Conversion of brain SPECT images between different collimators and reconstruction processes for analysis using statistical parametric mapping

To make it possible to share a normal database in single photon emission computed tomography (SPECT) studies, we developed a new method for converting a SPECT image in one physical condition to that in another condition for data acquisition and reconstruction. A Hoffman 3-dimensional brain phantom experiment was conducted to determine systematic differences between collimators and reconstruction processes. SPECT images for the brain phantom were obtained using fan-beam collimators with scatter and attenuation corrections and using parallel-hole collimators without any correction. Dividing these two phantom images after anatomical standardization by Statistical Parametric Mapping 99 (SPM99) created a 3-dimensional conversion map. This conversion map was applied to convert an anatomically standardized SPECT image using parallel-hole collimators without any correction to that using fan-beam collimators with scatter and attenuation corrections in eleven subjects who underwent sequential SPECT measurements using different collimators after injection of 99mTc ethyl cysteinate dimer. The SPM99 demonstrated adequate validity of this conversion in comparative analyses of these sequential SPECT images with different collimators. This may be a promising approach for further sharing of a normal database in SPECT imaging between different cameras.     

Dual-isotope SPECT using simultaneous acquisition of 99mTc and 123I radioisotopes: a double-injection technique for peri-ictal functional neuroimaging.

The acquisition of multiple radiotracer studies at different time points during a neurological event permits the study of different functional activation states in humans. Peri-ictal SPECT is a promising technique for localizing the epileptogenic zone and would be enhanced by the ability to acquire sequentially coregistered ictal and postictal SPECT images of a single seizure. This study was designed to develop and validate an accurate method for the simultaneous acquisition of 99mTc and 123I SPECT images of the brain. METHODS: A multicompartment, transaxial Hoffman brain-slice phantom was filled with 99mTc, 123I or a 3:1 mixture of the two isotopes. Planar and SPECT images were acquired by a dual-head gamma camera system equipped with parallel and fanbeam collimators, respectively. Thirty-two energy windows (2 keV width) were acquired over the energy range 120-184 keV. From the planar data, the signal-to-noise characteristics and crosstalk were measured for each energy window and used to devise an energy window acquisition strategy that was then applied to the SPECT data. Three summed energy windows were created: a primary 99mTc image (130-146 keV), a primary 123I image (152-168 keV) and a secondary 99mTc crosstalk image (134-140 keV). A fraction (0.041) of the 99mTc crosstalk image was subtracted from the 123I image. No crosstalk correction was performed on the primary 99mTc image. RESULTS: (a) Planar images: results showed 1.3% crosstalk in the 123I image compared with 19.7% for a 10% asymmetric energy window alone. 123I crosstalk into the 99mTc window was 2.79% and was relatively constant with changes in the location of the 99mTc energy window. (b) Tomographic images: results showed 1.51% 99mTc crosstalk in the 123I image compared with 12.44% for the uncorrected image and 3.70% 123I crosstalk in the 99mTc image. CONCLUSION: An effective technique for the simultaneous acquisition of 99mTc and 123I radiotracer distributions in the brain has been developed and validated in a phantom model and should have clinical application in peri-ictal functional activation studies of the brain.     

Simulating technetium-99m cerebral perfusion studies with a three-dimensional Hoffman brain phantom: Collimator and filter selection in SPECT neuroimaging

The choice of a collimator and the selection of a filter can affect the quality of clinical SPECT images of the brain. The compromises that 4 different collimators make between spatial resolution and sensitivity were studied by imaging a three-dimensional Hoffman brain phantom. The planar data were acquired with each collimator on a three-headed SPECT system and were reconstructed with both a standard Butterworth filter and a Wiener pre-filter. The reconstructed images were then evaluated by specialists in nuclear medicine and were also quantitatively analyzed with specific regions of interest (ROI) in the brain. All observers preferred the Wiener filter reconstructed images regardless of the collimator used to acquire the planar images. With this filter, the ultrahigh-resolution fan-beam collimator was the most subjectively preferable and quantitatively produced the highest contrast ratios. The findings support suggestions that higher resolution collimators are preferable to higher sensitivity collimators, and indicate that fan-beam collimators are preferable to parallel-hole collimators for clinical SPECT studies of cerebral perfusion. The results also suggest that the Wiener filter enhances the quality of SPECT brain images regardless of which collimator is used to acquire the data.     

