Fan-beam Collimators Research Articles

Study of the point spread function (PSF) for 123I SPECT imaging using Monte Carlo simulation

The iterative reconstruction algorithms employed in brain single-photon emission computed tomography (SPECT) allow some quantitative parameters of the image to be improved. These algorithms require accurate modelling of the so-called point spread function (PSF). Nowadays, most in vivo neurotransmitter SPECT studies employ pharmaceuticals radiolabelled with 123I. In addition to an intense line at 159 keV, the decay scheme of this radioisotope includes some higher energy gammas which may have a non-negligible contribution to the PSF. The aim of this work is to study this contribution for two low-energy high-resolution collimator configurations, namely, the parallel and the fan beam. The transport of radiation through the material system is simulated with the Monte Carlo code PENELOPE. We have developed a main program that deals with the intricacies associated with tracking photon trajectories through the geometry of the collimator and detection systems. The simulated PSFs are partly validated with a set of experimental measurements that use the 511 keV annihilation photons emitted by a 18F source. Sensitivity and spatial resolution have been studied, showing that a significant fraction of the detection events in the energy window centred at 159 keV (up to approximately 49% for the parallel collimator) are originated by higher energy gamma rays, which contribute to the spatial profile of the PSF mostly outside the ‘geometrical’ region dominated by the low-energy photons. Therefore, these high-energy counts are to be considered as noise, a fact that should be taken into account when modelling PSFs for reconstruction algorithms. We also show that the fan beam collimator gives higher signal-to-noise ratios than the parallel collimator for all the source positions analysed.

Evaluation of parallel and fan-beam data acquisition geometries and strategies for myocardial SPECT imaging

This study evaluates myocardial SPECT images obtained from parallel-hole (PH) and fan-beam (FB) collimator geometries using both circular-orbit (CO) and noncircular-orbit (NCO) acquisitions. A newly developed 4-D NURBS-based cardiac-torso (NCAT) phantom was used to simulate the /sup 99m/Tc-sestamibi uptakes in human torso with myocardial defects in the left ventricular (LV) wall. Two phantoms were generated to simulate patients with thick and thin body builds. Projection data including the effects of attenuation, collimator-detector response and scatter were generated using SIMSET Monte Carlo simulations. A large number of photon histories were generated such that the projection data were close to noise free. Poisson noise fluctuations were then added to simulate the count densities found in clinical data. Noise-free and noisy projection data were reconstructed using the iterative OS-EM reconstruction algorithm with attenuation compensation. The reconstructed images from noisy projection data show that the noise levels are lower for the FB as compared to the PH collimator due to increase in detected counts. The NCO acquisition method provides slightly better resolution and small improvement in defect contrast as compared to the CO acquisition method in noise-free reconstructed images. Despite lower projection counts the NCO shows the same noise level as the CO in the attenuation corrected reconstruction images. The results from the channelized Hotelling observer (CHO) study show that FB collimator is superior to PH collimator in myocardial defect detection, but the NCO shows no statistical significant difference from the CO for either PH or FB collimator. In conclusion, our results indicate that data acquisition using NCO makes a very small improvement in the resolution over CO for myocardial SPECT imaging. This small improvement does not make a significant difference on myocardial defect detection. However, an FB collimator provides better defect detection than a PH collimator with similar spatial resolution for myocardial SPECT imaging.

Speed up of an analytical algorithm for nonuniform attenuation correction by using PC video/graphics card architecture

A major task in quantitative SPECT (single photon emission computed tomography) reconstruction is compensation for object-specific attenuation, which is usually nonuniform. Mathematically this task is expressed as the inversion of the attenuated Radon transform. Novikov had derived an explicit inversion formula for the attenuated Radon transform for parallel-beam collimation geometry. In our previous work, we extended his work to variable focusing fan-beam (VFF) collimators. A ray-driven analytical inversion formula for VFF reconstruction with nonuniform attenuation was derived. The drawback of ray-driven methods is that they are time consuming. In this work, we proposed a fast implementation method, which includes algorithm optimization and acceleration by the texture-mapping architecture of PC graphics/video card. We further investigated the noise properties and associated artifacts of the analytical inversion formula. The artifacts were remarkably reduced when more projections were sampled to mitigate the problem of wide bandwidth of the discrete Hilbert transform. The reconstruction from noisy data demonstrated the accuracy and robustness of the presented ray-driven analytical inversion formula with dramatic speed acceleration by the PC graphics/video card.

