PET/CT: comparison of quantitative tracer uptake between germanium and CT transmission attenuation-corrected images.

In PET, transmission scanning for attenuation correction has most commonly been performed with an external positron-emitting radionuclide source, such as (68)Ge. More recently, combined PET/CT scanners have been developed in which the CT data can be used for both anatometabolic image formation and attenuation correction of the PET data. The purpose of this study was to assess the quantitative differences between CT-based and germanium-based attenuation-corrected PET images. METHODS: Twenty-eight patients with known or suspected cancer underwent whole-body (18)F-FDG PET/CT scanning for clinical diagnostic purposes. For each patient, attenuation maps were obtained from both the CT scan and the (68)Ge transmission data, and 2 different attenuation-corrected emission datasets were produced. Measured activity concentrations (both mean and maximum) from identical regions of interest in representative normal organs and in 36 pathologic foci of uptake were compared. RESULTS: CT-corrected emission images generally showed slightly higher radioactive concentration values than did germanium-corrected images (P < 0.01) for all lesions and all normal organs except the lung. Mean and maximum radioactivity concentrations were 4.3%-15.2% higher for CT-corrected images than for germanium-corrected images. Calculated radioactivity concentrations were significantly greater in osseous lesions than in nonosseous lesions (11.0% vs. 2.3%, P < 0.05, for mean value; 11.1% vs. 2.1%, P < 0.01, for maximum value). A weak positive correlation was observed between the CT Hounsfield units within the regions of interest and the percentage difference in apparent tracer activity in the CT-corrected images. CONCLUSION: Although quantitative radioactivity values are generally comparable between CT- and germanium-corrected emission PET images, CT-based attenuation correction produced radioactivity concentration values significantly higher than the germanium-based corrected values. These effects, especially in radiodense tissues, should be noted when using and comparing quantitative PET analyses from PET and PET/CT systems.     

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.     

CT vs 68Ge attenuation correction in a combined PET/CT system: evaluation of the effect of lowering the CT tube current

With the introduction of combined positron emission tomography/computed tomography (PET/CT) systems, several questions have to be answered.In this work we addressed two of these questions: (a) to what value can the CT tube current be reduced while still yielding adequate maps for the attenuation correction of PET emission scans and (b) how do quantified uptake values in tumours derived from CT and germanium-68 attenuation correction compare. In 26 tumour patients, multidetector CT scans were acquired with 10, 40, 80 and 120 mA (CT10, CT40, CT80 and CT120) and used for the attenuation correction of a single FDG PET emission scan, yielding four PET scans designated PET(CT10)-PET(CT120). In 60 tumorous lesions, FDG uptake and lesion size were quantified on PET(CT10)-PET(CT120). In another group of 18 patients, one CT scan acquired with 80 mA and a standard transmission scan acquired using 68Ge sources were employed for the attenuation correction of the FDG emission scan (PET(CT80), PET(68Ge)). Uptake values and lesion size in 26 lesions were compared on PET(CT80) and PET(68Ge). In the first group of patients, analysis of variance revealed no significant effect of CT current on tumour FDG uptake or lesion size. In the second group, tumour FDG uptake was slightly higher using CT compared with 68Ge attenuation correction, especially in lesions with high FDG uptake. Lesion size was similar on PET(CT80) and PET(68Ge). In conclusion, low CT currents yield adequate maps for the attenuation correction of PET emission scans. Although the discrepancy between CT- and 68Ge-derived uptake values is probably not relevant in most cases, it should be kept in mind if standardised uptake values derived from CT and 68Ge attenuation correction are compared.     

Optimized intravenous contrast administration for diagnostic whole-body 18F-FDG PET/CT.

