PET Scan
Positron emission tomography (Pet) was the first brain-imaging
technique to provide images of brain activity (functional brain
images) rather than images of brain structure (structural
brain images).
History of PET Scan
The first PET camera was built for human studies by Edward Hoffman,
Michael M. Ter-Pogossian, and Michael E. Phelps in 1973 at Washington
University, with DOE and NIH support. Phelps, who is often credited with
inventing PET, received the 1998 Enrico Fermi Presidential Award for his
work. The first whole-body PET scanner appeared in 1977. Today, there are
over 400 PET scanners in use worldwide.
Recent Developments
Instrumentation for positron emission tomography (PET) imaging has
experienced tremendous improvements in performance over the past 60 years
since it was first conceived as a medical imaging modality. Spatial resolution
has improved by a factor of 10 and sensitivity by a factor of 40 from the early
designs in the 1970s to the high-performance scanners of today.
Multimodality configurations have emerged that combine PET with
computed tomography (CT) and, more recently, with MR. Whole-body scans
for clinical purposes can now be acquired in under 10 min on a
state-of-the-art PET/CT.
Early Researchers using PET Scan
Two initial reports of the opportunities offered by the use of coincidence
detection of positron capture in medicine were published independently in
1951. Wrenn et al.4 from Duke University suggested the use of coincidence
detection, whereas Sweet5 from Boston referred to it in a more general report
on the use of nuclear disintegrations in the diagnosis and treatment of brain
tumors. A detailed publication on the application of the coincidence detection
for localizing brain tumors was published by Brownell and Sweet from
Massachusetts General Hospital (MGH) in 1953.6 A pair of sodium iodide
detectors [Fig. 1(a)], operating in coincidence, were placed on either side of
the head and scanned rectilinearly to derive the distribution of the
radioactivity. The application was to image the underlying distribution of the
positron emitter as, which leaks into the disrupted blood brain barrier caused
by the tumor, as shown in Fig. 1(b).
In 1961, a group at Brookhaven National Laboratory developed a ring of 32
sodium iodide-based coincidence detectors surrounding the head.7 At the
Donner Laboratory of the University of California, Berkeley, Anger,8 who
invented the gamma camera, used it in coincidence with a reference detector.
By the late 1960s, the MGH group led by Brownell9 had developed what they
termed the hybrid scanner by constructing static two-dimensional (2-D)
arrays of individual coincident detectors to view the brain. The image, while
still primarily 2-D, was recorded with much increased sensitivity over the
single pair of scanning probes. The MGH group went on to develop a device
known as the positron camera shown in Fig. 2, that had a field of view of
27 cm×30 cm, which became operational in the early 1970s.10 A unique
feature of the design, devised by Burnham and Brownell10, was a coding
scheme that allowed small sodium iodide crystals to be encoded by fewer,
larger photomultipliers, thereby reducing cost and improving spatial
resolution.
How it works
First, a person (or other animal) receives an injection of radioactive 2-DG. (The
chemical soon breaks down and leaves the cells. The dose given is harmless,
and over time gradually leaves the cells.) The person’s head is placed in a
machine similar to a CT scanner. When the radioactive molecules of 2-DG decay,
they emit subatomic particles called positrons, which meet nearby electrons. The
particles annihilate each other and emit two photons, which travel in directly
opposite paths. Sensors arrayed around the person’s head detect these photons,
and the scanner plots the locations from which these photons are being emitted.
From this information, the computer produces a picture of a slice of the brain,
showing the activity level of various regions in that slice.
PET scan of the heart
In addition, a PET scan can provide information on blood flow through the
coronary arteries into the heart muscle. A nuclear scan provides higher
resolution and imaging speeds and is becoming increasingly popular for
cardiac imaging.
