Albert Henry Roger Lonn

Comparison of 4-class and continuous fat/water methods for whole-body, MR-based PET attenuation correction

The goal of this study was to compare two approaches for MR-based PET patient attenuation correction (AC) in whole-body FDG-PET imaging using a tri-modality PET/CT & MR setup. Sixteen clinical whole-body FDG patients were included in this study. Mean standard uptake values (SUV) were measured for liver and lung volumes-of-interest for comparison. Maximum SUV values were measured in 18 FDGavid features in ten of the patients. The AC methods compared to gold-standard CT-based AC were segmentation of the CT (air, lung, fat, water), MR image segmentation with 4 tissue classes (air, lung, fat, water) and segmentation with air, lung and a continuous fat/water method. Results: The magnitude of uptake value differences induced by CT-based image segmentation were similar but lower on average than those found using the MRderived AC methods. The average liver SUV difference with that found using CTAC was 1.3%, 10.4% and 5.7% for 4-class segmented CT, 4-class MRAC and continuous fat/water MRAC methods, respectively. The average FDG-avid feature SUV max difference was -0.5%,1.7% and -1.6% for 4-class segmented CT, 4-class MRAC and continuous fat/water MRAC methods, respectively. Conclusion: The results demonstrated that both 4class and continuous fat/water AC methods provided adequate quantitation in the body, and that the continuous fat/water method was within 5.7% on average for SUV mean in liver and 1.6% on average for SUV max for FDG-avid features.

Truncation Artifact on PET/CT: Impact on Measurements of Activity Concentration and Assessment of a Correction Algorithm

OBJECTIVE Discrepancy between fields of view (FOVs) in a PET/CT scanner causes a truncation artifact when imaging extends beyond the CT FOV. The purposes of this study were to evaluate the impact of this artifact on measurements of 18F-FDG activity concentrations and to assess a truncation correction algorithm. MATERIALS AND METHODS Two phantoms and five patients were used in this study. In the first phantom, three inserts (water, air, bone equivalent) were placed in a water-filled cylinder containing 18F-FDG. In the second phantom study, a chest phantom and a 2-L bottle fitted with a bone insert were used to simulate a patients torso and arm. Both phantoms were imaged while positioned centrally (baseline) and at the edge of the CT FOV to induce truncation. PET images were reconstructed using attenuation maps from truncated and truncation-corrected CT images. Regions of interest (ROIs) drawn on the inserts, simulated arm, and background water of the baseline truncated and truncation-corrected PET images were compared. In addition, extremity malignancies of five patients truncated on CT images were reconstructed with and without correction and the maximum standard uptake values (SUVs) of the malignancies were compared. RESULTS Truncation artifact manifests as a rim of high activity concentration at the edge of the truncated CT image with an adjacent low-concentration region peripherally. The correction algorithm minimizes these effects. Phantom studies showed a maximum variation of -5.4% in the truncation-corrected background water image compared with the baseline image. Activity concentration in the water insert was 6.3% higher while that of air and bone inserts was similar to baseline. Extremity malignancies showed a consistent increase in the maximum SUV after truncation correction. CONCLUSION Truncation affects measurements of 18F-FDG activity concentrations in PET/CT. A truncation-correction algorithm corrects truncation artifacts with small residual error.

Evaluation of method to minimize the effect of X-ray contrast in PET-CT attenuation correction

In the Discovery ST PET-CT scanner, the attenuation map used for correction of the 511 keV emission attenuation is derived from the CT scan using a continuous conversion scale which has a range of slopes dependent on the CT kV and the CT number. High CT numbers are assumed to be bone, and contrast material which generates CT numbers in the range of bone results in over-estimated 511 keV attenuation values. For normal concentrations of contrast media, the elevation in the emission counts of contrast-enhanced organs is not significant, but artifacts can be produced when there are unusually high concentrations of contrast material, especially when the contrast has moved between the CT and the emission scan. In these cases it is possible to reduce the artifactually high emission counts using a tissue-contrast conversion scale derived from CT number and attenuation measurements made on various concentrations of contrast in phantoms and measurements of contrast enhancement in patients. If this conversion scale is applied to an image containing bone, the emission image is under-corrected for the bone attenuation. The effect on reconstructed activity of using either the bone scale or the contrast scale is evaluated in a range of organs. The use of the contrast scale results in a reduction of activity, which is not significant except in the vicinity of dense bone.

High Speed Reprojection And Its Applications

Reprojection is the process by which projections are produced from an image such that, if these projections are filtered and backprojected, they yield the original image. Because of the computational expense of reprojection, algorithms that employ this process have never been widely used. A method is presented that enables an unmodified backprojector to be used as a reprojector. Because backprojectors are designed to exploit the parallelism in the backprojection algorithm, the time required to obtain reprojections is significantly reduced. Another method, based on the Fourier Slice Theorem, is presented that enables a general purpose array processor to be used as a high speed reprojector. It is also shown that the parameters of the reprojection algorithm can be adjusted to decrease significantly the time required to perform an application that uses reprojection. Finally, two applications of reprojection in computed tomography are discussed.

Ultra low dose CT for attenuation correction in PET/CT

In this paper, we present an ultra low-dose CT acquisition and reconstruction technique that provides sufficient image quality for PET attenuation correction while keeping the CT dose to a minimum. With PET/CT, CT is used instead of radioactive transmission sources to acquire data for attenuation correction. While this results in higher radiation dose to the patient, the CT images are also viewed with the PET images and provide an anatomical reference to aid the physician in localizing tumors. We found that using low tube voltage and current results in severe shading and CT number shifts and causes artifacts and quantitative errors in PET images. By adding a simple low pass filter to CT reconstruction prior to the logarithmic step, shading and CT number shifts are reduced substantially. The resulting CT number accuracy and uniformity of the images with the proposed approach makes them acceptable for PET attenuation correction.

