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When to Select PET/CT versus Stand-Alone PET
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PET/CT scans result in more accurate interpretations of pathology, especially in the head and neck,
the abdomen, and the extremities |
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PET alone is recommended for brain studies because CT has less clinical utility than MRI for brain
evaluation |
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PET alone is appropriate for solitary pulmonary nodule evaluation if there is a recent
(within 1 month) CT scan
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PET alone may be considered for follow-up of treated, asymptomatic cancer patients when there is
a low suspicion for recurrence
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PET
and CT are complementary imaging techniques that, when used in
conjunction with one another, have better diagnostic accuracy than
either technique alone. PET imaging, using the glucose analog 18F-FDG
as a tracer, is functional imaging which detects sites at which
metabolic activity is significantly higher than the surrounding region
but contains little information on the anatomic location of that
activity. CT, on the other hand, provides little functional information
but combines high spatial resolution with good anatomical contrast.
When the images are co-registered, it is possible to identify the
anatomy of a “hot spot” and to distinguish between tumors and other
sites that may have high metabolic rate, such as muscle, brown fat, or
glands.
PET/CT
PET/CT
scanners incorporate both CT and PET detectors in a single gantry and
exam table. Because the patient remains in the same position throughout
both the CT and PET exams, automated software can accurately
co-register the PET and CT images to create fused PET/CT images
(Figures 1 and 2). Although software co-registration of data from
temporally separate PET and CT examinations may be possible, this
option is less accurate because the patient is not likely to assume
exactly the same position and because internal organs shift around.
Therefore, it easier to determine the anatomic structures associated
with high tracer activity in combined PET/CT images, which results in
fewer equivocal interpretations and reduced need for further testing.
This is especially significant in regions of complex anatomy, such as
in the abdomen and in the head and neck. In addition, PET/CT is more
convenient to the patient because the combined scans require only one
visit to the hospital and the imaging time is shorter than a PET scan
alone.
There is considerable evidence the combination of
PET and CT examinations is significantly more sensitive and specific
than either method alone. Furthermore, there is growing evidence that a
PET/CT examination demonstrates superior diagnostic performance than
separate PET and CT exams that are interpreted side-by-side. For
example, PET/CT is more accurate for tumor localization and assessing
tumor extent for staging non-small cell lung cancer and other solid
tumors, especially in the abdomen and pelvis, as well as for assessing
response to short-term therapy in gastrointestinal tumors. In the head
and neck, PET/CT images can easily distinguish between abnormalities
and physiological structures such as muscle and brown fat and hybrid
imaging has been shown to have superior sensitivity and specificity
compared to viewing side-by-side PET and CT images for the diagnosis
and staging of squamous cell carcinoma.
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Figure 1.
44 year old woman with squamous cell cancer of the anus status post
chemoradiation therapy presented for routine follow-up examination. FDG
PET from PET/CT did not show worrisome areas of uptake but a small
perirectal node (arrow) was seen on CT and on the fused PET/CT image
above . See Figure 2 for complete set of PET/CT images.
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A PET/CT scan can include a full diagnostic CT scan or an abbreviated
scan of the region of interest for localization purposes. If a patient
has had full diagnostic CT scan within the last month, a localization
scan is sufficient. In almost all other cases, a PET/CT examination
should include a full diagnostic CT examination. Table 1 shows the
recommended anatomy for the diagnostic CT examination for the most
common conditions.
In some instances, PET alone may be sufficient. If PET imaging is
required for the evaluation of a solitary lung nodule >8 mm in
diameter, the accuracy of software co-registration of PET and CT images
obtained temporally separately is comparable to that obtained from
PET/CT provided that a recent CT (within the last 30 days) is
available. However, software co-registration poses logistic challenges
in clinical practice and certainly a combined PET/CT scan is
recommended if there are no recent tomographic images.
A stand-alone PET scan is also recommended for brain studies because CT
has less clinical utility than MRI for brain disease. In addition, for
patients who weigh >250-300 lbs, separate stand-alone PET and CT
scans are preferable because they result in better PET image quality
than a PET/CT scanner.
Other circumstances in which PET alone may be considered include the
assessment of tumor response to therapy and for assessing certain
oncologic patients when there is a low suspicion for recurrence.
