Ultrasonography Imaging Technique in Veterinary Practice
DR MD MOIN ANSARI
Division of Veterinary Surgery and Radiology
Faculty of Veterinary Science and Animal Husbandry
SKUAST K, Shalimar-190025
Ultrasonography is most commonly used imaging technique in veterinary practice and forms an integral part of clinical diagnosis for many disorders. Ultrasonography uses high frequency sound viz. ultrasound waves to produce images of internal organs and other tissues. Ultrasound is characterized by sound waves with a frequency higher than the upper range of human hearing, approximately 20,000 cycles per second (20 kHz). Sound frequencies in the range of 2 to 10 MHz to create images of body structures based on the pattern of echoes reflected from the tissues and organs are commonly used in diagnostic examinations. A lot of advancements have been added to ultrasound imaging technology in recent years. Duplex and colour Doppler ultrasound, multi hertz high-resolution transducer, 3- dimensional ultrasound, 4- dimensional ultrasound, sono-endoscopic probes, ultrasound guided biopsy options have made the examination more versatile. Contrast enhanced ultrasonography with micro bubble contrast agent and tissue harmonic imaging (THI) are found very efficacious to study organ functioning and hemodynamic parameters. Ultrasonography is a potential armament in the hands of expert clinicians for diagnosis of various diseases/conditions in the light of clinical signs, laboratory findings and other diagnostic modalities. However, the examination takes skill and training to perform and to interpret. It is essential to produce an image of diagnostic quality that will allow the sinologist to differentiate between artifacts and real image. The sonologist must have a thorough knowledge of the instrumentation of the ultrasound machine and sound understanding of the anatomical position of the organ.
Principle:
A sound wave travels in a pulse and when it is reflected back it becomes an echo. It is the pulse-echo principle, which is used for ultrasound imaging. A pulse is generated by one or more piezoelectric crystals in an ultrasound transducer. When these crystals are stimulated electrically it changes its shape and produces sound waves of particular frequencies. The sound waves travel into the body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone). Some of the sound waves reflect back to the probe, while some travel on further until they reach another boundary and then reflect back to the probe. The reflected waves are detected by the probe and relayed to the machine. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (1540 m/s) and the time of the each echo’s return (usually on the order of millionths of a second). The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional (2D) image. In modern scanning systems, the sound beam is swept through the body many times per second, producing a dynamic, and real time image those changes as the transducer is moved across the body. This real-time image is easier to interpret and allows the examiner to scan continuously until a satisfactory image is obtained.
Interactions of Ultrasound with Matter:
Ultrasound interactions are determined by the acoustic properties of matter. As ultrasound energy propagates through a medium, interactions that occur include: Absorption: It occurs when the energy in the sound beam is absorbed by the tissues thereby converting it into heat. Absorption process forms the basis of therapeutic ultrasound.
Reflection: A portion of the ultrasound beam is reflected at tissue interface. The sound reflected back toward the source is called an echo and is used to generate the ultrasound image. The percentage of ultrasound intensity reflected depends in part on the angle of incidence of the beam. As the angle of incidence increases, reflected sound is less likely to reach the transducer
Refraction:
Refraction is the change in direction of an ultrasound beam when passing from one medium to another with a different acoustic velocity. Ultrasound machines assume straight line propagation, and refraction effects give rise to artifacts.
Scattering:
It occurs when the beam encounters an interface that is irregular and smaller than the sound beam. The portion of the beam that interacts with this interface is scattered in all the directions. Two closely related phenomenons occur, refraction and diffraction of which refraction is a common cause of artifacts. Acoustic scattering arises from objects within a tissue that are about the size of the wavelength of the incident beam or smaller, and represent a rough or non-specular reflector surface. As frequency increases, the non-specular (diffuse scatter) interactions increase, resulting in an increased attenuation and loss of echo intensity. Scatter gives rise to the characteristic speckle patterns of various organs, and is important in contributing to the grayscale range in the image
Attenuation:
Ultrasound attenuation, the loss of energy with distance traveled is caused chiefly by scattering and tissue absorption of the incident beam (dB). The intensity loss per unit distance (dB/cm) is the attenuation coefficient. Rule of thumb: attenuation in soft tissue is approx. 1 dB/cm/MHz. The attenuation coefficient is directly proportional to and increases with frequency. Attenuation is medium dependent.
