The lexicon of minimally invasive procedures in aesthetic medicine denotes dermal or subcutaneous injections, punctures, or small incisions that do not require surgery. Therapeutic goals are not standardized and are generally subjective. Most methods and techniques are designed to minimize aging related to skin laxity, rhytids, and contour abnormalities. Desirable therapeutic goals include volumizing, rejuvenation, restoration, resurfacing, and correction, not specific to one standard or facial feature, and therefore variable from individual to individual. This course will not attempt to modify terminology used in the published literature. Several excellent resources are available to familiarize this audience. For the purposes of this course, the terms aesthetic medicine and cosmetic medicine referenced in the literature are interchangeable [1,2,3].

Ultrasound is a type of mechanical (not electrical) energy collectively known as sound energy. The normal human range of sound is between16 Hz 20,000 Hz (1 MHz = 1 million cycles per second), with higher end of the spectrum seen in children and young adults. Ultrasonic frequencies beyond 20,000 Hz and can be applied in both therapeutic and diagnostic applications. The frequencies generally used in therapy are typically between 1–3 MHz for deeper tissues and 2–20 MHz in diagnostic ultrasound studies. Ultrasound beams are emitted by handheld devices, called transducers, which are mechanized high-frequency sound generators [13,14]. Figure 1 illustrates a schematic of a basic ultrasound system.

Ultrasound does not produce ionizing radiation, as in the case of x-ray or computed tomography scans. However, ultrasound does produce acoustic energy that does interact with human tissues, and is therefore considered relevant [40,41]. Bioeffects, both thermal and non-thermal in nature, need to be considered. Commercially available diagnostic ultrasound units are designed for safe application; no latent harmful biologic effects have been shown in humans [42]. However, scans should be performed setting the amount of acoustic exposure as low as reasonably achievable (known as the ALARA principle) [43].These biological effects are reported by the manufacturer as two main FDA-required standard preset values that appear on the image display as thermal index and mechanical index.

Thermal index (TI) is an indicator of the potential temperature increase resulting from the friction of tissues interfaces caused by acoustic energy [44]. Mechanical index (MI) is an indicator of non-thermal effects of cavitation from rarefaction of the ultrasound beam [45].

Fluid, as can be seen with cystic lesions, are completely devoid of signal (decreased echogenicity) and appears uniformly black (termed anechoic) due to decreased acoustic impedance of the ultrasound wave [49].

Soft tissue encompasses a gray-scale spectrum of possible acoustic properties (heterogeneous echogenicity), ranging from tendons and ligaments on the lower end of acoustic impedance, and appear darker (hypoechoic) to muscle and dermal layers. Subcutaneous fat has the highest acoustic impedance and appears the brightest (hyperechoic) [36].

Cosmetic fillers have continued to take center stage as safe and effective restoration of facial volume, recontouring, and additional collagen biostimulatory effect, leading to improved skin quality, as validated by their long-term safety, stability, and efficacy. The most commonly used temporary fillers are conjugated hyaluronic acid (HA) fillers and calcium hydroxylapatite (CaHa) [53]. Permanent injectable fillers, such as polymethylmethacrylate (PMMA), are less desirable because they do not break down (termed biodegradable) [54]. HA filler is a preferred agent because it is similar to naturally occurring HA in the body, which forms an integral part of the extracellular matrix for mechanical structure of various body tissues, including skin, muscle, and bone [55].

CaHA is a mineral commonly found in human teeth and bones that functions as a scaffold for collagen ingrowth. CaHA filler contains CaHA microspheres as biodegradable particles suspended in an aqueous carboxymethylcellulose gel carrier. Once injected, the carrier gel gradually resorbs; the microspheres stimulate a fibroblastic response resulting in active physiologic remodeling of the extracellular matrix and long-term collagen deposition around the implant that promotes volumizing [70,71]. The microspheres eventually degrade into calcium and phosphate ions and are then excreted. Filler duration is around 18 months [72]. Radiesse (formerly Radiance) is the only FDA-approved CaHA filler [73]. This viscoelastic filler is ideal for supraperiosteal and deep fat placement, clinically indicated for correcting moderate-to-severe soft-tissue defects, facial folds, and rhytids, including nasolabial folds [74].

