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PET Oncology: Lymphoma
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Krista Chambers, CNMT
Nuclear Medicine Technologist Student
Medical College of Georgia

Mary Anne Owen, MHE (RT)N
Program Director, Nuclear Medicine Technology
Medical College of Georgia



Patient History


The patient is a 42 year old female diagnosed with lymphoma. This exam was compared to the recent Computed Tomography scan of the neck and chest on December 22, 2004. The CT scan showed an increase in the number and size of lymph nodes in the neck that were more prominent than at the time of previous PET scan. The left supraclavicular region shows increase that extends to the thoracic inlet level into the upper mediastinum.



Brief History


The patient was instructed to fast with the exception of water from midnight the night before to the time of exam. This is required for all PET scans to minimize competitive inhibition FDG uptake and to decrease the heart uptake. When the patient arrived to the PET center, the procedure was explained to the patient along with verification of identity. An IV was started and made sure correct placement in the vein with a saline flush. The patient’s blood glucose level was checked to insure proper glucose levels. Glucose level between 90-150mg/dl is adequate for F18 FDG uptake and scanning. Levels too high will decrease uptake of the FDG in tumors. Once glucose level was approved, 15mCi fluorine FDG-18 was injected followed by a 250mL drip bag of saline. The patient relaxed in the room for 45 minutes post injection to ensure no muscle uptake. This is standard with all PET scans.



Radiopharmacy


15mCi fluorine FDG-18 injected then assayed in the dose calibrator for residual activity. Time of injection and then residual should be written down. The residual activity is important in the calculation of the standard uptake value.


Discussion (click to view)

F-18 FDG is the most widely used radiopharmaceutical for PET imaging due to the half life and localization to regional glucose metabolism.

 

Instructor's comment:

Q: What is the half life of F-18 FDG?
A: The half life of F-18 FDG is 110 minutes.

Malignant cells have the characteristic of increased glucose metabolism due to the number of cell surface glucose transporter proteins. FDG has energy of 511 KeV so proper shielding techniques should be utilized to reduce increase exposure.  FDG is a cyclotron produced isotope with a 110 minute half life.  When FDG is injected intravenously it localizes wherever glucose is being employed.  Normal biodistribution is seen in the genitourinary system, liver, myocardium, brain, soft tissue, and areas of the head and neck.  Amplified areas of activity could be consistent with malignancy.  A standard uptake value (SUV) measurement is calculated, which is used to determine if a lesion has increased uptake of the FDG.  An SUV greater than 2.5 has been shown to be consistent with malignancy. 


Technologists Concerns (click to view)

Imaging


Once the patient relaxed for 45 minutes in a quiet room she was instructed to void before scan to reduce artifacts of the bladder and for a better imaging of the pelvis area. Then she was instructed to remove any metal objects that would cause artifacts. The patient lay supine on the imaging table with arms over head to reduce attenuation in chest area and would not superimpose area of interest. The patient was then set up with laser markers from the mid thigh up to the forehead.

Instructor's Comment:
Q: Are there any caveats to assure accurate laser positioning?

A: To position the patient the only caveat is visualizing making sure the pelvis is included in the imaging. Usually if the laser is set in the mid thigh you are guaranteed a good position.

The patient was instructed to remain still and to breathe normally.



Instrumentation and Processing

A General Electric NXi Advance PET scanner used for imaging.

 

Instructor's Comment:

Q: Are there any caveats to assure accurate laser positioning?
A: The General Electric PET system uses a bismuth germinate crystal.

Six bed positions imaged at seven minutes each. Different systems have different standards for the bed positions. The General Electric system has a standard of an 8cm bed position. A Germanium rod source was used for attenuation correction in this particular imaging system.

Instructor's Comment:

Q: What is the purpose of attenuation correction?

A: The purpose of attenuation correction is to account for the different tissue densities. There is more severe photon attenuation in PET imaging than SPECT imaging so an attenuation map needed for non uniform densities in iterative algorithm.

Other attenuation correction sources that can be used are a Cesium point source or a CT scanner depending on the imaging system.  Images were reconstructed in the sagittal, axial, and coronal planes as well as 3-D reconstructed images.



Findings


Slight worsening in size and activity of the lymph nodes in the neck; right side less than the left compared to previous PET study. Slight decrease in the size and activity in multiple mediastinal lymph nodes compared to previous study. No new areas are seen in the axilla, abdomen, or inguinal regions.

Instructor's Comment:
Q: Was a standard uptake value reported? If not, is the study as diagnostically useful?
A: There was no standard uptake value reported on this study. This study is compared to previous PET scan done August of 2004.


References (click to view)

Bill Butts, MA, CNMT
PET Technologist
University Hospital


www.auntminnie.com