Recovery coefficients determination for partial volume effect correction in oncological PET/CT images considering the effect of activity outside the field of view

Objectives

The partial volume effect (PVE) has a great impact in quantitative PET/CT imaging. Correction methods have been recently proposed by many authors to make the image quantification more accurate. This work presents a methodology for determining the recovery coefficients (RCs) for PVE correction in PET/CT images. It was taken into account the radioactivity outside the field of view (FOV), which is expected in a patient image acquisition.

Methods

The NEMA image quality phantom and the NEMA scatter phantom were used. The phantoms were filled with ¹⁸F-FDG for different sphere-to-background ratios. The RCs have been determined from image acquisitions in a Siemens Biograph 16 Hi-Rez PET/CT scanner with and without the scatter phantom.

Results

The RC values that ranged from 0.38 to 1.00 without the scatter phantom exhibited a wider variation when this latter was taken into account (from 0.27 to 1.02). This more realistic estimation must be considered if one takes into account that an incorrect SUV measure in tumors leads to errors in the evaluation of the response to therapy based on PET/CT images.

Conclusions

The activity outside the FOV should be considered in RCs determination to improve the RC-based PVE correction method.     

Reconstruction from truncated projections in CT using adaptive detruncation

If the object exceeds the field of measurement (FOM) of a given CT scanner, severe artifacts may result. In this work, we propose an adaptive detruncation (ADT) method to reconstruct images from medical CT projections which are truncated in the transaxial direction. The truncated projections are extrapolated by estimating the convex hull of the patient. The ADT method allows us not only to achieve artifact-free images in the FOM but also to extend the images beyond the FOM, and can therefore be very attractive, for example, in PET/CT scanners for attenuation correction.     

Quantification and reduction of attenuation related artifacts in SPET by applying attenuation model during iterative image reconstruction: a Monte Carlo study

Photon attenuation is one of the main causes of the quantitative errors and artifacts in SPET. A transmission or CT based attenuation map is necessary to correct for the effects of attenuation accurately. In this research, some important attenuation related artifacts are described. A fast and memory efficient iterative algorithm is proposed for attenuation correction. Ordered subset expectation maximization (OSEM) algorithm with attenuation model was applied for image reconstruction. Monte Carlo simulation was used to create the projections in this study. Different voxel based phantoms with uniform and non-uniform activity distributions and attenuation maps were employed to evaluate the accuracy of this algorithm. The NCAT digital phantom was also used to investigate the attenuation effects on myocardial perfusion SPET in men and women. Projections free from the effect of attenuation were also simulated. The reconstructed image from these attenuation-free projections was considered as reference image. Our attenuation correction algorithm was evaluated by its ability to recover activity and to remove attenuation related artifacts. The mean-square-error (MSE) between reference and corrected image and image contrast were calculated for quantitative evaluation of this algorithm. A variety of attenuation related artifacts were observed. Moreover anterior wall of myocardial perfusion images of female phantom and inferior wall in male phantom were affected by attenuation. All of the attenuation related artifacts were removed after attenuation correction. Quantitatively, the MSE values between reference images and corrected images were reduced by about 900% for all phantoms. In conclusion, by applying our new method for incorporating attenuation model during OSEM, we were able to eliminate a variety of artifacts and errors, which is a necessary step for quantitative SPET.     

Respiration artifacts in whole-body 18F-FDG PET/CT studies with combined PET/CT tomographs employing spiral CT technology with 1 to 16 detector rows

