Real-time quantitative ex vivo direct autoradiography with 10 µm pixel resolution.

We present three new autoradiography methods to map positron emission rate of a bio-specimen slice with high resolution. One is based on LBNL scientific charge coupled device (CCD) and the other two are based on conventional CCDs. High conversion efficiency (100k e-h pairs / 0.5 MeV positron) and low dark current (1.75 × 10(-4) e-/pix/sec) can be achieved using the LBNL CCD. The theoretical calculations and preliminary experiments show that an 86 µm spatial resolution can be achieved when imaging a 100 µm thick tissue soaked with (18)F which produce higher energy positron. The main disadvantage of the LBNL CCD we tested is that a very low operating temperature is required to eliminate dark current. This dramatically increases the system cost. In addition, the integration time of the CCD needs to be short enough to avoid overlapping of the positron trajectories. Conventional CCDs have lower conversion efficiency (2k e-h pairs / 0.5 MeV positron) and higher dark current (200 e-/pix/sec), but are more cost-efficient and the requirement for the readout frequency is much lower. The conversion efficiency of the conventional CCD imager can be improved by 17 times by inserting a 100 µm layer of phosphor between the sample and the imager. However, the light emitted from the phosphor screen will be ~100 µm diameter, which severely degrades the spatial resolution. A high readout frequency is also required to avoid the overlapping. The CCD systems designed in this study will be used to map positron emission rate of bio-specimens such as cancerous tissues acquired in regular biopsy procedure. They can also be used to corroborate tracer kinetic modeling at a cellular level.

Multipole shimming of permanent magnets using harmonic corrector rings.

Shimming systems are required to provide sufficient field homogeneity for high resolution nuclear magnetic resonance (NMR). In certain specialized applications, such as rotating-field NMR and mobile ex situ NMR, permanent magnet-based shimming systems can provide considerable advantages. We present a simple two-dimensional shimming method based on harmonic corrector rings which can provide arbitrary multipole order shimming corrections. Results demonstrate, for example, that quadrupolar order shimming improves the linewidth by up to an order of magnitude. An additional order of magnitude reduction is in principle achievable by utilizing this shimming method for z-gradient correction and higher order xy gradients.

Region-selective effects of neuroinflammation and antioxidant treatment on peripheral benzodiazepine receptors and NMDA receptors in the rat brain.

Following induction of acute neuroinflammation by intracisternal injection of endotoxin (lipopolysaccharide) in rats, quantitative autoradiography was used to assess the regional level of microglial activation and glutamate (NMDA) receptor binding. The possible protective action of the antioxidant phenyl-tert-butyl nitrone in this model was tested by administering the drug in the drinking water for 6 days starting 24 hafter endotoxin injection. Animals were killed 7 days post-injection and consecutive cryostat brain sections labeled with [3H]PK11195 as a marker of activated microglia and [125I]iodoMK801 as a marker of the open-channel, activated state of NMDA receptors. Lipopolysaccharide increased [3H]PK11195 binding in the brain, with the largest increases (two- to threefold) in temporal and entorhinal cortex, hippocampus, and substantia innominata. A significant (> 50%) decrease in [125I]iodoMK801 binding was found in the same brain regions. Phenyl-tert-butyl nitrone treatment resulted in a partial inhibition (approx. 25% decrease) of the lipopolysaccharide-induced increase in [3H]PK11195 binding but completely reversed the lipopolysaccharide-induced decrease in [125I]iodoMK80 binding in the entorhinal cortex, hippocampus, and substantia innominata. Loss of NMDA receptor function in cortical and hippocampal regions may contribute to the cognitive deficits observed in diseases with a neuroinflammatory component, such as meningitis or Alzheimer's disease.

Kinetic analysis of 125I-iodorotenone as a deposited myocardial flow tracer: comparison with 99mTc-sestamibi.

