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Year : 2010  |  Volume : 20  |  Issue : 2  |  Page : 154-157
Radiation doses during chest examinations using dose modulation techniques in multislice CT scanner


Department of Radiology, Christian Medical College, Vellore - 632 004, TN, India

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Date of Web Publication6-May-2010
 

   Abstract 

Objective: To evaluate the radiation dose and image quality using a manual protocol and dose modulation techniques in a 6-slice CT scanner. Materials and Methods: Two hundred and twenty-one patients who underwent contrast-enhanced CT of the chest were included in the study. For the manual protocol settings, constant tube potential (kV) and tube current-time product (mAs) of 140 kV and 120 mAs, respectively, were used. The angular and z-axis dose modulation techniques utilized a constant tube potential of 140 kV; mAs values were automatically selected by the machine. Effective doses were calculated using dose-length product (DLP) values and the image quality was assessed using the signal-to-noise (SNR) ratio values. Results: Mean effective doses using manual protocol for patients of weights 40-60 kg, 61-80 kg, and 81 kg and above were 8.58 mSv, 8.54 mSv, and 9.07 mSv, respectively. Mean effective doses using z-axis dose modulation for patients of weights 40-60 kg, 61-80 kg, and 81 kg and above were 4.95 mSv, 6.87 mSv, and 10.24 mSv, respectively. The SNR at the region of the liver for patients of body weight of 40-60 kg was 5.1 H, 6.2 H, and 8.8 H for manual, angular, and z-axis dose modulation, respectively. Conclusion: Dose reduction of up to 15% was achieved using angular dose modulation and of up to 42% using z-axis dose modulation, with acceptable diagnostic image quality compared to the manual protocol.

Keywords: Dose modulation; MSCT; radiation dose

How to cite this article:
Livingstone RS, Pradip J, Dinakran PM, Srikanth B. Radiation doses during chest examinations using dose modulation techniques in multislice CT scanner. Indian J Radiol Imaging 2010;20:154-7

How to cite this URL:
Livingstone RS, Pradip J, Dinakran PM, Srikanth B. Radiation doses during chest examinations using dose modulation techniques in multislice CT scanner. Indian J Radiol Imaging [serial online] 2010 [cited 2020 May 27];20:154-7. Available from: http://www.ijri.org/text.asp?2010/20/2/154/63036

   Introduction Top


CT scan has emerged as a powerful tool for effective radiological diagnosis in a variety of diseases since it allows high-resolution three-dimensional images to be acquired in a short period of time. The significant increase in the use of CT scanners has raised concerns about the radiation doses that patients are exposed to and the consequent increase in the probability of occurrence of cancers. [1] The International Commission on Radiological Protection (ICRP) has recommended that patient doses should be kept as low as reasonably achievable (ALARA) consistent with clinical requirements, especially with regard to CT doses. [2] Effective doses generally depend on the individual technique factors as well as the physical size (i.e., weight) of the patients undergoing the CT procedures. [3] One of the easier methods of estimating effective doses is from the dose-length product (DLP) values displayed on the CT console. [4]

The purpose of dose reduction is to increase the benefit/risk ratio of CT, which is turning out to be a major source of exposure to man-made radiation [5] The radiation dose is directly proportional to the tube current at a constant tube potential, scanning time, and slice thickness. Methods of reducing the radiation dose to patients include varying tube current-time product (mAs), increasing the pitch, and reducing the beam energy according to patient body weights. [6] Automatic tube current modulation is one of the tools available in modern CT scanners for radiation dose reduction. This technique adjusts mAs to provide a constant level of image noise on the basis of patient size, attenuation profile, and scanning parameters, including kVp, beam pitch, table speed, and rotation time. [7] While using the dose-modulation technique, mAs can be decreased automatically for regions with lower attenuation whilst maintaining an acceptable level of image noise. [8] The angular-modulation technique involves varying the tube current as the x-ray tube rotates about the patient, while the z-axis modulation involves varying the tube current along the z-axis of the patient. [4] The current study compares the radiation dose and image quality achieved with the use of manual protocol settings with those obtained using the dose-modulation software available in a 6-slice CT scanner, which includes dynamic dose-modulation (D-DOM) and z-axis dose-modulation (Z-DOM) techniques.


   Material and Methods Top


Two hundred and twenty-one patients who underwent routine CT chest examinations using a 6-slice CT scanner (Brilliance, Philips Medical Systems, Netherlands) were included in the study. The tube potentials available in the machine were 90 kV, 120 kV, and 140 kV. Various other parameters, i.e., the total duration of the scan, field of view, and dose descriptors such as volume CT dose index (CTDI vol ), DLP, and pitch were displayed on the CT console of the 221 patients, 32 patients were scanned with manual protocol settings (constant tube potential and mAs of 140 kV and 120 mAs, respectively) irrespective of the patients' body weights. These factors were set manually so that an optimal image quality was obtained with the minimum radiation dose. One hundred and ten patients with D-DOM and 79 patients with Z-DOM techniques were scanned with a constant tube potential of 140 kV, with the mAs values automatically selected by the machine. The rotation time was set at 0.75 s, with a pitch of 1.1 and detector collimation of 6 Χ 3 mm; the reconstructed slice thickness was 5 mm. This study was part of a project funded by the Atomic Energy Regulatory Board (AERB) of India and was carried out after clearance by the ethical committee of the institution where the study was conducted.