Purpose Co-registration accuracy in combined whole-body (WB) PET/CT imaging is impaired by respiration-induced mismatches between the CT and the PET. Furthermore, PET/CT misregistration may bias the PET tracer distribution following CT-based attenuation correction (CT-AC). With the introduction of multi-row CT technology of up to 16 detector rows into PET/CT designs, we investigated the incidence of respiration artifacts in WB PET/CT examinations of patients who were unable to follow any breath-hold instructions.Methods A total of 80 WB studies from six international sites operating PET/CT tomographs with 1-, 2-, 4-, 6-, 8-, and 16-row spiral CT were included. PET/CT examinations were acquired with the patients breathing normally during both the CT and the PET scan, and CT-AC was performed routinely. All studies were anonymized and reviewed independently by three radiologists and three nuclear medicine specialists. We report the frequency and severity of artifacts on CT and PET for the thorax and the abdomen.Results In WB PET/CT imaging of normally breathing patients, significant gains in diagnostic image quality can be expected from employing CT technology with six or more detector rows. In our study, fewer PET images appear distorted than corresponding CT images, which is due to the limited propagation of only mild CT image artifacts after the resampling of the CT-based attenuation map during CT-AC.Conclusion In whole-body PET/CT imaging of normally breathing patients, respiration-induced artifacts are reduced in both magnitude and prominence for PET/CT systems employing CT components of six or more detector rows.     

Dosimetry and adequacy of CT-based attenuation correction for pediatric PET: phantom study

PURPOSE: To evaluate the dose from the computed tomographic (CT) portion of positron emission tomography (PET)/CT to determine minimum CT acquisition parameters that provide adequate attenuation correction. MATERIALS AND METHODS: Measurements were made with a PET/CT scanner or a PET scanner, five anthropomorphic phantoms (newborn to medium adult), and an ionization chamber. The CT dose was evaluated for acquisition parameters (10, 20, 40, 80, 160 mA; 80, 100, 120, 140 kVp; 0.5 and 0.8 second per rotation; 1.5:1 pitch). Thermoluminescent dosimetry was used to evaluate the germanium 68/gallium 68 rod sources. A phantom study was performed to evaluate CT image noise and the adequacy of PET attenuation correction as a function of CT acquisition parameters and patient size. RESULTS: The volumetric anthropomorphic CT dose index varied by two orders of magnitude for each phantom over the range of acquisition parameters (0.30 and 21.0 mGy for a 10-year-old with 80 kVp, 10 mAs, and 0.8 second and with 140 kVp, 160 mAs, and 0.8 second, respectively). The volumetric anthropomorphic CT dose index for newborn phantoms was twice that for adult phantoms acquired similarly. The rod source dose was 0.03 mGy (3-minute scan). Although CT noise varied substantially among acquisition parameters, its contribution to PET noise was minimal and yielded only a 2% variation in PET noise. In a pediatric phantom, PET images generated by using CT performed with 80 kVp and 5 mAs for attenuation correction were visually indistinguishable from those generated by using CT performed with 140 kVp and 128 mAs. With very-low-dose CT (80 kVp, 5 mAs) for the adult phantom, undercorrection of the PET data resulted. CONCLUSION: For pediatric patients, adequate attenuation correction can be obtained with very-low-dose CT (80 kVp, 5 mAs, 1.5:1 pitch), and such correction leads to a 100-fold dose reduction relative to diagnostic CT. For adults undergoing CT with 5 mAs and 1.5:1 pitch, the tube voltage needs to be increased to 120 kVp to prevent undercorrection.     

PET/CT imaging artifacts

The purpose of this paper is to introduce the principles of PET/CT imaging and describe the artifacts associated with it. PET/CT is a new imaging modality that integrates functional (PET) and structural (CT) information into a single scanning session, allowing excellent fusion of the PET and CT images and thus improving lesion localization and interpretation accuracy. Moreover, the CT data can also be used for attenuation correction, ultimately leading to high patient throughput. These combined advantages have rendered PET/CT a preferred imaging modality over dedicated PET. Although PET/CT imaging offers many advantages, this dual-modality imaging also poses some challenges. CT-based attenuation correction can induce artifacts and quantitative errors that can affect the PET emission images. For instance, the use of contrast medium and the presence of metallic implants can be associated with focal radiotracer uptake. Furthermore, the patient's breathing can introduce mismatches between the CT attenuation map and the PET emission data, and the discrepancy between the CT and PET fields of view can lead to truncation artifacts. After reading this article, the technologist should be able to describe the principles of PET/CT imaging, identify at least 3 types of image artifacts, and describe the differences between PET/CT artifacts of different causes: metallic implants, respiratory motion, contrast medium, and truncation.     