UNLABELLED: The goal of this investigation was to assess the accuracy of 7'-Z-[125I]iodorotenone (125I-iodorotenone) as a new deposited myocardial flow tracer and compare the results with those for 99mTc-sestamibi. METHODS: The kinetics of these two flow tracers were evaluated in 25 isolated, erythrocyte- and albumin-perfused rabbit hearts over a flow range relevant to patients. The two flow tracers and a vascular reference tracer (131I-albumin) were introduced simultaneously as a compact bolus through a port just above the aortic cannula in the absence of tracer recirculation. Myocardial extraction, retention, washout, and uptake parameters were computed from the venous outflow curves using the multiple-indicator dilution technique and spectral analysis. RESULTS: The extraction of 125I-iodorotenone was much higher than the extraction of 99mTc-sestamibi (0.84 +/- 0.05 vs. 0.48 +/- 0.10, respectively, P < 0.001). 125I-iodorotenone extraction was also less affected by flow than was 99mTc-sestamibi (P < 0.001). Net retention of 125I-iodorotenone was significantly greater than 99mTc-sestamibi net retention at 1 min (0.77 +/- 0.08 vs. 0.41 +/- 0.11, respectively, P < 0.001) and 26 min (0.46 +/- 0.13 vs. 0.27 +/- 0.11, respectively, P < 0.001) after tracer injection. Flow had less effect on 125I-iodorotenone net retention than on 99mTc-sestamibi net retention 1 min after tracer injection (P < 0.04). However, at 26 min, flow had an equivalent effect on the retention of both flow tracers (P < 0.4). The relationship between 125I-iodorotenone and 99mTc-sestamibi washout was complex and depended on elapsed time after isotope introduction and perfusion rate. Reflecting the favorable extraction and retention characteristics of 125I-iodorotenone, both its maximum myocardial uptake and its 26-min uptake were more closely related to flow than were those of 99mTc-sestamibi (P < 0.001 for both comparisons). CONCLUSION: The extraction and retention of 125I-iodorotenone were greater than those of 99mTc-sestamibi, making 125I-iodorotenone the superior flow tracer in the isolated rabbit heart.

Convection-enhanced delivery of AAV vector in parkinsonian monkeys; in vivo detection of gene expression and restoration of dopaminergic function using pro-drug approach.

Using an approach that combines gene therapy with aromatic l-amino acid decarboxylase (AADC) gene and a pro-drug (l-dopa), dopamine, the neurotransmitter involved in Parkinson's disease, can be synthesized and regulated. Striatal neurons infected with the AADC gene by an adeno-associated viral vector can convert peripheral l-dopa to dopamine and may therefore provide a buffer for unmetabolized l-dopa. This approach to treating Parkinson's disease may reduce the need for l-dopa/carbidopa, thus providing a better clinical response with fewer side effects. In addition, the imbalance in dopamine production between the nigrostriatal and mesolimbic dopaminergic systems can be corrected by using AADC gene delivery to the striatum. We have also demonstrated that a fundamental obstacle in the gene therapy approach to the central nervous system, i.e., the ability to deliver viral vectors in sufficient quantities to the whole brain, can be overcome by using convection-enhanced delivery. Finally, this study demonstrates that positron emission tomography and the AADC tracer, 6-[(18)F]fluoro-l-m-tyrosine, can be used to monitor gene therapy in vivo. Our therapeutic approach has the potential to restore dopamine production, even late in the disease process, at levels that can be maintained during continued nigrostriatal degeneration.

Brain size does not predict general cognitive ability within families.

Hominid brain size increased dramatically in the face of apparently severe associated evolutionary costs. This suggests that increasing brain size must have provided some sort of counterbalancing adaptive benefit. Several recent studies using magnetic resonance imaging (MRI) have indicated that a substantial correlation (mean r = approximately 0.4) exists between brain size and general cognitive performance, consistent with the hypothesis that the payoff for increasing brain size was greater general cognitive ability. However, these studies confound between-family environmental influences with direct genetic/biological influences. To address this problem, within-family (WF) sibling differences for several neuroanatomical measures were correlated to WF scores on a diverse battery of cognitive tests in a sample of 36 sibling pairs. WF correlations between neuroanatomy and general cognitive ability were essentially zero, although moderate correlations were found between prefrontal volumes and the Stroop test (known to involve prefrontal cortex). These findings suggest that nongenetic influences play a role in brain volume/cognitive ability associations. Actual direct genetic/biological associations may be quite small, and yet still may be strong enough to account for hominid brain evolution.

