Attenuation compensation for cardiac single-photon emission computed tomographic imaging: Part 1. Impact of attenuation and methods of estimating attenuation maps

Attenuation is believed to be one of the major causes of false-positive cardiac single-photon emission computed tomographic (SPECT) perfusion images. This article reviews the physics of attenuation, the artifacts produced by attenuation, and the need for scatter correction in combination with attenuation correction. The review continues with a comparison of the various configurations for transmission imaging that could be used to estimate patient specific attenuation maps, and an overview of how these are being developed for use on multiheaded SPECT systems, including discussions of truncation, noise, and spatial resolution of the estimated attenuation maps. Ways of estimating patient specific attenuation maps besides transmission imaging are also discussed.     

Multiple line source array for SPECT transmission scans: simulation, phantom and patient studies

Accurate attenuation and scatter corrections in quantitative SPECT studies require attenuation maps of the density distribution in the scanned object. These can be obtained from simultaneous emission/transmission scans. METHODS: A new method has been developed using a multiple line source array (MLA) for transmission scans, and its performance has been investigated using computer simulations and experimental data. The activity in the central lines of the MLA was higher than at the edges of the system, so that more transmission photons would be directed toward the thicker parts of the human body. A series of transmission-only and simultaneous emission/transmission studies were performed for different phantom configurations and human subjects. Attenuation maps were generated and used in reconstruction of attenuation-corrected emission images. RESULTS: The mu coefficients for attenuation maps obtained using the MLA system and simulated and experimental data display no artifacts and are qualitatively and quantitatively correct. For phantoms, the agreement between the measured and the true value of mu for water was found to be better than 4%. The attenuation-corrected emission images for the phantom studies demonstrate that the activity in the heart can be accurately reconstructed. A significant qualitative improvement was also obtained when the attenuation correction was used on patient data. CONCLUSION: Our results indicate that the MLA transmission source can be used in simultaneous transmission/emission imaging to generate accurate attenuation maps. These maps allow for performing an object-specific, attenuation correction of the emission images.     

Pulmonary perfusion and density gradients in healthy volunteers

The goal of this study was to measure regional pulmonary perfusion using SPECT and transmission tomography for attenuation correction and density measurements. METHODS: Regional pulmonary perfusion was studied after intravenous injection of radiolabeled particles in 10 supine healthy volunteers using SPECT. Transmission tomography was used to correct for attenuation, measure lung density and delineate the lungs. The effects of attenuation correction on pulmonary perfusion gradients were investigated. RESULTS: In perfusion measurements not corrected for attenuation, we found significant perfusion gradients in the direction of gravity but also significant gradients at isogravitational level. After correction for attenuation, the gravitational gradient was significantly greater than before correction, and gradients at isogravitational level were no longer observed. Perfusion in the ventral lung zone was half of that in the dorsal lung zone. Mean lung density was 0.28 +/- 0.03 g/ml, and density showed a significant increase in the direction of gravity and at isogravitational level. CONCLUSION: We found that SPECT perfusion studies of the lung not corrected for attenuation gave a false impression of nongravitational gradients and underestimate the gradient that is gravity-dependent. Transmission tomography, used for attenuation correction, also quantifies lung density and shows gravity dependent and nondependent density gradients.     