All CT examinations were performed as per the standard clinical protocol, with the area scanned extending from the apex of the lungs up to the adrenals. All patients were examined by physicians and were subjected to CT examinations of the chest only when clinically indicated. The patients were categorized into three groups based on their body weight: 40-60 kg (group A), 61-80 kg (group B), and ≥ 81 kg (group C). Patient weights were recorded prior to the CT examinations. All CT chest examinations were performed by injecting contrast agent intravenously, and contrast-enhanced images were acquired using a single breath-hold technique.

The effective doses were estimated by multiplying the DLP values by the normalized coefficient found in the European guidelines on quality criteria of CT, which is 0.017 mSv mGy−1 cm−1 . [9],[10] Periodic calibrations using 32 cm polymethyl methacrylate (PMMA) CTDI body phantom with a high-sensitivity 100-mm long pencil ion chamber (CTDI 100 , Victoreen, Ohio, USA) were performed to check the consistency of the CT console values.

Objective evaluation of image quality was based on the measurement of CT image noise of a uniformly attenuating region of the liver. For this purpose, the signal-to-noise ratio (SNR) was measured in a standard 1 cm 2 circular ROI from the CT console. The ROI selected was one that was not unduly influenced by the contrast in the blood vessels. Two experienced radiologists, blinded to the scanning techniques, independently reviewed each study. The lung, spleen, vessels, adrenals, and esophagus were assessed on a 5-point scale (1 - unacceptable; 2 - substandard; 3 - acceptable; 4 - above average; 5 - superior). All the CT images were assessed with constant window level and a window width of 60 and 400 HU using a high-resolution monitor in the workstation.


   Results Top


[Table 1] shows varying mAs values for D-DOM and Z-DOM for different body weights of patients. [Table 2], [Table 3] and [Table 4] show CTDI vol , DLP, effective dose, and SNR values for patients who underwent CT examinations of the chest. With the use of Z-DOM, the radiation doses were 11% higher than with the manual protocol, without significant variation in the image noise for group C patients. A reduction of 42.3% in radiation dose for group A patients and 20% for patients of group B was observed with the use of Z-DOM compared to the manual protocol. However, there was an increase of SNR by 42% and 29% with the use of Z-DOM for group A and group B patients respectively, compared to the manual protocol. With the use of D-DOM, the tube currents varied from 105-120 mAs, while with Z-DOM, they varied from 27-243 mAs. While using the D-DOM, the tube current-time product (mAs) was adjusted automatically according to patients' weight and did not exceed the preset parameter of 120 mAs.

Effective doses from D-DOM were found to be consistent, irrespective of the body weights, without significant change in the SNR values. Reduction of radiation dose of up to 21% was achieved with the use of D-DOM compared to the manual protocol. Compared to the use of Z-DOM, a 30% reduction in radiation dose with the use of D-DOM was noted for group C patients. One radiologist reported images from the manual protocol and D-DOM as above average (median = 4) for liver, spleen, adrenals, and esophagus and acceptable (median = 3) for vessels. For Z-DOM, this radiologist reported as acceptable (median = 3) the images of all the organs included in the study. Another radiologist reported images from liver, spleen, adrenals, esophagus, and vessels as acceptable (median = 3) for manual protocol, D-DOM, and Z-DOM.


   Discussion Top


In diagnostic radiological procedures, the radiation dose and image quality depend on the tube potential and tube current-time product, while in CT scan they depend on tube potential, tube current, scanning time, slice thickness, scanning volume, and pitch. Changes in tube potential affect CT tissue attenuation values and can change tissue contrast in a complex fashion. [11],[12] Modern CT scanners have automatic exposure control systems, which permit empirical adjustment of radiological technique factors according to the size of the patient. [13] The use of manual protocol settings would impart the same exposure to all patients, irrespective of the difference in patient sizes If a manual protocol is used, radiation exposures to thin patients can be reduced by selecting lower exposure parameters. In contrast, the use of dose-modulation techniques provides online tube current modulation, so that it is possible to adapt to varying patient sizes without the need for arbitrary selection of exposure values as in manual settings.

A pediatric CT study performed with an online angular-modulation technique showed a 26-43% reduction in the radiation dose compared with manual techniques. [14] Prasad et al. reported a dose reduction of 50% by reducing mAs values while maintaining a constant tube potential of 140 kVp. [11] Studies have shown a reduction of 16.9-27% in radiation dose for chest examination by using attenuation-based online tube current modulation. [15],[16] As compared with manual settings, a reduction of up to 42% in radiation dose, with acceptable image quality, was noted with the use of Z-DOM in the current study.

