Gabriela S. Seiler, DECVDI, DACVR
Department of Molecular Biomedical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC, USA
Diagnostic ultrasound is widely available in veterinary practice. Point-of-care ultrasound, also called FAST scanning, is a valuable method for initial assessment of a patient presented to the emergency room in order to determine if free fluid is present or not. If fluid is detected, sampling is indicated in most cases and ultrasound guidance is very helpful—in some cases necessary. Ultrasound in general is a very sensitive method to detect pathology, but unfortunately in most instances, it is also not specific enough to diagnose specific disease, or even to decide if a benign or malignant disease process is present. Tissue sampling is therefore often used to further characterize a lesion or an organ where disease is suspected. Ultrasound can also aid in performing cystocentesis, especially in patients that are obese or have a small urinary bladder. Diagnostic ultrasound in dogs and cats is enhanced by the opportunity to directly obtain tissue samples, but there are risks associated with the procedure such as hemorrhage, leakage of fluid from a cystic structure, or inadvertent puncture of gastrointestinal tract. It is therefore extremely important for ultrasonographers who do fine-needle aspiration (FNA) to have the expertise and experience to perform the sampling procedure safely.
Preparing the patient properly is the first step in ensuring success of the procedure. First of all, the lesion identified has to be determined to be accessible with ultrasound guidance. This means that there has to be a path between the lesion and skin surface where there are no other organ systems or vascular structures that have to be perforated to reach the target. The target lesion has to be superficial enough for the length of the needle used. Doppler ultrasound should be used to determine how vascular a lesion is and if there is a safe approach without large vessels that may represent a risk for hemorrhage. Sedation is recommended for fine-needle aspiration except for cystocentesis and abdominocentesis. FNA can be performed in the awake animal or with local anesthesia, but unexpected movement is more common, and the pain and discomfort of multiple aspirates for the patient should be avoided. Sedation protocols vary and should be selected based on the patient, and also based on accessibility and estimated risk of the procedure. For example, FNA or a large subcutaneous mass is much less risky and challenging than aspiration of the gallbladder or a lymph node. In some patients, general anesthesia may be necessary to completely avoid motion and to provide control of the respiratory movement. Respiratory movement particularly increases difficulty of fine-needle aspirates in the cranial abdomen (liver, gallbladder).
Fine-Needle Aspiration Techniques
Two basic techniques are available: free-hand technique and usage of a needle guide. A needle guide is a sleeve attached to the ultrasound probe that fixates the syringe in a specific position, and the needle enters the image in a predictable path that is often indicated by a line in the ultrasound image, depending on the system. The main limitation of this system is that the angle of needle entry is difficult to modify, and very superficial or very deep structures can be hard to reach. Free-hand technique is associated with more of a learning curve but is more flexible, as the angle of entry can be adjusted depending on the location of the target. Practice is of utmost importance for ultrasonographers who perform fine-needle aspirates for above-mentioned reasons. There are many ways to perform ultrasound-guided FNA. What all have in common is that the needle and ultrasound probe have to be aligned perfectly to allow visibility of the needle as it enters the tissue, and this requires practice. Slight malalignment will lead to the needle leaving the image plane, and if the sonographer is not sure where the tip of the needle is located, there is no way of knowing if structures other than the target are accidentally punctured. FNA can be performed right- or left-handed, depending on preference of the ultrasonographer. Care should be taken to always place the needle close to the ultrasound probe in order to enter the field of view, but far enough to not accidentally insert the needle into the rubber covering of the probe, which may cause expensive damage to the equipment.
Proficiency in Ultrasound-Guided FNA
Practice is the key to success. How to go about practice when first starting out? Initial attempts at FNA can easily be made using phantoms. There are commercially available ultrasound phantoms that have “lesions” inserted that are visible in the ultrasound image. Ultrasound phantoms can also be homemade using a plastic container filled with gelatin. “Lesions” can be hidden in the semi-hardened gelatin phantoms using fruits and vegetables or any other small item that is not gas-filled or made out of mineral/metal. Cut-off fingers of plastic gloves filled with water and knotted tightly make excellent cystic targets to practice cystocentesis. Once the ultrasonographer is comfortable reliably reaching targets in a phantom, and to always fully visualize the needle tip throughout the procedure, the move to a live patient is justified. Keep in mind that it is much easier to see the needle in a phantom than in a real patient where heterogeneity of the subcutaneous fat partially obscures the needle signal in some cases. Easy targets should be chosen first, such as cystocentesis or fine-needle aspirates of a large spleen or a large superficial mass that is not well perfused (check with Doppler ultrasound).
The most challenging types of FNA should only be attempted when the ultrasonographer is very comfortable with needle guidance. Lymph nodes can be challenging, as they are always closely associated with vascular structures. Additionally, they tend to move away from the needle as they are relatively loosely attached in the surrounding fat. Gallbladder aspirates carry the risk of bile peritonitis if leakage occurs after aspiration, and to make things more difficult the gallbladder is located very cranially and affected by respiratory motion even in sedated patients. Other higher-risk aspirates include lung aspirates where there is a risk of pneumothorax, and aspirates of highly perfused lesions such as thyroid carcinomas. Finally, prior to performing fine-needle aspirates, a risk-benefit analysis should always be made, keeping in mind possible complications and the sensitivity and specificity of FNA in different organ systems.
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