Magnetic resonance imaging of the bone marrow in hematological malignancies

The objective of this study was to determine the feasibility of using a fast (short-duration) transmission computed tomogram (TCT), acquired immediately before or after the emission CT, to correct for photon attenuation in cardiac SPECT. METHODS: The asymmetric fanbeam geometry with a 99mTc line source was used to acquire TCTs after conventional cardiac emission CT imaging on a triple-head SPECT system. The TCTs were reconstructed to generate patient-specific attenuation maps, which were used with an iterative maximum likelihood algorithm to reconstruct attenuation-corrected cardiac SPECT studies. The results of attenuation correction based on TCTs as short as 1 min were compared with long-duration transmission imaging for a phantom and several human studies. RESULTS: Attenuation correction based on asymmetric fanbeam TCT significantly improves the uniformity of images of a uniform tracer distribution in a cardiac-thorax phantom configured to simulate a large patient. By using a high-activity line source and a rapid camera rotation, a suitable attenuation map for this phantom can be obtained from a 4-min TCT. A similar result is obtained for patients with thorax widths of <40 cm. CONCLUSION: A sequential imaging protocol for acquiring a fast TCT can be used for attenuation correction of cardiac SPECT imaging. The sequential TCT can be acquired without significantly extending the duration of the imaging study. This method provides a way to perform attenuation correction on existing triple-head SPECT systems without extensively modifying the system.     

Significance of nonuniform attenuation correction in quantitative brain SPECT imaging

The purposes of this study were to develop a method for nonuniform attenuation correction of 123I emission brain images based on transmission imaging with a longer-lived isotope (i.e., 57Co) and to evaluate the relative improvement in quantitative SPECT images achieved with nonuniform attenuation correction. METHODS: Emission and transmission SPECT scans were acquired on three different sets of studies: a heterogeneous brain phantom filled with 1231 to simulate the distribution of dopamine transporters labeled with 2beta-carbomethoxy-3beta-(4-123I-iodophenyl)tropane (123I-beta-CIT); nine healthy human control subjects who underwent transmission scanning using two separate line sources (57Co and 123I); and a set of eight patients with Parkinson's disease and five healthy control subjects who received both emission and transmission scans after injection of 123I-beta-CIT. Attenuation maps were reconstructed using a Bayesian transmission reconstruction algorithm, and attenuation correction was performed using Chang's postprocessing method. The spatial distribution of errors within the brain was obtained from attenuation correction factors computed from uniform and nonuniform attenuation maps and was visualized on a pixel-by-pixel basis as an error image. RESULTS: For the heterogeneous brain phantom, the uniform attenuation correction had errors of 2%-6.5% for regions corresponding to striatum and background, whereas nonuniform attenuation correction was within 1%. Analysis of 123I transmission images of the nine healthy human control subjects showed differences between uniform and nonuniform attenuation correction to be in the range of 6.4%-16.0% for brain regions of interest (ROIs). The human control subjects who received transmission scans only were used to generate a curvilinear function to convert 57Co attenuation values into those for 123I, based on a pixel-by-pixel comparison of two coregistered transmission images for each subject. These values were applied to the group of patients and healthy control subjects who received transmission 57Co scans and emission 123I scans after injection of 123I-beta-CIT. In comparison to nonuniform attenuation correction as the gold standard, uniform attenuation with the ellipse drawn around the transmission image caused an approximately 5% error, whereas placement of the ellipse around the emission image caused a 15% error. CONCLUSION: Nonuniform attenuation correction allowed a moderate improvement in the measurement of absolute activity in individual brain ROIs. When images were analyzed as target-to-background activity ratios, as is commonly performed with 123I-beta-CIT, these outcome measures showed only small differences when Parkinson's disease patients and healthy control subjects were compared using nonuniform, uniform or even no attenuation correction.     

Noninvasive quantification of regional myocardial metabolic rate for oxygen by use of 15O2 inhalation and positron emission tomography. Theory, error analysis, and application in humans

