NEW YORK--Transrectal ultrasound is a valuable tool in the diagnosis of prostate cancer, but much can be done to optimize the procedure. W. Holt Sanders, MD, assistant professor of urology, Emory University School of Medicine, says that the basic principle is to "make sure the area of interest is within the focal range."
Speaking at a Lenox Hill Hospital conference on prostate cancer, Dr. Sanders explained that ultrasound penetration and resolution quality are inversely
proportionate. High-frequency sound waves penetrate poorly but offer good resolution; low-frequency waves penetrate farther but with poor resolution. "The frequency can therefore be increased to improve visualization of the posterior part of the gland, and decreased if necessary to better view the anterior zones of the prostate," he said.
Dr. Sanders pointed out that the majority of cancers (70%) originate in the peripheral zone of the gland, 25% in the transition zone, and 5% in the central zone. Tumors do not originate in the fibromuscular zone. With an adjustable machine, the frequency can be altered, depending on the area being viewed. On machines that are not adjustable, a mid-range of 7 mHz is usually used.
The ultrasound automatically enhances the image by time-gain compensation, he said. By amplifying more distant signals, visualization of more distant structures is improved.
Dr. Sanders also stressed the importance of knowing the focal range (the range of optimal resolution) of the ultrasound machine. With that knowledge, the operator can move the transducer so that the area of interest is within the focal range.
He offered a tip for correcting reverberation, a common and annoying aberration in the ultrasound image that occurs when sound waves hit an interface at a right angle: Avoid a right angle by moving the transducer off the midline, tilting it instead.