for the Veterinary Technician

Transcription

1 An Overview of Abdominal Ultrasound for the Veterinary Technician Valerie Gates, CVT, VTS (ECC) Learning Objective: The reader should gain a basic understanding of ultrasound, including physics, terminology, and artifacts which help the sonographer evaluate images. The participant will understand the various types and strengths of transducers, how they work, their orientation as it pertains to the patient and monitor, and movement across abdominal structures. This program was reviewed and approved by the AAVSB RACE program for 1 hour of continuing education in jurisdictions which recognize AAVSB RACE approval. Please contact the AAVSB RACE program if you have any comments/concerns regarding this program s validity or relevancy to the veterinary profession.

2 Ultrasound has become a common modality in the veterinary practice since it is non-invasive and more economical than some of the other modalities. In most veterinary practices, it is used mostly as a diagnostic tool for conditions occurring in the abdominal cavity. However, it can also be used to evaluate the thoracic cavity and musculoskeletal tissues. In an average practice, the veterinarian s time is consumed by making diagnoses, developing treatment plans, juggling exam rooms, surgery, etc. Many veterinarians may find themselves too busy to take time out of their busy schedule to sit for an ultrasound exam. The role of the veterinary technician can move beyond their normal duties to perform an ultrasound exam for that veterinarian. Images are documented in a variety of ways such as still photos, video tape for some older units, or realtime cine clips. Once the technician learns how to scan a patient in a prescribed manner, images are documented to present to the veterinarian. It is only when an abnormality is discovered that the veterinarian needs to be consulted. Ultrasound Physics Review To produce a complete ultrasound exam, it is important for the technician to understand some basic principles of ultrasound, terminology, and the importance of artifacts noted on the monitor. Choice of transducers for an anatomical location and adjustments of the control panel on the ultrasound unit will also contribute to the quality of the image. Sound waves are produced in the transducer which then enter the patient. While 99% of sound waves are lost as heat, a portion those ultrasound waves are refracted or absorbed by tissues. The remainder return to the transducer as echoes, which are converted to images of tissues and fluid structures on the screen. The speed of the soundwave traveling through the tissue depends on fluid content, tissue density, and the elasticity of the tissues. Certain terminology describes a sound beam as it travels, regarding its speed and interaction with various structures within the body. A sound beam may meet resistance as it passes through tissue. This is referred to as acoustic impedance and depends on the speed of the sound beam and thickness of the tissue. The strength of the echoes returning to the transducer is referred to as echogenicity. There are varying degrees of echogenicity

3 which are then compared to each other as they pertain to a structure. An organ that is brighter than another is considered hyperechoic to that tissue. Tissue that is darker than another is considered hypoechoic by comparison. Two adjacent structures having the same degree of brightness or echogenicity are determined to be isoechoic in comparison. No echoes are produced as sound passes through a fluid-filled structure such as the urinary bladder. This echogenicity is referred to as anechoic. The following chart may make it easier to compare the terms. Artifacts As sound passes through tissues, it loses some of its acoustic energy and is said to be attenuated. Aspects of attenuation may be affected by absorption within a tissue, reflection off a tissue, or a scattering of the sound beam through a tissue. 1 Higher frequency sound beams are attenuated more easily than low frequency beams; therefore, lower frequency beams penetrate deeper into tissues. As the ultrasound beam moves through a variety of tissues, a series of artifacts is created. Attenuation artifacts occur when there is a difference in the energy of sound as it passes through tissues. As tissues or structures attenuate the beam, there is less acoustic energy for tissues deep to those structures, resulting in dark focal areas seen distal to a structure on the monitor. This is referred to as shadowing and occurs when there is almost a complete absorption or reflection of sound produced by structures like bone or gas. Distant enhancement, also known as