Imaging serotonin transporters with 123I-ADAM brain SPECT in healthy non-human primates.

Serotonin transporters (SERTs) play a major role in modulating serotonergic neuronal function and are the target of many antidepressant drugs used in neuro-psychiatric disorders. To gain more information on the temporal distribution of SERTs, 2-([2-([dimethylamino]methyl)phenoxyl]thio)-5-[I]iodophenylamine (I-ADAM) single photon emission computed tomography (SPECT) was utilized in an in vivo imaging study using non-human primates. Two female monkeys (Macaca cyclopis) were studied. Eight brain SPECT imaging examinations, each 30 min in duration, were obtained after injection of 185 MBq of I-ADAM. Images were obtained using a dual-head gamma camera equipped with ultra-high resolution fan-beam collimators. In addition to visual inspection, the radio-uptake and specific uptake ratios (SURs) of midbrain (MB), thalamus (TH), striatum (ST), temporal and frontal cortices and the whole brain in reference to the corresponding magnetic resonance image at the eight time points were measured. The SUR of MB, using cerebellum (CB) as the reference tissue, was calculated as (MB - CB)/CB, in mean counts/pixel. The SURs of the other brain regions were similarly measured. There was relatively high uptake of I-ADAM in the MB and TH, moderate uptake in ST, lower uptake in the cerebral cortex, and almost no uptake in the CB. The image of MB could be easily identified at the first 30 min time point. It appeared that the SURs of MB, TH and ST reached equilibrium around 210 min after injection. No adverse reactions of the primates were found during and after imaging. Brain distribution of I-ADAM in the primate appeared consistent with the known distribution of SERTs. In conjunction with a high SUR in MB, TH and ST, we speculate that I-ADAM may be a potential radioligand for SPECT studies of serotonin transporters in humans.

An analytical inversion of the nonuniformly attenuated Radon transform with variable focal-length fan-beam collimators

Single photon emission computed tomography (SPECT) is based on the measurement of radiation emitted by a radiotracer injected into the patient. Because of photoelectric absorption and Compton scatter, the gamma photons are attenuated inside the body before arriving at the detector. A quantitative reconstruction must consider the attenuation, which is usually nonuniform. Novikov has derived an explicit inversion formula for the nonuniformly attenuated Radon transform for SPECT reconstruction of parallel-beam collimated projections. In this paper, we extend his research to variable focal-length fan-beam VFF collimator geometry. A ray-driven analytical formula for VFF reconstruction with nonuniform attenuation was derived. As a unified framework, this formula can be used for parallel-beam, fan-beam, and VFF collimators. Its accuracy is demonstrated by computer simulation experiments.

Analytical compensation for spatially variant detector response in SPECT with varying focal-length fan-beam collimators

The authors present a new method to compensate for the spatially variant detector response in single photon emission computed tomography (SPECT) with varying focal-length fan-beam (VFF) geometry. In this method, the deblurred projection is explicitly expressed as an integral of the collimator blurred data and the fully calibrated system kernel matrix with certain transforms. It needs a little more computation time than the conventional filtering techniques but provides much more accurate compensation. This method also represents a unified framework for detector response compensation in parallel-hole fan-beam and VFF SPECT with a circular scan orbit.

Regional binding index of the radiolabeled selective 5-HT2A antagonist 123I-5-I-R91150 in the normal canine brain imaged with single photon emission computed tomography.

