Diagnosis and Management of Mild Traumatic Brain Injury in Children: A Systematic Review.

Importance: In recent years, there has been an exponential increase in the research guiding pediatric mild traumatic brain injury (mTBI) clinical management, in large part because of heightened concerns about the consequences of mTBI, also known as concussion, in children. The CDC National Center for Injury Prevention and Control's (NCIPC) Board of Scientific Counselors (BSC), a federal advisory committee, established the Pediatric Mild TBI Guideline workgroup to complete this systematic review summarizing the first 25 years of literature in this field of study. Objective: To conduct a systematic review of the pediatric mTBI literature to serve as the foundation for an evidence-based guideline with clinical recommendations associated with the diagnosis and management of pediatric mTBI. Evidence Review: Using a modified Delphi process, the authors selected 6 clinical questions on diagnosis, prognosis, and management or treatment of pediatric mTBI. Two consecutive searches were conducted on PubMed, Embase, ERIC, CINAHL, and SportDiscus. The first included the dates January 1, 1990, to November 30, 2012, and an updated search included December 1, 2012, to July 31, 2015. The initial search was completed from December 2012 to January 2013; the updated search, from July 2015 to August 2015. Two authors worked in pairs to abstract study characteristics independently for each article selected for inclusion. A third author adjudicated disagreements. The risk of bias in each study was determined using the American Academy of Neurology Classification of Evidence Scheme. Conclusion statements were developed regarding the evidence within each clinical question, and a level of confidence in the evidence was assigned to each conclusion using a modified GRADE methodology. Data analysis was completed from October 2014 to May 2015 for the initial search and from November 2015 to April 2016 for the updated search. Findings: Validated tools are available to assist clinicians in the diagnosis and management of pediatric mTBI. A significant body of research exists to identify features that are associated with more serious TBI-associated intracranial injury, delayed recovery from mTBI, and long-term sequelae. However, high-quality studies of treatments meant to improve mTBI outcomes are currently lacking. Conclusions and Relevance: This systematic review was used to develop an evidence-based clinical guideline for the diagnosis and management of pediatric mTBI. While an increasing amount of research provides clinically useful information, this systematic review identified key gaps in diagnosis, prognosis, and management.

Regional and total skeletal measurements in the early postmenopause.

In a cross-sectional study of 70 early postmenopausal women, regional bone measurements were compared with total body calcium (TBCa). Spinal and forearm trabecular bone were mainly related to age and time since menopause. In contrast, TBCa and forearm integral (cortical and trabecular) and cortical bone were unrelated to age, although the time since menopause also had some influence. Forearm integral and cortical bone measurements were quite well correlated with TBCa (r = 0.84 and 0.73, respectively, P less than 0.001). The correlation between spinal bone measurements and any of the forearm measurements, even purely trabecular bone, was weak (r less than 0.52, P less than 0.001). Our results show quite clearly that forearm bone measurements cannot be used to predict bone density in the vertebrae. Loss of ovarian function affects bone in general, and trabecular bone in particular. Bone measurements at specific anatomical sites are clearly necessary for studies of metabolic bone diseases and their response to treatment.

Effects of 7 years of growth hormone replacement therapy in hypopituitary adults.

Short-term studies of GH replacement in adult hypopituitarism have usually demonstrated beneficial effects on body composition and circulating lipids, with neutral or occasionally adverse effects on glucose tolerance. Fasting hyperinsulinemia has been reported. GH effects on cardiac function have been variable. The effects of long-term GH therapy, taking into account the consequences of increasing age, are not fully known. Thirty-three hypopituitary, initially middle-aged adults were studied over a 7-yr period; 12 patients took GH therapy (mean, 0.7 mg daily) continuously (group A); 11 took GH for only 6-18 months, a minimum of 5 yr previously (group B); and 10 patients never received GH therapy (group C). Other pituitary replacement was maintained. Effects on anthropometry, body composition (by bioimpedance analysis, total body potassium, and dual energy x-ray absorptiometry), circulating lipids, glucose and insulin concentrations, cardiac 2-dimensional and Doppler echocardiography, and exercise tolerance were assessed before and after the treatment period. Continuous GH therapy had no significant effect on body weight, but it prevented the increase in waist circumference and waist to hip ratio that occurred in the patients without GH substitution (waist to hip ratio, group A, 0.87+/-0.08 at baseline, 0.85+/-0.09 at 7 yr; group B, 0.89+/-0.11 at baseline, 0.94+/-0.11 at 7 yr; P < 0.005 for GH effect; group C, 0.87+/-0.10 at baseline, 0.92+/-0.10 at 7 yr; P < 0.005 for GH effect). A GH-induced decrease in subscapular skinfold thickness was also observed. By bioimpedance analysis, GH therapy caused an increase in total body water and fat-free mass, and a decrease in the percent body fat. Although changes occurred with time in all groups, no significant additional GH therapy effects were observed on glucose tolerance, insulin concentrations, lipid levels, cardiac dimensions, echocardiographic diastolic function, or exercise tolerance. In conclusion, prolonged GH substitution in middle-aged hypopituitary adults causes a sustained improvement in body composition. Other benefits, e.g. on lipid levels and exercise tolerance, were not apparent at 7 yr when comparisons were made with GH-untreated hypopituitary controls. Potentially adverse effects on glucose tolerance and insulinemia did not develop with prolonged GH therapy.

