The ultrasound probes just described are for the traditional cart-based systems.
However, there are now handheld devices that connect to your smartphone and can simulate multiple probe types with just a click of the button. The Butterfly Ultrasound Device is an example of this (see below). From my experience, the footprint is slightly larger than the phased array and the weight of the probe is about 2-3 times more than a typical phased array. This increased weight is accounted for by the processor and the battery.
The endocavitary probe has a curvilinear footprint with a wide view but has a much higher frequency (8-13 MHz) than a curvilinear ultrasound probe. The image resolution of the endocavitary probe is exceptional, but like the linear probe, it must be adjacent to the structure of interest since it has such a high frequency/resolution, but poor penetration.
The most common POCUS applications for the endocavitary ultrasound probe are for intraoral (peritonsillar abscess) and transvaginal applications (early pregnancy, ovarian torsion, ovarian cyst, fibroids, ectopic pregnancy, etc). Make sure to always place a sterile endocavitary probe cover (condom or glove) prior to scanning.
The phased array (or sector array) transducer is commonly branded as the “cardiac probe” and has a frequency range from 1-5MHz. It has a similar frequency range as the curvilinear probe but has a smaller and flat footprint.
The advantage of this probe is that piezoelectric crystals are layered and packed in the center of the probe making it easier to get in-between small spaces such as the ribs (notice the extremely small pinpoint footprint on the ultrasound image below).
It is the ideal probe for cardiac scanning however it can perform all of the applications the curvilinear probe can as well (with less lateral resolution).
The curvilinear ultrasound probe has a frequency range of 2-5MHz. It is considered a low-frequency probe and has a large/wide footprint, allowing for better lateral resolution (compared to the phased array probe). The curvilinear ultrasound probe is often used for abdominal and pelvic ultrasound exams. However, it can also be used for cardiac and thoracic ultrasound exams but is limited by the large footprint and difficulty with scanning between rib spaces.
Here is what the Curvilinear probe looks like and how an ultrasound image will appear on the screen. Notice the curved nature of the ultrasound image.
The linear ultrasound probe is a high-frequency transducer (5-15 MHz) that will give you the best resolution out of all of the probes but is only able to see superficial structures. A general rule of thumb is that if you are going to ultrasound anything less than about 8cm, then use the linear probe. Anything above 8cm you won’t be able to see much.
The linear probe will give you a rectangular field of view that corresponds with its linear footprint:
Choosing the Right Ultrasound Probe based on Application
The single most important factor that will determine if you can get proper ultrasound images is choosing the correct ultrasound probe or transducer. Like with anything else you do, the right tool will be needed for the right situations. For example, if you used a linear probe, that has great resolution but minimal depth, you will not be able to visualize much if any of the heart.
So before you start scanning, always ask yourself these questions to help pick your ultrasound probe:
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?
Does it involve a procedure?
Does it involve a cavity (pelvic, peritonsillar abscess)
In this post we will go over the 4 most common Point of Care Ultrasound probes you will encounter (linear, curvilinear, phased array, and endocavitary probes). The table below lists when you should think about using each type of ultrasound probe.
Each ultrasound probe will have it’s pros and cons. Usually, the most important factors to decide on are resolution, penetration, and footprint size. Here is a figure showing how penetration and resolution are affected with respect to the frequency of the transducer.
The Ultrasound Probe “Footprint” refers to the area on the probe that comes in contact with the patient’s skin in order to produce an ultrasound image. It is located at the very tip of the probe and is usually has a soft “rubbery” feel. Depending on the application you may want a smaller or larger footprint. Regarding footprint width from largest to smallest it goes: Curvilinear > Linear > Phased Array.
The images below demonstrate the relative sizes and footprints of the 3 most commonly used ultrasound probes (Linear, Curvilinear, and Phased Array):