CT tube current for attenuation map in a combined PET/CT system: obese patient simulated phantom study

Objective

The CT portion of PET/CT provides attenuation correction of the PET emission scan. This study was performed to evaluate how much the CT tube current can be lowered while still providing attenuation maps on PET images.

Methods

Two body phantoms (outside diameters of 300 and 500?mm) were used to investigate, and PET/CT acquisitions were performed with an Aquiduo PCA-7000B (Toshiba Medical Systems, Otawara, Japan). The CT scan was performed with the following parameters (120?kVp; 0.5-s rotation; 10, 20, 40, 80, 160, 200, 320, 460?mA). After the CT scan, PET images for ¹⁸F-FDG (5.3?kBq/mL) were obtained for 4?min/bed position. The linear attenuation coefficients for ¹⁸F-FDG in 300- and 500-mm phantoms, pixel values and SD of CT images, radioactivity concentration values and hot- and cold-sphere contrast on PET images in the 500-mm phantom were evaluated.

Results

In the 300-mm phantom, all eight tube currents gave average linear attenuation coefficients of approximately 0.095?cm^?1. In contrast, the average linear attenuation coefficients of the 500-mm phantom at 10, 20, and 40?mA were significantly decreased (0.081, 0.087, and 0.092?cm^?1, respectively; p??1 of the other tube currents. Further, CT pixel values decreased 10 and 20?mA. Thus, the background radioactivity concentration values at 10 and 20?mA were substantially underestimated to be 57 and 80%, respectively (p?Conclusions Although the linear attenuation coefficients in the 300-mm phantom remained the same with varying CT tube currents, the 500-mm phantom yielded significant differences in the range 10?C40?mA. Therefore, the CT tube currents for attenuation correction should be adjusted over 40?mA in obese patients.     

Do implanted pacemaker leads and ICD leads cause metal-related artifact in cardiac PET/CT?

Artifacts related to metallic implants are an established limitation of CT-based attenuation correction (CT-AC) in PET/CT. However, the impact of metallic components of pacemaker leads and implantable cardioverter defibrillator (ICD) leads on the accuracy of cardiac PET has not been evaluated. The goal of this study was to investigate the magnitude of artifacts related to pacing and defibrillation leads in both phantom and patient studies. METHODS: Images were acquired on a PET/CT scanner using CT-AC and were compared with those obtained on a dedicated PET scanner using transmission source-based attenuation correction. Phantoms consisting of pacemaker leads and ICD leads submerged in uniform background activity solution were imaged, and regions were analyzed to measure radionuclide concentrations at known lead locations relative to background. In addition, 15 cardiac 18F-FDG patients (having either pacing leads, defibrillation leads, or both) were imaged on both PET/CT and PET scanners. Images were visually and quantitatively assessed to determine whether artifact related to the implanted leads was present and, if so, its severity relative to surrounding myocardium. RESULTS: In phantom studies, artifacts caused by pacing lead electrodes were barely noticeable, but artifacts arising from highly radioopaque ICD shock coil electrodes were clearly apparent. In the patient studies, no artifacts from pacing leads were identified. However, significant artifact was observed in 50% of the patient studies with ICD leads. In the affected areas, local myocardial uptake in PET/CT images using CT-AC was, on average, 30% higher than that in the corresponding PET images. CONCLUSION: Although pacemaker leads do not appear to cause artifact in cardiac PET/CT images, ICD leads frequently do result in artifacts of sufficient magnitude to impact clinical image interpretation. Accordingly, software-based corrections in CT-AC algorithms appear necessary for accurate cardiac imaging with PET/CT.     

