Attenuation correction for three-dimensional PET using uncollimated flood-source transmission measurements

An attenuation-correction method for three-dimensional PET imaging, which obtains attenuation-correction factors from transmission measurements using an uncollimated flood source, is described. This correction is demonstrated for two different phantoms using transmission data acquired with QPET, a rotating imaging system with two planar detectors developed for imaging small volumes. The scatter amplitude in the transmission projections was a maximum of 30%; to obtain accurate attenuation-correction factors the scatter distribution was first calculated and subtracted. The attenuation-corrected emission images for both phantoms indicate that their original uniform amplitudes have been restored. The attenuation correction adds only a small amount of noise to the emission images, as evaluated from the standard deviation over a central region. For the first phantom, with maximum attenuation of 48%, the noise added was 2.6%. The second phantom was attenuated by a maximum of 37%, and 1.9% noise was added. Because the transmission data are smoothed, some artifacts are visible at the edges of the phantom where the correction factors change abruptly within the emission image.     

The design and implementation of a motion correction scheme for neurological PET

A method is described to monitor the motion of the head during neurological positron emission tomography (PET) acquisitions and to correct the data post acquisition for the recorded motion prior to image reconstruction. The technique uses an optical tracking system, Polaris, to accurately monitor the position of the head during the PET acquisition. The PET data are acquired in list mode where the events are written directly to disk during acquisition. The motion tracking information is aligned to the PET data using a sequence of pseudo-random numbers, which are inserted into the time tags in the list mode event stream through the gating input interface on the tomograph. The position of the head is monitored during the transmission acquisition, and it is assumed that there is minimal head motion during this measurement. Each event, prompt and delayed, in the list mode event stream is corrected for motion and transformed into the transmission space. For a given line of response, normalization, including corrections for detector efficiency, geometry and crystal interference and dead time are applied prior to motion correction and rebinning in the sinogram. A series of phantom experiments were performed to confirm the accuracy of the method: (a) a point source located in three discrete axial positions in the tomograph field of view, 0 mm, 10 mm and 20 mm from a reference point, (b) a multi-line source phantom rotated in both discrete and gradual rotations through +/- 5 degrees and +/- 15 degrees, including a vertical and horizontal movement in the plane. For both phantom experiments images were reconstructed for both the fixed and motion corrected data. Measurements for resolution, full width at half maximum (FWHM) and full width at tenth maximum (FWTM), were calculated from these images and a comparison made between the fixedand motion corrected datasets. From the point source measurements, the FWHM at each axial position was 7.1 mm in the horizontal direction, and increasing from 4.7 mm at the 0 mm position, to 4.8 mm, 20 mm offset, in the vertical direction. The results from the multi-line source phantom with +/- 5 degrees rotations showed a maximum degradation in FWHM, when compared with the stationary phantom, of 0.6 mm, in the horizontal direction, and 0.3 mm in the vertical direction. The corresponding values for the larger rotation, +/- 15 degrees, were 0.7 mm and 1.1 mm, respectively. The performance of the method was confirmed with a Hoffman brain phantom moved continuously, and a clinical acquisition using [11C]raclopride (normal volunteer). A visual comparison of both the motion and non-motion corrected images of the Hoffman brain phantom clearly demonstrated the efficacy of the method. A sample time-activity curve extracted from the clinical study showed irregularities prior to motion correction, which were removed after correction. A method has been developed to accurately monitor the motion of the head during a neurological PET acquisition, and correct for this motion prior to image reconstruction. The method has been demonstrated to be accurate and does not add significantly to either the acquisition or the subsequent data processing.     

List-mode-based reconstruction for respiratory motion correction in PET using non-rigid body transformations

Respiratory motion in emission tomography leads to reduced image quality. Developed correction methodology has been concentrating on the use of respiratory synchronized acquisitions leading to gated frames. Such frames, however, are of low signal-to-noise ratio as a result of containing reduced statistics. In this work, we describe the implementation of an elastic transformation within a list-mode-based reconstruction for the correction of respiratory motion over the thorax, allowing the use of all data available throughout a respiratory motion average acquisition. The developed algorithm was evaluated using datasets of the NCAT phantom generated at different points throughout the respiratory cycle. List-mode-data-based PET-simulated frames were subsequently produced by combining the NCAT datasets with Monte Carlo simulation. A non-rigid registration algorithm based on B-spline basis functions was employed to derive transformation parameters accounting for the respiratory motion using the NCAT dynamic CT images. The displacement matrices derived were subsequently applied during the image reconstruction of the original emission list mode data. Two different implementations for the incorporation of the elastic transformations within the one-pass list mode EM (OPL-EM) algorithm were developed and evaluated. The corrected images were compared with those produced using an affine transformation of list mode data prior to reconstruction, as well as with uncorrected respiratory motion average images. Results demonstrate that although both correction techniques considered lead to significant improvements in accounting for respiratory motion artefacts in the lung fields, the elastic-transformation-based correction leads to a more uniform improvement across the lungs for different lesion sizes and locations.     