BACKGROUND: A method has been developed to measure the regional myocardial metabolic rate of oxygen consumption (rMMRO2) and oxygen extraction fraction (rOEF) quantitatively and noninvasively in humans by use of 15O2 inhalation and positron emission tomography. This article describes the theory, an error analysis of the technique, and procedures of the method used in a human feasibility study. METHODS AND RESULTS: Inhaled 15O2 is transported to peripheral tissues, where it is converted to 15O-labeled water of metabolism, which exchanges with the relatively large extravascular tissue space. Quantification of this buildup of radioactivity allows the calculation of rMMRO2 and rOEF. However, a correction for the spillover of the pulmonary gas radioactivity signal into myocardial regions is required and has been made by use of a gas volume distribution estimated from the transmission scan. This was validated by comparative measurements using the inert gas [11C]CH4 in four greyhounds. Spillover of the cardiac chamber radioactivity has been corrected for with an inhaled [13O]CO (blood volume) scan. The underestimation of myocardial radioactivity due to wall motion and thickness has been corrected for by use of values of tissue fraction obtained from the flow measurement [15OKCO2 scan). Values of rOEF were similar (within 4%) whether obtained from gas volume measurements determined from the transmission or [11C]CH4 scan data. 15O2 scan information from six healthy volunteers showed a clear distribution of myocardial radioactivity after the vascular and pulmonary gas 15O background was subtracted. Subsequent compartmental analysis resulted in values for rOEF and rMMRO2 of 0.60 +/- 0.11 and 0.10 +/- 0.03 mL.min-1.g-1 in the human myocardium at rest. CONCLUSIONS: The results of this study are in good agreement with established values. This is the first known approach to allow the direct quantitative determination of rOEF and oxygen metabolism to be made noninvasively on a regional basis.     

Evaluation of attenuation corrections using Monte Carlo simulated lung SPECT

SPECT (single photon emission computed tomography) images are distorted by photon attenuation. The effect is complex in the thoracic region due to different tissue densities. This study compares the effect on the image homogeneity of two different methods of attenuation correction in lung SPECT; one pre-processing and one post-processing method. This study also investigates the impact of attenuation correction parameters such as lung contour, body contour, density of the lung tissue and effective attenuation coefficient. The Monte Carlo technique was used to simulate SPECT studies of a digital thorax phantom containing a homogeneous activity distribution in the lung. Homogeneity in reconstructed images was calculated as the coefficient of variation (CV). The isolated effect of the attenuation correction was assessed by normalizing pixel values from the attenuation corrected lung by pixel values from the lung with no attenuation effects. Results show that the CV decreased from 12.8% with no attenuation correction to 4.4% using the post-processing method and true densities in the thoracic region. The impact of variations in the definition of the body contour was found to be marginal while the corresponding effect of variations in the lung contour was substantial.     

Fast transmission CT for determining attenuation maps using a collimated line source, rotatable air-copper-lead attenuators and fan-beam collimation

We describe a technique using a line source and a rotatable air-copper-lead assembly to acquire gamma transmission computed tomographic (TCT) data for determining attenuation maps to compensate SPECT emission scans. The technique minimizes problems associated with discriminating 99mTc transmission and 201Tl emission photons and requires only a modest increase in total study time. A 99mTc line source and a stacked foil ("multislat") collimator are placed near the focal line of a fan-beam collimator (114 cm focal length) mounted on one detector of a triple-camera SPECT system. We acquired TCT data of plastic rod and anthropomorphic thorax phantoms to investigate the capability of the line source and rotatable air-copper-lead attenuators to determine attenuation maps. The data were acquired with and without 5.4 MBq (145 microCi) of 201Tl placed in the myocardial chamber of the thorax phantom. Phantoms also were scanned using a curved transmission slab source mounted to a parallel-hole collimator. Fan-beam TCT images have improved resolution compared with parallel-beam TCT images. Two patient scans also were performed to evaluate the clinical usefulness of fan-beam TCT. The rotatable air-copper-lead attenuator method eliminates contamination of emission data by transmission photons and reduces spill-over of emission data into the transmission energy window for some cases. Results show the feasibility of using fast, sequential or interlaced transmission scans of a line source within a rotatable air-copper-lead attenuator assembly to obtain accurate attenuation maps for SPECT attenuation compensation.     

