Speaker
Dr
mohammad reza Ay
(Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran and Division of Nuclear Medicine, Geneva University Hospital, Geneva, Switzerland)
Description
The advent of dual-modality PET/CT imaging had great impact on improving the value
of diagnostic PET in localizing, evaluating and therapeutic monitoring of head and
neck cancer and equally valuable for other localizations that are difficult to
pinpoint. In addition, the use of CT images for CT-based attenuation correction
(CTAC) of PET data decreases overall scanning time and creates a noise-free
attenuation map (μmap). The most common CTAC procedure requires a bi-linear
calibration curve acquired under standard imaging conditions to convert the
patient’s CT image from low effective CT energy into an attenuation map at 511 keV.
Given that different tube voltages are used in current PET/CT scanning protocols
depending on patients size and the region under study, this work was designed to
provide answers to the legitimate question of the clinician or physicist: “what is
the magnitude of error due to acquiring CT at e.g. 80 kVp when the calibration curve
would be the manufacturer’s standard of 120 or 140 kVp?” and vice versa, that is,
acquiring CT images at specific tube voltages and varying the voltage for derivation
of calibration curves. The impact of using a single calibration curve on the
accuracy of CTAC for images acquired at different tube voltages was investigated
through quantitative analysis of created μmaps, generated attenuation correction
factors and reconstructed neurological PET emission data using experimental
anthropomorphic phantom and clinical studies. For CT images acquired at 80 and 140
kVp, an average relative difference of -2.9% and 0.7% with the images acquired at
120 kVp, respectively, was observed between absolute activity concentrations in five
regions of the anthropomorphic striatal phantom when CT images are converted using a
single calibration curve derived at 120 kVp. Likewise, an average relative
difference of 1.9% and -0.6% was observed when CT images are acquired at 120 kVp and
CTAC uses calibration curves derived at 80 and 140 kVp, respectively. It was
concluded that using a single calibration curve derived under standard scanning
conditions during the CTAC procedure to images acquired at different tube voltages
does not affect significantly the visual qualitative interpretation and quantitative
analysis of neurological PET emission images. The same behaviour was observed when
calibration curves are derived at different tube voltages and used for conversion of
CT images acquired at fixed tube voltage. These results might contribute to
alleviate the quality assurance procedures required for daily operation of PET/CT
scanners in a clinical environment.
Author
Dr
mohammad reza Ay
(Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran and Division of Nuclear Medicine, Geneva University Hospital, Geneva, Switzerland)
Co-author
Dr
Habib Zaidi
(Division of Nuclear Medicine, Geneva University Hospital, Geneva, Switzerland)
