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P K Marsden

Publications and source records attributed to P K Marsden.

32 records · Page 2Linked to original sources
PubMed1998

Clinical value of "ictal" FDG-positron emission tomography and the routine use of simultaneous scalp EEG studies in patients with intractable partial epilepsies.

PURPOSE: EEG is widely used during positron emission tomography (PET) to confirm the interictal state of the patient and assist in scan interpretation when a seizure occurs. Ictal scans usually reflect mixed interictal-ictal-postictal metabolic activity as seizures are brief in comparison to the 30-min uptake period of the tracer. We wished to determine whether routine EEG is justified and if seizures commonly affect the diagnostic information of the PET scan. METHODS: We examined the PET scans of 6 of 236 outpatients with intractable epilepsy with clinical and electrical evidence of a seizure during tracer uptake. We performed semi-quantitative analysis in 2 patients who had "ictal" and control interictal scans. RESULTS: Patients with single seizures lasting 23 s to 4 min [four complex partial seizures (CPS) and one absence seizure (AS)] had focal hypometabolism concordant with results of other investigations. One patient with complex partial status had irregular cortical uptake and focal hypometabolism, but the site of the ictal focus could not be confirmed. CONCLUSIONS: In this group of patients, seizures occurred infrequently during tracer uptake. The interpretation of the PET scan when single seizures occurred did not appear to be influenced by the continuous scalp EEG (CSEEG) recordings. The value of routine CSEEG in outpatients treated with medication should be reappraised, with potential cost savings. In rare circumstances in which a true ictal study occurs (complex partial status, epilepsia partialis continua, and repetitive CPS), PET scanning may be inconclusive and repeat interictal scanning should be pursued.

Adolescent↗
PubMed1997

PET and NMR dual acquisition (PANDA): applications to isolated, perfused rat hearts.

Positron emission tomography and nuclear magnetic resonance spectroscopy are non-invasive techniques that allow serial metabolic measurements to be obtained in a single subject. Significant advantages could be obtained if both types of scans could be acquired with a single machine. A small-scale PET scanner, designed to operate in a high magnetic field, was therefore constructed and inserted into the top half of a 7.3 cm bore, 9.4 T NMR magnet and its performance characterized. The magnetic field did not significantly affect either the sensitivity (approximately 3 kcps/MBq) or the spatial resolution (2.0 mm full width at half maximum, measured using a 0.25 mm diameter line source) of the scanner. However, the presence of the PET scanner resulted in a small decrease in field homogeneity. The first, simultaneous 31P NMR spectra (200, 80 degrees pulses collected at 6 s intervals) and PET images (transverse, mid-ventricular slices at the level of the mitral value) from isolated, perfused rat hearts were acquired using a specially designed NMR probe inserted into the bottom half of the magnet. The PET images were of excellent quality, enabling the left ventricular wall and interventricular septum to be clearly seen. In conclusion, we have demonstrated the simultaneous acquisition of PET and NMR data from perfused rat hearts; we believe that the combination of these two powerful techniques has tremendous potential in both the laboratory and the clinic.

Adenosine Triphosphate↗
PubMed1997

Simultaneous PET and MR imaging.

We have developed a prototype PET detector which is compatible with a clinical MRI system to provide simultaneous PET and MR imaging. This single-slice PET system consists of 48 2 x 2 x 10 mm3 LSO crystals in a 38 mm diameter ring configuration that can be placed inside the receiver coil of the MRI system, coupled to three multi-channel photomultipliers housed outside the main magnetic field via 4 m long and 2 mm diameter optical fibres. The PET system exhibits 2 mm spatial resolution, 41% energy resolution at 511 keV and 20 ns timing resolution. Simultaneous PET and MR phantom images were successfully acquired.

Equipment Design↗
PubMed1997

Adaptive and maladaptive behaviour in children with epileptic encephalopathies: correlation with cerebral glucose metabolism.