Standard application of CT intravenous contrast agents in combined PET/CT may lead to high-density artifacts on CT and attenuation-corrected PET. To avoid associated diagnostic pitfalls, we designed and compared different intravenous contrast injection protocols for routine whole-body PET/CT. METHODS: Whole-body PET/CT included a topogram and a single spiral CT scan (2-row) with or without intravenous contrast, followed by an emission scan. The CT scan was used for attenuation correction of the emission data. Four groups of 10 whole-body PET/CT referrals each were investigated: (A) no intravenous contrast agent, (B) biphasic injection (90 and 50 mL at 3 and 1.5 mL/s, respectively) of intravenous contrast (300 mg/mL iodine) and CT in the craniocaudal direction with a 30-s delay, (C) triple-phase injection (90, 40, and 40 mL at 3, 2, and 1.5 mL/s, respectively) in the craniocaudal direction with a 50-s delay, and (D) dual-phase injection (80 and 60 mL at 3 and 1.5 mL/s, respectively) in the caudocranial direction with a 50-s delay. CT image quality was assessed on a scale from 1 to 3, and CT and attenuation-corrected PET images were reviewed separately for contrast-induced artifacts. RESULTS: Average CT image quality was poorest for protocol A (1.0) but improved to 2.8 when using intravenous contrast agents (protocols B-D). Only protocols B and C resulted in contrast-induced image artifacts that were limited to the thorax. The most homogeneous intravenous contrast enhancement without high-density image artifacts on either CT or PET after CT-based attenuation correction was achieved with protocol D. CONCLUSION: Dual-phase intravenous contrast injection and CT in the caudocranial direction with a 50-s delay yields reproducible high image quality and is now used routinely for combined diagnostic PET/CT at our hospital.     

Head and neck imaging with PET and PET/CT: artefacts from dental metallic implants

Germanium-68 based attenuation correction (PET(Ge68)) is performed in positron emission tomography (PET) imaging for quantitative measurements. With the recent introduction of combined in-line PET/CT scanners, CT data can be used for attenuation correction. Since dental implants can cause artefacts in CT images, CT-based attenuation correction (PET(CT)) may induce artefacts in PET images. The purpose of this study was to evaluate the influence of dental metallic artwork on the quality of PET images by comparing non-corrected images and images attenuation corrected by PET(Ge68) and PET(CT). Imaging was performed on a novel in-line PET/CT system using a 40-mAs scan for PET(CT) in 41 consecutive patients with high suspicion of malignant or inflammatory disease. In 17 patients, additional PET(Ge68) images were acquired in the same imaging session. Visual analysis of fluorine-18 fluorodeoxyglucose (FDG) distribution in several regions of the head and neck was scored on a 4-point scale in comparison with normal grey matter of the brain in the corresponding PET images. In addition, artefacts adjacent to dental metallic artwork were evaluated. A significant difference in image quality scoring was found only for the lips and the tip of the nose, which appeared darker on non-corrected than on corrected PET images. In 33 patients, artefacts were seen on CT, and in 28 of these patients, artefacts were also seen on PET imaging. In eight patients without implants, artefacts were seen neither on CT nor on PET images. Direct comparison of PET(Ge68) and PET(CT) images showed a different appearance of artefacts in 3 of 17 patients. Malignant lesions were equally well visible using both transmission correction methods. Dental implants, non-removable bridgework etc. can cause artefacts in attenuation-corrected images using either a conventional 68Ge transmission source or the CT scan obtained with a combined PET/CT camera. We recommend that the non-attenuation-corrected PET images also be evaluated in patients undergoing PET of the head and neck.     