201Tl myocardial SPECT. First experiences with a simultaneous transmission-emission acquisition protocol for patient-specific attenuated correction

AIM: In this study our first clinical experiences with simultaneous transmission and emission acquisition in 201 TI myocardial SPECT (T/E-SPECT) are discussed. METHODS: The non-uniform attenuation (AK) was carried out with a triple-head camera (PRISM 3000, Picker Inc.) correction equipped with fanbeam collimators. A line source of 750 MBq 99mTc was used to construct the transmission profile. Prior to investigation patients got 80-120 MBq 201TI-chloride intravenously injected. RESULTS: The study comprises the evaluation of 40 patients, derived from the clinical routine. The investigation followed an usual one day protocol. Our results using T/E-SPECT reveal an almost equilibrated activity distribution between anterior and posterior myocardial wall. CONCLUSION: For this reason it is to be expected that T/E-SPECT provides more reliable information about the posterior myocardial wall, than the usual SPECT technique without attenuation correction.     

Human nickel exposure in an area polluted by nickel refining: the S?r-Varanger study

BACKGROUND: Because myocardial wall thickness is smaller than the spatial resolution of single photon emission computed tomography (SPECT) imaging, changes in myocardial wall thickness are related to changes in maximum pixel counts via the partial volume effect, allowing for quantification of regional systolic wall thickening. We have developed a new gated SPECT method for computing the global left ventricular ejection fraction (LVEF) based entirely on changes in maximum regional myocardial counts during systolic contraction. This new method is independent of endocardial edge detection or other geometric measurements. METHODS AND RESULTS: In 23 patients the gated SPECT method was validated by comparison with radionuclide angiography. The correlation between computed LVEFs was excellent (slope = 0.97, r = 0.91). The measurement of LVEF by gated SPECT was highly reproducible, with minimal intraoperator (slope = 0.97, r = 0.97) or interoperator (slope = 1.00, r = 0.97) variability. Measurements of regional thickening indexes were also reproducible, with a mean intraoperator correlation coefficient of 0.89 +/- 0.05 (range 0.79 to 0.95) for the 14 myocardial regions. Finally, the measurement of LVEF was not significantly influenced by changes in reconstruction filter parameters over a range of cutoff frequencies from 0.16 to 0.28. CONCLUSIONS: This new counts-based gated SPECT method for measuring global left ventricular systolic function correlates well with radionuclide angiography, is highly reproducible, and has theoretic advantages over geometric methods.     

Effect of correction function on image characteristics of Positologica: a positron CT device for the head

The effects of correction function on image characteristics were studied experimentally for a positron CT device Positologica. Correction functions were obtained by smoothing the Shepp and Logan function by convolution of Gaussian functions. Signal-to-noise ratio (SNR) and spatial resolution were measured for phantoms and related to the magnitude of smoothing. The relation between SNR and spatial resolution is also discussed. Some suggestions are made to indicate how to select correction functions for clinical images. A flow of data processing for Positologica is described together with an outline of its hardware.     

SPECT cerebral blood flow, MR imaging, and neuropsychological findings in traumatic head injury.

Investigated 16 patients with diffuse or contusional brain damage and 8 patients with focal lesions 5–22 mo postinjury, using single proton emission computed tomography (SPECT) cerebral blood flow (CBF) measurements and neuropsychological examination. All Ss were aged 16–64 yrs. Compared with 16 controls, the diffuse group showed significant differences on 13 of 24 measures after correction for premorbid differences, whereas the focal group was significantly impaired on only 3 tests after correction. SPECT apparently identified abnormalities not demonstrated on magnetic resonance (MR) imaging and vice versa. Abnormal regional CBF seemed to be related to neuropsychological defects. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Acetazolamide challenge and technetium-99m-ECD versus iodine-123-IMP SPECT in chronic occlusive cerebrovascular disease