Impact of improved attenuation correction featuring a bone atlas and truncation correction on PET quantification in whole-body PET/MR

Purpose

Recent studies have shown an excellent correlation between PET/MR and PET/CT hybrid imaging in detecting lesions. However, a systematic underestimation of PET quantification in PET/MR has been observed. This is attributable to two methodological challenges of MR-based attenuation correction (AC): (1) lack of bone information, and (2) truncation of the MR-based AC maps (μmaps) along the patient arms. The aim of this study was to evaluate the impact of improved AC featuring a bone atlas and truncation correction on PET quantification in whole-body PET/MR.

Methods

The MR-based Dixon method provides four-compartment μmaps (background air, lungs, fat, soft tissue) which served as a reference for PET/MR AC in this study. A model-based bone atlas provided bone tissue as a fifth compartment, while the HUGE method provided truncation correction. The study population comprised 51 patients with oncological diseases, all of whom underwent a whole-body PET/MR examination. Each whole-body PET dataset was reconstructed four times using standard four-compartment μmaps, five-compartment μmaps, four-compartment μmaps + HUGE, and five-compartment μmaps + HUGE. The SUV_max for each lesion was measured to assess the impact of each μmap on PET quantification.

Results

All four μmaps in each patient provided robust results for reconstruction of the AC PET data. Overall, SUV_max was quantified in 99 tumours and lesions. Compared to the reference four-compartment μmap, the mean SUV_max of all 99 lesions increased by 1.4 ± 2.5% when bone was added, by 2.1 ± 3.5% when HUGE was added, and by 4.4 ± 5.7% when bone + HUGE was added. Larger quantification bias of up to 35% was found for single lesions when bone and truncation correction were added to the μmaps, depending on their individual location in the body.

Conclusion

The novel AC method, featuring a bone model and truncation correction, improved PET quantification in whole-body PET/MR imaging. Short reconstruction times, straightforward reconstruction workflow, and robust AC quality justify further routine clinical application of this method.

     

Metallic artifacts caused by dental metal prostheses on PET images: a PET/CT phantom study using different PET/CT scanners

Objective  The objective of this study was to investigate the effects of computed tomography (CT) artifacts caused by dental metal prostheses on positron emission tomography (PET) images. Methods  A dental arch cast was fixed in a cylindrical water-bath phantom. A spherical phantom positioned in the vicinity of the dental arch cast was used to simulate a tumor. To simulate the tumor imaging, the ratio of the ¹⁸F-fluoro-deoxy-glucose radioactivity concentration of the spherical phantom to that of the water-bath phantom was set at 2.5. A dental bridge composed of a gold–silver–palladium alloy on the right mandibular side was prepared. A spherical phantom was set in the white artifact area on the CT images (site A), in a slightly remote area from the white artifact (site B), and in a black artifact area (site C). A PET/CT scan was performed with and without the metal bridge at each simulated tumor site, and the artifactual influence was evaluated on the axial attenuation-corrected (AC) PET images, in which the simulated tumor produced the strongest accumulation. Measurements were performed using three types of PET/CT scanners (scanners 1 and 2 with CT-based attenuation correction, and 3 with Cesium-137 (¹³⁷Cs)-based attenuation correction). The influence of the metal bridge was evaluated using the change rate of the SUVmean with and without the metal bridge. Results  At site A, an overestimation was shown (scanner 1: +5.0% and scanner 2: +2.5%), while scanner 3 showed an underestimation of −31.8%. At site B, an overestimation was shown (scanner 1: +2.1% and scanner 2: +2.0%), while scanner 3 showed an underestimation of −2.6%. However, at site C, an underestimation was shown (scanner 1: −25.0%, scanner 2: −32.4%, and scanner 3: −8.4%). Conclusions  When CT is used for attenuation correction in patients with dental metal prostheses, an underestimation of radioactivity of accumulated tracer is anticipated in the dark streak artifact area on the CT images. In this study, the dark streak artifacts of the CT caused by metallic dental prostheses may cause false negative finding of PET/CT in detecting small and/or low uptake tumor in the oral cavity.     

Clinically significant inaccurate localization of lesions with PET/CT: frequency in 300 patients.

