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Description
The Philips Allegro-PET & Gemini-PET/CT are whole body PET scanners, with the Gemini having the additional capability to perform whole body CT for attenuation correction and volume fusion. The Allegro/Gemini PET scanners are operated in 3D mode for multi-bed whole body acquisition.
The PET scanner systems of the Allegro/Gemini consist of a gantry with onboard analogue and digital processing, a VME based acquisition control system and a Sun Blade 1000/2000 workstation used for operator interface to define, control and process studies.
Scanner Specification - see Facilities
Acquisition subsystem specification
The Acquisition System is a VME based system interfaced to a Sun UltraSparc, it consists of;
- 28 detector modules
- 6 virtual detectors
- detector modules are summed in groups of (top) 6, 4, 4, (bottom) 6, 4, 4
- coincidence events determined from virtual detectors within 8 nsec window
- CRB (Coincidence Rebinner Board) to bin events into histogram memory
- Sorts coincidence events (detector pair indices), increments sinogram element at deterined plane, projection, view-angle
- Couch control system running VxWorks
- Motion control system to manage transmission source control, couch control
- Gantry communication
- Hardware tables downloaded to detector modules
- Energy correction
- Timing correction
- Geometric Distortion Correction
- 2 Gb histogram memory
- Dynamic histograms – multi-bed or multi-frame
- Listmode storage – typically 4 bytes/event => 500 Mevts
- 80 Gb system + sinogram storage disk
The Server workstation – Sun Blade 2000
- 80 Gb – study storage
- DDS-2 tape – system files archival
- AIT-2 tape – raw + processed data archival
- 2 Tb RAID – reconstructed images archived daily
Illustrated below is a schematic representation of the Allegro/Gemini acquisition system.
Scan Acquisition
Several scans are required in order to process an emission scan. These are divided into calibration scans and study scans.
The calibration scans are performed weekly and consist of a normalisation and blank scan.
- Calibration scans
- Normalisation scan; this is acquired with a uniform calibrated source placed in the FOV. It allows the
characterization of detector-detector sensitivities and therefore the correction for these varying efficiencies
- SUV (Standard Uptake Value) calibration; this is performed quarterly and provides the absolute quantification of
the camera to allow images to be reported in units of kBq/mL or more commonly in (near) dimensionless SUV units of
(kBq/mL)/(kBq/g). The calibration is performed by measuring a 9 Litre 18F-FDG filled phantom over a range
of activity concentrations to allow the deadtime correction to calculated over a range of singles rates
- Blank scan; this is acquired with the point-source extended and an empty FOV so as to characterise the point-source
activity. The blank scan is then used to correct the transmission scan so that an attenuation correction can be
generated
- Quality Control Scans
- GDR (Geometric Distortion Removal); this is checked daily by performing an emission scan with a point source in several positions to allow inspection of any position determination anomalies
- Energy Checks; this is measured daily by acquiring a listmode acquisition in a mode that records the energy of each
event. The pulse height centroid and width is checked and recalibration is flagged as being required if the centroid
or width exceed specified limits.
- PMT response; this is checked daily by performing a blank acquisition and inspecting the resultant scan for
anomalous behaviour
- Uniformity; this is checked weekly by performing an emission scan on a uniform 68Ge phantom and measuring the
differential uniformity
- Study scans
- Transmission Scan
- 30 sec rotation of 137Cs per bed position
- # transmission scans = # emission scans + 2
- Emission Scan
- 3 min per emission bed position
- # emission scans ~ study extent / 90mm
Image Reconstruction
- 2½D reconstruction
- 3D normalisation
- Rebinning of 3- to 2-D data
- SSRB (Single Slice Rebinning)
- FORE (Fourier Rebinning)
- 2D attenuation correction
- FBP (Filtered Back Projection)
- AW-OSEM (Attenuation Weighted - Ordered Subset Expectation Maximinisation)
- RAMLA2D
- 3D reconstruction
- 3D attenuation correction
- RAMLA3D
Introduction
The two features that distinguish PET from SPECT are; the increased count-rate efficiency of due to electronic rather than physical collimation and the ability to simply correct for attenuation effects of tissue.
