Extracorporeal shockwave lithotripsy (ESWL) first came into clinical use in the early 1980s, and now is a standard first-line treatment for nephrolithiasis. This procedure involves the application of a focused, high-intensity acoustic pulse to genitourinary stones, breaking them into small pieces that can then be passed spontaneously via the urine stream. Guidance systems are generally fluoroscopic, but ultrasound guidance may also be used. Sometimes a ureteral stent is placed to facilitate passage of the stone fragments. ESWL is the least invasive of the treatment modalities for nephrolithiasis, which includes percutaneous nephrolithotomy and laser-assisted ureteroscopy, and, in select circumstances, has an equivalent success rate. ESWL has also been applied to other stone diseases, such as gallstones and pancreatic stones, with less success and adoption for various reasons.
A typical acoustically generated shock wave is a very short pulse of about 5-μs duration, with near instantaneous (nanosecond) escalation to peak positive pressure, from 30 to 110 MPa. The positive portion of the pulse (shock front) has duration of about 1 μs, which is followed by a drop to a negative pressure (compressive phase) of about –10 MPa. Finally there is a short tail or tensile phase, familiar in the context of this text as a rarefaction of the acoustic wave. The pulse does not have a dominant frequency; rather, its energy is spread over a large range, typically from 100 kHz to 1 MHz.
Three different types of shockwave generators have been applied in clinical lithotripsy applications: electrohydraulic, electromagnetic, and piezoelectric. In the electrohydraulic generator, a high-voltage spark between two electrodes at the tip of a probe creates a hydraulic shockwave that can be focused on a calculus by an ellipsoid reflector. The electromagnetic lithotripter uses an electrical coil to generate the electrical pulse, which in turn creates a force on a metal plate to generate an acoustic wave. A concave plate is used to focus the acoustic wave in a predictable manner. Lastly, piezoelectric generators produce shockwaves via ultrasonic vibrations, resulting from the application of high-frequency electrical pulses to piezoelectric elements.
Numerous mechanisms of stone fragmentation have been proposed. The shockwaves propagate through the body with minimal collateral damage, as there is little difference in density among the soft tissues. At the stone-tissue/fluid interface, however, the marked difference in density causes the generation of compressive forces in the denser medium. At proper settings this energy is greater than the tensile strength of the stone, and the stone fragments. Spallation occurs when the shockwave passes through the stone and reflects from the rear border. This interface then inverts the positive pressure pulse, creating a tensile stress (as opposed to the compressive stress on the way in). The alternating compression and tension further weakens or fatigues the stone. Cavitation in the urine around the stone is seen in response to the tensile tail of the acoustic impulse. Collapse of microbubbles generates a microjet that impacts the stone surface. In addition, secondary shockwaves are generated with similar effect as the focused shockwave. Delivery of repeated shockwaves eventually leads to pulverization of the stone, which then ideally passes via the urine stream.
High-Intensity Focused Ultrasound (HIFU)
High-frequency diagnostic ultrasound has the advantage of precise definition of structures, albeit at the cost of increased attenuation of the signal and thus a limitation on imaging depth. When the acoustic energy is to be employed as therapy, and high incident intensities are desirable, this translates to a precise delivery of therapeutic ultrasound. In this case, the effect is predominantly thermal, with temperatures of approximately 60°C in the focal region when an exemplary intensity of 1,500 W/cm2 is applied for 1–2 seconds. This leads to instantaneous cell death and coagulative necrosis, with a margin of 6–10 cells between the focal zone and unaffected tissue. Incorporation of dual diagnostic and therapeutic imaging modes builds a targeted therapy system for tissue destruction. HIFU has been used historically as one of many treatments for prostate cancer, and is gaining popularity in treatment of tumors of the liver, kidney, uterus, and breast, as well as treatment for atrial fibrillation.
The need for precise temperature monitoring as an indicator of therapy progress has led to the development of MRI as a concurrent diagnostic imaging mode, to monitor therapy online. Unfortunately this is limited by motion artifact, equipment, and cost issues. Diagnostic ultrasound presents limited ability to monitor tissue changes related to temperature, but advances in elastography, ultrasound-stimulated acoustic emission, and radiation force measurement are potentially opening the door for accurate ultrasound-based therapy monitoring.
Although the primary effect of HIFU is coagulative necrosis by thermal absorption, investigation into utilizing the cavitation effect for enhanced heat delivery and reduced treatment times is underway. Previously avoided as unpredictable, controlled promotion of cavitation with more sophisticated monitoring and the introduction of microbubble-based contrast agents reduce treatment times and deliver more accurate beam shapes.
Many treatment options exist for prostate cancer, including traditional surgery, radiation therapy, cryosurgery, brachytherapy, and hormone therapy. HIFU emerged in the late 1980s as a viable option in appropriate circumstances of early-stage cancers. It is typically delivered via a dual diagnostic-therapeutic endorectal probe, with the patient under spinal or general anesthesia. Three-dimensional imaging advances have increased the accuracy of treatment, and advances in tissue signature detection allow monitoring of the effectiveness of the ablation in real time. Several commercial systems are available, delivering therapy in the frequency range of 3 to 4 MHz with intensities of 1300 to 220 W/cm2. These systems produce temperatures in the focal zone of around 60°C. Because of the ability to target therapy precisely, effect on adjacent structures such as the bladder neck or sphincter is greatly reduced, thus diminishing the risk of urinary dysfunction.
HIFU for treatment of symptomatic uterine fibroids was approved by the FDA in 2004. Again, HIFU confers an advantage over surgery in that it minimally invasively targets the diseased tissue only, sparing surrounding structures. HIFU treatment systems for uterine fibroids often employ magnetic resonance imaging for targeting and monitoring of tissue temperature as a gauge for therapy.
Given the success and widespread adoption of HIFU for treatment of cancer of the prostate, the technology has been applied to many other tumors including kidney, breast, brain, bone, liver, pancreas, rectum, and testes. The above-mentioned advantages apply to these cancers as well, avoiding radical surgery and surrounding tissue damage with precise treatment of the tumor, in some cases curative and in other cases palliative.
HIFU ablation for the minimally invasive treatment of atrial fibrillation offers an exciting alternative to the more invasive MAZE and energy (RF, microwave, and cryotherapy) ablative procedures. In addition, HIFU may be performed on a beating heart, thus obviating the need for coronary bypass. HIFU effectively destroys tissue in the focal region, thus disrupting transmission of abnormal electrical impulses. Several early case series reported successful ablation and return to normal rhythm at rates comparable to RF and cryotherapy, and better than microwave ablation, on the order of 70–80% in one year. There is controversy over disparate data, however, with concern that acoustic radiation force and acoustic cavitation in conjunction with inconsistent thermal deposition can increase the risk of lesion discontinuity and result in gap sizes that promote ablation failure.
In this technique, high-intensity focused ultrasound is used to liquefy subcutaneous fat before it is removed by suction. In contrast, conventional liposuction involves the operator (surgeon) dislodging the fat by moving a thin cannula in the subcutaneous space. UAL delivers acoustic energy in the frequency range of 22 to 36 kHz. The probe vibrates longitudinally, thus creating standing waves at the tip of the probe. Thermal, cavitation, and mechanical mechanisms have all been postulated to achieve the fat liquefaction effect. What is observed is a fragmentation of the adipose tissue and the formation of an emulsion, with the relative sparing of collagen structures, vessels, and nervous tissue. Advocates of UAL claim that this results in a smoother end result with less blood loss and less pain. However, certain complications, most importantly burns, heed careful use of the technology with proper training and instrumentation.