Measured attenuation correction methods

Accurate attenuation correction is a prerequisite for the determination of exact local radioactivity concentrations in positron emission tomography. Attenuation correction factors range from 4-5 in brain studies to 50-100 in whole body measurements. This report gives an overview of the different methods of determining the attenuation correction factors by transmission measurements using an external positron emitting source. The long-lived generator nuclide 68Ge/68Ga is commonly used for this purpose. The additional patient dose from the transmission source is usually a small fraction of the dose due to the subsequent emission measurement. Ring-shaped transmission sources as well as rotating point or line sources are employed in modern positron tomographs. By masking a rotating line or point source, random and scattered events in the transmission scans can be effectively suppressed. The problems of measured attenuation correction are discussed: transmission/emission mismatch, random and scattered event contamination, counting statistics, transmission/emission scatter compensation, transmission scan after administration of activity to the patient. By using a double masking technique simultaneous emission and transmission scans become feasible.     

Anatomically derived attenuation coefficients for use in quantitative single photon emission tomography studies of the thorax

Elimination of errors due to poor attenuation correction is an essential part of any quantitative single photon emission tomography (SPET) technique. Attenuation coefficients (_Tc) for use in attenuation correction of SPET data were determined using technetium 99m and cobalt 57 flood sources and using topographical information obtained from computed tomography (CT) scans and magnetic resonance (MR) images. In patients with carcinoma of the bronchus, the mean attenuation coefficient for ^99mTc was 0.096 cm^–1 when determined across a transverse section of the thorax at the level of the tumour by means of a ⁵⁷CO flood source (13 patients) and 0.093 and 0.074 cm^–1 as determined from CT scans for points in the centre of the tumour and contralateral normal lung, respectively (21 patients). In 18 patients with breast tumours, the mean attenuation coefficient for ^99mTc was 0.110 and 0.076 cm^–1 when determined from MRI cross-sections for points in the centre of the tumour and normal contralateral lung, respectively. This indicates significant overcorrection for attenuation when the conventional value of 0.12 cm^–1 is used. A value in the range 0.08–0.09 cm^–1 would be more appropriate for SPET studies of the thorax. An alternative approach to quantitative region of interest (ROI) analysis is to perform attenuation correction appropriate to the centre of each ROI (using topographical information derived from CT or MRI) on non-attenuation-corrected reconstructions.     

X-ray-based attenuation correction for positron emission tomography/computed tomography scanners

A synergy of positron emission tomography (PET)/computed tomography (CT) scanners is the use of the CT data for x-ray-based attenuation correction of the PET emission data. Current methods of measuring transmission use positron sources, gamma-ray sources, or x-ray sources. Each of the types of transmission scans involves different trade-offs of noise versus bias, with positron transmission scans having the highest noise but lowest bias, whereas x-ray scans have negligible noise but the potential for increased quantitative errors. The use of x-ray-based attenuation correction, however, has other advantages, including a lack of bias introduced from post-injection transmission scanning, which is an important practical consideration for clinical scanners, as well as reduced scan times. The sensitivity of x-ray-based attenuation correction to artifacts and quantitative errors depends on the method of translating the CT image from the effective x-ray energy of approximately 70 keV to attenuation coefficients at the PET energy of 511 keV. These translation methods are usually based on segmentation and/or scaling techniques. Errors in the PET emission image arise from positional mismatches caused by patient motion or respiration differences between the PET and CT scans; incorrect calculation of attenuation coefficients for CT contrast agents or metallic implants; or keeping the patient's arms in the field of view, which leads to truncation and/or beam-hardening (or x-ray scatter) artifacts. Proper interpretation of PET emission images corrected for attenuation by using the CT image relies on an understanding of the potential artifacts. In cases where an artifact or bias is suspected, careful inspection of all three available images (CT and PET emission with and without attenuation correction) is recommended.     

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.     