PLLA (Sculptra) is a biodegradable, biocompatible, synthetic polymer,historically used as resorbable sutures, orthopedic plates, urologic stents, and, later, for cosmetic use [75]. PLLA induces a subclinical inflammatory response that stimulates fibroblast proliferation, neocollagenesis, and type I collagen formation, leading to a progressive increase in dermal volume. PLLA provides global volume restoration in lean patients lacking sufficient dermal volume and thickness for space occupying fillers, an alternative to deep volumizing hyaluronic acid fillers that could be too expensive and difficult to use as suitable dermal scaffolding [76]. PLAA injection is recommended for the supraperiosteal injection in the temples, lateral brow, zygomatic area, maxillary area, mandibular area, mental area, and also in the subcutaneous fat in the mid-cheek regions and preauricular area [77].

PLLA presents with mixed or heterogeneous echogenicity, sometimes with acoustic shadowing. For practical purposes, the PLLA ultrasound appearance cannot be reliably distinguished from other cosmetic fillers.

The most important landmarks for aesthetic imaging involve the facial and submental regions, which supply the complex vascular structures and neurovascular bundles. These structures must be avoided in facial aesthetic injections commonly used in clinical practice, such as the reduction of facial wrinkles and contouring. Injectables include botulinum toxin, cosmetic fillers (such as hyaluronic acid and hydroxyapatite) [78].

Cosmetic fillers are used primarily for volume augmentation and contouring of facial concavities and hollows caused by atrophy and age-related collagen loss. Accidental injury to neurovascular bundles from inadvertent or misplaced injection can result in adverse clinical outcomes, such as vascular occlusion leading to tissue necrosis, and temporary or permanent sensory deficits [80,81].

More serious complications include rare peripheral nerve transection, with permanent anesthesia due to sensory nerve deficit, and compressive ischemic effects resulting in vasospasm with subsequent embolic phenomenon, blindness, and cerebral infarction [82,83].

Another practical point is that standard total body ultrasound units equipped with 15–20 MHz transducers and power Doppler have relatively larger footprints, making them unsuitable for highly contoured facial structures. Phillips Lumify, GE V-Scan Air, and Butterfly iQ all use small footprints suitable for facial aesthetics. Clarius and Mindray provide the highest possible resolution (with Clarius having a larger footprint and heavier transducer weight, which could be limiting factors).

The preferred protocol involves the use of smallest possible footprint transducer (for simple ergonomic reasons) in combination with a gel standoff pad, which partially mitigates the problem of decreased near field resolution, particularly with color Doppler vessel signal.

Anecdotal experience has suggested that HA fillers have a much longer lifespan than initially assumed. Prospective MRI studies have shown that HA fillers designed to dissolve within 6 to 12 months, surprisingly, can persist for up to 10 years post injection [86]. To date, there are no detailed clinical studies documenting a timeline of how long the dissolution process occurs in vivo, in human subjects [87]. Researchers have proposed a mechanism that the majority of dissolution occurs in the first initial weeks post-injection following a brief inflammatory response where hyaluronidase in produced by leukocytes causing a partial reduction in HA filler volume, a process that is dependent on the vascularity of the tissue. Once this inflammatory process has dissipated, the endogenous production of naturally occurring hyaluronidase is minimal [88]. Another consideration is a comparison between naturally occurring HA produced in the body, in vivo, compared to commercially prepared HA. Typical half-life of naturally occurring HA is one day or less in skin and dermis, compared with three weeks or less in cartilage. Commercial preparations of HA are typically cross-linked, making them more resistant to chemical breakdown, often lasting for months and years [89].

Typical ultrasound appearance of unwanted or residual HA filler is anechoic to hypoechoic, suggesting that the filler is reasonably well hydrated. One protocol consists of targeted treatment using microinjections of 15 IU, not exceeding 150 IU in total, administered in a radial pattern within unwanted (anechoic or hypoechoic) hydrated filler, dependent on volume of residual HA filler. Large quantities of HA filler (greater than 3 mL) could require repeat injections at one- or two-week intervals. Use of high-dose dose hyaluronidase flooding protocols (greater than 150 IU in a single session) is discouraged because of potential leakage into surrounding body tissues, risking an extended area of potential tissue breakdown. The effects of treatment are generally immediate, and a follow-up targeted ultrasound of the area shows no residual Ha filler (Figure 4) [98].