Acoustic Impedance (Z ):
is equal to density of the material times speed of sound in the material in which ultrasound travels. The differences between acoustic impedance values at an interface determine the amount of energy reflected at the interface.
Acoustic Shadowing of Sound Wave:
Total reflection of sound waves at interfaces of bone, calcification and gas-filled structures. No imaging of area distal to reflective interface and shadowing is displayed as a uniform black area distal to reflector. Incomplete shadowing may appear distal to stones and calculi.
Ultrasound Machine:
Large and small portable ultrasound machine suitable for different species of animals and type of examination are available in the market. Basic Components are Central Processing Unit (CPU); transducer probe; transducer pulse controls, display; keyboard/cursor; disk storage; printers.
Transducer (probe or scan head):
is a device that converts one form of energy to another. In U/S imaging this means converting electric energy into sound wave and vice versa. The frequency emitted by a particular transducer depends on the characteristics of the special piezoelectric crystal contained within the transducer. Transducer selection comes with experience, but general guidelines may help the new sonographer to select an appropriate frequency. For routine two- dimensional and M-mode echocardiography, cat and small dogs (< 7 kg) and small body parts (thyroid, breast), are usually examined initially with a higher frequency 7.5 to 10 MHz transducer. A 5 MHz transducer is used for most dogs and thick body parts (abdomen), except some large dosg (>50 kg), which may requires a a lower frequency 3.0-3.5MHz transducer. For optimal Doppler imaging, it may be necessary to sacrifice 2D image resolution and to use a lower frequency transducer than one would generally use for 2D or M –mode examination in the same patient.
Types of Transducers
Phased array transducers: A sector field of view is produced by firing multiple transducer elements (64 to 128 elements) in a precise sequence electronically. The beam can be steered in different directions and focussed at various levels, which allows the transducer to have a small size yet a wide field of view at deeper depths.
Linear array transducers:
a linear array transducer has multiple crystals (256 to 512) elements arranged in a line within a bar-shaped scan head. The narrow beam is swept through a rectangular field firing the transducers crystrals sequentially to produces the ultrasound beam. These transducers come in variety of sizes and frequencies. Small rectangular linear array transducers are commonly used for small parts (abdominal- extra thoracic) scanning and sonography of superficial structure (skin, mammary gland, joints and tendons) in small animals.
Curvilinear array transducers:
are linear arrays shapes into convex curves. They produce a sector image that has a wider field of view than that of linear arrays. These transducers also come in variety of sizes and frequencies suitable for many general purpose applications. Trapezoid convex (curved array is useful in abdominal ultrasonography and pregnancy diagnosis.
Mechanical sector scanners: the real scanner is named because the beam shape and resulting screen image produced by the transducer are triangular or pie shaped or sector shaped or fan shaped. A mechanical real-time sector scanner sweeps the beam through the field of view by movement of either solitary crystals or multiple crystals to generate real-time image. Mechanical sector scanners are less commonly used today because of the increased affordability of the various types of array scanners. Sector scanners, whether mechanical or electronic, have the disadvantage of limited near field visibility compared with linear array or curvilinear array transducers. Sector scanners are particularly useful in echocardiography, ultrasonography of intra pelvic and intra thoracic organs, brain, eyes, testes and joints.
There are three modes of echo display which are used frequently in clinical applications in veterinary medicine.
A-mode (amplitude mode):
is the least frequently used. Used for ophthalmic examinations and other applications requiring precise length and depth measurements are needed.
B-mode (brightness mode):
Displays the returning echoes as dots whose brightness or gray scale is proportional to the amplitude of the returned echo and whose position corresponds to the depth at which the echo originated along a single line (representing the beam’s axis) from the transdrucer. B-mode is usually displayed with the transducer positioned at the top of the screen and depth increasing to the bottom of the screen. Used most often in clinical practice and produces 2 dimensional reconstruction of the image slice.
M/TM- mode (Motion or Time-Motion mode):
is used for echocardiography along with B-mode to evaluate the heart. M-mode tracing usually record depth on the vertical axis and time on the horizontal axis. The image is oriented with the transducer at the top. The motion of the dots (changes in distance of reflecting interfaces from the transducer) is recorded with respect to time. The echo tracings produced with M-mode are useful for precise cardiac chamber and wall measurements and quantitative evaluation of mitral valve leaflets or wall motion with time.