Transmission scanning in emission tomography

Attenuation correction in single-photon (SPET) and positron emission (PET) tomography is now accepted as a vital component for the production of artefact-free, quantitative data. The most accurate attenuation correction methods are based on measured transmission scans acquired before, during, or after the emission scan. Alternative methods use segmented images, assumed attenuation coefficients or consistency criteria to compensate for photon attenuation in reconstructed images. This review examines the methods of acquiring transmission scans in both SPET and PET and the manner in which these data are used. While attenuation correction gives an exact correction in PET, as opposed to an approximate one in SPET, the magnitude of the correction factors required in PET is far greater than in SPET. Transmission scans also have a number of other potential applications in emission tomography apart from attenuation correction, such as scatter correction, inter-study spatial co-registration and alignment, and motion detection and correction. The ability to acquire high-quality transmission data in a practical clinical protocol is now an essential part of the practice of nuclear medicine. Received: 19 February 1998 / Accepted: 19 March 1998     

A method for calibrating three-dimensional positron emission tomography without scatter correction

Calibration for three-dimensional positron emission tomography (3D PET) using a uniform cylinder and cross-calibration with aliquots requires correction for scatter and attenuation. Thus the accuracy of thecalibration is dependent on the scatter correction method, and on the applicability of the scatter correction for different regions of the body. A method has been developed which provides a calibration which does not require correction for scatter or attenuation, making it generally applicable and independent of the scatter correction. The method has been previously described for measurement of the absolute sensitivity of tomography devices. This approach has been extended to give a calibration of the PET camera in air in units of kBq/pixel. The reconstructed images are multiplied by this factor to, give accurate activity concentrations, after attenuation and scatter correction. The method has been used with a fully 3D filtered back-projection (reprojection) algorithm and iterative convolution-subtraction scatter correction on data from an ECAT 953B. Using this method 3D PET images have been calibrated te, within ±5% accuracy, but this is highly dependent on the accuracy of the scatter correction. The method described here is practical and provides a means of calibrating a 3D PET system without the need for correction for scatter or attenuation of the calibration data.     

Impact of subject head motion on quantitative brain 15O PET and its correction by image-based registration algorithm

Objective

Subject head motion during sequential ¹⁵O positron emission tomography (PET) scans can result in artifacts in cerebral blood flow (CBF) and oxygen metabolism maps. However, to our knowledge, there are no systematic studies examining this issue. Herein, we investigated the effect of head motion on quantification of CBF and oxygen metabolism, and proposed an image-based motion correction method dedicated to ¹⁵O PET study, correcting for transmission–emission mismatch and inter-scan mismatch of emission scans.

Methods

We analyzed ¹⁵O PET data for patients with major arterial steno-occlusive disease (n = 130) to determine the occurrence frequency of head motion during ¹⁵O PET examination. Image-based motion correction without and with realignment between transmission and emission scans, termed simple and 2-step method, respectively, was applied to the cases that showed severe inter-scan motion.

Results

Severe inter-scan motion (>3 mm translation or >5° rotation) was observed in 27 of 520 adjacent scan pairs (5.2 %). In these cases, unrealistic values of oxygen extraction fraction (OEF) or cerebrovascular reactivity (CVR) were observed without motion correction. Motion correction eliminated these artifacts. The volume-of-interest (VOI) analysis demonstrated that the motion correction changed the OEF on the middle cerebral artery territory by 17.3 % at maximum. The inter-scan motion also affected CBV, CMRO₂ and CBF, which were improved by the motion correction. A difference of VOI values between the simple and 2-step method was also observed.

Conclusions

These data suggest that image-based motion correction is useful for accurate measurement of CBF and oxygen metabolism by ¹⁵O 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.     