Sensitivity, resolution and image quality with a multi-head SPECT camera.

This investigation sought to determine which collimation factors were most important in providing superior image quality with a three-headed SPECT device. The relationship between sensitivity, resolution and SPECT image quality was studied. Two different sets of parallel-hole collimators were used. The ultrahigh-resolution collimators have higher spatial resolution (8.9 versus 11.0 mm), but only 55% of the sensitivity of the high-resolution collimators. A phantom with hot rods was imaged with both collimator sets. Observers compared images with the ultrahigh-resolution collimators to images of varying counts with the high-resolution collimators and determined which high-resolution images matched the ultrahigh-resolution images in image quality. Eleven patient studies were acquired with both collimator sets for equal time, and observers chose which image set they preferred. Transverse images of brain and liver studies were simulated with varying resolution and counts and subjectively compared. The phantom study indicated that the improvement in resolution led to image quality comparable to increasing the number of counts by a factor of 2.5 to 3.4. The clinical studies showed that the ultrahigh-resolution collimators were preferred in a large majority of the cases. These trends were also seen in the simulation study. These results confirm that higher resolution collimators should be used with multihead SPECT devices. The improvement in resolution more than compensates for the loss in sensitivity, leading to an overall improvement in image quality.     

Cross-camera comparison of SPECT measurements of a 3-D anthropomorphic basal ganglia phantom

Purpose SPECT examinations of neurotransmitter systems in the brain have to be comparable between centres to generate a comprehensive data pool, e.g. for multicentre studies. Equipment-specific effects on quantitative evaluations and corresponding methods for compensation, however, have been insufficiently examined. Previous studies have shown that quantitative results may vary significantly according to the imaging equipment used, thereby affecting clinical interpretation of the data. The aim of this study was to determine correction factors for common camera/collimator combinations based on standardised measurements of an anthropomorphic 3D basal ganglia phantom to compensate for the effects of different SPECT camera/collimator equipment. The latter may serve as a model for human studies of the dopaminergic system. Methods The striatum and background chambers of a commercially available phantom (RSD Alderson) were filled with various ¹²³I concentrations encompassing specific striatum/background ratios from 0.6 to 16.1. This setup was imaged with the following four camera/collimator combinations: Siemens Multispect 3 fitted with LEHR and ¹²³I parallel-hole collimators, Siemens ECAM with LEHR parallel-hole collimators and Philips Prism 3000 fitted with LEHR fanbeam collimators, using standardised protocols for acquisition and reconstruction. All scans were automatically co-registered to a SPECT template of the phantom and quantified using a 3D volume of interest (VOI) map based on a CT scan of the phantom. All striatal/background ratios calculated by SPECT were compared with the true ratios calculated from the measurements in a well counter. Regression analyses were performed and recovery correction factors between measured and true ratios determined. Results The relation between true and measured ratios could be sufficiently described by a linear regression for each camera/collimator combination without relevant improvement when using second-order polynomial regression models. The recovery correction factors and standard errors were 2.04±0.04 for the Philips Prism 3000, 2.67±0.03 for the Siemens Multispect 3/LEHR parallel-hole collimators, 2.15±0.03 for the Siemens Multispect 3/¹²³I collimators and 2.81±0.03 for the Siemens ECAM. Percentage recovery ranged from 36% to 49%. Conclusion Measurements of a 3D basal ganglia phantom with various imaging devices revealed linear correlations between measured and true striatal/background ratios. Based on these findings, adjustment of quantitative results between different equipment seems possible, provided that acquisition, reconstruction and evaluation are adequately standardised. The use of identical evaluation methods in phantom and patient studies (comparable shape, size and location of the VOIs) might allow transfer of the calculated correction factors from phantom to studies of the dopaminergic system in patients.     