We compared the acetazolamide challenge test using 99mTc-ECD SPECT and 123I-IMP SPECT images in patients with chronic occlusive cerebrovascular disease. We also evaluated the usefulness of linearization correction for acetazolamide challenge test of 99mTc-ECD SPECT. METHODS: Twenty patients with unilateral chronic occlusive cerebrovascular disease (10 patients had middle cerebral arterial lesion and 10 had internal carotid lesion) were included in the study. Split-dose (a dose fractioning was 1:2), and sequential SPECT technique was used for 99mTc-ECD SPECT studies while only acetazolamide challenge test studies for 123I-IMP SPECT were performed. Permeability surface area product model (PS model) and back-diffusion model (Lassen's correction) were used for linearization correction of acetazolamide challenge with 99mTc-ECD SPECT. RESULTS: Six of 16 patients with reduced vasodilatory capacity in 123I-IMP SPECT were underestimated by 99mTc-ECD SPECT acetazolamide challenge test. Relative ECD uptake normalized by cerebellar uptake compared with IMP uptake showed a nonlinear relationship, indicating relatively less uptake in high flow range. The underestimations of limited vasodilatory capacity observed in 99mTc-ECD SPECT without linearization correction was modified by linearization algorithm. However, the effect of correction based on PS model was superior than that of Lassen's correction. The corrected 99mTc-ECD uptake ratio, based on PS model, and IMP uptake ratio demonstrated a better linear relationship than that of Lassen's correction. CONCLUSION: Technetium-99m ECD SPECT corrected based on the PS model is a better method of linearization for evaluating cerebrovascular reserve using acetazolamide challenge.     

Real-time continuous measurement of right ventricular volume using a conductance catheter

The authors propose using a multi-electrode conductance catheter to measure continuous right ventricular volume. True ventricular volume measurements are affected by four main sources of error. 1) field non-uniformity, 2) catheter curvature, 3) blood conductivity changes, and 4) leakage of current through surrounding tissues. Three-dimensional finite-element models were developed to investigate the effects of these sources of error and to devise schemes for correcting them. The models include an axisymmetric cylindrical model, a rectangular block model, and a heart model with left and right ventricular chambers. The heart model is built from conical primitives, with major dimensions derived from the literature. Finite-element simulations showed that volume measurements were underestimated due to field nonuniformity to as much as 1/25th actual volume in segments near the exciting electrodes. The extent of underestimation in a segment decreased with increasing distance of the segment from the exciting electrodes and increased for larger segmental volumes. Catheter curvature overestimated measured volume by as much as 4.5 times when the curvature was increased from 0.0 to 1.25 (from a straight catheter to a very curved one). The leakage of current through surrounding tissues overestimated volume by nearly 30%. The sensitivity of volume measurement to blood resistivity changes was found to be very high, at 70%. Correction factors established with the computer models compensate for field nonuniformity. Mathematical mapping of the curved catheter onto a fictitious straight catheter corrects for the catheter curvature error. Correction for both nonuniform field and catheter curvature allowed measurement of total ventricular volume with an error of 7%. Leakage current is determined by using different frequencies to build the catheter electric field and to separate tissue and blood resistance paths. Using this scheme, the percentage overestimation in volume measurement due to leakage could be determined with an accuracy of 85%. The proposed correction scheme for blood conductivity changes involves the in-vivo measurement of blood conductivity with the catheter itself. It was found that blood conductivity could be determined with insignificant error (< 0.5%) so long as the blood volume around the exciting electrodes had a radius of more than the electrode spacing.     

Attenuated cardiac sympathetic responsiveness during dynamic exercise in patients with heart failure

BACKGROUND: Cardiac norepinephrine (NE) spillover is increased in patients with chronic heart failure. This elevation is partly due to augmented NE release but also to reduced capacity for cardiac NE removal processes. In patients with mild to moderate heart failure, it is not known whether the described alteration in cardiac sympathetic function also affects cardiac NE spillover during intense sympathetic activation and whether other organs respond in proportion to the heart. METHODS AND RESULTS: Twenty-two patients with heart failure and 15 age-matched healthy subjects were studied. Whole-body and regional (NE) spillovers from the heart and kidneys were assessed at baseline and during supine cycling exercise (10 minutes) with the use of steady-state infusions of tritiated NE (isotope dilution). Cardiac performance was evaluated by means of catheterization of the right side of the heart. Cardiac NE spillover was higher (P < .05) at baseline in the patient group than in healthy subjects, whereas renal and whole-body NE spillovers were similar between the study groups. During exercise, cardiac NE spillover increased 13-fold (P < .05) in healthy subjects but only 5-fold (P < .05) in the cardiac failure group, the latter reaching a lower peak value (P < .05). In contrast, there was no difference between the study groups in either renal or whole-body NE spillover responsiveness to exercise. CONCLUSIONS: Patients with mild to moderate heart failure demonstrated a selective attenuation of cardiac sympathetic responsiveness during dynamic exercise. This attenuation may convey reduced inotropic and chronotropic support to the failing heart.     