Multi-dose crystalloid cardioplegia preserves intracellular sodium homeostasis in myocardium.

The goal of this study was to assess the effect of multi-dose St Thomas' cardioplegia on intracellular sodium homeostasis in a rat heart model. A new magnetic resonance method was applied which enable us to detect intracellular Na changes without chemical shift reagents. Three groups of isolated rat hearts were subjected to 51 min of ischemia and 51 min of reperfusion at 37 degrees C: Group 1-three infusions of St Thomas' cardioplegia every 17 min for 2 min (n=7); Group 2-single-dose infusion of cardioplegia at the beginning of stop-flow ischemia (n=8); and Group 3-clamp ischemia (n=3) without cardioplegia administration. Performance of the heart was assessed by rate-pressure product relative to the pre-ischemic level (RPP). An NMR method was applied which continuously detects the Na(i) concentration in the heart, using the ability of bound sodium to exhibit triple-quantum transitions and the growth of the corresponding signal when sodium ions pass from extracellular to intracellular space. Clamp ischemia without cardioplegia and 50 min of reperfusion left the heart dysfunctional, with Na(i) growth from the pre-ischemic level of 13.9+/-1.2 mM to 34.9+/-1.3 mM and 73. 9+/-1.9 mM at the end of ischemia and reperfusion, respectively. During single-dose cardioplegia the corresponding values for Na(i) were 30.2+/-1 mM and 48.5+/-1.7 mM (RPP=29%). Multiple infusions of cardioplegic solution resulted in a remarkable preservation of the heart's intracellular Na concentration with a non-significant increase in Na(i) during ischemia and only 16.7+/-1 mM, (P=0.01), after subsequent reperfusion (RPP=85%). The time course of Na(i) changes in the rat heart model demonstrates a prominent potential of multi-dose St Thomas' cardioplegia in preserving intracellular sodium homeostasis at 37 degrees C. The growth of Na(i) concentration during ischemia, as an indicator of the viability of the myocytes, can have a prognostic value for the heart's performance during reperfusion.

Dopamine transporter loss and clinical changes in MPTP-lesioned primates.

Single photon emission computed tomography (SPECT) and the dopamine (DA) transporter tracer, 2 beta-carboxymethoxy-3 beta-(4-iodophenyl)tropane ([123I]beta-CIT), were used to determine DA transporter density in 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-lesioned monkeys with varying degrees of parkinsonism. The clinical stage of parkinsonism corresponded to SPECT measures of striatal DA transporter density suggesting that more severe parkinsonism was associated with a greater degree of dopaminergic terminal degeneration. These findings are similar to those reported earlier using positron emission tomography (PET) and the DA metabolism tracer, 6-[18F]fluoro-L-m-tyrosine (FMT), indicating that both are good methods for evaluating nigrostriatal degeneration in MPTP primate models.

Imaging transgenic animals.

Transgenic and eugenic animals as small as 30 g can be studied non-invasively by radionuclides with resolutions of 1-2 mm, by MRI with resolution of 100 microns and by light fluorescence and bioluminescence with high sensitivities. The technologies of radionuclide emission, magnetic resonance imaging, magnetic resonance spectroscopy, optical tomography, optical fluorescence and optical bioluminescence are currently being applied to small-animal studies. These technologies and examples of their applications are reviewed in this chapter.

MR safety: past, present, and future from a historical perspective.

This article focuses on both static field effects and oscillating magnetic fields associated with MR imaging and spectroscopy. Contemporary experiments and theories on health effects demonstrate that currently MR imaging is practiced in a safe manner. Technological capabilities and medical science objectives, however, will lead to procedures that will challenge the thresholds of physiological effects. Thus progress in this field will require continual surveillance and better definitions of guidelines which at present are considered prudent but too restrictive.

PET instrumentation: what are the limits?

