3D vs 4D vs 7D vs 9D HIFU Explained

Nobody owns the D

Ask three suppliers what the seven in 7D HIFU stands for. You'll get three answers, and at least one will be a slogan. Seven cartridges. Seven depths. Seventh generation, a phrase with nothing testable inside it.

Now go looking for the rule that governs the word. There isn't one. In the United States these consoles are classified as focused ultrasound stimulator systems for aesthetic use under 21 CFR 878.4590, Class II with special controls, and you can read that classification start to finish without meeting a D. IEC 60601-2-62 sets basic safety for high intensity therapeutic ultrasound equipment. IEC TS 62556 says how the acoustic field gets measured and declared. Neither one knows what a 9D is either.

So the number is a badge. Badges aren't automatically dishonest. They're unaudited, and unaudited numbers drift upward.

What each badge tends to be counting

Below is how the four labels get used in the market, as seen across quotations and catalogues. Convention, not definition. Read every line as a question you still have to ask, never as a fact you've been handed.

3D

Oldest of the four. Usually a face-only console with a small fixed cartridge set: three depths in some cases, three undefined "modes" in others. A 3d hifu machine shipping 1.5, 3.0 and 4.5 mm covers the standard facial menu. One shipping a single 4.5 mm cartridge doesn't, and both are free to print the same digit on the lid.

4D

Hung on a facial console that has picked up body cartridges, a second handpiece, or both. It guarantees no four depths, no four frequencies, no four of anything you could hold a ruler against.

7D

Busiest badge in the category and the least consistent. Behind it you'll find seven cartridges in the case, or seven selectable depths, or handpieces plus bundled modes added together, or nothing countable whatsoever. Two consoles both wearing 7D can share no cartridge, no depth and no frequency. That's the entire problem in one sentence.

9D

Almost always a seven-count with something stacked on top. Two extra body cartridges. An RF, LED or EMS module folded into the tally. A 9d hifu machine can carry the very same three facial depths as the cheap 3D one. Start by asking which of the counted items are focused ultrasound at all.

See the pattern? Every tally mixes cartridges, depths, handpieces and bundled extras in whatever proportion flatters the total.

Picture two quotations on your desk, both badged 7D, within a few hundred dollars of each other. Machine A ships seven cartridges and only two of them reach the face. Machine B ships four, one being a 1.5 mm transducer you can book every week. Bigger number, thinner menu. Then look at HIFU cartridge life: if A's cartridges are rated for a fraction of B's shots and cost the same to replace, A charges you more per treatment for as long as you own it.

Millimetres are the number that treats people

A focused transducer converges sound to a point sitting a fixed distance from its face. Tissue there heats hard. Skin above it is spared. That distance is built into the cartridge, and turning the power up will not move it.

Skin is thin, which is why the figure carries so much weight. The skin anatomy training material in our engineering archive puts the epidermis at roughly 0.1 to 0.4 mm and the dermis at roughly 0.5 to 4 mm, with subcutaneous tissue below the dermis and fascia below that. At that scale a millimetre isn't a rounding difference. It's a different target.

Published work has settled on three facial depths. A 2025 narrative review in the Journal of Cosmetic Dermatology describes microfocused ultrasound delivered at 1.5, 3.0 and 4.5 mm, reaching dermis and subdermal tissue including the SMAS, at therapeutic temperatures of 60 to 70 degrees Celsius that form discrete thermal coagulation points. Frequency is a separate dial from focus depth. A 2015 Annals of Dermatology series ran probes at 4 MHz with a 4.5 mm focal depth, 7 MHz at 4.5 mm, and 7 MHz at 3.0 mm. Same depth, two frequencies. Ask for both figures, cartridge by cartridge.

Focus depth	Roughly what sits there	What it gets booked for
1.5 mm	Upper dermis	Fine lines, texture, work around the eyes
3.0 mm	Deep dermis	Cheek and jaw tightening, softening wrinkles
4.5 mm	SMAS fascia layer	The lift the client actually walked in asking for
6 to 16 mm	Subcutaneous fat and below	Body contouring: abdomen, arms, thighs

First three rows follow the 2025 review cited below. The 6 to 16 mm range is our own cartridge lineup.

Where the Pmise HF-01 sits, said plainly

The HF-01 HIFU platform is a 4D platform. Four is its badge, and by every argument above a badge buys you nothing, so here is the cartridge list from our device manuals instead, with included items kept apart from paid ones.

In the case, no extra invoice: 3.0 and 4.5 mm on the 4D handle; 1.5, 3.0 and 4.5 mm on the V-max handle.
Optional, priced separately: 6, 8, 10, 13 and 16 mm for the 4D handle; 8 and 13 mm for the V-max handle. All body work.