In the childhood epileptic encephalopathies mental impairment is common and severe. Traditional cognitive assessment is difficult because of the low level of performance, autistic features, and the unpredictable effect of seizures. An alternative is to measure adaptive and maladaptive behaviour using instruments administered to the caregivers. Adults with different types of dementia have characteristic patterns of cortical glucose hypometabolism. Thirty-two children were studied using visual and semiquantitative analysis of 18fluorodeoxyglucose positron emission tomographic (PET) scans. The Vineland Scales and the Conners' Questionnaires were used to assess adaptive and maladaptive behaviour. The mean adaptive behaviour composite score was 37.3+/-15.6; all but one subject had a low adaptive level. A profile of relative strength in socialisation and weakness in daily living skills emerged. Up to two-thirds of children had abnormal behaviour patterns, particularly attention-deficit disorders and hyperactivity. Adaptive and maladaptive behaviour was not related to the presence or absence of focal cortical PET abnormalities. However, adaptive behaviour scores showed an inverse correlation with the degree of metabolic abnormality in the frontal lobes.

Adaptation, Psychological↗
PubMed1997

Cortical and subcortical glucose metabolism in childhood epileptic encephalopathies.

OBJECTIVES: Nearly one third of children with cryptogenic epileptic encephalopathies have been reported to have focal cortical defects on 18fluorodeoxyglucose (FDG) PET. As diffuse cortical dysfunction and involvement of subcortical structures, particularly the thalami, is postulated to underlie the propensity to seizures in these conditions, the aim was to determine the frequency of bilateral and diffuse cortical metabolic defects and of subcortical metabolic abnormalities in the same patients. METHODS: The interictal uptake of FDG was studied in 32 children with epileptic encephalopathies. Using a semiquantitative technique, the ratio of uptake in cortical regions and subcortical structures to that in the cerebellum was compared with that of age matched historical controls. Uptake more than 2 SD above ("hypermetabolic") or below ("hypometabolic") that of age matched controls was considered abnormal. RESULTS: Diffusely abnormal cortical up-take (nearly always hypometabolic) occurred in almost two thirds of patients; in all but two of the remaining patients at least one cortical region showed significantly decreased uptake bilaterally. When analysed as age cohorts, the mean cortical:cerebellar FDG uptake was significantly lower than that of controls in all cortical regions (P<0.005). Ninety per cent of patients had evidence of relative thalamic hypometabolism and in each age group there was a significant reduction in relative thalamic FDG uptake compared with that of controls (P<0.005). In nine out of 11 patients with unilateral cortical hypometabolic defects thalamic FDG up-take was lower ipsilateral to the cortical abnormality. CONCLUSIONS: Diffuse cortical dysfunction is common in the epileptic encephalopathies and may reflect the underlying cause of the condition or arise as a consequence of uncontrolled seizures. Altered thalamic glucose metabolism is further evidence of subcortical involvement in these conditions.

Cerebellum↗
PubMed1997

A study of the effects of sumatriptan on myocardial perfusion in healthy female migraineurs using 13NH3 positron emission tomography.

Sumatriptan, a 5-HT1D receptor agonist, is believed to alleviate migraine attacks by extracerebral vasoconstriction. Chest pain associated with myocardial ischemia may occur after sumatriptan administration. We investigated the effect of a single 6-mg subcutaneous dose of sumatriptan on myocardial perfusion (MP) as measured by 13NH3 positron emission tomography (PET) in a randomized, double-blind, placebo-controlled, crossover trial at the Clinical PET Centre, Guy's and St. Thomas' Hospitals, London. Nineteen volunteer female migraineurs, age range 33 to 62 years, at low risk for ischemic heart disease were included. All bad undergone previous treatment with oral or intravenous sumatriptan. Patients were recruited by advertisement and referral from local neurology specialists. Each volunteer underwent two scanning sessions. On each occasion, a baseline dynamic 13NH3 PET scan was acquired followed by a 13NH3 PET scan 10 minutes after subcutaneous injection of placebo or 6 mg of sumatriptan. Regional MP was measured in five myocardial regions using the Patlak system of image analysis. The mean % change from baseline (+/-SD) in global MP after placebo was +9.5% +/- 18.0 and after sumatriptan was +6.6% +/- 18.8 (repeated measures ANOVA for treatment effect p = 0.56). There were no significant differences in MP changes from baseline observed in any of the five myocardial regions (treatment p = 0.32 to 0.84). These data suggest that in healthy female migraineurs, a single 6-mg subcutaneous dose of sumatriptan does not cause a significant change in regional or global myocardial perfusion.

Adult↗
PubMed1997

Visual and semiquantitative analysis of cortical FDG-PET scans in childhood epileptic encephalopathies.