PET/CT: artifacts caused by bowel motion

BACKGROUND AND AIM: In a combined positron emission tomography (PET) and computed tomography (CT) system, the CT images can be used for attenuation correction as well as for image fusion. However, quantitative and qualitative differences have been reported between CT based attenuation corrected PET and conventional transmission scan corrected PET images. The purpose of this study was to investigate potential differences in PET/CT caused by attenuation differences in bowel due to motion. METHODS: Twelve patients had PET/CT scans performed using 68Ge transmission and CT attenuation correction methods. Three emission imaging datasets were generated including CT corrected PET, Ge corrected PET, and the difference images (CT corrected PET minus Ge corrected PET). PET difference images were used to identify regions of mismatch and to quantify possible discordance between images by using standardized uptake values (SUVs). Using the Ge corrected PET as the standard, differences in emission images were classified as an overestimation (pattern A) or an underestimation (pattern B) in these difference images. RESULTS: One hundred and twenty-three mismatched areas were identified. Among them, overestimated areas in CT corrected image were detected in 36 regions (pattern A), while underestimated areas were evaluated in the remaining 87 regions (pattern B). The mean value of the difference in pattern A (mean +/- standard deviation = 0.84 +/- 0.44) was slightly higher than that in pattern B (0.60 +/- 0.23), and statistically significant. Six of 36 regions in pattern A had an SUV of greater than 2.5 in CT corrected PET but less than 2.5 in Ge corrected PET; two of 87 regions with pattern B demonstrated an SUV greater than 2.5 in Ge corrected PET and less than 2.5 in CT corrected PET. CONCLUSION: Physiological bowel motion may result in attenuation differences and subsequent differences in SUVs. Overestimation of fluorodeoxyglucose uptake should not be misinterpreted as disease.     

Frequent diagnostic errors in cardiac PET/CT due to misregistration of CT attenuation and emission PET images: a definitive analysis of causes, consequences, and corrections.

Cardiac PET combined with CT is rapidly expanding despite artifactual defects and false-positive results due to misregistration of PET and CT attenuation correction data-the frequency, cause, and correction of which remain undetermined. METHODS: Two hundred fifty-nine consecutive patients underwent diagnostic rest-dipyridamole myocardial perfusion PET/CT using (82)Rb, a 16-slice PET/CT scanner, helical CT attenuation correction with breathing and also at end-expiratory breath-hold, and averaged cine CT data during breathing. Misregistration on superimposed PET/CT fusion images was objectively measured in millimeters and correlated with associated quantitative size and severity of PET defects. Misregistration artifacts were defined as PET defects with corresponding misregistration on helical CT-PET fusion images that resolved after correct coregistration using a repeat CT scan, cine CT averaged attenuation during normal breathing, or shifted cine CT data that coregistered with PET data. RESULTS: Misregistration of standard helical CT PET images caused artifactual PET defects in 103 of 259 (40%) patients that were moderate to severe in 59 (23%) (P = 0.0000) and quantitatively normalized on cine or shifted cine CT PET (P = 0.0000). Quantitative misregistration was a powerful predictor of artifact size and severity (P = 0.0000), particularly for transaxial misregistration >6 mm occurring in anterior or lateral areas in 76%, in inferior areas in 16%, and at the apex in 8% of 103 artifactual defects. CONCLUSION: Misregistration of helical CT attenuation and PET emission images causes artifactual defects with false-positive results in 40% of patients that normalize on cine CT PET using averaged CT attenuation data during normal breathing comparable to normal breathing during PET emission scanning and shifting cine CT images to coregister visually with PET.     

Comparison of activity expression between PET using 68Ge-68Ga line source attenuation correction and PET-CT using CT attenuation correction: impact on emission images

OBJECTIVES: CT data can be used for both anatomical image and attenuation correction (CTAC) of PET data in PET-CT scanners. The CTAC method is useful for attenuation correction, because the CT scan time is much shorter than the external radionuclide (e.g., (68)Ge) transmission scan time. However, the energy of the X-rays from CT is not monoenergetic and is much lower than that of the external radionuclide source. In this study, we evaluated the differences between emission PET images reconstructed with CT-based and (68)Ge-based attenuation correction. METHODS: CT scans and (68)Ge-Transmission scans were acquired and used for attenuation correction (CTAC, MAC, and SAC). The PET emission scan time was 4 min. CT scans were acquired at 10, 20, 40, 80, and 160 mA. (68)Ge-Transmission scans were acquired at 1, 3, 5, 10, 20, 40, 60, and 300 min. The attenuation-corrected emission image using MAC on a 300 min transmission scan was defined as the reference image. Seven cylinders (30 mm diameter) were filled with (18)F-FDG placed in a heart-liver phantom with simulated pulmonary mass lesions. The PET value [counts/cc] was measured in circular regions of interest (ROI) over the cylindrical mass lesion. Averages [counts/cc], coefficients of variation [C.V.(%)], and ratios of difference [%Diff] from the reference value were calculated for all conditions. RESULTS: In the CT-Transmission, analysis of variance revealed no significant effect of CT current on the average and the C.V. In the (68)Ge-Transmission, the average and the C.V. changed in dependence on the acquisition time. All %Diff using CT-Transmission were small. It was shown that CT-Transmission is more appropriate than (68)Ge-Transmission.     