Evaluation of right and left ventricular volume and ejection fraction using a mathematical cardiac torso phantom

The availability of gated SPECT has increased the interest in the determination of volume and ejection fraction of the left ventricle (LV) for clinical diagnosis. However, the same indices for the right ventricle (RV) have been neglected. The objective of this investigation was to use a mathematical model of the anatomical distribution of activity in gated blood-pool imaging to evaluate the accuracy of two ventricular volume and ejection fraction determination methods. In this investigation, measurements from the RV were emphasized. METHODS: The mathematical cardiac torso phantom, developed to study LV myocardium perfusion, was modified to simulate the radioactivity distribution of a 99mTc-gated blood-pool study. Twenty mathematical cardiac torso phantom models of the normal heart with different LV volumes (122.3 +/- 11.0 ml), RV volumes (174.6 +/- 22.3 ml) and stroke volumes (75.7 +/- 3.3 ml) were randomly generated to simulate variations among patients. An analytical three-dimensional projector with attenuation and system response was used to generate SPECT projection sets, after which noise was added. The projections were simulated for 128 equidistant views in a 360 degrees rotation mode. RESULTS: The radius of rotation was varied between 24 and 28 cm to mimic such variation in patient acquisitions. The 180 degrees and 360 degrees projection sets were reconstructed using the filtered backprojection reconstruction algorithm with Butter-worth filtering. Comparison was made with and without application of the iterative Chang attenuation correction algorithm. Volumes were calculated using a modified threshold and edge detection method (hybrid threshold), as well as a count-based method. A simple background correction procedure was used with both methods. CONCLUSION: Results indicate that cardiac functional parameters can be measured with reasonable accuracy using both methods. However, the count-based method had a larger bias than the hybrid threshold method when RV parameters were determined for 180 degrees reconstruction without attenuation correction. This bias improved after attenuation correction. The count-based method also tended to overestimate the end systolic volume slightly. An improved background correction could possibly alleviate this bias.     

Importance of bone attenuation in brain SPECT quantification

The purpose of this study was to determine the effects of nonuniform attenuation on relative quantification in brain SPECT and to compare the ability of the Chang and Sorenson uniform attenuation corrections (UACs) to achieve volumetric relative quantification. METHODS: Three head phantoms (dry human skull, Rando and Radiology Support Devices (RSD) phantoms) were compared with a human head using a gamma camera transmission CT (gammaTCT) SPECT system and x-ray CT. Subsequently, the RSD phantom's brain reservoir was filled with a uniform water solution of 99mTc, and SPECT and gammaTCT data were acquired using fanbeam collimation. The attenuating effects of bone, scalp and head-holder in individual projections were determined by an analytical projection technique using the SPECT and gammaTCT reconstructions. The Chang UAC used brain and head contours that were segmented from the gammaTCT reconstruction to demarcate its attenuation map, whereas the Sorenson UAC fit slice-specific ellipses to the SPECT projection data. For each UAC, volumetric relative quantification was measured with varying attenuation coefficients (mus) of the attenuation map. RESULTS: Gamma camera transmission CT and x-ray CT scans showed that the dry skull and Rando phantoms suffered from a dried trabecular bone compartment. The RSD phantom most closely reproduced the attenuation coefficients of the human gammaTCT and x-ray CT scans. The analytical projections showed that the attenuating effects of bone, scalp and head-holder were nonuniform across the projections and accounted for 18%-37% of the total count loss. Volumetric relative quantification was best achieved with the Chang (zero iterations) attenuation correction using the head contour and mu = 0.075 cm(-1); however, cortical activity was found to be 10% higher than cerebellar activity. For all UACs, the optimal choices of mu were experimentally found to be lower than the recommended 0.12 cm(-1) for brain tissue. This result is theoretically supported here. CONCLUSION: The magnitude of errors resulting from uniform attenuation corrections can be greater than the magnitudes of regional cerebral blood flow deficits in patients with dementia, as compared with normal controls. This suggests that nonuniform attenuation correction in brain SPECT imaging must be applied to accurately estimate regional cerebral blood flow.     

