Design and evaluation of a positron emission tomograph: HEADTOME III

A high performance positron emission tomograph, HEADTOME III, with 480 bismuth germanate oxide detectors (13.4 X 25.0 X 40.0 mm), arranged in three rings of 750 mm diameter, with independent shadow masks and septa, is described. Image plane resolution at the center of the field of view (FOV) is 7.9 and 6.5 mm full width at half maximum (FWHM) in low resolution (LR) and high resolution (HR) mode, respectively. Axial resolution is 13.1, 11.3, and 9.1 mm FWHM at the center of FOV for direct planes of low quantitation (LQ), cross planes of LQ, and high quantitation (HQ) mode, respectively. Sensitivities evaluated by true events for a 68Ga 20 cm diameter cyclindrical pool were 34.5 (56.7), 16.4 (27.5), 19.4 and 9.5 kcps (muCi/ml)-1 with the direct (cross) planes of LQLR, direct (cross) planes of LQHR, and direct planes of HQLR and HQHR, respectively. The fraction of scattered coincidence in a cold spot phantom after software correction is 2.5% in LRLQ mode. Count rate linearity after software correction is within 1% up to 50 X 10(3) true events per second per plane with LRLQ mode.     

Long-term performance of a multiplanar positron emission tomograph

A protocol for the daily quality assurance (QA) of a multiplanar positron emission tomographic (PET) system was developed. This was implemented on a daily basis for the PC 4600 Neuro-PET, a multiplanar PET system designed for quantitative brain imaging. Sensitivity data collected as part of the protocol are presented for a 22-mo time period. These data show the need to periodically monitor instrument performance if meaningful quantitation is to be achieved. The methods presented have direct application to any quantitative multiplanar emission tomographic imaging program.     

Performance characteristics of Positologica III: a whole-body positron emission tomograph

A whole-body, multislice positron emission tomograph, Positologica III, has been constructed based on a sampling method "positology." The scanner consists of four continuously rotating detector rings with 192 bismuth germanate (BGO) crystals (12 X 24 X 24 mm) arranged at irregular intervals along each ring, providing seven tomographic images at 16 mm intervals simultaneously, for a total axial coverage of 12 cm. The patient aperture is 54 cm in diameter, and the field of view is 40 cm in diameter and 12 cm in depth. Newly developed dual photomultiplier tubes are used with 12 mm wide BGO crystals to provide a packing ratio of 0.894. The spatial resolution at the center of the field is 7.6 mm full width at half maximum, and 2.5 mm hot spots were delineated in the Derenzo phantom. The sensitivity for a 20 cm diameter cylindrical phantom is 34.2 and 52.2 kcps/muCi/ml for true planes and cross planes, respectively. In clinical studies the posterior papillary muscle of the heart was visualized in the myocardial scan, and the lung behind the pulmonary artery was also visualized in the pulmonary ventilation scan. These results suggest that our machine has sufficient resolution for most of the clinical studies of the body.     

Evaluation of the performance characteristics of the PC 4600 positron emission tomograph

The sensitivity, resolution, linearity, and count rate capability for the PC 4600 positron emission tomograph (PET), a neurological PET with five rings of 96 bismuth germanate crystals, are reported along with details of the design of this system. Phantom studies and preliminary human images demonstrate the clinical potential of this new instrument.     

Design concepts and preliminary performances of stationary-sampling whole-body high-resolution positron emission tomography: HEADTOME IV

Design concepts and preliminary performances of a stationary sampling high-resolution whole-body positron emission tomograph, HEADTOME IV, were reported. The system comprises four layers of detector ring which consists of 768 BGO crystals with 3 mm width, 96 photomultiplier tubes arrayed on 825 mm circle. A sufficiently fine sampling-interval allows data sampling without scan motion along the transaxial plane. But an axial motion (Z-motion) is installed to interlace between adjacent planes. Preliminary performance characteristics were an in-plane resolution of 4.5 mm in full width at half maximum (FWHM), an axial resolution of 9.5 mm in FWHM, sensitivities of 14 and 24 kcps/(microCi/ml) for direct and cross planes, respectively, with 20 cm diam. cylindrical flood phantom. A large scale cache memory prepared for each plane allows a realtime correction for the deadtime and the radionuclide decay, and a realtime calculation of the rate constants using the weighted integral method.     

A BGO detector unit for a stationary high resolution positron emission tomograph

A new encoding scheme was developed that is applicable to bismuth germanate (BGO) detector units for a high resolution positron emission tomograph. The detector unit is composed of eight equal-size, 3 mm wide BGO crystals coupled to a dual-module square photomultiplier tube (PMT) through a pair of light guides. The light guides divide the scintillation light between the two modules of the PMT, with a ratio dependent on the crystal of origin. Mean detector pair spatial resolution as measured by the point spread function was 3.9 mm full width at half maximum (FWHM) and reconstructed image spatial resolution was 4.8 mm FWHM at the center of the field of view. Timing resolution between two detector units was 6.0 ns FWHM. Energy resolution was 24% FWHM for 511 keV gamma rays. The finest spots of the Derenzo phantom were clearly resolved.     

