Variation in tube voltage dependence of X-ray CT image contrast attributed to the difference of object materials

Transcription

1 Variation in tube voltage dependence of X-ray CT image contrast attributed to the difference of object materials Tatsuo KONISHI, Yoshiyuki ASAI, Masanobu UEMURA, Koji YAMADA, and Masao MATSUMOTO Department of Central Radiology, Kinki University Hospital 377-2, Ono-Higashi, Osaka-Sayama, Osaka , Japan Division of Health Science, Graduate School of Medicine, Osaka University 1-7, Yamadaoka, Suita, Osaka , Japan Received on September 19, In final form on November 25, 2008 Abstract : The dependence of CT number and image contrast on the tube voltage, namely the photon energy in diagnostic CT imaging was theoretically investigated. The linear attenuation coefficients for various elements and mixtures were obtained for three cases that 1for the bremsstrahlung X-rays with an ideal detector, 2for the bremsstrahlung X-rays with an assumed detector, and 3for the monoenergetic X-rays, by usingthex-ray photon spectrum and the sensitivity spectrum of the CT detector. As a result, the CT number to several materials converged to zero with an increase of tube voltage in all cases, whereas that of some materials diverged from zero. This result indicates that the image contrast of some materials in the restricted energy range such as diagnostic range may be emphasized by using high tube voltage, as opposed to common knowledge. Keywords : CT number, image contrast, tube voltage, linear attenuation coefficient, X-ray spectrum X CTCT X 1[1-2] 2 [3]3 [4] 2 Dual Energy CT[5] CT CT X CT CT CT 80 kv140 kv CT CT CT CT CT X CT X X X X 2 CT X I 90

2 I EmaxI0E de 0 1 I0E X X E X E x Ie Ie EmaxI0E exp[ E x]de 2 0 I Ie X CT CT X I Ie x ln I 3 Ie I0E 6 80, 100, 120, 140 kv 1keV2 E National Institute of Standard and TechnologyNIST XCOM[7] 1keV x 5cm 34 CT VCT VCT 1000 m w w 4 w m CT Gd2O2S [8]Gd2O2S X [9] CT X SE E 1exp ene x 5 ene Gd2O2S x CT Gd2O2S Gd2O2S 120 mgcm 2 X E x X Ie Ie EmaxSE I0E exp[ E x]de 6 0 Ie 3 Ie4 Gd2O2S CT XCOM X 2 1keV CT XCOM X CT 1 I I0 2 I0 X 40, 60, 80, 100, 120, 140 kev 6 4 5cm X Table 1 s used for the calculation Category Element Hydrogen Oxygen Sodium Silicon Chorine Titanium Nickel Carbon, Graphite Neon Magnesium Phosphorus Calcium Chromium Copper Nitrogen Fluorine Aluminum Sulfur Scandium Cobalt Iodine Mixture A-150 Tissue-Equivalent Plastic B-100 Bone-Equivalent Plastic C-100 Air-Equivalent Plastic GI Tract Pancreas Spleen Adipose Tissue (ICRU-44 Blood, Whole (ICRU-44 Bone, Cortical (ICRU-44 Lung Tissue (ICRU-44 Tissue, soft (ICRU-44 Alanine Bakelite Cell Uncleic Kidney Skin Glass, Borosilicate Pyrex Air, DryICRU-44 Breast TissueICRU-44 Eye LensICRU-44 Muscle, SkeletalICRU-44 TestisICRU-44 Vol.25 No

3 Fig.1 Examples of the X-ray photon fluence spectra : before solid linesand after dotted linespenetrating 5-cm water phantom. Fig.2 Sensitivity spectrum for Gd2O2S detector calculated by equation5. a b Fig.3 Variation of CT images with the ideal detector accompanying a change of bremsstrahlung X-rays for tube voltages of 80, 100, 120 and 140 kv :act numbers of 14 materials,b6 image contrasts between two materials. 92

4 5 Gd2O2S 50 kev K 14 CT 6 AdiposeBreast Cell-UncleicB-100 Bone-Equivalent PlasticPyrexBonecortical CT CT CT GI TractBakelitePancreas, KidneySkinAlanine CT Bakelite Testis Spleen CT BreastPancreas BreastGI Tract SpleenCell Uncleic PancreasBakelite BakeliteTestis 80 kv 100 kv 140 kv TestisSpleen Gd2O2S CT CT Testis CT Gd2O2S CT Skin Gd2O2S CT CT X CT X CT Eye LensCarbon33 kev K Iodine CT CT CT CT CT 4VCT m w CT m w X CT X 3 1 CT Bakelite 80 kvx 30 kev70 kev140 kv X 40 kev100 kev CT Bone, cortical Bakelite CT Testis CT X CT CT CT m w BakeliteTestis 80 kv 100 kv 140 kv 80 kv Testis CT 100 kv Bakelite CT CT Gd2O2S Skin Gd2O2S K X X Gd2O2S CT Gd2O2S Vol.25 No

5 a b Fig.4 Variation of CT images with the Gd2O2S assumed detector accompanying a change of bremsstrahlung X-rays for tube voltages of 80, 100, 120 and 140 kv :act numbers of 14 materials,b6 image contrasts between two materials. K X X X CT X CT X CT Table 2 Photon energy kev Adipose Eye Lens Tissue, Soft Carbon B-100 Iodine Variation of CT numbers accompanying a change of monoenergetic X-ray for 7 photon energies

6 a CT CT 1 X 2 X Gd2O2S 3 X X CT CT CT CT b c Fig.5 Dependence of the ratiomwof linear attenuation coefficients on photon energy :a3 materials having a descending form for photon energy,b4 materials having an ascending form for photon energy, andc5 materials having an almost constant value for photon energy. The linear attenuation coefficients of all materials are normalized to that of water. CT CT [ 1 ] Funama Y: Radiation Dose Reduction without Degradation of Low-Contrast Detectability at Abdominal Multisection CT with a Low-tube Voltage Technique : Phantom Study, Radiology, 2373, , [ 2 ] Nakayama Y : Abdominal CT with Low Tube Voltage : Preliminary Observation about Radiation Dose, Contrast Enhancement, Image Quality,and Noise, Radiology, 237 3, , [ 3 ], INNERVISION, 2211, 36-42, 2007 [ 4 ] Takahashi N : Dual-Energy CT Iodine-Subtraction Virtual Unenhanced Technique to Detect Urinary Stone in an Iodine-Filled Collecting System, Am. J. Roentgenol., 190 3, , [ 5 ] The standard and Advanced Technologies of SOMATO CT, INNERVISION, 2211, 27-29, [ 6 ] Fewell TR, Shuping RE, and Healy KE : Handbook of Computed Tomography X-ray Spectra, pp.49-51, HHS Publication, FDA, U.S. Government Printing Office, Washington, D.C., [ 7 ] Berger MJ, Hubbell JH, Seltzer SM, et al.: NIST XCOM: Photon Cross Sections Database, http : //physics.nist.gov/ PhysRefData/Xcom/Text/XCOM.html (retrieved on 01 May 2008) [ 8 ] Willi AK : Computed Tomography, pp , Publicis Corporation Publishing, Erlangen, [ 9 ] Asai Y, Tanabe Y, Ozaki Y, et al. : Optimum tube voltage for the chest radiographs obtained by psychophysical analysis, Med. Phys., 2511, , Vol.25 No