High intensity focused ultrasound and non-invasive body contouring

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Chapter 51 High intensity focused ultrasound and non-invasive body contouring

Introduction

Liposuction (lipoplasty) first appeared as a procedure approximately 25 years ago as a technique to produce contour improvements with a minimally invasive approach. When it was introduced, it represented a quantum leap from the Pitanguy-style body contouring through excision of skin and fat deposits. Today, it remains a mainstay of esthetic surgery and one of the most requested esthetic procedures. Although a variety of attempts at noninvasive body contouring with energy-based technologies have been proposed, most had technological shortcomings, poorly-characterized outcomes, and/or a poor risk–benefit profile. Even in situations where multimodal approaches of mechanical force, laser, and radiofrequency energy have been used simultaneously, patients have not achieved uniformly satisfactory results.

The theory of a noninvasive approach for body contouring optimally involves the application of some sort of energy to ablate adipocytes and tighten collagen that is found within the midlamellar matrix of tissue (MLM). The question that follows is how does one accomplish this task and at the same time avoid fundamental problems with tissue thermodynamics from bulk heating of tissue or full-thickness epidermal injury. Energy-based systems that utilize bulk heating of tissue appear only minimally better than lipoplasty, but with substantive risk of tissue necrosis, chronic inflammation, and occlusion of vascular structures. Transcutaneous radiofrequency technologies become problematic due to treatment discomfort and risks of thermal injury to skin.

Pharmacologic approaches (injection lipolysis or “mesotherapy”) have been considered, yet remain problematic because of the small areas that are treated and the presence of chronic inflammation in tissues.1

Cold-based devices may improve contour, yet cannot be expected to tighten collagen in the MLM. These heat subtraction treatment devices appear to produce contour improvements through reduction of superficial fat and skin retraction only. External low-level or “cold” laser techniques have been reported as applicable for body contouring, yet with poorly-controlled studies and short, meaningless follow up periods.

This chapter will cover what appears to be a promising technology of high-intensity focused thermal ultrasound that is designed to improve body contour.

Ultrasonic Energy

High intensity, focused ultrasonic energy (HIFU) has been considered as a way to ablate tissue deep inside to body as a treatment for medical conditions (liver metastases, uterine fibroids, and atrial fibrillation.2 Ultrasound propagates through tissue at approximately 1540 m/s. Sonic energy, like light, can be focused. Pressure fluctuations lead to shearing motions and molecular-level interactions that produce heat. Because it is focused to a predetermined point within tissue, the energy can pass across the skin at relatively low fluences of 1–3 J/cm2, yet reach greater than 1000 J/cm2 within the focal zone. In this fashion, a “trackless” lesion is produced, without collateral damage to surrounding tissue or skin. Figure 51.1 demonstrates a Schlieren image of ultrasound waves that converge and diverge beyond a focal zone. Ultrasound can be focused at specific depths within the tissues as demonstrated in Fig. 51.2.

Temperature measurements taken within porcine tissue at the focal zone of a commercially available HIFU system (the LipoSonix® system, Medicis Technologies Corporation, Bothell, WA, USA, technical data) approach 70°C, while unaffected nearby tissues are normal (37°C).3

Nonfocused ultrasound is used for bulk heating of tissue by physical therapists in low fluences of 2–5 W/cm2. While this may have a therapeutic benefit for the treatment of musculoskeletal conditions, some devices are capable of bulk tissue heating in the 45–47°C range. This level of tissue heat is problematic in terms of skin burns and tissue necrosis. Currently, a dual-transducer ultrasound device is being studied to determine if it can produce body contour changes. Its use for body contouring would be considered off-label because these devices are approved for tissue heating. The safety and efficacy of external, nonfocused ultrasound as a device for noninvasive body contouring has not been determined. There have been other reports of the use of external, nonfocused ultrasound as an adjunct for the dispersion of wetting solutions used in surgical lipoplasty.4,5 Burns have been reported with nonfocused ultrasound.6

Tissue Thermodynamics

When considering energy-based applications for body contouring, it is important to remember that all energy ultimately becomes heat. The common approaches for energy-based body contouring are divided into technologies that produce bulk heating/cooling of tissues or deliver energy in a more targeted fashion, with minimal collateral damage risk to skin, vascular/lymphatic structures, or nerves.

A review of the biologic response of tissue to heating is important to understand how energy-emitting devices affect tissue and what would be considered reasonable safety parameters. It takes approximately 4.184 J of energy to raise 1 g of water 1°C in a linear fashion. Tissue response to heating at around 43°C becomes nonlinear when there is a break in the response curve. From this point upward, small increases in tissue heating result in profound tissue effects. Tissue damage has been noted at 44°C, blood flow stops at 45°C, and tissue necrosis will occur at 47°C. It takes tissue heating to 65–70°C to modulate collagen, which is way beyond thermal levels for tissue necrosis. Other factors that are involved in tissue injury relate to the rate of tissue heating and whether thermal protective measures are used (pulsed/pattern delivery of energy, lipoplasty wetting solution, or epidermal cooling).