This study evaluated lesion mislocalization between PET and CT on PET/CT studies when CT instead of germanium is used for attenuation correction (AC). METHODS: PET/CT scans were obtained for 300 clinical patients. Both CT and germanium scans were used to correct PET emission data. Cases were noted of suspected inaccurate localization of lesions on any of the 5 sets of images (PET using germanium AC [GeAC] fused and not fused with CT, PET using CT AC fused and not fused with CT, and PET with no AC [NAC]). Independent CT or MRI was used to determine true lesion locations. RESULTS: Six of 300 patients (2%) had lesion mislocalization when CT was used for AC or fusion. True liver dome lesions were mislocalized to the right lung base on PET/CT, likely because of a respiratory motion difference between PET and CT. No mislocalization was present on NAC PET or non-CT-fused GeAC PET images. CONCLUSION: Serious lesion mislocalization on PET/CT studies may occur, albeit very infrequently, when CT is used for either AC or fusion.     

Motion correction in PET/CT

Motion in PET/CT leads to artifacts in the reconstructed PET images due to the different acquisition times of positron emission tomography and computed tomography. The effect of motion on cardiac PET/CT images is evaluated in this study and a novel approach for motion correction based on optical flow methods is outlined. The Lukas-Kanade optical flow algorithm is used to calculate the motion vector field on both simulated phantom data as well as measured human PET data. The motion of the myocardium is corrected by non-linear registration techniques and results are compared to uncorrected images.     

Effective methods to correct contrast agent-induced errors in PET quantification in cardiac PET/CT.

In combined PET/CT studies, x-ray attenuation information from the CT scan is generally used for PET attenuation correction. Iodine-containing contrast agents may induce artifacts in the CT-generated attenuation map and lead to an erroneous radioactivity distribution on the corrected PET images. This study evaluated 2 methods of thresholding the CT data to correct these contrast agent-related artifacts. METHODS: PET emission and attenuation data (acquired with and without a contrast agent) were simulated using a cardiac torso software phantom and were obtained from patients. Seven patients with known coronary artery disease underwent 2 electrocardiography-gated CT scans of the heart, the first without a contrast agent and the second with intravenous injection of an iodine-containing contrast agent. A 20-min PET scan (single bed position) covering the same axial range as the CT scans was then obtained 1 h after intravenous injection of (18)F-FDG. For both the simulated data and the patient data, the unenhanced and contrast-enhanced attenuation datasets were used for attenuation correction of the PET data. Additionally, 2 threshold methods (one requiring user interaction) aimed at compensating for the effect of the contrast agent were applied to the contrast-enhanced attenuation data before PET attenuation correction. All PET images were compared by quantitative analysis. RESULTS: Regional radioactivity values in the heart were overestimated when the contrast-enhanced data were used for attenuation correction. For patients, the mean decrease in the left ventricular wall was 23%. Use of either of the proposed compensation methods reduced the quantification error to less than 5%. The required time for postprocessing was minimal for the user-independent method. CONCLUSION: The use of contrast-enhanced CT images for attenuation correction in cardiac PET/CT significantly impairs PET quantification of tracer uptake. The proposed CT correction methods markedly reduced these artifacts; additionally, the user-independent method was time-efficient.     

Use of segmented CT transmission map to avoid metal artifacts in PET images by a PET-CT device

BACKGROUND: Attenuation correction is generally used to PET images to achieve count rate values independent from tissue densities. The goal of this study was to provide a qualitative comparison of attenuation corrected PET images produced by a PET-CT device (CT, 120 kV, 40 mAs, FOV 600 mm) with and without segmentation of transmission data (ACseg+ and ACseg-respectively). Methods: The reconstructed images were compared to attenuation corrected images obtained with a high-energy transmission source (Cs-137 - 662 keV).Thirty oncologic patients were studied using CT and 137Cs for attenuation correction. All image data were acquired using the Gemini PET-CT scanner (Philips Medical Systems). It is an open PET-CT system that consists of the MX8000 multislice CT and the Allegro PET scanner arranged in a separable configuration. Images with ACseg+ and ACseg- were analyzed simultaneously in coronal, sagittal and transaxial planes. Two nuclear medicine physicians reviewed the image sets. Results: The image quality in the area of metal implants was better with ACseg+ than ACseg-, without metal induced artifacts generally observed in CT corrected images. Further the images with ACseg+ were qualitatively comparable to those obtained with 137Cs attenuation correction. Conclusions: In case of metal implants, PET studies corrected by CT should preferably use the ACseg+ method to avoid the image artifacts.     