Real time B-mode:
displays a moving gray scale image of cross sectional anatomy. This is accomplished by sweeping a thin, focused ultrasound beam across a triangular, linear or curvilinear field of view in the patient many times per second. The field is made up of many single B-mode lines. Sound pulses are sent out and echoes received back sequentially along each B-mode line of the field until a complete sector image is formed.
Echogenicity:
Anechoic (echo free): Tissues without acoustic interfaces appear as black area (fine or coarse).
Hypoechoic (echo poor):
Tissues with low echogenicity appear medium to dark grey.Tissues with medium echogenicity appear light to medium grey.
Hyperechoic (echo rich): Echo rich tissues, calcification and gas filled organs appear as white or light grey.
Image interpretation:
The proportion of sound waves reflected in represented on the ultrasound image by shades of gray ranging from black to white. The density of the tissue determines the shade of gray visualized on the screen. Gas and bone are barrier to ultrasound beam and appear white (hyperechoic) on the screen; ultrasound passes uninterrupted through fluid and appears black (anechoic) on screen; images of the soft tissues appear as shades of gray (hypoechoic) depending upon their proportion of fat, fibrous tissue and fluid.
Following order of decreasing echogenicity of body tissues and substances:
Bone, gas, organ boundaries (more echogenic)> structural fat, vessel walls>renal sinus>prostate> spleen> storage fat>liver>renal cortex>muscle>renal medulla>bile, urine (least echogenic).
Doppler works on Doppler principle to the processing of reflected ultrasound waves. The principal value of these studies is the delineation of the direction and velocity of blood flow in the heart and great vessels.
Artifacts in ultrasound:
Any density or mark on an ultrasound scan that is caused by something not belonging to the part of being imaged.
Artifacts can be divided into two categories:
Useless artifacts are produced by improper use of equipment, improper machine setting, improper scanning procedures or improper patient preparation. These artifacts usually affect the quality of images and therefore the interpretation.
Useful artifacts enhance accurate interpretation and are produced under proper technical conditions. The useful artifacts are a result of interactions ultrasound and matter.
Artifact Production:
Reverberation:
refers to the production of spurious echoes due to two or more reflectors in the sound path; the first reflector is usually the skin-transducer interface (external reverberation). Internal reflector such as bone or gas is also common causes of reverberation (internal reverberation). For example: gas filled bowel segments. The sound is entirely reflected back from the gas and then bounces back and forth between probe and the gas creating multiple echoes from one ultrasound transducer.
Mirror-image artifacts:
are produces by rounded, strongly reflective interfaces such as the diaphragm-lung interface. Part of the insonating beam is reflected back into the liver. The echoes from the liver return to the transducer along the same path via the diaphragm- lung interface. The ultrasound machine assumes that the sound pulse and the reflected echoes travel to and from the transducer in a straight line. A mirror image is produced in this erroneous position because of the increased round- trip time.
Side-lobe artifacts: lateral displacement of the structures not aligned with the sound beam is called side-lobe artifact. It is produced by minor beams of sound travelling out in directions different from the primary ultrasound beam. When side lobes of sufficient intensity interact with a highly reflective interface, the returning echoes are erroneously displaced along the path of the main ultrasound beam even though they did not originate within the main beam. Curved surfaces such as diaphragm, bladder or gall bladder and highly reflective interface such as with air are common conditions in which side lobe artifacts occur.
Acoustic shadowing:
appears as an area of low amplitude echoes (hypo echoic- to- anechoic area) created by structures of high attenuation. It occurs as a result of nearly complete reflection or absorption of the sound. This artifact can be produced by gas or bone. Urinary calculi, barium and gall stones creates a strong clean acoustic shadow.
Acoustic enhancement:
also called through –transmission represents a localized increase of the echo amplitude occurring distal to a structure of low attenuation. This is commonly seen distal to the gallbladder and urinary bladder.
Comets:
artifacts produced on film that resemble comets. They are usually caused by rust particles adhering to film during development.
