Choosing a Probe by Frequency

Choosing a probe by frequency: how frequency determines depth and detail, and how to match it to the examination.

Frequency decides depth and detail

Frequency is the most important single figure when choosing an ultrasound probe, because it decides two things at once: how deep the sound penetrates, and how much detail the image shows. Lower frequencies penetrate deeper but show less detail, while higher frequencies show more detail but penetrate less deeply. That trade-off cannot be avoided by any probe design, and it is the reason different examinations need different frequencies. A buyer who understands that trade-off can read a specification table correctly, while a buyer who does not may choose a probe that cannot reach the structures they need to image, or one that reaches them without enough detail.

In practical terms, abdominal imaging uses lower frequencies, typically in the range of a few megahertz, because the organs lie deep. Superficial imaging uses higher frequencies, because the structures lie close to the surface and detail matters more than depth. Cardiac imaging uses a middle range, because the heart lies at a moderate depth and must be imaged through narrow windows between the ribs. Those are general ranges, and the exact figures for a given model are on its product page.

Reading a specification table

A specification table usually gives frequency as a range or as a set of switched frequencies, and depth as a range or as a set of depth steps. Read both together: a probe with a frequency of 3.5 to 5 MHz and a depth of 90 to 305 mm is a deep probe, while a probe with a frequency of 18 to 24 MHz and depth steps of 10 to 25 mm is a shallow, high-detail probe. The number of elements also matters, because more elements generally give a better image, but elements alone do not compensate for the wrong frequency. A buyer should compare frequency and depth first, and treat other figures as secondary.

Be careful with series-level ranges. A series page may give a broad frequency range that covers several models, and that range must not be read as the specification of any single model. The product pages on this site distinguish series ranges from model specifications, and a buyer should always check the model table, not the series description, when they need a precise figure.

Matching frequency to common examinations

For abdominal examinations, a convex probe with a lower frequency is the usual choice, because the organs lie deep and the image must cover a wide area. For vascular, thyroid, breast, musculoskeletal and nerve examinations, a linear probe with a higher frequency is the usual choice, because the structures lie close to the surface and detail matters. For cardiac examinations, a phased-array probe with a middle frequency is the usual choice, because it must image through the ribs. For gynaecological examinations, a transvaginal probe with a middle-to-high frequency is used, because it sits close to the pelvic organs. Those are starting points, and the exact model should be checked against its published table.

Multipurpose probes complicate this picture in a useful way: they combine several probe types or several frequencies in one device, so one device can cover a mixed list of examinations. A dual-probe device offers two distinct probe types, while a single-probe device combines several functions in one head. The trade-off is that a combined device may not match a dedicated probe in every examination, and a buyer with a dominant examination type should compare the dedicated option before settling for a combined one.

High frequency in detail

High-frequency probes, at 18 MHz and above, deserve a separate note, because they are sometimes misunderstood. Their images show fine superficial detail, which suits facial work, small-part work and superficial vascular work, but their depth is limited to a few centimetres, so they cannot image deep abdominal structures. A buyer who orders a high-frequency probe for abdominal work will be disappointed, and a buyer who orders a low-frequency probe for facial work will miss the detail they need. The high-frequency product page on this site gives a complete example, with frequency, depth steps, size, weight and modes, and it shows how shallow a high-frequency device really is.

Frequency also interacts with the patient. Larger patients need lower frequencies to reach the same depth, while smaller patients can be imaged with higher frequencies. A practice that serves a varied patient population should therefore consider whether one frequency range covers its patients, or whether it needs more than one probe type.

A selection workflow

List the examinations the practice will perform, and note the depth and the detail each one needs. Match each examination to a probe type and a frequency range, using the general guidance above. Check the published specification tables for models that cover the list, and shortlist the models whose frequency and depth cover the most examinations. Confirm the remaining factors for the shortlisted models, including connectivity, battery, modes and price, and arrange a demonstration on the examinations that matter most. Only then ask for a quotation, and before payment confirm the model, specifications, lead time and price in writing.

Worked examples: matching probe to question

Consider four questions, and see how frequency guides the choice. First, a clinician wants to image the liver in an adult. The liver lies deep, so a convex probe with a lower frequency, in the range of a few megahertz, and a depth of several hundred millimetres, is the starting point. A high-frequency linear probe would not reach the liver, and its image would be useless for that question, however detailed it is for superficial work. The specification table for an abdominal model gives the figures to check, and the buyer should confirm them for the exact model.

Second, a clinician wants to image a thyroid nodule. The thyroid lies close to the surface, so a linear probe with a higher frequency, around 7 to 12 MHz, and a depth of tens of millimetres, is the starting point. A convex abdominal probe would reach the thyroid but would not show the detail needed to characterise a nodule, so it would be the wrong tool despite being able to produce an image. Detail, not just reach, decides this question, and the linear specification table gives the figures to check.

Third, a clinician wants to image the heart through the ribs. A phased-array probe is the starting point, because its small footprint fits between the ribs and its sector image covers the heart. The frequency is in the middle range, around 2 to 4 MHz, because the heart lies at a moderate depth. A linear probe would not fit the acoustic window well, and a convex probe would give a less suitable image shape, so the probe type matters as much as the frequency in this case.

Fourth, a clinician wants to image facial structures. The face lies very close to the surface, so a high-frequency linear probe, at 18 MHz and above, with depth steps of only tens of millimetres, is the starting point. That probe would be useless for abdominal work, and an abdominal probe would be useless for facial detail, which shows why one probe cannot serve every question. A practice that lists its questions first, and matches each one to a frequency and depth range, will choose correctly, while a practice that buys a probe first will often find that it serves only part of its list.