JULY 26--A new 'large-bore' (85-cm-diameter opening), x-ray imaging device, installed at the Mallinckrodt Institute of Radiology (St. Louis, MO) in Dec. 2000, allows patients to have a computed-tomography (CT) scan taken in the actual treatment position. Before now, many patients with breast cancer, lung cancer, or Hodgkin's lymphoma were not able to extend their arms into treatment position because of limitation of the bore size of the CT scanner. This technology offers the possibility of planning a patient's radiation treatment more effectively.
To get scanned, the patient lies on a table in treatment position, and the table then slides so that the body region of interest is inside a large vertical ring or 'bore' around which the x-ray camera and detection equipment rotate. However, conventional bores have a diameter of only 70 cm (about 28 inches). This makes it difficult for some patients to be placed in treatment position. For example, a breast cancer patient must lift an arm at almost right angles to her back, so that the x-ray camera can target the breast region and avoid other sensitive areas that do not contain the cancer.
At the meeting of the American Association of Physicists in Medicine (AAPM) in Salt Lake City, UT, this week, medical physicists will discuss the performance of the new large-bore scanner. Why wasn't this technology available before? Larger imaging devices are actually much harder to make, says medical physicist Sasa Mutic, an assistant professor of radiology at the Mallinckrodt Institute. In the larger-bore CT scanner, x-rays have to travel longer distances to the patient's body, and this degrades the quality of the image.
The new system's manufacturer and designer, Marconi Medical Systems (Cleveland, OH; www.picker.com/www/marconimed.nsf), solved this problem by making a specially designed spiral CT scanner. In this design, the x-ray tube rotates continuously around the patient in a helix pattern while the detectors are stationary. The design greatly increases the quality of the images. In addition, the new device allows for a larger 'scanner field of view' (SFOV) compared to traditional scanners. (The SFOV is the size of the largest object that can be seen on a CT scanner.) Larger SFOV allows for full imaging of cancer regions in large and obese patients, who sometimes have trouble entering the smaller openings. It also enables oncologists to get a more accurate view of a patient's body dimensions, which helps them calculate more accurately the appropriate radiation for cancer treatment.
The cost of the large-bore CT scanner is about $900,000, comparable to other systems used for the same purpose. It is now being used in several radiation centers across the USA and in Europe.