Radiation exposure during transmission measurements: comparison between CT- and germanium-based techniques with a current PET scanner

In positron emission tomographic (PET) scanning, transmission measurements for attenuation correction are commonly performed by using external germanium-68 rod sources. Recently, combined PET and computed tomographic (CT) scanners have been developed in which the CT data can be used for both anatomical-metabolic image formation and attenuation correction of the PET data. The purpose of this study was to evaluate the difference between germanium- and CT-based transmission scanning in terms of their radiation doses by using the same measurement technique and to compare the doses that patients receive during brain, cardiac and whole-body scans. Measurement of absorbed doses to organs was conducted by using a Rando Alderson phantom with thermoluminescent dosimeters. Effective doses were calculated according to the guidelines in the International Commission on Radiation Protection Publication Number 60. Compared with radionuclide doses used in routine 2-[fluorine-18]-fluoro-2-deoxy-d-glucose PET imaging, doses absorbed during germanium-based transmission scans were almost negligible. On the other hand, absorbed doses from CT-based transmission scans were significantly higher, particularly with a whole-body scanning protocol. Effective doses were 8.81 mSv in the high-speed mode and 18.97 mSv in the high-quality mode for whole-body CT-based transmission scans. These measurements revealed that the doses received by a patient during CT-based transmission scanning are more than those received in a typical PET examination. Therefore, the radiation doses represent a limitation to the generalised use of CT-based transmission measurements with current PET/CT scanner systems.     

The effect of metal artefact reduction on CT-based attenuation correction for PET imaging in the vicinity of metallic hip implants: a phantom study

Background

To determine if metal artefact reduction (MAR) combined with a priori knowledge of prosthesis material composition can be applied to obtain CT-based attenuation maps with sufficient accuracy for quantitative assessment of ¹⁸F-fluorodeoxyglucose uptake in lesions near metallic prostheses.

Methods

A custom hip prosthesis phantom with a lesion-sized cavity filled with 0.2 ml ¹⁸F-FDG solution having an activity of 3.367 MBq adjacent to a prosthesis bore was imaged twice with a chrome–cobalt steel hip prosthesis and a plastic replica, respectively. Scanning was performed on a clinical hybrid PET/CT system equipped with an additional external ¹³⁷Cs transmission source. PET emission images were reconstructed from both phantom configurations with CT-based attenuation correction (CTAC) and with CT-based attenuation correction using MAR (MARCTAC). To compare results with the attenuation-correction method extant prior to the advent of PET/CT, we also carried out attenuation correction with ¹³⁷Cs transmission-based attenuation correction (TXAC). CTAC and MARCTAC images were scaled to attenuation coefficients at 511 keV using a trilinear function that mapped the highest CT values to the prosthesis alloy attenuation coefficient. Accuracy and spatial distribution of the lesion activity was compared between the three reconstruction schemes.

Results

Compared to the reference activity of 3.37 MBq, the estimated activity quantified from the PET image corrected by TXAC was 3.41 MBq. The activity estimated from PET images corrected by MARCTAC was similar in accuracy at 3.32 MBq. CTAC corrected PET images resulted in nearly 40 % overestimation of lesion activity at 4.70 MBq. Comparison of PET images obtained with the plastic and metal prostheses in place showed that CTAC resulted in a marked distortion of the ¹⁸F-FDG distribution within the lesion, whereas application of MARCTAC and TXAC resulted in lesion distributions similar to those observed with the plastic replica.

Conclusions

MAR combined with a trilinear CT number mapping for PET attenuation correction resulted in estimates of lesion activity comparable in accuracy to that obtained with ¹³⁷Cs transmission-based attenuation correction, and far superior to estimates made without attenuation correction or with a standard CT attenuation map. The ability to use CT images for attenuation correction is a potentially important development because it obviates the need for a ¹³⁷Cs transmission source, which entails extra scan time, logistical complexity and expense.     