Refraction:
it occurs when incident sound wave traverses tissues of different acoustic impedances. This may cause a reflector to be improperly displayed. Refraction between the spleen or liver and the adjacent fat and creates the duplication of the organ.
Manipulating Artifacts:
Proper patient preparation with 12 hours fasting of filling the bladder and part of gastrointestinal tract with fluid.
Choosing proper frequency of transducers for the organ of interest.
Adjusting the power, gain and time-gain compensation setting correctly.
Avoiding other electrical devices or radiofrequency signal interference.
Applications for Ultrasonography:
o Pregnancy diagnosis – as early as 14 days conceptual fluid is seen in equine, in canines as early as 20 days.
o Twin pregnancies, pregnancy losses, pseudopregnancy, ovarian cysts and haematoma, ovarian tumours and infection.
o The organs which can be scanned usefully are liver, kidney, urinary bladder, spleen, uterus, ovaries, teat and udder.
o Merit of ultrasound is its ability to characterize internal parenchymal contrast radiography, angiography or exploratomy laparotomy
• Biopsy guidance for internal masses and cytocentesis (liver and kidney biopsies, pericardium and chest drainage)
• Bone heal monitoring
• Cardiac evaluations (assessment of blood flow and supply).
Strength of ultrasonography
o Non-invasive
o Essentially non-toxic
o Free from radiation hazards
o usually does not require general anaesthesia or sedation
o Provide quick instant and dynamic visualization
o Allows precise location of biopsy needle
o Fetal viability
o Real time scanning – see movement/motion (dynamic imaging);
o Relatively inexpensive compared to other modes of investigation, viz: CT or MRI.
Weakness of ultrasonography
o Repetitive exam.
o Can’t evaluate some extra abdominal structures (i.e. spine)
o High level of skill and experience is needed to acquire good- quality images and make accurate diagnosis.
o Must know anatomy very well.
Therapeutic Applications
o Physical therapy and treatment of Cancer.
o Cataract treatment by phacoemulsification.
o Break up kidney stones by lithotripsy.
o Intervenional biopsy
o Contrast-enhanced ultrasound.
Imaging Approaches
In trans abdominal scanning, the transducer is placed on ventral or lateral wall in small sized animal. In transrectal scanning, the transducer is taken per rectum under a rubber sleeve in large animals. In transurethral or transvaginal scanning special sector or radical endoluminal transducer are used to evaluate mucosal surface of urethra, bladder, vagina, cervix or uterus. Transthoracic scanning is commonly used to scan heart and major vessels, lungs and dioaphragm. Sonoendoscopic transducers are also available which may allow transluminal evaluation of hollow visceral organs.
Abdominal Ultrasonography:
In abdominal scanning various abdominal organs are amenable to sonographic scanning: viz: hepatobiliary (hepatic size, abscess, cyst, tumour, cirrhosis, calculi, obstruction, infection of gall bladder and bile duct), spleen (abscess, tumour, infraction and splenic size), gastro- intestinal tract (foreign body, obstruction, tumors, mucosal pattern, peristalsis, gastric and intestinal emptying time), calculi in kidney and urinary bladder, hydronephrosis, bladder tumour, renal cortex and medulla are well visualized in sonographic scanning.
Reproductive Ultrasonograpphy:
Ultrasonography has been used to its fullest extent in animal reproduction to study ovarian follicular dynamics, pregnancy diagnosis, fetal wellbeing including fetal sexing, ovum pick-up technology (OPU) and in the detection of reproductive diseases. The uterus appears as a well-defined, tubular structure, with hypoechoic to anechoic lumen. Spectral, color flow and power Doppler imaging now facilitate physiologic interpretations of vascular dynamics over time. Pregnancy is diagnosed by the presence of anechoic vesicular conceptus. Ultrasoonography gives definite evidence of fetal viability in allowing visualization of fetal heartbeat, fetal development, fetal sexing and expected date of parturition. Program and formula are available to predict gestational age (GA) with parameters like gestational sac diameter (GSD), head diameter (HD), crown rump length (CRL) and body diameter (BD).
Gestational age in the dog:
GA= (6 x GSD) + 20: GA=( 3 x CRL) + 27. Greater than 40 days:
GA= (15 x HD) + 20: GA =(7 x BD) + 29.