through transmission occurs when there is an increased strength of sound due to the lower attenuation as it passes through structures, allowing more acoustic energy to reach the deeper tissues. Fluid filled structures such as the urinary bladder, gallbladder, or a cyst appear brighter. These are examples of low attenuation, which do not absorb much acoustic energy, causing enhancement of the beam that reaches tissues deeper to them. Some curved surfaces cause the ultrasound beam to change direction, which is referred to as refraction. The diaphragm is an example of a highly reflective surface which causes part of the sound beam to be reflected while part crosses the boundary. The rest of the sound is reflected to the transducer to create an image on the monitor. Refraction results in three different types of artifacts. At the diaphragm, the most common is the mirror image artifact where a flipped copy of part of the normal image is shown on the other side of the reflector which may mimic the liver and gall bladder as well. 2 The second type is often seen at the margins of the kidneys, causing a dark streak from the edge of the structure. This is referred to as edge shadowing, and it is caused by sound waves striking a curved surface being deflected in another direction. The final form of refraction may cause apparent displacement of a section of tissue from the normal location due to different velocities of sound within the structures. This is referred to as a propagation speed error. An example would be the spleen overlying the left kidney, causing the kidney to appear dented. Some artifacts are developed by the movement of a sound wave and are classified as propagation artifacts. An example occurs when a second ultrasound beam is produced off the axis of the main sound beam. It may occur at a curved and reflective surface such as the diaphragm, or a curved surface such as the urinary bladder. These artifacts are examples of side lobe artifacts and are echoes within structures such as the gallbladder or urinary bladder. They may appear as sludge. 1 Another type of propagation artifact is called a reverberation artifact where the sound beam bounces back and forth between two reflective surfaces, such as gas or metal. An example of an artifact involving gas or metal is referred to as a ring-down or comet tail artifact. Knobology Each ultrasound machine contains basic features on its panel or control bars and varies based on the manufacturer. The price of the ultrasound unit increases with the quality of its features. Some common controls noted on every ultrasound unit include image mode, power, overall gain, time gain compensation (TGC), depth, focus, measurement, annotations, freeze/cine, and image reverse. The first control to be considered is selection of the image mode. Selection is determined by the area to be scanned. B-Mode is brightness mode and displays the returning echoes to the transducer as dots on the monitor in a two-dimensional presentation. The image is displayed as a pie shape with the narrow aspect at the top of the screen representing the probe and the widest area the deeper structures. When scanning the heart, M-Mode is displayed as a strip at the bottom of the screen and the 2D image created by the transducer at the top. Measurements of the cardiac wall are obtained from the tracing, and an evaluation of the valve and wall motion is made from the transducer image. The ultrasound beam is directed over specific areas both in sagittal and cross-section evaluating the heart s various structures. Another control is the voltage or power button and is adjusted to pulsate the crystals in the transducer. The higher the voltage, the higher the intensity of the beam moving through the tissues. By setting the power as low as possible, the resolution is better and the artifacts are minimal. Choosing a proper transducer frequency for the depth being scanned allows this to occur. The overall Gain and Time Gain Compensation (TGC) on the control panel strengthens the returning echoes. The Overall Gain strengthens the returning echoes and overall brightness while the TGC is a series of sliding controls that adjusts the brightness from the near (top) to far (bottom) fields independently of each other. As the sound beam progresses into deeper structures, its strength weakens. Therefore, the TGC is gradually increased as the sound travels deeper. The sliders on the TGC control move to the left to decrease, and to the right to increase the echo strength. With some newer machines, this loss of strength is automatically compensated by the machine, negating the need to adjust the TGC levels except in extreme situations.

