ULTRASOUND AS A DIAGNOSTIC TOOL
Ultrasound has become a useful diagnostic tool, both in the emergency setting and in field of Internal Medicine. Ultrasound can be useful in determining the location or tissue of origin of abdominal masses, identifying thickening to the gastric or intestinal walls, loss of normal layering to the intestinal or gastric walls, intussusception or dilation of the intestines. The architecture of the liver, spleen, pancreas, kidneys, bladder and adrenal glands can all be assessed in more detail (Figure 1). Free fluid can be identified in smaller amounts than can be appreciated on abdominal radiographs.
Figure 1: An ultrasound image of a nodule within the spleen of a dog. This mass effect is too small to distort the shape of the spleen and would not readily be seen on a radiograph.
History: Acoustics, sound waves and echoes have fascinated scientists for centuries. Leonardo De Vinci, Galileo Galilei, Sir Isaac Newton to name a few. In 1877, Lord Raleigh published a description of sound as a mathematical equation. The piezo-electric effect was discovered by the Jacques and Pierre Curie a few years later. This is defined as an electric potential that is generated when mechanical pressure is applied to a quartz crystal.
More advances were made in the early 1900s leading to the development of a high frequency ultrasonic device by Langevin and Chilowsky called a hydrophone. This machine sent and received ultrasonic signals underwater. The Titanic disaster and World War I accelerated research resulting in the development of SONAR (Sound Navigation and Ranging) and RADAR (Radio Detection and Ranging) using electromagnetic waves.
The first application of ultrasound in medicine was published in 1942 by Karl and Friedrich Dussik in Vienna. Over the following decades ultrasound has evolved to include two, three and four dimensional applications, as well as color-flow Doppler.
Physical Properties of Ultrasound:
Ultrasound is characterized by sound waves with a frequency higher than the upper range of human hearing. Audible sound frequency for humans is between 50 to 20,000 kilohertz (1 kHz = 1000 cycles per second). Most machines use transducers that emit frequencies from 2 to 15 megahertz (1 MHz = 1 million cycles per second). Crystals in the transducer (probe) vibrate to emit low-intensity, high frequency sound waves. The crystal emits ultrasound waves about 1% of the time. These waves are directed at body tissues. As the waves strike the tissues, some waves are reflected back to the crystals, which act as a receiver 99% of the time. Then the probe receives these returning signals it converts these "echoes" into electrical energy. The computer interprets this energy into an image, displaying various shades of grey on the computer screen. The stronger the returning echo, the brighter the point is on the screen, i.e. tissues that are more reflective will appear brighter than tissues that absorb sound. The time between emission of the impulse and the return of the reflected echoes depends on the depth traveled and the density of the tissue the echoes travel through. The computer will calculate all this information and correlate it with a specific site on the monitor.
Images are processed in two main "modes," B-mode (Brightness mode) and M-mode (Motion mode). B-mode places dots on the screen which correspond to the depth of the origin of the echo. M-mode focuses a smaller bean of sound over a time period. This
mode is primarily used in echocardiography and to monitor blood flow. B-mode will still show motion in general.
The depth to which sound penetrates into soft tissue is directly related to the frequency employed. Higher frequency (better resolution) sound waves are attenuated more than lower frequency waves.
Meaning any attempt to improve resolution by increasing the frequency will invariably decrease penetration.
Terminology:
- Echogenic/echoic - means that most of the sound is being reflected back to the transducer, images appear lighter or white
- Echotexture - the image pattern of the tissue being examined
- Anechoic - describes tissues that do not reflect any sound waves, thus appearing black (i.e. fluid)
- Hyperechoic - tissue that reflects more sound back to the transducer and appears brighter than surrounding tissue
- Hypoechoic - tissue that absorbs more sound and appears darker than surrounding tissue
- Isoechoic - tissues that appear the same as surrounding tissue
- Homogenous - uniform
- Heterogeneous - mixed
Acoustic impedance is the ability of living tissue to impede the transmission of sound. This quality varies dependent upon the density of the tissue, thus making it possible to distinguish between types of tissues and organs (i.e. kidney vs. liver vs. bladder). Also, since the density of individual organs differ, the speed (velocity) at which the sound waves travel through each organ (and are reflected back to the transducer) varies slightly. This helps refine the image and give more architectural detail so that we can distinguish the renal cortex from medulla or the layering to the gastrointestinal tract, etc. (Figure 2).
Figure 2: Small intestinal loop with a normal
wall thickness and wall layering.
FAST³: Focused Assessment with Sonography for Trauma, Triage, and Treatment. This is a rapid, non-invasive and easily performed procedure for the assessment of intra-abdominal or intra-thoracic injury leading to fluid accumulation that can be done during the initial examination and stabilization of the pet.
