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This study aimed to examine auditory and visual cortical activations in bilateral cochlear implant recipients using [(18)F]-FDG positron emission tomography. We aimed to compare the activations from use of the first implant alone, the second implant alone, and both implants together. When both implants were activated simultaneously, summation of cortical activity did not occur. The first and second implants demonstrated evidence of developing distinct neural networks. The first implants show stronger bilateral recruitment of the auditory areas than the second implants. Visual cortical activations occur in response to stimulation of the second but not the first implants. When both implants were activated together, there were no visual activations suggesting interaction between the first- and second-implant networks. These findings add to the existing knowledge of plasticity following cochlear implantation and demonstrate a variability of these processes that was previously unreported.

This study aimed to investigate changes in auditory and visual cortical activity over the first year following cochlear implantation using (18)F-fluorodeoxyglucose positron emission tomography. Subjects underwent scanning prior to the initial implant activation (control), after one to two months of implant use (early activation) and after one year of implant use (late activation). All subjects had activation of the auditory cortices. Group analysis using Statistical Parametric Mapping package SPM99 showed these became more focused over the first year of implant use. There was no evidence of left hemispheric dominance at any stage post implantation.Visual cortical activations were highly variable between patients and did not increase significantly between early and late activations. Taken together, our results lead us to suggest that the neural processes that occur during the first year of auditory rehabilitation following cochlear implantation vary between individuals to a greater extent than previously reported.

The performance of a novel type of animal PET camera, the quad High-Density Avalanche Chamber (HIDAC) was assessed for a non-rotating 16-module system. Spatial resolution was 1.0 mm, and invariant within a standard deviation ⩽5%. Absolute sensitivity was 0.95%, and the scatter-background corrected sensitivity was 0.75%. The count rate capability was linear at typical activities used in animal imaging, with a 20% loss at 11.5 MBq. The camera demonstrates small regions of radiotracer uptake with excellent detail in the mouse.

Conventional methods for dealing with attenuation and scatter in PET can limit the reconstructed image quality, particularly if the attenuating medium is large (as in whole body 3D PET). In such cases, often a substantial scatter subtraction is performed followed by amplification of the remaining data (to correct for attenuation) resulting in noisy reconstructions. More recent iterative reconstruction methods include the attenuation in the system model in conjunction with either pre-scatter subtraction or a separate addition of the scatter component after each application of the forward model. This work compares these more conventional approaches of including attenuation and scatter to the case where attenuation and scatter information are both included within the system matrix used by the expectation maximization maximum likelihood (EM-ML) algorithm. For this case all acquired data are used and regarded as a source of information by the reconstruction algorithm. Multiple realisations of simulated data sets have been used to compare the performance of the unified attenuation and scatter model with other methods. For a large attenuating medium and low counts there are notable differences between the four main ways of including attenuation and scatter within the reconstruction-with full pre-correction of the data being inferior compared to all the other methods, and the method which models scatter and attenuation within the system matrix showing some advantages. This work suggests that if regularisation of the EM algorithm is carried out by early termination of the iterative process, the subtraction method is the better approach among the techniques considered. In contrast, if a post-reconstruction smoothing approach to regularisation is to be used (whereby highly iterated, noisy image estimates are smoothed), the full modeling method for attenuation and scatter yields the better results, albeit at the computational cost of many more iterations being required