Rigid-body transformation of list-mode projection data for respiratory motion correction in cardiac PET

High-resolution cardiac PET imaging with emphasis on quantification would benefit from eliminating the problem of respiratory movement during data acquisition. Respiratory gating on the basis of list-mode data has been employed previously as one approach to reduce motion effects. However, it results in poor count statistics with degradation of image quality. This work reports on the implementation of a technique to correct for respiratory motion in the area of the heart at no extra cost for count statistics and with the potential to maintain ECG gating, based on rigid-body transformations on list-mode data event-by-event. A motion-corrected data set is obtained by assigning, after pre-correction for detector efficiency and photon attenuation, individual lines-of-response to new detector pairs with consideration of respiratory motion. Parameters of respiratory motion are obtained from a series of gated image sets by means of image registration. Respiration is recorded simultaneously with the list-mode data using an inductive respiration monitor with an elasticized belt at chest level. The accuracy of the technique was assessed with point-source data showing a good correlation between measured and true transformations. The technique was applied on phantom data with simulated respiratory motion, showing successful recovery of tracer distribution and contrast on the motion-corrected images, and on patient data with C15O and 18FDG. Quantitative assessment of preliminary C15O patient data showed improvement in the recovery coefficient at the centre of the left ventricle.     

Respiratory motion correction for PET oncology applications using affine transformation of list mode data

Respiratory motion is a source of artefacts and reduced image quality in PET. Proposed methodology for correction of respiratory effects involves the use of gated frames, which are however of low signal-to-noise ratio. Therefore a method accounting for respiratory motion effects without affecting the statistical quality of the reconstructed images is necessary. We have implemented an affine transformation of list mode data for the correction of respiratory motion over the thorax. The study was performed using datasets of the NCAT phantom at different points throughout the respiratory cycle. List mode data based PET simulated frames were produced by combining the NCAT datasets with a Monte Carlo simulation. Transformation parameters accounting for respiratory motion were estimated according to an affine registration and were subsequently applied on the original list mode data. The corrected and uncorrected list mode datasets were subsequently reconstructed using the one-pass list mode EM (OPL-EM) algorithm. Comparison of corrected and uncorrected respiratory motion average frames suggests that an affine transformation in the list mode data prior to reconstruction can produce significant improvements in accounting for respiratory motion artefacts in the lungs and heart. However, the application of a common set of transformation parameters across the imaging field of view does not significantly correct the respiratory effects on organs such as the stomach, liver or spleen.     

Replacement correction factors for electron measurements with a parallel-plate chamber

The AAPM Task Group dosimetry protocol is ambiguous regarding the replacement correction factors Prepl to be applied to electron measurements with parallel-plate chambers. By intercomparison with a cylindrical chamber whose Prepl values at dmax may be calculated from Task Group 21, the Prepl values for a PTW/Markus parallel-plate chamber have been determined in the range of mean incident energies of 5-11 MeV. The Prepl values for this chamber are found to differ significantly from unity, if one assumes that the cylindrical chamber values are valid.     

Calibration of photodiode measurements of cell motion by a transmission optical lever method

A method of calibrating photodiode measurements of cell motion is described. A transparent glass plane is inserted perpendicular to the light path, in front of the photodiode. When the glass plane is rotated, changing the angle between the plane and the light path, the image is displaced. The rotatable glass plane acts as an optical lever to simulate cell motion. The optical lever is easy to construct and offers excellent linearity and accuracy.     

