What Extracorporeal Shockwave Lithotripsy is Used For
Extracorporeal shockwave lithotripsy is the most commonly prescribed treatment for kidney stones. The technique uses shockwaves to break up stones, so that they can easily pass through the urinary tract. Most people can resume normal activities within a few days. Complications of extracorporeal shockwave lithotripsy include blood in the urine, bruising, and minor discomfort in the back or abdomen.
In extracorporeal shockwave lithotripsy, shockwaves that are created outside the body travel through the skin and body tissues until they hit the denser kidney stones. After the stones have been hit, they will break down into sand-like particles that are easily passed through the urinary tract in the urine.
How Does a Lithotripter Work?
The lithotripter attempts to break up the stone with minimal collateral damage, by using an externally-applied, focused, high-intensity acoustic pulse. The sedated or anesthetized patient lies down in the apparatus’ bed, with the back supported by a water-filled coupling device placed at the level of kidneys. A fluoroscopic x-ray imaging system or an ultrasound imaging system is used to locate the stone and aim the treatment. The first generation lithotripter has a half ellipsoid-shaped piece that opens toward the patient. The acoustic pulse is generated at the ellipsoidal focal point that is furthest from the patient and the stone positioned at the opposite focal point receives the focused shock wave. The treatment usually starts at the equipment’s lowest power level, with a long gap between pulses, in order to accustom the patient to the sensation. The length of gap between pulses is also controlled to allow cavitation bubbles to disperse, minimizing tissue damage.
Second and later generation machines use an acoustic lens to focus the shock wave. This functions much like an optical lens, focusing the shock wave at the desired loci. The frequency of pulses are currently left at a slow rate for more effective comminution of the stone and to minimize morbidity, while the power levels are then gradually increased, in order to break up the stone. The final power level usually depends on the patient’s pain threshold and the observed success of stone breakage. If the stone is positioned near a bone, this treatment may be more uncomfortable because the shock waves can cause a mild resonance in the bone which can be felt by the patient. The sensation of the treatment is likened to an elastic band twanging off the skin. Alternatively, the patient may be sedated during the procedure. This allows the power levels to be brought up more quickly and a much higher pulse frequency, sometimes above 100 shocks per minute.
The successive shock wave pressure pulses result in direct shearing forces, as well as cavitation bubbles surrounding the stone, which fragment the stones into smaller pieces that then can easily pass through the ureters or the cystic duct. The process takes about an hour. A ureteral stent (a kind of expandable hollow tube) may be used at the discretion of the urologist. The stent allows for easier passage of the stone, by relieving obstruction and through passive dilatation of the ureter.
How the Shockwaves are Generated
There are three different ways to generate the shockwaves:
- Electrohydraulic: The original method of shockwave generation was electrohydraulic, meaning that the shockwave is produced via spark-gap technology. In an electrohydraulic generator, a high-voltage electrical current passes across a spark-gap electrode located within a water-filled container. The discharge of energy produces a vaporization bubble, which expands and immediately collapses, generating a high-energy pressure wave.
- Electromagnetic: In an electromagnetic generator, a high voltage is applied to an electromagnetic coil, similar to the effect in a stereo loudspeaker. This coil, either directly or via a secondary coil, induces high-frequency vibration in an adjacent metallic membrane. This vibration is then transferred to a wave-propagating medium (often water) to produce shockwaves.
- Piezoelectric: The piezoelectric generator takes advantage of the piezoelectric effect. Piezoelectric ceramics or crystals, set in a water-filled container, are stimulated via high-frequency electrical pulses. The alternating stress/strain changes in the material create ultrasonic vibrations, resulting in the production of a shockwave.
The Lithotripter Focusing System
The focusing system is used to direct the generator-produced shockwaves at a focal volume. The basic geometric principle used in most lithotripters is that of an ellipse. Shockwaves are created at one focal point and converge at the second focal point. The target zone, or blast path, is where the shockwaves are concentrated and fragmentation occurs.
Focusing systems differ, depending on the shockwave generator used. Electrohydraulic systems used the principle of the ellipse; a metal ellipsoid directs the energy created from the spark-gap electrode. In piezoelectric systems, ceramic crystals arranged within a hemispherical dish direct the produced energy toward a focal point. In electromagnetic systems, the shockwaves are focused with either an acoustic lens or a cylindrical reflector.
The Lithotripter Imaging System
Imaging systems are used to localize the stone and to direct the shockwaves onto the calculus, as well as to track the progress of treatment and to make alterations as the stone fragments. The two methods commonly used to localize stones include fluoroscopy and ultrasonography:
- Fluoroscopy, which is familiar to most urologists, involves ionizing radiation to visualize calculi. As such, fluoroscopy is excellent for detecting and tracking calcified and otherwise radio-opaque stones, both in the kidney and the ureter.
- Ultrasonographic localization allows for visualization of both radiopaque and radiolucent renal stones and the real-time monitoring of lithotripsy. Most second-generation lithotripters can use this imaging modality, which is much less expensive to use than radiographic systems. Although ultrasonography has the advantage of preventing exposure to ionizing radiation, it is technically limited by its ability to visualize ureteral calculi, typically due to interposed air-filled intestinal loops. In particular, smaller stones may be difficult to localize accurately.