UNLABELLED: The optimal method for analyzing PET scans in children being considered for epilepsy surgery is unresolved: Fully quantified methods are invasive, and the required controls are generally unavailable. We sought to compare visual inspection with semiquantitative analysis for the detection of cortical metabolic defects. METHODS: Thirty-two children with cryptogenic epileptic encephalopathies were studied prospectively with 18F-fluorodeoxyglucose (FDG) PET. Visual inspection was performed on separate occasions by independent observers. Four-millimeter circular regions of interest were used to sample radiotracer uptake in selected cortical regions. Asymmetry between homologous regions were calculated to detect focal abnormalities. Bilateral and diffuse abnormalities were assessed by comparing the ratio of cortical-to-cerebellar uptake in patients with historical age-matched controls. The sensitivity and specificity of visual inspection was compared with that of semiquantitative analysis for the detection of focal, bilateral and diffuse cortical metabolic abnormalities. RESULTS: Visual inspection revealed full inter-rater agreement for the presence of major focal abnormalities. The sensitivity and specificity for visual inspection compared to semiquantitative analysis were 77% and 92%, respectively, with semiquantitative analysis often revealing abnormalities to be more extensive than had been suspected visually. Compared with semiquantitative analysis, visual inspection had a low sensitivity but high specificity for the detection of bilateral and diffuse hypometabolism. CONCLUSION: Semiquantitative analysis gives clinically useful information additional to that obtained from visual inspection.

Brain↗
PubMed1996

Optimization of noise-equivalent count rates in 3D PET.

We have used noise-equivalent count (NEC) rates to optimize count rate performance for 3D acquisition in PET in a wide range of situations, with particular reference to imaging of the torso. We have also compared NEC performance for 2D and 3D acquisition in order to establish the conditions under which 3D mode offers an improvement over 2D mode. Measurements were performed on four tissue-equivalent phantoms ranging in size from that of an infant's head (13 cm diameter) to that of an obese adult's chest (37 cm x 48 cm). Count rate data were acquired as a function of phantom size, activity in the field of view, lower energy discriminator level (LLD) and acquisition mode, and NEC rates were derived as a function of these variables. The LLD at which the highest NEC rate is obtained shows a dependence both on phantom size and on the activity in the field of view both for 2D and for 3D acquisition. The relative advantage of 3D mode over 2D mode, at the optimum LLD setting, is also strongly dependent both on activity in the field of view (FOV) and on the phantom size. The main limiting factors for 3D NEC rates are detector dead-time for small phantoms and random coincidences for large phantoms. The 3D NEC rate is more than twice as great as the 2D NEC rate when less than 60 MBq is present in the FOV for all phantoms except the largest, in which case a ratio of two is only achieved for activities less than 25 MBq. For the smallest phantom, 3D/2D NEC ratios of greater than 3.5 are obtained when the activity in the FOV falls below 10 MBq.

Adult↗
PubMed1989

Image quantification with a large area multiwire proportional chamber positron camera (MUP-PET).

A large area multi wire proportional chamber positron camera system is under evaluation for clinical Nuclear Medicine investigation using isotopes both from the house generators (68Ga) and from remote cyclotrons (18F, 124I, 64Cu and 81Rb). Images are reconstructed using a fully three dimensional (3D) algorithm and exhibit equal resolution in all three orthogonal directions. The axial field of view of 15 cm is large enough to allow coverage of the whole brain. This paper discusses the performance of the MUP-PET system with particular emphasis on quantification and the development of an attenuation correction scheme for a large area detector system. The distribution of scattered and accidental events is investigated and observed to contribute a fairly uniform background to the image. Dead time correction factors are calculated from an analytical expression obtained by considering the various sources of dead time present in the system. Following correction for attenuation, accidentals, scattered events and dead time, reconstructed phantom images show a strong linear correlation (r = 0.998) between count density and regional isotope concentration. The extension of the methods to clinical studies is discussed.

Image Processing, Computer-Assisted↗
PubMed1989

The performance of a multiwire proportional chamber positron camera for clinical use.