4 The area of interest can be made to fill the screen by decreasing the depth. This will bring a structure more to the near field for a closer examination. Conversely, increasing the depth allows an overall view of a structure in the far field, such as the liver. Here, a large mass may be compared to the overall size of the organ being examined. When an image is optimal for capture, the freeze control is pressed. This control may have a different name depending on the manufacturer. Measurements may now be obtained. If there is motion seen in the image that s been captured, the cine control may be activated to roll the image backwards frame by frame until the preferred image is captured and saved. Depending on the manufacturer, the cine control may be associated with the trackball or have a separate control. Once an image is frozen, there are several choices for measuring a structure, including distance(s), elliptical, circumference, or by tracing a structure. The measurement begins by placing the cursor at one edge of the structure, setting the placement, and dragging it to the opposite end to record the length. The cursor lines up on the opposite axis for the width and the process is repeated to record the measurement. Once the measurements are obtained, the structures appearing on the screen are labeled by opening the dropdown menu marked annotations. A title is selected from the list and is transferred to the point of interest on the monitor. on the monitor. The marker points cranially while scanning a structure in the sagittal plane with the patient positioned in dorsal recumbency. The top of the monitor corresponds to the skin surface and is referred to as the near field. The bottom of the screen demonstrates the more dorsal structures and is referred to as the far field. The left side of the monitor corresponds to cranial and the right side of the screen corresponds to caudal. In the transverse or cross-sectional plane the left side of the screen becomes the right side of the patient as the transducer s ID marker denotes. Conversely, the right side of the screen corresponds to the left side of the patient with the orientation at the top and bottom of the monitor cranial and caudal, respectively. Transducer Selection Choosing a proper transducer (probe) to examine an area of interest may assist with the reduction of artifacts. However, understanding the mechanics of a transducer is a precursor to choosing the proper probe. Sound is produced by the transducer in pulses which energize hundreds of piezoelectric crystals located in the transducer head. These crystals send pulses of ultrasound waves through the body by vibrating when voltage is applied by a pulser. The transducer acts as both a transmitter and a receiver. The image on the monitor is formed when echoes return to the probe after traveling through the tissues. The frequency range by the transducer depends on both the size and characteristics of the crystals. Transducers produce a pie shape or a rectangular shape on the monitor. The curvilinear probe produces the pie shape and examines deeper structures in the far field (3cm-30cm) with a lower range of frequencies. The rectangular or linear probe provides detail for the near field (2cm-9cm) at a range of higher frequencies. Array probes signal high and low frequencies created by the crystals, and are designed as linear, small and large curvilinear, and phased array, which range an average of 1-13 MHz in frequency collectively. Sound is produced by the transducer in pulses which energize hundreds of piezoelectric crystals located in the transducer head. These crystals send pulses of ultrasound waves through the body by vibrating when voltage is applied by a pulser. Transducer Orientation Every ultrasound unit contains a control to reverse the image viewed on the monitor. The screen orientation is usually located on the upper left corner of the image on the monitor when scanning the abdomen. However, when the heart is to be scanned, the image is reversed so the cranial aspect of the heart is located on the right side of the screen. Sonographers may scan the abdomen with the patient in either a ventrodorsal position, oblique, or in lateral recumbency. Each transducer has an identification marker located to the side of the transducer head which corresponds to the icon

5 Table 1: Echogenicity Comparisons ANECHOIC No Echoes Not Echogenic Black HYPOECHOIC Fewer Echoes Less Echogenicity Darker HYPERECHOIC More Echoes More Echogenicity Brighter ISOECHOIC N/A The Same Echogenicity Same As Compared Tissue The rule of thumb when deciding on a transducer is to use the highest frequency possible to penetrate a desired depth. The frequency of the probe may be changed to accommodate a specific depth. The same probe may also be used for a more superficial area on the same patient. The higher the frequency, the more detailed the image is on the screen. However, depth of field is limited. With a lower frequency, depth may be achieved but higher detail may be sacrificed. In choosing a good overall probe for a small animal clinic, a microconvex or small curved array may be an ideal choice. Its frequency ranges from 3-9 MHz and can penetrate up to 15 cm in depth. 