Encyclopedia of Health and Medicine

418 Appendix VIII
realign themselves. The MRI machine then detects
the rate at which the nuclei return to their natural
alignment. A computer constructs multidimensional
images based on this data.
MRI is particularly effective for detecting abnormal
tissue within the body, such as tumors, tears to
muscles, and neurologic injury or deterioration.
Because of the electromagnetic disruption the MRI
machine temporarily causes, people who have
implanted pacemakers and other devices, metal
hardware (such as to repair fractures), permanent
prostheses (such as an artificial eye or COCHLEAR
IMPLANT), and certain other circumstances cannot
undergo MRI. It is essential to remove all clothing
and items that may contain metal; the person wears
a hospital gown during the procedure.
MRI is painless and takes 15 minutes to an hour
depending on the area of the body being scanned.
Sometimes the doctor may choose to administer an
intravenous injection of a contrast medium to
enhance the images the MRI produces. The MRI
machine is very loud and surrounds the person
during the procedure. Some people find the experience
of the procedure disconcerting because of
these factors. The technologist performing the MRI
can provide methods to minimize this. MRI does
not have any adverse side effects.
radionuclide scan A nuclear medicine procedure
that measures the rate of deterioration of
low-level radioactive isotopes to present images of
the cellular function of organs such as the BRAIN,
BONE, LIVER, THYROID
GLAND, and GALLBLADDER.
Radionuclide scans involve exposure to radiation.
Before the scan, the person receives an intravenous
injection of a small amount of fluid, typically
a glucose (sugar) solution, “tagged” with the
appropriate radioisotope (the radionuclide). Cells
throughout the body uptake, or take in, the
tagged glucose molecules. The attached radioisotope
molecules deteriorate as the body uses the
glucose.
Cells in various organs and structures use glucose
at known rates; measuring the rate helps
doctors to determine whether there is abnormal
function such as tumors or healing (increased glucose
use). Slowed uptake may indicate degenerative
disorders or problems with healing.
During the scan the person lies on a procedure
table and the gamma camera or other device passes
over the area of the body being evaluated. The procedure
may take 15 to 90 minutes. There is usually
no need to change out of regular clothes. The risks
of radionuclide scans are minimal. The radioisotopes
dissipate rapidly, so the radiation does not
remain in the body very long. Specialized types of
radionuclide scans include positron emission
tomography (PET) scan and single photon emission
computed tomography (SPECT).
ultrasound Also called ultrasonography, a
diagnostic procedure that uses high-frequency
sound waves (beyond the frequency human hearing
can detect) to create images of internal organs
and structures. Ultrasound does not involve exposure
to radiation. Ultrasound is painless and is
especially effective for evaluating hollow structures
within the body such as the GALLBLADDER,
urinary BLADDER, and arteries and veins. Doppler
ultrasound is a technique that presents moving
images, such as the flow of BLOOD or the movement
of a FETUS within a pregnant woman’s
uterus. Ultrasound is also useful for detecting cysts
and tumors in structures such as the OVARIES, TESTI-
CLES, BREASTS, and PROSTATE GLAND. Doctors sometimes
use ultrasound to guide the placement of
biopsy instruments.
Ultrasound typically requires no advance
preparation, though pelvic ultrasound may
require a full urinary bladder. The procedure is
painless. During the procedure, the sonographer
applies a warm gel to the surface of the skin over
the area being scanned. The gel improves the conductivity
of sound signals. The sonographer gently
presses a transducer against the skin and moves it
in a particular pattern. The transducer emits ultrasound
waves, which “echo” from the structures
within the body. The transducer then picks up the
echoes and transmits them back to the ultrasound
machine, which creates representational images
from them.
Some ultrasound procedures involve placing
the transducer within a natural body opening
such as the VAGINA, RECTUM, or ESOPHAGUS to provide
focused examination of key structures that
are deeper within the body. Transesophageal
ultrasound, for example, can provide close examination
of the heart. ECHOCARDIOGRAM is another
type of ultrasound that specifically examines the
HEART.