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.     

Respiration-induced attenuation artifact at PET/CT: technical considerations

Combined positron emission tomographic (PET)/computed tomographic (CT) scanners allow the use of CT data for attenuation correction of PET images. Eight patients with cancer underwent PET/CT scanning. Transmission scanning was performed with conventional attenuation correction and with CT scanning during maximum inspiration and normal expiration. Image quality was visually compared and fluorine 18 activities were measured in volumes of interest in the lung and myocardium. Analysis of variance for repeated measures revealed a significant decrease (P =.0001) in measured activities between PET images corrected with CT data acquired during maximum inspiration and those corrected with the conventional attenuation correction method or with CT data acquired during normal expiration. Deep inspiration during CT can result in severe deterioration in the final PET image.     

PET/CT with intravenous contrast can be used for PET attenuation correction in cancer patients

Purpose If the CT scan of a combined PET/CT study is performed as a full diagnostic quality CT scan including intravenous (IV) contrast agent, the quality of the joint PET/CT procedure is improved and a separate diagnostic CT scan can be avoided. CT with IV contrast can be used for PET attenuation correction, but this may result in a bias in the attenuation factors. The clinical significance of this bias has not been established. Our aim was to perform a prospective clinical study where each patient had CT performed with and without IV contrast agent to establish whether PET/CT with IV contrast can be used for PET attenuation without reducing the clinical value of the PET scan.Methods A uniform phantom study was used to document that the PET acquisition itself is not significantly influenced by the presence of IV contrast medium. Then, 19 patients referred to PET/CT with IV contrast underwent CT scans without, and then with contrast agent, followed by an ¹⁸F-fluorodeoxyglucose whole-body PET scan. The CT examinations were performed with identical parameters on a GE Discovery LS scanner. The PET data were reconstructed with attenuation correction based on the two CT data sets. A global comparison of standard uptake value (SUV) was performed, and SUVs in tumour, in non-tumour tissue and in the subclavian vein were calculated. Clinical evaluation of the number and location of lesions on all PET/CT scans was performed twice, blinded and in a different random order, by two independent nuclear medicine specialists.Results In all patients, the measured global SUV of PET images based on CT with IV contrast agent was higher than the global activity using non-contrast correction. The overall increase in the mean SUV (for two different conversion tables tested) was 4.5±2.3% and 1.6±0.5%, respectively. In 11/19 patients, focal uptake was identified corresponding to malignant tumours. Eight out of 11 tumours showed an increased SUV_max (2.9±3.1%) on the PET images reconstructed using IV contrast. The clinical evaluation performed by the two specialists comparing contrast and non-contrast CT attenuated PET images showed weighted kappa values of 0.92 (doctor A) and 0.82 (doctor B). No contrast-introduced artefacts were found.Conclusion This study demonstrates that CT scans with IV contrast agent can be used for attenuation correction of the PET data in combined modality PET/CT scanning, without changing the clinical diagnostic interpretation.     

Cine CT for attenuation correction in cardiac PET/CT.