The influence of the time activity variation on dynamic SPECT images in camera based SPECT

Image quality of dynamic single photon emission computed tomography (SPECT) using a rotating gamma camera is dependent on the time activity variation of the tracer such as accumulation and excretion in the object's organ. Especially at the early time after injection of radionuclide, artifacts may occur strongly in the SPECT images. Simulated and experimental projection data of line sources and Jaszczak phantom were altered by sequentially weighting the projections with a function that varied linearly with time. With a variation of object activity given by linearly decaying functions, the main effect observed on the SPECT images obtained from simulated line sources was an elliptical deformation on the object. If the changing rate (R (t + 1)-R(t))/R(t) x 100 remained within 20% during acquisition, this deformation of SPECT images of line sources was not noticeable visually and resolution (FWHM) of line sources scarcely was degraded. In renal dynamic SPECT study using 99mTc-DTPA, the image quality of the first scan (30 sec) was considerably degraded. However, the changing rates after the third scan were less than 20% on the mean of 10 kidneys and the image quality was not noticeable visually.     

Angulation errors in parallel-hole and fanbeam collimators: computer controlled quality control and acceptance testing procedure.

Angulation errors in collimators of 1 degrees or even less can seriously diminish the resolution of SPECT images. We have developed a computer-controlled quality control procedure that can be used for acceptance testing and regular routine checks. METHODS: Using a marker point source and a computer-controlled x-y positioning table, we investigated 7 parallel-hole and 3 fanbeam collimators. The results are presented as collimator surface maps, which are easy to interpret visually. RESULTS: The measurement accuracy for absolute angulation errors was better than 0.32 degrees. Regional variations in channel tilt could be detected with an accuracy better than 0.16 degrees. Six parallel-hole collimators were found acceptable for high-resolution SPECT imaging. For a parallel-hole collimator that had to be replaced because of nonoptimal image quality, our measurements clearly identified regions of directionally uniform angulation errors. Two fanbeam collimators showed slight concavities. CONCLUSION: Automation of the measurement and evaluation process make this procedure suitable for both acceptance tests and routine quality control checks. It can be applied to parallel-hole, fanbeam, converging, and diverging collimators, regardless of their individual geometry. No technical collimator specifications are needed. Our results reveal subtle mechanical deformations of collimators. They also show that for a detailed investigation, angulation error surface maps should be used to discover regional preferences in channel orientation.     

Clinically important characteristics of maximum-likelihood reconstruction.

SPECT images of a Jaszczak rod phantom, a single-slice Hoffman brain phantom and a uniform water-bath were acquired. Simulated noisy bar phantoms incorporating depth-dependent attenuation and blur were produced and compared to simulations with depth-independent attenuation and blur, as is the case in PET. Following iterative maximum-likelihood reconstruction, regularization was performed with use of Gaussian filters. While correction for attenuation is achieved in approximately 10 iterations, spatial resolution in the SPECT reconstructions, quantified by contrast in the bar simulations and by visual inspection of the real data, was highly nonuniform, being poorest at the center and improving toward the periphery. Image resolution continued to improve well beyond 50 iterations when regularization was applied that maintained a constant signal-to-noise ratio. Contrast in the simulated PET data also improved with increasing iterations, but the PET data showed uniform contrast throughout the transaxial slices at all numbers of iterations.     

Quantitation of iodine-123-beta-CIT dopamine receptor uptake in a phantom model.

OBJECTIVE: The purpose of this study was to determine the effects of technical factors such as collimation and filtration on the measurement of 123I-beta-CIT uptake in the striatum. METHODS: All SPECT studies were performed using a brain phantom containing striata within a bone- and tissue-equivalent skull. The effects of collimator resolution and septal penetration were assessed from 99mTc and 123I studies containing variable activities in the striata and background regions. Optimum attenuation coefficients (mu) were determined from studies containing uniform activity in the brain. RESULTS: For 99mTc, mu was 0.095 cm-1 and 0.07 cm-1 for parallel-hole and fanbeam collimators, respectively. For 123I, these values dropped to 0.09 cm-1 and 0.00 cm-1 (zero) for medium-energy and fanbeam collimators, respectively. Striatal uptake was significantly underestimated, particularly for medium-energy and general-purpose collimators. With 99mTc, fanbeam collimation gave a 50% increase in the measured striatal uptake, compared to medium-energy collimation. However, with 123I, this gain was eliminated by septal penetration and scatter. Increasing transaxial slice thickness, ROI size and decreasing filter cutoff frequency all degraded apparent striatal uptake. CONCLUSION: Partial volume effects, combined with the averaging effects of increasing slice thickness and ROI size, are the most significant factors affecting measurement of striatal uptake of 123I-beta-CIT. The increased resolution of low-energy high-resolution collimators, compared to a medium-energy collimator, is offset by the increased septal penetration and scatter.     