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| Table 1. Guidelines for the Selection of PET/CT and PET Examinations: |
| Patient Condition |
Recommended Approach* |
| Thymoma staging |
PET/CT including diagnostic CT of the chest PLUS other affected body part, if any |
| Solitary pulmonary nodule evaluation** |
PET/CT including diagnostic CT of the chest |
| Head & neck cancers (Tongue, larynx, thyroid, parotid, etc) |
PET/CT including diagnostic CT of neck and chest |
Abdominal cancers (most)
Esophageal cancer
Lung cancer staging
Mesothelioma staging
Sarcoma |
PET/CT including diagnostic CT of chest, abdomen, and pelvis |
Breast cancer
Lymphoma
Melanoma |
PET/CT including diagnostic CT of neck, chest, abdomen, and pelvis |
| Brain disease |
PET only |
*If
the patient has received a diagnostic CT examination within the past 30
days, the PET/CT examination need only include a CT scan of the
anatomical region of the abnormality for localization purposes.
**PET alone is sufficient if patient has had diagnostic chest CT within past 30 days. |
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Figure 2. Complete
set of PET/CT images (A, B, and C) from same examination as in Figure
1. CT image from PET/CT showed a small perirectal node (A, arrow).
Fusion images (B, arrow) confirmed mild uptake within the node. PET
images (C) alone did not show worrisome areas of uptake.
Follow-up of the same patient 2 months later (D, E, F, arrows)
confirmed growth and further uptake in the node. This case demonstrates
how small size is a limitation of PET and how the high structural
resolution and anatomical localization of CT can help PET. |
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Limitations
Although
most tumors have a high metabolic rate and take up FDG avidly, there
are exceptions such as carcinoid tumors, well-differentiated low grade
tumors such as bronchioalveolar cell carcinoma in the lung, and most
mucinous cancers. Therefore, a negative FDG-PET scan is not an absolute
indication of a benign lesion. In addition, the spatial resolution of
PET is about 4-6 mm, inferior to that of other imaging methods, such as
CT, which means that tumors must be at least 7 mm before they can be
detected. A list of conditions that are recognized as suitable for
evaluation with PET is shown in Table 2.
False positive
diagnoses can arise from high FDG uptake into tissues that have
unusually high metabolic rates, such as inflammatory lesions.
Therefore, PET should generally not be performed until 2-3 months after
radiation therapy and the interpreting physician needs to have all
pertinent clinical information concerning recent surgery, chemotherapy,
bone fractures or other trauma, as well as the date of the last
menstrual period in pre-menopausal women.
Patient Preparation
In general, patients are required to fast for 6 hours prior to a
FDG-PET or FDG- PET/CT scan but may continue to take regular
medication. Diabetic patients should fast for 4 hours before the study,
and should eat just prior to fasting. While fasting, water is permitted
but no sweetened beverages. If the patient is on an IV, there should be
no glucose in the IV solution in the hours preceding the scan. This is
necessary to minimize the amount of insulin circulating in the blood
and, therefore, maximize FDG uptake into the metabolically active
cells. All patients will have a blood glucose level measured before the
scan. In a diabetic patient, a blood glucose level below 200 mg/100ml
is acceptable. Above that level, the scan will be performed only if the
referring physician does not believe that is possible for the patient
to safely lower his or her glucose level.
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PET/CT Procedure
Patients receive an injection of 18F-FDG-PET about an hour before the
start of imaging to allow time for metabolic uptake of the tracer. Both
before injection and between injection and imaging, physical activity
or repetitive movement should be avoided to avoid unnecessary uptake
into the muscles. At the start of the examination, the patient is
positioned comfortably on the imaging table and is asked to stay
motionless for the duration of the imaging procedure. A low radiation
dose non-contrast transmission CT scan is performed first, which
provides data to correct for attenuation for the PET scan. The FDG-PET
scan is performed next that, depending on the size of the patient,
takes about 25-35 minutes for a routine whole-body scan, which entails
imaging of the neck, chest, abdomen, and pelvis. After the PET scan is
complete, intravenous contrast is administered to the patient for a
standard radiation dose diagnostic CT scan if it has been
requested.
The radiation exposure for a PET/CT scan is about 110 mrem for a whole
body low-dose attenuation correction scan, 2,600 mrem for a whole body
(neck/chest/abdomen/pelvis) diagnostic CT scan, and about 1,600 mrem
for the PET portion which is from injection of 18F (15 mCi) in an
average sized patient. 18F has a half-life of 109 minutes and is
effectively fully decayed within a few hours of administration. If
radiation exposure is a concern, it is possible perform essentially a
PET only scan by using the PET/CT scanner with a non-diagnostic, low
quality CT not reliable for making CT diagnoses but sufficient for PET
attenuation purposes.