Count them. Seven focus depths on a console labelled 4D. Read that twice, because it's exactly the confusion this guide exists to clear up: the depth count and the D-number are unrelated quantities, and treating them as the same thing is how buyers overpay. A competitor with an identical depth list is perfectly free to print 9D on the lid. Relabelling moves a sticker. It never moves a transducer.

Output runs 4 to 7 MHz, and one shot lays down twelve lines. Set those beside whatever else you're weighing: millimetres against millimetres, megahertz against megahertz, lines against lines.

Focused ultrasound isn't the only way to heat collagen. Bulk radio frequency warms a wider area more gently, which is why the RF-01 skin tightening system sits beside HIFU rather than underneath it, and our technology notes compare how each modality carries its energy into tissue.

Five numbers and one file

Ask for	Why it decides the purchase	Answer that should worry you
Focus depth per cartridge, in mm	Depth picks the tissue. Nothing else does.	"Multi-depth" with no figures behind it
Transducer frequency in MHz, per cartridge	Shapes the focal spot; depth alone won't tell you	One frequency quoted for the whole console
Lines and shots per pass	Sets how long a full face takes, so how many you can book	Vague praise for a "fast mode"
Rated shot count per cartridge	The denominator of your running cost	A round number with no test behind it
Replacement cartridge price, landed	The numerator. Ask lead time in the same breath.	"We can discuss that later"
Compliance file in your exact model number	Decides whether it clears customs and whether you can legally treat	A certificate issued for a different model

Buyers skip that last row, and it's the one that bites hardest. Clearance doesn't travel. A US 510(k) under 21 CFR 878.4590 says nothing about Europe, where equipment intended to reduce, remove or destroy adipose tissue falls under Section 4 of Annex XVI of the Medical Device Regulation and is classified IIb by Implementing Regulation (EU) 2022/2346. Ask by document name: registration status in the market you'll operate in, a declaration of conformity carrying your model number, test reports against IEC 60601-1 and IEC 60601-2-62, an acoustic output declaration per IEC TS 62556. A manufacturer worth buying from answers that with paper, not adjectives.

The comparison, in five moves

Two quotations, side by side, in this order.

Cross out the D-numbers. Both of them. They aren't evidence of anything.
List face depths only, included ones first. Count the depths that serve treatments you'll genuinely sell. A depth arriving as a paid extra belongs in the cost column, not the feature list.
Work out cost per shot, cartridge by cartridge. Replacement price divided by rated shots, calculated separately for each cartridge you'll use weekly. Averaging a cheap 4.5 mm against a dear 3.0 mm hides the thing you're trying to see.
Multiply by a real treatment. The 2015 Annals of Dermatology series delivered a mean of about 420 shots per patient across seven facial areas. Use that as a yardstick and your cost per full face drops straight out.
Project twelve months. Sessions you honestly expect each month, times twelve, plus the console price. Compare the totals, not the stickers.

The console is a one-off. Cartridges are a subscription you never signed. Skip that last step and you'll find out in month seven.

What no spec sheet can tell you

Ultrasound doesn't chase melanin the way light does, so skin type won't drive your settings the way it would on an IPL or a laser. Don't read that as a free pass. A 2025 meta-analysis in the Aesthetic Surgery Journal pooled 42 studies and reported erythema, oedema and swelling, followed by bruising and tenderness, as the most common adverse events, all of them mild or moderate; across the 785 patients assessed for pain, the pooled mean score was 4.85 on a 0 to 10 scale. Post-inflammatory hyperpigmentation stays a genuine concern in deeper phototypes whenever heat lands in the dermis, so screen carefully and stay conservative on a first pass.

Whether a particular face needs lifting at all, or volume, or something no device can deliver, is a clinical judgement about an individual. That belongs with a qualified practitioner, not a comparison table. Contraindications work the same way: implants, fillers, active infection, a history of keloids.

One last tell. A supplier who talks at length about why their machine carries the number it carries, and never once gives you a millimetre figure, has already answered your real question.

Ask for the sheet, not the badge

Five numbers and one file. That's the whole guide. Ask us for the HF-01 cartridge sheet and that's what comes back, with the compliance documents for your market beside it. The HF-01 product page carries the console specifications; cartridge economics travel by email, since they depend on the treatment mix you plan to sell.

Evidence and further reading

Educational material for equipment buyers and operators. It is not medical advice, an operating protocol or a promise of clinical outcome.

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3D, 4D, 7D and 9D HIFU: What the Numbers Mean