This report has emphasized the attributes of positron emission tomography (PET) through a discussion of the historical development with attention to limitations or factors that are of importance in using and further developing this technology. As is the case for all nuclear detector developments, the factors that require consideration are spatial resolution, uniformity of resolution, sensitivity, distortions (attenuation), background noise (scatter and randoms), image volume, data acquisition capabilities (count-rate saturation), and limitations based on allowable radiation doses to the subject. Forty years ago, the fact that dual gamma-cameras could not handle the count-rates from the short half-life radionuclides that had clinical applications at that time (ie, 15O, 11C, 13N) precluded their acceptance in nuclear medicine. With the advent of 18F applications particularly with FDG in oncology, this limitations was no longer a barrier. Twenty years ago and until recently, the promise of time-of-flight PET has been stifled by the fact that the appropriately fast scintillator BaF2 had too low an efficiency (low density) to be useful in improving the signal to noise of a time-of-flight tomograph over contemporary systems. With the development of dense scintillators with high light output and high speed such as LSO30 the time-of-flight potentials are now once again worth pursuing. Twenty years ago systems that theoretically would have improved sensitivity by minimal or no septa with spherical geometric arrangements of detectors were ignored because it appeared that scatter backgrounds would lead to a signal to noise less than 1. But in the last 5 years, cylindrical systems without speta have shown that noise effective sensitivity improvements of a factor of 4 can be realized. With time-of-flight additional improvements in sensitivity will be realized. Horizons for detector development include discovery of new scintillators, new methods of registering scintillation light, deployment of larger field of view systems and methods of compensating for scatter, randoms, attenuation, and irregular sampling associated with new geometries which can encircle most of the body. The expected limit for PET is 2 mm isotropic resolution for the head and appendages including joints and breasts. Clinical realization of this resolution for the thorax and abdomen requires compensation for motion and even in this area strategies are underdevelopment which rely on the improvement in sensitivity being realized by 3D systems.

Estimating glucose metabolism using glucose analogs and two tracer kinetic models in isolated rabbit heart.

The purpose of this investigation was to 1) evaluate the relative accuracy of the Sokoloff and Patlak tracer kinetic models in estimating glucose metabolic rate (GMR) in the presence and absence of insulin; 2) evaluate the effect of nutritional state on the lumped constant (LC); and 3) compare the kinetics of 2-fluoro-2-deoxy-D-[14C]glucose (FDG) and 2-deoxy-D-[3H]glucose (DG) membrane transport and phosphorylation. The experimental preparation was the isolated, red blood cell-albumin-perfused rabbit heart. Our results showed that both tracer kinetic models provided GMR estimates that correlated well with the Fick method (for FDG, R = 0. 84 and 0.91 for the Sokoloff and Patlak models, respectively); nutritional state did not affect the LC; and FDG and DG have different transport and/or phosphorylation parameters. We also observed that 1) the addition of a fourth compartment to the Sokoloff model reduced the mean squared error between measured and modeled data by a factor of 7.4; 2) a longer time (21.8 min) was required to obtain a linear phase of the Patlak plot than is allowed in clinical studies; and 3) accurate GMR estimates were obtained only by using different LCs reflecting insulin's presence or absence. Our results indicate potential sources of error in the use of FDG and positron emission tomography to quantify GMR in patients.

Sodium TQF NMR and intracellular sodium in isolated crystalloid perfused rat heart.

The feasibility of monitoring intracellular sodium changes using Na triple quantum filtered NMR without a chemical shift reagent (SR) was investigated in an isolated rat heart during a variety of interventions for Na(i) loading. Perfusion with 1 mM ouabain or without K+ present in the perfusate for 30 min produced a rise of the Na TQF signal with a plateau of approximately 190% and approximately 228% relative to the preintervention level, respectively. Stop-flow ischemia for 30 min resulted in a TQF signal growth of approximately 147%. The maximal Na TQF signal increase of 460% was achieved by perfusion without K+/Ca2+, corresponding to an elimination of the Na transmembrane gradient. The observed values of Na NMR TQF growth in the physiological and pathological ranges are in agreement with reported data by other methods and have a linear correlation with intracellular sodium content as determined in this study by Co-EDTA method and by sucrose-histidine washout of the extracellular space. Our data indicate that the increase in Na TQF NMR signal is determined by the growth of Na(i), and the extracellular Na contribution to the total TQF signal is unchanged at approximately 64%. In conclusion, Na TQF NMR without using SR offers a unique and noninvasive opportunity to monitor alterations of intracellular sodium. It may provide valuable insights for developing cardioprotective strategies and for observing the effects of pharmaceutical treatments on sodium homeostasis.