PET and MR imaging in a neuro-behçet syndrome

Positron emission tomography (PET) and magnetic resonance imaging (MRI) studies were performed on a case of neuro-Behçet's syndrome. In accordance with the clinical signs, FDG PET (using¹⁸F-labeled 2-F-2-desoxyglucose) revealed disseminated storage defects in the cerebrum and cerebellum. Focal regions of enhanced signal intensity were demonstrated in the parietal white matter of the cerebrum in T2-weighted images and in the brain stem by MRI.This article was presented at the 1st EEC workshop on accuracy determination in PET, January 19–20th. 1989 Pisa, Italy (COMAC-BME Concerted Project Characterization and Standardization of PET Instrumentation)     

PET/CT imaging: Effect of respiratory motion on apparent myocardial uptake

Background  Positron emission tomography (PET) attenuation correction (AC) using computed tomography (CT) can be affected by respiratory motion: hi-speed CT captures 1 point of the respiratory cycle while PET emission data averages many cycles. We quantified the changes in apparent myocardial uptake due to this respiratory-induced CT attenuation mismatch. as]Methods  Twenty-two patients undergoing fluorine-18 fluorodexyglucose (FDG) PET/CT received 3 sequential CT scans at normal resting end-inspiration (CT _INSPIR ), ending expiration (CT_EXPIR), and at midvolume between end-expiration and end-inspiration (CT _MIDVOL ). A pneumotachometer measured absolute changes in lung volume. Seven subjects also underwent a 3-minute transmission scan with a ⁶⁸Ge rotating rod source (RRS). The PET emission data set was reconstructed up to 4 times using CT _EXPIR , CT_INSPIR, CT _MIDVOL , and RRS AC maps. Relative heart position and cardiac uptake was measured for each CT attenuation correction. Results  Respiratory motion produced marked changes in global and regional myocardial uptake. Changes were large in the lateral and anterior regions at the lung-soft tissue interface (up to 30% using CT _INSPIR compared to CT _EXPIR for AC) and smaller in the septal region (10% or less). Data corrected with CT _EXPIR agreed best with the RRS. Conclusion  Respiratory effects can introduce large inhomogeneities in apparent myocardial uptake when CT is used for attenuation correction.     

The performance of<Superscript> 18</Superscript>F-fluorodeoxyglucose positron emission tomography in small solitary pulmonary nodules

Solitary pulmonary nodule (SPN, intraparenchymal lung mass <3 cm) is often a diagnostic challenge. This study was performed to evaluate the diagnostic accuracy of ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG PET) in radiologically indeterminate SPN 10 mm on spiral CT. Between August 1997 and March 2001, we identified all patients with radiologically indeterminate SPNs 10 mm who were referred for FDG PET imaging at the VU University Medical Centre. All PET scans were retrospectively reviewed by an experienced nuclear medicine physician. PET was considered positive in cases with at least moderately enhanced focal uptake, and otherwise as negative. Lesions were considered benign on the basis of histology, no growth during 1.5 years or disappearance within at least 6 months. Thirty-five patients with 36 SPNs 10 mm in diameter at clinical presentation were identified (one patient had two metachronous lesions). In 13 of 14 malignant nodules and in two of 22 benign nodules, diagnosis was confirmed by histology. Prevalence of malignancy was 39%. PET imaging correctly identified 30 of 36 small lesions. One lesion proved to be false negative on PET (CT: 10 mm), and in five lesions, PET scans proved to be false positive. Specificity was 77% (17/22; 95% CI: 0.55–0.92), sensitivity 93% (13/14; 95% CI: 0.66–1.0), positive predictive value 72% (13/18; 95% CI: 0.46–0.90) and negative predictive value 94% (17/18; 95% CI: 0.73–1.0). This retrospective study suggests that FDG PET imaging could be a useful tool in differentiating benign from malignant SPNs 10 mm in diameter at clinical presentation. Such results may help in the design of larger prospective trials with structured clinical work-up.     

PET attenuation coefficients from CT images: experimental evaluation of the transformation of CT into PET 511-keV attenuation coefficients

