Regional access to acute ischemic stroke intervention.

BACKGROUND AND PURPOSE: Benefit-risk ratios from recombinant tissue plasminogen activator (rtPA) therapy for acute ischemic stroke demonstrate lack of efficacy if intravenous administration is commenced beyond 3 hours of symptom onset. We undertook to enhance therapeutic effectiveness by ensuring equitable access to rtPA for patients affected by acute ischemic stroke within a 20 000 km(2) population referral base served by a tertiary facility. METHODS: Representatives of all provider groups involved in emergency medical services developed a Regional Acute Stroke Protocol (RASP), a coordinated regional system response by dispatch personnel, paramedics, physicians, community service providers, emergency and inpatient staff in community hospitals, and the tertiary facility acute stroke team. RESULTS: As of July 26, 1999, all ambulance services in Southeastern Ontario began bypassing the closest hospital to deliver patients meeting the criteria for the RASP to the Kingston General Hospital. At 12 months, approximately 403 ischemic strokes have occurred in the region, the RASP has been activated 191 times, and 42 patients have received rtPA. CONCLUSIONS: We conclude that (1) acute stroke patients in Southeastern Ontario have improved access to interventions for stroke care; (2) geography of the region is not a barrier to access to interventions for patients with acute stroke; and (3) acute ischemic stroke patients treated with rtPA account for 5% of all acute strokes and 10% of all ischemic strokes in this region.

Performance of QPET, a high-resolution 3D PET imaging system for small volumes.

QPET is a positron imaging system developed at Queen's University for high-resolution, 3D imaging of small volumes; it includes a pair of planar gamma-ray detectors 25.4 cm square, which rotate about a central axis with a quasi-cylindrical geometry. The authors describe the performance of this system. Basic characteristics of the detectors are evaluated: a spatial sampling of 1 mm, a quantum efficiency of 9.3% (for 511 keV gamma rays with normal incidence), and a time resolution of 88 ns. Models are developed to characterize the system deadtime and the sensitivity in terms of the noise-equivalent counting rate. With an 8 cm diameter spherical source, the noise-equivalent counting rate reaches a maximum at just over 3 kcps for an activity concentration of 2 muCi/cc; the random coincidence events and the deadtime losses both contribute significantly and the scatter contribution is small. Spatial resolution and uniformity over the field of view are evaluated by imaging short and long line sources; a spatial resolution of 2.7 mm in the transverse directions and 2.0 mm in the axial direction is achieved, with excellent uniformity throughout the field of view. The detector response is amplitude invariant across a 20 cm transverse diameter and a 9 cm axial length with the acceptance angle limited to +/-25 degrees in the axial direction. As an example of the imaging capabilities of QPET, the authors show 3D images of (18)F uptake in the bones of a rat, showing the excellent spatial resolution. This system is best suited to limited-volume applications where high counting rates are not necessary, but where high spatial resolution and uniform detector response are priorities.

Reversible parkinsonism induced by hypercalcemia and primary hyperparathyroidism.

We describe a patient who had the recent onset of both primary hyperparathyroidism and parkinsonism. Surgical removal of a parathyroid adenoma was followed by spontaneous resolution of the parkinsonism. Hypoparathyroidism and other disorders of calcium metabolism are recognized causes of secondary parkinsonism, while hyperparathyroidism is not. We review the relevant literature, which supports the hypothesis that hypercalcemia may have induced cytotoxic changes in the basal ganglia of this patient.

Removing the interference error between attenuation correction and scatter subtraction in 3D PET imaging.

A method to remove the interference between attenuation correction and scatter subtraction has been developed for the QPET 3D imaging system at Queen's University. Because the detector system has more than 10(10) lines of response, we reconstruct the image by first backprojecting, then filtering. We correct for attenuation at backprojection by weighting each event by the inverse of the attenuation factor calculated by reprojection through an attenuation image. Since the scatter background has not been corrected at backprojection time, this has the side effect that a fraction of the detected scattered events get incorrectly weighted. When a scatter subtraction is subsequently applied, the correction is inaccurate because the scatter distribution has been modified by the attenuation correction procedure. The residual interference error in the reconstructed image is a distorted image of the attenuator. An approximation to this error is obtained by reprojecting through the attenuation image, backprojecting with appropriate weights, then reconstructing. This image is then scaled and multiplied by the calculated scatter distribution to obtain an estimate of the interference error. Both simulations and measurements indicate that for our system, this method provides a reasonable approximation of the interference error in the image.

A deconvolution scatter correction for a 3-D PET system.

A method to remove the scattered background from a reconstructed image by deconvolution with a point response function which includes the scatter contribution is presented. The amplitude of the scattered response function is obtained by constraining a region of the corrected image to zero average amplitude. This method assumes that the shape of scatter distribution is shift invariant and independent of the shape of the scattering object and the distribution of the positron activity. The validity of these approximations for the QPET geometry was tested using simulations. An average scatter response function for the system was obtained from these simulations and compared with results from measurements. The method was tested using experimental data from an irregularly shaped acrylic phantom. It was simple to implement and resulted in a satisfactory correction of the scattered background for a small-volume system.

Accurate attenuation correction for a 3D PET system.

An accurate attenuation correction has been developed for a small-volume three-dimensional positron emission tomography (PET) system. Transmission data were measured as twenty-four 2D slices which were reconstructed and combined to form a 3D attenuation image. Emission data were reconstructed using a backproject-then-filter technique, and each event was corrected for attenuation at backprojection time by a reprojection through the attenuation image. This correction restores the spatial invariance of the point response function, thus allowing a valid deconvolution and producing an undistorted emission image. Scattering corrections were not applied to either the transmission or the emission data but simulation studies indicated that scattering made only a small contribution to the attenuation measurement. Results are presented for two phantoms, in which transmission scans of 57,500 and 18,700 events/slice were used to correct emission images of 5.2 and 2.8 million events. Although the attenuation images had poor statistical accuracy and a resolution of 13 mm, the method resulted in accurate attenuation-corrected images with no degradation in image resolution (which was 3 mm for the first emission image), and with little effect on image noise.

Computerized cranial tomography in Wilson disease.

Hepatolenticular degeneration (Wilson disease) is suspected by the clinical picture and confirmed by characteristic laboratory demonstration of impaired copper metabolism. Three patients with Wilson disease involving the basal ganglia were shown to have abnormalities on computerized tomography (CT) scan, whereas four other patients without signs of cerebral involvement had normal brain scans. Wilson disease may be added to the long list of diseases to which the EMI scan may make a useful diagnostic contribution.

Metabolic responses to status epilepticus in the rat, cat, and mouse.

Prolonged sustained seizure activity (status epilepticus) was created in rats and cats using paralysis and ventilation to prevent muscular contraction and its secondary systemic effects. Under physiologic control, seizure activity was maintained for 30, 60, and 120 min. At this time the brains were frozen using the in situ technique and the cortical tissue was analyzed for energy-related metabolites. The alteration of metabolites found at these times was similar to that previously described in the first 10 min of seizure activity. No evidence was found of any significant or progressive derangement of oxidative metabolism. A progressive lactic acidemia developed in spite of adequate arterial oxygen tensions. In contrast, when mice received a similar dose of the convulsant and were allowed to convulse freely in an oxygen-enriched environment, major derangements of energy metabolism were found which were progressive and persisted following recovery for at least 18 h.