Evaluation of a 3D reconstruction algorithm for multi-slice PET scanners

Brain activation associated with changes in heart rate, heart rate variability, and plasma catecholamines during rectal distention

OBJECTIVE

To test the hypothesis that gut stimulation provokes autonomic arousal via activation of regional cerebral cortices. How the human brain processes interoceptive signals and forms initial autonomic arousal is one of the key questions to be answered in research on emotion.

METHODS

Twelve healthy males participated in this study. A barostat bag was inserted in the rectum and intermittently inflated with 0, 20, or 40 mm Hg at random for 80 seconds. H(2)(15)O positron emission tomography (PET) of the brain, electrocardiography, and blood sampling for catecholamines were performed. Changes in regional cerebral blood flow were interpreted using statistical parametric mapping.

RESULTS

Rectal distention with 40 mm Hg induced a significant increase in heart rate, low frequency (LF)/high frequency (HF) ratio of heart rate variability, and plasma adrenaline. Activated brain areas that were associated with increased heart rate during rectal distention were the right insula, right operculum, right dorsolateral prefrontal cortex, putamen, thalamus, periaqueductal gray, and cerebellum (p < .001, uncorrected), whereas those that were associated with an increased LF/HF ratio were the bilateral insula, putamen, thalamus, midbrain, pons, and cerebellum (p < .001, uncorrected). Activated brain areas that were associated with increased plasma adrenaline were the right insula, right orbitofrontal cortex, right parahippocampal gyrus, putamen, thalamus, periaqueductal gray, pons, and cerebellum (p < .001, uncorrected).

CONCLUSION

Our results suggest that the right insula and the related body mapping regions may form the functional module of sympathetic arousal in response to gut stimulation.

Classical conditioned response of rectosigmoid motility and regional cerebral activity in humans

The relationship between the central processes of classical conditioning and conditioned responses of the gastrointestinal function is incompletely understood in humans. We tested the hypothesis that the rectosigmoid motility becomes conditioned with anticipatory painful somatosensory stimulus and that characteristic brain areas become activated during anticipation. In nine right-handed healthy male subjects, a loud buzzer (CS, conditional stimulus) was paired with painful transcutaneus electrical nerve stimulation to the right hand (unconditional stimulus). Rectosigmoid muscle tone measured by the barostat as the intrabag volume, phasic contractions of the bowel measured as the number of phasic volume events (PVEs), and regional cerebral blood flow assessed by positron emission tomography (PET), were measured before and after conditioning. Following conditional trials, the bag volume after CS alone did not show significant changes between before and after the stimulus, but the number of PVEs after 2-minute interval of the CS alone was significantly greater than that before the stimulus (P < 0.05). The PET data showed the conditioning elicited significant cerebral activation of the prefrontal, anterior cingulate, parietal and insula cortices (P < or = 0.001, uncorrected). Rectosigmoid motility can be conditioned with increase in phasic contractions in humans.

Clinical validation of fully 3-D versus 2.5-D RAMLA reconstruction on the Philips-ADAC CPET PET scanner

INTRODUCTION

Fully three-dimensional (3-D) reconstruction algorithms have the potential to better utilize PET data acquired in true 3-D acquisition modes, as compared to 2-D approaches involving data reduction through rebinning procedures followed by a 2-D or 2.5-D reconstruction. Our previous studies, involving simulated and real phantom data from PET scanners working purely in 3-D acquisition mode, showed that the fully 3-D reconstruction approaches bring appreciable improvement on physical measures and visual image quality as well. Reconstruction procedures utilized in those studies were based upon the row action maximum likelihood algorithm (RAMLA) implemented on our clinical scanners. The purpose of this study was to investigate if the fully 3-D RAMLA reconstruction software brings an appreciable improvement in clinical image quality and reader confidence.

MATERIALS AND METHODS

Ninety patient scans acquired on the Philips CPET scanner at our institution were reconstructed using 2.5-D RAMLA and 3-D RAMLA with differing reconstruction parameters. These scans were blindly presented to four experienced PET scan readers who graded various aspects of image quality.

RESULTS

Our study concluded that 3-D RAMLA on the CPET scanner reduces artifacts and image noise and improves clinical confidence in interpreting PET images.

CONCLUSION

This study led to the routine use of 3-D RAMLA reconstruction on the CPET scanner at our institution.

Brain activity during distention of the descending colon in humans

Brain-gut interaction is considered to be a major factor in the pathophysiology of irritable bowel syndrome. However, only limited information has been provided on the influence of gastrointestinal tract stimulation on the brain. Our aim in this study was to determine the specific regions of the brain that are responsible for visceral perception and emotion provoked by distention of the descending colon in humans. Fifteen healthy males aged 22 +/- 1 participated in this study. Using a colonoscope, a balloon was inserted into the descending colon of each subject. After sham stimulation, the colon was randomly stimulated with bag pressures of 20 and 40 mmHg, and regional cerebral blood flow was measured by [(15)O] positron emission tomography. The subjects were asked to report visceral perception and emotion using an ordinate scale of 0-10. Colonic distention pressure dependently induced visceral perception and emotion, which significantly correlated with activation of specific regions of the brain including the prefrontal, anterior cingulate, parietal cortices, insula, pons, and the cerebellum. In conclusion, distention of the descending colon induces visceral perception and emotion. These changes significantly correlate with activation of specific regions in the brain including the limbic system and the association cortex, especially the prefrontal cortex.

