How does the procedure work?
Ultrasound imaging is based on the same principles involved in the sonar used by bats, ships at sea and anglers with fish detectors. As the sound passes through the body, echoes are produced that can be used to identify how far away an object is, how large it is, its shape and its consistency (fluid, solid or mixed). The ultrasound transducer functions as both a generator of sound (like a speaker) and a detector (like a microphone). When the transducer is pressed against the skin it directs inaudible, high-frequency sound waves into the body. As the sound echoes from the body’s fluids and tissues the transducer records the strength and character of the reflected waves. With Doppler ultrasound the microphone captures and records tiny changes in the sound wave’s pitch and direction of the sound. These echoes are instantly measured and displayed by a computer, which in turn creates a real-time picture on the monitor. The “live” images of the examination are usually recorded on videotape b
The ultrasound transducer generates sound waves that pass through the skin and also serves as a microphone to record the returning sounds—the echoes. When pressed against the skin, the transducer directs high-frequency sound waves toward the veins being studied, and records any changes in the pitch and direction of the returning echoes. The bounce-back echoes, called the signature, are automatically measured by the computer and converted electronically to a picture that shows what is happening at that instant—creating a so-called “real time” image on the monitor screen. These images can be videotaped, or they may be frozen in time to obtain still pictures. If a Doppler study is done, changes in blood flow can be displayed in color on the screen and actually heard as a change in pitch.
To begin the procedure, a small amount of radioactive glucose (or similar tracer) is injected into your bloodstream. There is no danger to you from this injection. Glucose (also known as sugar) is a common substance every cell in your body needs in order to function. Radioactive glucose must pass multiple quality control measures before it is used for any patient injection. The radiation exposure associated with PET is similar to that associated with a conventional CT scan. After the injection, you will wait approximately an hour, while the injection material is distributed throughout your body. Then, you will be asked to lie on a table that passes slowly through the scanner. The scanner resembles a CT scanner.
Exposing the patient to radio waves in a strong magnetic field generates data that are used by a computer to create images of tissue slices that may be viewed in any plane or from any direction. The magnetic field lines up atomic particles called protons in the tissues, which are then spun by a beam of radio waves and produce signals that are picked up by a receiver in the scanner. It is these signals that are processed by the computer to produce images. The resulting images are very sharp and detailed, and so are able to detect tiny changes from the normal pattern that are caused by disease or injury. Special settings are used to image various structures, such as arteries in the case of MRA.
A. Before the examination begins, a radioactive substance is produced in a machine called a cyclotron and attached, or tagged, to a natural body compound, most commonly glucose, but sometimes water or ammonia. This process is called radiolabeling. Once this attached substance is administered to the patient, the radioactivity begins to breakdown in the body, resulting in the release of energy that is detected by the PET scanner. Different colors or degrees of brightness on a PET image represent different levels of body function. For example, because healthy tissue uses glucose for energy, it accumulates some of the radiolabeled glucose, which will show up as background areas on the PET images. Cancerous tissue, which uses more glucose than normal tissue, will absorb more of the substance and appear brighter on the PET images. Scientifically speaking, the radioactive substance delay leads to the ejection of positive electrons (called positrons and sometimes referred to as anti-matter). A
