The two patients enter the hospital medical scanning unit hoping for opposite outcomes. Vanessa Q., who has suffered several early pregnancy losses, hopes that an ultrasound exam will reveal a living embryo in her still-flat abdomen. Michael P., a 16-year-old who has been suffering from terrible headaches, is to undergo a magnetic resonance imaging (MRI) scan to assure his physician (and himself!) that the cause of the headache is not a brain tumor.
Both ultrasound and magnetic resonance imaging scans are noninvasive procedures that provide images of soft internal structures. Ultrasonography uses high-frequency sound waves that are beyond the range of human hearing. A technician gently presses a device called a transducer that emits sound waves against the skin and moves it slowly over the surface of the area being examined, in this case, Vanessa's abdomen.
The sound waves travel into the body, and when they reach a border between structures of slightly different densities, some of the waves reflect back to the transducer. Prior to the exam, Vanessa drank several glasses of water. Her filled bladder will intensify the contrast between her uterus (and its contents) and nearby organs. Other sound waves continue into deeper tissues, and some of them are reflected back by still other interfaces. As the reflected sound waves reach the transducer, they are converted into electrical impulses that are amplified and used to create a sectional image of the body's internal structure on a viewing screen. This is known as a sonogram.
Glancing at the screen, Vanessa yelps in joy. the image looks only like a fuzzy lima been with a pulsating blip in the middle, but she knows it is the image of an embryo--and its heart is beating!
Vanessa's ultrasound exam takes only a few minutes, whereas Michael's MRI scan takes an hour. First he receives an injection of a dye that provides contrast so that a radiologist examining the scan can distinguish certain brain structures. Then, a nurse wheels the narrow bed on which Michael lies into a chamber surrounded by a powerful magnet and a special radio antenna. the chamber, which looks like a metal doughnut, is the MRI instrument. As Michael settles back and closes his eyes, a technician activates the device.
The magnet generates a magnetic field that alters the alignment and spin of certain types of atoms within Michael's brain. At the same time, a second rotating magnetic field causes particular types of atoms (such as the hydrogen atoms in body fluids and organic compounds) to release weak radio waves with characteristic frequencies. The nearby antenna receives and amplifies the radio waves, which are then processed by a computer. Within a few minutes, the computer generates a sectional image based on the locations and concentrations of the atoms being studied. The device continues to produce data, painting portraits of Michael's brain in the transverse, coronal, and sagittal sections.
Michael and his parents nervously wait two days for the expert eyes of a radiologist to interpret the MRI scan. Happily, the scan shows normal brain structure. Whatever is causing Michael's headaches, it is not a possibly life-threatening brain tumor.