Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA
Percutaneous ultrasound-guided sample collection is used to obtain needle aspirates and tissue core biopsy specimens for a more definitive diagnosis due to the nonspecific nature of most ultrasound abnormalities. Common examples are hepatomegaly, diffusely hyperechoic liver, multifocal liver and splenic nodules. Increased echogenicity and nodules have many differential diagnoses from benign to malignant. While ultrasound screens and identifies diffuse and focal and multifocal lesions, it is nonspecific. Ultrasound-guided tissue sampling offers real-time visualization of needle placement for precise sampling of a selected portion of an organ or a focal abnormality, such as a mass lesion. Common clinical indications in animals include evaluation of enlarged lymph nodes, masses, nonspecific hepatopathies and splenic disease.
Patient Selection and Preparation
Animals should have a complete sonographic examination performed of the body region. Complete workup with blood work is also imperative, and underlying coagulopathies should be screened for. Animals with coagulopathies should not have tissue sampling performed as a general rule.
Sedation is not always necessary, and many animals can have the liver and spleen sampled with a fine-needle aspirate without it. However, risk should not be taken, and if the animal is not cooperative, painful, or panting heavily, then sedation should be performed for FNA. On the other hand, sedation is always recommended for Tru-Cut biopsy sampling.
The approach to the organ should be investigated carefully. Check the probe position so that the needle has the shortest path between the skin and the lesion.
Never pass between two body cavities or another organ to sample the lesion or organ.
Typically, cavitary lesions can be sampled with FNA, but not a Tru-Cut biopsy. Solid parenchymal organs or lesions can have FNA performed.
Vascularity is not typically a contraindication to sampling. For example, the kidney is highly vascular but can have a Tru-Cut biopsy performed very safely. Thyroid carcinoma is very vascular and bleeds readily when sampled, but the bleeding is self-limiting. For fine-needle aspiration (FNA), an 18- to 20-gauge needle is inserted into the selected site with ultrasound guidance, most commonly using a free-hand technique. Tissue core biopsies for histopathology are obtained using a larger needle (Tru-Cut®) with an automatic biopsy device, which allows the operator to control the depth of needle placement and length of sample obtained. A 14- to 18-gauge needle is suggested depending on the location and size of the lesion being sampled. Generally, 2–3 specimens are obtained for histopathologic evaluation. As a general rule, 18-gauge biopsy needles are preferred in small dogs and cats and small organs, and 14–16-gauge needles for larger dogs and large organs like the liver.
Technique for FNA
Clip and clean the skin with a cleaning solution like Hibiscrub or other skin prep and wipe with alcohol. Use a 3- or 6-ml syringe with the plunger pulled back 1 ml. Determine the probe position and place the needle in the same plane as the scan plane of the probe (middle of the probe) and use a 45-degree angle. Place the tip of the needle in the skin at the midpoint and edge of the probe and be careful to not pierce the probe housing. Insert the needle and make short in-and-out (woodpecker-like) excursions with the needle while watching the screen to make sure the needle is visible just under the skin so that when it is further inserted, it can be well visualized. Once the needle position is confirmed, advance the needle to the lesion and stab the needle in and out a few times, without aspirating, then withdraw the needle without suction applied. Remove the needle, draw the plunger on the syringe back, and reattach and spray the contents of the needle onto a clean glass slide.
Use a different syringe and needle for each organ and do not place the glass slides of different lesions and organs near each other when spraying the samples onto the slides, as this can cause cells from one region to be sprayed onto the slides of another organ, confusing the diagnosis.
If the stab technique does not result in enough material on the slide, then repeat the procedure and use aspiration with the plunger during the stabbing movement to attempt to get more cells in the needle. This can lead to more blood dilution, however.
Technique for Tru-Cut Biopsy
Typically, the liver and kidney are the main organs of interest for Tru-Cut biopsy procedures. They are larger and more amenable than smaller organs.
Lesions <4 cm in size are typically not suited for Tru-Cut biopsies. Small livers can be difficult to biopsy (cats, small dogs), and one should not hesitate to send those animals to laparoscopy for surgical biopsies.
The main difference from FNA is that the needle is larger and the animal should be sedated. A small nick with an 11 blade in the skin is necessary to advance the needle through the skin, and some force is required to penetrate the body wall. Once the needle is through the body wall, place the tip of the needle on the surface of the organ to be sampled and line it up with the plane of the ultrasound image.
Measure the surface of the organ with calipers to see how deep 2 cm is (typical penetration depth) to be sure that the needle will not strike a vessel or other vital structure deep to the lesion being sampled.
Fire the spring-loaded device and remove the needle from the organ. Open the needle sample volume notch and remove the delicate tissue core sample by running a stream of sterile saline over it, and the sample will slide into a reciprocal or slide. Put the sample in formalin directly afterward. Check the patient for focal fluid accumulation around the biopsy site. The PCV may also be checked periodically a few hours later to monitor for blood loss. It is suggested to perform biopsies in the morning and observe the animal in the hospital for the day to monitor for blood loss. This is typically not necessary with an FNA, however.