PET Facility at the University of Pittsburgh Medical Center (USA), and
CTI PET Systems Inc. (USA)
The combined PET/CT tomograph will have the unique capability of operating either in PET or in CT mode.
PET data will be intrinsically aligned to anatomical information from the X-ray CT without the use of external markers or internal landmarks.
The quantitation of the PET data can be improved by using the low noise CT transmission information during the correction of the PET data for self-attenuation and for contamination from scattered photons.
The high resolution CT images could be used to guide the 3D reconstruction of the PET data, thereby improving the quality of the reconstructed PET images.
Functional Imaging: Positron Emission Tomography (PET)
While still in an early phase, the role of PET imaging in oncology research and patient care is clearly growing [Strauss, 1991] . The ability of PET to add unique functional information to that obtained with conventional anatomical-based modalities, such as CT, is generating considerable interest [Wahl, 1993]. For example, [18F]-fluoro-deoxy-glucose, or FDG, has been used to image the distribution of glucose uptake in various regions of the body. Any abnormal tumour metabolism can then be detected on the basis of increased FDG accumulation in the region suspected of a malignancy.
Many primary tumours have spread at the time of the diagnosis and the identification of distant metastases can crucially influence patient management and predict survival probability. Therefore, FDG PET is being increasingly used in whole-body oncological imaging [Dahlbom, 1992] . By performing a PET scan of a large axial extent of the body (whole-body scan) distant disease [Hoh, 1993] and lymph node involvement [Hoh, 1997; Hoh, 1994; Moog, 1997] can potentially be detected by increased FDG accumulation (figure1).
Figure 1. Series of coronal views from a whole-body scan with PET showing multiple distant lymphnode involvements from primary cancer.
[Strauss, 1991] "The application of PET in clinical oncology", JNM (32): 623-8.
[Wahl, 1993] ""Anatometabolic" tumor imaging: Fusion of FDG PET with CT or MRI to localize foci of increased activity", JNM (34): 1190-7.
[Dahlbom, 1992] "Whole-body PET: Part I. Methods and Performance Characteristics", JNM (33): 1191-9.
[Hoh, 1997] "Whole-body FDG-PET Imaging for Staging of Hodgkin's Disease and Lymphoma", JNM (38): 343-8.
[Hoh, 1994] "Utility of FDG Whole-body PET in the staging of Hodgkin's disease and lymphoma", JNM (35): 221P.
[Moog , 1997] "Lymphoma: Role of Whole-body 2-deoxy-2-[F-18]fluoro-D-glucose (FDG) PET in Nodal Staging", Radiology (203): 795-0.
Anatomical Imaging: Computed Tomography (CT)
Computed Tomography is considered the standard technique used in cancer diagnosis. CT detects differences in the attenuation of the sections of the human body that are exposed to external X-rays. Since the attenuation coefficients are proportional to the physical densities of the organs being imaged, CT images contain information about the morphology of the patient (figure 2). Current state-of-the-art CT tomographs typically achieve a spatial resolution of 1 mm. Though most malignancies are of bigger extent they usually alter their metabolic activities prior to any morphological changes. Therefore, CT alone is not a safe measure of malignancy in particular for very small lesions [Ferguson, 1986].
Figure 2. Transaxial slice from Computed Tomography study of a patient with bronchogenic carcinoma (arrow) with pretracheal mediastinal lymphadenopathy.
[Ferguson, 1986] "Regional accuracy of Computed Tomography of the mediastinum in staging of lung cancer", JThor Cardio Surg(91): 498-4.
Dual-modality Imaging: PET and CT
A major difficulty of whole-body FDG PET imaging is that the demonstration of increased glucose uptake is limited in value without accurate anatomical localization of the tracer concentration (figure 1). In the discrimination of a benign from a malignant mass, a CT scan typically identifies the borders and size of the lesion but is less informative about the stage and metabolic activity of the lesion itself. Therefor, some combination of the corresponding PET and CT images of the same region of the patient seem desirable with respect to an accurate diagnosis.
Image Alignment
Recent work [Wahl, 1993] in aligning CT scans with PET FDG functional images has clearly demonstrated the importance of combining anatomy and function in organs other than the brain for which most of the known registration techniques have been developed. While the brain remains fixed in the skull, the position of organs such as the liver or the colon may depend upon the precise way in which the patient lies on the bed. Thus, PET-CT, or other post-hoc alignment procedures may be affected by different internal relationships and deformations within the body.
However, coregistration of CT and PET data (figure 3) that were acquired with different scanners did allow a more accurate evaluation and reliable diagnosis and has been shown, in a number of situations, to completely change patient management [Valk, 1996] .
Figure 3. Examples of co-registered CT (left) and PET (right) images of a primary hepatocellular carcinoma (arrow). The increased information from the aligned CT and PET images is evident.
Using a single dual-modality tomograph the increased diagnostic usefullness of combining functional (PET) and anatomical (CT) images can be further enhanced by eliminating the well-known problems of co-registering two independent data sets that are acquired with two different tomographs and typically on two different days.
[Wahl, 1993] ""Anatometabolic" tumor imaging: Fusion of FDG PET with CT or MRI to localize foci of increased activity", JNM (34): 1190-7.
[Valk, 1996] "Cost-effectiveness of PET imaging in clinical oncology", Nuc Med Biol (23): 737-3.
- - using higher sensitivity 3D PET imaging,
- - combining functional (PET) and anatomical (CT) information, and
- - developing CT based correction methods of the PET data.
