Linear Probes - do I really need one?

05.09.2023

This month our Account Manager Q&A session is with Megan Howe, our Account Manager for the Midlands!

One of the most important steps to set yourself up for ultrasound success is choosing the correct ultrasound probe. Like with anything else, you need to pick the right tool for the job to get the best results.

Each probe will have pros and cons. Usually, the most important factors to decide on are resolution, penetration and footprint size.

These are some questions to think about:

• What application am I using the ultrasound machine for?
• How deep are the structures I’m trying to visualize?
• How big or small of a footprint do I need?

Q. Can you give me a quick guide to the different probe options?

A. Sure, the common choices of probes are:

• High frequency probes (10-15MHz)High frequency probes provide better resolution but have less penetration. They are therefore ideal for superficial structures (2-4cm).
  
• Mid-range frequency probes (5-10MHz)Midrange frequency probes are chosen when looking at slightly deeper structures (5-10cm).
  
• Low frequency probes (2-5 MHz)Low frequency probes are favoured for those deeper structures although the detail within the image will be significantly poorer.

You need to select the correct probe that provides the best resolution for the required depth. Having a selection of probes allows for greater flexibility with what you can see.

Q. Are there different types of linear probes?

A. Yes, there are two main types of linear probes: wideband linear array probes and hockey stick linear array probes.

• Wideband linear array probes are the most common choice. They are identified by their long straight-line acoustic lens on the face of the probe. These probes utilise high frequency ranges, this results in detail when imaging shallow structures.
• Hockey stick linear array probes get their name from their appearance. The acoustic lens sits on the thin straight foot of the stick. These probes have very high frequency ranges which allows extra detailed shallow imaging, but their penetration ability is poor.

We cannot forget the linear rectal probes, these guys work hard for our farm and equine colleagues. The imaging principles remain the same, just these probes are generally inserted rectally for image acquisition.

Field of view from a linear rectal probe. Image credit: Erika Wierman, DVM, 2022, Flat Linear vs. Curved Rectal Probe: How Do I Choose?, retrieved August 24, 2023, from: https://www.eimedical.com/blog/flat-linear-vs.-curved-rectal-probe-

Bovine Fetus – approx 71 days (left) and Equine Fetus – approx 26 days (right)

Q. When would I use which?

A. Wide band linear probe – Linear probes are good for superficial structures and for eye examinations.

One limitation is that they often have quite large footprints which may not be ideal.

Linear hockey stick probe – Hockey stick probes have an extremely small footprint making them a great choice for scanning exotic species, musculoskeletal structures in small animals and eyes.

Rectal linear probe – Although cows and horses are large animals, when scanning rectally, you are only a few centimetres away from reproduction and areas of the GI tract. For this reason, you can afford to use a high frequency transducer for much greater detail.

Q. When would you recommend reaching for a linear probe?

A. A general rule of thumb is that if you are going to ultrasound anything less than about 6cm deep, then use the linear probe. Anything deeper than 6cm you won’t be able to see much.

There are lots of scenarios where the linear probe is likely to be the best tool for the job, below are just some ideas to get you started:

• Vascular access.
• Musculoskeletal—tendons, ligaments, joints, bones & muscle integrity.
• Superficial abdominal structures (especially in cats) – SI and Kidney.
• Skin and soft tissue examination for abscess, foreign body etc.
• Ophthalmic imaging.
• Nerve blocks.
• Lung screening, evaluation of the pleural line (pneumothorax, interstitial fluid)
• Thyroid examination.
• Ultrasound-guided procedures (e.g. arthrocentesis, paracentesis, thoracentesis etc.)
• Exotics (and smaller patients).

Linear Rectal:

• Dairy and equine reproductive health (pre-breeding: Uterus and ovaries)
• Pregnancy diagnosis and maintenance (foetal sexing and twin management).
• Colic / equine GI examination – imaging of bowel wall thickness, structure, luminal content and contractile function can be assessed.

Anatomical location of structures can be reviewed.

Deeper abdominal organs can be examined, such as kidney, spleen and adrenal glands can be inspected.

Q. That’s a lot of information – can you give me a summary of linear probes?

These are great tools for:

• High frequency (detail) BUT poor penetration.
• Shallow structures (<6cm).
• Easily identified by their rectangular images.
• Wide band, hockey stick or rectal are the common options to choose from.

To answer the question – Do I need one?

It is difficult to say, but there are lots of ways a linear probe can justify its place within your ultrasound toolbox if you get the opportunity to add one.