Where's the Fat?
At Howard University in Washington, D.C., chemist George
Gassner records information as animal scientist Al Mitchell (left) and
university scientist Hua Fu Song examine a cross-sectional magnetic resonance
image from the abdominal area of a pig. The sharp images of the MRI's let
scientists see how a particular swine beeding line looks-in terms of
fat-to-lean ratio, or how a particular diet is affecting an animal's body. In
the background, animal geneticist Armin Scholz monitors the pig within the MRI
No one could call Al Mitchell's Sundays relaxing.
First, there's the task of negotiating a truckload of a dozen or so hefty
hogs through downtown Washington, D.C., traffic. Then, Mitchell gets to help
lift each of the hogs onto a high tabletwiceat their destination,
before heading home with his squealing load at the end of a long, long day.
Mitchell's no moonlighting hog farmer; he's an animal scientist with USDA's
Agricultural Research Service.
An even bigger surprise is the hogs' destination on their Sunday jaunts:
Howard University, where they undergo magnetic resonance imaging
(MRI)the same high-tech procedure medical doctors use to detect internal
disorders such as brain tumors in humans.
Since 1985, Mitchell has collaborated with Paul Wang of Howard University on
using MRI's to analyze body composition of research hogs while they're still on
The sharp images of the MRI's let scientists see how a particular swine
breeding line looksin terms of fat-to-lean ratio, or how a particular
diet is affecting an animal's bodyand still use that animal for further
"Before MRI's, there really was no accurate way of measuring the body
composition of a live animal," explains Mitchell, who is with the ARS
Growth Biology Laboratory at Beltsville, Maryland. "The MRI provides a lot
of information you can't get with other techniques, such as the amount of fat
on the animal, the distribution of that fat, or the volume of muscle in
specific muscle groups.
Using DEXA, or dual-energy x-ray absorptiometry, ARS animal
scientist Al Mitchell can noninvasively measure the body composition of live,
anesthetized pigs. (K5908-9)
Magnetic resonance involves placing a specimensuch as a 90-kilogram
(200-pound) porkerinside a device with a strong magnetic field, which
causes certain naturally magnetized atoms in the specimen to orient themselves
in the field, much as a compass needle lines up with the poles in the Earth's
While the atoms are lined up, a short pulse of radio waves is emitted to
give them a little push. When that pulse ends, the atoms bounce back into
alignment, giving off a faint radio signal that's picked up by the magnetic
resonance device. These signals are transmitted to a computer for analysis and
converted into a visual image.
"Since this is not a destructive technique, we can follow changes in
the animal's body composition as it grows," notes Mitchell.
While the MRI's are as clear as a pork chop on a plate, obtaining them is no
picnic. Each pig is anesthetized, then lifted onto the magnetic resonance
device's table. Once a pig is in place, a body section up to 40 centimeters
long is scanned at increments of one centimeter; then the pig must be
repositioned on the table for another scan. At an average of nearly 13 minutes
per scan and four scans per pig, a 15-pig day seems interminable.
Mitchell's studies have focused on the impact of diet on pigs, while
colleague Armin M. Scholz has used the MRI's to study body type of pigs
susceptible to porcine stress syndrome. A visiting scientist from Humboldt
University in Berlin, Germany, Scholz says PSS-positive pigs are leaner.
A DEXA scan of the live pig discloses fat and lean tissue
"In my diet study, I scanned 601 pigs, starting at 20 kilograms [about
44 pounds] each, and I followed them to 60 kilograms [about 132 pounds],"
Mitchell recalls. "I slaughtered animals from each weight group and did
chemical analyses of the tissue to determine if the MRI was presenting an
accurate picture of fat and lean. There was a good match between the chemical
analyses and what I saw on the MRI's."
Mitchell can also check body composition closer to home with a Dual-Energy
X-ray Absorptiometry (DEXA) unit next to his Beltsville lab. Unlike the MRI,
which presents images of cross-sectional "slices" of the pig's body,
the DEXA is a whole-body scan without the distinct internal views.
DEXA uses two different energy-levels of X-rays to differentiate between
lean and fat, providing percentages and grams of fat and lean tissue throughout
the whole body or in a particular area, such as a front or rear leg.
"This system is often used for human body composition studies, but this
is the first time it's been used extensively for farm animals," says
Mitchell. "DEXA images are not as precise as those in an MRI, but the
unit's less expensive and easier to work with technically. Plus, you can get an
image of a 90-kilogram pig in 35 minutes.
"These techniques would not be used by the farmerthe equipment is
still too expensive," Mitchell says. "But as research tools, they
provide a way for us to evaluate an animal without harming it. You don't want
to have to slaughter a valuable animal just to see how it's growing."
By Sandy Miller Hays, ARS.
D. Mitchell is with the USDA-ARS
Biology Laboratory, 10300 Baltimore Avenue, Beltsville, MD 20705-2350;
phone (301) 504-8868, fax (301) 504-8623.
"Where's the Fat?" was published in the
July 1995 issue of
Agricultural Research magazine.