Manual segmentation of 129 x-ray CT transverse slices of a living male human has been done and a computerized 3-dimensional volume array modeling all major internal structures of the body has been created. Each voxel of the volume contains a index number designating it as belonging to a given organ or internal structure. The original x-ray CT images were reconstructed in a 512 x 512 matrix with a resolution of 1 mm in the x,y plane. The z-axis resolution is 1 cm from neck to midthigh and 0.5 cm from neck to crown of the head. This volume array represents a high resolution model of the human anatomy and can serve as a voxel-based anthropomorphic phantom suitable for many computer-based modeling and simulation calculations.

We report the use of high-speed magetic resonance imaging to follow the changes in image intens in the human visual cortex during stimulation by a flashing checkerboard stimulus. Measurements were made in a 2.1-T, 1-m-diameter magnet, part of a Bruker Biospec spectrometer that we had programmed to do echo-planar imaging. A 15-cm-diameter surface coil was used to transmit and receive signals. Images were acquired during periods of stimulation from 2 s to 180 s. Images were acquired in 65.5 ms in a 10-mm slice with in-plane voxel size of 6 x 3 mm. Repetition time (TR) was generally 2 s, although for the long flashing periods, TR = 8 s was used. Voxels were located onto an inversion recovery image taken with 2 x 2 mm in-plane resolution. Image intensity increased after onset ofthe stimulus. The mean change in signal relative to the prestimulation level (AS/S) was 9.7% (SD = 2.8%, n = 20) with an echo time of 70 ms. Irrespective of the period of stimulation, the increase in magnetic resonance signal intensity was delayed relative to the stimulus. The mean delay measured from the start of stimulation for each protocol was as follows: 2-s stimulation, delay = 3.5 s (SD = 0.5 s, n = 10) (the delay exceeds stimulus duration); 20-to 24-s stimulation, delay = 5 s (SD = 2 s, n = 20).Functional mapping of the brain by nuclear magnetic resonance (NMR) methods has recently been demonstrated by two techniques. The first method uses a bolus of paramagnetic contrast agent (gadolinium/diethylenetriaminepentaacetic acid) as a tracer for blood volume (1). The agent is injected into the arm and produces variations in local blood volume magnetic susceptibility as the bolus passes through the capillary bed of the brain. The changes in susceptibility are monitored by using a transverse relaxation time (T2)-weighted, fast magnetic resonance-imaging sequence, such as echo-planar imaging (EPI) (2) or fast low-angle shot (FLASH) (3). This method is subject to the same assumptions as all kinetic tracer methods (4), but within these restrictions it gives a quantitative result. With this method Belliveau et al. (5) have made measurements in the human visual cortex and demonstrated an increase in blood volume during stimulation. The main drawback of the technique is the need for at least two injections of contrast agent to measure the resting and activated brain states.The second method relies solely on the paramagnetic effects of hemoglobin. In the oxygenated state (HbO2), the molecule is diamagnetic, but once dissociated from oxygen (deoxyhemoglobin; Hb), it becomes paramagnetic. Thus, changes in Hb concentration will also result in magnetic susceptibility changes in the capillaries. Complications may arise because the paramagnetic agent is not evenly distributed throughout the blood but is compartmentalized inside the erythrocytes. Ogawa et al. (6) have studied the effect of inhaled 02 content on the intensity of gradient-echo images ofrat brain at 7 T. They have shown that a decrease in oxygen content produces dark streaks in the i...

Summary: Kinetic methods were used to obtain regional estimates of benzodiazepine receptor concentration Abbreviations used: BZ, benzodiazepine; CT, computed to mography; CV, coefficient of variation; PET, positron emission tomography; SA, specific activity. 656Method I. In Methods II and III , the ko./2/SA parameter was specifically constrained using the Method I value of kon and the volunteer's values Off2 and low SA (CillLmol) . Four parameters were determined simultaneously using Method II . In Method III, K/k2 was fixed to the inhibi tion value and only three parameters were estimated. Method I provided the most variable results and conver gence problems for regions with low receptor binding. Method II provided results that were less variable but very similar to the Method I results, without convergence problems . However, the K,lk2 ratios obtained by Method II ranged from 1.07 in the occipital cortex to 0.61 in the thalamus. Fixing the K,lk2 ratio in Method III provided a method that was physiologically consistent with the fixed value of f2 and resulted in parameters with considerably lower variability . The average Bmax values obtained using Method III were 100 ± 25 nM in the occipital cortex, 64 ± 18 nM in the cerebellum , and 38 ± 5.5 nM in the thal amus; the average Kd was 8.9 ± 1.0 nM (five brain re gions).

Summary: The aim of this work was to study the feasi bility and reproducibility of in vivo measurement of ben zodiazepine receptors with single photon emission com puterized tomography (SPECT) in the baboon brain. Ar terial and brain regional activities were measured for 420 min in three baboons after single bolus injection of the benzodiazepine antagonist [1 2 3I]iomazenil. Data were fit to a three-compartment model to derive the regional bind ing potential (BP), which corresponds to the product of the receptor density, (B m aJ and affinity (l/Kd. Regional BP values (from 114 in striatum to 241 in occipital) were in good agreement with values predicted from in vitro studies. Constraining the regional volume of distribution of the nondisplaceable compartment to the value mea sured during tracer constant infusion experiments in ba-