The Rutherford Appleton Laboratory clinical positron camera consists of two opposed multiwire proportional chambers (MWPCS) mounted on a rotating gantry capable of performing routine nuclear medicine studies. The system has operated since the end of 1986 with complete reliability. It has a sensitivity of 37 kcps MBq-1 cm3 per axial cm for a 20 cm diameter uniformly filled cylinder of activity. The best spatial resolution obtainable is about 6 mm, although in practice images are smoothed in order to reduce statistical noise with a corresponding decrease in resolution. Cross-plane rays are utilised during image reconstruction and the resulting three-dimensional images exhibit the same spatial resolution in three orthogonal directions over a large cylindrical field of view 15 cm high by 30 cm in diameter. The maximum data-taking rate is limited to 1.5 to 2 kcps at present due to deadtime in the read-out system. The performance of the system is described with particular reference to the problems of imaging with the large fractions of random and scattered events which are a consequence of using large-area detectors. Images of phantoms and patients are presented and proposed modifications to the camera are described.

Algorithms↗
PubMed1987

Measurement of radiation dose to the thyroid using positron emission tomography.

Measurements of the functioning volume of thyroid tissue have been made in 22 patients undergoing radioiodine therapy for thyrotoxicosis, using a prototype multiwire proportional counter positron camera. Tomographic images were produced of the distribution of 124I in the thyroid. Functioning volumes were found to be in the range 21-79 cm3 with volume errors of the order of +/- 4% to +/- 14%. Radioiodine uptake varied from 28% to 98%. Using a value of 6 days for the effective half-life of radioiodine in hyperactive thyroids, radiation doses from a standard therapy administration of 75 MBq of 131I varied from 11 to 48 Gy (compared with a recommended 50-70 Gy). In five cases PET imaging showed a non-uniform distribution of radioiodine in thyroids thought to have uniform uptake from conventional pinhole scintigraphy.

Humans↗
PubMedDate not supplied

Evaluation of myocardial perfusion using positron emission tomography in infants following a neonatal arterial switch operation.

This study was performed to examine the use of positron emission tomography (PET) as a method of evaluating myocardial perfusion after the arterial switch operation for correction of transposition of the great arteries. Eleven asymptomatic patients (median age 2.3 years, range 1.3-4.3 years) post successful neonatal arterial switch repair for transposition underwent cardiac PET scanning using N(13) ammonia before and after dipyridamole infusion. Reconstructed data from static scans were analyzed for regional perfusion defects before and after pharmacological stress. Simultaneous assessment of coronary flow before and after stress was performed using a Patlak graphical analysis of data from dynamic scans. Results obtained from PET scanning were correlated with patterns of coronary artery anatomy, electrocardiogram (ECG) recordings, and echocardiographic evaluation. PET scanning demonstrated normal distribution of myocardial perfusion before and after stress in all but one patient, who was found to have a discrete inferior transmural perfusion defect. The defect was well correlated with perioperative ECG changes and a complicated postoperative course. Myocardial blood flow before dipyridamole (0.690 ml/min/g) was similar to reported adult rest values. There was a small but significant (p < 0.002) increase in myocardial blood flow after dipyridamole stress with a mean coronary flow reserve of 1.19 (+/-0.103). Echocardiographic evaluation failed to demonstrate significant wall motion abnormalities in any of the patients. Cardiac PET scanning is a reliable noninvasive method for evaluation of myocardial perfusion in small children. In this study, the incidence of myocardial perfusion defects after the arterial switch operation is lower than previously reported. The data obtained concerning coronary flow and coronary flow reserve after the arterial switch need to be interpreted with caution because normal data in children are not available.

Ammonia↗
PubMedDate not supplied

The development of high-efficiency cathode converters for a multiwire proportional chamber positron camera.

A high-efficiency cathode converter for 511-keV photons has been developed for incorporation into a multiwire proportional chamber (MWPC) positron camera. The converter consists of a honeycomb pattern produced in a 1-mm-thick lead sheet to leave lead walls with a thickness of approximately 60 micron. The converter also serves as the cathode of an MWPC, the gap between the converter and the anode wire plane being 2.5 mm. This small gap results in a high secondary electron extraction efficiency without the need for additional drift voltages. Measurements of the efficiencies of a plane converter and of two types of structured converters in a single section MWPC are described and the efficiency is found to increase in proportion to the converter surface area. This result justifies the use of a simple theoretical model whereby an extrapolation to the efficiency of a detector consisting of a stack of 20 MWPC sections, each section having two converters, is made. The efficiency of this proposed system is calculated to be 17% for 511-keV photons.

Beta Particles↗