It is not intended to be an extensive examination of all the internal organs and is not designed to detect parenchymal injury. Its main purpose is to look for the presence of free fluid in the shortest time possible. It is used for the detection of free abdominal fluid, pleural and pericardial effusions, and free air in either the abdominal or thoracic cavities. This diagnostic should be considered as an extension of the physical examination while patients are undergoing resuscitative therapy to provide a diagnosis and help direct treatments. It may also be used to help retrieve samples of any fluid for analysis.
The conditions in which FAST³ emergency ultrasound can be of benefit are post-traumatic abdominal effusions, suspected abdominal mass effects leading to abdominal effusions, pleural effusion, pericardial effusion, pneumothorax, and pyometra. Examination can usually be done in about 5-10 minutes. The terms "AFAST³" and "TFAST³" are used in veterinary medicine for abdominal and thoracic scans, respectively. A diagram of an AFAST³ scan is shown in Figure 3. The scan is performed in right lateral recumbency preferably. The fur is NOT shaved to save time. Ultrasound gel and/or isopropyl alcohol are applied to part the fur. Alcohol should NOT be applied if electrical defibrillation is anticipated.
Figure 3: A schematic of the AFAST³ scan.
1) Diaphragmatic-Hepatic (DH) view. This view is also used to image the pleural and pericardial spaces.
2) Spleno-Renal (SR) view. This view is also used as a window into the retroperitoneal space.
3) Cysto-Colic (CC) view.
4) Hepato-Renal (HR) view. This view is often a site for obtaining abdominocentesis.
Full Abdominal Ultrasound: A more thorough ultrasound evaluation may be required in certain situations such as patients with abdominal masses, ascites, vomiting, anorexia, fever, etc. Vomiting is a common clinical sign of many disease states. Foreign body obstruction, acute gastritis, renal disease, hepatic disease, hypoadrenocortisism, pancreatitis, inflammatory bowel disease and neoplasia are just a few
possible causes of vomiting. Ultrasound can often be a useful diagnostic tool in determining the cause of vomiting.
In gastritis vomiting is usually acute in onset. It is suspected to be caused by gastric mucosal insult or inflammation of the gastric mucosa. In almost all cases inflammation is not proven by histopathology. In many cases the cause can be inferred from the history. The most common causes of acute gastritis are dietary indiscretion, foreign bodies, medications or toxins, parasites, or other systemic disease. Biopsy is rarely necessary as these patients as they either resolve their symptoms on their own or with minimal supportive and symptomatic therapy. Therapy usually involves fluids, antacids, diet change and anti-emetic medication. Most of the time owners are instructed to withhold food and offer small amounts of water for a period of 12-24 hours. Small amounts of water orally are usually tolerated even in the face of intermittent vomiting episodes. Dehydrated patients can benefit from subcutaneous fluid therapy if they are being treated on an out-patient basis. A bland diet is also often recommended. If the signs resolve in 1 to 2 days with or without supportive therapy, the tentative diagnosis of acute gastritis is considered correct. Those patients that present with additional clinical signs such as diarrhea, melena, hematemesis, etc. may require a more thorough diagnostic evaluation.
Foreign bodies are often seen more in young dogs or cats, though older pets are not exempt especially if dietary indiscretion or garbage raiding has occurred in the past. Toys, rocks, coins, string, sewing needles with thread, grasses, cloth, corn cobs, acorns...the list is endless. Some of these can be seen on radiographs easily. Others are more subtle and may require abdominal ultrasound. Often the exact foreign body cannot be "visualized" by ultrasound however there may be other clues such as dilation of segments of the intestinal tract, material coursing through the pylorus into the duodenum, or acoustic shadowing indicating that there is an object through which ultrasound waves cannot pass (Figure 4). Sometimes endoscopy is useful in removing the object from the esophagus or stomach without the need for a full abdominal exploratory surgery. If the material is too large, potentially caustic, or has passed into the duodenum or beyond, abdominal exploratory surgery is recommended (View video, "Bush", 1-year old, female Laborador, repeated vomiting).
Figure 4: An ultrasound image of the descending duodenum in a dog with repeated vomiting. The actual object is not clearly defined however there is acoustic shadowing distal to the object within the lumen of the duodenum casting a shadow.
Layering of the intestinal mucosa can be seen on ultrasound. Normally, five layers can be appreciated: the hyperechoic lumen/mucosal interface, the hypoechoic mucosa, the hyperechoicsubmucosa, hypoechoicmuscularispropria and the hyperechoic serosa (Figure 5). The intestinal wall measures 2-3mm in thickness and up to 5 mm in larger breed dogs. The duodenal wall measures up to 5 or 6 mm in thickness. Thickening can be noted at any point along the gastrointestinal tract or be diffuse.