Blepharoplasty is a procedure to rejuvenate the upper and lower eyelids, either by surgical or non-surgical methods, and represents the fourth most common cosmetic surgical procedure in the United States [104]. RF-micro needling (such as AGNES technique) offers a viable non-surgical alternative for achieving long-term youthful and natural appearance to the eye and periorbital area [105]. Volume preservation and enhancement, rather than excessive tissue removal, define the modern blepharoplasty methodology [106]. The goal in the upper lid is to decrease eyelid laxity and increase visibility of the pretarsal space by micro-contouring [107]. The goal in the lower lid is to create a smooth lower lid surface and seamless transition into the cheek,combining micro-contouring with fat removal of the eye bag, if required [108].

Another indication for ultrasound is to address excessive periorbital swelling related to over-correction/over-filling of the tear trough area under the eye using cosmetic filler. This is the result of incorrect placement versus migration of filler above the orbicularis retaining ligament or due to superficial filler placement under the skin [109]. The mechanism of this phenomenon is unclear and difficult to treat clinically [110]. The only solution is to dissolve the filler and then replace the filler using modified (superolateral) technique [111].The orbicularis retaining ligament forms the superior border of the nasolabial and medial fat compartments, which combine to form, collectively, the malar fat pad, a recognized structure affected by midface aging [102]. Cosmetic fillers attempt to rejuvenate the midface by elevating and repositioning the malar fat pad via supraperiosteal injection [103].

Hyaluronidase is the first-line treatment of vascular occlusion [125]. Published treatment protocols range from high-dose flooding regimen with a mean dose of 500 IU for ischemic area of 3 x 3 cm, versus ultrasonically guided pulsing of 35–60 units into the area daily for three to eight treatment sessions until normal skin color returns [126]. High doses increase the risk of retinal toxicity with inadvertent intravascular injections of the enzyme in the periorbital region. In most cases complete recovery is generally encountered in less than 12hours, somewhat longer in chin necrosis from injury to the larger submental artery. The mean time for complete reversal of vascular occlusion is 72 hours,with a maximum of 7 days. Successfully treated patients encountered no delayed ulceration, crusting, or skin damage [127,128].Ultrasound guidance offers a distinct advantage of precision delivery of lower doses, with the benefit of increased safety.

The superficial temporal artery is a branch of the external carotid artery and divides into frontal and parietal branches at the cranial portion of the zygomatic arch. The parietal branch is more medial and superficial to the temporal fascia, overlying the temporalis muscle, and must be avoided [131]. Inadvertent direct injection and/or compressive ischemia can cause retrograde embolization of the ophthalmic artery and blindness [132]. The superficial temporal artery must be carefully palpated and marked; cannula placement should be medial when placing polydioxanone (PDO) threads for midface lifts. The superficial temporal artery is visible with color Doppler ultrasound, when performed with proper technique (Figure 6).

The dorsal nasal artery is a terminal branch of the ophthalmic artery and emerges from the medial orbital rim along the nasal septum [138]. The angular artery is the terminal portion of the facial artery and located at the lateral margin of the ala of the nose [138].

The paranasal area becomes a target in aesthetic practice, particularly with regard to injection of cosmetic fillers in achieving a "liquid rhinoplasty" or nasal contouring (Figure 7).Unlike other areas of cosmetic filler injections, midline nasal injections should be injected deep rather than superficial, because vascularity is more superficial, avoiding nasal bridge and ala [139]. Safe zones include midline middle third and lateral zone between the borders of the lateral nose and nasofacial groove. Avoid superficial injections of the alar tip and alar groove (avoid the crease) [139]. Nasal injection is the leading cause of tissue necrosis (Figure 8) [139,140].

The facial artery is located 15 mm lateral to the lip commissure and divides into the superior labial artery in the upper lip and inferior labial artery of the lower lip (Figure 10). These arteries are typically deep and run between the orbicularis oris muscle and oral mucosa [149]. General rules when addressing this facial area are [20,149]: