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CHAPTER 1 - BRAIN SPECT IMAGING
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An Introduction
On November 8, 1895 in Wurzburg Germany, Wilhelm Roentgen, a physicist at the University
of Wurzburg, was working late in his laboratory. In an experiment he had been conducting
before dinner, he had been sending an electric current through a tube. Without warning, a
crystalline material on the other side of the room started emitting light. This made no
sense to him. The rays produced by the tube could only travel a few centimeters; there was
no way that they could travel all the way across the room to the crystal. Where was the
light from the crystal coming from? When Roentgen came back after dinner, he tried the
experiment again. This time he blackened the room, blocking out all light from the windows
and covering the tube with black cardboard so no light could possibly escape when he sent
another electric current through it. Yet still the crystalline material on the other side
of the lab emitted visible light. Roentgen realized that it was caused by some kind of
rays coming from the tube that were far more penetrating than he thought. Since he had
never seen this phenomenon reported in the literature before and didn't know what the
emitted rays were, he named them X-rays to signify their unknown nature.
Roentgen invited his wife Bertha into his lab to witness the experiment. The week before
Christmas he made an X-ray of the bones in her left hand -- the first X-ray ever of the
human body.
After Roentgen published a short paper on the phenomenon, the newspapers sensationalized
his discovery. The physicist's life was never the same again. In 1901 he won the first
Nobel Prize in physics. An intensely private man, he did not relish the attention he got
for his discovery. He certainly could not predict the impact his discovery would have on
the lives of many millions of people.
Many remarkable inventions and discoveries came out of the second half of the 19th century
in science, but it would be hard to overestimate the sensation Roentgen created with his
skeletal photographs and the impact of his discovery on medicine, for it provided a way to
see into the body without cutting it open. It was to be many years before scientists
understood the true nature of X-rays. Roentgen did not, at the time, realize that what he
had done was to cause the crystal in his laboratory to emit visible radiation, when it was
struck by X-rays from the vacuum tube. By bombarding the atoms of the crystal with the
high energy photons from the X-rays, he had knocked the electrons of the crystal's
atoms out of orbit. Whenever electrons move back from a higher energy orbit into a lower
energy orbit they emit photons. This process is called electromagnetic radiation.
Depending on where the emission is on the electromagnetic spectrum, it will be visible or
not visible to the naked eye. In Roentgen's case, the electromagnetic rays were
clearly visible.
Roentgen's first paper on the subject described about 40 different properties of his
newly discovered X-rays. In 1896, when another scientist, Henri Becquerel, read
Roentgen's paper he noticed that these properties had a number of similarities to
those he himself had observed in an unusual rock in his possession. When he first observed
the rock, he did not realize it was emitting its own energy. He accidentally exposed some
photographic plates to his uranium rock and he noticed changes consistent with
Roentgen's discovery. But where Roentgen had accidentally "created"
radiation with his device, Becquerel was the first to discover the principal of naturally
occurring radioactivity.
Marie Curie, one of Becquerel's students, found that certain samples of uranium had
higher levels of activity than other samples. Upon investigation she discovered the
reason: other elements, polonium (which gives off 700 times more radiation than uranium)
and radium (which gives off a million more times more radiation than uranium) were mixed
in with the uranium ore. Radium and polonium were important in that they alerted
scientists to the fact that there were particles in nature that produced their own energy,
as opposed to everything else on Earth, which require energy from an outside source -- the
sun. For the next 40 years, other naturally occurring radioactive elements were
discovered.
Radium was the first radioactive material ever used in medicine. However, it has an
extremely long half-life (time it takes an isotope to decay from a level of radioactivity
down to half that level). Simply for the sake of science, no one was interested in
injecting a long-lived radioactive isotope into the body that was going to remain
destroying cells for years to come. Therefore, when the medical possibilities for
radioactive substances as a detection agent were recognized, scientists realized they
needed to find an isotope that would do the job without doing any significant
damage.
In Marie Curie's day, however, they were stuck with whatever nature had made
available, and the radioactive materials occurring naturally had a half-life that lasted
many, many years. It was important to scientists to be able to use radioactive substances
with properties that allowed them to be safe inside the human body. In other words, they
needed isotopes that would assist in understanding function and then disappear. Irene
Curie (Marie's daughter) recognized this and found a way to artificially create
radioactive material. Eventually technetium, was discovered and proved to be a very good
short-acting isotope. It is the isotope we use in our lab. Of course, they could not
possibly have known how to produce mass quantities of radioisotopes at the turn of the
century. It was not until World War II, after the Manhattan Project developed the atomic
bomb, that science was able to achieve that. The experimental nuclear reactor furnished a
rich source of neutrons that generated radioisotopes in large quantities at a relatively
low cost. From then on, there was no scarcity of radioactive material, for national
defense or scientific research.
Many other important discoveries along the way helped nuclear medicine get where it is
today. As early as 1903, Alexander Graham Bell suggested the first clinical use of
radioactive material. In a letter he suggested that possibly radium in a sealed glass tube
could be inserted near a tumor in a patient. One of the most important discoveries,
however, was made in 1927 in Boston by Herman Blumgart and his colleagues who used a
diluted solution of radon to study circulation. By measuring how fast the diluted Radon
flowed from one side of the body to the other, they were able to measure circulation and
cardiac functions. Consequently they were the first to use radioactive isotopes to measure
physiological functions in the body, and their discovery ushered in the "age of
nuclear medicine." The studies of Dr. Blumgart and others conducted in the 1920s in
observing the transportation of radioactive elements in the body lead to the conclusion
that radioactive material could be used as a tracer. The "age of nuclear
medicine" has created new and safer ways to treat people suffering from disease and
injury.
What Is SPECT?
What is SPECT? It is an acronym for Single Photon Emission Computerized Tomography. SPECT is
a sophisticated nuclear medicine study that looks directly at cerebral blood flow and
indirectly at brain activity (or metabolism). In this study, a radioactive isotope (which,
as we will see, is akin to a myriad of beacons of energy or light) is bound to a substance
that is readily taken up by the cells in the brain.
A small amount of this compound is injected into the patient's vein where it runs
throughout the blood stream and is taken up by certain receptor sites in the brain. The
patient then lies on a table for 14-16 minutes while a SPECT "gamma" camera
rotates slowly around his head. The camera has special crystals that detect where the
compound (signaled by the radioisotope acting like a beacon of light) has gone. A
supercomputer then reconstructs 3-D images of brain activity levels. The elegant brain
snapshots that result offer a sophisticated blood flow/ metabolism brain map. With these
maps, physicians have been able to identify certain patterns of brain activity that
correlate with psychiatric and neurological illnesses.
SPECT studies belong to a branch of medicine called nuclear medicine. Nuclear (refers to
the nucleus of an unstable or radioactive atom) medicine uses radioactively tagged
compounds (radiopharmaceuticals), because the unstable atoms emit gamma rays when they
decay acting like a beacon of energy or light from each location where they go. An
unstable atom is always looking for stability, and it will keep changing or degrading,
until it reaches its most stable form. At each step of decay, it emits a gamma ray
(portion of energy). Scientists can detect those gamma rays with film or special crystals
and can record an accumulation of the number of beacons that have decayed in each area of
the brain. These unstable atoms are essentially tracking devices - they track which cells
were most active and had the most blood flow and those cells which are least active and
have the least blood flow.
Nuclear medicine studies measure the physiological functioning of the body, and they can
be used to diagnose a multitude of medical conditions: heart disease, certain forms of
infection, the spread of cancer, and bone and thyroid disease. My own area of expertise in
nuclear medicine, the brain, uses SPECT studies to help in the diagnosis of head trauma,
dementia, atypical or unresponsive mood disorders, strokes, seizures, the impact of drug
abuse on brain function and atypical or unresponsive aggressive behavior.
During the late70s and 80s SPECT studies were being replaced in many cases by the
sophisticated anatomical CAT and later MRI studies. The resolution of those studies was
far superior to SPECT as far as seeing tumors, cysts and blood clots. In fact, they nearly
eliminated the use of SPECT studies altogether. Yet despite their clarity, CAT scans and
MRIs could offer only images of a static brain, and its anatomy; they gave little or no
information on the activity in a working brain. It was analogous to looking at the parts
of a car's engine without being able to turn it on. In the last decade it has become
increasingly recognized that many neurological and psychiatric disorders are not disorders
of the brain's anatomy, but problems in how it functions.
Two technological advancements have encouraged the use, once again, of SPECT studies.
Initially, the SPECT cameras were single-headed, and they took a long time to scan a
person's brain (up to an hour). People had trouble holding still that long, and the
images were fuzzy, hard to read (earning nuclear medicine the nickname "unclear
medicine") and they did not give much information about the functioning deep within
the brain. Then multi-headed cameras were developed which were able to image the brain
much faster and with enhanced resolution. The advancement of computer technology also
allowed for improved data acquisition from the multi-headed systems. The brain SPECT
studies of today, with their higher resolution, can see into the deeper areas of the brain
with far greater clarity and show what CAT scans and MRIs cannot - how the brain actually
functions.
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