2D imaging is done with the septa in for their whole body images and 3D imaging with the septa out for their brains. Septa out of the field leaves the count rate wide open for maximum information acquisition. "In 2D mode, the acquired data are organized in a series of parallel slices that can be reconstructed independently. Data for 3D PET must be rebinned to resemble 2D data before reconstruction, or a 3D reconstruction algorithm must be used."(1) Every morning the camera is set up to automatically begin QC at a certain time. Septa out of the field leaves the count rate wide open for maximum information acquisition. Every morning the camera is set up to automatically begin QC at a certain time. Every day it performs the blank scan with the Ge68 rod sources. This is equivalent to our uniformity floods in general Nuclear Med. If the computer finds defects in the blank scan it will perform a Normalization scan which will calibrate everything and make it right again. The FOV used with this system is divided up into 47 planes based on detectors; this configuration has something to do with how the images are automatically processed and how they are acquired.

Instructor's Comments:

Q: What does it mean when data for #D PET must be rebinned?
Q: How and Why?

These FDG studies are acquired with a step and shoot mode at 13 seconds at each stop. Data acquired each emission scan is 25 to 30 million paired events- this is not including the random and scatter counts.

The reconstruction used during processing is Iterative reconstruction. They use OSEM which stands for Ordered Subsets for Expected Maximum. This is a mathematic formula that is used to speed up reconstruction. It takes a piece of data and reconstructs it with guesses over and over...the image/data subset should improve each time. During reconstruction, the Coronal and Sagittal slices are created "on the fly" because the 3D and transaxial are the only true projections.

Instructor's Comments:
Q: Why?

This system has incorporated a very large window by setting a lower level discriminator of 250keV and an upper level discriminator of 650keV.(2)

Instructor's Comments:
Q: What is the significance of the discriminators?

The way the windows, time and energy, are set is very important for both the patient and the physician. The more accurate the windows, the higher the image quality. This gives the physician a higher level of confidence when reading the study and this improves the outcome for the patient with an accurate reading.

Glioblastomas are the most common central nervous system tumors. This one is consistent with the majority of Glioblastomas in that it was found in the lateral hemispheres. (3) Many of these types of tumors are poorly demarcated making a complete resection difficult. Rapid growth and the presence of many mitotic cells contribute to FDG uptake in this tumor.(4)

There is a new positron imaging radiopharmaceutical being studied for it’s possible affinity for certain brain tumors. It is an amino acid tagged with F18 and is known as FACBC. The procedure is basically the same as that for FDG brain scans. There are hopes that this pharmaceutical will make it easier to distinguish cerebral malignancies since the FACBC should not have normal cerebral uptake. FDG is intensely absorbed by the brain often making it a bit muddy for the radiologist to call a malignancy confidently.

(1)Fahey, F. (2002) Data Acquisition in PET Imaging. J. Nucl. Med. Technol. 2002 30: 39-49 Accessed 6/23/02. Available Online

(2)Delicia Votaw, CNMT, Michael White, CNMT. Margie Jones, CNMT, Kim Egeler, CNMT. Emory University Hospital, PET Center Core Facility. Interviews. 3/18/02.

(3)Vanderbilt Medical Center. Neurology: Glioblastoma. Available Online. Accessed 6/3/02

(4)Williams, S., Nuclear Medicine on the Internet. Positron Emission Tomography PET. Available Online. Accessed 6/3/02