Development of a new method of high-speed rotation multiplied projection single photon emission computed tomography of the triple-headed type: lung MAA SPECT based on phantom study

PURPOSE: A chest phantom study was conducted to evaluate the image quality of newly developed high-speed rotation multiplied projection-single photon emission computed tomography (HSRMP-SPECT) images. MATERIALS AND METHODS: HSRMP-SPECT images of a chest phantom consisting of a simulated lung structure filled with 5000 ml of water containing 185 MBq Tc-99m-pertechnetate, and several small 11 mm simulated lung nodules of glass balls and one large 35 mm simulated lung nodule of a plastic sphere filled with water were obtained using a triple-headed SPECT system. During image acquisition, this phantom was regularly moving in the head-to-caudal direction with a range of 12 mm at a frequency of 15 cycles/min to simulate respiratory motion, and 360 degrees projection data of this moving phantom was acquired with an image acquisition time of 20 sec, which was repeated 10 times. To eliminate the setting time between projection and acquisition of multiple temporal samples of data, each detector was continuously rotated in the clockwise direction for 20 sec around a 120-degree arc. On the perspective SPECT images reconstructed from various numbers of the 20-sec projection data, the perfusion heterogeneity of the simulated lungs and perfusion defect clarity of the simulated nodules were assessed by the coefficient of variation (CV) of pixel counts and the defect-to-lung radioactivity ratios, respectively. The results were compared with those on conventional SPECT images of the moving phantom obtained with a data acquisition time of 8 min, and SPECT images of the standing phantom obtained with the same data acquisition time. RESULTS: The average CV value of 0.28+/-0.01 on the SPECT image reconstructed from 5 projection data sets was not significantly different from that of 0.27+/-0.01 on the SPECT image reconstructed from 10 projection data sets (p<0.05). The perfusion defect contrast of the simulated nodules obtained from 5 projection data was significantly higher than that on conventional SPECT images (0.50 vs. 0.73) . CONCLUSIONS: The present phantom study indicated that HSRMP-SPECT could be a useful technique for quickly obtaining high-quality SPECT images of a moving subject, thereby improving perfusion defect clarity in comparison with the conventional technique. This technique may have potential utility for obtaining high-quality breath-hold SPECT images of the chest in clinical practice.     

Three-dimensional SPECT simulations of a complex three-dimensional mathematical brain model and measurements of the three-dimensional physical brain phantom

We have developed a three-dimensional computer simulation of SPECT imaging. We have applied the simulation procedure to the realistic mathematical Hoffman three-dimensional brain model to generate the projection data (in the absence of attenuation, scatter, or noise) of both a parallel-hole and a multidetector SPECT system with point-focusing collimators. The simulated projection data were then reconstructed using standard software. The projection data resulting from the distribution of grey matter alone, or grey and white matter, were simulated. The results of these simulations indicate the existence of significant qualitative and quantitative artifacts in reconstructed human brain images. For example, the reconstructed values for grey matter along a cortical circumferential profile in a transverse slice through the basal ganglia varied by a factor of 2.40 (parallel-hole) and 2.99 (point-focusing), although the original grey matter values were identical in all cortical regions in the model. We have compared the simulated reconstructed images with those obtained by imaging the physical three-dimensional Hoffman brain phantom, which was constructed based upon the same set of data from which the mathematical three-dimensional Hoffman brain model was derived. Although the simulation did not include all of the degrading factors present in the physical imaging, the two images were in good agreement, indicating the applicability of the simulation to a realistic situation and the importance of the detector resolution effect.     

Application of a medium-energy collimator for I-131 imaging after ablation treatment of differentiated thyroid cancer

Purpose

High-energy (HE) collimators are usually applied for I-131 imaging after ablation treatment of differentiated thyroid cancer (DTC). However, purchase of HE collimators has been avoided in many nuclear medicine departments because the HE collimators are more expensive than other collimators. In this study, we compared the I-131 imaging using HE- and medium-energy (ME) collimators, which is more versatile than HE collimators.