The pattern of the specific 5-HT2A (5-hydroxytryptamine 2A receptor) antagonist 123I-5-I-R91150 was measured in 10 healthy dogs without neurologic and behavior abnormalities. Eight cortical regions (left and right fronto-, temporo-, parieto-, and occipitocortical area), one global subcortical region (including the thalamic system) were compared with a reference region lacking receptors; that is, the cerebellum. The 123I labeled radioligand was injected intravenously 100-200 minutes before acquisition. Both transmission and emission data were obtained with a triple head gamma camera equipped with high-resolution fanbeam collimators. The emission data were corrected for scatter and attenuation. To delineate different cerebral regions more accurately, the regions of interest (ROI) defined in a former study on brain perfusion measured with 99mTc-ethyl cysteinate dimer (ECD) in the same dogs were used. The co-registration of the 99mTc-ECD and the 123I-5-I-R91150, obtained from each dog, was realized with the help of corresponding transmission maps. By normalizing each regional cerebral activity to the activity observed in the cerebellum, the regional radioactivity (binding index) could be relatively quantified. Highest brain uptake was noted in the frontocortical brain areas (right: 1.85, left: 1.89), followed by the temporocortical region (right: 1.58, left: 1.56). Least uptake was noted in the more caudal and middle brain regions [occipito- (right: 1.46, left: 1.41), parietocortical (right: 1.30, left: 1.26), and striatal region (1.19)]. No gender nor age influence was noted in this series. The 123I labeled serotonin-2A receptor ligand seems to have similar cortical binding in the normal canine brain, as shown in humans and other animal species. A frontocortical to occipitocortical (rostrocaudal) binding index gradient was identified within the dog, which has not been seen in imaging studies from humans and other animal species. The significance of these results will need further investigation. This normative data can be used to compare regional brain uptake of the 123I-radioligand to dogs with behavioral disorders related to the serotonergic system, in future studies.

Dual SPECT of dopamine system using [ 99mTc]TRODAT-1 and [ 123I]IBZM in normal and 6-OHDA-lesioned formosan rock monkeys

Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by a severe loss of the dopaminergic neurons in the substantia nigra pars compacta. In this study, we evaluated pre- and post-synaptic binding sites of the dopamine system in three normal and one parkinsonian monkeys using simultaneous [ 99mTc]TRODAT-1 and [ 123I]IBZM imaging. The parkinsonian monkey was induced by injecting 6-hydroxydopamine (6-OHDA) bilaterally into the medial forebrain bundle under MRI guidance. [ 99mTc]TRODAT-1 (targeting dopamine transporters) and [ 123I]IBZM (targeting D 2/D 3 receptors) were administered almost simultaneously and the SPECT images were acquired over 4 h using a dual-headed gamma camera equipped with ultra-high resolution fan-beam collimators. Data were obtained using energy window of 15% centered on 140 keV for 99mTc in conjunction with 10% asymmetric energy window in a lower bound at 159 keV for 123I. Single SPECT studies of [ 99mTc]TRODAT-1 and [ 123I]IBZM were also performed. We found a comparable image quality and uptake ratios between single- and dual-isotope studies. There are higher TRODAT-1 uptakes in the control monkeys than the 6-OHDA-lesioned monkey. The uptake of [ 123I] IBZM showed no significant difference between controls and 6-OHDA-lesioned monkey. Our results suggest that dual isotope imaging using [ 99mTc]TRODAT-1 and [ 123I]IBZM may be a useful means in evaluating the changes of both pre- and post-synaptic dopamine system in a primate model of parkinsonism.

Demonstration of energy-coded Compton scatter tomography with fan beams for one-sided inspection

An instrument is demonstrated whereby radiographic images of a sample's electron density are compiled from the information encoded in the energy spectra of gamma rays backscattered from one side of the sample. It is assumed that access is restricted to only one surface of the object under inspection. Use of energy coding allows imaging in a fan beam rather than independent interrogation of individual volume elements. The Multiplexed Compton Scatter Tomograph instrument consists of an array of high-energy-resolution detectors and fan beam collimators. Instrument signals are converted to electron density images using a penalized weighted least squares image reconstruction algorithm coupled with a deterministic system model that includes effects of Doppler broadening. The proof-of-principle instrument is demonstrated on aluminum samples. In an 8 mm thick sample with a 4 mm void in its center, contrast recovery of 90% is achieved. In a 10 mm thick sample with a 3 mm void at the back about 85% of the contrast is recovered.