Accurate attenuation correction for a 3D PET system

An accurate attenuation correction has been developed for a small-volume three-dimensional positron emission tomography (PET) system. Transmission data were measured as twenty-four 2D slices which were reconstructed and combined to form a 3D attenuation image. Emission data were reconstructed using a backproject-then-filter technique, and each event was corrected for attenuation at backprojection time by a reprojection through the attenuation image. This correction restores the spatial invariance of the point response function, thus allowing a valid deconvolution and producing an undistorted emission image. Scattering corrections were not applied to either the transmission or the emission data but simulation studies indicated that scattering made only a small contribution to the attenuation measurement. Results are presented for two phantoms, in which transmission scans of 57,500 and 18,700 events/slice were used to correct emission images of 5.2 and 2.8 million events. Although the attenuation images had poor statistical accuracy and a resolution of 13 mm, the method resulted in accurate attenuation-corrected images with no degradation in image resolution (which was 3 mm for the first emission image), and with little effect on image noise.     

Scatter correction for three-dimensional PET based on an analytic model dependent on source and attenuating object

Three-dimensional positron emission tomography admits a significant scatter fraction due to the large aperture of the detectors, and requires accurate scatter subtraction. A scatter-correction method, applicable to both emission and transmission imaging, calculates the projections of the single-scatter distribution, using an approximate image of the source and attenuating object. The scatter background is subtracted in projection space for transmission data and in image space for emission data, yielding corrected attenuation and emission images. The accuracy of this single-scatter distribution is validated for the authors' small imaging system by comparison with Monte Carlo simulations. The correction is demonstrated using transmission and emission data obtained from measurements on the authors' QPET imaging system using two acrylic phantoms. For the transmission data, generated with a flood source, errors of up to 24% in the linear attenuation coefficients resulted with no scatter subtraction, but the correction yielded an accurate value of mu =0.11+or-0.01 cm-1. For the emission data, the corrected images show that the scattered background has been removed to within the level of the background noise outside the source. The residual amplitude within a cold spot in one of the phantoms was reduced from 21% to 3% of the image amplitude.     

Attenuation correction for small animal PET tomographs

Attenuation correction is one of the important corrections required for quantitative positron emission tomography (PET). This work will compare the quantitative accuracy of attenuation correction using a simple global scale factor with traditional transmission-based methods acquired either with a small animal PET or a small animal x-ray computed tomography (CT) scanner. Two phantoms (one mouse-sized and one rat-sized) and two animal subjects (one mouse and one rat) were scanned in CTI Concorde Microsystem's microPET Focus for emission and transmission data and in ImTek's MicroCAT II for transmission data. PET emission image values were calibrated against a scintillation well counter. Results indicate that the scale factor method of attenuation correction places the average measured activity concentration about the expected value, without correcting for the cupping artefact from attenuation. Noise analysis in the phantom studies with the PET-based method shows that noise in the transmission data increases the noise in the corrected emission data. The CT-based method was accurate and delivered low-noise images suitable for both PET data correction and PET tracer localization.     

Attenuation correction of PET images with interpolated average CT for thoracic tumors

To reduce positron emission tomography (PET) and computed tomography (CT) misalignments and standardized uptake value (SUV) errors, cine average CT (CACT) has been proposed to replace helical CT (HCT) for attenuation correction (AC). A new method using interpolated average CT (IACT) for AC is introduced to further reduce radiation dose with similar image quality. Six patients were recruited in this study. The end-inspiration and -expiration phases from cine CT were used as the two original phases. Deformable image registration was used to generate the interpolated phases. The IACT was calculated by averaging the original and interpolated phases. The PET images were then reconstructed with AC using CACT, HCT and IACT, respectively. Their misalignments were compared by visual assessment, mutual information, correlation coefficient and SUV. The doses from different CT maps were analyzed. The misalignments were reduced for CACT and IACT as compared to HCT. The maximum SUV difference between the use of IACT and CACT was ～3%, and it was ～20% between the use of HCT and CACT. The estimated dose for IACT was 0.38 mSv. The radiation dose using IACT could be reduced by 85% compared to the use of CACT. IACT is a good low-dose approximation of CACT for AC.     

红外热像仪恒温源实时校正方法的研究

提出了一种利用恒温源对红外热像仪进行实时温度校正的方法.这种方法,可以有效地消除外界各种因素和仪器自身噪声对采集信号的干扰,实现热态图像物理真实温度的测量和热图间的有效对比.     