PET radiotracers which incorporate longer-lived radionuclides enable biological processes to be studied over many hours, at centres remote from a cyclotron. This paper examines the radioisotope characteristics, imaging performance, radiation dosimetry and production modes of the four copper radioisotopes, ( 60)Cu,( 61)Cu,( 62)Cu and( 64)Cu, to assess their merits for different PET imaging applications.Spatial resolution, sensitivity, scatter fraction and noise-equivalent count rate (NEC) are predicted for( 60)Cu,( 61)Cu,( 62)Cu and( 64)Cu using a model incorporating radionuclide decay properties and scanner parameters for the GE Advance scanner. Dosimetry for( 60)Cu,( 61)Cu and( 64)Cu is performed using the MIRD model and published biodistribution data for copper(II) pyruvaldehyde bis(N(4)-methyl)thiosemicarbazone (Cu-PTSM).(60)Cu and( 62)Cu are characterised by shorter half-lives and higher sensitivity and NEC, making them more suitable for studying the faster kinetics of small molecules, such as Cu-PTSM.( 61)Cu and( 64)Cu have longer half-lives, enabling studies of the slower kinetics of cells and peptides and prolonged imaging to compensate for lower sensitivity, together with better spatial resolution, which partially compensates for loss of image contrast.( 61)Cu-PTSM and( 64)Cu-PTSM are associated with radiation doses similar to [(18)F]-fluorodeoxyglucose, whilst the doses for( 60)Cu-PTSM and( 62)Cu-PTSM are lower and more comparable with H(2) (15)O.The physical and radiochemical characteristics of the four copper isotopes make each more suited to some imaging tasks than others. The results presented here assist in selecting the preferred radioisotope for a given imaging application, and illustrate a strategy which can be extended to the majority of novel PET tracers.

This study aimed to investigate the relationship between outcome following cochlear implantation and auditory cortical activation. It also studied the effects of length of implant use and duration of deafness on the auditory cortical activations. Cortical activity resulting from auditory stimulation was measured using [ 18 F]FDG positron emission tomography. In a group of 18 experienced adult cochlear implant users, we found a positive correlation between speech perception and activations in both the primary and association auditory cortices. This correlation was present in a subgroup of experienced implant users but absent in a group of new implant users with similar speech perception abilities. There was a significant negative correlation between duration of deafness and auditory cortical activation. This study gives insights into the relationship between implant speech perception and auditory cortical activation and the influence of duration of preceding deafness and implant experience.

The key performance measures of resolution, count rate, sensitivity and scatter fraction are predicted for a dedicated BGO block detector patient PET scanner (GE Advance) in 2D mode for imaging with the non-pure positron-emitting radionuclides 124I, 55Co, 61Cu, 62Cu, 64Cu and 76Br. Model calculations including parameters of the scanner, decay characteristics of the radionuclides and measured parameters in imaging the pure positron-emitter 18F are used to predict performance according to the National Electrical Manufacturers Association (NEMA) NU 2-1994 criteria. Predictions are tested with measurements made using 124I and show that, in comparison with 18F, resolution degrades by 1.2 mm radially and tangentially throughout the field-of-view (prediction: 1.2 mm), count-rate performance reduces considerably and in close accordance with calculations, sensitivity decreases to 23.4% of that with 18F (prediction: 22.9%) and measured scatter fraction increases from 10.0% to 14.5% (prediction: 14.7%). Model predictions are expected to be equally accurate for other radionuclides and may be extended to similar scanners. Although performance is worse with 124I than 18F, imaging is not precluded in 2D mode. The viability of 124I imaging and performance in a clinical context compared with 18F is illustrated with images of a patient with recurrent thyroid cancer acquired using both [124I]-sodium iodide and [18F]-2-fluoro-2-deoxyglucose.

Methodology for PET system modeling using image-space techniques in the expectation maximization (EM) algorithm is presented. The approach, applicable to both list-mode data and projection data, is of particular significance to EM algorithm implementations which otherwise only use basic system models (such as those which calculate the system matrix elements on the fly). A basic version of the proposed technique can be implemented using image-space convolution, in order to include resolution effects into the system matrix, so that the EM algorithm gradually recovers the modeled resolution with each update. The improved system modeling (achieved by inclusion of two convolutions per iteration) results in both enhanced resolution and lower noise, and there is often no need for regularization-other than to limit the number of iterations. Tests have been performed with simulated list-mode data and also with measured projection data from a GE Advance PET scanner, for both [/sup 18/F]-FDG and [/sup 124/I]-NaI. The method demonstrates improved image quality in all cases when compared to the conventional FBP and EM methods presently used for clinical data (which do not include resolution modeling). The benefits of this approach for /sup 124/I (which has a low positron yield and a large positron range, usually resulting in noisier and poorer resolution images) are particularly noticeable.

