Jennifer Brisson, DVM, DACVR
Massachusetts Veterinary Referral Hospital, Woburn, MA
Posted on 2016-11-29 in Diagnostic Imaging
The use of ultrasound is widespread and becoming a standard of care in veterinary medicine across the diversity of practices, and not just specialty medicine. Understanding basic ultrasound physics is paramount to success in image optimization during patient evaluation. While there are proprietary differences and generational differences in the user controls of an ultrasound machine, the following basics are present in each system.
Introduction to Knobology
Dynamic application and manipulation of the basic controls will provide for enhanced image optimization and increased abnormality conspicuity.
Adjustment of power is generally limited to the start of the exam, if manipulated at all. This should be set to a relative standard for each ultrasound machine. Keeping power the same across the diverse patient caseload contributes to the identification of abnormal tissue pathology.
Gain should be manipulated for each patient, and often throughout the individual exam. The more variable penetration in depth required during a patient examination, the more manipulation of the gain control may be needed. Gain controls the amplification of echoes returning to the probe, which is used in image formation. Gain controls the overall level of brightness of the image. Specifically 2D gain establishes the gray scale brightness across the entire image.
The Time-Gain-Compensation controls, usually a series of slides, allow for manipulation of gain segmentally by depth. Adjustment of the TGC slides allows the user to lessen the enhancement of the returning near field echoes, and increase the enhancement of the far field echoes, allowing for a more uniform display in the shade of gray across the tissue of interest/scan field of view. Alternatively, adjustment of the TGC slides allows for optimal gain settings to be set for a lesion or structure of interest, and minimize the signal from the surrounding structures.
Focal Zone should be set to (1) the depth of the lesion of interest, or (2) the maximal depth of an organ. This control should be dynamically changed as needed during patient examination, for full organ evaluation and lesion interrogation. Focal zone is the portion of the ultrasound beam with maximum lateral resolution. Lateral resolution is the ability to define two points perpendicular to the ultrasound beam as separate and distinct. The smaller the lateral distance between two points, the greater the lateral resolution. This contributes greatly to lesion conspicuity.
Image optimization starts at probe (transducer) selection. Diagnostic ultrasound ranges from 2-18MHz. Higher frequency transducers offer greater resolution, greater fine detail, with less depth penetration. High-frequency transducers refer to those with an ultrasound frequency greater than 10MHz. These are often in linear configuration. Low or mid-range frequency transducers (2-8 or 5-10MHz respectively) offer greater depth penetration and more versatile use, but less detail resolution. Select the transducer appropriate to patient size for the desired examination. Select the transducer with the maximal resolution for the depth required.
Following transducer selection, set depth to the desired field of view. This should be manipulated continuously to evaluate entire organs and specific structures. With changes in the field of view, manipulation of the focal zone should then be considered. This will optimize lesion conspicuity and image resolution. For some newer ultrasound units, this may change automatically to match depth field of view. And ultimately fine tuning of gain will enhance an image.
Focused Assessment with Sonography in Trauma (FAST)
The FAST ultrasound technique was first developed in the human emergency room to assess patients following blunt force trauma for the presence of effusions. This groundbreaking work demonstrated the invaluable tool ultrasound could be used in a triage setting. From that initial study, numerous others have further demonstrated the importance of ultrasound in initial and recurring patient assessments across diverse ailments. Point of care ultrasound is fast, minimally invasive, dynamic, and can be performed by non-radiologists with ease.
A modified abdominal FAST technique was first described in veterinary medicine in 2004 by Boysen SR, et al. Since then this technique has become widely utilized and is routinely incorporated into patient assessments at veterinary emergency and specialty care facilities. Detection of abnormalities on FAST exams can aid in diagnosis and direction of additional diagnostics. Abnormalities, such as effusions, can be sampled via abdominocentesis, and then monitored over time to assess for progression or resolution.
The FAST technique for the abdomen consists of a four quadrant approach. The patient may be in variable recumbency depending on clinical presentation. Dorsal recumbency evaluation of the abdomen is recommended if possible. Alternatively, bilateral lateral recumbency evaluations can be performed. The first part of the four quadrant approach is to place the transducer in a subxiphoid position and evaluate the hepatic and perihepatic space. Evaluation should extend beyond the depth of the diaphragm if possible. This allows for evaluation of both the caudal thorax and cranial abdomen simultaneously. The second transducer position is mid left lateral abdomen to evaluate the perisplenic and left perirenal region. The third transducer position is prepubic to evaluate the peripelvic structures, including the urinary bladder and caudolateral abdominal recesses. The fourth transducer position is mid right lateral abdomen to evaluate the right perirenal region.
A FAST technique has also been developed for the thorax and consists of a five-point approach, however also depends on patient recumbency. If possible, bilateral assessments should be made with the patient in sternal recumbency. The cranioventral thorax and pericardial space should be evaluated bilaterally via a para-axillary intercostal approach. The caudodorsal thorax should be evaluated bilaterally via an intercostal approach. It is recommended to start in the 8th and 9th intercostal spaces dorsally, and then adjust cranially as needed given breed conformation to view caudal dorsal pleural space and lobar parenchyma. If permissible, a subxiphoid evaluation completes the five-point assessment for interrogation of the ventral pericardial space and caudal pleural space.
These techniques can be easily incorporate into daily practice of the primary care veterinarian for patients presenting with diverse illnesses. Understanding and application of the basic ultrasound controls can increase the yield of diagnostic information from FAST exams of the thorax and abdomen.
- Boysen SR, Rozanski EA, Tidwell AS, et al. Evaluation of a focused assessment with sonography for trauma protocol to detect free abdominal fluid in dogs involved in motor vehicle accidents. J Am Vet Med Assoc 2004;225:1198-204.
- Lisciandro GR. Abdominal and thoracic focused assessment with sonography for trauma, triage, and monitoring in small animals. J Vet Emerg Crit Care 2011;21:104-22.
About the author
Dr. Brisson received her Doctor of Veterinary Medicine with thesis, from Tufts University, Cummings School of Veterinary Medicine in 2004. She completed a one-year internship in small animal medicine and surgery at the Veterinary Referral and Emergency Center in Norwalk, CT, followed by a three-year residency in Diagnostic Imaging at Tufts University in July of 2008. Dr. Brisson became board certified by the American College of Veterinary Radiology in September 2009.
Dr. Brisson has received extensive training in all modalities of veterinary radiology, including magnetic resonance imaging (MRI), computed tomography (CT), radiography, nuclear medicine, and ultrasound. Her professional interests in diagnostic radiology include abdominal MRI, gastrointestinal and hepatobiliary ultrasound, and thoracic computed tomography. Dr. Brisson also has a strong professional interest in Interventional Radiology.
Dr. Brisson joined the radiology department at Massachusetts Veterinary Referral Hospital in July 2009. In 2014 she was promoted to Assistant Medical Director for the surgical services.