In many instances, a quick and painless exam can be performed without ionizing radiation. The exam is called a sonogram, or ultrasound scan. Ultrasound is defined as sound with a frequency greater than 20,000 cycles/sec (Hertz, or Hz), which is the upper limit of audible sound. Sonography for diagnostic imaging purposes employs frequencies of one million to twenty million cycles per second.
These are produced by a device known as a transducer, which is placed directly on, and occasionally within, the patient. The device contains a material that vibrates upon receiving a voltage charge. Sound waves are then created and either transmitted, refracted, or reflected back to a receiver. The degree of reflection is based on applied frequency, the sound velocity, and the acoustic impedance of the tissue. In other words, different tissues will have different responses to the sound waves. Various transducers have been optimized for specific body parts. Major applications of ultrasound include obstetrical imaging, abdominal imaging, and Doppler imaging, a variant technique used in heart and vascular diagnosis. In some instances, biopsies are performed under the guidance of ultrasound, permitting more accurate and less invasive tissue sampling.