Evaluation of the geometric, scatter, and septal penetration components in fan-beam collimators using Monte Carlo simulation

The quantitative analysis of single-photon emission computed tomography data requires an accurate determination of the collimator point spread function (PSF). The aim of this paper is to characterize fan-beam collimators' PSFs by using Monte Carlo simulation. Given a particular collimator configuration, a detailed hexagonal hole array was generated and information describing its geometry was stored in a lookup table. When a photon crossed the collimator front plane, a hole array was placed around its impact position using this table. Each photon was then tracked up to the detector surface by using the Monte Carlo code PENELOPE and its associated geometry handling routines. Particle counters were defined that score the probability of impact on the detector as a function of the final photon position. Four sets of counters were employed so as to differentiate contributions to the geometric, septal penetration, coherent (Rayleigh), and incoherent (Compton) scatter components. Furthermore, sensitivity quantification was calculated for different source locations. Monte Carlo results were compared with sensitivity values obtained experimentally, and good agreement was found. Our results show that for /sup 99m/Tc imaging, the geometric component represents about 95% of the fan-beam PSF, whereas the incoherent scattering component is negligible.

Geometrical response modeling in fan-beam collimators-a numerical simulation

Iterative reconstruction algorithms require modeling of the point spread function (PSF) of the collimator in order to correct the loss of resolution in single photon emission computed tomography (SPECT). A method for modeling the geometric response of a fan-beam collimator is proposed. The geometric PSF is obtained by numerical simulation based on ray-tracing techniques. The versatility of the numerical simulation allows us to generate the array of holes with a distance between them varying as an angular function. This enables us to reproduce the real distribution of holes. The integral of the simulated PSF was calculated in order to determine the relative sensitivity and Gaussian functions were fitted to obtain the resolution. Functions depending on the intrinsic collimator parameters and on the coordinates of the source were fitted to the individual PSF. To validate the functions proposed, a comparison between the predicted values and those obtained from experimental measures was made. Our findings show that the predicted values for relative sensitivity and resolution accurately match the experimental ones, with a correlation coefficient of 0.992 for the sensitivity and 0.994 for the resolution.

Improved coronary disease detection with quantitative attenuation-corrected Tl-201 images

Background. The normal distribution of myocardial tracer activity is different in attenuation-corrected images compared with uncorrected images. We therefore postulated that quantitation of attenuation-corrected thallium 201 images with direct comparison to a database of healthy subjects could improve detection of coronary artery stenoses. Methods and Results. In 49 patients with angiographic evidence of coronary artery disease and 69 patients with a less than 5% likelihood of coronary artery disease, tomographic Tl-201 myocardial imaging was performed by means of a triple-headed camera with fan-beam collimators and the images were processed with attenuation correction, with attenuation and Compton scatter correction, and without correction. Images were subjectively interpreted in a blinded manner. From the group of 69 patients with a low likelihood of coronary disease, scintigraphic data from the first 20 men and the first 20 women were used to generate normal reference ranges for each of the 3 image-processing methods. Data from the 49 patients with angiographic coronary disease and from the remaining 29 patients with a low likelihood of coronary disease were then analyzed quantitatively in comparison to the gender-matched normal databases. With visual interpretation, attenuation-corrected images yielded an improved detection rate compared with uncorrected images (79% vs 46%, P = .008) for 70% or greater left anterior descending coronary artery stenoses. Attenuation-corrected images also provided an improved normalcy rate for the right coronary artery territory (91% vs 75%, P = .006) and an improved normalcy rate when the images were analyzed overall by patient (88% vs 74%, P = .009). With quantitative analysis, attenuation correction yielded an improved detection rate for patients with a significant coronary artery stenosis compared with uncorrected images (90% vs 71%, P = .016). Defect extent was also increased by attenuation correction. Compared with attenuation correction alone, the addition of scatter correction yielded no incremental improvement in detection rate or normalcy rate. Attenuation-scatter correction did produce a nonsignificant trend toward detection of more of the total number of stenotic arteries compared with uncorrected images (60 vs 48 of 79 arteries). Conclusions. With the use of a triple-headed camera with fan-beam collimation, visual analysis of attenuation-corrected Tl-201 images improved detection of left anterior descending coronary artery disease without loss of detection in other coronary territories. Furthermore, quantitative analysis of attenuation-corrected Tl-201 images improved the overall detection rate for coronary artery disease in patients without compromise of the normalcy rate. (J Nucl Cardiol 2002;9:52-61.)