The CT data acquired in combined PET/CT studies provide a fast and essentially noiseless source for the correction of photon attenuation in PET emission data. To this end, the CT values relating to attenuation of photons in the range of 40-140 keV must be transformed into linear attenuation coefficients at the PET energy of 511 keV. As attenuation depends on photon energy and the absorbing material, an accurate theoretical relation cannot be devised. The transformation implemented in the Discovery LS PET/CT scanner (GE Medical Systems, Milwaukee, Wis.) uses a bilinear function based on the attenuation of water and cortical bone at the CT and PET energies. The purpose of this study was to compare this transformation with experimental CT values and corresponding PET attenuation coefficients. In 14 patients, quantitative PET attenuation maps were calculated from germanium-68 transmission scans, and resolution-matched CT images were generated. A total of 114 volumes of interest were defined and the average PET attenuation coefficients and CT values measured. From the CT values the predicted PET attenuation coefficients were calculated using the bilinear transformation. When the transformation was based on the narrow-beam attenuation coefficient of water at 511 keV (0.096 cm(-1)), the predicted attenuation coefficients were higher in soft tissue than the measured values. This bias was reduced by replacing 0.096 cm(-1) in the transformation by the linear attenuation coefficient of 0.093 cm(-1) obtained from germanium-68 transmission scans. An analysis of the corrected emission activities shows that the resulting transformation is essentially equivalent to the transmission-based attenuation correction for human tissue. For non-human material, however, it may assign inaccurate attenuation coefficients which will also affect the correction in neighbouring tissue.     

PET transmission tomography using a novel nonlocal MRF prior

In positron emission tomography, transmission scans can be performed to estimate attenuation correction factors (ACFs) which are in turn used to correct the emission scans. And such an attenuation correction is crucial for quantitatively accurate PET reconstructions. The prior model used in this work was based on our assumption that the attenuation values vary smoothly, with occasional discontinuities at anatomical borders. And on the other hand, long acquisition or scan times, although alleviating the noise effect of the count-limited scans, are blamed for patient uncomfortableness and movements. So, transmission tomography often suffers from the noise effect because of the short scan time. Thus reconstruction which is capable of overcoming the noise effect is highly needed. In this article, we apply the nonlocal prior Bayesian reconstruction method in PET transmission tomography. Resulting experimentations validate that the reconstructions using the nonlocal prior can reconstruct better transmission images and overcome noise effect even when the scan time is relatively short.     

MRI-based attenuation correction for PET/MRI: a novel approach combining pattern recognition and atlas registration

For quantitative PET information, correction of tissue photon attenuation is mandatory. Generally in conventional PET, the attenuation map is obtained from a transmission scan, which uses a rotating radionuclide source, or from the CT scan in a combined PET/CT scanner. In the case of PET/MRI scanners currently under development, insufficient space for the rotating source exists; the attenuation map can be calculated from the MR image instead. This task is challenging because MR intensities correlate with proton densities and tissue-relaxation properties, rather than with attenuation-related mass density. METHODS: We used a combination of local pattern recognition and atlas registration, which captures global variation of anatomy, to predict pseudo-CT images from a given MR image. These pseudo-CT images were then used for attenuation correction, as the process would be performed in a PET/CT scanner. RESULTS: For human brain scans, we show on a database of 17 MR/CT image pairs that our method reliably enables estimation of a pseudo-CT image from the MR image alone. On additional datasets of MRI/PET/CT triplets of human brain scans, we compare MRI-based attenuation correction with CT-based correction. Our approach enables PET quantification with a mean error of 3.2% for predefined regions of interest, which we found to be clinically not significant. However, our method is not specific to brain imaging, and we show promising initial results on 1 whole-body animal dataset. CONCLUSION: This method allows reliable MRI-based attenuation correction for human brain scans. Further work is necessary to validate the method for whole-body imaging.     

A method for postinjection PET transmission measurements with a rotating source

A method is presented for obtaining accurate positron emission tomography transmission measurements after tracer injection. A transmission scan is performed using a rotating source immediately before or after a conventional emission scan. Sinogram windowing, which removes most scattered and random coincidences, also removes most of the emission counts contaminating the transmission measurement. Data from the emission scan can be used to subtract the remaining emission counts to produce accurate transmission measurements. For studies with moderate to low emission count rates (e.g., fluorodeoxyglucose) there is little increase in noise in the resulting attenuation correction factors. This method was tested in experiments with phantoms and a rotating source simulator and validated against conventional ring transmission measurements. Applications of the technique can significantly shorten the time between transmission and emission studies, and thereby reduce the likelihood of patient motion and increase scanning throughput.     

