Dual-Headed Gamma Camera Coincidence Imaging:   The Clinical Reality

1. Three major developments have occurred that has lead to the wide spread use of F-18 FDG in PET imaging (mostly in the area of oncology)
1. FDG regulations and its production
1. 110 minute T1/2
2. Commercial production
3. Delivery to clinical sites within 2 – 4 hours from the site of production
4. Approval by HCFA for reimbursement of F-18 FDG studies
• March 1998 Pulmonary nodules and staging malignant lesions in the lungs
• In 1999 three additional diseases where approved
• Recurrent colorectal cancer
• Recurrent malignant melanoma
• Malignant lymphoma
• Other indications are currently being reviewed
• Insurance carriers have followed HCFA’s recommendations
• Additional indications are reimbursed based on pre-approval by the referring physician
• Based on result in literature that identify F-18 FDG, as a tumor imaging agent, has an impact on the patient and is cost effectiveness
5. Dual head, multi-function gamma cameras allows the use of single gamma and coincidence PET imaging
• Problems/limitations with the multi-function gamma camera coincidence imaging
• Lower sensitivity (counts) 3/8 inch NaI crystal has a sensitivity (efficiency) of 3%
• Sensitivity is improved by increasing crystal thickness to 5/8 inch (sensitivity 6%)
• The thicker crystal reduces intrinsic resolution for single emission imaging
• Decreases by 10%
• No apparent impact on routine diagnostic imaging (ex.   bone and cardiac)
• Increases sensitivity of higher energy radionuclides (ex. 67Ga and 111In)
• Gated 201Tl SPECT does not seem to be a problem
• Digital detector improves system sensitivity
• Can accommodate up to 2.5 million counts per second (non-digital detectors are limited to 1.5 million counts per second)
• Better stability
•  Remote service with utilization of a modem
• Segmentation of the detector from one large unit (crystal)
• Divided into four quadrants
• Improves sensitivity by 1.6
• Utilization of 3D acquisition instead of 2D
• There is debate as to where or not this improves system sensitivity
• 3D does improve count rate by 4 to 5 times more than an 2D acquisition
• Disadvantage – performance of a 3D detector drops around the peripheral areas of the detection requiring image overlapping (30 to 50%)
• Disadvantage – Ratio of the scatter to the true coincidence is much higher with 3D (50%) as compared to 2D acquisition (15%)
• Scatter is corrected by collecting a second window at 330 keV ( Compton window) in addition to the 511 keV (gamma window) and applying a special computer algorithm for appropriate correction
• Iterative reconstruction is used instead of filtered back projection for image processing which improves image quality
• Attenuation correction also improves image quality
• Attenuation process using a single source
• Transmission source using 137Cs
• Source moves across each head of the camera
• Rotates around the patient for 90 projections
• Takes about 4 minutes to acquire
• Attenuation process could include multiple sources
• Moves across the field of view
• Takes 32 projections
• Takes approximate the same amount of time as the single source
• Another attenuation correction approach is the use of an x-ray CT tube on the camera gantry and requires about 9 minutes of acquisition
• Field of view and total acquisition time effect system sensitivity
• Dual head gamma camera’s field of view is 38 to 40 cm vs. 15 to 18 cm dedicated PET system resulting in a 2.5 gain in sensitivity
• Acquisition of the dual head gamma camera varies
• 24 minutes with 3D acquisition
• 32 to 40 minutes with 2D acquisition
• Dedicated PET is 8 to 10 minutes
• The net results is that a dual head, multi-function system will have 45 to 50% of the counts that would be in a dedicated PET system
• High count rate images are needed to detect small lesions and further research is required with the dual head gamma camera system to improve detection
• Objectives a the multi-function dual head gamma camera
• Perform routine nuclear medicine procedures
• Coincidence PET imaging procedures
• Suited for smaller hospitals that lack an adequate patient load
• Economically more affordable
• How do you not compromise the quality of patient?
• Identifying benign vs. cancerous lesions
• The size of the pulmonary nodule
• Dual and dedicated can detect cancerous lesions down to 5 mm
• Dedicated PET is 95% accurate in detecting cancerous lesions that are 1 cm in size and 100% at 1.5 cm
• Dual head system has similar accuracy at 1.5 cm, however, only 75% accurate if the lesion was under 1.5 cm
• Every positive lesion has to go to biopsy
• In general, lesions that are negative and are 1.5 or greater in size may not require a biopsy, however, those lesions that are less than 1.0 cm and negative, should require biopsy
• Spiral CT can detect lesions between 5 to 8 mm, but the question is what is the accuracy of the dedicated PET system with lesions of this size?
• Hence, both dual head and dedicated face similar challenges
• Staging malignant disease
• If any lymph nodes pickup FDG this directs the surgeon to biopsy these nodes
• If mediastinal nodules are negative with PET the surgeon must still biopsy these nodes to identify possible microscope spread of disease
• Negative predicted value with a dual head system is 50% and with a dedicated PET system it is better than 80%
• Other roles of PET include:   Evaluating treatment response, detection of early recurrence, and differentiating recurrence from post operative changes or fibroses
• The most important role in the future maybe the evaluating treatment response
• Quantification of the standard uptake value
• Quantification of tumor to background ratio
• Visual interpretation
• Evaluation of chemotherapy maybe accomplished after only 1 or 2 cycles of chemotherapy
• CT and MRI require longer periods of time to identify if the tumor has decreased in size
• Summarize
• Dual-head gamma camera imaging has been developed and has progressively improved in a fast pace over the last 3 years
• It is an acceptable phase but additional improvements are needed
• Good alternative to a dedicated PET system, but one should consider
• Economic standpoint
• Physicians understanding its limitations

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