Respiratory motion artifacts on PET emission images obtained using CT attenuation correction on PET-CT

PET-CT scanners allow generation of transmission maps from CT. The use of CT attenuation correction (CTAC) instead of germanium-68 attenuation correction (Ge AC) might be expected to cause artifacts on reconstructed emission images if differences in respiratory status exist between the two methods of attenuation correction. The aim of this study was to evaluate for possible respiratory motion artifacts (RMA) in PET images attenuation corrected with CT from PET-CT in clinical patients. PET-CT scans were performed using a Discovery LS PET-CT system in 50 consecutive patients (23 males, 27 females; mean age 58.2 years) with known or suspected malignancy. Both CTAC and Ge AC transmission data obtained during free tidal breathing were used to correct PET emission images. Cold artifacts at the interface of the lungs and diaphragm, believed to be due to respiratory motion (RMA), that were seen on CTAC images but not on the Ge AC images were evaluated qualitatively on a four-point scale (0, no artifact; 1, mild artifact; 2, moderate artifact; 3, severe artifact). RMA was also measured for height. Curvilinear cold artifacts paralleling the dome of the diaphragm at the lung/diaphragm interface were noted on 84% of PET-CT image acquisitions and were not seen on the (68)Ge-corrected images; however, these artifacts were infrequently severe. In conclusion, RMA of varying magnitude were noted in most of our patients as a curvilinear cold area at the lung/diaphragm interface, but were not diagnostically problematic in these patients.     

Feasibility of deep-inspiration breath-hold PET/CT with short-time acquisition: detectability for pulmonary lesions compared with respiratory-gated PET/CT

Objectives

Deep-inspiration breath-hold (DIBH) PET/CT with short-time acquisition and respiratory-gated (RG) PET/CT are performed for pulmonary lesions to reduce the respiratory motion artifacts, and to obtain more accurate standardized uptake value (SUV). DIBH PET/CT demonstrates significant advantages in terms of rapid examination, good quality of CT images and low radiation exposure. On the other hand, the image quality of DIBH PET is generally inferior to that of RG PET because of short-time acquisition resulting in poor signal-to-noise ratio. In this study, RG PET has been regarded as a gold standard, and its detectability between DIBH and RG PET studies was compared using each of the most optimal reconstruction parameters.

Methods

In the phantom study, the most optimal reconstruction parameters for DIBH and RG PET were determined. In the clinical study, 19 cases were examined using each of the most optimal reconstruction parameters.

Results

In the phantom study, the most optimal reconstruction parameters for DIBH and RG PET were different. Reconstruction parameters of DIBH PET could be obtained by reducing the number of subsets for those of RG PET in the state of fixing the number of iterations. In the clinical study, high correlation in the maximum SUV was observed between DIBH and RG PET studies. The clinical result was consistent with that of the phantom study surrounded by air since most of the lesions were located in the low pulmonary radioactivity.

Conclusion

DIBH PET/CT may be the most practical method which can be the first choice to reduce respiratory motion artifacts if the detectability of DIBH PET is equivalent with that of RG PET. Although DIBH PET may have limitations in suboptimal signal-to-noise ratio, most of the lesions surrounded by low background radioactivity could provide nearly equivalent image quality between DIBH and RG PET studies when each of the most optimal reconstruction parameters was used.     