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.

Neutron-activation measurement of metabolic activity of sodium in the human hand.

Turnover of sodium in the human hand was studied by neutron activation. One hand of each subject was irradiated with a 1.5-rad dose of partially thermalized fast neutrons. The activity of 24Na was measured at intervals from 3 min to 48 hr after irradiation. The loss of sodium from the hand during this period can be described either by two exponentials or by a single power function. The latter description involves only two disposable factors compared with four in the former. The rate of loss of sodium was found, on the average, to be greater in patients suffering from bone disease than in normal subjects. Neutron activation is a powerful method for studying sodium turnover because the sodium atoms are labeled simultaneously and with equal probability, regardless of the turnover time of individual bodily compartments.

Metabolic activity of sodium, measured by neutron activation, in the hands of patients suffering from bone diseases: concise communication.

Turnover of sodium in the human hand was studied by neutron activation. Patients suffering from various metabolic abnormalities affecting the skeleton, who were undergoing routine neutron activation for the measurement of calcium, were investigated along with a group of healthy volunteers. Neutron activation labels the sodium atoms simultaneously and with equal probability regardless of the turnover time of individual body compartments. The loss of sodium can be described either by a sum of two exponentials or by a single power function. Distinctions between patients and normal subjects were not apparent from the exponential model but were brought out by the power function. The exponent of time in the latter is a measure of clearance rate. The mean values of this parameter in (a) a group of patients suffering from acromegaly: (b) a group including Paget's disease, osteoporosis, Cushing's disease, and hyperparathyroidism; and (c) a group of healthy subjects, were found to be significantly different from each other.

Generation of myocardial factor images directly from the dynamic oxygen-15-water scan without use of an oxygen-15-carbon monoxide blood-pool scan.

UNLABELLED: The measurement of regional myocardial blood flow (MBF) with H2(15)O and PET requires an additional C15O blood-pool scan for the purpose of region of interest (ROI) definition. This additional scan results in a substantially increased radiation dose, study duration and risk of movement artifacts. Therefore, a method was developed to generate myocardial factor images directly from the dynamic H2(15)O study without the need for a C15O scan. METHODS: The factor sinograms were generated by means of linear dimension reduction of the dynamic sinograms, where the required variate and covariate factors (myocardial and blood time-activity curves) were modeled from the lung time-activity curve. The factor images were generated by iterative reconstruction. RESULTS: No significant difference was found between MBF values from ROIs drawn on the traditional images (using the C15O scan) and those drawn on the factor images. CONCLUSION: It is possible to generate myocardial images directly from the dynamic H2(15)O study, so that the C15O scan can be omitted from MBF studies. The proposed method is robust and results in nearly optimal signal-to-noise ratios in the factor images.

Influence of smoking, hypertension, and sex on the phenotypic expression of familial intracranial aneurysms in siblings.