Materials and methods

To simulate DTC patients with extra-thyroid beds, a phantom of acrylic containers containing I-131 was used. To simulate patients with thyroid beds, four phantoms representing extra-thyroid beds were arranged around the phantom representing normal thyroid tissues. Patients administered 1.11 or 3.70 GBq NaI-131 were also evaluated. Whole-body imaging and SPECT imaging of the phantoms and patients performed using HE-general-purpose (HEGP) and ME-low-penetration (MELP) collimators, and full-width at half maximum (FWHM) and percent coefficient of variation (%CV) were measured.

Results

In the extra-thyroid beds, FWHM and %CV with MELP were negligibly different from those with HEGP in whole-body imaging. Although FWHM with MELP was a little different from that with HEGP in SPECT imaging, %CV with MELP was significantly higher than that with HEGP. In the thyroid beds, only an extra-thyroid bed including higher radioactivity was identified in whole-body imaging with both collimators. Although SPECT images with MELP could not clarify extra-thyroid beds with low radioactivity, HEGP could identify them. In patients, although some whole-body images with MELP could not detect extra-thyroid beds, whole-body imaging with HEGP and SPECT imaging with both collimators could detect them.

Conclusions

Although HEGP is the best collimator for I-131 imaging, MELP is applicable for not only whole-body imaging but also SPECT imaging.     

Description of a prototype emission-transmission computed tomography imaging system.

We have developed a prototype imaging system that can perform simultaneous x-ray transmission CT and SPECT phantom studies. This system employs a 23-element high-purity-germanium detector array. The detector array is coupled to a collimator with septa angled toward the focal spot of an x-ray tube. During image acquisition, the x-ray fan beam and the detector array move synchronously along an arc pivoted at the x-ray source. Multiple projections are obtained by rotating the object, which is mounted at the center of rotation of the system. The detector array and electronics can count up to 10(6) cps/element with sufficient energy-resolution to discriminate between x-rays at 100-120 kVp and gamma rays from 99mTc. We have used this device to acquire x-ray CT and SPECT images of a three-dimensional Hoffman brain phantom. The emission and transmission images may be superimposed in order to localize the emission image on the transmission map.     

SPECT图像重建方法对Hoffmann模型图像质量的影响

目的探讨SPECT显像不同断层重建方法对Hoffmann模型图像质量的影响。方法采用放射性线源及Hoffmann模型,进行SPECT配平行孔低能高分辨准直器断层采集。对线源图像和Hoffmann模型图像均用滤波反投影（FBP）法和有序子集最大期望值迭代（OSEM）法进行断层重建。对线源重建图像计算2种重建算法的半高宽（FWHM）值,视觉评价2种方法重建的Hoffmann模型图像,并比较2种重建方法的重建时间和模型特定感兴趣区（ROI）。采用SPSS15．0软件,2种重建方法的脑叶及基底节ROI与小脑ROI计数比值行两独立样本t检验。结果平行孔低能高分辨准直器采集的线源断层图像经FBP法和OSEM法重建,FWHM值分别为18．77mm,12．62mm。OSEM法重建所得Hoffmann模型总的图像质量、基底节区的显示以及核团分辨程度均优于FBP法。FBP法和OSEM法重建时间分别为80s和100s,均在临床允许范围之内。OSEM迭代与FBP法脑叶及基底节ROI与小脑ROI比值差异无统计学意义（t=-0．332,P=0．750）。结论在现有软硬件技术条件下,平行孔低能高分辨准直器配合OSEM法优于FBP法,可获得空间分辨率和图像质量较好的Hoffmann模型重建图像。     

Optimized acquisition and processing protocols for I-123 cardiac SPECT imaging

BACKGROUND: Deconvolution of septal penetration (DSP) has been developed to improve quantification so as to allow the use of low-energy high-resolution collimators for iodine 123 cardiac single photon emission computed tomography (SPECT) imaging. The purpose of this study is to optimize its acquisition and processing protocols. METHODS AND RESULTS: Planar images of a 9-compartment phantom loaded with variable radioactive concentrations were acquired to derive optimal scatter compensation scaling factors for 20% and 15% photopeak energy window configurations, respectively. A cardiac phantom, loaded with high and low heart-to-calibration ratios (HCRs), respectively, was imaged with both configurations. Repeated acquisitions were done for medium-energy all-purpose collimators for comparison. Critical frequencies for Butterworth filtering were optimized by use of defect contrast and normal short-axis uniformity as selection indices. HCRs were calculated with planar projection and different reconstruction methods, respectively, and then compared with the true HCRs. SPECT produced more accurate HCRs than planar imaging. With the optimized parameters for scatter compensation and filtering, the 2 energy window configurations yielded similar results. Iterative reconstructions with DSP yielded more accurate HCRs than other reconstructions without DSP. CONCLUSION: The optimized protocols based on DSP show promise that quantification of I-123 cardiac SPECT imaging can be achieved with the widely available low-energy high-resolution collimators.     

