Physician Information on PET/CT Scans
General Information about PET/CT
PET/CT in Types of Cancer
PET/CT in Heart and Brain Disease
Positron Emission Tomography (PET) is rapidly
becoming a major diagnostic imaging modality used predominantly in determining
the presence and severity of cancers, neurological conditions, and cardiovascular
disease. It is currently the most effective way to check for cancer recurrences.
Studies demonstrate that PET offers significant advantages over other forms
of imaging such as CT or MRI scans in diagnosing disease. Last year more than
200,000 PET scans were performed at more than 700 sites around the country.
A PET scanner consists of an array of detectors
that surround the patient. Using the gamma ray signals given off by the injected
radionuclide, PET measures the amount of metabolic activity at a site in the
body and a computer reassembles the signals into images. Cancer cells have higher
metabolic rates than normal cells, and show up as denser areas on a PET scan.
PET is useful in diagnosing certain cardiovascular and neurological diseases
because it highlights areas with increased, diminished or no metabolic activity,
thereby pinpointing problems.
The imaging in PET is all indirect. Like CT, MRI, and SPECT, PET relies on
computerized reconstruction procedures to produce tomographic images.
It is performed by means of detecting positron-emission by use of tomography.
Two ways in which radionuclides decay that will reduce excess positive charge
on the nucleus include the neutralization of a positive charge with the negative
charge of an electron or the emission of a positron from the nucleus. The positron
will then combine with an electron from the surroundings and annihilate. Upon
annihilation both the positron and the electron are then converted to electromagnetic
radiation. This electromagnetic radiation is in the form of two high-energy
photons which are emitted 180 degrees away from each other. It is this annihilation
radiation that can be detected externally and is used to measure both the quantity
and the location of the positron emitter.
Simultaneous detection of two of these photons by detectors on opposite sides
of an object places the site of the annihilation on or about a line connecting
the centers of the two detectors. At this point mapping the distribution of
annihilations by computer is allowed. If the annihilation originates outside
the volume between the two detectors, only one of the photons can be detected,
and since the detection of a single photon does not satisfy the coincidence
condition, the event is rejected. Simultaneous detection provides a precise
field of view with uniform sensitivity. This occurs because wherever the disintegration
takes place between the two detectors, the photons must in sum have traveled
the full interdetector distance in order that the event be recorded.
PET is considered particularly effective in
identifying whether cancer is present or not, if it has spread, if it is responding
to treatment, and if a person is cancer free after treatment. Cancers for which
PET is considered particularly effective include lung, head and neck, colorectal,
esophageal, lymphoma, melanoma, breast, thyroid, cervical, pancreatic, and brain
as well as other less-frequently-occurring cancers.
- Early
Detection: Because
PET images biochemical activity, it can accurately characterize a tumor as
benign or malignant, thereby avoiding surgical biopsy when the PET scan is
negative. Conversely, because a PET scan images the entire body, confirmation
of distant metastasis can alter treatment plans in certain cases from surgical
intervention to chemotherapy.
- Staging
of Cancer: PET
is extremely sensitive in determining the full extent of disease, especially
in lymphoma, malignant melanoma, breast, lung, colon and cervical cancers.
Confirmation of metastatic disease allows the physician and patient to more
accurately decide how to proceed with the patient's management.
- Checking
for recurrences:
PET is currently considered to be the most accurate diagnostic procedure to
differentiate tumor recurrences from radiation necrosis or post-surgical changes.
Such an approach allows for the development of a more rational treatment plan
for the patient.
- Assessing
the Effectiveness of Chemotherapy:
The level of tumor metabolism is compared on PET scans taken before and after
a chemotherapy cycle. A successful response seen on a PET scan frequently
precedes alterations in anatomy and would therefore be an earlier indicator
of tumor response than that seen with other diagnostic modalities.
