10 March 2009

CAT scanner

The invention:

 A technique that collects X-ray data from solid,
opaque masses such as human bodies and uses a computer to
construct a three-dimensional image.

The people behind the invention:

Godfrey Newbold Hounsfield (1919- ), an English
electronics engineer who shared the 1979 Nobel Prize in
Physiology or Medicine
Allan M. Cormack (1924-1998), a South African-born American
physicist who shared the 1979 Nobel Prize in Physiology or
James Ambrose, an English radiologist

A Significant Merger

Computerized axial tomography (CAT) is a technique that collects
X-ray data from an opaque, solid mass such as a human body
and uses a sophisticated computer to assemble those data into a
three-dimensional image. This sophisticated merger of separate
technologies led to another name for CAT, computer-assisted tomography
(it came to be called computed tomography, or CT). CAT
is a technique of medical radiology, an area of medicine that began
after the German physicistWilhelm Conrad Röntgen’s 1895 discovery
of the high-energy electromagnetic radiations he named “X
rays.” Röntgen and others soon produced X-ray images of parts of
the human body, and physicians were quick to learn that these images
were valuable diagnostic aids.
In the late 1950’s and early 1960’s, Allan M. Cormack, a physicist
at Tufts University in Massachusetts, pioneered a mathematical
method for obtaining detailed X-ray absorption patterns in opaque
samples meant to model biological samples. His studies used narrow
X-ray beams that were passed through samples at many different angles.
Because the technique probed test samples from many different
points of reference, it became possible—by using the proper mathematics—
to reconstruct the interior structure of a thin slice of the object
being studied.
Cormack published his data but received almost no recognition
because computers that could analyze the data in an effective fashion
had not yet been developed. Nevertheless, X-ray tomography—
the process of using X-rays to produce detailed images of thin
sections of solid objects—had been born. It remained for Godfrey
Newbold Hounsfield of England’s Electrical and Musical Instruments
(EMI) Limited (independently, and reportedly with no
knowledge of Cormack’s work) to design the first practical CAT

A Series of Thin Slices

Hounsfield, like Cormack, realized that X-ray tomography was
the most practical approach to developing a medical body imager. It
could be used to divide any three-dimensional object into a series of
thin slices that could be reconstructed into images by using appropriate
computers. Hounsfield developed another mathematical approach
to the method. He estimated that the technique would make
possible the very accurate reconstruction of images of thin body sections
with a sensitivity well above that of the X-ray methodology
then in use. Moreover, he proposed that his method would enable
researchers and physicians to distinguish between normal and diseased
tissue. Hounsfield was correct about that.
The prototype instrument that Hounsfield developed was quite
slow, requiring nine days to scan an object. Soon, he modified the
scanner so that its use took only nine hours, and he obtained successful
tomograms of preserved human brains and the fresh brains
of cattle. The further development of the CAT scanner then pro-
ceeded quickly, yielding an instrument that required four and onehalf
minutes to gather tomographic data and twenty minutes to
produce the tomographic image.
In late 1971, the first clinical CAT scanner was installed at Atkinson
Morley’s Hospital in Wimbledon, England. By early 1972,
the first patient, a woman with a suspected brain tumor, had been
examined, and the resultant tomogram identified a dark, circular
cyst in her brain. Additional data collection from other patients
soon validated the technique. Hounsfield and EMI patented the
CAT scanner in 1972, and the findings were reported at that year’s
annual meeting of the British Institute of Radiology.
Hounsfield published a detailed description of the instrument in
1973. Hounsfield’s clinical collaborator, James Ambrose, published
on the clinical aspects of the technique. Neurologists all around the
world were ecstatic about the new tool that allowed them to locate
tissue abnormalities with great precision.
The CAT scanner consisted of an X-ray generator, a scanner unit
composed of an X-ray tube and a detector in a circular chamber
about which they could be rotated, a computer that could process
all the data obtained, and a cathode-ray tube on which tomograms
were viewed. To produce tomograms, the patient was placed on a
couch, head inside the scanner chamber, and the emitter-detector
was rotated 1 degree at a time. At each position, 160 readings were
taken, converted to electrical signals, and fed into the computer. In
the 180 degrees traversed, 28,800 readings were taken and processed.
The computer then converted the data into a tomogram (a
cross-sectional representation of the brain that shows the differences
in tissue density). A Polaroid picture of the tomogram was
then taken and interpreted by the physician in charge.


Many neurologists agree that CAT is the most important method
developed in the twentieth century to facilitate diagnosis of disorders
of the brain. Even the first scanners could distinguish between
brain tumors and blood clots and help physicians to diagnose a variety
of brain-related birth defects. In addition, the scanners are believed
to have saved many lives by allowing physicians to avoid
the dangerous exploratory brain surgery once required in many
cases and by replacing more dangerous techniques, such as pneumoencephalography,
which required a physician to puncture the
head for diagnostic purposes.
By 1975, improvements, including quicker reaction time and
more complex emitter-detector systems, made it possible for EMI to
introduce full-body CAT scanners to the world market. Then it became
possible to examine other parts of the body—including the
lungs, the heart, and the abdominal organs—for cardiovascular
problems, tumors, and other structural health disorders. The technique
became so ubiquitous that many departments of radiology
changed their names to departments of medical imaging.
The use of CAT scanners has not been problem-free. Part of
the reason for this is the high cost of the devices—ranging from
about $300,000 for early models to $1 million for modern instruments—
and resultant claims by consumer advocacy groups that
the scanners are unnecessarily expensive toys for physicians.
Still, CAT scanners have become important everyday diagnostic
tools in many areas of medicine. Furthermore, continuation of the
efforts of Hounsfield and others has led to more improvements of
CAT scanners and to the use of nonradiologic nuclear magnetic resonance
imaging in such diagnoses.

Godfrey Newbold Hounsfield

On his family farm outside Newark, Nottinghamshire, England,
Godfrey Newbold Hounsfield (born 1919), the youngest
of five children, was usually left to his own devices. The farm,
he later wrote, offered an infinite variety of diversions, and his
favorites were the many mechanical and electrical gadgets. By
his teen years, he was making his own gadgets, such as an electrical
recording machine, and experimenting with homemade
gliders and water-propelled rockets. All these childhood projects
taught him the fundamentals of practical reasoning.
During World War II he joined the Royal Air Force, where
his talent with gadgets got him a position as an instructor at the
school for radio mechanics. There, on his own, he built his an oscilloscope
and demonstration equipment. This initiative caught
the eye of a high-ranking officer, who after the war arranged a
scholarship so that Hounsfield could attend the Faraday Electrical
Engineering College in London. Upon graduating in 1951,
he took a research position with Electrical and Musical Instruments,
Limited (EMI). His first assignments involved radar and
guided weapons, but he also developed an interest in computers
and in 1958 led the design team that put together England’s
first all-transistor computer, the EMIDEC 1100. This experience,
in turn, prepared him to follow through on his idea for computed
tomography, which came to him in 1967.
EMI released its first CT scanner in 1971, and it so impressed
the medical world that in 1979 Hounsfield and Allan M. Cormack
shared the Nobel Prize in Physiology or Medicine for the
invention. Hounsfield, who continued to work on improved
computed tomography and other diagnostic imagining techniques,
was knighted in 1981.

See also : Amniocentesis; Electrocardiogram; Electroencephalogram;
Mammography; Nuclear magnetic resonance; Pap test;X-ray computed tomography.