Contents
- Critical Resources
The Operating Room of the Future
Critical Resources
The Operating Room of the Future
Ultrasound as a scalpel
A technology that progressed rapidly from discovery to clinical implementation is magnetic resonance–guided focused ultrasound. Physicians typically use ultrasound to image tissues in the body (see "An Imaging Primer"); when high-frequency sound waves bounce off internal tissues, the echoes they produce can be analyzed as images of tissues. However, it has been known for more than 60 years that focusing a beam of ultrasound to a particular point in the body could be used to destroy or remove a tissue — akin to performing surgery but without a scalpel.
In the early 1990s, Kullervo Hynynen and Jolesz, both at Brigham and Women's Hospital at the time, showed that this type of "surgery" could be performed inside a magnetic resonance imaging (MRI) scanner to guide and monitor the procedure. Use of the MRI allows surgeons to carefully plan where to point the ultrasound and to immediately visualize the results of the operation. In addition, because energy from the ultrasound beam produces heat, a special application of MRI allows surgeons to monitor the temperature in the target tissue and maintain it within a certain range.
Hynynen and Jolesz have taken their research to the clinic. Research by the Focused Ultrasound Laboratory core at NCIGT led to the development of an instrument, called ExAblate 2000, for transmitting focused ultrasound waves. This instrument was approved by the U.S. Food and Drug Administration (FDA) in 2004 for the removal of fibroids.
Fibroids are generally benign muscular tumors that grow in the wall of the uterus. Most women with fibroids do not have symptoms, but for those whose fibroids cause pain or severe bleeding, removal of the uterus is one of the few options available to them. But with focused ultrasound, a fibroid can be removed without surgery.
A woman lies facedown on the MRI scanner, and beneath her the ExAblate 2000 ultrasound transducer transmits a focused stream of waves for up to several hours. The heat generated by these waves gradually destroys the fibroid, while the MRI scanner continuously monitors the temperature of tissues in the uterus as well as the position of the fibroid. The recovery time for the patient is reduced, and she is able to return to normal activities more quickly than after traditional surgery.
Because of the success of focused ultrasound with uterine fibroids, NCIGT researchers are testing the same technology on different organs, such as the prostate or brain, by modifying the transducer and how the treatment is delivered. These procedures are at various stages of testing in animals or in clinical trials in patients.
Focused ultrasound is also proving useful for the delivery of medicines to the brain. Humans have a collection of tightly packed epithelial cells within the blood vessels leading to the brain — called the blood-brain barrier — which prevents most larger molecules, including drugs, from passing through the bloodstream into the brain. Researchers in the Focused Ultrasound Laboratory core have developed a technology that uses focused ultrasound to poke temporary holes in the blood-brain barrier to allow drugs through. So far, the strategy has been successful in delivering chemotherapy for brain tumors in animal models.
AN IMAGING PRIMER
Functional magnetic resonance imaging reveals areas of the brain that become active when an individual performs a particular task. This type of imaging is used to create maps of human brain function.
Magnetic resonance imaging (MRI) uses a powerful magnetic field and pulses of radio wave energy to construct images of the structures and organs inside the body. MRI can provide a greater level of detail for some areas of the body than can other imaging systems. It is especially useful in neurological, musculoskeletal, cardiovascular, and oncological imaging. The strength of the magnetic field in an MRI scanner is measured in Tesla units. The standard MRI systems used for patient care imaging are 1.5 Tesla. But more powerful and faster 3-Tesla MRI systems are becoming more common in hospitals and research institutions.
X-rays are a form of ionizing radiation that can travel through the body. When X-rays strike a film, they produce a picture. Dense tissues in the body, such as bones, absorb many of the X-rays and appear white on the film, whereas less dense structures appear as different shades of gray.
Functional MRI (fMRI) measures signal changes in the brain due to changing activity. Increased neural activity causes an increased demand for oxygen and thus an increase in the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin, which can be detected as a stronger MR signal.
Computed tomography (CT) uses X-rays and computer processing to generate a three-dimensional image of the inside of the body from a series of images taken from different angles. CT scanning is a good tool for examining bone and calcifications within the body or such structures as blood vessels.
Ultrasound consists of high-frequency sound waves that can produce images when they reflect off organs and other structures in the body. The most well-known imaging application for ultrasound is producing pictures of fetuses in the womb.
Positron emission tomography (PET ) detects gamma rays emitted by a positron-emitting radionuclide (tracer) ingested by a patient. Images of tracer concentration in three-dimensional space within the body are reconstructed by computer analysis. The reconstruction is typically accomplished with the aid of a CT scan performed on the patient during the same session in the same machine.