The prototype design combines a fully 3D PET system and a state-of-the-art spiral CT scanner. The PET tomograph which is the partial ring tomograph, ECAT ART has been described in detail in [Bailey, 1997]. It consists of two partial rings, or detector arcs, of BGO crystals that rotate with 30 rpm around the centre of the field of view, thus collecting a complete set of views that is intrinsic to a full-ring tomographic system. It was anticipated that the gaps between the detector arcs of the ECAT ART would be sufficient to hold the major parts of a CT tomograph, that is the X-ray tube and the opposite arc of X-ray detectors. The first design concept is shown in figure 4.
Figure 4. First design concept of the combined PET/CT tomograph. The ECAT ART is shown with the two detector arcs highlighted in yellow. The gaps in which the X-ray tube (arrow upper right) and the CT detector arc (arrow lower left) were anticipated are also shown.
During the design phase we developed the idea of mounting the ART components on a separate aluminum ring that is attached to the back of the CT gantry as shown in figure 5. That design became the design of choice since the CT components had to remain as little altered as possible. The major advantage, however, to scan the patient in two imaging modalities with the same scanner and without moving the patient between the scans remains an intrinsic option of this design for a PET/CT tomograph.
Figure 5. Final design of the PET/CT tomograph with the CT and PET system arranged axially offset as shown in the centre drawing. The PET components (right) are physically attached to the rotating parts of the CT (left) but can be operated individually. The axial offset between the centres of the FOV is about 60 cm. Note the X-ray tube/detector assemblu highlighted in the picture of the CT scanner (left) and the opposite PET detector blocks (darkened in the right picture).
With the current design (figure 5) we will be able to acquire both functional and anatomical information of an axial range of the patient of 100 cm, which is enough to cover the whole-body range in a typical PET scan that extents from the upper neck to the lower abdomen or upper thighs (figure 6).
Figure 6. Whole-body imaging range over which both anatomical and functional information can be acquired using the PET/CT scanner. The patient does not need to be moved between scans and the same body shape and position is guaranteed throughout the entire imaging process.
The combined tomograph will rotate continuously during the acquisition. To ensure steady power supply and data transfer between the tomographs and the processing units slip-ring technology is employed. Slip-rings are typically copper rings that are mounted to the rotating part of the tomograph. Graphite brushes that are fixed to the stationary gantry have contact with the individual slip rings (figure 9). These brushes ensure steady power supply to the rotating components of the tomograph.
Figure 9. Slip-rings and graphite brushes in the SOMATOM AR.SP CT scanner. Slip-rings ensure steady power supply to the rotating components of the tomograph and often data are transferred through slip-rings from the gantry to the external processing unit.
The slip-rings for the CT are shown in figure 9. Two rings are used for the power supply and four rings are reserved for data transfer from the rotating detector unit to the external image processing station. These slip rings are mounted in the transverse plane of the CT tomograph in contrast to the ECAT ART scanner where a similar arrangement of slip rings (not shown here) is mounted in the axial direction of the scanner. Note that while both data and power are transferred for the CT part through the slip-rings, the ECAT ART only receives the power supply for the rotating components (figure 10) through the slip rings while the PET data are transmitted via optical couplers.
Figure 10. View on some of the rotating components of the ECAT ART PET tomograph.
The name for the first prototype, SMART Oncology, of a combined PET/CT tomograph derives from the combination of a CT scanner from Siemens' SOMATOM series and CTI's ECAT ART PET tomograph. Some characteristic scan parameters for the SMART systems are given in table 1.
Table 1. Some characteristic parameters of the PET/CT tomograph.
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Parameter |
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detector material in-plane spatial resolution slice width voltage current scatter fraction peak NEC |
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[Bailey, 1997] "ECAT ART - a continuously rotating PET camera: performance characteristics, initial clinical studies and installation considerations in a nuclear medicine department", Eur J Nuc Med (24): 6-15,.
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submission of grant application |
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start of the project (May) |
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investigation of CT based attenuation correction of PET data |
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design phase of the combined tomograph |
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installation of the Somatom AR.SP at CTI Knoxville, USA |
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construction of PET/CT tomograph begins at CTI Knoxville, USA | |
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construction complete (February) |
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testing and validation phase begins (March) | |
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shipment to UPMC Pittsburgh, USA (April) | |
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first patient scan (June) |
During the first two months of operation of the PET/CT scanner we studied ten clinical patients. Although the CT scans were acquired without the application of contrast agents the overall image quality if sufficient in most cases to correlate the PET and CT data.
We find that owing to the axial offset of the PET and the CT field-of-view bed deflection becomes a critical issue with respect to aligning corresponding PET and CT volumes. The bed deflection depends on the patient weight and on how far the bed is extended into the scanner, i.e bed position. We have designed a bed support that will be installed by the end of August to overcome the problem of the bed deflection..
All data are corrected for attenuation based on the CT information [Beyer, 1995] . Scatter correction was performed only in a few studies. Images were generally reconstructed iteratively using FORE/OSEM. We find that overall image quality is better than image quality from a standard ART (using rod transmission sources) and 768 ns standard block integration time.
[Beyer, 1995] "Attenuation correction for a combined 3D PET/CT scanner", presented at the 1995 International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine, Aix-les-Bains, France.
- Lung cancer
- Esophageal cancer
- Pancreatic cancer
- Renal cancer
- Image of the year 1999
For further information about this project please contact David Townsend.
UPMC Health System - Positron Emission Tomography Facility / Phil Greer / Web Master / greer@pet.upmc.edu