A radiologist specializing in the relevant anatomy as well as a nuclear
medicine specialist, routinely read PET/CT images in a joint
interpretation format, and issue separate, but concordant, reports for
the PET/CT scans. |
| Table 2. Applications of FDG PET Covered by Medicare |
| Cancer |
Diagnosis |
Staging |
Restaging |
Monitoring Tumor Response |
Non-Small Cell Lung Cancer
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Yes |
Yes |
Yes |
No |
| Esophageal Cancer |
Yes |
Yes |
Yes |
No |
| Colorectal Cancer |
Yes |
Yes |
Yes |
No |
| Lymphoma |
Yes |
Yes |
Yes |
No |
| Melanoma |
Yes |
Yes |
Yes1 |
No |
| Breast Cancer |
No |
Yes2 |
Yes3 |
Yes4 |
| Head and Neck Cancers5 |
Yes |
Yes |
Yes |
No |
| Thyroid Cancers |
No |
Yes6 |
No |
No |
| Cervical Cancer |
No |
Yes7 |
No |
No |
| Other |
| Fronto-temporal dementia |
Yes |
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| Alzheimer’s disease |
Yes |
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| Myocardial viability |
Yes8 |
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| Myocardial perfusion |
Yes |
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1Evaluation of regional nodes not covered.
2Staging breast cancer patients for distant metastasis; initial diagnosis and staging of axillary nodes are not covered.
3Restaging breast cancer patients with loco-regional recurrence or metastasis.
4Monitoring
tumor response to treatment for locally advanced and metastatic breast
cancer when a change in therapy is anticipated.
5Not central nervous system.
6Restaging
of recurrent or residual thyroid cancers of follicular cell origin
previously treated by thyroidectomy and radioiodine ablation, have a
serum thyroglobulin >10ng/ml and negative I-131 whole body scan.
7Staging
in patients with newly diagnosed and locally advanced cervical cancer
with no extra-pelvic metastasis on conventional imaging tests.
8Only following negative inconclusive SPECT |
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Scheduling
PET and PET/CT is currently available on the main campus only. An
additional PET/CT scanner is scheduled to be installed in the Chelsea
Imaging Center in the fall of 2007. PET/CT examinations may be
scheduled online through Radiology Order Entry system, (http://mghroe
) or by calling 617-724-9729 (4-XRAY).
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Further Information
For further questions on PET/CT, please contact ,
Director of Nuclear Medicine (617-726-8294) or ,
(617-726-8743), Staff Radiologist in the Division of Abdominal Imaging
and Intervention, MGH Department of Radiology.
We would like to thank Drs. Fischman, Blake, and James A. Scott, M.D., Nuclear Medicine for their advice on this issue.
This article provided useful information about the appropriate use of imaging studies:
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References
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of PET, CT, and dual-modality PET/CT imaging for monitoring of imatinib
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Antoch, G, Saoudi, N, Kuehl, H, Dahmen, G, Mueller, SP, Beyer, T, Bockisch, A, Debatin, JF and Freudenberg, LS. (2004) Accuracy
of whole-body dual-modality fluorine-18-2-fluoro-2-deoxy-D-glucose
positron emission tomography and computed tomography (FDG-PET/CT) for
tumor staging in solid tumors: comparison with CT and PET. J Clin Oncol 22: 4357-68
Blodgett, TM, Meltzer, CC and Townsend, DW. (2007) PET/CT: form and function. Radiology 242: 360-85
Bybel, B, Brunken, RC, Shah, SN, Wu, G, Turbiner, E and Neumann, DR. (2006) PET and PET/CT imaging: what clinicians need to know. Cleve Clin J Med 73: 1075-87
Krishnasetty, V, Fischman, AJ, Halpern, EL and Aquino, SL. (2005) Comparison of alignment of computer-registered data sets: combined PET/CT versus independent PET and CT of the thorax. Radiology 237: 635-9
Lardinois, D, Weder, W, Hany, TF, Kamel, EM, Korom, S, Seifert, B, von Schulthess, GK and Steinert, HC. (2003) Staging of non-small-cell lung cancer with integrated positron-emission tomography and computed tomography. N Engl J Med 348: 2500-7
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