Acoustical Society of America
158th Meeting Lay Language Papers
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Ultrasound to help kidney stones pass
Wei Lu - uwweil@gmail.com
Center for Industrial and Medical Ultrasound, Applied Physics Lab.
Univ. of Washington, Seattle, WA 98105.
Anup Shah - anupshah@u.washington.edu
Dept. of Urology
Univ. of Washington School of Medicine
1959 NE Pacific St., Box 356510, Seattle, WA 98195
Michael R. Bailey - bailey@apl.washington.edu
Bryan W. Cunitz - bwc@apl.washington.edu
Peter J. Kaczkowski - peter@apl.washington.edu
Oleg A. Sapozhnikov
Center for Industrial and Medical Ultrasound, Applied Physics Lab.
Univ. of Washington, Seattle, WA 98105
Popular version of paper 2pBB2
Presented Tuesday afternoon, October 27, 2009
158th ASA Meeting, San Antonio, TX
In medical treatment, kidney stones are broken and either removed surgically or allowed to pass out of the body with the urine. Often fragments remain. Specifically, fragments often remain in the lower pole of the kidney where they must overcome gravity to exit the kidney. Residual fragments act as a site of re-growth for future stone formation and 50 percent of patients with residual stones return for treatment within five years. The goal of this project is to use ultrasound to push stones and stone fragments within the kidney to move them toward the exit of the kidney and facilitate their passage.
The problem of residual fragments was recognized early in the history of lithotripsy, which is the common treatment of breaking stones with shock waves from outside the body and then allowing the fragments to pass. In fact, early on some doctors gave repeat lithotripsy treatments with the idea of stirring up the pieces to make them pass. The effectiveness was debated and either way, this technique fell out of favor as side effects from lithotripsies were recognized that wouldn’t justify these extra treatments. Today, commonly only stones smaller than 10 cm are treated with lithotripsy because larger stones create too many pieces for the kidney to pass and are removed surgically instead. After lithotripsy, some urologists rap patients on the back and roll them over in what is called “percussion and inversion” treatment with the idea of freeing fragments for passage. The effectiveness of percussion and inversion treatment is debated.
Our solution relies on using a pushing force created by an ultrasound beam. Ultrasound waves can create a force that pushes on an object in the direction the waves propagate by transferring momentum carried by the waves to the object such as a stone. This force called radiation force, or when measured over an area, radiation pressure, is increasingly being used in many applications. Previous ASA talks have described applications now used commercially including levitating objects for non-contact fabrication, characterization of ultrasound sources by the output force, and new ultrasound imaging techniques to record the stiffness of tissue by pushing on it. In fact, the kidney stone can be seen with x-ray or ultrasound imaging to jump around during lithotripsy because ofthe radiation force from the shock waves.
We built a new system that combines a commercial ultrasound imager for visualizing the stone and a focused ultrasound device for pushing the stone. The active end is a hand held applicator that is put in contact with the patient’s skin and manipulated to find and then push the stone. Two conventional and readily available imaging modes are used: brightness or B-mode, and Color Flow Doppler mode. Brightness shows the bright reflection from the stones, which are hard compared to the surrounding tissue and fluid. In Doppler, the stone appears as a bright spot of rapidly changing color , and can be targeted using this “twinkling artifact.” Doppler is usually used to detect blood flow and then label it with red or blue color on the image. With twinkling artifact, something in the ultrasound reflection from the stone confuses the Doppler processing and causes the imager to display the stone as a flickering mosaic of color. In other research we are investigating why the twinkling artifact occurs and are developing methods to make it a reliable imaging tool; we reported preliminary results at the last ASA meeting in Portland, Oregon.
Specifically in the work reported here, natural and artificial stones were implanted in tissue-mimicking phantoms and in pigs, targeted with ultrasound guidance, pushed with focused ultrasound radiation force, and the stone movement recorded with x-ray fluoroscopy and ultrasound. Stones were commonly moved 1-2 centimeters in about 1 second and were moved from the lower pole to the kidney exit, the ureteropelvic junction, or even into the ureter, the tube leading to the bladder. The appended movies show the motion filmed in a phantom and observed with x-ray in a phantom and a kidney. Figure 1 and the Power Point animations help explain the arrangement. The phantom is a translucent gel with red-colored water in a space mimicking the lower and mid poles of the kidney. The 3-mm stone rests in the bottom of the small chamber and is pushed up in to the larger chamber as seen with the camera and x-ray. Figure 1c shows the fluid spaces of the lower portion of the kidney, which appear dark because of addition of an x-ray contrast fluid that then drips out of the kidney. The lower pole and the stone in the movie are again down and to the left in the image and the ultrasound pushes the stone up to the right and actually out of the kidney into the ureter.
Figure 1. (a) photograph of kidney phantom, (b) x-ray image of stone in the phantom, and (c) x-ray image showing the lower portion of kidney collecting system.
WATCH a camera movie of a stone being moved from the lower to the upper chamber in the phantom.
WATCH an X-ray fluoroscopy movie of a stone being moved from the lower to the upper chamber in the phantom.
WATCH an X-ray fluoroscopy movie of a stone being moved from the lower pole through the mid-pole and out the kidney.
DOWNLOAD a power point presentation with animations summarizing information presented here.
Although ultrasound can thermally and mechanically damage tissue, after exposure to the radiation force ultrasound, injury to the kidney was not observed on visual inspection or in samples viewed under a microscope. Thus, our technique to push stones appears effective and safe. This is not unexpected as a calculation of the ultrasound pulse intensity required to lift a submerged 1-cm stone against gravity yields a pulse intensity within the range used safely in diagnostic ultrasound. And pulses need last only a fraction of a second to produce a pushing force sufficient to move a stone or stone fragment.
W. Lu, A. Shah, B.W. Cunitz, P.J. Kaczkowski, O.A. Sapozhnikov, and M.R. Bailey, “Radiation pressure from ultrasound to help kidney stones pass,” J. Acoust. Soc. Am., 126, (4, Pt. 2) 2213 2009.