Clinical studies of cerebral blood flow in Alzheimer's disease.

Studies of Alzheimer's disease using single-photon emission computed tomography (SPECT) and positron emission tomography (PET) have found reductions in blood flow and glucose metabolism in temporal and parietal cortex. In 50 AD patients who underwent neuropsychological testing and SPECT perfusion imaging, we found significant correlations between perfusion and performance on the Mini-Mental Status Examination in the frontal and parietal lobes. In addition, specific correlations between perfusion in the frontal lobes and performance on tests of frontal lobe ability were noted. These findings, while suggesting the importance of perfusion measures in determining clinical features of the disease, do not clearly define perfusion changes as primary, since similar findings have been seen when metabolism is studied. In a separate group of 5 AD patients and 16 controls, we used PET with the perfusion tracer HIPDM and examined cerebrovascular reactivity to carbon dioxide inhalation. We found that in multiple brain regions, including the temporal lobes, AD patients showed robust and significant increases in perfusion in response to carbon dioxide that did not differ from the response seen in the controls. Taken together, these results show that while perfusion changes are important in AD, they are not clearly either primary or limiting.

Effects of specific sodium/hydrogen exchange inhibitor during cardioplegic arrest.

BACKGROUND: The accumulation of intracellular sodium during myocardial ischemia couples an inappropriate calcium influx and depressed cardiac recovery during subsequent reperfusion. The effects of the selective sodium/ hydrogen exchange inhibitor HOE 694 are evaluated during myocardial ischemia and reperfusion. METHODS: Ten isolated rat hearts were subjected to a 2-minute infusion of St. Thomas' cardioplegia +/- 1 mumol/L HOE 694 followed by 50 minutes' normothermic (37 degrees C) global ischemia. Intracellular sodium accumulation was continuously measured using triple quantum filtered 23Na nuclear magnetic resonance spectroscopy without chemical shift reagents. Hemodynamic variables were assessed before and after ischemia. RESULTS: The addition of 1 mumol/L HOE 694 to St. Thomas cardioplegic solution (n = 5) attenuated the accumulation of intracellular sodium after 50 minutes' ischemia (160.5% +/- 9.1% versus 203.4% +/- 10.9% [mean +/- standard error], HOE 694 versus control, respectively; p = 0.014) and after the initial reperfusion period (first 30 minutes) (288.7% +/- 10.2% versus 335.9% +/- 10.3%; p = 0.008). HOE 694-treated hearts showed significantly improved postischemic recovery of left ventricular developed pressure (53.5% +/- 8.4% versus 26.4% +/- 6.6%; p = 0.036) and rate-pressure product (40.2% +/- 6.9% versus 13.2% +/- 5%; p = 0.014). Postischemic recovery of coronary flow was not significantly different between the two groups (68.6% +/- 5.9% versus 55.5% +/- 4.6%, HOE 694 versus control, respectively; p = 0.11). CONCLUSIONS: The addition of 1 mumol/L HOE 694 to cardioplegic solution attenuates the increase of intracellular sodium during myocardial ischemia and early reperfusion. This is coupled with an improved recovery of contractile function, possibly as a result of decreased sodium and calcium overload of ischemic myocardium.

Kinetic analysis of rubidium and thallium as deposited myocardial blood flow tracers in isolated rabbit heart.

