A PRIMER ON MEDICAL IMAGING -- PART TWO
by Egon Weck 

This month FDA Consumer offers the second of two articles on the wide array 
of radiological techniques that physicians can use to help them "see" inside
the body.  The first article, in last month's issue, covered techniques that
use X-rays, though often in ways far different from the traditional X-ray 
machine.  Part two covers non-X-ray techniques, some of which work without
potentially hazardous ionizing radiation. 

Advances in X-ray technology have vastly increased the range and precision of 
medical imaging.  Meanwhile, research scientists have developed entirely new
imaging technologies, some of which enable physicians to look inside the body 
without subjecting it to potentially harmful ionizing radiation, such as
X-rays.  They also enable doctors to "see" beyond the capabilities of 
X-rays.  And in some instances, they can be used to study body functions such 
as metabolism as well as anatomical features. 

Sound Waves from the Navy 

The Navy has long used sonar, a system of underwater detection based on 
ultra-high frequency sound waves, to locate submarines and other underwater 
objects.  Like sonar, ultrasound medical imaging, or sonography, relies on
the echoes of inaudible, high-frequency sound waves.

Medical sonography has been in use in the United States since the early 
1960s.  To make sonograms (sound "pictures"), ultrasound waves are
transmitted from a wand-like probe called a transducer, which is passed back
and forth in contact with the skin over target areas such as the liver or a 
kidney.  The ultrasound waves bounce off the internal organs and echo back to 
the transducer.  Other equipment converts the echoes electronically into a
picture on a TV screen where they can be monitored, recorded on videotape, or 
photographed. 

Sonograms can't provide fine structural details, but they can show the size 
and shape of an organ.  And they can reveal cysts and tumors as well as 
abnormalities of the heart. 

Ultrasound, however, is ineffective in imaging the lungs, bones or brain. 
And obesity or large scarred areas pose obstacles to ultrasound imaging.

Obstetricians have turned to ultrasound as a safe alternative to X-rays.
However, while no harmful effects on the human fetus have been documented,
experts recommend that it be used only in cases of clear medical necessity. 
For example, some question the use of ultrasound if the sole motive is to 
predict the baby's sex.  From fetal sonograms, obstetricians can often
predict twins, locate the placenta, identify abnormalities, help prepare for
a Caesarean birth, and tell the position and age of the fetus.

Sonograms are used in other applications: to identify aneurisms--dangerous
outpouchings of the aorta or other arteries; to locate blocked bile ducts,
gallstones and liver disorders, such as cysts, tumors and abscesses; to 
identify abnormalities and diseases of the pancreas, kidneys and thyroid. 

With a technique called Doppler ultrasound, specialists can detect abnormal 
rates of blood flow that betray blockages or narrowing of blood vessels and 
blood clots.

Newer ultrasound probes can be used inside the vagina for a closer look at a
fetus, or inside the rectum to detect signs of colon cancer, or to view the 
prostate gland.  Specialized ultrasound probes have also proven useful in 
diagnosing the cause of female infertility. 

Radiation from the Inside Out 

Nuclear medicine emerged after World War II when radionuclides (radioactive 
isotopes, which emit ionizing radiation) became available.  At first, 
radiation from radionuclides was used to destroy cancerous tissue inside the
body.  In 1963, however, a body scanner using radionuclides was developed.
Unlike an X-ray machine, which beams radiation at the body from outside, a
nuclear scan places the source of radiation inside the patient. 

To prepare for nuclear scans (also known as scintigrams), a very small and
virtually harmless amount of a radionuclide is administered by mouth, 
injection or inhalation.  A variety of radionuclides, such as technecium and
thallium, are available; each has a special affinity for a different organ or 
part of the body. 

A "camera" or scanning device then picks up the radiation being emitted from
the body and transforms it into an image.  Nuclear scans lack the clear 
definition of structure visible on an X-ray.  But they can reveal areas of an 
organ, such as the liver, that are not functioning normally.

Some radionuclides that concentrate in diseased areas, such as tumors, show 
them as hot spots on a scintigram.  Others concentrate in healthy,
functioning tissues to reveal areas of disease as cold spots. 

Nuclear scans of bones can enable doctors to detect bone tumors long before 
they show up on X-rays.  They also aid in diagnosing bone injury, infection,
and arthritis.  Nuclear scans are also used to locate blood clots in the
lungs, and scans of the liver help to diagnose cirrhosis, hepatitis, tumors,
cysts and abscesses.  In the brain they can uncover tumors and areas damaged
by stroke.

