements can be made on individual pups. Measurements of the head or thorax size for example can be compared to normal tables, thus allowing an estimation of the age of the pups to be made. This allows the time of whelping to be predicted. This procedure is commonly applied to pregnant women but to date has only been done in one breed of dog (the Labrador retriever). Since there is such a large size difference between breeds normal values for one breed are of little use to others. Abnormalities of pregnancy Regular ultrasound monitoring of bitches has demonstrated that there is probably a higher incidence of resorption than is generally appreciated. This information is interesting but has limited value since in most cases little can be done to prevent resorption once it has started. Indeed it may be dangerous to attempt this since in many cases there is a good reason for the resorption. Foetal distress and death may be noted with ultrasound. This may help to decide whether to perform a caesarean operation when assessing a bitch with a prolonged whelping. If dead pups are present, gas may be detected within the uterus. In man, many foetal abnormalities can be detected using ultrasound. This is helped by the large size of the foetus and the fact that single foetuses are most common. In the dog it is difficult to examine all foetuses in mid and late pregnancy. This hinders the use of the technique for the diagnosis of foetal abnormalities, although recently a case of hydrops foetalis was diagnosed. Experience may allow more foetal abnormalities to be detected. Pyometra Cases of pyometra can usually be diagnosed on the clinical signs. However, it is not uncommon for pregnant bitches at times to have a vaginal discharge and refuse food. In most cases of pyometra the uterus contains fluid which appears black upon ultrasound examination. The wall of the uterus is thickened and the distended loops are easily identified. In some cases the fluid contains debris and appears grey. The uterus containing fluid can be distinguished from a pregnant uterus by the lack of foetal tissue, and from intestine since there is usually no gas, and peristaltic contractions do not occur. Some bitches accumulate thin fluid or blood within the uterus (hydrometra and haematometra); these cases cannot be distinguished from pyometra. Other uterine abnormalities It may be possible to image cases of stump pyometra which occasionally occur in spayed bitches. Uterine tumours may be found but these are very rare and their appearance varies depending upon the nature of the tumour. The normal ovary The bitch's ovary is difficult to image when not active. During proestrus small follicles can be identified upon its surface. The follicles appear as small fluid filled (black) structures which increase in si7e as the bitch approaches ovulation. The structures formed by the follicle after ovulation (corpora lutea) also contain fluid. This means that it is not possible to reliably determine the timing of ovulation using ultrasound. Ovarian tumours Ovarian tumours can often be diagnosed clinically since they either cause signs of continual oestrus or create marked accumulations of fluid within the abdomen. Ultrasound of these cases is often diagnostic since the tumours are larger than normal ovaries and often contain fluid and can be easily imaged. Cystic ovaries The majority of ovarian cysts in the bitch are of little significance, They frequently arise from the ovarian bursa and are inactive. These structures may be found with ultrasound, and it may be difficult to distinguish between ovarian cysts and tumours. Usually if the ovaries are being examined there is a clinical suspicion of some problem. In these cases it is often necessary to remove the ovary and submit it for histopathological investigation. Examination of the dog The testicles The testes of the dog are easily examined. They can be palpated and measured but their internal structure cannot be examined without either biopsy or the use of ultrasound. It has been shown that ultrasound examination of testes is not damaging to semen production. Testicular ultrasonography is most easily achieved with the dog in the standing position. The hair is not usually clipped, but copious amounts of acoustic gel are applied to the scrotum. Testicular tissue appears dark grey on ultrasound and has white speckling throughout its substance which is due to the presence of fibrous tissue. The epididymis can be imaged adjacent to the testicle, its tail being more easy to image than its body. The relative sizes of epididymis and testis might be useful for the diagnosis of certain abnormalities. Measurement of testicle size has been claimed to be useful as a method of determining the semen quality of dogs. Ultrasound can be used for this purpose, but unfortunately it is not true that testicular size is a good indicator of semen quality. Many infertile dogs have testicles which are of normal size. Testicular abnormalities The majority of testicular abnormalities can be detected by palpation of the testes. Cases of testicular tumours have been identified using ultrasound; these disrupt the normal architecture of the testicular parenchyma and may have fluid filled cavities. The tumours are often discrete and easily identified, although tumours such as seminomas may affect the whole testis. In such cases a change in the ultrasound appearance may be noted, as well as changes in testicular size. Recently some dogs have been examined which have small fluid pockets within the testicle. These 'testicular cysts' were not known to exist in dogs. Small cysts are unlikely to affect fertility but the significance of large cysts is not yet known. Further work in this area is being undertaken. Ultrasonography may be helpful in cases of scrotal swelling. Small scrotal hernias may be identified by the presence of loops of small intestine whilst testis enlargement and oedema formation occur in testicular torsion. The prostate gland The prostate gland of the normal dog is located within the pelvis. This means that examination is hindered somewhat by the pelvic bones. If the prostate is enlarged for any reason it usually moves further forwards in the abdomen and can be more easily imaged. However, in certain circumstances the entire prostate is best imaged by placing a transducer into the rectum. This requires sedation or anaesthesia. The normal prostate is pear-shaped and bright grey in appearance with white fibrous stippling present throughout. The normal size of the prostate gland may differ between different breeds of dog. To date these normal variations have not been quantified. Prostatic abnormalities In most entire adult males the prostate gland enlarges during life. In older dogs this normal enlargement is frequently noted and is called benign prostatic hyperplasia. Small focal regions of fluid accumulation may be noted within the prostate gland although it usually maintains its normal shape and smooth outline. Cases of chronic and acute prostatitis can be diagnosed using ultrasound. Often the gland is enlarged and changes in its ultrasound appearance are noted. It is difficult to differentiate these changes from prostatic neoplasia using ultrasound alone unless the tumour is discrete. Ultrasound is most useful for the diagnosis of paraprostatie cysts. These are usually well defined and because they contain fluid they appear black with a grey margin.
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