List-mode data preserves all sampling information in three-dimensional (3-D) PET imaging and can reduce storage requirements for short-time frame acquisitions. List-mode expectation maximization-maximum likelihood (EM-ML), which has been implemented in a number of forms (such as the EM algorithm for list-mode maximum likelihood, the FAIR algorithm and COSEM), is an obvious choice to reconstruct from such data sets when the statistics are low. However, these methods can be slow for large quantities of list-mode data and it is desirable to accelerate them. This work investigates the use of subsets in combination with a relaxation parameter for 3-D list-mode EM reconstructions. Results show that two iterations through the list-mode data are sufficient to yield good quality reconstructions. Furthermore, if counting statistics are good, just one iteration may prove sufficient, opening the way for real-time iterative reconstruction.

There are major potential advantages in non-invasive measurement of preclinical tumour biology and therapeutic response in clinically relevant, internal body sites, notably the ability to follow outcome in individual animals rather than averaging results from groups. We have exploited positron emission tomography (PET) to determine the feasibility of detecting liver metastases in B6D2F1 mice using fluorine-18 fluorodeoxyglucose ([18F]FDG) both before and after treatment by the novel cytotoxic agent, combretastatin A-4. The normal distribution of [18F]FDG in the absence of disease was characterised, with the clear delineation of the brain, the heart and the urinary bladder in all studies. In untreated mice with liver metastases, a strong correlation (r 2 = 0.98) was found between the quantitative estimates of [18F]FDG uptake obtained by analysis of PET images, and those obtained from ex vivo assay of liver plus metastases excised immediately after imaging. In this first series, the effective limit of resolution was in livers containing a number of small metastases (range 8–14) with a single volume equivalent of approximately 200 mm3. PET image analysis was concordant with histological measurements in showing that single intraperitoneal doses of combretastatin A-4 resulted in an average 30% volume destruction of metastatic mass by 24 h following administration.

Most existing techniques for determining volumes on single-photon-emission computed tomography (SPECT) data employ thresholding, two-dimensional edge detection, or manual delineation of edges. These methods, however, are limited in both accuracy and applicability. In seeking to overcome these limitations, a truly three-dimensional (3D) second-derivative-based algorithm which can be implemented with relative ease has been developed. The method incorporates 3D matrix operators; these are convoluted with the SPECT count data in order to produce a 3D voxel map whose data elements correspond to the second derivative of counts in the image. This map is then searched, a suitable derivative-based edge-defining criterion being applied to each voxel position, in order to locate the derivative surface boundary which defines the volume. Validation is obtained using phantom data from 99Tcm-filled bottles of volumes 200, 580 and 2500 cm3 placed within a body-sized tank containing background activities set to give a range of contrasts between 1.00 and 0.75 (i.e. background 0% to 25%). The performance of the algorithm is encouraging: the volumes of the two larger bottles are determined to within a 3% accuracy without the need for any prior calibration, and the results obtained over all bottle sizes are found to be contrast independent to within approximately 4%.

Cochlear implantation is generally accepted as a successful means of restoring auditory sensation to profoundly deaf individuals. Although most patients can expect a satisfactory outcome following implantation, some have poor speech perception outcomes. This investigation used [18F]-fluorodeoxyglucose positron emission tomography to measure cortical activity resulting from auditory stimulation in seven ‘good’ and four ‘poor’ cochlear implant recipients. Activations were significantly greater in both the primary and association cortices in the good compared with the poor implant users. We suggest that the ability to access the more specialised speech processing abilities of the auditory association cortices helps determine outcome following cochlear implantation.

