Hurwitz's theorem (number theory): Difference between revisions
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The '''computed tomography dose index''' (CTDI) is a commonly used radiation exposure index in [[Computed tomography|X-ray computed tomography]] and is reported by the CT manufacturers to scan personnel for each exam. The CTDI can be used in conjunction with patient size to determine the [[absorbed dose]]. The CTDI and absorbed dose may differ by more than a factor of two for small patients such as children.<ref>{{cite journal|last=McCollough|first=C. H.|coauthors=Leng, S.; Yu, L.; Cody, D. D.; Boone, J. M.; McNitt-Gray, M. F.|title=CT Dose Index and Patient Dose: They Are Not the Same Thing|journal=Radiology|date=18 April 2011|volume=259|issue=2|pages=311–316|doi=10.1148/radiol.11101800|url=http://radiology.rsna.org/content/259/2/311.full|accessdate=22 June 2012}}</ref> | |||
==Definitions== | |||
It is defined by the US [[Department of Health and Human Services]] (HHS) as the average dose imparted by a single axial acquisition to a standard 100-mm pencil chamber dosimeter inside a PMMA phantom over the width of 14 CT slices: | |||
<math>CTDI=\frac{1}{nT}\int_{-7T}^{7T}{D(z)dz}</math> | |||
where <math>n</math> is the number of slices acquired, <math>T</math> is the slice thickness and <math>D(z)</math> is the [[radiation dose]] measured at position <math>z</math> along the scanner's main axis. | |||
This equation is often used in a modified form | |||
<math>CTDI_{100}=\frac{1}{nT}\int_{-50 mm}^{50 mm}{D_a(z)dz}</math>. | |||
The dose absorbed in air <math>D_a(z)</math> can be conveniently assessed with a 100-mm standard pencil dose chamber. Typically, the dose distribution within the body cross section imparted by a CT scan is much more homogeneous than that imparted by radiography, but is still somewhat larger near the skin than in the body center. Therefore, a third measure, the ''weighted'' CTDI was introduced: | |||
<math>CTDI_w=\frac{1}{3} CTDI_{100}^{central} + \frac{2}{3} CTDI_{100}^{peripheral}.</math> | |||
Similar measures with yet wider chambers are useful for CT systems with large numbers of detector rows.<ref>[[#refGeleijns2009|Geleijns 2009]]</ref> | |||
== Notes == | |||
{{reflist|2}} | |||
== References == | |||
* <cite id=refRothenberg2000>Rothenberg, LN and Pentlow, KS (2000). "CT dosimetry and radiation safety", Categorical Course in Diagnostic Radiology Phyics: CT and US Cross-sectional Imaging, LW Goldman and JB Fowlkes, eds., pp171-188, RSNA, Oak Brook, IL</cite> | |||
* <cite id=refGeleijns2009> J Geleijns, M Salvadó Artells, P W de Bruin, R Mather, Y Muramatsu and M F McNitt-Gray (2009). "Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner", Phys. Med. Biol. 54, pp3141-3159</cite> | |||
[[Category:X-ray computed tomography]] | |||
Revision as of 16:24, 9 January 2014
The computed tomography dose index (CTDI) is a commonly used radiation exposure index in X-ray computed tomography and is reported by the CT manufacturers to scan personnel for each exam. The CTDI can be used in conjunction with patient size to determine the absorbed dose. The CTDI and absorbed dose may differ by more than a factor of two for small patients such as children.[1]
Definitions
It is defined by the US Department of Health and Human Services (HHS) as the average dose imparted by a single axial acquisition to a standard 100-mm pencil chamber dosimeter inside a PMMA phantom over the width of 14 CT slices:
where is the number of slices acquired, is the slice thickness and is the radiation dose measured at position along the scanner's main axis.
This equation is often used in a modified form
.
The dose absorbed in air can be conveniently assessed with a 100-mm standard pencil dose chamber. Typically, the dose distribution within the body cross section imparted by a CT scan is much more homogeneous than that imparted by radiography, but is still somewhat larger near the skin than in the body center. Therefore, a third measure, the weighted CTDI was introduced:
Similar measures with yet wider chambers are useful for CT systems with large numbers of detector rows.[2]
Notes
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References
- Rothenberg, LN and Pentlow, KS (2000). "CT dosimetry and radiation safety", Categorical Course in Diagnostic Radiology Phyics: CT and US Cross-sectional Imaging, LW Goldman and JB Fowlkes, eds., pp171-188, RSNA, Oak Brook, IL
- J Geleijns, M Salvadó Artells, P W de Bruin, R Mather, Y Muramatsu and M F McNitt-Gray (2009). "Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner", Phys. Med. Biol. 54, pp3141-3159
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