Positron emission tomography--a new technique for studies of the central nervous system

Neuroimaging for drug addiction and related behaviors

In this review, we highlight the role of neuroimaging techniques in studying the emotional and cognitive-behavioral components of the addiction syndrome by focusing on the neural substrates subserving them. The phenomenology of drug addiction can be characterized by a recurrent pattern of subjective experiences that includes drug intoxication, craving, bingeing, and withdrawal with the cycle culminating in a persistent preoccupation with obtaining, consuming, and recovering from the drug. In the past two decades, imaging studies of drug addiction have demonstrated deficits in brain circuits related to reward and impulsivity. The current review focuses on studies employing positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and electroencephalography (EEG) to investigate these behaviors in drug-addicted human populations. We begin with a brief account of drug addiction followed by a technical account of each of these imaging modalities. We then discuss how these techniques have uniquely contributed to a deeper understanding of addictive behaviors.

Reduced midbrain dopamine transporter binding in male adolescents with attention-deficit/hyperactivity disorder: association between striatal dopamine markers and motor hyperactivity

BACKGROUND

The hypothesis that altered dopamine transmission underlies hyperactive-inattentive behavior in children with attention-deficit/hyperactivity disorder (ADHD) is based on genetic studies and the efficacy of psychostimulants. Most of previous positron emission tomography (PET) and single photon emission tomography (SPET) studies have shown altered binding of dopamine markers in the basal ganglia. Yet, the functional role of the neurochemical disturbances are poorly understood. The purpose of our study was to examine dopamine transporter (DAT) and dopamine D2 receptor (D2R) binding in adolescents with ADHD and to search for its relationship with cognitive functions as well as locomotor hyperactivity.

METHODS

Twelve adolescents with ADHD and 10 young adults were examined with PET using the selective radioligands [11C]PE2I and [11C]raclopride, indexing DAT and D2R density. The simplified reference tissue model was used to calculate binding potential (BP) values. Attention and motor behavior were investigated with a continuous performance task (CPT) and motion measurements.

RESULTS

The BP value for [11C]PE2I and [11C]raclopride in the striatum of children with ADHD did not differ from that of the young adult control subjects. In the midbrain, however, the BP values for DAT were significantly lower (16%; p = .03) in children with ADHD. Dopamine D2 receptor binding in the right caudate nucleus correlated significantly with increased motor activity (r = .70, p = .01).

CONCLUSIONS

The lower BP values for DAT in the midbrain suggest that dopamine signaling in subjects with ADHD is altered. Altered dopamine signaling might have a causal relationship to motor hyperactivity and might be considered as a potential endophenotype of ADHD.

Functional imaging of the brain by infrared radiation (thermoencephaloscopy)

A technique for thermal imaging of the animal and human brain cortex using an infrared optical system is described. Thermoencephaloscopy (TES) is based on improved thermovision and image processing techniques and allows two-dimensional, contact-free, dynamic and non-invasive recording of background and evoked cortical activity through an unopened skull. Activated (heated) and deactivated (cooled) zones of the cerebral cortex are revealed. The instrumental temporal resolution of TES is 40 msec (25 maps sec-1), the spatial resolution is up to 70 x 70 microns pixel-1. The diameter of the smallest recordable active region of the cortex is 200-300 microns. TES allows to detect the position, size and sequence of activation of precisely located specific cortical zones, and to measure their dynamics before, during and after sensory and direct cortical stimulation, motor acts and conditioning (associative learning). TES effects were recorded in rats, rabbits, cats, monkeys and humans. Waves were found spreading over the cortex with a speed up to 33 mm sec-1 along trajectories specific for the sensory modality and the site of stimulation. Some pathological processes in the brain are detectable by TES: spreading depression; stress; catalepsy; experimental tumors; and epileptic focuses. The main mechanisms of thermal responses recorded by TES are discussed: neural activity; local metabolism of units; local cerebral blood flow; and thermoconductivity in the activated zones of the cortex. Thermoencephaloscopy is a dynamic, non-invasive, contact-free, comparatively cheap, simple and inexpensive method of neuroimaging with a relatively high temporal and spatial resolution and sensitivity. It can be a useful tool in basic neuroscience and medicine.

Developments in tomographic methods for biological trace element research

Neutron-induced gamma-ray emission tomography for quantitative determination of concentration and distribution of elements in a selected plane through a biological specimen is briefly explained and applied by way of illustration to the analysis of gallstones. A system capable of carrying out studies of the binding site of 75Se is different matrices using time differential perturbed angular correlation spectroscopy is also briefly described. Developments in the detector technology of positron emission tomography have allowed small-diameter imaging devices to be built for in vivo preclinical evaluation of new tracers in small animals and are discussed in the context of a proposed experiment combining the techniques mentioned above.