4 The higher frequency linear probe is recommended as a second probe for smaller breeds, cats, or more superficial, smaller anatomical parts such as the thyroid gland. Scanning Technique There are three basic movements made with the transducer during an abdominal ultrasound. In one movement, the sonographer slides the transducer across a structure for an overall view. In another, the contact surface of the probe is stationary and the probe will rock or sweep back and forth slowly over the same area to view the structure in a different plane. The last movement is rotation of the probe, which may give a slightly oblique view of a structure or view it in a transverse, or cross-sectional plane. Each structure must first be examined in a sagittal plane followed by a cross-sectional plane. It cannot be stressed enough that probe movements must be made slowly and minimally. It does not take more than a millimeter or two to change the location being viewed. The abdomen must be scanned in the same manner each time and for every patient. This trains the human eye to note subtleties. For a ventral approach, the patient is placed in dorsal recumbency to mimic a VD radiograph of the abdomen. When first learning to ultrasound, this allows the sonographer to easily think in three dimensions while scanning. The patient is clipped from the xyphoid process caudally to the inguinal area and laterally to each edge of the ventral abdomen. The patient must be clipped close to the skin since hair creates artifacts. Unlike scanning the patient in lateral recumbency, the ventrally clipped area will not be visible as the animal moves in the standing position. Warm water or alcohol is then applied to the clipped abdomen, followed by a generous portion of coupling gel. Since the gel is water soluble, multiple repeat applications may be necessary throughout the scan. When initially learning to ultrasound the abdomen, the periphery of the abdomen is scanned first, followed by the medial structures. Once the sonographer becomes more experienced, the choice to change the order of scanning may arise. Many sonographers start with the liver, stomach to duodenum, pancreas, spleen, left kidney and adrenal, urinary bladder and so on. Again, it is important to scan in the same manner each time. The natural order of scanning for those new to ultrasound begins with the right liver, gallbladder, mid-liver, left liver, stomach, and spleen. It then proceeds to the left kidney and adrenal, urinary bladder, right kidney, and adrenal. Once the periphery scan is completed, the duodenum, pancreas, small intestine, lymph nodes, and vessels are examined. Standard baseline images are captured by the technician, completing the exam. The veterinarian may wish to send images to a radiologist for input on the study. The abdominal structures are examined in both sagittal and cross-sectional planes as well as specified standard slices. Abnormalities require cine clips or video for the veterinarian to observe in addition to still images for the client. Specific sections of the organs must always be examined since they may be affected by abnormalities. Baseline measurements are obtained of various structures throughout the ultrasound exam. Both kidneys, adrenals, prostate, and uterus are measured in both axes, while the walls of the bladder, duodenum, stomach, and representative samples of small intestines are measured from the lumen to the outermost layer. Any abnormalities such as masses or cysts are also measured in length and width. Solitary abnormalities or those that are more generalized must be recorded by the technician in the form of still images and cine clips for the veterinarian to evaluate. Any known abnormality must be brought to the veterinarian s attention during the time of the study.

6 Understanding the anatomy of the patient will assist veterinary technicians in becoming valuable sonographers. Practicing on personal pets or spay and neuter patients for an extra 5-10 minutes of anesthesia time will expedite the learning process. The following images must be consistently captured to complete the abdominal study. Images of the liver include, but are not limited to, the right lobes including the gallbladder, mid-liver and porta hepatis, and left liver lobes. The stomach will come into the field of view at the left to medial liver lobes. The stomach wall must be measured in an area where the stomach wall is stretched as opposed to a completely empty stomach due to contraction of the rugal folds. The spleen is imaged in three sections with abnormalities as additional views. The first section is the head of the spleen, followed by the hilar area, and the tail. The splenic vein is observed exiting the hilus and may be surrounded by echogenic fat which is a normal finding. The tail of the spleen should taper evenly. There is a minimum of six images obtained per kidney. The first sagittal image demonstrates both the cortex and medullary areas. The medulla has a hypoechoic scalloped appearance. A thin membrane surrounds the kidney. Sliding the probe medially allows the echogenic hilar area or renal pelvis to be visualized. Sliding the probe slightly lateral will demonstrate the lateral cortex. The cross-sectional images begin with a 90 turn counterclockwise. The hilus appears as a fibrous, echogenic linear area exiting the kidney medially. The probe slides to the cranial and caudal poles to further examine the cortex. The adrenal glands lie medial to the cranial poles of the kidneys. The left is generally located where