The ability to electronically collimate is due to the more sophisticated nature of photon detection, by detecting the 0.511 MeV pair photons resulting from positron annihilation.
Positron emitters
Four positron emitters are used routinely with the Austin Hospital PET facility. The table below outlines the properties of interest.
| Isotope |
b+ energy (MeV) |
b+ range (mm) |
1/2-life |
Applications |
| 11C |
0.96 |
1.1 |
20.3 min |
receptor studies |
| 15O |
1.70 |
1.5 |
2.03 min |
stroke/activation |
| 18F |
0.64 |
1.0 |
109.8 min |
oncology/neurology |
| 124I |
2.1350/1.5323 |
1.7/1.4 |
4.5 days |
oncology |
Resolution effects
There are a number of factors determining the imaging resolution of the reconstructed PET images. These can be categorised as follows:
- positron range
- annihilation non-collinearity
- detector size
- block effect ~ 2mm
Γ = 1.25*( Γ range2 + Γnon-colinearity2 + Γdet-size2 + Γblock2)1/2
The 1.25 factor accounts for the effect of reconstruction, this expression was determined empirically by Moses et al. (J.Nucl.Med vol 34, pp 101P, 1993)
The positron range contribution to the effective resolution is a result of the uncertainty in the point of the positron emission. Positron paths follow a random path so that the integrated positron range is much larger than the separation between the positron annihilation and the positron emission point. For the isotopes of interest, the FWHM range is 1.0 - 1.5 mm.
The non-collinearity term arises due to the probability of positrons annihilating whilst in motion. The quoted angle between the two 0.511 MeV gammas is 180 +/- 0.25 degrees. With a ring diameter of Ds, the resultant effect on resolution is 0.0022Ds. For the Philips Allegro/Gemini cameras, ds = 800 mm => 1.76 mm
The detector size response is given by a FWHM equal to half the detector element width, for the Philips Allegro/Gemini, the transaxial crystal size if 4 mm giving a detector element contribution to the resolution of 4/2 = 2 mm
The block-effect is due to the multiplexed nature of the position determination.
For 18F, this gives: 1.25 * (0.892 + 1.762 + 22 + 22) = 4.31 mm
Attenuation Correction
Photons emitted are absorbed as they pass through the body matter. To quantify this, given I0 (cps) photons being emitted (where I0(x) = ρ(x).dV ), then the effect of travelling through a medium is to reduce the number of counts detected, the source intensity is attenuated and is described by;
or casting this as a probability,
That is, the probability that a photon emitted at point x=x' will be detected at x=0 is P1(x').
For PET, there are two photons as emitted as illustrated in the figure below (detector 1 at x=0, detector 2 at x=a, volume element v at x=x'). The probability of detection by the 2nd detector is;
So the probability that both will be detected is;
P = P1(x').P2(a-x') =
By taking the -ve of the natural log, we have:
which when back-projected gives us a mu-map. The attenuation coefficient
m(x) is a function of the attenuating medium's physical properties ( atomic number and density) and the energy of the photon (
m(x,E)=s(E).r(x)). For water and 0.511 MeV photons, mwater(0.511MeV) =0.09695 cm-1, which gives a half-depth, the depth at which 1/2 the initial photons have been absorbed, of 7.15 cm.
As is evident from above, the Line Of Response (LOR) can be corrected for attenuation effects by measuring the attenuation fraction anywhere along the LOR. The most convenient place to measure the attenuation is then outside the imaging FOV. This is achieved with a rotating 137Cs point source and is measured prior to or post emission.
From the above, you can see that in order to generate an attenuation correction, we need a measure of I0, the transmission point source activity. This is the purpose of taking a blank scan and they are acquired weekly as part of the scanner QA.
Randoms Contribution
Scatter Contribution
Work in progress
Performance Measures
Work in progress
Disclaimer: Any information given on these pages has been done so to the best knowledge of the Authors, however, the Authors in no way are liable for any mis-information possibly presented therein.
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