OBJECTIVE: To investigate the effects of smoking, hypertension, and sex on the phenotypic expression of familial intracranial aneurysms (FIAs). METHODS: We retrospectively reviewed the case records of 806 consecutive patients undergoing aneurysm surgery at our institution (1986-1995) and discovered 24 families with at least two affected siblings. Prevalence rates for the smoking, hypertension, and sex risk factors in these nuclear families were compared with those of patients with sporadic intracranial aneurysms (SIAs) and population-based control patients. RESULTS: Affected family members with FIAs exhibited prevalence rates of smoking and hypertension (74% and 43%, respectively) that tended to be higher than those of population-based control patients (52% [P < 0.005] and 36% [P = not significant (NS)], respectively) and comparable to those of patients with SIAs (64% [P = NS] and 40% [P = NS], respectively). A positive association existed between FIA formation and female sex but was somewhat less strong than that observed in the SIAs (59% FIAs, 71% SIAs, 50% control patients). In addition, the prevalence rates of smoking, hypertension, and female sex were higher in affected family members with FIAs than in their unaffected siblings (58% [P < 0.05], 28% [P = 0.06], and 39% [P < 0.05], respectively). Individuals in families with expressed FIAs who had high aneurysmal penetrance had a greater tendency to be smokers, hypertensive, and female (74%, 59%, and 55%, respectively) than did their low-penetrance counterparts (61% [P = 0.1], 27% [P < 0.05], and 45% [P = NS], respectively). CONCLUSION: Together these data suggest that hypertension, smoking, and female sex increase the likelihood that a member of a family with an expressed FIA will have an aneurysm. These observations may prove helpful in guiding the use of screening studies and encouraging education about the potential risks of continued tobacco use and untreated hypertension in this patient cohort.

Noise equivalent count measurements in a neuro-PET scanner with retractable septa.

The noise-equivalent count-rate (NEC) performance of a neuro-positron emission tomography (PET) scanner has been determined with and without interplane septa on uniform cylindrical phantoms of differing radii and in human studies to assess the optimum count rate conditions that realize the maximum gain. In the brain, the effective gain in NEC performance for three-dimensions (3-D) ranges from >5 at low count rates to approximately 3.3 at 200 kcps (equivalent to 37 kcps in 2-D). The gains of the 3-D method assessed by this analysis are significant, and are shown to be highly dependent on count rate and object dimensions.

Fully three-dimensional reconstruction for a PET camera with retractable septa.

A fully 3-D reconstruction algorithm has been developed to reconstruct data from a 16 ring PET camera (a Siemens/CTI 953B) with automatically retractable septa. The tomograph is able to acquire coincidences between any pair of detector rings and septa retraction increases the total system count rate by a factor of 7.8 (including scatter) and 4.7 (scatter subtracted) for a uniform, 20 cm diameter cylinder. The reconstruction algorithm is based on 3-D filtered backprojection, expressed in a form suitable for the multi-angle sinogram data. Sinograms which are not measured due to the truncated cylindrical geometry of the tomograph, but which are required for a spatially invariant response function, are obtained by forward projection. After filtering, the complete set of sinograms is backprojected into a 3-D volume of 128x128x31 voxels using a voxel-driven procedure. The algorithm has been validated with simulation, and tested with both phantom and clinical data from the 953B.

Effect of size and composition of the body on absolute measurement of calcium in vivo.

The effect of size and composition of the body on the measurement of total calcium in absolute units by neutron activation analysis is described. The neutron beam, produced from a cyclotron, was of mean energy 7.5 MeV. Counting of the activities induced in a patient was carried out with a multidetector whole body counter. Calibration of the system was achieved by using two phantoms of different sizes containing known quantities of calcium. The dimensions of various phantom sections were varied to determine the effect on efficiency of activation, an approximately linear relationship with body thickness being found. The influence of body habitus on counting efficiency was assessed by counting different patients who had been given known quantities of 42K. The effects of (a) the non-uniform distribution of calcium and (b) the variation in density of different tissues on both activation and counting efficiency are discussed. It is estimated that the 68% confidence limits on the measurement of 49Ca count rate per unit neutron fluence is +/- 3.0% and on the measurement of calcium in grams +/- 8.2%.

Calculation of single detector efficiencies and extension of the normalization sinogram in PET.

A fast iterative method is presented for calculating single detector efficiencies for positron emission tomographs. These efficiencies can be used to extend the normalization scan to areas outside that covered by the normalization source. The root mean square (rms) error of the calculated single detector efficiencies decreases exponentially with the number of iterations. Thirty iterations per normalization image are sufficient and take about 1 s on a SUN Classic. The geometry factors are composed of factors which only depend on the distance from the centre of the field-of-view (FOV) and of factors which show a more complex pattern over the normalization sinogram. The geometry factors are specific to each scanner. On the ECAT 931 scanner the complex part of the geometry factors showed a diamond shaped pattern (caused by the varying sensitivity of single detectors in a detector block with varying angle of incidence) and S-shaped curves (representing attenuation caused by supporting rods for the ring source). The coefficient of variation of the diamond-shaped pattern was 4% for detectors farthest from the centre of the FOV. Extensions of the normalization scan may, therefore, contain a relative rms error of about 4% if the applied geometry factors only take the distance from the centre of the FOV into account.