Mapping energy metabolism in jaw and tongue muscles during chewing

Investigators have used positron emission tomography with 18F-fluoro-D-deoxyglucose to obtain information not only for the diagnosis of cancers, but also for researching physiology in skeletal muscles. The aim of this study was to evaluate the activities of the jaw and tongue muscles during gum-chewing. Five volunteers aged 32-61 years were studied by positron emission tomography. They were requested to chew two pieces of chewing gum for 30 min after intravenous injection of 18F-fluoro-D-deoxyglucose. 18F-fluoro-D-deoxyglucose uptake in the intrinsic tongue muscle was significantly (p < 0.05) higher than that in the masseter, temporal, and medial pterygoid muscles. Heterogeneous uptake of 18F-fluoro-D-deoxyglucose was observed in the masticatory muscles. In addition, the tongue exhibited higher activity than the masticatory muscles. In conclusion, positron emission tomography with 18F-fluoro-D-deoxyglucose appeared to be a useful technique for investigating the physiologic activities of the skeletal muscles, which have been difficult to examine by conventional methods.

Oral 18F-fluoro-2-deoxyglucose for primate PET studies without behavioral restraint: demonstration of principle

We describe a method of orally administering 18F-fluoro-2-deoxyglucose (FDG) for positron emission tomography (PET) scans to determine local cerebral metabolic rates for glucose (LCMRGlc), normalized to that of whole brain, in fully conscious, non-restrained primates. Oral FDG-PET studies were performed in both non-restrained and chaired monkeys, and in one human where results could be compared with traditional intravenous FDG administration. The oral route of FDG administration gave images and whole brain-normalized PET LCMRGlc results comparable to those obtained by the intravenous route. This oral FDG-PET method may provide a useful means by which to obtain measures of LCMRGlcs for brain structures, relative to each other, in non-restrained, non-drugged primates in field and laboratory studies. This method might also have clinical applications for PET studies of children.

Three-dimensional image reconstruction for PET by multi-slice rebinning and axial image filtering

A fast method is described for reconstructing volume images from three-dimensional (3D) coincidence data in positron emission tomography (PET). The reconstruction method makes use of all coincidence data acquired by high-sensitivity PET systems that do not have inter-slice absorbers (septa) to restrict the axial acceptance angle. The reconstruction method requires only a small amount of storage and computation, making it well suited for dynamic and whole-body studies. The method consists of three steps: (i) rebinning of coincidence data into a stack of 2D sinograms; (ii) slice-by-slice reconstruction of the sinogram associated with each slice to produce a preliminary 3D image having strong blurring in the axial (z) direction, but with different blurring at different z positions; and (iii) spatially variant filtering of the 3D image in the axial direction (i.e. 1D filtering in z for each x-y column) to produce the final image. The first step involves a new form of the rebinning operation in which multiple sinograms are incremented for each oblique coincidence line (multi-slice rebinning). The axial filtering step is formulated and implemented using the singular value decomposition (SVD). The method has been applied successfully to simulated data and to measured data for different kinds of phantom (multiple point sources, multiple discs, a cylinder with cold spheres, and a 3D brain phantom).

An evaluation of exact and approximate 3-D reconstruction algorithms for a high-resolution, small-animal PET scanner

MicroPET is a low-cost, high-resolution positron emission tomography (PET) scanner designed for imaging small animals. MicroPET operates exclusively without septa, acquiring fully three-dimensional (3-D) data sets. The performance of the projection-reprojection (3DRP), variable axial rebinning (VARB), single slice rebinning (SSRB), and Fourier rebinning (FORE) methods for reconstruction of microPET data were evaluated. The algorithms were compared with respect to resolution, noise variance, and reconstruction time. Results suggested that the 3DRP algorithm gives the best combination of resolution and noise performance in 9 min of reconstruction time on a Sun UltraSparc I workstation. The FORE algorithm provided the most acceptable accelerated method of reconstruction, giving similar resolution performance with a 10%-20% degradation in noise variance in under 2 min. Significant degradation in the axial resolution was measured with the VARB and SSRB methods, offsetting the decrease in reconstruction time achieved with those methods. In-plane angular mashing of the 3-D data before reconstruction led to a 50% reduction in reconstruction time but also introduced unacceptable tangential blurring artifacts. This thorough evaluation of analytical 3-D reconstruction techniques allowed for optimal selection of a reconstruction method for the diverse range of microPET applications.

