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Reduction of Patient Dose in CT Adjustment of Tube Current to Patient Size

Prtic, Gordana (2003)
Medical Physics Programme
Abstract (Swedish)
Motivationen med studien var den alldeles för höga stråldosen som används vid CTundersökningar. I de allra flesta fallen anpassas man inte rörströmmen efter patientens storlek och därmed får smalare patienter en onödigt hög stråldos.

Målet var att reducera stråldosen till patienter som undersöks med datortomografi utan att minska på bildkvalité, genom att anpassa rörströmmen efter patientens storlek.

Studien visar att 60 kg patient can be examined with only half the dose of an 80 kg patient and still the same level of image quality can be achieved. En stor dos reducering kan göras för smalare patienter genom att använda ett lämpligt val av scan parametrar
Abstract
In this study we have investigated the possibilities of dose reduction in CT at a constant level of image quality by adjustment of the tube current to the phantom size. The motivation with the study is the high radiation doses to the patients and the increasing frequency of CT examinations and the variety of examinations in CT, and that the same tube current is used for different patient sizes. According to UNSCEAR (2000) the effective dose from CT examinations are in general relatively high, typically 1- 30 mSv. These doses can often approach or exceed levels known to increase the probability of cancer. The conclusion based on the results from a survey of the frequency of radiological examination in Sweden during the period 1998-99... (More)
In this study we have investigated the possibilities of dose reduction in CT at a constant level of image quality by adjustment of the tube current to the phantom size. The motivation with the study is the high radiation doses to the patients and the increasing frequency of CT examinations and the variety of examinations in CT, and that the same tube current is used for different patient sizes. According to UNSCEAR (2000) the effective dose from CT examinations are in general relatively high, typically 1- 30 mSv. These doses can often approach or exceed levels known to increase the probability of cancer. The conclusion based on the results from a survey of the frequency of radiological examination in Sweden during the period 1998-99 (Leitz, 1999), is that a relatively small fraction of all examinations (11 %) gives rise to a large contribution to the collective effective dose (68 %).The aim of the study was to reduce the patient dose without reducing the image quality. This is possible with adjustment of the tube current to the patient size. To establish the relation between phantom size, image quality and dose we scanned tree different phantoms, two of these were the standard CT dosimetry phantoms (16 and 32 diameter) and the third was a similar phantom with 24 cm diameter. The scans were made on a Siemens Somatom Plus 4, with standard scan technique for abdomen. To estimate the image quality at different levels of dose, we made measurements with reduced mAs and determined the signal-to-noise ratio, SNR. Finally for the different phantom sizes, we determined the necessary dose to keep the SNR at the same level as when the body phantom was imaged with 185 mAs. The results of the study showed that the mAs settings can be reduced to about 41 mAs in the phantom with diameter 24 cm (the medium size phantom) and to 20 mAs in the 16 diameter phantom (the small phantom) and still get the same level of SNR as in the body phantom at 185 mAs. This means that by using a lower mAs setting for smaller patients a considerable dose saving can be achieved. Determinations of SNR in patient images have also been performed. Images of twenty-four patients who had a CT examination of the liver were evaluated. The result was that the dose to the patient can be reduced to half for patients weighing about 60 kg and still get the same SNR as for an 80 kg patient with the original mAs setting. With the knowledge from this study, a measuring-tape graded with mAs setting can be produced. Before examining a patient the technician would measure the circumference of the body part of interest for the patient with the mAs-tape to find what mAs setting should be used. More than a 50 percent reduction in patient dose is possible by appropriate choice of scan parameters. (Less)
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author
Prtic, Gordana
supervisor
organization
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Röntgen
language
English
id
2156896
date added to LUP
2011-09-13 12:59:54
date last changed
2011-12-06 14:54:54
@misc{2156896,
  abstract     = {In this study we have investigated the possibilities of dose reduction in CT at a constant level of image quality by adjustment of the tube current to the phantom size. The motivation with the study is the high radiation doses to the patients and the increasing frequency of CT examinations and the variety of examinations in CT, and that the same tube current is used for different patient sizes. According to UNSCEAR (2000) the effective dose from CT examinations are in general relatively high, typically 1- 30 mSv. These doses can often approach or exceed levels known to increase the probability of cancer. The conclusion based on the results from a survey of the frequency of radiological examination in Sweden during the period 1998-99 (Leitz, 1999), is that a relatively small fraction of all examinations (11 %) gives rise to a large contribution to the collective effective dose (68 %).The aim of the study was to reduce the patient dose without reducing the image quality. This is possible with adjustment of the tube current to the patient size. To establish the relation between phantom size, image quality and dose we scanned tree different phantoms, two of these were the standard CT dosimetry phantoms (16 and 32 diameter) and the third was a similar phantom with 24 cm diameter. The scans were made on a Siemens Somatom Plus 4, with standard scan technique for abdomen. To estimate the image quality at different levels of dose, we made measurements with reduced mAs and determined the signal-to-noise ratio, SNR. Finally for the different phantom sizes, we determined the necessary dose to keep the SNR at the same level as when the body phantom was imaged with 185 mAs. The results of the study showed that the mAs settings can be reduced to about 41 mAs in the phantom with diameter 24 cm (the medium size phantom) and to 20 mAs in the 16 diameter phantom (the small phantom) and still get the same level of SNR as in the body phantom at 185 mAs. This means that by using a lower mAs setting for smaller patients a considerable dose saving can be achieved. Determinations of SNR in patient images have also been performed. Images of twenty-four patients who had a CT examination of the liver were evaluated. The result was that the dose to the patient can be reduced to half for patients weighing about 60 kg and still get the same SNR as for an 80 kg patient with the original mAs setting. With the knowledge from this study, a measuring-tape graded with mAs setting can be produced. Before examining a patient the technician would measure the circumference of the body part of interest for the patient with the mAs-tape to find what mAs setting should be used. More than a 50 percent reduction in patient dose is possible by appropriate choice of scan parameters.},
  author       = {Prtic, Gordana},
  keyword      = {Röntgen},
  language     = {eng},
  note         = {Student Paper},
  title        = {Reduction of Patient Dose in CT Adjustment of Tube Current to Patient Size},
  year         = {2003},
}