Quantitative evaluation of measurement accuracy for three-dimensional angiography system using various phantoms

PURPOSE: The purpose of this study was to evaluate the spatial resolution and accuracy of three-dimensional (3D) distance measurements performed with 3D angiography using various phantoms. MATERIALS AND METHODS: With a 3D angiography system, digital images with a 512 x 512 matrix were obtained with the C-arm sweep, which rotates at a speed of 30 degrees/second. A 3D comb phantom was designed to assess spatial resolution and artifacts at 3D angiography and consisted of six combs with different pitches: 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, and 1.0 mm. Frame rate, field of view (FOV) size, reconstruction matrix, and direction of the phantom were changed. In order to investigate the accuracy of 3D distance measurements, aneurysm phantoms and stenosis phantoms were used. Aneurysm phantoms simulated intracranial saccular aneurysms and parent arteries; 2-mm- or 4-mm-inner-diameter cylinder and five different spheres (diameter: 10, 7, 5, 3, 2 mm) were used. Stenosis phantoms were designed to simulate intracranial steno-occlusive diseases; the nonpulsatile phantoms were made of four cylinders (diameter: 3.0, 3.6, 4.0, 5.0 mm) that had areas of 50% and 75% stenosis. The dimensions of the spheres and cylinders were measured on magnified multiplanar reconstruction (MPR) images. RESULTS: The pitch of the 0.5 mm comb phantom was identified clearly on 3D images reconstructed with a frame rate of 30 frame/sec and 512(3) reconstruction mode. In any reconstruction matrixes and any angles of the phantom, the resolution and artifacts worsened when frame rates were decreased. With regard to the angle of the phantom to the axis of rotational angiography, spatial resolution and artifacts worsened with increase in angle. Spatial resolution and artifacts were better with a FOV of 7 x 7 inch than with one of 9 x 9 inch. All spheres on the aneurysm phantom were clearly demonstrated at any angle; measurement error of sphere size was 0.3 mm or less for 512(3) reconstruction. In 512(3) reconstruction, the error of percent stenosis was 3% or less except for a cylinder diameter of 3.0 mm and 5% for a cylinder diameter of 3.0 mm. CONCLUSION: Spatial resolution of the reconstructed 3D images in this system was 0.5 mm or less. Measurement error of sphere size was 0.3 mm or less when 512(3) reconstruction was used. When using proper imaging parameters and postprocessing methods, measurements of aneurysm size and percent stenosis on the reconstructed 3D angiograms were substantially reliable.     

CT-based attenuation correction in the calculation of semi-quantitative indices of [<Superscript>18</Superscript>F]FDG uptake in PET

The introduction of combined PET/CT systems has a number of advantages, including the utilisation of CT images for PET attenuation correction (AC). The potential advantage compared with existing methodology is less noisy transmission maps within shorter times of acquisition. The objective of our investigation was to assess the accuracy of CT attenuation correction (CTAC) and to study resulting bias and signal to noise ratio (SNR) in image-derived semi-quantitative uptake indices. A combined PET/CT system (GE Discovery LS) was used. Different size phantoms containing variable density components were used to assess the inherent accuracy of a bilinear transformation in the conversion of CT images to 511 keV attenuation maps. This was followed by a phantom study simulating tumour imaging conditions, with a tumour to background ratio of 5:1. An additional variable was the inclusion of contrast agent at different concentration levels. A CT scan was carried out followed by 5 min emission with 1-h and 3-min transmission frames. Clinical data were acquired in 50 patients, who had a CT scan under normal breathing conditions (CTAC(nb)) or under breath-hold with inspiration (CTAC(insp)) or expiration (CTAC(exp)), followed by a PET scan of 5 and 3 min per bed position for the emission and transmission scans respectively. Phantom and patient studies were reconstructed using segmented AC (SAC) and CTAC. In addition, measured AC (MAC) was performed for the phantom study using the 1-h transmission frame. Comparing the attenuation coefficients obtained using the CT- and the rod source-based attenuation maps, differences of 3% and <6% were recorded before and after segmentation of the measured transmission maps. Differences of up to 6% and 8% were found in the average count density (SUV(avg)) between the phantom images reconstructed with MAC and those reconstructed with CTAC and SAC respectively. In the case of CTAC, the difference increased up to 27% with the presence of contrast agent. The presence of metallic implants led to underestimation in the surrounding SUV(avg) and increasing non-uniformity in the proximity of the implant. The patient study revealed no statistically significant differences in the SUV(avg) between either CTAC(nb) or CTAC(exp) and SAC-reconstructed images. The larger differences were recorded in the lung. Both the phantom and the patient studies revealed an average increase of approximately 25% in the SNR for the CTAC-reconstructed emission images compared with the SAC-reconstructed images. In conclusion, CTAC(nb) or CTAC(exp) is a viable alternative to SAC for whole-body studies. With CTAC, careful consideration should be given to interpretation of images and use of SUVs in the presence of oral contrast and in the proximity of metallic implants.     