Measurement of total body nitrogen and oxygen by irradiation with cyclotron neutrons and 'delayed' gamma ray counting.

Measurement of total body nitrogen is assuming increasing importance in the nutritional evaluation of seriously ill patients. Nitrogen has been previously measured either by counting (i) the annihilation radiation from 13N immediately after neutron irradiation with 14 MeV neutrons or (ii) the 'prompt' gamma rays from thermal neutron capture by 14N during irradiation with 14 MeV neutrons or with those produced by isotopic sources or a cyclotron. The present work describes studies into the feasibility of measuring 13N produced by irradiation with a neutron beam from the MRC Cyclotron. A complication of this method is that 13N is also produced in a reaction with 16O. Direct measurement of oxygen by use of the reactions 16O(n, p)16N or 16O(n, 2n)15O enables this interference to be estimated. The former reaction is possible with both 14 meV and cyclotron-produced neutrons but the 7.1 s half-life of 16N requires detectors to be placed in or very close to the irradiation site. In our particular circumstances this is not possible but the more energetic cyclotron neutron spectrum allows the production of 15O which has a half-life of 2.05 min and can be measured in a remote whole-body counter. A disadvantage with the cyclotron beam, in comparison with 14 MeV neutrons, is that a higher dose is required for similar accuracy. A reproducibility of about 4% is obtained with a dose equivalent of 0.01 Sv.

Three-dimensional performance of a small-diameter positron emission tomograph.

Recently, the initial 2D physical characterization of a small-diameter positron emission tomograph, designed specifically for scanning of small laboratory animals, was reported. The physical characteristics of the tomograph operating in 3D mode have now been measured and compared to those obtained in 2D mode. In 3D, the transaxial resolution was measured as 2.4 +/- 0.1 mm full width at half maximum (FWHM) and 6.7 +/- 1.1 mm full width at tenth maximum (FWTM) at the centre of the transaxial field of view (FOV). These values degraded to 4.5 +/- 0.3 mm (tangential) and 6.6 +/- 0.3 mm (radial) FWHM and 10.6 +/- 1.6 mm (tangential) and 11.2 +/- 2.1 mm (radial) FWTM, respectively, at a distance of 40 mm from the centre of the transaxial FOV. The axial resolution was measured as 4.6 +/- 0.1 mm FWHM and 15.0 +/- 0.5 mm FWTM at the centre of the transaxial FOV, increasing to 5.0 +/- 0.1 mm FWHM and 18.8 +/- 3.7 mm FWTM at a radial distance of 40 mm from the centre of the transaxial FOV. These resolutions are similar to those obtained for the tomograph operating in 2D mode. The sensitivity of the tomograph operating in 3D was 4.31 x 10(4) counts s-1 MBq-1 at 250-850 keV compared to 0.995 x 10(4) counts s-1 MBQ-1 in 2D at the same energy thresholds. In this energy window the noise equivalent count rate peaked at 4.1 x 10(4) counts s-1, compared to 1.03 x 10(4) counts s-1 in 2D. A scatter fraction of 30.2% at 250-850 keV was measured for a 18F line source centered in a 60 mm diameter water filled phantom in 3D, compared to 31.0% in 2D for the same scanning geometry and energy thresholds. A comparison was made between 2D and 3D kinetic analyses for a group of five anaesthetized rats scanned using [11C] SCH 23390, a marker of dopamine D1 receptors. The integrity of the results was maintained between 2D and 3D data sets, though in 3D there was a significant reduction in the standard error of the fitted parameter. The results demonstrate that, with regard to sensitivity, there are significant gains in the physical performance of this tomograph when operating in 3D compared to 2D mode and that the quantification of PET studies of small animals using the 3D data reflects this.

Correction for scatter in 3D brain PET using a dual energy window method.

A method for scatter correction using dual energy window acquisition has been developed and implemented on data collected with a brain-PET tomograph operated in the septa retracted, 3D mode. Coincidence events are assigned to (i) an upper energy window where both photons deposit energy between 380 keV and 850 keV or (ii) a lower energy window where one or both photons deposit within 200 keV and 380 keV. Scaling parameters are derived from measurements of the ratios of counts from line sources due to scattered and unscattered events in the two energy windows in head-sized phantoms. A scaled subtraction of the two energy windows produces a distribution of scatter which is smoothed prior to subtraction from the upper energy window. In phantoms, the correction was found to restore the uniformity, contrast and linearity of activity concentration. Relative activity concentrations were restored to within 7% of their true values in a multicompartment phantom. The method was found to provide accurate correction for scattered events arising from activity outside the direct detector field of view. In a three-compartment phantom containing water, 18F and 11C scanned in dynamic, multiframe mode, the half-lives of the two isotopes were restored to within 2% of their true value.