An enhanced reconstruction algorithm to extend CT scan field‐of‐view with z‐axis consistency constraint

PURPOSE To further improve the image quality, in particularly, to suppress the boundary artifacts, in the extended scan field-of-view (SFOV) reconstruction. METHODS To combat projection truncation artifacts and to restore truncated objects outside the SFOV, an algorithm has previously been proposed based on fitting a partial water cylinder at the site of the truncation. Previous studies have shown this algorithm can simultaneously eliminate the truncation artifacts inside the SFOV and preserve the total amount of attenuation, owing to its emphasis on consistency conditions of the total attenuation in the parallel sampling geometry. Unfortunately, the water cylinder fitting parameters of this 2D algorithm are inclined to high noise fluctuation in the projection samples from image to image, causing anatomy boundaries artifacts, especially during helical scans with higher pitch (≥1.0). To suppress the boundary artifacts and further improve the image quality, the authors propose to use a roughness penalty function, based on the Huber regularization function, to reinforce the z-dimensional boundary consistency. Extensive phantom and clinical tests have been conducted to test the accuracy and robustness of the enhanced algorithm. RESULTS Significant reduction in the boundary artifacts is observed in both phantom and clinical cases with the enhanced algorithm. The proposed algorithm also reduces the percent difference error between the horizontal and vertical diameters to well below 1%. It is also noticeable that the algorithm has improved CT number uniformity outside the SFOV compared to the original algorithm. CONCLUSIONS The proposed algorithm is capable of suppressing boundary artifacts and improving the CT number uniformity outside the SFOV.

Towards single ultra-low dose CT scan for both attenuation map creation and localization in PET-CT application

We have proposed the use of a single ultra-low dose CT scan for both attenuation map creation and localization purpose for PET applications. An adaptive filtering algorithm specifically for such imaging task has been developed to adequately reduce image noise and streaking artifacts and to provide clear delineation of major organs in low signal scanning conditions. Simulated ultra-low dose CT scans were used to evaluate the low-dose scan proposal and the associated adaptive filtering algorithm. The results demonstrated that with the use of a sophisticated pre-processing algorithm, we could utilize a single ultra-low dose CT scan to obtain accurate attenuation map and to provide excellent localization information for PET applications.

Estimation of mean lung attenuation for use in generating PET attenuation maps

In PET/MR the attenuation map required for PET reconstruction is obtained by segmenting MR images and assigning linear attenuation coefficients (LAC) to the segmented regions. Some tissues such as brain, muscle and fat have relatively uniform attenuation coefficients but the lung attenuation varies within and between patients. The variation in mean lung density was measured in a group of patients and found to correspond to LAC in the range from 0.02cm-1 to 0.04cm-1. The effect of this range of densities was measured on the PET reconstruction and was found to produce a substantial range of PET activities in the lung. It is proposed that a better estimate of lung density could be obtained from measuring the relative size of the lung. The feasibility of this is assessed by correlating lung volume with density in a group of patients.

Randoms from singles estimation for long PET scans

We have previously described the algorithm for estimating the contribution of random coincidences to the PET dataset using a set of singles counters, one per detector element, which accumulate the single events transmitted to the coincidence processor over the course of an acquisition frame. In this algorithm, the effect of isotope decay during the acquisition was neglected, as it was shown to be small (<1% change for an acquisition time of one-half the isotope half-life). While this is sufficient for most PET acquisitions, it may not be for scans that are long compared to the isotope half-life, such as static or gated 82Rb (75 sec half-life) acquisitions of several minutes duration. In several sample 82Rb scans from a whole-body PET/CT scanner, the singles-based algorithm underestimated the total randoms by 33–45% compared with the randoms estimate from a delayed coincidence window. We have developed a correction term for the previous algorithm to account for activity changes during long-duration PET frames. This correction term produces a more accurate estimate of randoms from singles which also takes into account the effect of the decaying activity on the deadtime of the scanner and the effect of the intrinsic singles rate and resulting deadtime from the background activity present in a lutetium-based scintillator material. The resulting correction term is a function of the measured total singles, the average deadtime and the duration of the frame, the average intrinsic counts per crystal and intrinsic deadtime per block (set to zero for a non-lutetium containing detector) and the isotope half-life. Applying this correction term to the studies described above reduced the difference between the single-based estimate and the delayed window randoms counts to 1–3%.

Evaluation of CT Field of View Restoration for PET-CT Attenuation Correction

The Discovery ST PET-CT scanner corrects the attenuation of the 511 keV PET emission using an attenuation map derived from a CT scan which is acquired with a 50 cm detector field of view. The PET scanner uses a 70 cm reconstruction field of view, and to support attenuation correction over the 70 cm diameter, the CT data are extended using a previously published method. The restored CT data have some errors in the extrapolated outline and the density in the extrapolated region, but the integral attenuation along the PET lines of response are restored with an acceptable error for the purpose of emission attenuation correction. The effect of the truncation on the reconstructed PET activity and the reduction in these effects with the restoration algorithm was measured using an active PET phantom. The use of the truncated attenuation map resulted in a loss of activity outside the truncated field of view and also had an effect on the activity within the 50 cm field of view. The use of the extrapolated CT images for attenuation correction results in a considerable improvement in the accuracy of the reconstructed activity.