Probe Selection and Machine Settings for Guided Injection

Written by Dr. Glenn Barnes

Updated September 2026 ·8 min read

Choosing the wrong probe is the most common reason a scan looks unusable, and it is entirely avoidable. The decision comes down to one number: how deep the target is.

This guide follows the teaching of Dr. Glenn Barnes, a primary care sports medicine physician and diagnostic and interventional ultrasound specialist.

The depth rule

Three probes cover almost everything in non-spinal interventional practice, and Barnes assigns them by depth.

Deeper than five or six centimetres — use a curvilinear probe. Lower frequency penetrates further at the cost of resolution. It is identifiable by its curved face, and Barnes names its targets specifically: sacroiliac joints, hips, and glenohumeral joints in larger patients — anything typically greater than three or four centimetres deep.

Two to six centimetres — use a high-frequency linear probe. Barnes notes these are “the most common” injections, which makes this the workhorse.

Very superficial or very small targets — use a higher-frequency linear probe with a smaller footprint. His targets here are hands, wrists and toes, generally two centimetres or less. A small footprint matters as much as the frequency, because a long probe face cannot sit flat on a finger.

Frequency and penetration trade against each other. Higher frequency gives better resolution but less depth. Choosing a curvilinear probe for a wrist wastes resolution; choosing a linear probe for a deep hip in a larger patient produces an image where the target is not visible at all.

Echogenicity, and reading what you see

Images are described by how much sound a tissue reflects back, which Barnes calls echogenicity.

Hyperechoic tissue reflects a great deal and appears bright — bone being the obvious example, which is why bone gives such a clear, sharp bright line.

Hypoechoic tissue reflects very little and appears dark. Barnes's example is telling: an area of tendinosis is hypoechoic. Degenerate tendon appears darker than healthy tendon, which is what makes ultrasound useful for the diagnosis and not merely for guidance.

Gain, and the settings worth understanding

Machines carry a great many controls and most can be ignored at first. Barnes starts with gain.

Gain is brightness. As he puts it, it is “increasing or decreasing the contrast on the screen. Turning it up will make the picture whiter. Turning it down will make the picture” darker.

The practical point is that gain changes the appearance of the image without changing what is in the patient. A structure that looks abnormally dark may be under-gained rather than pathological, which matters when the dark area you are looking at is supposed to indicate tendinosis.

Extended field of view

A feature worth knowing because it solves a specific problem: the probe is shorter than the structure.

Extended field of view takes many images as the probe is swept along and digitally stitches them into one. Barnes's examples are a complete Achilles tendon or a complete rotator cuff — structures you cannot otherwise see whole.

It is a documentation and communication tool as much as a diagnostic one. A single image showing a whole tendon with the lesion in context is more useful in a report than three separate views.

The scanning hand: two fingers down

The most immediately useful technical instruction in the course, and the one most beginners get wrong.

Ultrasound gel makes the probe slippery. Barnes is blunt about the consequence: unless the hand holding the probe is stable against the patient, “it will slip and slide all over the place, making diagnosis and needle visualisation impossible.”

His rule: the hand holding the transducer has at minimum two fingers down on the patient. That anchors the probe and converts hand movement into “fine motor control and adjustment of your position with purposeful movements that are small.”

He explicitly advises against the alternative grips commonly adopted. And for diagnostic imaging, he recommends holding the probe in the dominant hand.

The patient is part of the examination

A point easy to lose once the screen has your attention.

Barnes recommends eliciting feedback throughout, on the grounds that although live scanning requires competence, “the patient can provide critical feedback to aid in the diagnosis and the intervention.”

Sonographic findings are common in asymptomatic people. Asking whether the probe is sitting on the spot that hurts is what connects an image finding to the complaint, and it costs nothing.

Looking after an expensive instrument

Capable units still run over $10,000, and the probe is the fragile part.

Barnes recommends commercially available probe cleaners, which “will preserve the function and lifespan of your very expensive investment.”

And one specific warning worth heeding: do not use adhesives such as Tegaderm on probe heads. It degrades the probe over time and results in costly repairs or replacement. Where a barrier is needed, a proper probe cover is the answer — covered in sterile technique for ultrasound-guided injection.

What adoption actually costs

Barnes is candid about the barriers, which is useful when building a business case.

Capable units remain over $10,000, though cost is falling. Staff need training. Documentation requirements attach.

But the obstacle he identifies as largest is not money: “perhaps the largest obstacle to the implementation of ultrasound into clinical practice is the learning curve associated with its introduction.”

His mitigations are practical — proper training, practice outside the clinic on volunteers, practice kits, and slow steady integration rather than a hard switch. He adds one that is easy to overlook: educating local referral sources that you now offer bedside ultrasound, which generates the case volume that builds the skill.

The credentialing, documentation and reimbursement side is covered in musculoskeletal ultrasound for pain practice.