A new technique for modelling multiple-order Compton scatter which uses the absolute probabilities relating the image space to the projection space in 3D whole body PET is presented. The details considered in this work give a valuable insight into the scatter problem, particularly for multiple scatter. Such modelling is advantageous for large attenuating media where scatter is a dominant component of the measured data, and where multiple scatter may dominate the total scatter depending on the energy threshold and object size. The model offers distinct features setting it apart from previous research: (1) specification of the scatter distribution for each voxel based on the transmission data, the physics of Compton scattering and the specification of a given PET system; (2) independence from the true activity distribution; (3) in principle no scaling or iterative process is required to find the distribution; (4) explicit multiple scatter modelling; (5) no scatter subtraction or addition to the forward model when included in the system matrix used with statistical image reconstruction methods; (6) adaptability to many different scatter compensation methods from simple and fast to more sophisticated and therefore slower methods; (7) accuracy equivalent to that of a Monte Carlo model. The scatter model has been validated using Monte Carlo simulation (SimSET).

Background: Between 1 and 2 per cent of head and neck squamous cell carcinoma patients will reveal no evidence of a primary malignancy. The management of this group poses many problems, including the morbidity associated with wide field irradiation as well as the difficulty in treatment when a primary does emerge. The aim of this study was to assess the use of fluoro-deoxy-glucose positron emission tomography (FDG-PET) imaging in patients presenting with an unknown head and neck primary and to consider its routine use in such patients.Methods: We enrolled 25 patients into our study over a four year period. They all presented with a histologically proven, metastatic, squamous cell carcinoma of the neck for which no primary could be found despite full clinical, endoscopic and radiological evaluation with computed tomography (CT) and/or magnetic resonance imaging (MRI). Additionally, all the patients underwent imaging using FDG-PET. The images were interpreted by two radiologists experienced in PET imaging.Results: A primary was identified in nine of the 25 patients (42 per cent); however, of these patients, six had false positive results and only three patients were true positives with supportive histology. In the remaining 16 patients, no abnormality was identified on CT, MRI or PET. Of these 16 patients, two eventually displayed a primary carcinoma, the other 14 patients remaining without evidence of any primary.Conclusion: Despite the high number of positive PET scans, the actual true positive rate was 3/9 (33 per cent); conversely, the true negative rate was 14/16 (88 per cent). We conclude from this study that there is a role for FDG-PET in the patient with an unknown head and neck primary, particularly in the context of a negative PET scan.

An approach to an analytic system model for the true component of 3D PET data offering the accuracy of a Monte Carlo model run for an infinite time is presented. The system model, which is used in the expectation maximisation (EM) algorithm, accounts for the 3D nature of the emission process in which the geometric sensitivity to a point source varies along and across the tube of each line of response (LOR). Therefore the system model cannot be based just on line or tube integrals as it is for the model of transmission scanning. By accounting for the varying sensitivity within each LOR in the reconstruction process unbiased and quantitatively exact images can be achieved without using calibration or scaling factors after the reconstruction process. Two approaches to the model are investigated, i.e., LOR- and voxel-driven which are validated using high statistics Monte Carlo simulation (SimSET).

In statistical image reconstruction for PET, the reconstructed image quality depends on the system matrix as well as the scatter correction method used, especially for the case of a large attenuating medium where the measurement process is dominated by photon attenuation and scatter. Accurate system and scatter modeling can improve image quality, but whatever the method employed systematic and/or random errors will always exist in the system model, inevitably impacting final reconstructed image quality. Theoretical expressions have been derived to study the error propagation from the scatter response function to the reconstructed images for the case of maximum likelihood (ML) reconstruction. The effect of system and scatter modeling errors for three different scatter correction methods are considered: a) scatter subtraction, b) adding scatter as a constant term to the forward model and c) a unified model where the scatter is completely modeled within the system matrix itself. First order approximations are used to derive the theoretical expressions for the error propagation, which account for errors in both the system matrix and the scatter estimates (when used outside the system matrix). These expressions are validated using simulated data. A close agreement is found between the measured and theoretically derived error images, with the unified system model being least sensitive to the errors. The theoretical expressions are useful to determine the required accuracy for the system matrix and scatter estimation.