Effect of scattering and spatial resolution on SPECT quantification values: experimental study using phantoms and clinical application]

The relative SPECT values are often inaccurate by the scattering and limited spatial resolution of single photon emission CT (SPECT). These effects were studied using phantoms and some attentions on clinical application were investigated. Using cylindrical phantom divided into six compartments filled with various radioactivities, the linear correlation between SPECT value and radioactivity, and also correlation with partial reduction of radioactivity were identified. But the SPECT value was relatively increased in proportion to the reduction of radioactivity due to the increase of scattering contribution. The SPECT value represented lower radioactivity when the cortical thickness was smaller than two times of FWHM and represented half radioactivity when the cortical thickness was equal to FWHM. Excellent correlation between SPECT value and radioactive partial volume averaging of brain with CSF was recognized using our hand made phantom simulating various degree of atrophic brain. It is very important to compare SPECT image with X-CT or MR image to avoid misreading taking the above mentioned effects due to scattering and limited spatial resolution.     

Collimator selection, acquisition speed, and visual assessment of 131I-tositumomab biodistribution in a phantom model

(131)I-Tositumomab has been used in treating patients with non-Hodgkin's lymphoma. It is generally recommended that high-energy collimators be used to image patients before they receive (131)I-tositumomab therapy, to determine the effective half-life for therapeutic dose and gross biodistribution. Because many nuclear medicine departments do not possess high-energy collimators, this study was designed to assess the suitability of using medium-energy collimators. The effect of scanning speed was also investigated, in an attempt to optimize the acquisition time. METHODS: Measurements were taken using an elliptic anthropomorphic torso phantom and an organ-scanning phantom fitted with fillable spheres (1-5 cm in diameter) and organ inserts. Three phantom studies were performed with differing initial (131)I concentrations in the organs, the spheres, and the thoracic and abdominal chambers. Images were acquired with both high-energy and medium-energy collimators and at acquisition speeds of 20 and 100 cm/min. The half-life for each combination (study/collimator/speed) was calculated from a linear fit of the data. The contrast of the tumor sphere was assessed using 2 identical regions, placed on and beside the sphere, and averaged over several time points. Biodistribution and image quality were visually assessed by 2 independent observers. RESULTS: Measured half-life values and visual assessment of biodistribution showed no significant difference between the 2 collimators (P = 0.32) or acquisition speeds (P = 0.85). A significant difference in the contrast of the tumor spheres was observed between the 2 collimators (P < 0.01) but not between acquisition speeds. Visual assessment of the images showed increased noise on the image acquired at 100 cm/min, although this noise did not affect lesion detectability. CONCLUSION: Measured half-life is not significantly different between the 2 collimators; hence, calculation of the residence time would be nearly the same. Medium-energy collimators can be used to accurately calculate the (131)I-tositumomab therapeutic dose and detect alterations in biodistribution.     

Feasibility of dual-isotope coincidence/single-photon imaging of the myocardium.