Design and performance characteristics of a whole-body positron transaxial tomograph.

A whole-body positron-emission transaxial tomograph (PETT III) is described in detail and evaluated in terms of resolution, accuracy, and efficiency. The PETT III utilizes annihilation coincidence detection to provide spatial resolution; high sensitivity is achieved by using 48 Nal(tl) detectors set in a hexagonal array with a multiple-coincidence logic. The assumptions and approximations made in the reconstruction and their effect on image quality are discussed. Phantom studies shows the depth-independent resolution and response of PETT III, as well as its ability to recover activity distribution quantitatively in the cross section measured. Images obtained with patients and normal volunteers show the potential clinical utility of PETT III.     

Performance characteristics of a whole-body positron tomograph

This paper describes an investigation of some of the important physical characteristics of a whole-body positron tomograph consisting of two rings of bismuth germanate detectors of dimensions 5.6 mm X 30 mm X 30 mm (512/ring). The resolution applicable to in vivo imaging is six mm or more, depending on radionuclide and reconstruction filter and is very uniform over the field of view normally used. Axial resolution can be varied by moving side collimation (maximum approximately 16 mm FWHM with interplane septa). A simple scheme has been devised to correct for loss of true coincidence events with varying count rate based on the total random and multiple coincidence rates. It is concluded that correction for scattered radiation should be implemented for reliable quantitation.     

Continuous-slice PENN-PET: a positron tomograph with volume imaging capability

The PENN-PET scanner consists of six hexagonally arranged position-sensitive Nal(TI) detectors. This design offers high spatial resolution in all three dimensions, high sampling density along all three axes without scanner motion, a large axial acceptance angle, good energy resolution, and good timing resolution. This results in three-dimensional imaging capability with high sensitivity and low scatter and random backgrounds. The spatial resolution is 5.5 mm (FWHM) in all directions near the center. The true sensitivity, for a brain-sized object, is a maximum of 85 kcps/microCi/ml and the scatter fraction is a minimum of 10%, both depending on the lower level energy threshold. The scanner can handle up to 5 mCi in the field of view, at which point the randoms equal the true coincidences and the detectors reach their count rate limit. We have so far acquired [18F]FDG brain studies and cardiac studies, which show the applicability of our scanner for both brain and whole-body imaging. With the results to date, we feel that this design results in a simple yet high performance scanner which is applicable to many types of static and dynamic clinical studies.     

Design considerations for a positron emission transverse tomograph (PETT V) for imaging of the brain.

Imaging of the brain by positron emission tomography can be optimized for sensitivity by dedicating the design of the tomograph to this application. We have designed a multislice positron emission tomograph (PETT V) for imaging the human brain and the whole body of small experimental animals. The detector system of PETT V consists of a circular array of 48 NaI(Tl) scintillation detectors, each fitted with two photomultiplier tubes, with one dimensional positioning capability. Suitable sampling is achieved by rotation of the circular array of detectors and by a wobbling motion of the detector circle. The proposed system is capable of providing seven slices simultaneously, with a spatial resolution in the plane of the slice from 7 to 15 mm and with slice thicknesses of 7 and 14 mm. The minimum scanning time is 1 sec. The estimated overall sensitivity of PETT V is 350,000 counts/sec/mCi in a 20 cm diameter phantom for a resolution of approximately 1.5 x 1.5 cm. The system is under construction.     

The influence of tomograph sensitivity on kinetic parameter estimation in positron emission tomography imaging studies of the rat brain

We investigated the influence of tomograph sensitivity on reliability of parameter estimation in positron emission tomography studies of the rat brain. The kinetics of two tracers in rat striatum and cerebellum were simulated. A typical injected dose of 10 MBq and a reduced dose of 1 MBq were assumed. Kinetic parameters were estimated using a region of interest (ROI) analysis and two pixel-by-pixel analyses. Striatal binding potential was estimated as a function of effective tomograph sensitivity (S(eff)) using a simplified reference tissue model. A S(eff) value of > or =1% was required to ensure reliable parameter estimation for ROI analysis and a S(eff) of 3-6% was required for pixel-by-pixel analysis. We conclude that effective tomograph sensitivity of 3% may be an appropriate design goal for rat brain imaging.     

Whole-body positron emission tomography: Part I. Methods and performance characteristics.

