I have an ultrasound machine-how do I aspirate stuff?

You just finished an abdominal ultrasonographic examination and found a lesion or fluid that you would like to aspirate or biopsy. Or maybe you found a lesion in the thorax or on an extremity on which you want to perform an ultrasound-guided aspirate or biopsy. Here's a guide to getting a good sample with ultrasonographic guidance.

Preliminary considerations 

First, determine whether the patient's physical condition is sufficient to allow for an aspirate or a biopsy. In most cases, the reward of having a cytologic diagnosis outweighs the risk of the procedure. The risks for each patient will be slightly different. The size and location of the lesion is another factor in deciding whether or not you can obtain a sample. 

Coagulopathies and anemia are of most concern when considering puncturing the body wall and internal organs with a needle. Some patients may need transfusions or other types of supportive care before the procedure. 

Another big concern is patient motion when performing this procedure. When you choose a sedative, one that causes the least amount of respiratory panting is desired. But a sedative is usually indicated since some animals may lie on the ultrasound table perfectly still for the whole examination, but when a needle is introduced close to the skin, they start moving. 

Site localization 

Once you locate the lesion you want to sample, determine whether you can safely access the lesion. You want the lesion to be as close to the transducer as possible. If you are not sure if your needle will reach the lesion, you can measure from the upper corner of the image to the center of the lesion. Remember, a 2.5-in needle is 6.35 cm. Sometimes you can add a little pressure to the transducer and bring a lesion closer to the transducer. 

The next step is to make sure no critical structures are between the transducer and the target. We are primarily trying to avoid blood vessels and bowel. If a critical structure is in the projected path of the needle, try readjusting the relationship of the transducer to the lesion. Sometimes this means rotating the transducer 90 degrees. Sometimes the lesion is blocked by critical structures, and you are simply not able to sample the lesion safely. 

Some ultrasound machines have an on-screen needle guide, which is for use with a biopsy guide. The biopsy guide is a device that clips onto selected transducers and guides the path of the needle under the transducer. The ultrasound machine displays the projected path of the needle on the screen. 

Fine-needle aspiration equipment 

You will need:

> 22- to 25-ga, 1.5- to 2.5-in needles

> A spinal needle with stylet if you have a lot of normal tissue to traverse before reaching the lesion (an animal with a lot of fat in the body wall or falciform area, or simply a lesion that is deep within the liver)

> A 6- to 12-ml syringe, with or without an extension set

> A second person to apply suction if you use the suction method (see below) with an extension set between the needle and the syringe.

Biopsy device equipment 

Multiple biopsy devices are on the market. Most of the devices use needles specific to the device. Typically, the needles are 16- to 20-ga. The biopsy devices are spring-loaded and must be set before inserting the needle into the patient. Some devices have a safety so you can prevent premature firing of the device. The typical throw on a biopsy needle is 12 to 20 mm. Before you fire the biopsy device, make sure that there is 12 to 20 mm of tissue to sample, otherwise the needle will go too deep and you will sample unwanted tissue. 

Some types of biopsy devices have needles that rapidly fire into the tissue, cut a core sample and then retract back into the needle sheath. Other types of biopsy devices allow the user to advance a stylet into the lesion, allowing you to know the extent that the device will sample.

Patient preparation 

Clip the hair over site, which has usually already been done because you just performed the ultrasonographic examination. Clean the site to remove ultrasound gel since it can affect certain stains. We use a small amount of rubbing alcohol as the ultrasound contact medium. However, some ultrasound manufacturers will void your warranty if you use alcohol on transducers for their machines. Be sure to check with your sales representative. 

Technique 

For biopsies, make sure the device is spring-loaded. For both fine-needle aspirations and biopsies, start the needle about 45 degrees to the transducer on the skin. If the lesion is shallow, you may start at an angle more parallel to the skin. If the lesion is deeper, you may start more perpendicular to the skin. 

Enter the skin in front or behind the transducer in the plane of the ultrasound beam. Do not enter the skin from the side of the transducer as this limits the amount of the needle you will be able to see. More specifically, you will likely not see the tip of your needle, thus you will not know if you are within the lesion or not.

Initially, you want the needle to remain very shallow until you can verify the location of the needle tip and the direction the needle will travel. Move the transducer to find the needle's location instead of moving the needle to find the transducer. When the tip of needle is located and you are happy with its projected path, advance into the lesion. 

Peck method. Once the tip of the needle is in the lesion, move the needle up and down, like a chicken pecking at feed, loading the needle with cells. After a few passes, remove the needle and spread the cells onto a glass slide. Repeat as needed. The Peck method is my preferred method.

Suction method. Whether you are using an extension set between the needle and the syringe or not, once the tip of the needle is in the lesion, apply suction to the needle by pulling on the syringe plunger. Release the pressure. After a few cycles, remove the needle and spread the cells onto a glass slide. Repeat this process as needed. The suction method is preferred when lesions are small and there is not enough room to move the needle up and down.

Preparing the sample 

Fine-needle aspirates are blown onto a glass slide and then smeared. Biopsy samples may be rolled onto a glass slide, creating touch preps, before the samples are placed in formalin. A 22-ga needle is useful for teasing samples off biopsy devices.

When performing biopsies, have the containers ready before you start the procedure. Some laboratories prefer the biopsy samples be put into a plastic cassette before placement in formalin. Other laboratories may prefer samples be placed directly in formalin without a cassette. Check with your laboratory for its preferences.

Wm Tod Drost, DVM, DACVR 

Department of Veterinary Clinical Sciences

College of Veterinary Medicine

The Ohio State University 

Columbus, OH 43210