In dual-modality PET/CT systems, the CT scan provides the attenuation map for PET attenuation correction. The current clinical practice of obtaining a single helical CT scan provides only a snapshot of the respiratory cycle, whereas PET occurs over multiple respiratory cycles. Misalignment of the attenuation map and emission image because of respiratory motion causes errors in the attenuation correction factors and artifacts in the attenuation-corrected PET image. To rectify this problem, we evaluated the use of cine CT, which acquires multiple low-dose CT images during a respiratory cycle. We evaluated the average and the intensity-maximum image of cine CT for cardiac PET attenuation correction. METHODS: Cine CT data and cardiac PET data were acquired from a cardiac phantom and from multiple patient studies. The conventional helical CT, cine CT, and PET data of an axially translating phantom were evaluated with and without respiratory motion. For the patient studies, we acquired 2 cine CT studies for each PET acquisition in a rest-stress (13)N-ammonia protocol. Three readers visually evaluated the alignment of 74 attenuation image sets versus the corresponding emission image and determined whether the alignment provided acceptable or unacceptable attenuation-corrected PET images. RESULTS: In the phantom study, the attenuation correction from helical CT caused a major artifactual defect in the lateral wall on the PET image. The attenuation correction from the average and from the intensity-maximum cine CT images reduced the defect by 20% and 60%, respectively. In the patient studies, 77% of the cases using the average of the cine CT images had acceptable alignment and 88% of the cases using the intensity maximum of the cine CT images had acceptable alignment. CONCLUSION: Cine CT offers an alternative to helical CT for compensating for respiratory motion in the attenuation correction of cardiac PET studies. Phantom studies suggest that the average and the intensity maximum of the cine CT images can reduce potential respiration-induced misalignment errors in attenuation correction. Patient studies reveal that cine CT provides acceptable alignment in most cases and suggest that the intensity-maximum cine image offers a more robust alternative to the average cine image.     

Utility of inspiratory diagnostic CT scan after CT/PET study in the detection of pulmonary nodules

We present the case of a 57-year old woman diagnosed of papillary thyroid carcinoma and treated with thyroidectomy followed by radioiodine (I-131) on two occasions. Follow-up radioiodine scan showed disease in right cervical region, confirmed by fine needle aspiration (FNA) and treated with lymphadenectomy. Due to thyroglobulin elevation, I-131 scan negative and inconclusive cervical ultrasonography/CT scan, we conducted a CT/PET study that confirmed cervical disease. An additional CT scan that was performed on maximum-inspiration showed four micro-nodules, one of which was not detected by the CT scan on shallow breathing (CT/PET). Post-treatment (I-131) scan confirmed uptake in these localizations. Good fusion between PET and CT images that avoids the errors of attenuation correction, especially in the lung bases, is necessary for correct image interpretation of the CT/PET study. Shallow breathing is necessary in order to obtain optimal image fusion with the CT/PET study, although this is not the best to evaluate pulmonary parenchyma in which an additional inspiratory CT scan improves detection of the pulmonary nodules.     

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.     

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.     

Reducing radiation dose in rest-stress cardiac PET/CT by single poststress cine CT for attenuation correction: quantitative validation.

Cardiac PET/CT is optimized by cine CT with dedicated shift software for manual correction of attenuation-emission misregistration. Separate rest and stress CT scans incur greater radiation dose to patients than does standard helical PET/CT or "pure" PET using rotating rod attenuation sources. To reduce radiation dose, we tested quantitative accuracy of using a single poststress cine CT attenuation scan for reconstructing rest perfusion images to eliminate resting CT attenuation scans. METHODS: A total of 250 consecutive patients underwent diagnostic rest-dipyridamole myocardial perfusion PET/CT with (82)Rb and a 16-slice PET/CT scanner using averaged cine CT attenuation data during breathing at rest and stress. After correcting for any attenuation-emission misregistration, we quantitatively compared resting perfusion images reconstructed using rest cine CT attenuation data with the same resting emission data reconstructed with poststress cine CT attenuation data. Automated software quantifying average regional quadrant activity, severity, size, and combined size and severity of perfusion defects was used for this comparison. RESULTS: Resting perfusion images reconstructed using rest cine CT attenuation data were quantitatively comparable to resting images reconstructed with poststress cine CT attenuation data with no clinically significant differences. Twenty-five (10%) of 250 cases required shifting of stress cine CT attenuation data to achieve optimal attenuation-emission coregistration with resting perfusion data. Eliminating rest CT attenuation scans reduced CT radiation dose by 50% below rest-plus-stress cine CT protocols. CONCLUSION: Resting perfusion images reconstructed using poststress cine CT attenuation data are quantitatively comparable to resting images reconstructed with resting cine CT attenuation data. Eliminating the rest CT scan reduces CT radiation dose by 50%.     