PET scan of the brain
A PET scan uses injected radioactive material to visualize the active regions
of the brain. PET scans can be used to detect cancerous tumor cells or to
diagnose conditions such as epilepsy. By injecting a small amount of
radioactive sugar (a sugar) into a vein, the PET scanner rotates the body and
creates a picture of the glucose metabolism in the brain. The image of a
malignant tumor cell is brighter because it is more active and absorbs more
glucose than a normal cell.
Once the radioactive material has been injected into the vein, it takes 3 to 90
minutes for the radioactive material to settle into the brain tissues. During this
time, the patient will be asked to remain calm and quiet and not to talk or
move much. The real scan will take 30 to 45 minutes. Due to the small
amount of the radioactive material used in this test.
The benefit of PET scans in cancer diagnosis
Due to the biological nature of disease and PET being a biological imaging
test, PET is able to detect and screen most cancers, often even before they
are detectable by other tests. Because cancer cells use more glucose to grow
than normal tissues, they appear brighter in the picture. Clinical research has
demonstrated that PET is more effective than conventional imaging in
diagnosing, staging, and monitoring (restaging) various types of cancer.
Recently, CMS has expanded its scope to include many additional tumor
types and confirmed PET’s role in previously covered tumor types.
The most significant current application of PET technology is its
use in identifying the distribution in the brain of molecules of
interest (e.g., particular neurotransmitters, receptors, or
transporters)—see Camardese et al. (2014). This is readily
accomplished by injecting volunteers with radioactively labeled
ligands (ions or molecules that bind to other molecules under
investigation). Then, PET technology can be used to document
the distribution of radioactivity in the brain.
PET Scan and Neurological Disorders
PET/CT scans are the only way doctors can tell if a person has
Alzheimer's, Huntington's, or Pick's. Huntington's is a degenerative brain
disease that gradually reduces a person's ability to move, think, talk, and
reason. Pick's is not known exactly what causes it, but it affects the front
and back of the brain, leading to a gradual, irreversible decline similar to
Alzheimer's. Prior to PET scans, Pick's was only diagnosed after an
autopsy. PET/CT can also show if memory loss is caused by depression, or
vascular dementia. Vascular dementia is the second most common type of
dementia, and it usually results from a stroke that damages the part of the
brain that controls memory or emotion. PET/CT also shows if the tumor is
cancerous, if it's cancerous, and if it's scar tissue.
PET/CT has made numerous discoveries about Parkinson's effects on the
brain, which is an essential part of finding a cure for Parkinson's disease.
PET/CT is able to accurately diagnose Parkinson's disease, and distinguish
between Parkinson's disease and other movement disorders.
Patients with epilepsy typically respond to medications that control or
eliminate severe seizures. If medication does not control seizures, surgery
may be necessary. PET can help doctors avoid unnecessary surgery by
helping them decide if surgery is the right treatment.
Conclusion (PET scan)
One of the disadvantages of PET scanners is their operating cost. For reasons of safety, the
radioactive chemicals that are administered have very short half-lives; that is, they decay and
lose their radioactivity very quickly. For example, the half-life of radioactive 2-DG is 110 minutes;
the halflife of radioactive water (also used for PET scans) is only 2 minutes. Because these
chemicals decay so quickly, they must be produced on site, in an atomic particle accelerator
called a cyclotron. Therefore, the cost of the PET scanner must be added to the cost of the
cyclotron and the salaries of the personnel who operate it. Another disadvantage of PET scans
is the relatively poor spatial resolution (the blurriness) of the images. The temporal resolution is
also relatively poor. The positrons being emitted from the brain must be sampled for a fairly long
time, which means that rapid, short-lived events within the brain are likely to be missed. These
disadvantages are not seen in functional MRI, described in the next paragraph. However, PET
scanners can do something that functional MRI scanners cannot do: measure the concentration
of particular chemicals in various parts of the brain.
For most purposes, PET scans have been replaced by functional magnetic
resonance imaging (fMRI), which is less expensive and less risky. Stan-
dard MRI scans record the energy released by water molecules after removal of
a magnetic field.
