Background/introduction
Computed Tomography (CT) values characterize the linear attenuation coefficient of the tissue in each volume element relative to the µ-value of water.
The Hounsfield Units (HU) or CT numbers are defined as a normalized index of X-ray attenuation based on a scale of –1000 HU (air) to +1000 HU (bone),
including water equalto 0 HU at standard temperature and pressure [1].
According to most investigators,
healthy liver tissue CT numbers must be between 50 HU and 70 HU [2,
3].
Some clinical recommendations advocate the...
Description of activity and work performed
Phantom study
Our study involved the CT Electron Density Phantom (CIRS,
Norfolk,
model 062).
This phantom consists in two body parts made of soft tissue equivalent epoxy resin.
The use of inserts with known elemental composition and Relative Electron Density (RED) indicated by the phantom’s manufacturer allowed a conversion of the RED value to the associated CT attenuation values of each pixel [14].
The following RED values were included in the phantom: 0.190,
0.489,
0.949,
0.976,
1.043,
1.052,
1.117 and 1.512.
The RED values equal...
Conclusion and recommendations
Cross-calibration of tissue-to-noise ratiosand absolute attenuation values between several CT scanners should be determined prior to patient studies.
Large discrepancies in liver CT attenuation can occur during lifetime of the same multislice CT scanner.
Radiologists must be careful when using absolute CT numbers for diagnosis.
Specific calibration phantoms are highly recommended for quantitative applications as a reference.
Personal/organisational information
Dr Isabelle FITTON,
PhD inRadiation and Imagingin Medicine
European Georges Pompidou Hospital
Department of Radiology
20-40 rue Leblanc
75015 Paris
FRANCE
email:
[email protected]
References
[1] “Computed medical imaging.
Nobel lecture,
December 8,
1979.” Hounsfield GN.
J Comput Assist Tomogr. 1980; 4(5):665-74.
[2] “The essential physics of medical imaging” Bushberg JT.,
Seibert JA.,
Leidholdt EM.,
et al.
2012; 3rd edition.
Philadelphia,
PA: Lippincott Williams & Wilkins.
[3] “Computed tomography: fundamentals,
system technology,
image quality,
applications” Kalender WA.
2011; 3rd edition Erlangen,
Germany: Publicis Publishing.
[4] “CYP17 polymorphism as a risk factor of tamoxifen-induced hepatic steatosis in breast cancer patients” Ohnishi T.,
Ogawa Y.,
Saibara T.,
Nishioka A.,
Kariya S.,
Fukumoto...