SPECT and Stroke

The single photon emission computed tomography (SPECT) scan is a refinement 
of nuclear scanning.  SPECT employs some of the same radionuclides, but it
uses a more sophisticated camera to pick up the radiation.  SPECT resembles 
the CAT scan inasmuch as the signals picked up by the "camera" are fed to a 
computer, which performs countless computations and transmits the results to
a TV screen to produce either a slice-like cross-section or a 3-D image.

While some of the radionuclides used in nuclear scans are employed in SPECT,
newer ones have been developed especially for SPECT.  One new injectable
imaging agent, called SPECTamine, is specifically designed to pass intact 
through the blood-brain barrier (which keeps many chemicals out of the
brain).  When used by skilled specialists, the new agent can help make quick, 
accurate assessments of the effects of a stroke, showing which blood vessels
have been affected and the nature and extent of brain damage. 

A Magnet Stronger Than Earth's

At the heart of a magnetic resonance imaging (MRI) machine is a large 
cylindrical magnet that may weigh many tons.  The patient is carefully
positioned inside the cylinder.  When the machine is turned on, the patient 
is subjected to a powerful--though apparently harmless--magnetic field
thousands of times stronger than the Earth's. 

The magnetic field acts on the atomic nuclei in the cells of the patient's
body, lining them up like compass needles.  The MRI machine then sends out a
pulsed radio signal bumping the displaced nuclei out of line.  When the radio 
pulse stops, the nuclei return to their original displaced position.  As they 
do, they emit a faint signal, a phenomenon called nuclear magnetic
resonance.  Each element emits a distinct signal. 

Since 75 percent of the body is composed of water and the water molecule is 
made up mostly of hydrogen, the hydrogen atom is often the target of MRI. 
The signals from the hydrogen nuclei differ, depending on the types of
tissues the atoms inhabit.  The hydrogen nucleus of a water molecule in 
normal tissue will behave differently than one in cancerous tissue, for 
example.

The nature, strength and duration of the signals is picked up by MRI's
detector coils.  The signals are processed by computer, and an image is 
projected on a TV screen.  Like other advanced imaging techniques, MRI scans
produce a cross-sectional view, or "slice," through the target area.

By changing the settings on the detector coils, the image can be changed. 
For example, one setting may image the outline of a tumor.  Another will show 
the insides of the tumor in great detail, offering clues as to whether it is
benign or malignant.

Developed in the early 1980s, MRI was first used to image the brain and 
spinal cord.  It has now proved useful in diagnosing many conditions, 
including: disorders of the brain and nervous system; bone, joint and muscle
disorders; tumors; heart and blood vessel problems; and cancer of the 
reproductive organs, liver, kidneys, lymph nodes, bladder, pancreas, and
vocal cords.

PET for Early Signs 

In diseases like cancer, by the time the structural damage shows up on
X-rays, it may be too late to effect a cure.  So medical investigators are
constantly looking for ways to detect early signs of disease. 

Another drawback with X-rays is that mental and nervous system disorders
seldom produce visible anatomical changes.  What is needed is a form of 
medical imaging capable of visualizing metabolic processes.  Enter PET: 
positron emission tomography. 

PET scans employ radionuclides with positrons attached to them.  Positrons
are subatomic particles that resemble electrons but carry a positive instead
of a negative charge.  When a positron collides with an electron, the 
particles are annihilated and transformed into two photons (photons are a 
form of radiant energy).  Because they travel in opposite directions, the 
source of each pair of photons can be identified with great precision.

As in other forms of tomography, a computer processes the information picked
up by a PET scanner and produces an image on a TV screen.  The resulting
image can be color-coded to differentiate distinct areas of the target. 

To prepare for a PET scan, a positron-labeled compound is administered, often 
by inhalation.  The positron tagging is carried out in a machine called a 
cyclotron, which generates charged atomic particles.  The tagged compounds
emit their positrons in a matter of minutes, so it takes highly skilled teams 
working with nearby cyclotrons to perform a PET scan. 

The cost of the skilled teams and the equipment has made the scans very 
expensive.  There are only about two dozen PET scan facilities in the United
States, and the technology is still largely under investigation, according to 
the American College of Radiology.

"PET's great promise rests with the capability of imaging metabolic rather
than anatomical detail," notes Dr.  Ronald G.  Evens, professor and head of 
radiology at Washington University's School of Medicine in St.  Louis.
"Processes such as oxygen uptake, blood flow, glucose metabolism, and drug
interaction could give us early warning to diseases that produce no 
anatomical changes. 

"For example, we know that cancers start in small metabolic patterns.  It 
would be extremely useful to get a handle on these early signs," Dr.  Evens 
explains. 

PET scans are being applied to the study of brain function and disorders such 
as schizophrenia and Parkinson's disease.  In other areas, the scans are
giving medical scientists a closer look at the mechanism that causes strokes
and heart attacks, and the clogging process that gradually narrows arteries.