Bulk heating of tissue via laser fiber, unfocused ultrasound, or radiofrequency can raise tissue temperatures far beyond the range to produce thermal necrosis, yet not high enough to modulate collagen (65–70°C). External temperature measurements are notoriously unreliable in measuring heat levels in deeper layer tissues. Not all surgeons will pay attention to temperature measurements when using devices that perform bulk heating of tissue. Outcome studies published show only minimal incremental benefit with bulk heating technologies over existing approaches for surgical lipoplasty.

Surgical lipoplasty with ultrasound utilizes a thermal injury protective approach of wetting solutions that prevent excess tissue heating.7 Solid probe devices will fragment tissue at lower energy levels and pulsed energy reduces the possibility of thermal damage.

HIFU will produce a direct thermal effect that results in ablation of adipocytes and thermal modulation of collagen fibers that are contained within the MLM. Animal studies with thermocouples implanted in the HIFU target zone demonstrate that temperature levels high enough to accomplish the intended effect (65–70°C) are reached and that nearby tissue outside of the focal zone is not heated. HIFU with the LipoSonix device is delivered in a grid-like pattern where pulses of energy are applied to tissue and nearby unheated tissue is used to dissipate heat. This is similar to a pattern generator used by lasers for fractional skin treatments. In both situations, there is enough untreated tissue nearby that serves as a heat sink to prevent bulk heating and bulk tissue damage.

Tissue Effect and Biology of Thermal HIFU

HIFU produces an immediate coagulative necrosis of tissue within its focal zone. A thermal effect is also produced in the collagen component of the MLM. Within the tissue treated with HIFU, a hemorrhagic appearance of the tissue is initially noted, as documented in Fig. 51.3. Extracellular lipid and cell debris is removed by macrophages and transported to the liver via lymphatic channels. Figure 51.4 demonstrates foamy macrophages that are removing cell debris and extracellular lipid. Collagen remodeling occurs over time. The majority of remodeling is completed by week 12, according to animal studies. Within the preclinical and pivotal studies for the LipoSonix device, ecchymosis was frequently seen, yet resolved by 12–16 days post-treatment. Tissue lumpiness or contour irregularities were not encountered. The HIFU lesions within treated tissue heal over time, as shown in Fig. 51.5.

The biologic effect of thermal HIFU appears to be different than other forms of body contouring that use bulk heating of tissue. While adipocyte necrosis results in the release of extracellular lipids, fat necrosis and chronic inflammation following HIFU treatment do not occur. There is minimal collateral damage to tissue, with no known effect on arterioles or lymphatic channels. A patent arteriole that crosses the HIFU treatment zone is seen in Fig. 51.6. Laser frequencies (900–1400 nm) used for lipoplasty (via fiberoptic delivery) are attracted to hemoglobin and water chromophores. This will inflict collateral damage on vascular and lymphatic structures. HIFU, like all other forms of sonic energy, can produce self-limited dysesthesias in cutaneous sensory nerves that are located within the treatment zone.

Thermal HIFU as A Noninvasive Body Contouring Device

Initial studies have established that thermal HIFU has a predictable biologic effect for adipocyte ablation and thermal modulation of collagen contained within the MLM. Safety studies in both preclinical studies, pilot studies and a pivotal study demonstrate that subjects treated with HIFU do not show abnormalities in clinical chemistry laboratory tests that evaluate blood lipids, liver function, inflammatory markers or coagulation during the post-treatment interval.8

Within the preclinical studies for the LipoSonix device, dosimetry was optimized in the 47–59 J/cm2 range. While higher energy fluences may be considered, greater levels are reported to be more uncomfortable and do not necessarily produce enhanced outcomes. In the pilot and pivotal studies it was determined that three passes of the treatment head with total tissue dosimetry in the range of 141–177 J/cm2 during a single treatment session would produce a clinical outcome with acceptable patient comfort levels.

The LipoSonix device utilizes a microprocessor-controlled positioning device that moves the transducer head in a water bath to create an X–Y grid pattern that is 5 × 5 cm in dimension, as shown in Fig. 51.7. The transducer head and pattern generating device is seen in Fig. 51.8. Prior to treatment, the areas to be treated are marked with a 5 × 5 cm grid that facilitates placement of the treatment head, as shown in Fig. 51.9. The ultrasonic transducer is energized as it moves through the grid pattern to produce rows of thermal HIFU lesions deep within the treatment zone. Water is the optimal coupling agent between the treatment head and the patient’s skin, as shown in Fig. 51.10

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