Regional brain perfusion in 10 normal dogs measured using Technetium-99m ethyl cysteinate dimer spect.

Single photon emission computed tomography (SPECT) of the brain using perfusion tracers allows estimation of regional brain perfusion. This allows in vivo examination of brain function in the setting of neuropsychologic and pathophysiologic changes. However functional imaging data on brain perfusion in dogs are limited. Hence, the aim of this study was to determine the scintigraphic regional perfusion pattern of the normal canine brain. Ten healthy shepherd type dogs were injected with 925 MBq Technetium-99m ethyl cysteinate (ECD) 20 minutes before the examination. Acquisition was performed using a triple head gamma camera equipped with fanbeam collimators. Uniform attenuation correction and triple energy window correction were applied. Computed tomographic images were obtained from the same dogs, reoriented along the orbito-meatal axis and SPECT perfusion data were coregistered to the CT-volume data. Based on morphological and suggested brain divisions, regions-of-interest (ROIs) were defined for the bilateral frontocerebral, temporocerebral, parietocerebral, occipitocerebral, cerebellar, thalamic, and striatal area. Regional count density was normalized on total counts. All dogs had the highest uptake in the thalamic/striatal area compared to a rather homogeneous cerebral uptake. No significant left/right count differences were found, but a rostro-caudal gradient (+12-13%) was present. In this group, age and gender did not influence the perfusion pattern.

Transmission image reconstruction and redundant information in SPECT with asymmetric fanbeam collimation

Two novel approaches for the reconstruction of asymmetric fanbeam transmission computed tomography data are discussed. The first, called the hybrid approach, involves a Fourier-based rebinning of the fanbeam data into parallel-beam data, Reconstruction then proceeds by use of filtered backprojection (FBP). The second approach, called generalized fanbeam filtered backprojection (GFFBP), involves direct fanbeam FBP reconstruction of a modified fanbeam sinogram. In both cases, the data are multiplied by weight functions that seek to appropriately normalize redundant data while exploiting them for noise reduction. The GFFBP approach is found to have resolution-noise tradeoffs superior to those of the hybrid approach for low degrees of smoothing, although for the higher levels of smoothing likely to be of interest in practical situations, the difference between the approaches is negligible. However, GFFBP's distance-dependent fanbeam backprojection factor also produced a high-intensity peripheral artifact that impinged slightly upon the object of interest. Because it ultimately makes use of parallel-beam FBP for reconstruction, the hybrid approach avoids this artifact.

Importance of the choice of the collimator for the detection ofsmall lesions in scintimammography: a phantom study