Evaluation of myocardial perfusion with tracers such as thallium and rubidium is based on the assumption that tissue tracer content is proportional to flow. The purpose of this study was to evaluate the relationship between flow and tissue tracer content of 201Tl and 83Rb in the isolated perfused rabbit heart. 83Rb (86-day half-life), an isotope that is not used clinically, was used as a subsitute for 82Rb (76-s half-life) to improve the accuracy and precision of data acquisition. The multiple indicator-dilution technique was employed with two independent computational approaches. The first approach explicitly deconvolved 201Tl and 83Rb venous concentration curves by the intravascular reference tracer curve. The second approach used a conventional analysis. Both approaches showed that there was more early washout of 83Rb than 201Tl and that the heart retained 201Tl better than 83Rb within 2 min after isotope introduction. These data indicate that 201Tl is a better perfusion tracer than 83Rb in the isolated rabbit heart.

Alignment of volume MR images and high resolution [18F]fluorodeoxyglucose PET images for the evaluation of patients with brain tumors.

PURPOSE: The goal of the study was to investigate the use of automated registration techniques for interpretation of volume MR and high resolution FDG-PET images that were obtained from patients with brain tumors. METHOD: Twenty-one patients with brain tumors were studied on one or more occasions using MRI and high resolution FDG-PET. The data were aligned using automated volume- and surface-matching algorithms. Composite images comprising the resliced pre- and postgadolinium spoiled GRE, T2-weighted SE, and PET data were constructed to correlate intensities of regions on the PET images with regions that corresponded to normal gray matter, white matter, and gadolinium enhancement. RESULTS: The accuracy of registration between the MR and PET images was estimated to be within 1-2 mm based upon the distance between surfaces of the outside of the head. In 12 of the 24 examinations, there were diagnoses of recurrent tumor, with only 5 of these exhibiting regions of higher FDG uptake than normal gray matter. For 19 of the 24 studies, the anatomic context provided by the registered MR images was found to be important in distinguishing recurrent tumor from necrosis based upon FDG uptake. CONCLUSION: The automated alignment was found to be an important factor in interpreting the high resolution PET images. This was particularly true for small lesions close to the cortex and for situations where FDG uptake had been reduced by prior treatment with radiation therapy.

Four-dimensional 1H and 23Na imaging using continuously oscillating gradients.

A class of fast magnetic spectroscopic imaging methods using continuously oscillating gradients for four-dimensional (three spatial and one spectral) localization is introduced. Sampling may start immediately following the application of an RF excitation pulse, thus enabling measurement of spin density, chemical shift, and relaxation rates of short-T2 species. For spatial localization, steady-state sinusoidal gradient waveforms are used to sample a ball in k space. The two types of trajectories presented include: (1) continuously oscillating gradients with continuously rotating direction used for steady-state free-precession imaging and (2) continuously oscillating gradients followed by a spoiler directed along discrete projections. Design criteria are given and spatial-spectral and spatial-temporal reconstruction methods are developed. Theoretical point-spread functions and signal-to-noise ratios are derived while considering T2*, off-resonance effects, and RF excitation options. Experimental phantom, in vivo, and in vitro 1H and 23Na images collected at 2.35 T are presented. The 1H images were acquired with isotropic spatial resolution ranging from 0.03 to 0.27 cm3 and gradient-oscillation frequencies ranging from 600 to 700 Hz, thus allowing for the separation of water and lipid signals within a voxel. The 23Na images, acquired with 500 and 800 Hz gradient waveforms and 0.70 cm3 isotropic resolution, were resolved in the time domain, yielding spatially localized FIDs.

Strategies for extraction of quantitative data from volumetric dynamic cardiac positron emission tomography data.

The ability of positron emission tomography (PET) to serve as a useful myocardial perfusion indicator is well established. We describe a methodology for obtaining reliable quantitative kinetic parameters from dynamic cardiac PET data. Reconstructed images of the myocardium are subdivided into three-dimensional volumes of interest which are used to obtain quantitative measures of myocardial perfusion over physiologically meaningful anatomical regions. The quantitation technique rigorously models the uncertainty of estimated parameters while compensating for effects such as patient motion and partial volumes to arrive at model parameters with well-established confidence intervals.