Strategies to improve 3D whole-body PET image reconstruction

An algorithm is described for three-dimensional whole-body (3DWB) image reconstruction in positron emission tomography. For whole-body applications, improvements to the popular fixed-axial-acceptance-angle technique are achieved by combining axially adjacent projection data available with a long-axis data set. Time-consuming reprojection of unmeasured oblique lines of response is reduced or eliminated by axial overlap of bed positions, while pixel variance and reconstructed axial resolution are made more uniform by the overlap. Improvements in noise and resolution uniformity are accompanied by gains in reconstruction efficiency, and may be optimized against increased acquisition-time due to overlapping acquisition segments and reduced axial coverage. An 11-detector-ring overlap improves axial uniformity in coronal images of a long, uniform cylinder from 23% to 8% with uniform axial resolution. Associated with a 37% improvement in reconstruction time is a 34% reduction in axial coverage for four bed positions. A smaller degree of overlap is found to provide the best trade-off between image uniformity, total scan duration, and reconstruction time because of a proportionally greater reduction in reprojected lines of response. Using a sample optimization scheme, we find a three-ring overlap is best for a 60 cm axial field of view and a five-ring overlap for an 80 cm axial field of view.

Functional MR and PET imaging of rolandic and visual cortices for neurosurgical planning

Magnetic resonance (MR) imaging has recently been used to demonstrate physiological activation of the human brain. This development is of considerable interest to the neurosurgeon planning procedures near brain regions involving specific functions. In the present study, rolandic and visual cortices were imaged with a conventional 1.5-tesla clinical MR imager using a spoiled gradient-recalled acquisition in the steady state sequence. Two patients, one with a right frontal astrocytoma and the other with a left parietal meningioma, underwent MR imaging of rolandic cortex while performing a repetitive finger apposition task. Two patients with complex partial seizures referable to the temporal and occipital regions underwent MR imaging of visual cortex while exposed to repetitive photic stimulation (8.3 Hz). Significant signal intensity changes up to 15% between the activation and rest conditions were observed near the surgical targets at the expected anatomical location of the rolandic and visual cortices. In two of these cases activation measured by MR was compared and found similar to the activation measured at the same plane by H2(15)O positron emission tomography (PET). These results suggest that functional MR and PET techniques can be used to obtain preoperative brain mapping in individual patients considered for neurosurgical procedures.

Methodological aspects of brain activation studies: cerebral blood flow determined with [15O]butanol and positron emission tomography

In this methodological study, a procedure for measuring regional CBF (rCBF) with positron emission tomography and 15O-labelled tracers is optimized. Four healthy volunteers were subjected to eight studies with use of [15O]butanol as a tracer: four times while reading aloud and four times while reading silently from a phonologically balanced list of single words. The gain from these repeated intra-individual studies of the same activation state (fractionation) was demonstrated in terms of noise-equivalent counts in a phantom study. A computerized brain atlas was used to reformat the images to a common anatomical representation, thereby minimizing the effects of inter- and intra-individual anatomical and positional variations. This allowed the formation of inter- and intra-individual average subtraction images with error estimates. Differences between the two activation states were detected with use of an exploratory significance map based on a paired Student's t test. The results compared well with Friston's method of determining levels of statistical significance. No difference was obtained when comparing results from rCBF images and images generated from measurement of uptake of the tracer. The paradigm chosen for activation was shown to yield a constant activation level during the repeated measurements (i.e., no habituation).

Signal-to-noise ratios for attenuation correction in PET imaging

Attenuation correction is an important part of accurate image reconstruction in positron tomography. The usual correction method involves direct measurement of attenuation correction factors (ACFs). A reconstruct-reproject method, which has been suggested as providing superior noise properties, is sometimes employed; an attenuation image is first reconstructed from the measurement and then ACFs are obtained by reprojection through this image. Here the authors present a model which follows the signal-to-noise ratio (SNR) through the attenuation correction by both the direct and reconstruct-reproject methods. This model is applicable to both 2D and 3D imaging geometry, but applies to the central elements of emission and transmission objects with circular symmetry and constant amplitude. For this simplified geometry, the model predicts that the SNR of the emission image following attenuation correction is the same for both direct and reconstruct-reproject methods, although the SNRs of the ACFs are themselves substantially different. The authors also present the measured SNR at the various steps of attenuation correction for both the direct and reconstruct-reproject methods using simulated transmission and emission data. The measured SNRs agree with the model; no significant difference between the direct and reconstruct-reproject SNRs was observed.

Improved detection of focal cerebral blood flow changes using three-dimensional positron emission tomography

Removal of the interplane septa and configuration of a typical multislice PET scanner to accept all possible coincidence lines of response leads to a fivefold increase in sensitivity. This can be of value in regional CBF studies using bolus 15O-labeled water injections, allowing the injected dose to be reduced by a factor of 4, while maintaining the same number of noise equivalent counts. Thus, for a given cumulative dose limit, four times as many studies can be performed in a single subject. Data from the three-dimensional Hoffman brain phantom, closely matched to count rates seen in human studies, show that for an identical cumulative dose, the noise in subtraction (stimulus minus baseline) images can be reduced by a factor of 2 using three-dimensional data acquisition, with appropriate fractionation of the dose. This improvement is dependent on axial position due to the sensitivity characteristics of three-dimensional scans; however, there is a significant gain in the signal-to-noise ratio (S/N) in all image planes. Studies performed in a human subject demonstrate how the factor of 2 gain in S/N leads to improved detectability of activation sites in PET subtraction images.