Correction for oral contrast artifacts in CT attenuation-corrected PET images obtained by combined PET/CT.

Recent studies have shown increased artifacts in CT attenuation-corrected (CTAC) PET images acquired with oral contrast agents because of misclassification of contrast as bone. We have developed an algorithm, segmented contrast correction (SCC), to properly transform CT numbers in the contrast regions from CT energies (40-140 keV) to PET energy at 511 keV. METHODS: A bilinear transformation, equivalent to that supplied by the PET/CT scanner manufacturer, for the conversion of linear attenuation coefficients of normal tissues from CT to PET energies was optimized for BaSO(4) contrast agent. This transformation was validated by comparison with the linear attenuation coefficients measured for BaSO(4) at concentrations ranging from 0% to 80% at 511 keV for PET transmission images acquired with (68)Ge rod sources. In the CT images, the contrast regions were contoured to exclude bony structures and then segmented on the basis of a minimum threshold CT number (300 Hounsfield units). The CT number in each pixel identified with contrast was transformed into the corresponding effective bone CT number to produce the correct attenuation coefficient when the data were translated by the manufacturer software into PET energy during the process of CT attenuation correction. CT images were then used for attenuation correction of PET emission data. The algorithm was validated with a phantom in which a lesion was simulated within a volume of BaSO(4) contrast and in the presence of a human vertebral bony structure. Regions of interest in the lesion, bone, and contrast on emission PET images reconstructed with and without the SCC algorithm were analyzed. The results were compared with those for images obtained with (68)Ge-based transmission attenuation-corrected PET. RESULTS: The SCC algorithm was able to correct for contrast artifacts in CTAC PET images. In the phantom studies, the use of SCC resulted in an approximate 32% reduction in the apparent activity concentration in the lesion compared with data obtained from PET images without SCC and a <7.6% reduction compared with data obtained from (68)Ge-based attenuation-corrected PET images. In one clinical study, maximum standardized uptake value (SUV(max)) measurements for the lesion, bladder, and bowel were, respectively, 14.52, 13.63, and 13.34 g/mL in CTAC PET images, 59.45, 26.71, and 37.22 g/mL in (68)Ge-based attenuation-corrected PET images, and 11.05, 6.66, and 6.33 g/mL in CTAC PET images with SCC. CONCLUSION: Correction of oral contrast artifacts in PET images obtained by combined PET/CT yielded more accurate quantitation of the lesion and other, normal structures. The algorithm was tested in a clinical case, in which SUV(max) measurements showed discrepancies of 2%, 1.3%, and 5% between (68)Ge-based attenuation-corrected PET images and CTAC PET images with SCC for the lesion, bladder, and bowel, respectively. These values correspond to 6.5%, 62%, and 66% differences between CTAC-based measurements and (68)Ge-based ones.     

PET/CT artifacts

There are several artifacts encountered in positron emission tomography/computed tomographic (PET/CT) imaging, including attenuation correction (AC) artifacts associated with using CT for AC. Several artifacts can mimic a 2-deoxy-2-[18F] fluoro-d-glucose (FDG) avid malignant lesions and therefore recognition of these artifacts is clinically relevant. Our goal was to identify and characterize these artifacts and also discuss some protocol variables that may affect image quality in PET/CT.