A technique for modelling multiple Compton scatter as the absolute probabilities which relate the image space to the projection space in 3D whole body PET system is presented. Such modelling is advantageous for large attenuating media where scatter is a dominant component of the measured data, and where multiple scatter may constitute up to about half of the total scatter depending on the energy settings and attenuating object. The model developed in this work goes beyond the limitations of previous methods in three distinct ways: (1) specification of the scatter distribution for every voxel based on calculation of actual probabilities, using the transmission data, the physics of Compton scattering and the specification of a given PET system (independence from a true activity estimate); (2) in principle no scaling or iterative process is required to find the scatter distribution; (3) explicit multiple scatter modelling; (4) easily adaptable to time-of-flight PET scatter estimation. The model can be included in a system matrix for statistical image reconstruction methods (avoiding the additive or subtractive approaches to scatter compensation), and of course the method can be adapted to a whole host of differing scatter compensation methods of varying speed and complexity. The proposed, high accuracy model has been validated using Monte Carlo simulation (SimSET).

The very high resolution quad-HIDAC PET scanner has four detector banks each with four planar high density avalanche chambers, capable of collecting billions of lines-of-response (LORs), with very high spatial resolution, good absolute sensitivity and a large field of view (170 mm times 170 mm times 280 mm). However, it is crucial to correct for degrading factors introduced by the scanner's geometry and detection. A plane converter efficiency estimation method based on acceptance regions is presented, in which it is possible to represent a plane converter as a set of detection efficiency areas. LORs registered in a particular region in coincidence with another in the opposite plane converter are registered in a square array named a cooccurrence matrix, which is the basis of the method. The reciprocal of these factors are used as a multiplicative correction within the list-mode EM algorithm, event by event. A methodology to obtain the system-geometric correction factors is described and included within the reconstruction process as a normalisation component. Reconstructed images from a uniform 18F cylinder source (3 cm diameter, 5 cm length) and a 68Ge line source (15.5 cm length), were used to assess the region-efficiency and system-geometry correction.

A voxel-based approach to 3D PET analytic system modeling which projects the unit sphere of Klein-Nishina probabilities onto the cylindrical detection sphere, is proposed and under development. The aim is to offer the accuracy of a Monte Carlo model but without the noise and computational limitations. The model, incorporating both photon attenuation and scattering, defines PET system matrix elements based only on knowledge of a transmission scan and the specifications of a given PET system. No knowledge of the true activity is required to define the model. Reconstruction with such a unified analytic model makes full use of all the information contained in the measured data (both true and scatter events). No scatter subtraction, or scatter addition to the forward model output, are required. For 3D PET with large attenuating media, full use of the acquired information results in lower noise levels in the final image. The core of the technique models object scatter and attenuation via the Klein-Nishina cross-section and the transmission scan. However, the model must also include component-based normalization which accounts for scanner geometry as well as the crystal type and their arrangement.

Conventional methods for dealing with attenuation and scatter can degrade the reconstructed image quality, particularly if the attenuating medium is large (as in whole body 3D PET). In such cases, a substantial scatter subtraction is performed followed by amplification of the remaining data (to correct for attenuation), which results in noisy reconstructions. More recent methods used with iterative reconstruction include the attenuation in the system model in conjunction with either pre-scatter subtraction or a separate addition of the scatter component after each application of the forward model. This work compares these more conventional approaches to including attenuation and scatter within the EM algorithm with a fully unified scatter and attenuation model - whereby all attenuation and scattering effects are included within the system matrix. For this case all acquired data are used and regarded as information by the reconstruction algorithm. This work indicates that for a large attenuating medium there are notable differences between the four ways of including attenuation and scatter within the reconstruction - fully pre-correction of the data is inferior compared to all the other methods. For the case of simple shift-invariant Gaussian model of scatter - subtraction, additive and unified model methods shows similar variance-bias characteristics. Multiple realisations of simulated data sets have been used to compare the performance of this model with other methods.