Attenuation compensation in cerebral 3D PET: effect of the attenuation map on absolute and relative quantitation

It is generally well accepted that transmission (TX)-based non-uniform attenuation correction can supply more accurate absolute quantification; however, whether it provides additional benefits in routine clinical diagnosis based on qualitative interpretation of 3D brain positron emission tomography (PET) images is still the subject of debate. The aim of this study was to compare the effect of the two major classes of method for determining the attenuation map, i.e. uniform versus non-uniform, using clinical studies based on qualitative assessment as well as absolute and relative quantitative volume of interest-based analysis. We investigated the effect of six different methods for determining the patient-specific attenuation map. The first method, referred to as the uniform fit-ellipse method (UFEM), approximates the outline of the head by an ellipse assuming a constant linear attenuation factor (=0.096 cm^–1) for soft tissue. The second, referred to as the automated contour detection method (ACDM), estimates the outline of the head from the emission sinogram. Attenuation of the skull is accounted for by assuming a constant uniform skull thickness (0.45 cm) within the estimated shape and the correct value (0.151 cm^–1) is used. The usual measured transmission method using caesium-137 single-photon sources was used without (MTM) and with segmentation of the TX data (STM). These techniques were finally compared with the segmented magnetic resonance imaging method (SMM) and an implementation of the inferring attenuation distributions method (IADM) based on the digital Zubal head atlas. Several image quality parameters were compared, including absolute and relative quantification indexes, and the correlation between them was checked. The qualitative evaluation showed no significant differences between the different attenuation correction techniques as assessed by expert physicians, with the exception of ACDM, which generated artefacts in the upper edges of the head. The mean squared error between the different attenuation maps was also larger when using this latter method owing to the fact that the current implementation of the method significantly overestimated the head contours on the external slices. Correlation between the mean regional cerebral glucose metabolism (rCGM) values obtained with the various attenuation correction methods and those obtained with the gold standard (MTM) was good, except in the case of ACDM (R ²=0.54). The STM and SMM methods showed the best correlation (R ²=0.90) and the regression lines agreed well with the line of identity. Relative differences in mean rCGM values were in general less than 8%. Nevertheless, ANOVA results showed statistically significant differences between the different methods for some regions of the brain. It is concluded that the attenuation map influences both absolute and relative quantitation in cerebral 3D PET. Transmission-less attenuation correction results in a reduced radiation dose and makes a dramatic difference in acquisition time, allowing increased patient throughput.     

Impact of metallic dental implants on CT-based attenuation correction in a combined PET/CT scanner

Our objective was to study the effect of metal-induced artifacts on the accuracy of the CT-based anatomic map as a prerequisite for attenuation correction of the positron emission tomography (PET) emission data. Twenty-seven oncology patients with dental metalwork were enrolled in the present study. Data acquisition was performed on a PET/CT in-line system (Discovery LS, GE Medical Systems, Milwaukee, Wis.). Attenuation correction of emission data was done twice, using an 80-mA CT scan (PET_CT80) and a ⁶⁸Ge transmission scan (PET_68Ge). Average count in kBq/cc was measured in regions with and without artifacts and compared for PET_CT80 and PET_68Ge. Data analysis of region of interests (ROIs) revealed that the ratio (ROIs PET_CT80/ROIs PET_68Ge) and the difference (ROIs PET_CT80 minus ROIs PET_68Ge) had a higher mean of values in regions with artifacts than in regions without artifacts (1.2±0.17 vs 1.06±0.06 and 0.68±0.67 vs 0.15±0.17 kBq/cc, respectively). For most of the studied artifactual ROIs, the PET_CT80 values were higher than those of the PET_68Ge. Attenuation correction of PET emission data using an artifactual CT map yields false values in regions nearby artifacts caused by dental metalwork. This may falsely estimate PET quantitative studies and may disturb the visual interpretation of PET scan. Electronic Publication