Figure 5:
Normal layering to the small intestine as noted on an abdominal ultrasound. 1) mucosal interface 2) mucosa 3) submucosa
4) muscularis propria 5) serosa
Infiltrative disease such as plasmocytic lymphocytic enteritis and lymphangiectasia may cause thickening of the bowel wall, but usually without disruption of the wall layers. Large cell lymphoma will usually cause a mass effect somewhere along the intestinal tract, whereas small cell lymphoma causes a diffuse thickening (Figure 6).
Lymphadenopathy may be seen with IBD and lymphoma. Ultrasound provides insight to which areas may be more affected (stomach, duodenum, ileum, etc.). This in turn can help distinguish if endoscopy can be effectively used to obtain quality biopsy samples versus abdominal surgery to obtain full thickness biopsies.
Ultrasound plays an important role in veterinary medicine. Whether it is used to assess chronic disease or in the emergency setting, ultrasound provides useful information that has helped change and direct therapies for many diseases.
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Tech Tip
Easier to breathe on my back!
BRACHYCEPHALIC AIRWAY SYNDROME (BAS)
Brachycephalic breeds have become more popular in recent years. We now recognize early intervention surgically, at a young age, will avoid the more difficult and expensive surgical procedures when these dogs are older. The initiating abnormality is stenotic nares. This is followed by elongation of the soft palate and everted laryngeal saccules. We now do much more upper airway surgery than we did in the past. Early in the disease surgically removing the tissue narrowing the nasal openings and later in the disease when the classic triad is seen: Stenotic nares, Elongated soft palate, and Everted laryngeal saccules.
Technicians play a key role in facilitating effective treatments to patients suffering from Brachycephalic airway syndrome (BAS). As exam room technicians you will begin to recognize the very narrowed nasal openings in young puppies when receiving their early vaccinations. As surgical technicians you are a vital part of the anesthetic and surgical management of the disease. BAS is a condition affecting short-headed dogs and cats. These patients may suffer from stenotic nares (narrowed nostrils), elongated soft palate, everted laryngeal saccules, and hereditary hypoplastic tracheas. Pug, Pekinese, Maltese, Boston Terriers, Shih Tzu, French Bulldogs, and English Bulldogs are common canine breeds affected, and the Persian and Himalyan are among the cats. The symptoms are classic of many upper respiratory conditions, including inspiratory stridor and stertorous breathing, cyanosis, hyperthermia, exercise intolerance, excitability, leading to collapse in severely affected patients. Owner's may also report coughing, gagging, and vomiting.
Sedatives, such as acepromazine are often recommended to help relieve anxiety and excitement, as well as reduce the incidence of regurgitation. A complete physical exam, including auscultation of the chest and tracheal sounds, along with tracheal palpation for abnormalities is done on all patients. Right and left lateral, and ventrodorsal chest radiographs are taken to check for evidence of aspiration pneumonia or heart disease. Lateral cervical radiographs should be taken to determine tracheal diameter there as well, as they can have both cervical and thoracic hypoplastic tracheas. Radiographs can be taken with the patient under light sedation, such as butorphanol (0.2-0.4 mg/kg) and acepromazine (0.01-0.03 mg/kg) given IM or IV and flow by oxygen delivered via face mask. Because there is a risk of vagal stimulation with many of these patients, an anticholinergic, such as atropine or glycopyrrolate (0.1 mg/kg IM), is given intramuscularly (IM) as a premedication to prevent bradycardia. Metoclopramide (Reglan) can be used to help reduce the incidence of regurgitation.
Other considerations for technicians is to always use a laryngoscope during oral exams and intubations. Just because you are capable of intubating without the aid of a laryngoscope, it does not mean you should. Light is necessary in recognizing potential irregularity or irritations of the oral cavity, that may be missed in the dark. Always have oxygen and a variety of endotracheal tubes (ETT) available when administering sedation to patients affected by airway disease, often the ETT size is over estimated for patients with hypoplastic tracheas. It is helpful to have a rigid stylet, such as a polypropylene urinary catheter, to aid in the intubations of cats or small dogs. Many of these patients are administered steroids, so nonsteroidial anti- inflammatory drugs, NSAIDS, should be avoided due to the risk of GI ulceration that can lead to GI perforation.