Because PET measures metabolism, as opposed
to MRI or CT, which "see" structure, it can be superior to these modalities,
particularly in separating tumor from benign lesions, and in differentiating
malignant from non-malignant masses such as scar tissue formed from treatments
like radiation therapy. PET is often used in conjunction with an MRI or CT scan
through "fusion" to give a full three-dimensional view of an organ
and the location of cancer within that organ. Newer PET scanners are being made
that are a combination of PET/CT devices.
PET's ability to measure metabolism also has
significant implications in diagnosing Alzheimer's disease, Parkinson's disease,
epilepsy and other neurological conditions, because it can vividly illustrate
areas where brain activity differs from the norm.
Alzheimer's Diagnosis: Until recently, autopsy
has been considered the only definitive test for Alzheimer's disease (AD). Recent
studies indicate that PET can supply important diagnostic information and confirm
an Alzheimer's diagnosis (Journal of Nuclear Medicine, November 2000). When
comparing a normal brain versus an AD-affected brain on a PET scan, a distinctive
image appears in the area of the AD-affected brain. This pattern is seen very
early in the AD course. Conventionally, the confirmation of AD is a long process
of elimination that averages between two and three years of diagnostic and cognitive
testing. Early diagnosis can provide the patient access to therapies, which
are more effective earlier in the disease.
- PET
also is useful in differentiating Alzheimer's disease from other forms of
dementia disorders, such as vascular dementia, Parkinson's disease, Huntington's
disease, etc.
- Epilepsy:
PET is one of the most accurate methods available to localize areas of the
brain causing epileptic seizures and to determine if surgery is a treatment
option.
By measuring both blood flow (perfusion) and
metabolic rate within the heart, physicians using PET scans can pinpoint areas
of decreased blood flow such as that caused by blockages, and differentiate
muscle damage from living muscle, which has inadequate blood flow (myocardial
viability). This information is particularly important in patients who have
had previous myocardial infarction and who are being considered for a revascularization
procedure.
PET scan charges range from $1200-$3500, depending
on the type of scan. Insurance companies will cover the cost of many PET scans.
Medicare reimburses for PET scans for the following cancers: colorectal, lung,
lymphoma, and melanoma, head and neck and esophageal cancers, and also for refractory
seizures (epilepsy). Medicare will begin PET reimbursement to initially stage,
to determine recurrence and to measure effectiveness of treatment of breast
cancer as well as for myocardial viability. These new reimbursement categories
become effective October 1, 2002. Medicare is constantly updating reimbursements,
so visit the SNM website (www.snm.org)
to find the latest information.
In the 1970's PET scanning was formally introduced
to the medical community. At that time it was seen as an exciting new research
modality that opened doors through which medical researchers could watch, study,
and understand the biology of human disease.
In 1976, the radiopharmaceutical fluorine-18-2-fluoro-2-deoxyglucose
(FDG), a marker of sugar metabolism with a half-life of 110 minutes, enabled
tracer doses to be administered safely to the patient with low radiation exposure.
The development of radiopharmaceuticals like FDG made it easier to study living
beings, and set the groundwork for more in-depth research into using PET to
diagnose and evaluate the effect of treatment on human disease. To perform PET
studies in the late 1970's, a large staff was needed: physicists to run the
cyclotron that produces the F-18 and to oversee the scanner, chemists to make
the tracers such as FDG, and dedicated, specialist physicians.
During the 1980's the technology that underlies
PET advanced greatly. Commercial PET scanners were developed with more precise
resolution and images. As a result, many of the steps required for producing
a PET scan became automated, and able to be performed by a trained technician
and experienced physician, thereby reducing the cost and complexity of the procedure.
Smaller, self-shielded cyclotrons were developed, making it possible to install
cyclotrons at more locations.
Until recently a PET center required a cyclotron
and a radiochemistry laboratory on site to produce the FDG. As a result there
was a scarcity of centers. However, there are now multiple sites that make FDG
and distribute it to the centers that only need to have a PET scanner to perform
the imaging study.
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