We have previously shown that labelled water positron emission tomography (H215O PET) can be used to identify regional cerebral blood flow (rCBF) changes in the human brain during volitional swallowing. (18F) fluorodeoxyglucose (FDG PET), by comparison, uses a glucose analogue to quantitatively measure regional cerebral glucose metabolism (rCMRglc) rather than rCBF. The main advantage of FDG PET is improved spatial resolution, and because of its pharmacodynamic properties, activation can be performed external to the scanner, allowing subjects to assume more physiologic positions. We therefore conducted a study of the brain's metabolic response while swallowing in the erect seated position, using FDG PET. Eight healthy male volunteers were studied with a randomised 2 scan paradigm of rest or water swallowing at 20-second intervals for 30 minutes. Data were analysed with SPM99 using multisubject conditions and covariates design. During swallowing, analysis identified increased rCMRglc ( P

We are interested in imaging cell death in vivo using annexin V radiolabeled with 124 I. In this study, [ 124 I]4IB-annexin V and [ 124 I]4IB-ovalbumin were made using [ 124 I] N -hydroxysuccinimidyl-4-iodobenzoate prepared by iododestannylation of N -hydroxysuccinimidyl-4-(tributylstannyl)benzoate. [ 124 I]4IB-annexin V binds to phosphatidylserine-coated microtiter plates and apoptotic Jurkat cells and accumulates in hepatic apoptotic lesions in mice treated with anti-Fas antibody, while [ 124 I]4IB-ovalbumin does not. In comparison with 124 I-annexin V, [ 124 I]4IB-annexin V has a higher rate of binding to phosphatidylserine in vitro, a higher kidney and urine uptake, a lower thyroid and stomach content uptake, greater plasma stability and a lower rate of plasma clearance. Binding of radioactivity to apoptotic cells relative to normal cells in vitro and in vivo appears to be lower for [ 124 I]4IB-annexin V than for 124 I-annexin V.

A noninvasive method of measuring programmed cell death in the tumors of cancer patients using positron-emission tomography (PET) would provide valuable information regarding their response to therapeutic intervention. Our strategy is to radiolabel annexin V, a protein that binds to phosphatidylserine moieties that are translocated to the external leaflet of plasma membranes during apoptosis. We developed a phosphatidylserine-ELISA capable of distinguishing wild type from point mutant annexin V that is known to have a lower phosphatidylserine binding affinity. A maltose-binding protein/annexin V chimera was synthesized and purified with high yield using amylose resin. We showed that it bound to phosphatidylserine in the ELISA as well as to that exposed on apoptotic Jurkat cells; therefore, it was used in the development of a method for radiolabeling annexin V using iodine radionuclides. MBP-annexin V retained its phosphatidylserine binding properties on direct iodination, but at high levels of oxidizing agents (iodogen and chloramine T), its specificity for phosphatidylserine was compromised. (124)I-MBP-annexin V was successfully used to image Fas-mediated hepatic cell death in BDF-1 mice using PET. In conclusion, we have shown that MBP-annexin V and the phosphatidylserine ELISA are useful tools for the development of methods for radiolabeling annexin V for PET imaging.