the renal artery meets the aorta. The right appears to be part of the vena cava as it is bordered so closely medially that they appear to overlap. Measurements are obtained both in length and width. The bladder, prostate, and uterus are in the same vicinity when scanning. The urinary bladder is imaged in both planes at the apex and at the trigone. Measurements of the bladder wall are obtained by slightly rocking the probe until the wall becomes sharp in appearance. The trigone image is captured as it tapers caudally. Just caudal to the bladder lies the prostate which is also imaged in both planes. The transverse view allows the sonographer to observe its bi-lobed appearance and to obtain measurements. Here, the lobes of the prostate are compared for texture and size. The uterus lies between the colon and the bladder. It may be easier to image in the transverse plane initially. By rotating the probe 90 clockwise, the sagittal plane may be viewed. The pancreas may be difficult to find since it has similar echogenicity to the surrounding mesenteric tissue. Understanding the anatomy will help locate the pancreas. The body of the pancreas lies between the pyloric antrum and the duodenum. The right limb is ventral to the right kidney and medial to the descending duodenum. The left limb lies between the greater curvature of the stomach and the transverse colon with the spleen to its left. 3 Breed variation will slightly affect the location of the pancreas. The right limb of the pancreas in the cat is smaller. Since the right limb lies along the duodenum, it is usually easier to find the pancreas in the transverse plane. The duodenum will hug the right kidney and the right limb of the pancreas is located medial to the duodenum in transverse. The duodenum can be measured from the lumen to the outermost serosal layer. Once the duodenum has been measured, it is good practice to measure a few representative samples of small intestines and lymph nodes. The small intestines are imaged methodically from the duodenum across the abdomen, sliding a little caudal, across the abdomen again and so on. The loops are compared to each other for wall echogenicity and uniformity of the wall layers. Lymph nodes appear in clusters along arteries and veins throughout the body and come in all shapes and sizes. Since they act as filters to trap bacteria, toxins, and cancer cells for elimination from the body, lymph nodes are affected by inflammation or disease. They usually become large and hypoechoic, but may occasionally be mixed in echogenicity. When examining the lymph nodes, it is important to observe them in both planes to confirm their shapes and location. Understanding the anatomy of the patient will assist veterinary technicians in becoming valuable sonographers. Practicing on personal pets or spay and neuter patients for an extra 5-10 minutes of anesthesia time will expedite the learning process. This allows technicians to become familiar with the normal sonographic anatomy, enabling them to more easily notice abnormalities. Veterinarians are then free to concentrate on other pressing issues. References 1. Mattoon JS, Nyland TG. Fundamentals of diagnostic ultrasound. In: Mattoon, JS, Nyland TG, ed. Small Animal Diagnostic Ultrasound. 3rd ed. St. Louis, MO: Saunders, an imprint of Elsevier, Inc; 2015: Lisciandro GR. Focused Ultrasound Techniques for the Small Animal Practitioner. Wiley Blackwell; Trevail T. Imaging the Pancreas. Veterinary Ireland Journal. 2015;5: may 2015.pdf. Accessed Dec 20, 2016

7 Article Questions 1. When doing an ultrasound scan of the abdomen, echogenicity refers to a. The strength of the echoes returning to the transducer b. The lack of echoes visualized in an organ c. The image created when it hits a highly reflective surface d. A control on the ultrasound unit 2. Time gain compensation is a control which a. Reverses the image b. Determines the mode to use c. Adjusts the brightness from the near to far field independently of one another d. Strengthens the returning echoes for overall brightness 3. A type of artifact that bounces back and forth between two reflective surfaces such as gas or metal is referred to as a a. Attenuation artifact b. Distant enhancement c. Side lobe artifact d. Reverberation artifact 4. What percentage of the ultrasound beam is lost as heat while the remainder is converted to an image on the monitor? a. 79 % b. 99 % c. 59 % d. 89 % 5. Through transmission is also known as a. Shadowing b. Ring-down c. Distant enhancement d. Side lobe QUIZ ONLINE CONTINUING EDUCATION visit VetMedTeam.com and log in with your Vet Med Team Profile. Valerie Gates, CVT, VTS (ECC) Val has instructed imaging to veterinary and technician students at the University of Illinois Veterinary Teaching Hospital for 15 years. She then moved to Colorado to enjoy the mountains and work in a Specialty Hospital in Internal Medicine department for 7 years, followed by the Emergency Medicine department for 5 years. Val received her VTS in Emergency and Critical Care and is presently working for Echosphere Ultrasound/We See You Support, where she instructs veterinarians and their staff on the art of ultrasound via the internet. When not working, she spends time with her Friesian mare and 2 cats (who occupy my lap until they decide they re done). A puppy is soon to join the mix, which will be sure to infuriate the kitties!