Application of intravenous contrast in PET/CT: does it really introduce significant attenuation correction error?

The current perception of using contrast-enhanced CT (CECT) for attenuation correction (AC) is that of caution, as it might lead to erroneously elevated (18)F-FDG uptake on the PET scan. This study evaluates in vivo whether an intravenous iodinated contrast agent produces a significant AC artifact in the level of standardized uptake value (SUV) changes in PET/CT. METHODS: Fifty-four patients referred for whole-body (WB) PET/CT scans were enrolled and subdivided into 2 groups. In part I, 26 patients had a single WB PET scan that was corrected for attenuation using noncontrast and intravenous CECT obtained before and after the emission data, respectively. The final PET images were compared for any visual and SUV maximum (SUV(max)) measurement difference. This allowed analysis of the compatibility of the scaling processes between the 2 different CTs and the PET. The SUV(max) values were obtained from ascending aorta, upper lung, femoral head, iliopsoas muscle, spleen, liver, and the site of pathology (total, 193 regions). Part II addressed whether intravenous contrast also influenced the PET emission data. For that purpose, the remaining 28 patients underwent a limited plain CT scan from lung base to lower liver edge, followed by a 1-bed PET scan of the same region and then a WB intravenous contrast CT scan in tandem with a WB PET scan. SUV(max) values were obtained at the lung base, liver, spleen, T11 or T12 vertebra, and paraspinal muscle (total, 135 regions). The data obtained from pre- and post-intravenous contrast PET scans were analyzed as in part I. RESULTS: There was no statistically significant elevation of the SUV level in the measured anatomic sites as a whole (part I: mean SUV(max) difference = 0.06, P > 0.05; Part II: mean SUV(max) difference = -0.02, P > 0.05). However, statistically significant results as a group (mean SUV(max) difference = 0.26, P < 0.05)--albeit considered to be clinically insignificant--were observed for areas of pathology in the part I study. No abnormal focal increased (18)F-FDG activity was detected as a result of the intravenous contrast in both parts of this examination. CONCLUSION: No statistically or clinically significant spuriously elevated SUV level that might potentially interfere with the diagnostic value of PET/CT was identified as a result of the application of intravenous iodinated contrast.     

Application of oral contrast media in coregistered positron emission tomography-CT

OBJECTIVE: Coregistration of positron emission tomography (PET) and CT images results in significantly improved localization of abnormal FDG uptake compared with PET images alone. For delineation of intestinal structures, application of oral contrast media is a standard procedure in CT. The influence of oral contrast agents in PET imaging using CT data for attenuation correction was evaluated in a comparative study on an in-line PET-CT system. SUBJECTS AND METHODS: Sixty patients referred for PET-CT were evaluated in two groups. One group of 30 patients received oral Gastrografin 45 min before data acquisition. The second group received no contrast medium. PET images were reconstructed, using CT data for attenuation correction. Image analysis was performed by two reviewers in consensus, using a 4-point scale comparing FDG-uptake in the gastrointestinal tract in PET images of both groups. Furthermore, correlation of FDG uptake and localization of contrast media in the intestinal tract in CT images were determined. RESULTS: No significant difference in FDG uptake in PET images in all regions of the gastrointestinal tract except the ascending colon was seen in both groups. No correlation was found in the location of increased FDG uptake and contrast media in the CT images. CONCLUSION: An oral contrast agent can be used for coregistered PET-CT without the introduction of artifacts in PET.     