99mTc methoxyisobutylisonitrile planar scintimammography (SMM) is mostly performedusing low-energy high-resolution (LEHR) parallel collimators. We studiedwhether using a different collimator could improve the detection of small(<1.5 cm) lesions for which SMM sensitivity is poor. Thirty fourbreast phantom configurations were considered, either with hot spheressimulating lesions or without any spheres. For each configuration, fourplanar acquisitions were performed using LEHR, low-energy ultrahigh-resolution (LEUHR), high-resolution fan-beam (HRFB) and ultrahigh-resolution fan-beam (UHRFB) collimators. Images corresponding to the20% and 10% energy windows and to the Jaszczak subtraction werecalculated. A database including 156 borderline images was derived. Aftertraining, 10 observers scored the images for the presence of a sphere. Theperformances in sphere detection were studied using receiver operatingcharacteristic (ROC) analysis. For all types of image, the area under theROC curve was highest with the UHRFB collimator and lowest with either theLEUHR or the HRFB collimator. For the 10% energy window imagesconventionally used in SMM, the detection sensitivities averaged 91%,73%, 60% and 55% for the UHRFB, LEHR, HRFB and LEUHR collimatorsrespectively, for the same specificity of 64%. We conclude that detectionof small tumours in planar SMM might be significantly improved by using aUHRFB collimator instead of an LEHR collimator.

Diagnostic accuracy of simultaneous acquisition of transmission and emission data with technetium-99m transmission source on thallium-201 myocardial SPECT.

This study evaluates not only the clinical usefulness but also the problems in attenuation correction for thallium-201 (Tl-201) myocardial SPECT by means of simultaneous transmission and emission data acquisition in the detection of coronary artery disease (CAD). A three-detector SPECT system equipped with a Tc-99m line source and fan-beam collimators was used for simultaneous transmission and emission data acquisition for Tl-201 myocardial SPECT in 73 patients (18 patients for normal database and 55 patients for the evaluation of diagnostic accuracy). Attenuation-corrected (AC) images and non-attenuation-corrected (NC) images were reconstructed with an iterative maximum-likelihood estimation-corrected (ML-EM) algorithm. Both sets of images were reoriented into the short axis. Normal database polar maps were constructed from the AC and NC images for quantitative analysis. There was a significant difference in specificity between NC and AC images in the RCA territory and those in specificity and accuracy in the LCX territory. There was no significant difference in sensitivity found between NC and AC images in either territory, but sensitivity in both territories tended to decrease with attenuation correction. In the LAD territory, there were various changes in sensitivity and specificity observed with attenuation correction in cases with each quantitative criterion. Diagnostic performance of significant stenosis in the RCA and LCX territories quantitatively improved with attenuation correction because of an increase in specificity, but no significant improvement in diagnostic performance was obtained in the LAD territory with attenuation correction. We recommend combined interpretation of AC and NC images and careful evaluation of any SPECT image by means of transmission computed tomography.

Quantitation of neuroreceptors: a need for better SPECT imaging

Existing methods to determine the binding potential in brain receptor and transporter studies with single-photon emission computed tomography (SPECT) show a marked dependence on the physical performance of the scanner and on the regions of interest (ROIs) drawn. We examined the influence of the physical performance on the binding potential. The authors tested three gamma cameras (single-, dual- and triple-head) and two reconstruction techniques (conventional filtered back-projection and iterative reconstruction method) to determine the binding potential in brain receptor studies. Both human and organ-like phantom studies were performed. The binding potential was completely dependent on the imaging resolution, on the reconstruction technique used and on the ROIs drawn. The results of the phantom study revealed that the striatum-to-cerebellum ratio was only 37% of the actual one for the single-head, 63% for the dual-head and 72% for the triple-head gamma camera. The multiple head gamma camera with fan-beam collimators and iterative reconstruction with the attenuation based scatter correction is an imaging system of choice which introduces clinically more relevant images and is able to distinguish smaller differences in radioactivity at early stage of disease. However, for quantitative purposes the improvement without the use of the recovery coefficient is not great enough to restore the binding potentials to those of the true ones.

