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Dosimetry and Adequacy of CT-based Attenuation Correction for Pediatric PET: Phantom Study¹

¹ From the Division of Nuclear Medicine and Department of Radiology, Children's Hospital Boston, 300 Longwood Ave, Boston, MA 02115 (F.H.F., K.J.S., S.T.T.); Division of Nuclear Medicine, Beth Israel Deaconess Medical Center, Boston, Mass (M.R.P.); Division of Nuclear Medicine, Brigham and Women's Hospital, Boston, Mass (R.E.Z.); and Department of Radiology, University of California at Davis, Davis, Calif (R.D.B.). Received April 21, 2006; revision requested June 23; revision received July 12; accepted July 21; final version accepted September 1. Address correspondence to F.H.F. (e-mail: frederic.fahey{at}childrens.harvard.edu).

View larger version (15K): [in this window] [in a new window] [Download PPT slide]   Figure 1a: Graphs show CT noise in the NU2-94 phantom in terms of the standard deviation of the CT numbers for five background regions (ROIs 1–5). (a) Graph shows values as a function of tube current, with tube voltage of 80 kVp. (b) Graph shows values as a function of tube voltage, with tube current of 160 mA. In both cases, the rotation speed was 0.8 second per rotation and the pitch was 1.5:1. ROI 4 is in the center of the phantom, and its values are slightly noisier due to photon attenuation.

View larger version (14K): [in this window] [in a new window] [Download PPT slide]   Figure 1b: Graphs show CT noise in the NU2-94 phantom in terms of the standard deviation of the CT numbers for five background regions (ROIs 1–5). (a) Graph shows values as a function of tube current, with tube voltage of 80 kVp. (b) Graph shows values as a function of tube voltage, with tube current of 160 mA. In both cases, the rotation speed was 0.8 second per rotation and the pitch was 1.5:1. ROI 4 is in the center of the phantom, and its values are slightly noisier due to photon attenuation.

View larger version (32K): [in this window] [in a new window] [Download PPT slide]   Figure 2: Graph shows contribution to PET noise from CT-based attenuation correction. All scans except where otherwise indicated were acquired with 80 kVp, 0.8 second per rotation, and 1.5:1 pitch. For the torso phantom, scans also were acquired with 80 kVp, 10 mA, 0.5 second per rotation, and 1.5:1 pitch. The standard deviation of 10 realizations of the CT scan at each set of acquisition parameters was used to determine the CT contribution to the PET noise for the NU2-94 (NEMA NU2) and torso phantoms. For the NU2-94 phantom, the coefficient of variation was less than 1% in all cases. For the torso phantom, the coefficient of variation ranged from less than 1% to 7.65%.

View larger version (51K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Adequacy of CT-based attenuation correction in the uniform NU2-94 phantom. A, Transverse CT scan for 80 kVp, 10 mA, 0.5 second per rotation, and 1.5:1 pitch. B, Transverse CT scan for 140 kVp, 160 mA, 0.8 second per rotation, and 1.5:1 pitch. C, Transverse PET scan reconstructed by using the CT scan in A for CT-based attenuation correction. D, Transverse PET scan reconstructed by using the CT scan in B for CT-based attenuation correction. Note that the reconstructed PET data in C and D are essentially identical.

View larger version (60K): [in this window] [in a new window] [Download PPT slide]   Figure 4: Adequacy of CT-based attenuation correction in the torso phantom. A, Transverse CT scan for 80 kVp, 10 mA, 0.5 second per rotation, and 1.5:1 pitch. B, Transverse CT scan for 140 kVp, 160 mA, 0.8 second per rotation, and 1.5:1 pitch. C, Transverse PET scan reconstructed by using the CT scan in A for CT-based attenuation correction. D, Transverse PET scan reconstructed by using the CT scan in B for CT-based attenuation correction. Note that the use of very low–dose CT with the larger phantom leads to undercorrection with CT-based attenuation correction.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 5a: Effect of CT noise on adequacy of CT-based attenuation correction. (a) Graph shows relationship of CT noise (graph at bottom), CT number, and µ511 transformation. The graph at left shows the transformed distribution of CT numbers. The wide range in CT numbers leads to an underestimation in µ511. (b) Plot of CT noise (standard deviation) versus µ511. CT scans of all eight of the tissue-equivalent CT phantoms were acquired, and these were processed into attenuation correction files and reconstructed to yield estimates of µ511. Both calculated (based on a Gaussian model) and measured data were plotted, and there was very good agreement between the calculated and measured data. The slight bias is most likely a result of the inadequacy of the Gaussian noise assumption.

View larger version (15K): [in this window] [in a new window] [Download PPT slide]   Figure 5b: Effect of CT noise on adequacy of CT-based attenuation correction. (a) Graph shows relationship of CT noise (graph at bottom), CT number, and µ511 transformation. The graph at left shows the transformed distribution of CT numbers. The wide range in CT numbers leads to an underestimation in µ511. (b) Plot of CT noise (standard deviation) versus µ511. CT scans of all eight of the tissue-equivalent CT phantoms were acquired, and these were processed into attenuation correction files and reconstructed to yield estimates of µ511. Both calculated (based on a Gaussian model) and measured data were plotted, and there was very good agreement between the calculated and measured data. The slight bias is most likely a result of the inadequacy of the Gaussian noise assumption.

View larger version (45K): [in this window] [in a new window] [Download PPT slide]   Figure 6a: Effect of patient size on adequacy of CT-based attenuation correction. (a) Graph shows µ511 versus patient size for 80 kVp, 10 mA, 0.5 second per rotation, and 1.5:1 pitch. An appropriate value for µ511 (>0.0900 cm–1) was obtained for all pediatric phantoms but not for the adult phantoms. (b) Graph shows µ511 versus tube voltage, tube current, and rotation speed for the adult phantoms. All CT data were acquired with 1.5:1 pitch. Note that, for 10 mA, 120 kVp is necessary and that, for 80 kVp, 40 mA would be necessary for adequate CT-based attenuation correction.

View larger version (36K): [in this window] [in a new window] [Download PPT slide]   Figure 6b: Effect of patient size on adequacy of CT-based attenuation correction. (a) Graph shows µ511 versus patient size for 80 kVp, 10 mA, 0.5 second per rotation, and 1.5:1 pitch. An appropriate value for µ511 (>0.0900 cm–1) was obtained for all pediatric phantoms but not for the adult phantoms. (b) Graph shows µ511 versus tube voltage, tube current, and rotation speed for the adult phantoms. All CT data were acquired with 1.5:1 pitch. Note that, for 10 mA, 120 kVp is necessary and that, for 80 kVp, 40 mA would be necessary for adequate CT-based attenuation correction.