This method was less accurate for toy, miniature and giant breeds. A correction factor of +1day should be applied to the gestational age prediction for small body weight (<9 kg) bitches and -2 days for giant body weight (>40 kg) bitches.
Ovum pick-up (OPU) technology: reproductive technologies like in-vivo embryo production, production of cloned or transgenic animals and establishment of occyte banks have made a giant stride in recent years. The requirement of OPU comprised of an ultrasound scanner with a transvaginal transducer, needle guidance system adn suction pump. Diseases of female reproduction tract like pyometra, mucometra, hydrometra, fetal mummification and ovarian tumours can be detected by ultrasonography. Physiological blood flow pattern of canine testis and prostate gland is documented sonographically. Testicular hypoplasia, benign prostatic, hyperplasis in canine are frequently diagnoses by ultrasonography.
Cardiac ultrasonography/ Echocardiography:
When ultrasound is used to image the heart it is referred to as an echocardiogram, allows physicians to see detailed structures of the heart, including chamber size, heart function, the valves of the heart, as well as the pericardium. Echocardiography uses 2D, 3D and Doppler imaging to create pictures of the heart and visualize the blood flowing through each of the four heart valves. In emergency situations echocardiography is quick, easily accessible and able to be performed at the bedside making it the modality of choice for many physicians. Stress echocardiography is gaining ground as an evaluating tool for performance in racing horses.
Limb ultrasonography:
Evaluation of tendons and ligaments of both forelimbs and hind limbs is extensively used in racehorses to certify their fitness. It has been proved to be a superior imaging modality to detect adhesions, tearing and inflammation of these structures.
Besides, ultrasonography is being used for evaluation of endocrine glands, salivary glands, eyes, tissue healing, teat and udder.
Ultrasound-guided Biopsy
Percutaneous biopsy of abdominal organs like liver, spleen, kidney, prostrate, abdominal mass, lesions etc. under ultrasound guidance is well documented. Biopsy site is prepared for aseptic intervention. The transducer is covered with a sterile sleeve and gel. Once a good ultrasonographic image is the relevant organ is obtained, a biopsy needle is introduced through the abdominal wall, either via a clip-on guide at an angle of 15-300 to the transducer. The point of the needle can be directed precisely into the mass/lesion under guidance of the image on monitor. A variety of needles are available for biopsy collection. After sample collection the needle is withdrawn.
Three Dimensional (3-D) Ultrasound
Two-dimensional ultrasonography relies on the acquisition of images in multiple scan planes from which to build a mental 3 D images. Ultrasound waves are directed from multiple angles and waves are reflected back and captured, providing very detailed 3-dimensional images of the baby. Help in diagnosing certain conditions (such as a cleft lip) that may not be visible with 2D. Manipulated in a number of ways (rotation, zooming) to allow unprecedented examination of the ultrasonographic images.
2 types of 3 D ultrasonography
A) Featured based construction (surface rendering): provides a profile of the surface of the structure.
B) Voxel based reconstruction (volume rending): each pixel acquired in the 2-D ultrasonography images is placed into the proper 3-D.
Four Dimensional (4-D) ultrasonography
The process of streaming 3-D images into live, real-time video of the baby. This allows viewers to see real-time motion. It is a newest form of 3D and provides functional data in the 3-D. Applied in the echocardiography and neurosonology.
References:
Ansari, M.M. 2011. Advances and application of diagnostic ultrasonography in Veterinary practice-a review. Livestock Line. 5 (3): 11-14.
Ansari, M.M. 2014. Essentials of Veterinary Diagnostic Imaging (A book for both undergraduate and postgraduate level). First edition, New India Publishing Agency, New Delhi 110 034, India.
Bushberg, J.T., Seibert, J.A., Leidholdt, E.M and Boone.J.M .2002. The essential physics of medical imaging,Second edition,Lippincott Williams and Wilkins p.1-15.
Denny, P. P. and Heaton, B. 1999. Physics for Diagnostic Radiology. USA: CRC Press.
Jerrold, T., Bushberg, J., Anthony S., Edwin M. L. and John, M. B 2002. The essential physics of medical imaging. Lippincott Williams & Wilkins.
Nyland, T.G. and Marroon, J.S. 2002. Veterinary diagnostic ultrasound. 2nd ed. Philadelphia, Lea and Fabiger.
