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PHYSICS Table of Contents   
Year : 2000  |  Volume : 10  |  Issue : 1  |  Page : 19-20
CT: Caution on radiation dose


Medical Physics Dept., IRCH, All India Institute of Medical Science, IR Cancer Hospital, New Delhi, India

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Keywords: CT- Radiation dose, Patient doses in CT

How to cite this article:
Rehani M M. CT: Caution on radiation dose. Indian J Radiol Imaging 2000;10:19-20

How to cite this URL:
Rehani M M. CT: Caution on radiation dose. Indian J Radiol Imaging [serial online] 2000 [cited 2019 Aug 18];10:19-20. Available from: http://www.ijri.org/text.asp?2000/10/1/19/30625
The imaging technique with the Nobel Prize-computerized tomography (CT) has retained its value and popularity despite threats from MRI, SPECT and PET. The widespread use of this technique has the potential to make the largest overall contribution to radiation doe to the patients from amongst all diagnostic medical X-ray modalities. For example, in the United Kingdom, CT constitutes about 2 to 3% of all examinations but contributes to about 20 to 30% of the total patient dose from the use of medical X-rays [1]. The Royal College of Radiologist states that CT now probably contributes almost half of the collective dose from all examinations [2]. In the UK, the average rate of increase of CT workload is reported to be 24% per year against 2.3% for all types of medical and dental x-ray examinations [3]. There is increasing demand for chest and head CTs despite developments in other imaging modalities.

Ever since its discovery in 1972 by G. Hounsfield, who was awarded the Nobel Prize in 1979, the CT has made quantum jumps both in its widespread application in the realm of medicine and in technological improvisations. The advances are such that it is possible, with the spiral technique, to carry out whole CT examinations of the chest within a single breath-hold as against a few minutes in earlier systems. The frustrating experience of trying to obtain diagnostic CT scans in patients who are restless, uncooperative or, for whatever reasons, unable to remain immobile during the scanning sequence is a thing of the past. In conventional CT, the scanner rotates 360, the patient moves into the gantry by a small increment, the scanner begins its next 360 turn and the scan-move-scan sequence goes on. This slow and tedious process has been replaced by continuous imaging. The X-ray tube in the gantry rotates continuously while the patient too moves continuously and not incrementally through the gantry, thus forming a spiral. Since its, introduction in 1989 [4] the spiral technique has completely revolutionized CT with resultant immense benefit. The speed coupled with the convenience with which CT scanning can be performed by modern CT scanners is leading to the temptation to obtain head-to-pelvic or even head-to-toe examinations.


   What Is The Magnitude Of The Radiation Dose? Top


Although doses vary widely from one centre to another, a typical CT scan of the chest gives a radiation dose equivalent to 400 chest radiographs (chest CT 8 mSv, chest radiography = 0.02 mSv) and a high resolution CT scan of chest by conventional scanner (thin slices, high mA) gives a dose of more than 600 chest radiographs [2]. In India, since calcium tungstate screens are in use at most places, the chest radiograph dose can be taken to be 0.05 mSv in place of 0.02 mSv. CT examinations of the thoracic spine, routine chest, mediastinum, routine abdomen, liver, pancreas, kidney, lumbar spine and pelvis are associated with effective doses of > 5-15 mSv (100-300 chest X-rays if chest X-ray dose is taken as 0.05 mSv). CT fluoroscopy, CT angiography and a number of interventional procedures using CT are associated with doses in the range of 15-25 mSv. The average radiation dose a radiologist gets in India is upto 1 mSv/year. Thus a single CT procedure implies a patient dose, equivalent to 5-25 years of work in a radiology department. In addition, superficial organs such as the breast, eyes, thyroid and testes get higher radiation doses even though they are seldom the target of diagnostic procedures. They are needlessly irradiated during radiological procedures of the thorax, cervical spine, head, sinus and pelvis. Few radiologists are aware that conventional diagnostic chest CTs impart a radiation dose of 20-50 mGy to the breasts of an average-sized women. This is equivalent to 10-25 two view mammographic examinations and to about 100 chest radiographs [5]. The eye lens dose in head CT is around 25 mGy, 70 mGy during scanning of the sinuses, 10 to 130 mGy in CT of orbital trauma and upto 215 mGy for a pituitary examination [6].


   What Are The Implications Of High Doses ? Top


Delivery of 10 mGy to a woman's breast before age 35 fractionally (multiplicatively) increases her risk of breast cancer by 13.6% over the expected spontaneous rate for the general population [7]. For 50 mGy, the figure for the general population [7]. For 50 mGy, the figure exceeds 60%. In long-term follow-up (average 26 years) of 1030 patients with scoliosis who had received an average dose of 130 mGy of breast radiation from whole-spine radiography during adolescence, Hoffman et al . [8] found that 11 had developed breast cancer. On the basis of population demography, however, only six were expected to have developed cancer. The dose limit to prevent opacification of the lens in persons occupationally exposed to radiation has been set at 150 mGy per year [9] and this limit is also recommended by the International Commission on Radiological Protection [10]. Even though the statutory dose limits do not apply to the irradiation of patients for medical purposes, this dose may be exceeded in certain CT examinations and thus control of dose to the lens of the eye is necessary.


