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


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.


CONCLUSIONS
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.
Fahey
Frederic H
FH
Division of Nuclear Medicine, Department of Radiology, Children's Hospital Boston, MA 02115, USA. frederic.fahey@childrens.harvard.edu
Palmer
Matthew R
MR
Strauss
Keith J
KJ
Zimmerman
Robert E
RE
Badawi
Ramsey D
RD
Treves
S Ted
ST
D000328:Adult; D016477:Artifacts; D049628:Body Size; D002648:Child; D005710:Gallium Radioisotopes; D005857:Germanium; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography; D011829:Radiation Dosage; D011868:Radioisotopes; D013819:Thermoluminescent Dosimetry; D014057:Tomography, X-Ray Computed
D005710:Gallium Radioisotopes; D011868:Radioisotopes; D005857:Germanium