Abnormal regulation of apoptosis is an important pathogenic mechanism in many diseases including cancer. Techniques to assess apoptosis in living organisms are limited and, in the case of solid organs, restricted to histological examination of biopsy samples. We investigated the use of (124)I-annexin V, which binds to phosphatidylserine (PS) on the surface of apoptotic cells, as a potential positron emission tomography (PET) radioligand for the noninvasive measurement of apoptosis in vivo. Annexin V and a similar-sized protein, ovalbumin, were directly labelled with (124)I. We report the validation of (124)I-annexin V in vitro and in an animal model of liver apoptosis that has not previously been used to test iodinated annexin V. Also, for the first time, we report metabolite analysis of (124)I-annexin V and the correlation of (124)I-annexin V uptake with apoptotic density (AD). Sixfold more (124)I-annexin V was associated with Jurkat cells after apoptosis induction, indicating that PS binding by annexin V was preserved after iodination. (124)I-ovalbumin did not demonstrate increased uptake in apoptotic cells. In normal BDF-1 mice, the radioligand was rapidly cleared, but some in vivo dehalogenation resulted in the accumulation of activity in the thyroid and stomach content. PET images demonstrated uptake of (124)I-annexin V but not (124)I-ovalbumin in apoptotic liver lesions. In vivo (124)I-annexin V uptake, derived from PET images, correlated with histologically derived AD (r=.86, P

A method for modeling and recovering resolution in the expectation maximization (EM) algorithm via two image-space convolutions per iteration is presented. The approach is of particular significance to EM algorithm implementations which otherwise only use basic system models, such as those which calculate the system matrix elements on the fly rather than using pre-calculated values. The first convolution is applied to a copy of the image estimate prior to forward projecting this copy. The second is performed on the backprojected correction image prior to its use to multiplicatively update the original (not convolved) image estimate. The inclusion of resolution-modeling convolution at these two stages corresponds to extending the system transfer matrix, and the EM algorithm gradually recovers the modeled resolution with each update. The approach does not constitute regularization in any way, but rather the improved system modeling results in both enhanced resolution and lower noise, and there is often no need for regularization - other than to limit the number of iterations. Tests have been performed with simulated list-mode data and also with measured phantom and patient projection data from a GE Advance PET scanner, for both [/sup 18/F]-FDG and [/sup 124/I]-NaI. The method demonstrates improved image quality in all cases when compared to the conventional FBP and EM methods presently used for clinical data (which do not include resolution modeling). The benefits of this approach for I-124 (which has a low positron yield and a large positron range, usually resulting in noisier and poorer resolution images) are particularly noticeable.

High resolution 3D PET scanners with high count rate performance, such as the quad-HIDAC, place new demands on image reconstruction algorithms due to the large quantities of high precision list-mode data which are produced. Therefore a reconstruction algorithm is required which can, in a practical time frame, reconstruct into very large image arrays (submillimetre voxels, which range over a large field of view) whilst preferably retaining the precision of the data. This work presents an algorithm which meets these demands: Regularized One-Pass List-mode EM (ROPLE). The algorithm operates directly on list-mode data, passes through the data once only, accounts for finite resolution effects in the system model and also includes regularization. The algorithm performs multiple image updates during its single pass through the list-mode data, corresponding to the number of subsets that the data have been split into. The algorithm has been assessed using list-mode data from a quad-HIDAC, and is compared to the analytic reconstruction method 3D RP.

List-mode data preserves all sampling information from 3D PET imaging, and reduces storage requirements for multiple time frame acquisitions. List-mode EM-ML, which has been implemented in a number of forms (such as the EM algorithm for list-mode maximum likelihood, the FAIR algorithm and COSEM), is an obvious choice to reconstruct from such data sets when the statistics are low. However, these methods can be slow for large quantities of mode data, and it is desirable to accelerate them. This work investigates the use of subsets in combination with a relaxation parameter for 3D list-mode EM-ML reconstructions. Results show just two iterations through the list-mode data are sufficient to aid good quality reconstructions. Furthermore, if counting statistics are good, just one iteration may prove sufficient, opening the way for real-time iterative reconstruction.