   Are Doses Going Up? Top


In a nationwide study conducted by NRPB in UK, Wall and Hart have reported a 30% reduction in patient doses for common radiological procedures compared to those 10 years ago, but an increase in patient dose of about 35% for abdominal and pelvic CT examination [11]. The individual patient doses are becoming higher because of

a)Volume scanning in place of discrete slice-by-slice scanning,

b)Thinner slices,

c)Overlap sections,

d)Liberal inclusion of wider areas of the body,

e)Demand for shorter scanning times,

f)Demand for higher resolution (high-resolution CT),

g)Better quality image as a result of higher exposure factors and

h)Frequent repeat studies on the same patient.


   What Can Be Done To Reduce The Doses ? Top


There is increasing evidence in the clinical and radiation protection literature of practical techniques by which manufacturers, the CT facility and referring physicians can better control radiation dose from CT procedures without loss in clinical usefulness. From the manufacturer, one can expect: equipment that can continuously adjust dose factors as the body slides through the gantry the so- called smart technique; signals whenever the operator chooses high dose factors in manual adjustments; monitoring and display of radiation dose; development of dedicated equipment with an upper limit of mA; fixed dose rate at defined settings of dose controlling factors like kVp/mAs; and improvements in detector and filters.

From radiology departments, one can expect low-dose and ultra-low dose techniques; optimal and judicious choice of slice thickness, pitch and area to be scanned and organ shielding techniques (e.g, for breast, eyes, thyroid and gonads). The reports in the literature indicate substantial radiation dose saving without affecting image quality. For example, low dose CT of the chest involves using lower mAs [12],[13], omitting the frontal 90 in head scans and the use of an angular rotation of 270 rather than 360 to minimize eye lens dose [14], use of lower mAs for the pelvic portions which contain high contrast organs, whenever the abdomen and pelvis are to be scanned together. The encouraging results in dose reduction achievable at the level of radiographer/radiologist need wider acceptance. From the referring physician one can expect to develop criteria for clinical justification jointly with radiology colleagues, appropriate choice of imaging techniques and evaluation of practice. The publication of the Royal College of Radiologists [2] in this respect serves a good purpose wherein guidelines for doctors have been stipulated for fairly comprehensive clinical situations. When to use which imaging modality for optimal yield is deliberated upon in the RCR pamphlet. There is a definite need to ensure safer utilization of this otherwise excellent imaging modality.

 
   References Top

1.Shrimpton PC, Jones DG, Hillier MC et al . Survey of CT practice in the UK, Part 2: Dosimetric aspects. Chilton: NRPB R-249, London: HM Stationary Office, 1991.  Back to cited text no. 1    
2.The Royal College of Radiologists. Making the best use of department of clinical radiology- guidelines for doctors, Fourth edition. London: The Royal College of Radiologists, 1998  Back to cited text no. 2    
3.National Radiological Protection Board. Protection of the patient in x-ray-computed tomography and further statements on radon affected areas. Documents of NRPB 1992; 3 (4).  Back to cited text no. 3    
4.Kalender WA, Seissler W, Vock P. Single-breathhold spiral volumetric CT by continuous patient translation and scanner rotation. Radiology 1989; 173: 414.  Back to cited text no. 4    
5.McCollough Cynthia H, Liu Hui H. Breast dose during electron-beam CT: measurement with film dosimetry. Radiology 1995; 196: 153-157.  Back to cited text no. 5    
6.Maclennan AC. Radiation dose to the lens from coronal CT scanning of the sinuses. Clin Radiol 1995; 50: 265-267.  Back to cited text no. 6  [PUBMED]  
7.Hopper KD, King SH, Lobell ME et al . The breast: in plane x-ray protection during diagnostic thoracic CT-Shielding with Bismuth radioprotective garments. Radiology 1997; 205: 853-858.  Back to cited text no. 7    
8.Hoffman DA, Lonstein JE, Morin MM et al . Breast cancer in women with scolisis exposed to multiple diagnostic x-rats. J. Natl Cancer Iinst, 1989; 81: 1307-1312.  Back to cited text no. 8    
9.Rehani MM, Govinda Rajan KN. (Eds) Radiation protection for doctors, Association of Medical Physicists of India, 1997; 25.  Back to cited text no. 9    
10.ICRP Publication 60. 1990 Recommendations of the International Commission on Radiological Protection. Annals of ICRP 1991; Oxford: Pergamon Press.   Back to cited text no. 10    
11.Wall BF, Hart D. Revised radiation doses for typical X-ray examinations, report on a recent review of doses to patients from medical X-ray examinations in the UK by NRPB. Br J Radiol 1997;70:437-439.  Back to cited text no. 11  [PUBMED]  [FULLTEXT]
12.Naidich DP, Marshall CH, Gribbin C, Arams RS, McCauley DI. Low dose CT of the lungs: preliminary observations. Radiology 1990; 175: 729-731.  Back to cited text no. 12  [PUBMED]  
13.Lee Soo Kyung, Primack L Steven, Staples Catherin A, Mayo John R. Chronic infiltrative lung disease: comparison of diagnostic accuracies of radiography and low-and conventional dose thin-section CT. Radiology 1994; 191: 669-673.  Back to cited text no. 13    
14.Robinson Alan. Radiation protection and patient doses in diagnostic radiology. In: Grainger RG and Allison DJ, Eds. A Textbook of Medical Imaging, New York, Churchill Livingstone, 1997; 169-189.  Back to cited text no. 14    

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Correspondence Address:
M M Rehani
Medical Physics Dept., IRCH, All India Institute of Medical Science, IR Cancer Hospital, New Delhi
India
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Source of Support: None, Conflict of Interest: None


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