The effect of activity outside the direct field of view in a 3D-only whole-body positron tomograph.

The ECAT EXACT3D (CTI/Siemens 966) 3D-only PET tomograph has unprecedented sensitivity due to the large BGO (bismuth germanate) detector volume. However, the consequences of a large (23.4 cm) axial field-of-view (FOV) and the need for a patient port diameter to accommodate body scanning make the device more sensitive to photons arising from activity outside the direct (coincidence) FOV. This leads to relatively higher deadtime and an increased registration of random and scatter (true) coincidences. The purpose of this study is to determine the influence of activity outside the FOV on (i) noise-equivalent counts (NEC) and (ii) the performance of a 'model-based' scatter correction algorithm, and to investigate the effect of side shielding additional to that supplied with the tomograph. Annular shielding designed for brain scanning increased the NEC for blood flow (H[2]15O) measurement (integrated over 120 s) by up to 25%. For 11C tracer studies, the increase is less than 5% over 120 min. Purpose-built additional body shielding, made to conform to the shape of a volunteer, reduced the randoms count rate in a heart blood flow measurement (H[2]15O) by about 30%. After scatter correction the discrepancy between ROI count ratios for compartments within the 20 cm diameter 'Utah' phantom differed by less than 5% from true (sampled) activity concentration ratios. This was so with or without activity outside the FOV and with or without additional side shielding. Count rate performance is thus improved by extra shielding but more improvement is seen in head than in body scanning. Measurement of heart blood flow using bolus injections of H(2)15O would benefit from the use of detectors with lower deadtime and superior timing resolution such as LSO (lutetium oxyorthosilicate).

Experience with fully 3D PET and implications for future high-resolution 3D tomographs.

The aim of this paper is to report on experience with 3D positron emission tomography (PET) in our institute where we have three 3D scanners, of which two operate exclusively in 3D mode (ECAT ART, EXACT 3D). Fully 3D PET requires attention to a number of factors which are not as problematic in 2D PET. Firstly, 3D tomographs designed for whole-body acquisition suffer from a large single-photon field of view, extending well beyond the coincidence field of view. Single photons from outside the coincidence field of view increase the dead time and random coincidence rates, and contribute scattered events. For brain studies, we have extended the lead side shielding at the ends of the tomograph to mitigate against these effects, and this has dramatically improved the count rate performance. This approach is not as effective for whole-body scanning. In addition, operating in 3D without septa necessitates new approaches to transmission scanning, as measurements using positron emitters such as 68Ge/68Ga have the unfavourable characteristics of high dead time and high scatter. Both of our fully 3D scanners use 137Cs for single-photon transmission measurements, although the data are treated differently. On the ECAT ART, a combination of physical and electronic collimation effectively reduces transmission scatter to acceptable levels. On the EXACT 3D physical collimation is not as readily implemented and therefore segmentation and reassignment of the histogrammed attenuation (mu) values is employed to produce unbiased attenuation correction factors in 3D. Many of the lessons learnt with these BGO (bismuth germanate) based tomographs will be applicable to the next generation of systems using faster detectors such as lutetium oxyorthosilicate (LSO).

Physical characteristics of the ECAT EXACT3D positron tomograph.