Respiration-averaged CT for attenuation correction in canine cardiac PET/CT.

Heart disease is a leading cause of death in North America. With the increased availability of PET/CT scanners, CT is now commonly used as a transmission source for attenuation correction. Because of the differences in scan duration between PET and CT, respiration-induced motion can create inconsistencies between the PET and CT data and lead to incorrect attenuation correction and, thus, artifacts in the final reconstructed PET images. This study compared respiration-averaged CT and 4-dimensional (4D) CT for attenuation correction of cardiac PET in an in vivo canine model as a means of removing these inconsistencies. METHODS: Five dogs underwent respiration-gated cardiac (18)F-FDG PET and 4D CT. The PET data were reconstructed with 3 methods of attenuation correction that differed only in the CT data used: The first method was single-phase CT at either end-expiration, end-inspiration, or the middle of a breathing cycle; the second was respiration-averaged CT, which is CT temporally averaged over the entire respiratory cycle; and the third was phase-matched CT, in which each PET phase is corrected with the matched phase from 4D CT. After reconstruction, the gated PET images were summed to produce an ungated image. Polar plots of the PET heart images were generated, and percentage differences were calculated with respect to the phase-matched correction for each dog. The difference maps were then averaged over the 5 dogs. RESULTS: For single-phase CT correction at end-expiration, end-inspiration, and mid cycle, the maximum percentage differences were 11% +/- 4%, 7% +/- 3%, and 5% +/- 2%, respectively. Conversely, the maximum difference for attenuation correction with respiration-averaged CT data was only 1.6% +/- 0.7%. CONCLUSION: Respiration-averaged CT correction produced a maximum percentage difference 7 times smaller than that obtained with end-expiration single-phase correction. This finding indicates that using respiration-averaged CT may accurately correct for attenuation on respiration-ungated cardiac PET.     

Effects of nonionic intravenous contrast agents at PET/CT imaging: phantom and canine studies

PURPOSE: To investigate the effects of intravenous contrast agents on quantitative values obtained with a combined positron emission tomographic (PET) and computed tomographic (CT) scanner by using several phantoms and a dog. MATERIALS AND METHODS: Fluorine 18 fluorodeoxyglucose (FDG) was mixed with different concentrations of contrast agent with the same syringe (phantom 1), and the phantom was scanned. After image reconstruction with various attenuation maps, radioactivity concentrations were compared. Then, FDG solutions with (phantom 2) or surrounded by (phantom 3) various concentrations of contrast agent were scanned repeatedly, and radioactivity concentration was compared. Finally, PET and CT with and without contrast agent were performed in a dog. PET images were reconstructed by using different attenuation maps, and radioactivity concentrations were compared. The radioactivity concentration on germanium 68 (68Ge)-based corrected images was regarded as standard, and percentage bias, defined as difference divided by measured activity of 68Ge-based corrected images, was assessed. The relationship between the concentration of contrast agent and the percentage bias was assessed with the Pearson coefficient r, and the significance of correlations was evaluated with the Fisher z test. RESULTS: All phantom studies demonstrated that presence of a contrast agent resulted in overestimation of emission data. CT numbers showed a strong positive correlation with the percentage bias in phantoms 2 (r = 0.999) and 3 (r = 0.987); the maximum percentage bias at 1,360 HU reached approximately 45%. These effects were independent of FDG concentration. In a canine model, presence of a contrast agent also increased emission activity, but the percentage bias was less than 15% in the liver and smaller in all other organs except the kidney (26%). CONCLUSION: High concentrations of a contrast agent caused considerable overestimation of apparent tracer activity in phantom studies; however, the emission bias was relatively modest in vivo, except in areas with very high contrast agent concentrations.