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Method for transforming CT images for attenuation correction in PET/CT imaging.
Jonathan P J Carney1, David W Townsend, Vitaliy Rappoport
1Department of Medicine, University of Tennessee Medical Center, Knoxville, TN 37920, USA. carneyjp@upmc.edu
Medical Physics
|May 16, 2006
Summary
A new method accurately converts CT Hounsfield units (HU) to 511 keV linear attenuation values. This tube-voltage-dependent scheme improves positron emission tomography (PET) data reconstruction accuracy, especially for bone.
Area of Science:
- Medical Imaging Physics
- Radiological Sciences
- Nuclear Medicine Technology
Background:
- Accurate attenuation correction is crucial for quantitative positron emission tomography (PET) data reconstruction.
- Computed tomography (CT) Hounsfield units (HU) require transformation to 511 keV linear attenuation coefficients for PET attenuation correction.
- Variations in CT scanner protocols, specifically X-ray tube voltage (kVp), affect HU values and subsequent PET accuracy.
Purpose of the Study:
- To develop and validate a tube-voltage-dependent scheme for converting CT-derived Hounsfield units (HU) to 511 keV linear attenuation values.
- To improve the accuracy of attenuation correction in PET/CT imaging across different scanners and kVp settings.
- To demonstrate the impact of kVp-dependent scaling on reconstructed PET activity levels, particularly for dense materials like bone.
Main Methods:
- A Gammex 467 electron density phantom was scanned on five different CT scanners (Siemens, GE, Hitachi, Toshiba) across a kVp range of 80-140 kVp.
- Hounsfield units (HU) for various tissue substitutes were recorded and compared to known 511 keV linear attenuation values.
- A transformation function was derived, incorporating a threshold and kVp-dependent parameters (a and b), to convert HU to 511 keV attenuation values.
Main Results:
- A kVp-dependent transformation scheme was established, yielding different functions for values below and above approximately 50 HU.
- The proposed method significantly improved reconstructed PET activity levels in phantom studies, reducing errors by up to 40% for bone at 80 kVp.
- Patient studies demonstrated high accuracy, with a linear fit of 140 kVp scaled data to 80 kVp scaled data yielding R² = 0.999, indicating consistent results.
Conclusions:
- The developed tube-voltage-dependent scheme provides accurate conversion of CT HU to 511 keV linear attenuation values for PET attenuation correction.
- This method ensures consistent PET quantitative accuracy across different CT scanners and kVp settings, enhancing diagnostic reliability.
- The approach effectively addresses discrepancies caused by varying CT acquisition parameters, leading to improved PET/CT imaging outcomes.