In vivo [11C]flumazenil-PET correlates with ex vivo [3H]flumazenil autoradiography in hippocampal sclerosis

By using [11C]flumazenil-positron emission tomography ([11C]FMZ-PET), we have previously shown that reductions of central benzodiazepine receptors (cBZRs) are restricted to the hippocampus in mesial temporal lobe epilepsy (mTLE) caused by unilateral hippocampal sclerosis (HS). Receptor autoradiographic studies on resected hippocampal specimens from the same patients demonstrated loss of cBZRs that was over and above loss of neurons in the CA1 subregion. Here, we report the first direct comparison of in vivo cBZR binding with [11C]FMZ-PET and ex vivo binding using [3H]FMZ autoradiography. We applied a magnetic resonance imaging-based method for partial volume effect correction to the PET images of [11C]FMZ volume of distribution ([11C]FMZ Vd) obtained in 10 patients with refractory mTLE due to unilateral, histologically verified HS. Saturation autoradiography was performed on the hippocampal specimens obtained from the same patients, allowing calculation of receptor availability ([3H]FMZ Bmax). After correction for partial volume effect, [11C]FMZ Vd in the body of the epileptogenic hippocampus was reduced by a mean of 42.1% compared with normal controls. [3H]FMZ Bmax, determined autoradiographically from the same hippocampal tissue, was reduced by a mean of 42.7% compared with control hippocampi. Absolute in vivo and ex vivo measurements of cBZR binding for the body of the hippocampus were significantly correlated in each individual. Our study demonstrates that reduction of available cBZR on remaining neurons in HS can be reliably detected in vivo by using [11C]FMZ-PET after correction for partial volume effect.     

Using ARROWSMITH: a computer-assisted approach to formulating and assessing scientific hypotheses

The usefulness of Xe-133 and Tc-99m-MAA single photon emission computed tomography (SPECT) in identifying areas to be resected during video-assisted thoracoscopic lung reduction surgery for emphysema was examined. Twenty-nine patients with advanced emphysema were examined using Xe-133 and Tc-99m-MAA SPECT prior to and following surgery. For the Xe-133 dynamic SPECT, patients inhaled Xe-133 gas for 6 minutes. Equilibrium and subsequent washout SPECT images were acquired every 30 seconds for 6 to 7 minutes during spontaneous breathing. Ventilation was quantified by Xe-133 clearance time (T1/2) in addition to visual assessment. The patients underwent unilateral thoracoscopic volume reduction in the regions with abnormal Xe-133 retention and Tc-99m-MAA defect. All patients demonstrated marked, heterogeneous Xe-133 retention and Tc-99m-MAA defects preoperatively. The worst functioning areas were identified as nonventilated and noflow areas, or areas with air trapping and low perfusion. These changes were found even in patients with diffuse and symmetrical impairments on chest CT. After surgery, most of these "target areas" disappeared and pulmonary function tests demonstrated significant improvement. T1/2 correlated closely with the percent predicted FEV1 (%FEV) and 6-minute walk distance before and after surgery (p<0.0001). Xe-133 and Tc-99m-MAA SPECT imaging was useful in identifying "target areas" in the emphysematous lung. Directed unilateral thoracoscopic volume reduction based on these SPECT images is an effective treatment for emphysema.