Human brain measures of clinical pain: a review. II. Tomographic imagings

This paper (Parts I and II) reviews the measures employed in studying the brain neurophysiological activities of clinical pain. In Part II, these measures include the imaging and measurement of brain blood flow and hemodynamics in various regions of the brain, the scanning of gross and fine brain structures by computerized axial tomography or magnetic resonance imaging, and the imaging and measurement of brain metabolic changes, energy uptake, and receptors bindings through positron emission tomography or single-photon emission computerized tomography. Molecular chemical transformation by the nuclear magnetic resonance analysis of tissue changes and analgesic-receptor interactions is also noted. Most studies of the cerebral measures of traumatic and pathophysiological pain reported in the literature are concerned with headache. The relationships of brain activities among sensory processes of nociception, subjective experience of pain intensity and quality, emotional reaction, and cognitive coping often are complex and not well elucidated in man. Although significant changes in the cerebral physiological parameters are frequently reported in pain patients, the specificity and sensitivity of these measures as objective markers for human pain, reviewed from Part I and Part II together, has not yet been conclusively defined. Also, normative data basis and criteria for classifying abnormality of these brain measures must be established and their validity and reliability be carefully examined so that they can be confidently applied in diagnosis and management of clinical pain. Nevertheless, advancement on measurement of temporal dynamics in 3-D topographic mapping of cortical activities and source localization modeling, together with tomographic imaging of neurochemical metabolisms in the brain will further our scientific understanding of cerebral pain mechanisms. A window on the brain of human pain is being opened.

2D and 3D imaging of small animals and the human radial artery with a high resolution detector for PET

The performance of a pair of multicrystal, high-resolution, bismuth germanate (BGO) block detectors for positron emission tomography (PET) has been investigated. Utilizing the detectors at a separation of 100 mm, the spatial resolutions and count rate response of the block have been measured. These measurements indicate maximum spatial resolutions of 3.6 and 4.5 mm (FWHM) in the two axes of the block and a maximum coincidence rate of 3400 cps. The system has been used to observe the regional kinetics of positron-emitting radionuclides in the rat brain and the human radial artery. Designs for a small diameter, no-septa tomograph incorporating these detectors have been considered. Simulations demonstrate the possibilities of following tracer uptake within the rat brain and radial artery. Three-dimensional tomographic data-sets of a 20-mm uniform cylinder, obtained from rotating the two blocks, indicate good uniformity in the field of view (FOV) of 25 mm.

Dynamic monitoring of [11C]diprenorphine in rat brain using a prototype positron imaging device

The present work tests the feasibility of using the most recently developed positron emission tomograph detector technology to image positron-emitting radioligands in small experimental animals. A prototype imaging device, using two opposing multicrystal, high-resolution (approximately 4 mm) block detectors of bismuth germanate to produce a 2-dimensional image in the centre of the field of view, is described. To evaluate the probe's potential as a non-invasive experimental tool, the dynamic regional distribution of the established opiate receptor ligand, [11C]diprenorphine was determined in rat brain following intravenous injection. The distribution of counts in the images was consistent with the localisation of diprenorphine binding sites and the specificity of the signal obtained was confirmed by administration of non-radioactive diprenorphine and naloxone. Although the signal-to-noise ratio was reduced compared with data obtained by post mortem dissection, the dynamic data acquisition capabilities of the system demonstrate the feasibility of monitoring the kinetics of ligand binding in individual animals and encourages further design of a small-diameter detector system with tomographic capabilities.

Rapid feasibility studies of tracers for positron emission tomography: high-resolution PET in small animals with kinetic analysis

The development of methods for production of a radiotracer for use in human studies with positron emission tomography (PET) is often a time-consuming process of optimizing radiolabelling yields and handling procedures. Sometimes the radiotracer is not the original drug, but rather a derivative with unknown in vivo pharmacological properties. We have developed a fast and simple method of testing putative new PET tracers in vivo in small animals. The procedure has been validated in rats with different PET tracers with known kinetic and pharmacological properties ([2-18F]2-fluoro-2-deoxy-D-glucose, [N-methyl-11C]Ro 15-1788, and [15O]butanol). The tracer concentration in arterial blood was continuously measured to obtain the brain input function. Following image reconstruction of the scans, time-activity curves of selected regions of interest were generated. Estimations of CMRglc (1.0 +/- 0.2 mumol g-1 min-1), CBF (1.4 +/- 0.4 ml g-1 min-1) and transport rate constants for [N-methyl-11C]Ro 15-1788 (K1 = 0.44 +/- 0.01 ml g-1 min-1 and k2 = 0.099 +/- 0.005 min-1) as well as calculated first pass extraction (0.32 +/- 0.1) are in reasonable agreement with literature values. Small animal studies require minimal amounts of radioactivity and can be performed without sterility and toxicology tests. They may serve as a preliminary basis for radiation safety calculations because whole body scans can be performed even with a head scanner.(ABSTRACT TRUNCATED AT 250 WORDS)