When the airway is obstructed by stenotic nares and the amount of air required by the lung is not achieved, the pressure on the area is increased. The increase in pressure acts like a vacuum and pulls on the soft palate and surrounding tissues. Stenotic nares greatly reduces the amount of air the patient can breathe. Surgical treatment is required to resolve the clinical signs. The surgery option available for stenotic nares varies but the ultimate result is the same, a larger nasal passage. Surgical repair is recommended at 3 -4 months of age, but can be done as early as 9 weeks in clinically affected patients. The sooner stenotic nares are fixed, the less likely the patient will have to be treated for elongated soft palate and everted laryngeal saccules. An alar fold (obstructive nasal folds ) resection can be performed on very young dogs. Because the alar folds are too small to allow primary wedge removal and closure with sutures we no longer suture the tissue. Following removal of the nasal folds, at any age, they heal well without suturing. Laser can be used, however the owner should be warned the nares will be white afterwards but will turn back to the original color (usually black) within 2-4 months.
Dogs with elongated soft palates will suck the soft palate back during inspiration, covering the larynx. A computed topography (CT) evaluation of the soft palates of brachycephalic breeds were shown to be thicker than non-brachycephalic breeds. The soft palate it considered too long if it hangs down 1-3mm below the level of the epiglottis. During a soft palate resection surgery the patient is intubated, positioned in sternal recumbancy and the head is elevated so the mandible can hang open. Another method to keep the mouth open during pharyngeal/laryngeal surgery is to place two equal size mouth gags on the canine teeth to hold the mouth open. These can be held by the surgical tech to position the head so the surgeon can see the pharynx and larynx well during surgery. A bright, narrow focus light source is necessary for good visualization by the surgeon. The redundant soft palate tissue is excised, traditionally, by a cut and sutures technique, and a 3-0 or 4-0 monofilament absorbable suture ( PDS ) is placed to approximate the wound and control hemorrhage. Laser and radiofrequency cautery are both acceptable alternatives, often much faster than the traditional method and have similar clinical outcomes.
Laryngeal saccules are located behind the arytenoid cartilages in the larynx and when everted they block the opening of the larynx. They are lateral to each vocal cord and " bulge " or "balloon " out obstructing the larynx.The surgeon may elect to remove the saccules if they are significantly blocking the airway. Often the patient needs to be extubated for this procedure, so IV anesthetics (i.e. Propofol) should be available during this procedure. The surgeon can simply remove the saccules with long scissors or cup-forceps. There is no surgical treatment for hypoplastic trachea.
BAS patients are at risk for aspiration pneumonia when heavily sedated. In dogs the aspiration can be silent, so a rapid recovery and late ETT extubation is recommended. If there are any concerns, the patient's neck is shaved and prepped in case an emergency tracheostomy is needed. If the soft palate has been shortened a soft food diet is recommended for 10-14 days post-op. Steroids; prednisone (0.5-1.0 mg/kg PO) or dexamethesone (1 mg/kg IV) is given to decrease edema and inflammation after surgery. Antibiotics are recommended prophylactically for an appropriate period of time. The outcome is favorable in young dogs when treated early for stenotic nares. If nares surgery is not done at a young age then it is often necessary to correct the nares, soft palate and laryngeal saccule protrusion in adulthood. At either age the results are often dramatic. They snore much less, can exercise more easily, become more tolerant of warm summer days, thus enjoying life more.
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Ultrasound Corner
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The following video is presented by Dr. Brian Poteet. It explains the case of "Bush" a 1 year old, female Laborador that presented for repeated vomiting. He compares radiographs images to ultrasound images.
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Continuing Education Opportunies
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All our lectures provide 2 hours of Continuing Education Credits. You can register online through our websites, Boston Veterinary Specialists and Cape Cod Veterinary Specialists. A meal is provided during each lecture. Your technicians are welcome as well.
BVS:
Dr. William Henry:
February 13, 2013, "Newer Palliative Treatments for Osteogenic Sarcoma"
Dr. Catherine Briere:
March 6, 2013, "Hip Dysplasia" CCVS:Dr. Kimberly Bebar:February 12, 2013, "Doc, Fluffy just stopped breathing!" Now what?: RECOVERing pets from cardiopulmonary arrest. Part II: a CPCR hands-on lab at CCVS. It is the first hands on lab at CCVS. Dr. Elizabeth Martin:
March 19, 2013, "Hospital hygiene and infection control" Dr. Katherine Westcott:April 23, 2013, "Immune-mediated polyarthropathy (IMPA) in dogs" Dr. Louisa Rahilly: May 21, 2013, "Steroids in veterinary medicine: Friend or Foe." Dr. Daniel Beaver: June 4, 2013, "Hip Dysplasia" |
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Visit Our Newsletter Archive
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Read our December newsletter article - Part Two: Osteogenic Sarcoma by visiting our
newsletter archive!
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