Methods for whole-body PET imaging have been developed to provide a clinical tool for the detection and evaluation of primary and metastatic cancers. The axial FOV of the PET system is extended by imaging at multiple bed positions to cover the whole body. In typical rectilinear PET scans, only a small fraction of the data is collected to form two-dimensional projection images. In this work, 100% of the projection data was collected to form the two-dimensional projection images. These projection images were generated for continuous angles over 180 degrees by resorting sinogram data. In addition, tomographic images were formed by using filtered backprojection reconstruction without attenuation correction. Coronal and sagittal cuts were then extracted from the three-dimensional data set. The tomographic images were reconstructed to a resolution of 10.8 mm in all dimensions because of statistical limitations of the data. Both methods of image formation resulted in images of high quality with the tomographic reconstruction providing the highest contrast and resolution. An acquisition time of 1-2 min/bed position after a 10-mCi injection of [18F]fluoride ion or [18F]FDG was found to give a sufficient number of counts for producing images of good resolution and contrast, from a total scanning time of 32-64 min.     

Aspects of three dimensional reconstruction for a multi ring positron tomograph

An important feature of multi ring positron tomographs is the inter plane septa, the purpose of which is to reduce random and scattered coincidences. In general, such septa also eliminate the coincidence lines of response between pairs of detectors more than one ring apart. The operation of a camera without septa must result in an increase not only in the true coincidence rate, but also in the singles, and therefore in the dead time and randoms rate, and in the scattered coincidences. A configuration option in the coincidence hardware of the 8 ring, 15 slice ECAT 931/08-12 enables a full set of 64 sinograms to be acquired when the septa are removed. The detector normalisation and transmission data for studies with the septa out can be obtained using a rotating pin source. To take maximum advantage of the additional signal, the emission data must be reconstructed using a fully three dimensional reconstruction algorithm. This paper presents an analysis of some phantom studies acquired without septa and reconstructed in three dimensions. The results are compared with data acquired with septa for the same phantoms imaged under similar conditions. It is found that, with the septa removed, the signal to noise for a uniform, 20 cm diameter cylinder improves by a factor of 2.8 in the centre of the field of view, whereas in regions distant from the centre in the axial direction, the signal to noise decreases due to the increase in scatter and randoms. An improvement in signal to noise is observed in 6 cm of the 10 cm axial length of the tomograph.     

Physical aspects of cardiac scanning with a block detector positron tomograph

Physical aspects relating to cardiac scanning are described for an eight ring (15 plane) positron tomograph consisting of BGO block detectors (CTI/Siemens 931-08/12). Performance parameters were derived from a cylindrical heart phantom having a "myocardial" wall of thickness varying from 3 mm to 27 mm. This phantom was inserted into a chest phantom consisting of simulated chest wall, lungs, and arms. Recovery coefficients for myocardial thicknesses of 10 mm and 15 mm were 0.75 and 0.9, respectively. Division by the transmission minus "blood pool" (extravascular density) image was found to give a variation of corrected myocardial counts within +/- 5% when transmission data were smoothed. The on-line dead time correction algorithm was found to be accurate to within 5% up to 20 mCi (740 MBq) in the axial field of view (FOV) (10.8 cm) in the central chamber of the heart phantom. However, the correction factor at this rate is approximately 3, which would imply poor use of administered dose. Counts in the image due to scatter are approximately 2% in the (cold) central cavity of the heart phantom relative to counts/pixel in the active myocardium. The presence of phantom arms in the FOV was found to have only a small effect on mean pixel counts and noise in the heart phantom image, as did movement of the arms within a reasonable range.     

18F-Fluoride positron emission tomography and positron emission tomography/computed tomography

(18)F-Fluoride is a positron-emitting bone-seeking agent, the uptake of which reflects blood flow and remodeling of bone. Assessment of (18)F-fluoride kinetics using quantitative positron emission tomography (PET) methods allows the regional characterization of lesions of metabolic bone diseases and the monitoring of their response to therapy. It also enables the assessment of bone viability and discrimination of uneventful and impaired healing processes of fractures, bone grafts and osteonecrosis. Taking advantage of the favorable pharmacokinetic properties of the tracer combined with the high performance of PET technology, static (18)F-fluoride PET is a highly sensitive imaging modality for detection of benign and malignant osseous abnormalities. Although (18)F-fluoride uptake mechanism corresponds to osteoblastic activity, it is also sensitive for detection of lytic and early marrow-based metastases, by identifying their accompanying reactive osteoblastic changes, even when minimal. The instant fusion of increased (18)F-fluoride uptake with morphological data of computed tomography (CT) using hybrid PET/CT systems improves the specificity of (18)F-fluoride PET in cancer patients by accurately differentiating between benign and malignant sites of uptake. The results of a few recent publications suggest that (18)F-fluoride PET/CT is a valuable modality in the diagnosis of pathological osseous conditions in patients also referred for nononcologic indications. (18)F-fluoride PET and PET/CT are, however, not widely used in clinical practice. The limited availability of (18)F-fluoride and of PET and PET/CT systems is a major factor. At present, there are not enough data on the cost-effectiveness of (18)F-fluoride PET/CT. However, it has been stated by some experts that (18)F-fluoride PET/CT is expected to replace (99m)Tc-MDP bone scintigraphy in the future.