Backprojected images offer compact and accurate storage of three-dimensional (3-D) positron emission tomography (PET) data and so are potentially very suited to high-resolution 3-D PET scanners. However, there are very few methods available for correcting backprojected images for the effects of scatter and random events. Scatter correction methods usually involve experimental measurement of scatter response functions or accurate simulations, and are thus limited by the accuracy of the model used. In contrast, the approach proposed here uniquely adapts a general form of a combined scatter and random response function. This method inherently does not rely upon accurate determination of the scatter and random response functions but does, however, rely upon knowledge of the attenuating object's outline. The technique is able to adapt to different imaging situations and account for detector scatter, and in particular is readily implemented in backprojection space. The method has been implemented for a small-volume HIDAC (II) PET scanner.

One early effect of the treatment of tumours by the new modality photodynamic therapy (PDT) is a reduction in tumour glucose levels. We have employed the widely used positron-emitting glucose analogue flurorine-18 fluoro-2-deoxy-D-glucose ([18F]-FDG), to determine whether, in principle, PDT-induced injury might be delineated non-invasively and quantitatively by positron emission tomography (PET). The scanner was of the high-density avalanche-chamber (HIDAC) type with a resolution of 2.6 mm. Subcutaneous T50/80 mouse mammary tumours, sensitised by haematoporphyrin ester, were illuminated by graded doses of interstitial 630 nm light. Thirty hours later, any remaining viable tumour was detected (a) by region-of-interest analysis of the PET images and (b) by gamma counting the excised tumour. PET measurements of % uptake of [18F]-FDG into tumour correlated closely with ex vivo gamma counting (slope=0.976, r2=0. 995), validating the in situ method. Uptake into untreated, control tumours was 3.8%+/-1.1% of the injected activity. Uptake of [18F]-FDG into treated tumours decreased by 0.7% for every 100 mm3 reduction in remaining viable histological volume. Outcome was further compared with that measured by (a) T2-weighted proton imaging on a 4.7-T magnetic resonance imaging (MRI) system and (b) histological analysis of subsequently sectioned tumours. PET using [18F]-FDG described the absolute volume of surviving tumour histological mass to the same degree as high-resolution MRI. The conclusion of these initial studies is that PET with [18F]-FDG, although non-specific, quantitatively described at early times the extent of tumour destruction by PDT.

Thirty-seven patients with chest pain have been studied by exercise thallium tomography and coronary arteriography to assess the accuracy of thallium tomography in the identification of patients with coronary artery disease and to evaluate whether knowledge of clinical data improves this accuracy. Thallium tomography was also used to identify which coronary arteries were stenosed. Thallium tomography when compared with coronary arteriography has a sensitivity of 97%, a specificity of 75%, and an overall accuracy of 92% for the detection of coronary artery disease. When clinical information was made available, these results improved to a sensitivity of 100%, a specificity of 75%, and an overall accuracy of 95%. In the identification of which coronary arteries were stenosed, thallium tomography had a sensitivity of 85%, a specificity of 89%, and an overall accuracy of 86%. These results show that exercise thallium tomography can be an accurate non-invasive screening test in patients with chest pain thought to be angina.

High-resolution three-dimensional (3-D) positron emission tomography (PET) scanners with high count rate performance, such as the quad-high density avalanche chamber (HIDAC), place new demands on image reconstruction algorithms due to the large quantities of high-precision list-mode data which are produced. Therefore, a reconstruction algorithm is required which can, in a practical time frame, reconstruct into very large image arrays (submillimeter voxels, which range over a large field of view) whilst preferably retaining the precision of the data. This work presents an algorithm which meets these demands: one-pass list-mode expectation maximization (OPL-EM) algorithm. The algorithm operates directly on list-mode data, passes through the data once only, accounts for finite resolution effects in the system model, and can also include regularization. The algorithm performs multiple image updates during its single pass through the list-mode data, corresponding to the number of subsets that the data have been split into. The algorithm has been assessed using list-mode data from a quad-HIDAC and is compared to the analytic reconstruction method 3-D reprojection (RP) with 3-D filtered backprojection.

The quad-HIDAC PET scanner offers unparalleled high resolution PET imaging capability (