Single‐voxel MRS with prospective motion correction and retrospective frequency correction

Subject motion during MRS investigations is a factor limiting the quality and the diagnostic value of the spectra. The possibility of using external motion tracking data to correct for artefacts in MR imaging has been demonstrated previously. In this paper the utility of prospective motion correction for single‐voxel proton MRS is investigated. The object motion data are used in real time to update the position of the spectroscopy voxel during the acquisition prior to every sequence repetition cycle. It is not, however, sufficient to update the voxel position alone due to shim changes accompanying subject motion. Adverse effects of frequency shifts induced by subject motion are effectively suppressed by the interleaved reference scan method. Copyright © 2010 John Wiley & Sons, Ltd.     

Attenuation correction of PET cardiac data with low-dose average CT in PET/CT

We proposed a low-dose average computer tomography (ACT) for attenuation correction (AC) of the PET cardiac data in PET/CT. The ACT was obtained from a cine CT scan of over one breath cycle per couch position while the patient was free breathing. We applied this technique on four patients who underwent tumor imaging with 18F-FDG in PET/CT, whose PET data showed high uptake of 18F-FDG in the heart and whose CT and PET data had misregistration. All four patients did not have known myocardiac infarction or ischemia. The patients were injected with 555-740 MBq of 18F-FDG and scanned 1 h after injection. The helical CT (HCT) data were acquired in 16 s for the coverage of 100 cm. The PET acquisition was 3 min per bed of 15 cm. The duration of cine CT acquisition per 2 cm was 5.9 s. We used a fast gantry rotation cycle time of 0.5 s to minimize motion induced reconstruction artifacts in the cine CT images, which were averaged to become the ACT images for AC of the PET data. The radiation dose was about 5 mGy for 5.9 s cine duration. The selection of 5.9 s was based on our analysis of the respiratory signals of 600 patients; 87% of the patients had average breath cycles of less than 6 s and 90% had standard deviations of less than 1 s in the period of breath cycle. In all four patient studies, registrations between the CT and the PET data were improved. An increase of average uptake in the anterior and the lateral walls up to 48% and a decrease of average uptake in the septal and the inferior walls up to 16% with ACT were observed. We also compared ACT and conventional slow scan CT (SSCT) of 4 s duration in one patient study and found ACT was better than SSCT in depicting average respiratory motion and the SSCT images showed motion-induced reconstruction artifacts. In conclusion, low-dose ACT improved registration of the CT and the PET data in the heart region in our study of four patients. ACT was superior than SSCT for depicting average respiration motion in a patient study.     

Computer phantom study of brain PET glucose metabolism imaging using a rotating SPECT/PET camera

Positron emission tomography (PET) with [18F] fluoro-deoxy-glucose (FDG) provides information about glucose metabolism and is used to measure tissue glucose kinetics in the brain. The recent interest in hybrid SPECT/PET systems emerged as a practical approach to reduce the high cost of purchasing a dedicated ring-detector PET system. We have implemented interpolation methods for processing the projection data that could potentially reduce artifacts when reconstructing a dynamic imaging sequence in a PET study from a dual-head rotating SPECT/PET system. The computer simulations predict that parameter estimates from the dedicated PET system will be superior to results using the rotating camera system. However, the rotating camera system using projection interpolation may approach the accuracy of the dedicated PET system if the data noise is below 20%.     

A convolution-subtraction scatter correction method for 3D PET

3D acquisition and reconstruction in positron emission tomography (PET) produce data with improved signal-to-noise ratios compared with conventional 2D slice-oriented methods. However, the sensitivity increase is accompanied by an increase in the number of scattered photons and random coincidences detected. This paper presents a scatter correction technique for 3D PET data where an estimate of the scattered photon distribution is subtracted from the data before reconstruction. The scatter distribution is estimated by iteratively convolving the photopeak projections with a mono-exponential kernel. The method accounts for the 3D acquisition geometry and nature of scatter by performing the scatter estimation on 2D projections. The assumptions of the method have been investigated by measuring the variation in the scatter fraction and the scatter function at different positions in a cylinder. Both parameters were found to vary by up to 50% from the centre to the edge of a large water-filled cylinder. Despite this, in a uniform cylinder containing water with different concentrations of radioactivity, scatter was reduced from 25% in a non-radioactive region to less than 5% using the convolution-subtraction method. In addition, the relative concentration of a cylinder containing an increased concentration, which was underestimated by almost 50% without scatter correction, was within 5% of the true concentration after correction.