It was observed that while using Z-DOM, the radiation dose increased according to the patients' body weights because the mAs values exceeded the preset limit of 120 mAs. There was no significant change in SNR values in Z-DOM, when compared to manual settings, for patients weighing above 80 kg. Both the radiologists reported that the images were of acceptable quality for clinical diagnosis.

In conclusion, a significant reduction in radiation dose to patients, without compromising diagnostic image quality, is possible by using dose-modulation techniques for CT of the chest. The present study reveals that dose reduction of up to 15% can be achieved using D-DOM and up to 42% using Z-DOM, with acceptable image quality. The use of Z-DOM imparted higher doses for patients weighing above 80 kg. Dose modulation software is now available in most modern CT scanners and use of this should be encouraged. The current study suggests that z-axis dose modulation should be used for young adults and for patients of groups A and B and angular dose modulation for group C patients.


   Acknowledgement Top


Authors would like to express their gratitude to Atomic Energy Regulatory Board of India for having provided financial support to this work.

 
   References Top

1.Brenner DJ, Hall EJ. Computed tomography-an increasing source of radiation exposure. N Engl J Med 2007;357:2277-784.  Back to cited text no. 1  [PUBMED]  [FULLTEXT]  
2.Zhu X, Yu J, Huang Z. Low dose chest CT: optimizing radiation protection for patients. AJR Am J Roentgenol 2004;183:809-16.   Back to cited text no. 2  [PUBMED]  [FULLTEXT]  
3.Huda W, Scalzetti EM, Roskopf M. Effective doses to patients undergoing thoracic computed tomography examinations. Med Phys 2000;27:838-44.   Back to cited text no. 3  [PUBMED]    
4.McCollough HC, Bruesewitz MR, Kofler JM Jr. CT Dose reduction and dose management tools: overview of available options. Radiographics 2006;26:503-12.   Back to cited text no. 4      
5.Kalra MK, Maher MM, Toth TL, Hamberg LM, Blake MA, Jo-Anne S, et al. Strategies for CT radiation dose optimization. Radiology 2004;230:619-28.   Back to cited text no. 5      
6.Michael F. McNitt-Gray. AAPM/RSNA physics tutorial for residents: topics in CT. Radiation Dose in CT. Radiographics 2002;22:1541-53.   Back to cited text no. 6      
7.Kalra MK, Rizzo S, Maher MM, Halpern EF, Toth TL, Jo Anne S, et al. Chest CT performed with Z-axis modulation: Scanning protocol and radiation dose. Radology 2005;237:303-8.  Back to cited text no. 7      
8.Rizzo S, Kalra MK, Schmidt B, Schmidt B, Tejas D, Suess CT, et al. Comparison of angular and combined automatic tube current modulation technique with constant tube current CT of the abdomen and pelvis. AJR Am J Roentgenol 2006;186:673-9.   Back to cited text no. 8      
9.European Commission. European guidelines on quality criteria for computed tomography. Brussels, Belgium. Report EUR 16262 EN, 1999.  Back to cited text no. 9      
10.Huda W, Nickoloff EL, Boone JM. Overview of patient Dosimetry in diagnostic radiology in the USA for the past 50 years. Med Phys 2008;35:5713-28.  Back to cited text no. 10  [PUBMED]    
11.Prasad SR, Wittram C, Jo Anne S, McLoud T, Rhea J. Standard-dose and 50%-reduced-dose chest CT: comparing the effect on image quality. AJR Am J Roentgenol 2002;179:461-5.   Back to cited text no. 11      
12.Mayo JR, Aldrich J, Muller NL. Radiation exposure at chest CT: a statement of the fleischner society. Radiology 2003;228:15-21.  Back to cited text no. 12      
13.Huda W, Scalzetti EM, Levin G. Technique factors and image quality as functions of patient weight at abdominal CT. Radiology 2000;217:430-5.   Back to cited text no. 13  [PUBMED]  [FULLTEXT]  
14.Greess H, Lutze J, Nomayr A, Wolf H, Hothorn T, Kalender WA, et al. Dose reduction in sub second mulislice spiral CT examination of children by online tube current modulation. Eur Radiol 2004;14:995-9.   Back to cited text no. 14      
15.Tack D, Maertelaer VD and Gevenois AP. Dose reduction in multidetector CT using attenuation - based online tube current modulation. AJR Am J Roentgenol 2003;181:331-4.  Back to cited text no. 15      
16.Greess H, Wolf H, Baum U, Lell M, Pirkl M, Kalender W, et al. Dose reduction in computed tomography by attenuation-based on-line modulation of the tube current: evaluation of six anatomical regions. Eur Radiol 2000;10:391-4.  Back to cited text no. 16  [PUBMED]  [FULLTEXT]  

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Correspondence Address:
Roshan S Livingstone
Department of Radiology, Christian Medical College, Vellore - 632 004, TN
India
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Source of Support: Atomic Energy Regulatory Board of India, Conflict of Interest: None


DOI: 10.4103/0971-3026.63036

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]

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