The 'EXACT3D' positron tomograph, which is now in routine clinical research use, was developed with the aim of achieving unprecedented sensitivity, high spatial and temporal resolution and simplicity of design using proven detector technology. It consists of six rings of standard detector blocks (CTI/Siemens EXACT HR+) with 4.39 mm x 4.05 mm x 30 mm elements, giving an axial field of view (FOV) of 23.4 cm. This extended FOV and the absence of interplane septa and retractable transmission rod sources has allowed greatly simplified gantry and detector cassette design. Operation in exclusive 3D mode requires an alternative to the conventional coincidence method for transmission scanning, and a single photon approach using a hydraulically driven 137Cs point source has been implemented. The tomograph has no other moving parts. A single time frame of data without any compression is very large (> 300 Mbyte) and two approaches are employed to overcome this difficulty: (a) adjacent sinograms can be summed automatically into different combinations and (b) listmode (event-by-event) acquisition has been instituted, which is both storage efficient (particularly for acquisition of sparse data sets) and maximizes temporal resolution. The high-speed I/O and computing hardware can maintain a sustained acquisition rate of about 4 million coincidence events per second. A disadvantage of the large axial FOV in 3D is the increased sensitivity to activity outside the coincidence FOV. However, this can be minimized by additional side shielding. The mean spatial resolution is 4.8 +/- 0.2 mm FWHM (transaxial, 1 cm off-axis) and 5.6 +/- 0.5 mm (axial, on-axis). Its absolute efficiency is 5.8% for a line source in air (just spanning the axial FOV) and 10% for a central point source (with thresholds of 350-650 keV). For a uniform 20 cm diameter cylinder, the efficiency is 69 kcps kBq(-1) ml(-1) (after subtraction of a scatter fraction of 42%). Sensitivity relative to the EXACT HR+ (with four rings of blocks) is 2.5 (3D) and 12 (2D) times respectively. The rate of random events in blood flow studies in the brain and body, using 15O-labelled water, can be controlled by limiting the administered dose and inserting additional side shielding.

Physical performance of a positron tomograph for brain imaging with retractable septa.

Performance characteristics of a new design of positron tomograph with automatically retractable septa for brain imaging have been studied. The device, consisting of block BGO detectors (8 x 8 elements per block), has a ring diameter of 76 cm and an axial FOV of 106.5 mm. The in-plane resolution is on average 5.8 mm and 5.0 mm (FWHM) for stationary and wobble sampling, respectively, over the central 18 cm of the transaxial FOV. Its unique feature is the capability of data acquisition both in the 'conventional' 2D mode (with septa) or 3D mode (septa retracted) where coincidences between any of the 16 detector rings are acquired. When scattered events are subtracted, the efficiency for a 20 cm diameter uniform cylinder increases overall by a factor of 4.8 between 2D (septa extended) and 3D modes. For a 20 cm phantom the trues/singles ratio is higher for 3D than for 2D but for a given unscattered trues rate, the randoms rate in 3D is higher. At 380 keV the scatter fraction within a 20 cm cylinder is 10% (septa extended) and 36% (retracted). In spite of the increase in scatter when septa are retracted, the increased efficiency in the 3D mode of acquisition yields distinct advantages, particularly in the many studies where tracer concentration is low and consequently where dead time and random rates are less important.

The design and physical characteristics of a small animal positron emission tomograph.

A small diameter positron emission tomography, designed specifically for small animal studies, was constructed from existing, commercially available, bismuth germanate (BGO) detectors and electronics. The scanner consists of 16 BGO detector blocks arranged to give a tomograph with a diameter of 115 mm and an axial field of view (FOV) of 50 mm. Each block is cut to produce eight (axial) by seven (radial) individual detector elements. The absence of interplane septa enables the acquisition of 3D data sets consisting of 64 sinograms. A 2D data set of 15 sinograms, consisting of eight direct and seven adjacent cross planes, can be extracted from the 3D data set. Images are reconstructed from the 2D sinograms using a conventional filtered backprojection algorithm. Two methods of normalization were investigated, based on either a rotating 68Ge rod source, or a uniform 68Ge plane source, with a uniform cylindrical 18F phantom. Attenuation of the emitted photons was estimated using a rotating 68Ge rod source. The transaxial resolution of the tomograph was measured as 2.3 mm full width at half maximum (FWHM) and 5.6 mm full width at tenth maximum (FWTM) at the centre of the FOV, degrading to 6.6 mm (radial) and 4.4 mm (tangential) FWHM and 10.4 mm (radial) and 14.4 mm (tangential) FWTM at 40.0 mm from the centre of the FOV. The axial slice width was 4.3 mm FWHM, 10.3 mm FWTM at the centre of the transaxial field of view and 4.4 mm FWHM, 10.6 mm FWTM at 20.0 mm from the centre of the FOV. A scatter fraction of 31.0% was measured at 250-850 keV, for an 18F line source centred in a 60 mm diameter, water-filled phantom, reducing to 20.4% and 13.8% as the lower energy discrimination was increased to 380 keV and 450 keV, respectively. The count rate performance was measured using a noise equivalent count rate method, and the linearity of the dead time correction was confirmed over the count rates encountered during routine scanning. In 2D mode, the absolute sensitivity of the tomograph was measured as 9948 counts s-1 MBq-1 at 250-850 keV, 8284 counts s-1 MBq-1 at 380-850 keV and 6280 counts s-1 MBq-1 at 450-850 keV.