Hybrid PET scanners offer the possibility of obtaining myocardial viability information from coincidence imaging of the positron emitter (18)F-FDG and perfusion measurements from a single-photon tracer-potentially simultaneously. This new approach is less costly and more readily available than dedicated PET and offers potential for improved FDG resolution and sensitivity compared with SPECT with 511-keV collimators. Simultaneous imaging of the coincidence and single-photon events offers the further advantages of automatic image registration and reduced imaging time. However, the feasibility of simultaneous coincidence/single-photon imaging or even immediately sequential imaging is unknown. In this study, the potential of using standard low-energy high-resolution (LEHR) collimators with hybrid PET to obtain coincidence and SPECT data was assessed. METHODS: Phantom and human studies were performed to investigate the effect of LEHR collimators on FDG coincidence imaging with a hybrid PET system, the effect of the presence of (99m)Tc during FDG coincidence imaging with LEHR collimators, and the effect of the presence of FDG during (99m)Tc SPECT imaging. RESULTS: FDG images were somewhat degraded (a measure of myocardial nonuniformity increased 10%) with LEHR collimators. With 148 MBq (4 mCi) (99m)Tc present during FDG imaging of a phantom, image quality was maintained and the number of detected coincidences changed by <5%. With (99m)Tc/(18)F whole-body ratios of 7:1, crosstalk from (18)F photons accounted for the majority of counts in the (99m)Tc SPECT images and resulted in severe artifacts. The artifacts were decreased with a simple crosstalk correction scheme but remained problematic. CONCLUSION: (99m)Tc/(18)F ratios of at least 9:1 and state-of-the-art reconstruction and crosstalk correction are likely to be required to perform immediately sequential coincidence/single-photon imaging of the myocardium with clinically useful results. Additional challenges remain before simultaneous imaging of coincidence events and single photons can be realized in practice.     

Truncation correction of fan beam transmission data for attenuation correction using parallel beam emission data on a 3-detector SPECT system

BACKGROUND: When the simultaneous transmission computed tomography (TCT)/single photon emission CT (SPECT) acquisition protocol is applied to myocardial studies using a 3-detector SPECT, the narrow effective field of view of a fan beam collimator used for TCT acquisition may cause truncation artifacts on TCT images. In this paper, we propose a new method of correcting for the truncation of TCT. METHODS: The truncated parts of the TCT projection data are corrected using quadratic functions, based on the properties that the integral of non-truncated TCT projection data is constant at any projection angle and the position of the centre of gravity is focused on a fixed point. The usefulness of our method was investigated in phantom and human studies using a 3-detector SPECT equipped with one cardiac fan beam collimator for TCT and two parallel beam collimators for SPECT. We used Tl as a tracer for SPECT and Tc as an external source for TCT. RESULTS: The phantom and human studies showed that our method can adequately correct for the truncation of TCT data acquired using a fan beam collimator in a 3-detector SPECT, as long as there is no truncation in SPECT data. CONCLUSION: Our method appears to be useful for improving the SPECT images obtained using simultaneous TCT/SPECT acquisition in a 3-detector SPECT. However, further studies will be necessary to establish the clinical usefulness of this method.     

Characterization of septal penetration in 511 keV SPECT

BACKGROUND: Single photon emission computed tomography (SPECT) with 511 keV photons is a challenging modality and collimators for this purpose require trade-offs among resolution, sensitivity and septal penetration. While PET is the modality of choice for imaging at 511 keV, there are some procedures, e.g., dual-isotope imaging, in which 511 keV SPECT has a role. AIM: To measure the imaging properties of a VPC-93 SPECT collimator designed for imaging at 511 keV and to isolate the effects of septal penetration. METHODS: NaI gamma camera projection images of (18)F (511 keV) and (99m)Tc (140 keV) point sources were measured and the corresponding modulation transfer functions calculated. The projection images were reconstructed via filtered back-projection to obtain the tomographic three-dimensional (3-D) point spread function. Differences between the 511 and 140 keV results were attributed mainly to septal penetration. Contrast measurements were made separately using (18)F and (99m)Tc of a 20 cm phantom containing hot spheres and a warm background. Both isotopes were also used in imaging studies of a 3-D Hoffman brain phantom. RESULTS: Reconstructed 511 keV point source images were spatially extended with more than half of the total reconstructed counts appearing away from the point source region. The number of false counts contained in the image as a function of distance from the true source location remains approximately constant for large distances out to at least 14 cm. Septal penetration results in a rapid roll-off with spatial frequency of collimator response. The response of the collimator to 511 keV photons falls to half of its 0-frequency response at 0.03 cm(-1). For 140 keV photons this value is 0.20 cm(-1). A result is reduced image contrast as measured in the phantom sphere studies. Septal penetration causes image degradation through large-scale blurring. Image noise characteristics are modified and correlations are extended into many transaxial planes. CONCLUSIONS: Both 2-D and 3-D point spread functions for 511 and 140 keV photons using the VPC-93 collimator have been measured. Septal penetration unfavourably affects image resolution and changes image noise characteristics. Without compensation, the effects of septal penetration are readily apparent in images of real objects.