Investigation of the vertebrobasilar arterial system in vertigo by vestibulocochlear test, SPECT and angiography

Objective: Perfusion of the areas of cochleovestibular structures was investigated using Tc-99m HMPAO single photon emission computerized tomography (SPECT) in the young patients with vertigo presumed to have peripheral vestibular abnormalities and compared the results with cochleovestibular test results, and vertebrobasilar angiography findings. Methods and patients: The study was performed on 20 patients with vertigo and 17 control subjects who had performed vertebral arteriography due to another disease. The patients with vertigo presumed to be on peripheral vestibular disorder were included in the study. Systemic and otorhinolaryngological examination were carried out. Audiovestibular function was evaluated in all patients and control subjects. Brain SPECT was performed with a one-head rotating gamma camera system from 64 projections into a 64×64 matrix, using low energy and high-resolution fan beam collimators with a sampling time of 30 s. The images taken from cerebellum, temporal, parietal and occipital lobes which are supplied by a vertebrobasilar system were compared visually with the other side and asymmetry in the perfusion was searched. After cochleovestibular tests and laboratory had been completed, vertebral arteriography was performed bilaterally in all patients. Results of digital subtraction angiography (DSA) and the vestibulocochlear test were compared with images of SPECT. Data analysis was done with Yates χ 2 and percentages of sensitivity and specificity were calculated. Results: There was vestibular test and cochlear test abnormalities in eight and ten of the patients with vertigo, respectively. SPECT imaging showed hypoperfusion and decreased regional blood flow in 12 of the patients. There was statistically significant differences between the study and control groups in SPECT ( P<0.05). Five patients showed abnormal findings in DSA examinations. Conclusion: It is believed that SPECT may be helpful in the detection of vascular pathology in the patients with vertigo.

Transmission CT reconstruction for offset fan beam collimation

Offset fan beam collimation has shown to be an effective means of obtaining transmission CT (TCT) data for SPECT attenuation correction. This paper compared three methods for handling the truncation problem and reconstructing this TCT data: iterative Os-FM and filtered backprojection (FBP) with two different pre-processing approaches. The first approach (FBP1) extends the fan beam projection by sinogram interpolation, and the second (FBP2) uses parallel beam rebinning. With a TCT spatial resolution phantom, FBP2 demonstrated a small degree of resolution loss which was equalized across methods after post-reconstruction filtering. With a thorax phantom, all methods performed adequately at high count level; however, at low count level, FBPI showed degraded image quality due to its handling of zero count projection bins. This finding was corroborated in patient images. It is concluded that at practical count levels, the iterative OS-FM method offers a clear advantage in TCT image quality compared to FBP methods.

Regional cerebral distribution of [Tc-99m] hexylmethylpropylene amineoxine in patients with progressive aphasia.

Progressive aphasia is a prominent clinical feature of several neurodegenerative disorders. This study used hexylmethylpropylene amineoxine (HMPAO) single photon emission computed tomography (SPECT) to estimate blood flow in areas of the brain that mediate language in patients with progressive aphasia and matched control subjects. The patient population consisted of four men and 12 women with a mean +/- SD age of 69.1 +/- 7.6. Of these, eight were classified as having a nonfluent form of aphasia, whereas the other eight had a fluent form. The patients were compared to 16 healthy volunteers who were studied with an identical protocol. The SPECT images of the brain were acquired with 740 MBq (20 mCi) of Tc-99m-labeled HMPAO on a triple-headed gamma camera equipped with fan beam collimators. The images were analyzed with a set of standardized templates. Mean counts per pixel in 33 regions of interest were compared to the mean counts in the whole supratentorial brain. A laterality index was determined for homotopic regions using the equation 100 x (R - L)/(1/2 x (R - L)). Patients with progressive aphasia had several regions of significantly decreased HMPAO uptake in the left cortex when compared to the homotopic regions on the right. The most prominent deficit in the nonfluent group, as determined by the laterality index, were found in the left dorsolateral prefrontal region (p < 0.05), whereas the most prominent deficits in the group with fluent aphasia were found in the left temporal and parietal language centers (p < 0.05). The left subcortical nuclei were differentially affected, particularly in patients with nonfluent aphasia. The HMPAO SPECT indicates that multiple regions of the left hemisphere are dysfunctional in patients with progressive aphasia. The pattern of perfusion deficits in patients with fluent aphasia appears to be distinct from the pattern in patients with nonfluent aphasia.